1 00:00:20,230 --> 00:00:21,230 *36c3 preroll music* 2 00:00:21,230 --> 00:00:24,140 Herald: Our next talk's topic is the Large Hadron Collider infrastructure talk. You 3 00:00:24,140 --> 00:00:29,770 probably know the Large Hadron Collider over at CERN. We heard quite a bit of it 4 00:00:29,770 --> 00:00:37,080 in the recent talks. This time we will have a deep dive into the infrastructure. 5 00:00:37,080 --> 00:00:43,250 You can assume our next speakers are doing a great job. Basically, it's pretty 6 00:00:43,250 --> 00:00:48,360 obvious because we are not stucked into a great into a giant supermassive black 7 00:00:48,360 --> 00:00:55,530 hole. So please welcome with a very warm applause, Severin and Stefan. 8 00:00:55,530 --> 00:01:01,200 *Applause* 9 00:01:01,200 --> 00:01:08,530 Severin: Yeah. Hello, everyone. Thanks for coming. So many people here, quite nice. 10 00:01:08,530 --> 00:01:12,580 In the last couple of years we had several talks about, yeah, basically the physics 11 00:01:12,580 --> 00:01:19,330 perspective of LHC, how physicists analyze data at LHC, how physicists store all the 12 00:01:19,330 --> 00:01:25,009 data, et cetera. And we would like to give like more an engineering perspective of 13 00:01:25,009 --> 00:01:30,039 the whole LHC. So three years ago we had a talk by Axel about how physicists analyze 14 00:01:30,039 --> 00:01:35,311 massive big data and then last year we had a talk conquering large numbers at LHC by 15 00:01:35,311 --> 00:01:40,369 Carsten and Stefanie. And we would, as I've mentioned already, we would like to 16 00:01:40,369 --> 00:01:47,069 give like more an engineering perspective. We are Stefan and Severin. We're both 17 00:01:47,069 --> 00:01:50,420 electrical engineers working at CERN. Stefan is working in the experimental 18 00:01:50,420 --> 00:01:54,929 physics microelectronics section and he will give a second talk tomorrow about 19 00:01:54,929 --> 00:01:59,700 designing high reliability digital electronics together with Szymon tomorrow 20 00:01:59,700 --> 00:02:05,349 morning at 11:30. And I'm, as I mentioned already, also working at CERN. I'm working 21 00:02:05,349 --> 00:02:10,119 in the electronics for machine protection section. I will describe briefly later. A 22 00:02:10,119 --> 00:02:17,629 short disclaimer; the LHC is a pretty big machine and we try to explain it as good 23 00:02:17,629 --> 00:02:21,870 as possible. 45 minutes is not really enough to talk about everything because I 24 00:02:21,870 --> 00:02:26,209 think you can basically take one of the topics we are talking here about now and 25 00:02:26,209 --> 00:02:31,069 talk for 45 minutes only about one specific topic, but we try to give an 26 00:02:31,069 --> 00:02:36,680 overview as good as possible. So imagine you want to build an accelerator in your 27 00:02:36,680 --> 00:02:41,540 backyard. OK, maybe not in your backyard because LHC is quite big, so 27 kilometers 28 00:02:41,540 --> 00:02:47,379 in diameter is quite big, but basically we figured out three main challenges you have 29 00:02:47,379 --> 00:02:50,680 to take. First of all, we have to accelerate particles because otherwise 30 00:02:50,680 --> 00:02:54,959 it's not a particle accelerator. Second, we have to keep the particles on a 31 00:02:54,959 --> 00:02:59,400 circular trajectory. And then third, we have to make sure that the particles which 32 00:02:59,400 --> 00:03:03,709 are inside our beam tube or beam pipe don't collide with anything which is 33 00:03:03,709 --> 00:03:08,939 there, for example, to beam pipe itself, air molecules, etc. And the solution we 34 00:03:08,939 --> 00:03:12,760 adopted for LHC there is, that we accelerate the particles with a high power 35 00:03:12,760 --> 00:03:16,980 radio frequency cavities. Then we have a beam control system which is quite 36 00:03:16,980 --> 00:03:22,010 sophisticated using superconducting magnets and then we have the beam pipe 37 00:03:22,010 --> 00:03:26,599 itself, which is evacuated, so it's under vacuum conditions to avoid any collisions 38 00:03:26,599 --> 00:03:33,069 we have inside with gas molecules, etc. A brief overview about the location itself. 39 00:03:33,069 --> 00:03:39,139 So probably many of you know already that CERN is next to Geneva. So it's in the 40 00:03:39,139 --> 00:03:43,079 western-southern part of Switzerland. When we zoom in a little bit more, then we have 41 00:03:43,079 --> 00:03:49,900 here an artificial like picture of LHC itself in the red circle there. To put it 42 00:03:49,900 --> 00:03:53,480 a little bit in a perspective, we have a relatively big airport there. You can see 43 00:03:53,480 --> 00:03:58,940 there, it's a 2200 metre long runway. We have Geneva Lake next to it. And that's 44 00:03:58,940 --> 00:04:03,469 only one small part of Geneva Lake, but nevertheless, and what also quite nice, we 45 00:04:03,469 --> 00:04:10,450 see Mont Blanc from LHC, er, from CERN. When we zoom in a little bit more, then we 46 00:04:10,450 --> 00:04:14,680 basically have the big, circular collider there. That's LHC itself. And we have pre- 47 00:04:14,680 --> 00:04:19,570 accelerators, I will talk in a few minutes about. Basically we have two main 48 00:04:19,570 --> 00:04:23,300 campuses: we have Meyrin Site, which is in Switzerland, and we have to Prevessin 49 00:04:23,300 --> 00:04:28,850 site, which is in France. Then at LHC itself, we have eight service points. We 50 00:04:28,850 --> 00:04:32,669 also call it just points, to briefly go through all of them; we have point one 51 00:04:32,669 --> 00:04:38,970 where we have the experiment called ATLAS, one of the big and major experiments at 52 00:04:38,970 --> 00:04:45,410 LHC. Then at the exactly opposite side of ATLAS, we have CMS at point five. Then we 53 00:04:45,410 --> 00:04:47,780 have a little bit smaller experiment, which is ALICE. It was basically 54 00:04:47,780 --> 00:04:51,950 constructed for lead ion runs. We will talk about this later. And then we have 55 00:04:51,950 --> 00:04:55,790 another relatively small experiment, it's called LHCb. And that's the only non- 56 00:04:55,790 --> 00:05:00,810 symmetrical experiment at LHC. These are, I think, the four experiments you already 57 00:05:00,810 --> 00:05:06,170 maybe heard of. Then there are four or three other smaller experiments. We have 58 00:05:06,170 --> 00:05:10,690 LHCf at point one, it's a forward scattering experiment at point one. So 59 00:05:10,690 --> 00:05:15,250 basically, they're taking data like scattered particles from ATLAS itself. 60 00:05:15,250 --> 00:05:19,871 Then we have TOTEM. It's also a forward scattering experiment and point five, then 61 00:05:19,871 --> 00:05:26,850 we have, sorry, we have MOEDAL, which is the experiment at point eight. They're 62 00:05:26,850 --> 00:05:33,260 looking for magnetic mono-poles. Then we have TOTEM, sorry for that, at point five. 63 00:05:33,260 --> 00:05:37,880 And then we have a relatively new experiment which is called PHASER. It's 64 00:05:37,880 --> 00:05:41,570 actually under construction and it will be used, starting from 2021 and it's forward 65 00:05:41,570 --> 00:05:47,720 scattering experiment, which, where they try to detect neutrinos. Then we have 66 00:05:47,720 --> 00:05:52,810 point four, there we have the RF cavities to accelerate the particle beam itself. We 67 00:05:52,810 --> 00:05:57,450 have the beam dump area. So when there is like a fault in a machine or we just want 68 00:05:57,450 --> 00:06:01,800 to dump the beam, then we used the mean dump system at point six. And then we have 69 00:06:01,800 --> 00:06:07,730 two more general service areas. It's point three and point seven. LHC would not be 70 00:06:07,730 --> 00:06:12,100 possible without the pre-accelerator complex. So we have a relatively big one 71 00:06:12,100 --> 00:06:17,350 and it's also sometimes relatively old. On the left hand side of the slide you can 72 00:06:17,350 --> 00:06:24,680 see LINAC2, it's an old linear accelerator which was used until last year. It's not 73 00:06:24,680 --> 00:06:27,920 now phased out. And now we have LINAC4, which is also a linear accelerator and it 74 00:06:27,920 --> 00:06:34,970 has a little bit higher acceleration. Then we have the proton synchrotron booster. 75 00:06:34,970 --> 00:06:40,310 It's the first circular collider. So you can see two pictures there. What is 76 00:06:40,310 --> 00:06:44,560 relatively special about PSB is that we have there two, sorry, four beam pipes 77 00:06:44,560 --> 00:06:49,970 instead of just one beam pipe. Then we have the proton synchrotron accelerator, 78 00:06:49,970 --> 00:06:56,620 which is the next stage for acceleration. It then has only one one beam pipe. And 79 00:06:56,620 --> 00:07:01,920 then we are going from PS we are going to SPS, which is the super proton synchrotron, 80 00:07:01,920 --> 00:07:07,740 which is has circumferences of seven kilometers. There we basically accelerate 81 00:07:07,740 --> 00:07:13,230 the particles the last time and then they are injected it in the LHC itself. We 82 00:07:13,230 --> 00:07:17,970 mentioned a few accelerators already, basically all everything we just 83 00:07:17,970 --> 00:07:23,040 highlighted here. But CERN is a little bit more. So CERN is famous for LHC, I would 84 00:07:23,040 --> 00:07:27,280 say. But there is much more than only the LHC. So only about 15 percent of the 85 00:07:27,280 --> 00:07:30,880 protons, which are accelerated in the pre- accelerator complex, are really going to 86 00:07:30,880 --> 00:07:36,620 the LHC itself. So there is much more: there is material science, there is anti 87 00:07:36,620 --> 00:07:43,460 matter research and all different other kinds of research going on. Of course, 88 00:07:43,460 --> 00:07:47,190 everything has to be controlled. It's called a CCC, the CERN control center. 