1 00:00:08,070 --> 00:00:09,720 MARKUS KLUTE: Welcome back to 8.701. 2 00:00:09,720 --> 00:00:13,560 So in this lecture, we talk about nuclear fusion. 3 00:00:13,560 --> 00:00:16,320 And what we mean by fusion is the energy production 4 00:00:16,320 --> 00:00:18,300 by two light nuclei fusing together 5 00:00:18,300 --> 00:00:21,900 to produce a heavier one which is more tightly bound. 6 00:00:21,900 --> 00:00:26,850 And again, we can understand this from the empirical mass 7 00:00:26,850 --> 00:00:28,680 formula, OK? 8 00:00:28,680 --> 00:00:30,720 So the difficulty in nuclear fusion 9 00:00:30,720 --> 00:00:34,020 is that we now have to overcome the Coulomb 10 00:00:34,020 --> 00:00:35,480 barrier from the other side. 11 00:00:35,480 --> 00:00:38,700 So we have to bring two light nuclei together, have 12 00:00:38,700 --> 00:00:45,620 to overcome the Coulomb barrier in order to form this heavier 13 00:00:45,620 --> 00:00:49,600 and more tightly bound state. 14 00:00:49,600 --> 00:00:51,370 You might think that you can just 15 00:00:51,370 --> 00:00:55,000 take a two beams of protons like we do at the LHC, 16 00:00:55,000 --> 00:01:00,110 bombard them, and create heavier nuclei. 17 00:01:00,110 --> 00:01:02,440 But the problem is that most of the nuclei 18 00:01:02,440 --> 00:01:04,840 will scatter elastically, and will not 19 00:01:04,840 --> 00:01:08,510 lead to a new-bound state. 20 00:01:08,510 --> 00:01:11,300 So the practical way to overcome the Coulomb barrier 21 00:01:11,300 --> 00:01:19,700 is by creating a confined mixture and supplying heat, 22 00:01:19,700 --> 00:01:25,010 such that the thermal energy is enough to overcome the Coulomb 23 00:01:25,010 --> 00:01:26,410 barrier. 24 00:01:26,410 --> 00:01:30,260 You can estimate how much energy is needed. 25 00:01:30,260 --> 00:01:35,540 If you, for example, assume a Coulomb barrier of about 5 MeV, 26 00:01:35,540 --> 00:01:40,370 this implies temperatures of 5 times 10 to the 10 Kelvin. 27 00:01:40,370 --> 00:01:43,760 OK, so that's really, really hot, right? 28 00:01:43,760 --> 00:01:48,800 If you compare this to typical temperatures within stars, 29 00:01:48,800 --> 00:01:53,710 you'll find that those are only 10 to the 8 Kelvin. 30 00:01:53,710 --> 00:01:56,420 So now you ask, why does it still work? 31 00:01:56,420 --> 00:02:01,400 Why do we see fusion within a stellar medium? 32 00:02:01,400 --> 00:02:04,760 And the answer to this is, again, quantum tunneling, 33 00:02:04,760 --> 00:02:07,070 and to some degree, it's also the fact 34 00:02:07,070 --> 00:02:11,000 that when we have a medium of a specific temperature, 35 00:02:11,000 --> 00:02:15,850 the kinetic energy of the particle involved 36 00:02:15,850 --> 00:02:18,810 follows a mixture of energy distribution. 37 00:02:18,810 --> 00:02:22,130 So you find some particles which have enough energy 38 00:02:22,130 --> 00:02:24,680 to overcome the Coulomb potential. 39 00:02:24,680 --> 00:02:29,720 Even so, the mean value would be below. 40 00:02:29,720 --> 00:02:33,320 The processes within the sun, they 41 00:02:33,320 --> 00:02:36,980 are dominated by so-called proton-proton cycle, or PPI 42 00:02:36,980 --> 00:02:37,970 cycle. 43 00:02:37,970 --> 00:02:40,150 And this happens in a number of steps. 44 00:02:40,150 --> 00:02:43,370 You start with hydrogen or protons-- 45 00:02:43,370 --> 00:02:46,190 the core of the sun is basically a plasma. 46 00:02:46,190 --> 00:02:49,525 So we can forget about electrons in this context. 47 00:02:49,525 --> 00:02:53,620 So we have two protons fusing together 48 00:02:53,620 --> 00:02:57,220 to a deuteron, which is a proton and a neutron, 49 00:02:57,220 --> 00:02:59,020 via the weak interaction. 50 00:02:59,020 --> 00:03:03,880 Here, you find for the first time, again, the neutrinos 51 00:03:03,880 --> 00:03:06,150 being produced in the sun. 52 00:03:06,150 --> 00:03:10,180 Then, you supply the deuterium again. 53 00:03:10,180 --> 00:03:12,340 And together with the proton, you 54 00:03:12,340 --> 00:03:14,440 are able to produce the helium. 55 00:03:14,440 --> 00:03:16,480 And then the helium, again, is being 56 00:03:16,480 --> 00:03:19,240 used to supply the third step in this. 57 00:03:19,240 --> 00:03:22,495 Helium-3 is supplied to produce helium-4. 