89 00:07:47,190 --> 00:07:53,310 It's located at, the Prevessin site; looks like that. Basically, we have, four 90 00:07:53,310 --> 00:07:58,660 Cs looking to each other and there the operators are sitting 24/7 and operate the 91 00:07:58,660 --> 00:08:03,650 whole machine. So basically, the whole pre-accelerator complex, all the energy 92 00:08:03,650 --> 00:08:09,540 cryogenics and LHC itself. Before you ask, everything is running on Scientific Linux. 93 00:08:09,540 --> 00:08:13,770 So we have basically our own Linux distributed distribution, which is used 94 00:08:13,770 --> 00:08:19,280 there and it of course, it's open source. Talking about the LHC beam itself, we have 95 00:08:19,280 --> 00:08:22,830 two beams: one is running clockwise and the other one is to running anti- 96 00:08:22,830 --> 00:08:27,340 clockwise because we don't have a fixed target experiment where we basically let 97 00:08:27,340 --> 00:08:30,890 the accelerated particles colliding with like a fixed target, like metal or 98 00:08:30,890 --> 00:08:35,930 something like that. We have controlled collisions at four points, we mentioned 99 00:08:35,930 --> 00:08:42,180 before. Most of the year we have proton runs. So we have protons and protons 100 00:08:42,180 --> 00:08:46,520 colliding each towards each other. And then we have at the end of the year, 101 00:08:46,520 --> 00:08:52,280 nearly starting from November to December, we have lead ion run. The protons itself 102 00:08:52,280 --> 00:08:56,870 is not really like a fixed, straight line of particles. We have something called 103 00:08:56,870 --> 00:09:01,810 bunches. You can imagine a little bit like spaghetti. It's basically the same length 104 00:09:01,810 --> 00:09:06,949 of a spaghetti, but it is much thinner than a spaghetti. And each bunch, when you 105 00:09:06,949 --> 00:09:11,490 have a proton run, then each bunch consists of approximately 100 billion 106 00:09:11,490 --> 00:09:15,689 protons. And when you have lead ion runs, then we have approximately 10 million lead 107 00:09:15,689 --> 00:09:25,680 ions in LHC. And last year we operated with 2565(sic) bunches in the LHC itself. 108 00:09:25,680 --> 00:09:29,259 The LHC tunnel. We already talked about the tunnel itself. It is 27 kilometers 109 00:09:29,259 --> 00:09:33,839 long and you can see maybe a little bit on this graph, that we have some, we have 110 00:09:33,839 --> 00:09:38,920 eight straight sections and we have eight arcs in the tunnel. Basically the straight 111 00:09:38,920 --> 00:09:42,560 sections are always there, where we have like service cavities or we have areas and 112 00:09:42,560 --> 00:09:48,069 also the experiments. Because it's not so good visible in this picture, I put the 113 00:09:48,069 --> 00:09:51,700 picture here. Basically, that's a straight section of LHC. You can basically just see 114 00:09:51,700 --> 00:09:56,100 the beam pipe itself, with aluminum foil around it, and there are also no magnets. 115 00:09:56,100 --> 00:10:01,339 And when we look in the arc section of LHC, then you see here the arc itself and 116 00:10:01,339 --> 00:10:06,529 I think it's quite famous picture of LHC itself because we have blue dipole magnets 117 00:10:06,529 --> 00:10:12,509 there. The tunnel itself is an old tunnel, used previously by LEP, the large electron 118 00:10:12,509 --> 00:10:17,990 proton collider. It has a diameter of 3.8 metres and the circumference is 119 00:10:17,990 --> 00:10:24,449 approximately 27 kilometers. Inside the tunnel we have, first of all, cryogenics, 120 00:10:24,449 --> 00:10:29,350 so we have big tubes, stainless steel tubes to carry all the cryogenic. So 121 00:10:29,350 --> 00:10:33,889 liquid helium and gaseous helium. Then we have the magnet itself to bend the 122 00:10:33,889 --> 00:10:38,809 particles and then we have electrical installations to carry like signals from 123 00:10:38,809 --> 00:10:43,779 the magnets to have safety systems, electricity, etc, etc. Geography is a 124 00:10:43,779 --> 00:10:49,829 little bit complicated in the area because we have in the western part of LFC we have 125 00:10:49,829 --> 00:10:55,029 the Jura mountain range and this Jura mountain range has a relatively hard 126 00:10:55,029 --> 00:11:00,190 material. It's not made out of, not made, but nature. I mean it's a limestone, so 127 00:11:00,190 --> 00:11:04,110 it's relatively complicated to dig into this material, in comparison to all the 128 00:11:04,110 --> 00:11:08,749 other areas at LHC. So when you would basically put a straight section of LHC, 129 00:11:08,749 --> 00:11:14,059 then you have to dig much more into the relatively hard limestone. So that's why 130 00:11:14,059 --> 00:11:18,480 it was decided that the LEP or LHC tunnel is tilted a little bit. So we have a tilt 131 00:11:18,480 --> 00:11:23,499 angle of one 1.4 percent there. The depth is approximately between 50 meters at 132 00:11:23,499 --> 00:11:32,000 point one or point eight, up to 170 metres deep at point four. We already talked a 133 00:11:32,000 --> 00:11:34,519 little bit about magnets, but we would like to go a little bit more in the 134 00:11:34,519 --> 00:11:40,639 details now. So why do we need magnets? Um, maybe you learned at school that when 135 00:11:40,639 --> 00:11:43,559 you have a magnetic field and you have charged particles and you can bend 136 00:11:43,559 --> 00:11:48,730 particles around an arc in a magnetic field. Depending on the charge of the 137 00:11:48,730 --> 00:11:52,009 particles, you bend them around on the right side or left side, that's this 138 00:11:52,009 --> 00:11:57,870 famous right hand and left side rule, you maybe learned during school. And at LHC we 139 00:11:57,870 --> 00:12:02,449 cannot use a normal magnets like typical magnets. We have to use electromagnets 140 00:12:02,449 --> 00:12:06,160 because normal magnets would not be strong enough to build in an electromagnetic 141 00:12:06,160 --> 00:12:11,759 field which is feasible to bend the particles around the whole tunnel. At LHC 142 00:12:11,759 --> 00:12:16,819 we use, in the dipole magnets, a magnetic field of 8.3 Tesla. And to do this we need 143 00:12:16,819 --> 00:12:24,329 a current of 11850 amps. We have basically two different types of magnets. We have 144 00:12:24,329 --> 00:12:29,050 bending magnets. So the dipole magnets I mentioned quite often already. And then we 145 00:12:29,050 --> 00:12:31,689 have injection and extraction magnets. They are also dipole magnets there, but 146 00:12:31,689 --> 00:12:35,620 they are a little bit differently constructed, because the injection and 147 00:12:35,620 --> 00:12:39,699 extraction magnets have to be quite fast, because they have to basically be powered 148 00:12:39,699 --> 00:12:47,350 up at full, the full magnetic field in several microseconds. Then we have higher 149 00:12:47,350 --> 00:12:50,660 order magnets which are quadrupole pool, magnet, sextupole magnets and octupole 150 00:12:50,660 --> 00:12:54,690 magnets, et cetera, et cetera. And they are used for focusing and defocusing the 151 00:12:54,690 --> 00:13:01,449 beam itself. In total, we have 1200 dipole magnets at LHC. We have around 850 152 00:13:01,449 --> 00:13:06,619 quadrupole magnets and we have 4800 higher order magnets. But they are normally quite 153 00:13:06,619 --> 00:13:12,559 short or so, shorter than the the other magnets. The dipole magnets consist of two 154 00:13:12,559 --> 00:13:20,060 apertures. They are used to bend to beam around, so I already said. In the middle 155 00:13:20,060 --> 00:13:24,519 of the magnet itself we have a cold bore. So there is basically there are the 156 00:13:24,519 --> 00:13:28,600 particles flying around. Then there is a metallic structure. You can see this in 157 00:13:28,600 --> 00:13:33,279 the picture. It is just a shiny metallic sphere you see there. And then we have 158 00:13:33,279 --> 00:13:38,720 next to the cold bore, we have the tool, the two apertures to bend the particle and 159 00:13:38,720 --> 00:13:43,850 build the magnetic field itself. The dipole magnets have a length of 50 meters 160 00:13:43,850 --> 00:13:47,889 and the manufacturing precision is plus minus one fine, one point five percent, 161 00:13:47,889 --> 00:13:53,660 er, one point five millimeters. Then we have quadrupole magnets. They are used for 162 00:13:53,660 --> 00:13:58,519 focusing and defocusing the beam. The problem is that we have bunches, were are 163 00:13:58,519 --> 00:14:04,440 basically equally charged particles inside. And the Coloumb force tells us, that when 164 00:14:04,440 --> 00:14:08,970 we have equally charged particles, then they are basically want to fade out from 165 00:14:08,970 --> 00:14:12,649 each other and in the end they would just hit the beam pipe itself and we could 166 00:14:12,649 --> 00:14:17,200 maybe destroy the beam or cannot do any collisions. So what we do is we use a 167 00:14:17,200 --> 00:14:22,470 quadruple magnets as, yeah, similar to lenses, because we can focus and defocus 168 00:14:22,470 --> 00:14:28,490 the beam. The quadrupole magnets, the name already suggested it, that we have 169 00:14:28,490 --> 00:14:32,380 basically four apertures. So we have on the left and the right side two and then 170 00:14:32,380 --> 00:14:38,459 we have on top and bottom we have also a few of them. To go a little bit into 171 00:14:38,459 --> 00:14:42,220 detail about the focusing and defocusing scheme. In the beginning we have a 172 00:14:42,220 --> 00:14:47,240 particle beam which is not focused, but we want to focus it. Then we go to the first 173 00:14:47,240 --> 00:14:53,679 quadrupole magnet. So we focus the beam. And this is only done in one axis. That's 174 00:14:53,679 --> 00:14:57,209 a little bit a problem. So, in the second axis we don't have any focus and we have a 175 00:14:57,209 --> 00:15:00,990 defocusing effect there. And then we have to use a second quadrupole magnet for the 176 00:15:00,990 --> 00:15:05,709 other axis, in this case the Y-axis to focus the beam even further. And you can 177 00:15:05,709 --> 00:15:10,800 even see this here in the Z-axis, that's basically the cut off the beam itself. You 178 00:15:10,800 --> 00:15:14,050 can also see that in the beginning we have on the left side, we have an unfocused 179 00:15:14,050 --> 00:15:17,929 beam and then we focus it in one axis, so we have like a little bit more ellipse and 180 00:15:17,929 --> 00:15:22,069 then we focus in the other direction, then we have a different ellipse. So we have to 181 00:15:22,069 --> 00:15:25,509 use several quadruple magnets in a row to really focus the beam in the way we want 182 00:15:25,509 --> 00:15:33,190 to have it. In the LHC magnets, we have quite high currents. We we need these 183 00:15:33,190 --> 00:15:39,179 currents, because otherwise we cannot bend to the very high energetic particle beam 184 00:15:39,179 --> 00:15:44,050 and to use normal conducting cable, it would not be possible to basically build a 185 00:15:44,050 --> 00:15:47,929 magnet out of it. So what we do is, we use materials which are called superconducting 186 00:15:47,929 --> 00:15:54,170 materials, because they're for very good effect. They go to basically zero 187 00:15:54,170 --> 00:16:00,410 resistance at a specific temperature point. And after this point or when we 188 00:16:00,410 --> 00:16:08,149 basically go lower, then the current can flow without any losses inside of it. But 189 00:16:08,149 --> 00:16:10,860 to reach the state, we have to cool the magnets quite heavily, which is not so 190 00:16:10,860 --> 00:16:16,889 easy, but it can be done. And on the right side you basically see a very historic 191 00:16:16,889 --> 00:16:22,429 plot. That was 1911 in Denmark, a researcher called Heike Onnes detected for 192 00:16:22,429 --> 00:16:27,220 the first time superconducting effect in mercury. And it was detected at 4.19 193 00:16:27,220 --> 00:16:32,509 Kelvin. To show you a little bit the comparison between a normal conducting 194 00:16:32,509 --> 00:16:37,189 cable and a superconducting cable, as we put the picture here. So that is basically 195 00:16:37,189 --> 00:16:43,529 the same amount of cable you need to use to carry, the thirteen thousand amps and 196 00:16:43,529 --> 00:16:47,509 to do the same or to transport the same amount of energy we also can use a very 197 00:16:47,509 --> 00:16:51,130 small superconducting cable and I think it's quite obvious why we use here 198 00:16:51,130 --> 00:16:57,639 superconducting cables. At the LHC we use Niobium tin(sic) as material. And this 199 00:16:57,639 --> 00:17:01,470 material basically goes into a superconducting state at 10 Kelvin. But to 200 00:17:01,470 --> 00:17:07,530 have a safe operation to LHC, we have to cool it down at 1.9 Kelvin. 201 00:17:07,530 --> 00:17:11,500 Superconducting magnets have some benefits, but also some downsides, so 202 00:17:11,500 --> 00:17:16,439 sometimes they change their state because there are small rigid vibrations and the 203 00:17:16,439 --> 00:17:20,650 magnet or the temperature's not precise enough or the current is too high, then 204 00:17:20,650 --> 00:17:25,240 they change that state and it's called "Quench". And, we basically can detect a 205 00:17:25,240 --> 00:17:29,570 Quench when we measure the voltage across the magnet, because the resistance changes 206 00:17:29,570 --> 00:17:33,519 at this point. So when there is a Quench then the resistance changes quite rapidly, 207 00:17:33,519 --> 00:17:38,330 in milliseconds and we can detect this voltage rise with sophisticated 208 00:17:38,330 --> 00:17:42,530 electronics. On the right side, you see a board I'm working on. So basically here, 209 00:17:42,530 --> 00:17:47,610 we have a measuring system to measure the voltage across the magnet. And then we 210 00:17:47,610 --> 00:17:54,340 have a detection logic implemented in FPGA to basically send triggers out and open an 211 00:17:54,340 --> 00:17:58,549 Interlock loop. Interlock loop is a system at LHC. You can imagine that little bit 212 00:17:58,549 --> 00:18:03,269 like a cable going around the whole tunnel and there are thousands of switches around 213 00:18:03,269 --> 00:18:08,510 this Interlock loop. And as soon as one of the detection systems basically opens to 214 00:18:08,510 --> 00:18:13,669 the interlock loop, then basically the whole machine will be switched off. And 215 00:18:13,669 --> 00:18:16,530 what means switched off is basically, that we will power down the power converter, 216 00:18:16,530 --> 00:18:20,039 but then the energy is still in the superconducting magnet and it has to be 217 00:18:20,039 --> 00:18:24,190 taken out of the superconducting magnet. And therefore, we use dump resistors to 218 00:18:24,190 --> 00:18:30,600 extract the energy. And here you can see a picture of such a dump resistor. It's 219 00:18:30,600 --> 00:18:34,960 quite big. It's in a stainless steel tube, oil cooled. It's approximately three or 220 00:18:34,960 --> 00:18:41,470 four meters long. And basically, when there was a Quench, and the energy was 221 00:18:41,470 --> 00:18:44,480 extracted via these resistors, the whole resistor is heated up by several hundred 222 00:18:44,480 --> 00:18:50,179 degrees and it needs several hours to cool it down again. Power converters; the power 223 00:18:50,179 --> 00:18:54,679 converters are used to power the magnet itself. So they can produce a current of 224 00:18:54,679 --> 00:19:01,110 approximately 13000 amps and a voltage of plus minus 190 volts. And you can see a 225 00:19:01,110 --> 00:19:06,310 picture how here, how big it is. One downside with the power converters is that 226 00:19:06,310 --> 00:19:10,269 they have to be, not downside but one difficulty is, that they have to be very 227 00:19:10,269 --> 00:19:16,210 precise, because every instability in the current would have or has a direct effect 228 00:19:16,210 --> 00:19:20,290 on the beam stability itself. So basically the beam would be not diverted in the 229 00:19:20,290 --> 00:19:27,169 right amount of length. So that's why they have to be very precise and have to have a 230 00:19:27,169 --> 00:19:32,409 very precise stability. So here I just pointed out, like in 24 hours, the power 231 00:19:32,409 --> 00:19:36,700 converter is only allowed to have a deviation of 5 ppm. And in comparison, for 232 00:19:36,700 --> 00:19:41,970 13000 amps we have a deviation of 65 milli amps. So the power converters have to be 233 00:19:41,970 --> 00:19:46,600 very precise. And to do that, we had to develop our own ADC, because at the time 234 00:19:46,600 --> 00:19:51,270 when LHC was built, there was no ADC on the market which was able to have this 235 00:19:51,270 --> 00:19:55,900 precision and also the whole ADC is put into a super-precise temperature 236 00:19:55,900 --> 00:20:02,970 controlled areas and it is calibrated quite regularly. Okay, cryogenics. We 237 00:20:02,970 --> 00:20:05,520 already talked about that we have superconducting magnets and they have to 238 00:20:05,520 --> 00:20:11,340 be cooled down quite low. So the superconducting magnets we have at LHC has 239 00:20:11,340 --> 00:20:17,230 have to be cooled down to 1.9 Kelvin. And we are doing this when we like start the 240 00:20:17,230 --> 00:20:21,610 LHC then we cool down on the first hand with liquid nitrogen. So approximately six 241 00:20:21,610 --> 00:20:26,429 thousand tonnes of liquid nitrogen are put through the magnets to cool them down 242 00:20:26,429 --> 00:20:33,260 to 18 Kelvin and afterwards we cool the magnets down with liquid helium. And 243 00:20:33,260 --> 00:20:38,960 liquid helium is at 1.9 or 1.8 Kelvin. And to put it a little bit in a comparison, 244 00:20:38,960 --> 00:20:42,200 outer space, so when we measure like the temperature of space, we have 245 00:20:42,200 --> 00:20:47,799 approximately 2.7 Kelvin in outer space. So LHC is much colder than outer space. 246 00:20:47,799 --> 00:20:52,289 The whole cooldown needs approximately one month and each dipole magnet, which is 15 247 00:20:52,289 --> 00:20:57,010 meters long, shrinks several centimeters during that. Which also has to be taken 248 00:20:57,010 --> 00:21:02,870 into account, because otherwise pipes would break. The cryogenic system is that 249 00:21:02,870 --> 00:21:08,980 we have at each of the eight points at LHC we have compressors to cool down the 250 00:21:08,980 --> 00:21:13,860 liquid helium or the helium itself. And then we compress the helium and pump it 251 00:21:13,860 --> 00:21:17,970 down. We have one gaseous helium stream, which is at 15 Kelvin and we have liquid 252 00:21:17,970 --> 00:21:24,490 helium stream at approximately 4.5 Kelvin. And then we pump it underground and then 253 00:21:24,490 --> 00:21:28,580 we have something called Cold Compression System. And the Cold Compression System 254 00:21:28,580 --> 00:21:36,090 even further reduces the pressure of the helium that we have in the end a helium, 255 00:21:36,090 --> 00:21:40,980 which is at 1.8 Kelvin. So it can really cool down the magnet itself. And helium 256 00:21:40,980 --> 00:21:45,600 has a very interesting effect because at 2.1 Kelvin, it becomes something called 257 00:21:45,600 --> 00:21:52,350 superfluid. So it basically can run around like holes, for example, or walls. It can 258 00:21:52,350 --> 00:21:57,679 basically flow against gravity, which is quite interesting. And it has also very 259 00:21:57,679 --> 00:22:03,860 high thermal conductivity and that's also why we use superfluid helium here. And 260 00:22:03,860 --> 00:22:08,370 that's why we cool down the whole magnets that low. And one other interesting effect 261 00:22:08,370 --> 00:22:13,570 is also that the LHC tilt angle, which is 1.4 percent, has to be taken into account 262 00:22:13,570 --> 00:22:18,320 because we have very low pressure inside all the tubes or all the system, at 16 263 00:22:18,320 --> 00:22:24,539 millibars. But we have sometimes to pump the helium against gravity or going down. 264 00:22:24,539 --> 00:22:28,481 So we also have to take into account the LHC tilt angle to not have wrong pressure 265 00:22:28,481 --> 00:22:34,519 levels at the whole LHC itself. Okay. Stefan: All right! So, you probably 266 00:22:34,519 --> 00:22:38,649 already got the idea, that what we've done in the last 20 minutes, was only solve the 267 00:22:38,649 --> 00:22:42,250 first of the three challenges we had, which was actually bending the beam around 268 00:22:42,250 --> 00:22:49,269 the circular trajectory. So I'm trying to go to the other challenges we have lined 269 00:22:49,269 --> 00:22:54,350 up in the beginning. And the first one of that is the actual acceleration of the 270 00:22:54,350 --> 00:22:59,700 particle beam. And large synchrotrons, e.g. like the LHC, they use radio 271 00:22:59,700 --> 00:23:05,299 frequency or RF systems to do this acceleration. And I'm just going to do a 272 00:23:05,299 --> 00:23:10,510 quick recap of the LHC beam and RF and how they interact. So Severin mentioned 273 00:23:10,510 --> 00:23:15,600 already that the particles in LHC actually come in bunches. So in like packets that 274 00:23:15,600 --> 00:23:20,309 contain about hundred billion protons and those bunches are spaced when they are 275 00:23:20,309 --> 00:23:26,470 running around the LHC approximately 25 nanoseconds apart. And starting from that 276 00:23:26,470 --> 00:23:31,250 the tasks of the RF system are basically twofold. It first has to ensure that these 277 00:23:31,250 --> 00:23:35,630 bunches are kept tightly together in a process that we call longitudinal 278 00:23:35,630 --> 00:23:39,700 focusing. And the second task is to care for the actual acceleration of the 279 00:23:39,700 --> 00:23:44,570 particle bunches. So from their injection energy, when they come from one of the 280 00:23:44,570 --> 00:23:49,039 pre-accelerators up to their final energy, that they are supposed to collide at 281 00:23:49,039 --> 00:23:55,549 during the physics run. So in general, you can imagine RF as being a quickly 282 00:23:55,549 --> 00:24:03,260 alternating electric and magnetic field components. And in the LHC, this RF energy 283 00:24:03,260 --> 00:24:07,700 is basically injected into what is called a cavity, which is a resonant structure. 284 00:24:07,700 --> 00:24:11,520 And there the particle beams travels through, while the field quickly 285 00:24:11,520 --> 00:24:16,690 alternates and the RF signal, or the energy, basically interacts with the 286 00:24:16,690 --> 00:24:21,750 particle beam. So perhaps you know that the protons are positively charged and 287 00:24:21,750 --> 00:24:26,210 then a negative polarity of the field would attract these protons, while the 288 00:24:26,210 --> 00:24:32,320 positive field location would basically move them away. And this has ... well, 289 00:24:32,320 --> 00:24:36,260 after just injecting and with the frequency of this RF field being the same 290 00:24:36,260 --> 00:24:41,390 as the speed that the particles actually go round the LHC, solves the first of the 291 00:24:41,390 --> 00:24:45,110 two problems, which was the the focusing because actually the particles that are 292 00:24:45,110 --> 00:24:49,370 too slow arrive only when the field is already changed to the opposite polarity 293 00:24:49,370 --> 00:24:52,990 and actually get accelerated a bit, while the particles that are too fast, they are 294 00:24:52,990 --> 00:24:58,000 actually being decelerated a bit. And this is a process that we call the longitudinal 295 00:24:58,000 --> 00:25:03,490 focusing, which makes sure that the bunches stay neatly packed together. And 296 00:25:03,490 --> 00:25:06,090 of course this would be relatively inefficient if we would only change the 297 00:25:06,090 --> 00:25:12,399 polarity of this field once for each of the proton bunches that pass by. Which is 298 00:25:12,399 --> 00:25:15,870 why we do it ten times. So the polarity basically changes ten times or the 299 00:25:15,870 --> 00:25:21,370 frequency is ten times higher than the bunch crossing frequency. And by doing 300 00:25:21,370 --> 00:25:25,960 that, we make sure that the change of this field is much faster and therefore the 301 00:25:25,960 --> 00:25:33,070 particle bunches are packed much closer together and the focusing is better. So 302 00:25:33,070 --> 00:25:35,389 here you can see these cavities that were shown in the previous picture as a 303 00:25:35,389 --> 00:25:39,899 schematic, how they're actually placed in the tunnel. So eight of these huge 304 00:25:39,899 --> 00:25:43,850 cavities are used per beam and they are the actual thing that is used to couple 305 00:25:43,850 --> 00:25:49,730 the RF energy into the beam and transfer it to the particles. They are also 306 00:25:49,730 --> 00:25:54,179 operating superconductively, so at cryogenic temperatures, to reduce the 307 00:25:54,179 --> 00:25:59,610 thermal stress and the losses that would otherwise occur in their materials. And 308 00:25:59,610 --> 00:26:01,270 these are actually – even though they are so big, similar to the magnets that had to 309 00:26:01,270 --> 00:26:05,820 be very precisely manufactured – these also have very small manufacturing 310 00:26:05,820 --> 00:26:12,070 tolerances and have to be precisely tuned to the RF frequency that is used to 311 00:26:12,070 --> 00:26:17,440 inject. So and the second part of this, that actually produces this high power RF 312 00:26:17,440 --> 00:26:22,420 signal. For that is used what we call Klystrons. So Klystrons are basically RF 313 00:26:22,420 --> 00:26:29,330 amplifiers. They are built from high power RF vacuum tubes and they amplify this 400 314 00:26:29,330 --> 00:26:34,289 MHz signal that is used to transfer energy to the particles. And each of those 315 00:26:34,289 --> 00:26:40,100 Klystrons produces about 300 kW of power and you can probably imagine how much that 316 00:26:40,100 --> 00:26:43,889 power for an individual unit that is, if you know that your microwave oven has like 317 00:26:43,889 --> 00:26:49,460 2 or 3 kW. And of course, as we have eight cavities per beam and one Klystron always 318 00:26:49,460 --> 00:26:54,840 feeds one cavity, we in total have 16 of those Klystrons and they are in principle 319 00:26:54,840 --> 00:27:03,659 able to deliver a total energy of 4.8 MW into the LHC beam to accelerate it. But if 320 00:27:03,659 --> 00:27:06,640 we take a small step back for now, we have only solved the first of the two problems, 321 00:27:06,640 --> 00:27:12,510 which was to keep the bunches neatly focused. Because currently the particles 322 00:27:12,510 --> 00:27:17,429 have been injected and the frequency is at some specific frequency and actually they 323 00:27:17,429 --> 00:27:22,400 are only running basically in sync, the two. So what we do after all the particle 324 00:27:22,400 --> 00:27:26,410 bunches from the pre-accelerators have been injected into LHC, is that we ever so 325 00:27:26,410 --> 00:27:30,700 slightly increase the frequency, which of course also means that the particles need 326 00:27:30,700 --> 00:27:35,399 to accelerate together with the RF signal. And this is the mechanism that we use to 327 00:27:35,399 --> 00:27:40,159 accelerate them actually. And the change, that is required to do this, is very tiny, 328 00:27:40,159 --> 00:27:44,419 actually. So it is less than a thousandth of a percent sometimes, that is used to 329 00:27:44,419 --> 00:27:48,379 change the frequency to actually make them go so much faster. So from their 330 00:27:48,379 --> 00:27:54,070 relatively low injection energy up to the top energy plateau that they need to have 331 00:27:54,070 --> 00:28:00,409 to produce the actual physics collisions. And an interesting question to ask here is 332 00:28:00,409 --> 00:28:03,990 where does this signal actually comes from if it needs to be so precisely tuned to 333 00:28:03,990 --> 00:28:10,250 some specific frequency? Who generates it or who controls it? And that opens up the 334 00:28:10,250 --> 00:28:15,669 whole complex of the timing of the LHC, of the machine. So actually this first signal 335 00:28:15,669 --> 00:28:21,460 that I mentioned, this RF signal, it originates in a Faraday cage. So an 336 00:28:21,460 --> 00:28:25,880 especially shielded area somewhere on the Prévessin site of CERN. And from there it 337 00:28:25,880 --> 00:28:33,440 is distributed to the low-level RF subsystem with the Klystrons and the 338 00:28:33,440 --> 00:28:38,690 cavities. But inside this room, there are also a number of other signals generated. 339 00:28:38,690 --> 00:28:42,720 The first one of that being this Bunch Crossing Clock, which is the actual clock 340 00:28:42,720 --> 00:28:48,240 that signals one pulse, basically every time, it changes polarity one time a 341 00:28:48,240 --> 00:28:54,640 proton bunch moves across a specific location inside the LHC. And another one 342 00:28:54,640 --> 00:28:59,470 is the so-called orbit clock, which always indicates the start of the first or when 343 00:28:59,470 --> 00:29:04,990 one proton bunch has basically re-arrived at the same position and has completed one 344 00:29:04,990 --> 00:29:10,820 orbit. And you may ask the question why this is an important piece of information. 345 00:29:10,820 --> 00:29:16,529 But if you think back to this image that Severin has already shown, about the 346 00:29:16,529 --> 00:29:21,789 accelerator complex, the big challenge that all this brings is also the whole 347 00:29:21,789 --> 00:29:24,909 synchronization of all these machines. Because you have to imagine that while 348 00:29:24,909 --> 00:29:29,790 these proton bunches run around the LHC and new ones are supposed to be injected 349 00:29:29,790 --> 00:29:34,000 from the outside, from another pre- accelerator, this has to be very precisely 350 00:29:34,000 --> 00:29:37,840 synchronized. So all these pre-accelerator systems actually share a common 351 00:29:37,840 --> 00:29:42,840 synchronized timing system that allows them to precisely inject a new packet of 352 00:29:42,840 --> 00:29:49,380 bunches at the right position, at the right location into the LHC. And this a 353 00:29:49,380 --> 00:29:53,309 bit how such a timing distribution system looks like. It is only a very small 354 00:29:53,309 --> 00:29:56,430 excerpt of what it looks like, but it gives you an idea that somewhere 355 00:29:56,430 --> 00:30:01,100 underground in the LHC there is rooms full of equipment that is just used to 356 00:30:01,100 --> 00:30:06,110 distribute timing signals between different parts of the accelerator. And of 357 00:30:06,110 --> 00:30:11,909 course, as CERN is forward-thinking and realized that future colliders will need 358 00:30:11,909 --> 00:30:15,400 quite a bit more of all this synchronization and that the requirements 359 00:30:15,400 --> 00:30:20,050 for how precisely everything needs to be synchronized is ever growing, they 360 00:30:20,050 --> 00:30:23,300 actually developed their own timing distribution standard which is also 361 00:30:23,300 --> 00:30:28,270 openly available and available for everybody to use. So if you're interested, 362 00:30:28,270 --> 00:30:34,320 look that up. But of course, not only the accelerator itself is interested in this 363 00:30:34,320 --> 00:30:40,309 information about what particles are where and how quickly they interact or how 364 00:30:40,309 --> 00:30:45,050 quickly they go around. But also all the experiments need this information, because 365 00:30:45,050 --> 00:30:50,070 in the end they want to know "Okay, has a collision occurred at some specific time 366 00:30:50,070 --> 00:30:54,940 in my experiment?" and actually providing this timing information about when bunches 367 00:30:54,940 --> 00:30:59,789 have crossed their experiment locations is also vital for them to really time tag all 368 00:30:59,789 --> 00:31:06,149 their collision data and basically track which bunches were responsible for what 369 00:31:06,149 --> 00:31:11,559 kind of event or what event throughout their whole signal storage and processing 370 00:31:11,559 --> 00:31:17,120 chain, let's say. Good. So that is basically challenge 2 out of the way. So 371 00:31:17,120 --> 00:31:19,889 that was the acceleration of the actual particles and all the associated issues 372 00:31:19,889 --> 00:31:25,899 with timing. And the third issue we mentioned was that the particles need to, 373 00:31:25,899 --> 00:31:31,330 let's say, be kept from colliding with anything but themselves or the other beam. 374 00:31:31,330 --> 00:31:36,210 And that is what we, why we need vacuum systems for. So, again, it is not as 375 00:31:36,210 --> 00:31:41,200 simple as just putting a vacuum somewhere. Of course not. Because in fact, there is 376 00:31:41,200 --> 00:31:45,740 not only one vacuum system at LHC, but there are three. So, the first two of 377 00:31:45,740 --> 00:31:51,090 those are perhaps a bit less interesting to most of us. They are mainly insulation 378 00:31:51,090 --> 00:31:56,919 vacuum systems that are used for the cryogenic magnets. So they isolate, 379 00:31:56,919 --> 00:32:02,789 basically thermally isolate the magnets at those very cool temperatures from the 380 00:32:02,789 --> 00:32:08,789 surrounding air to avoid them getting more heat load than they need to. And there is 381 00:32:08,789 --> 00:32:11,830 an insulation vacuum also for the helium distribution lines that are actually 382 00:32:11,830 --> 00:32:16,299 distributing, delivering the helium to these magnets. And then the third one, 383 00:32:16,299 --> 00:32:19,820 which is perhaps the most interesting one, is the beam vacuum. So the one where 384 00:32:19,820 --> 00:32:24,999 actually the beam circulates inside the LHC. And this is a cross section of what 385 00:32:24,999 --> 00:32:30,242 this beam vacuum typically looks like. So it is approximately this size, so a very 386 00:32:30,242 --> 00:32:37,470 ... handful, let's say. And the question you may ask "OK, if I want to keep all the 387 00:32:37,470 --> 00:32:41,590 like the particles in my particle beam from colliding with anything they are not 388 00:32:41,590 --> 00:32:47,340 supposed to, for example, rest molecules of remaining air there, how many molecules 389 00:32:47,340 --> 00:32:51,240 can there still be?" So somebody has to make up that number. And typically you 390 00:32:51,240 --> 00:32:55,880 express this as a quantity called the beam lifetime, which basically says if you 391 00:32:55,880 --> 00:33:00,870 were only to keep those particles circulating in the accelerator, how long 392 00:33:00,870 --> 00:33:04,620 would it take until they have all dispersed and lost their energy due to 393 00:33:04,620 --> 00:33:10,129 colliding with rest gas molecules? And it was decided that this should be at a value 394 00:33:10,129 --> 00:33:15,230 of 100 hours, is what the beams should basically be able to circulate without 395 00:33:15,230 --> 00:33:19,340 collisions, without being lost. And this gave the requirement for pressures down to 396 00:33:19,340 --> 00:33:24,700 about 100 femtobar, which is a very small, very, very tiny fraction of the 397 00:33:24,700 --> 00:33:29,679 atmospheric pressure we have here, which is about 1 bar. And to actually get to 398 00:33:29,679 --> 00:33:34,019 this level of vacuum, it requires multiple stages and multiple components to actually 399 00:33:34,019 --> 00:33:42,399 get there. So the initial vacuum inside these beam tubes, which are basically 400 00:33:42,399 --> 00:33:48,610 going throughout the whole LHC tunnel, has the volume of approximately the Notre-Dame 401 00:33:48,610 --> 00:33:53,800 cathedral. So the first step of getting all the air out of these beam tubes is 402 00:33:53,800 --> 00:34:00,179 using turbomolecular pumps. And then there needs to be more mechanisms to reduce the 403 00:34:00,179 --> 00:34:04,000 pressure even further, because these pumps are not able to reduce the pressure to the 404 00:34:04,000 --> 00:34:09,231 levels required. And they actually use a relatively clever trick to do that, which 405 00:34:09,231 --> 00:34:16,169 is the use of cryopumping. So the, ... I cannot show that? Okay. So the outer wall 406 00:34:16,169 --> 00:34:20,380 of this beam pipe cross section that you see here is actually also where the very 407 00:34:20,380 --> 00:34:27,230 cold helium inside the magnets is outside of. And what that does is, it leads to an 408 00:34:27,230 --> 00:34:31,589 effect called cryopumping. So actually any rest gas molecule that hits this wall 409 00:34:31,589 --> 00:34:35,990 actually condenses there. And as the molecules condense there, they are of 410 00:34:35,990 --> 00:34:40,510 course removed from the atmosphere inside this beam pipe, which removes them from 411 00:34:40,510 --> 00:34:44,750 the atmosphere and increases the quality of the vacuum. And with the use of this 412 00:34:44,750 --> 00:34:47,760 and then the warm sections, the use of getter coatings, which are basically able 413 00:34:47,760 --> 00:34:53,609 to trap gas molecules, you are able to reach the crazy vacuum levels that are 414 00:34:53,609 --> 00:34:59,020 required to make this happen. But they realized also during the design that one 415 00:34:59,020 --> 00:35:05,430 big problem – for the first time in an accelerator – another effect will create a 416 00:35:05,430 --> 00:35:08,540 significant problem for the vacuum, which is the generation of synchrotron 417 00:35:08,540 --> 00:35:15,240 radiation. So synchrotron radiation is a byproduct of when you do bend a particle 418 00:35:15,240 --> 00:35:20,220 beam, it results in a phenomenon called synchrotron radiation. And when this 419 00:35:20,220 --> 00:35:24,369 synchrotron radiation, as it goes straight on and is not bent, hits the walls of this 420 00:35:24,369 --> 00:35:30,170 vacuum system, or in this case of the beam pipe, it actually liberates molecules from 421 00:35:30,170 --> 00:35:33,810 there and reintroduces them into the vacuum, which of course then makes the 422 00:35:33,810 --> 00:35:40,230 vacuum worse again. An additional problem that gives the synchrotron radiation is, 423 00:35:40,230 --> 00:35:44,960 that it also gives a significant heat load, and if you need to dissipate all 424 00:35:44,960 --> 00:35:49,660 this heat that is generated through the very cold helium, this is not a very 425 00:35:49,660 --> 00:35:53,820 efficient process. Because making this helium so cool, is actually a very energy 426 00:35:53,820 --> 00:35:58,590 intensive process. And just removing a single watt of thermal power through the 427 00:35:58,590 --> 00:36:03,230 superfluid helium costs about 1 kW of energy. So that is not the most efficient 428 00:36:03,230 --> 00:36:07,770 part. And this is why the cross-section you have just seen includes another large 429 00:36:07,770 --> 00:36:11,200 component, which also technically belongs to the vacuum system, which is called the 430 00:36:11,200 --> 00:36:15,940 beam screen. And this beam screen is basically another tube running inside the 431 00:36:15,940 --> 00:36:20,760 beam pipe, of which we have, of course, two, which run inside the magnet cold 432 00:36:20,760 --> 00:36:25,200 bores. And it shields the synchrotron radiation heat load from the outer walls, 433 00:36:25,200 --> 00:36:30,550 which are at 1.8 Kelvin, while this pipe itself is actively cooled to only about 20 434 00:36:30,550 --> 00:36:36,240 Kelvin of temperature, which is much more efficient to dissipate this heat. So it is 435 00:36:36,240 --> 00:36:39,440 basically a steel tube about one millimeter thick. It has these pumping 436 00:36:39,440 --> 00:36:46,920 holes, where hydrogen gas molecules can go out of, and on the inside it has a copper 437 00:36:46,920 --> 00:36:51,970 coating, which is used to reduce its electrical resistance, which is required 438 00:36:51,970 --> 00:36:55,530 because the beam, while it circulates, also induces current that would otherwise 439 00:36:55,530 --> 00:37:00,180 flow inside this tube, which is really, if you think about it, only a simple tube and 440 00:37:00,180 --> 00:37:03,950 it would increase the heat load again. So a lot of engineering already has to go 441 00:37:03,950 --> 00:37:11,579 into a very simple piece of ... a thing like that. So after having spoken so much 442 00:37:11,579 --> 00:37:16,450 about all the things required to just make a beam circulate and accelerate and so on, 443 00:37:16,450 --> 00:37:20,590 now it's probably also time to talk a little bit about the beam itself and how 444 00:37:20,590 --> 00:37:27,220 to control it and how to instrument, how to measure things about this beam. Even 445 00:37:27,220 --> 00:37:32,080 without going yet about collisions and doing actual physics experiments. So the 446 00:37:32,080 --> 00:37:36,760 first important bit that is able to basically control or influence the beam 447 00:37:36,760 --> 00:37:41,540 here is what's called the beam cleaning or collimation system. So typically such a 448 00:37:41,540 --> 00:37:47,050 particle beam is not very clean. It always travels associated with what is called 449 00:37:47,050 --> 00:37:51,640 halo of particles around this core area that is less than a millimeter wide where 450 00:37:51,640 --> 00:37:57,140 most of the intensity is focused. And these particles outside we want to remove, 451 00:37:57,140 --> 00:38:00,260 because they otherwise would be lost inside the magnets and for example, would 452 00:38:00,260 --> 00:38:05,640 lead to quenches of the superconducting magnets. And for collimation, we basically 453 00:38:05,640 --> 00:38:10,819 use small slits that are adjustable and are located at two main locations of the 454 00:38:10,819 --> 00:38:15,100 LHC. So they have collimation systems there, with vertical and horizontal slits 455 00:38:15,100 --> 00:38:21,590 that can be adjusted in width, in order to scrape off all the particles that they do 456 00:38:21,590 --> 00:38:25,319 want to get rid of and extract out of the beam, while only the core part can 457 00:38:25,319 --> 00:38:30,490 circulate and produce clean collisions without any background, that otherwise 458 00:38:30,490 --> 00:38:36,110 would need to be accounted for. And then there is a whole other open topic of beam 459 00:38:36,110 --> 00:38:39,710 instrumentation. So when you run a particle accelerator, you want to measure 460 00:38:39,710 --> 00:38:45,220 various quantities and performance figures of such a beam. And that is crucial for a 461 00:38:45,220 --> 00:38:48,730 correct operation and for the highest performance, getting the highest 462 00:38:48,730 --> 00:38:52,331 performance from an accelerator. And there are a lot of different types of those, and 463 00:38:52,331 --> 00:38:58,920 I want to go quickly about ... over why we have them and what we do with them. So the 464 00:38:58,920 --> 00:39:03,730 first and most basic measurement you want to do, is the beam current measurement. So 465 00:39:03,730 --> 00:39:08,550 the beam current is a basic accelerator beam intensity measurement. So it gives 466 00:39:08,550 --> 00:39:13,260 you an idea of how strong the beam that is running inside your accelerator is. And it 467 00:39:13,260 --> 00:39:18,000 is measured using these DCCTs or DC current transformers. And their basic 468 00:39:18,000 --> 00:39:22,230 principle of operation is, that while the particles move through this torus, which 469 00:39:22,230 --> 00:39:26,069 is actually a coil or a transformer, induces a voltage there that you can 470 00:39:26,069 --> 00:39:30,579 measure and then use to quantify the intensity of this beam. And the big 471 00:39:30,579 --> 00:39:34,760 challenge here is that the dynamic range, this instrument needs to capture, is 472 00:39:34,760 --> 00:39:39,930 really large, because it has to operate from the lowest intensity pilot injection 473 00:39:39,930 --> 00:39:46,119 beams up to the full energy, full number of bunches running inside the LHC. So it 474 00:39:46,119 --> 00:39:51,580 has to cover six orders of magnitude of measurement dynamic range. Then the second 475 00:39:51,580 --> 00:39:57,130 thing when talking about collisions is the luminosity measurement. So luminosity is a 476 00:39:57,130 --> 00:40:02,210 quantity basically said to measure the rate of interaction of the particle beams. 477 00:40:02,210 --> 00:40:06,710 So to give you an idea of how often interactions happen inside the experiments 478 00:40:06,710 --> 00:40:11,280 or where you want them to happen. And this measurement is used to first of all, 479 00:40:11,280 --> 00:40:14,461 adjust this interaction rate to a target value, which is optimal for the 480 00:40:14,461 --> 00:40:19,109 experiments to function and to equalize the interaction rates in different 481 00:40:19,109 --> 00:40:23,970 experiments. So different experiments also are specified to have the same interaction 482 00:40:23,970 --> 00:40:29,650 rate so they can get the same let's say statistical quality of their data. So it's 483 00:40:29,650 --> 00:40:33,869 used to equalize those. And then as a third thing, this system is also used to 484 00:40:33,869 --> 00:40:37,510 measure the crossing angle of the beam. So as you may know, at some point, when the 485 00:40:37,510 --> 00:40:42,610 beams are collided, they collide at an angle, that is very small. And this angle 486 00:40:42,610 --> 00:40:47,559 is actually measured also very precisely in order to adjust it correctly. And it is 487 00:40:47,559 --> 00:40:50,190 measured to less than a thousandth of a degree, which is again a very impressive 488 00:40:50,190 --> 00:40:54,640 feat, given that the detection principle of this measurement is only measurement of 489 00:40:54,640 --> 00:40:58,700 some neutral particles that are a result of the particle interaction of the beam 490 00:40:58,700 --> 00:41:05,080 ... of the collision. Okay, so that is number two. Then number three that we have 491 00:41:05,080 --> 00:41:09,610 is the beam position monitor. Because along the LHC, you also always want to 492 00:41:09,610 --> 00:41:14,940 know, where the beam is at any given time. So you want to measure the position of the 493 00:41:14,940 --> 00:41:19,190 beam inside the beam pipe in order to optimally adjust it to the position you 494 00:41:19,190 --> 00:41:23,740 want to have it. And for that we use these beam position monitors of which we have 495 00:41:23,740 --> 00:41:27,910 more than a thousand installed along the LHC. So they are typically capacitive 496 00:41:27,910 --> 00:41:31,780 probes or electromagnetic strip lines. As you can see on top and bottom 497 00:41:31,780 --> 00:41:36,790 respectively. And they basically are distributed along the LHC and provide 498 00:41:36,790 --> 00:41:40,300 position of the particle beam along the accelerator, which can then be used to 499 00:41:40,300 --> 00:41:47,640 tune, for example, the magnets. All right. Then we have beam profile. So after the 500 00:41:47,640 --> 00:41:51,059 position, that gives you an idea where the beam is, you also want to know its 501 00:41:51,059 --> 00:41:57,329 intensity distribution. Basically when you do a cut through the beam pipe somewhere, 502 00:41:57,329 --> 00:42:02,480 you want to know how the intensity profile looks like. And for that we have basically 503 00:42:02,480 --> 00:42:07,380 two measurement systems. One measures the profile in X and Y directions. So if you 504 00:42:07,380 --> 00:42:10,400 really would do a cut and it gives you something like this and it's, for example, 505 00:42:10,400 --> 00:42:14,950 done with wire scanners, which is literal, very thin wire that is moved through the 506 00:42:14,950 --> 00:42:20,290 beam. And then the current that the beam moving through this wire, generates is 507 00:42:20,290 --> 00:42:25,280 used to generate such a profile map, when scanning with this wire. The other one is 508 00:42:25,280 --> 00:42:29,750 the longitudinal profile, which gives you an idea about the quality of your RF 509 00:42:29,750 --> 00:42:34,040 system and there you want to know how the intensity profile of your beam looks like. 510 00:42:34,040 --> 00:42:37,690 If you were looking only at one spot of the accelerator and the beam would pass 511 00:42:37,690 --> 00:42:43,230 by, and you would basically see over time how the intensity looks like. And then as 512 00:42:43,230 --> 00:42:47,980 the last bit of beam instrumentation, there is the beam loss monitors. So they 513 00:42:47,980 --> 00:42:53,170 are these yellow tubes, that are located on the outside of mostly all the magnets, 514 00:42:53,170 --> 00:42:57,951 of the dipole and quadrupole magnets and so on. Again, there's more than a thousand 515 00:42:57,951 --> 00:43:03,230 of those. And the idea here is that you need a lot of detectors that are basically 516 00:43:03,230 --> 00:43:07,819 small ionization chambers, which detect any showers of secondary particles that 517 00:43:07,819 --> 00:43:12,720 are generated when one of the high energy protons are lost somewhere in the magnet 518 00:43:12,720 --> 00:43:16,760 materials. So these are really used for protection of the system, because if a 519 00:43:16,760 --> 00:43:21,109 specific threshold of energy loss is detected, then the accelerator needs to be 520 00:43:21,109 --> 00:43:26,220 quickly shut down. Which is why they have to react in a matter of nanoseconds in 521 00:43:26,220 --> 00:43:31,260 order to keep the accelerator safe. Because any interaction of the particle 522 00:43:31,260 --> 00:43:36,059 beam with for example, the magnets could just destroy huge amounts of money and of 523 00:43:36,059 --> 00:43:41,870 time that would be needed to rebuild. And as a last and final thing, we have spoken 524 00:43:41,870 --> 00:43:46,260 one or two times already about shutting down the LHC. Which sounds also trivial at 525 00:43:46,260 --> 00:43:51,799 first, but really is not. So, the last thing here is, what we call the Beam Dump. 526 00:43:51,799 --> 00:43:56,589 So the energy content that is contained in those particular beams, it can be used, 527 00:43:56,589 --> 00:44:02,950 could be used if it were shot on a copper target, it could just melt 1000 kilograms 528 00:44:02,950 --> 00:44:07,599 or one ton of copper instantly. So during beam extraction, the process of getting 529 00:44:07,599 --> 00:44:12,359 the particle beam outside out of the LHC, this energy needs to be dissipated 530 00:44:12,359 --> 00:44:16,990 somehow. And for that, this special Beam Dump area is constructed. So there are 531 00:44:16,990 --> 00:44:20,670 fast kicker magnets, that are used to ..., that are able to ramp up in a really, 532 00:44:20,670 --> 00:44:24,930 really short amount of time of microseconds. And then the beam is 533 00:44:24,930 --> 00:44:30,309 carefully and in a controlled manner directed into a set of concrete blocks, 534 00:44:30,309 --> 00:44:35,339 that is basically big enough to dissipate all this energy, when required. And in the 535 00:44:35,339 --> 00:44:38,800 process of doing so, it also heats up to about 800 degrees Celsius, and then of 536 00:44:38,800 --> 00:44:45,530 course, also needs the associated time to cool down again. Good. So as you may or 537 00:44:45,530 --> 00:44:50,040 may not know, currently the LHC is not in operation. So LHC currently is undergoing 538 00:44:50,040 --> 00:44:56,099 its second long shutdown phase, or LS2. But what we do when the LHC is in 539 00:44:56,099 --> 00:44:59,961 operation, is that we have these status dashboards, that you can see here, that 540 00:44:59,961 --> 00:45:04,970 are distributed all around CERN, and can be used by anyone, any passer-by, to 541 00:45:04,970 --> 00:45:09,980 basically monitor what the current operation mode or the current situation of 542 00:45:09,980 --> 00:45:15,609 the accelerator is. And can be used also to quickly see if like an operator needs 543 00:45:15,609 --> 00:45:19,990 to go somewhere or is needed, or how the shift planning for the next shift works 544 00:45:19,990 --> 00:45:24,430 out and so on. And on the right side you would see what this currently looks like. 545 00:45:24,430 --> 00:45:30,990 So basically black screen saying next beam expected in spring 2021. And the good 546 00:45:30,990 --> 00:45:33,980 thing about these status pages is that you can actually see them from your home, 547 00:45:33,980 --> 00:45:40,390 because they're also openly available, as most of the stuff we do at CERN. So if you 548 00:45:40,390 --> 00:45:44,510 are interested, then perhaps in a year from now or a bit longer than a year, it 549 00:45:44,510 --> 00:45:47,670 would be quite interesting to follow all the commissioning process of when they are 550 00:45:47,670 --> 00:45:54,130 trying to start the LHC back up, and follow that process from your home. 551 00:45:54,130 --> 00:45:58,109 Otherwise, if you now feel the urge to maybe visit CERN, pay some of the things 552 00:45:58,109 --> 00:46:01,980 we talked about a visit, or are just generally interested, CERN offers a 553 00:46:01,980 --> 00:46:06,970 variety of tours free of charge. So if you're interested in that, visit that web 554 00:46:06,970 --> 00:46:10,819 site and we would be happy to welcome you there. And with that, thank you very much 555 00:46:10,819 --> 00:46:14,569 for your attention. 556 00:46:14,569 --> 00:46:17,530 *Applause* 557 00:46:17,530 --> 00:46:24,470 Severin: Punktlandung. Herald: Thank you, Stefan and Severin. If 558 00:46:24,470 --> 00:46:29,869 you have questions, there are six microphones in the room. Please make a 559 00:46:29,869 --> 00:46:34,660 queue, and we start with the Signal Angel. Signal Angel, please, first question. 560 00:46:34,660 --> 00:46:39,630 Signal Angel: There is said to be a master red button for shutting down the whole 561 00:46:39,630 --> 00:46:45,859 system in case of heavy problems. How often did you push it yet? 562 00:46:45,859 --> 00:46:49,059 Stefan: Master red button? Severin: Master button ... 563 00:46:49,059 --> 00:46:55,160 Signal Angel: Like a shut down button. Severin: I cannot really understand you. I 564 00:46:55,160 --> 00:46:59,270 think the question was about how often basically we used the Beam Dump system to 565 00:46:59,270 --> 00:47:01,050 basically get rid of the beam, is that correct? 566 00:47:01,050 --> 00:47:04,280 Signal Angel: I guess so. Stefan: He said there is a master button. 567 00:47:04,280 --> 00:47:06,280 Signal Angel: I guess so. Stefan: I think there's a master button in 568 00:47:06,280 --> 00:47:08,280 the... Severin: There is not only one master 569 00:47:08,280 --> 00:47:11,790 button, there are several master buttons. These are switches, called beam interlock 570 00:47:11,790 --> 00:47:17,530 switch. Basically, at every operator's screen, there is basically one beam 571 00:47:17,530 --> 00:47:23,690 interlock switch. I don't know. I think sometimes they get rid of the beam just 572 00:47:23,690 --> 00:47:29,579 because, I mean. When we have LHC at full operation – Stefan talked about the 573 00:47:29,579 --> 00:47:33,799 luminosity – so what is happening, that in the beginning we have a very high amount 574 00:47:33,799 --> 00:47:39,920 of luminosity, So many particles collide on each other. But over time, like after 575 00:47:39,920 --> 00:47:45,230 12 or 15 hours or whatever, basically the luminosity ..., so the amount of particles 576 00:47:45,230 --> 00:47:49,119 which collide with each other, is going down and down. So the luminosity 577 00:47:49,119 --> 00:47:53,770 decreases. And then at some point in time, basically the operators decide, that they 578 00:47:53,770 --> 00:47:58,950 will now get rid of the actual beam, which is inside LHC and basically will recover 579 00:47:58,950 --> 00:48:02,290 the whole machine and then restart the machine again. And this is done sometimes, 580 00:48:02,290 --> 00:48:08,170 I don't know, every 12 hours, sometimes after 24 hours. Something like that, yes. 581 00:48:08,170 --> 00:48:11,380 Herald: Cool. And microphone number four, I think. 582 00:48:11,380 --> 00:48:17,559 Q: Yes. So where's the energy coming from? So do you have your own power plant, or 583 00:48:17,559 --> 00:48:20,119 so? Severin: So, no, not really, not really. 584 00:48:20,119 --> 00:48:24,349 Basically, we get all the power from the French grid. So we have relatively big 585 00:48:24,349 --> 00:48:32,640 power trails coming from the French grid. So we get 450 kV of power there. So 586 00:48:32,640 --> 00:48:35,390 basically the voltage is quite high and then we have our own transformers on site. 587 00:48:35,390 --> 00:48:39,609 And I think only, ... a little bit smaller fraction of the energy is coming from the 588 00:48:39,609 --> 00:48:43,940 Swiss grid. So basically we use most of the energy which is coming from the French 589 00:48:43,940 --> 00:48:46,940 grid. Q: Okay. Thank you. 590 00:48:46,940 --> 00:48:48,940 Herald: Thank you for your question. And microphone number one, please. 591 00:48:48,940 --> 00:48:57,280 Q: Hi. Thank you for your presentation. If I'm not wrong, you say the beam can warm a 592 00:48:57,280 --> 00:49:04,599 block of concrete to 800 Celsius. Would it be possible to use it as a weapon? 593 00:49:04,599 --> 00:49:09,960 Stefan: *laughs* Very likely not. And CERN very much condemns these actions in any 594 00:49:09,960 --> 00:49:13,690 form, I guess. So CERN operates in a purely peaceful mission and would never 595 00:49:13,690 --> 00:49:18,000 think about using their particle beams as a weapon. And even if they could, it is 596 00:49:18,000 --> 00:49:21,720 probably not the most practical thing to do, I guess. *laughs* 597 00:49:21,720 --> 00:49:26,710 Herald: But if your telephone is again hanging up, you can destroy it, right? 598 00:49:26,710 --> 00:49:29,090 Stefan: *laughs* Herald: And microphone number six, I 599 00:49:29,090 --> 00:49:33,390 think. Q: Yes. So you said, you can stop in 600 00:49:33,390 --> 00:49:39,980 nanoseconds, but just the light would go just 30 centimeters, you know, a 601 00:49:39,980 --> 00:49:45,110 nanosecond. How will you be able to stop in this small time? 602 00:49:45,110 --> 00:49:49,220 Stefan: Ah, no, no. So what I was talking about is that these magnets that are used 603 00:49:49,220 --> 00:49:54,770 to extract the beam out of the LHC, they have reaction times, or ramp up times that 604 00:49:54,770 --> 00:50:01,020 are in the order of 1, 2, 3 microseconds. So not nanoseconds, but microseconds. And 605 00:50:01,020 --> 00:50:06,329 really only then basically the particles still circulate, worst case one full turn, 606 00:50:06,329 --> 00:50:10,430 and only then moving outside of the accelerator. 607 00:50:10,430 --> 00:50:17,349 Herald: And microphone number one again. Q: So do you have any photos of the front 608 00:50:17,349 --> 00:50:22,609 of the dump block? It has to look like it's got hit a lot. 609 00:50:22,609 --> 00:50:26,110 *laughter* Severin: No, not really. I think it's one 610 00:50:26,110 --> 00:50:31,819 of the only pictures we could find about, the Beam Dump system. And these areas, I 611 00:50:31,819 --> 00:50:37,059 think it's not really opened any more. So since operation of LHC, which was in 612 00:50:37,059 --> 00:50:43,329 basically LHC started in 2008, and since then, the Beam Dump system was not opened 613 00:50:43,329 --> 00:50:48,860 again because it's completely sealed in stainless steel. And that's why it wasn't 614 00:50:48,860 --> 00:50:52,339 opened anymore. Heral: Cool. Question from the interwebs. 615 00:50:52,339 --> 00:50:59,329 Signal: Regarding power supply. How do you switch or fine-control the currents? Are 616 00:50:59,329 --> 00:51:03,460 you using classic silicone transistors, off-the-shelf IGBTs? 617 00:51:03,460 --> 00:51:07,230 Sverin: Um, yes. *laughs* *laughter* 618 00:51:07,230 --> 00:51:12,530 Severin: Yes. Uh, the system was developed at CERN. And I think that's quite common 619 00:51:12,530 --> 00:51:16,280 at CERN that we basically developed all the technology at CERN or try to develop 620 00:51:16,280 --> 00:51:20,250 nearly everything at CERN. But then production, for example, is put into 621 00:51:20,250 --> 00:51:25,750 industry. And yes, these are relatively classical power converters. The 622 00:51:25,750 --> 00:51:29,609 interesting or like challenging part about the current power converters is really 623 00:51:29,609 --> 00:51:32,859 that the current has to be measured quite precisely and also controlled quite 624 00:51:32,859 --> 00:51:39,430 precisely so there we use also DCC TS. Which we have also mentioned before. But 625 00:51:39,430 --> 00:51:42,290 basically all this controlled mechanism there. That's one of the big challenges 626 00:51:42,290 --> 00:51:47,980 there. Herald: Cool. Microphone number one again. 627 00:51:47,980 --> 00:51:55,609 Q: You talked about the orbit clock that detects when the bunch is completed one 628 00:51:55,609 --> 00:52:00,200 round. How is it possible to detect which is the first bunch? 629 00:52:00,200 --> 00:52:03,980 Stefan: Yeah. So it is it is actually not detected, but this clock is actually 630 00:52:03,980 --> 00:52:07,470 something that is constructed. So we basically what we do is, we count these 631 00:52:07,470 --> 00:52:13,250 cycles of the of the RF cycle. Maybe I can open this slide. So somewhere there is a 632 00:52:13,250 --> 00:52:18,510 counter that basically knows how many 40 MHz clock cycles a full rotation takes. 633 00:52:18,510 --> 00:52:21,770 And then at some point decides this is number one. And that's also where they 634 00:52:21,770 --> 00:52:26,180 start counting when they inject bunches into the LHC. So there's no marker, let's 635 00:52:26,180 --> 00:52:29,510 say. But there is a certain structure to the beam. So you could potentially do 636 00:52:29,510 --> 00:52:33,320 that. So, for example, for these longer periods where the kicker magnets need to 637 00:52:33,320 --> 00:52:36,910 ramp up, they have something they call the abort gap. So a number of bunches that are 638 00:52:36,910 --> 00:52:41,350 never filled but are always kept empty. So the magnets have enough time to deflect 639 00:52:41,350 --> 00:52:44,460 the beam when the next bunch comes around. So you could probably measure that, but 640 00:52:44,460 --> 00:52:47,260 it's much easier to do it the other way around. 641 00:52:47,260 --> 00:52:54,570 Herald: Microphone number four, please. Q: You said you had quite tight needs for 642 00:52:54,570 --> 00:53:01,230 the timing clock. Is it tight enough? That the speed of light was the limit with the 643 00:53:01,230 --> 00:53:03,589 distances between locations or that was not a concern? 644 00:53:03,589 --> 00:53:08,680 Stefan: No, it is a concern. So because just distributing a cable for 27 645 00:53:08,680 --> 00:53:14,869 kilometers produces like just considerable run times of electrical signals. All the 646 00:53:14,869 --> 00:53:18,150 delays of all the cables need to be measured precisely for their delay and 647 00:53:18,150 --> 00:53:22,849 then calibrated out so all the experiments get their clocks at the right time, 648 00:53:22,849 --> 00:53:27,400 shifted, compensated for the delay time that it just takes to get the signal 649 00:53:27,400 --> 00:53:33,000 there. Herald: And again, the interwebs. 650 00:53:33,000 --> 00:53:40,210 Signal Angel: Is it too dangerous to stand near the concrete cooling blocks, like 651 00:53:40,210 --> 00:53:46,980 radioactive wise or, I don't know. Severin: Yes. 652 00:53:46,980 --> 00:53:49,090 *laughter* Stefan: Not recommended. 653 00:53:49,090 --> 00:53:54,859 Severin: Not recommended. We have a very good interlock system. Also, the doors, 654 00:53:54,859 --> 00:53:58,270 all the doors have switches. So basically when one door is basically like opened 655 00:53:58,270 --> 00:54:03,059 then basically the whole machine will be shut down. So we have very critical and 656 00:54:03,059 --> 00:54:11,319 safety related access system at LHC. Maybe you watch Angels and Demons. This 657 00:54:11,319 --> 00:54:16,760 Hollywood movie that we have, the eye scanners are shown. It's a little bit. I 658 00:54:16,760 --> 00:54:22,430 mean, it's Hollywood. But, we have eye scanners. So iris scanners. So every time 659 00:54:22,430 --> 00:54:25,780 like we want to go to the tunnel, for example, then we have to let also our iris 660 00:54:25,780 --> 00:54:30,000 be scanned because otherwise we will not be able to go to the tunnel. So there's a 661 00:54:30,000 --> 00:54:33,890 very sophisticated access system to really go to the tunnel. So when there is 662 00:54:33,890 --> 00:54:36,810 operation, the whole tunnel access is completely blocked. 663 00:54:36,810 --> 00:54:40,510 Herald: Good, microphone number one, please. 664 00:54:40,510 --> 00:54:47,780 Q: What is the exact reason to have each of the experiments, every side. I mean, so 665 00:54:47,780 --> 00:54:52,080 far apart on the LHC. I mean, on opposite sides. 666 00:54:52,080 --> 00:54:57,609 Severin: Um, basically, you are talking about Atlas and CMS. The reason for that 667 00:54:57,609 --> 00:55:02,060 is because when, these two experiments were constructed, there was a little bit 668 00:55:02,060 --> 00:55:10,060 of fear that particles basically interact at the two experiments. So that they 669 00:55:10,060 --> 00:55:13,670 really are like the most far away. We like to have a very big distance from each 670 00:55:13,670 --> 00:55:17,930 other. So there is no interaction between them. That's why we basically put them at 671 00:55:17,930 --> 00:55:20,869 point one and point five. That's the reason why. 672 00:55:20,869 --> 00:55:25,549 Herald: If I can see it correctly. Microphone number five. 673 00:55:25,549 --> 00:55:32,030 Q: Yes, hello. I've seen that you're also using the CAN bus. What are you using the 674 00:55:32,030 --> 00:55:37,190 CAN bus for in CERN? Stefan: I know of at least one use, but it 675 00:55:37,190 --> 00:55:42,539 is inside an experiment. So there are, as far as I know, investigations under way to 676 00:55:42,539 --> 00:55:47,610 use the CAN bus to do the actual control of the detectors of one experiment. I 677 00:55:47,610 --> 00:55:51,059 don't know if there is a use inside the accelerator itself. So apart from the 678 00:55:51,059 --> 00:55:55,790 experiments. But perhaps if you come by afterwards we can find one. 679 00:55:55,790 --> 00:55:59,539 Q: Thank you. Herald: Microphone number one. 680 00:55:59,539 --> 00:56:05,829 Q: Do you have any official data about how many tons of duct taper used in 681 00:56:05,829 --> 00:56:08,480 daily operations? *laughter* 682 00:56:08,480 --> 00:56:13,750 Severin: No. No. Herald: What about zip ties? 683 00:56:13,750 --> 00:56:21,460 Severin: Many. Yeah. Millions. Billions. Herald: Okay. As far as I can see... Ah, 684 00:56:21,460 --> 00:56:25,470 the intercepts again with a question. Signal Angel: Do you know your monthly 685 00:56:25,470 --> 00:56:27,470 power bill? *laughter* 686 00:56:27,470 --> 00:56:31,470 Severin: No, not no. No, sorry. Stefan: No. But it is, I think, in fact 687 00:56:31,470 --> 00:56:37,960 the contribution of France, which is the main contributor in terms of energy. That 688 00:56:37,960 --> 00:56:41,440 it is part of their contribution to contribute the electricity bill basically 689 00:56:41,440 --> 00:56:44,440 instead of paying money to CERN. That's as far as I know. 690 00:56:44,440 --> 00:56:48,480 Severin: Yes. And also we shut down the LHC and the accelerator complex through 691 00:56:48,480 --> 00:56:52,859 like the wintertime. And one of the reasons for that is because electricity is 692 00:56:52,859 --> 00:56:56,940 more expensive during wintertime in France than in summer. 693 00:56:56,940 --> 00:57:03,529 Herald: In this case, I can't see any other questions. I have a maybe stupid 694 00:57:03,529 --> 00:57:09,780 question. You said earlier you have to focus and defocus the beam. But as we 695 00:57:09,780 --> 00:57:12,529 know, you accelerated already the particles. Why do we have to focus the 696 00:57:12,529 --> 00:57:17,020 beam? Severin: Because every time when we have a 697 00:57:17,020 --> 00:57:21,910 dipole magnet, then basically we bend the particle around an arc. But then we also 698 00:57:21,910 --> 00:57:24,720 defocus a little bit. And also, the coulomb force is still a problem because 699 00:57:24,720 --> 00:57:29,770 we have equally charged particles in the bunch or in the whole beam itself. So they 700 00:57:29,770 --> 00:57:33,539 will by themselves basically go out of each other. And if you would not focus it 701 00:57:33,539 --> 00:57:36,299 again, then basically we would lose the whole beam in the end. 702 00:57:36,299 --> 00:57:44,630 Herald: Oh, thank you. I don't see any questions. Internet? In this case thank 703 00:57:44,630 --> 00:57:49,730 you very, very much, Stefan and Severin. Please. With a warm applause. The Large 704 00:57:49,730 --> 00:57:50,730 Hadron infrastructure talk. 705 00:57:50,730 --> 00:57:51,730 *Applause* 706 00:57:51,730 --> 00:57:52,989 subtitles created by c3subtitles.de in the year 20??. Join, and help us!