58 00:03:22,495 --> 00:03:28,130 So this then-- the end product is a helium-4 here, and energy. 59 00:03:28,130 --> 00:03:30,520 You combine all of those three steps, 60 00:03:30,520 --> 00:03:33,550 you find that you start with four protons. 61 00:03:33,550 --> 00:03:39,070 You produce helium-4 plus positrons, neutrinos, photons, 62 00:03:39,070 --> 00:03:41,450 and energy. 63 00:03:41,450 --> 00:03:46,150 In fact, as I was just saying, this all 64 00:03:46,150 --> 00:03:49,630 happens within the hot plasma. 65 00:03:49,630 --> 00:03:51,820 The positrons are basically annihilated 66 00:03:51,820 --> 00:03:55,270 with electrons, which are part of the plasma, 67 00:03:55,270 --> 00:04:00,170 adding another MeV of energy to this. 68 00:04:00,170 --> 00:04:04,760 All right, this is one, and the dominant energy production 69 00:04:04,760 --> 00:04:05,870 mechanism within the sun. 70 00:04:05,870 --> 00:04:07,790 But it's not the only one. 71 00:04:07,790 --> 00:04:11,510 Also quite interesting is the so-called carbon cycle. 72 00:04:11,510 --> 00:04:15,360 It's contributing about 3% to the sun's energy output. 73 00:04:15,360 --> 00:04:20,630 So here, carbon basically works as a catalyst. 74 00:04:20,630 --> 00:04:25,670 So you have a carbon and, again, a proton producing nitrogen. 75 00:04:25,670 --> 00:04:29,300 The nitrogen produces carbon-13, carbon-13 together 76 00:04:29,300 --> 00:04:33,770 with a proton nitrogen-14, 14, nitrogen-14 with a proton 77 00:04:33,770 --> 00:04:39,230 oxygen-15, and oxygen-15, nitrogen-15, and then, 78 00:04:39,230 --> 00:04:42,980 last but not least, nitrogen-15 together with a proton 79 00:04:42,980 --> 00:04:48,950 produces carbon-12 again, and helium. 80 00:04:48,950 --> 00:04:54,450 So you see that the carbon-12 is the catalyst here, 81 00:04:54,450 --> 00:04:57,840 which is used to produce helium and energy. 82 00:04:57,840 --> 00:05:01,940 So if you produce this cycle, that you combine this cycle, 83 00:05:01,940 --> 00:05:08,090 you find that, again, from four protons or four hydrogen atoms, 84 00:05:08,090 --> 00:05:12,530 you produce helium, positrons, again, neutrinos, photons, 85 00:05:12,530 --> 00:05:15,410 and energy-- very similar, it's visible, 86 00:05:15,410 --> 00:05:18,470 to this combined chain, with the exception 87 00:05:18,470 --> 00:05:19,970 that there is one additional photon. 88 00:05:19,970 --> 00:05:24,200 And again, here, the positrons supply additional energy 89 00:05:24,200 --> 00:05:27,730 and they annihilate with electrons. 90 00:05:27,730 --> 00:05:30,170 Great, so we have seen that we do two things. 91 00:05:30,170 --> 00:05:34,840 We create heavier forms of metal starting from hydrogen, 92 00:05:34,840 --> 00:05:36,370 and we produce energy. 93 00:05:36,370 --> 00:05:38,580 And so this is the energy production mechanism 94 00:05:38,580 --> 00:05:39,360 within the sun. 95 00:05:42,060 --> 00:05:43,600 This is nice. 96 00:05:43,600 --> 00:05:47,700 You produce-- you start with hydrogen or deuterium. 97 00:05:47,700 --> 00:05:51,300 Those are two elements which are very abundant. 98 00:05:51,300 --> 00:05:52,795 And you produce energy. 99 00:05:52,795 --> 00:05:54,420 So the question comes up whether or not 100 00:05:54,420 --> 00:05:57,300 you can actually use this on Earth, 101 00:05:57,300 --> 00:05:59,280 in a controlled environment, in order 102 00:05:59,280 --> 00:06:03,420 to produce energy and solve many of the ongoing issues we 103 00:06:03,420 --> 00:06:05,760 have on this planet. 104 00:06:05,760 --> 00:06:07,360 There are several efforts underway. 105 00:06:07,360 --> 00:06:09,900 And this goes back all the way to the '50s 106 00:06:09,900 --> 00:06:12,580 of the last century. 107 00:06:12,580 --> 00:06:16,890 The most prominent one currently is the so-called ITER project, 108 00:06:16,890 --> 00:06:19,350 which is international collaboration, 109 00:06:19,350 --> 00:06:23,940 and a project where one tries to build a fusion 110 00:06:23,940 --> 00:06:27,760 reactor in France. 111 00:06:27,760 --> 00:06:31,390 I can already tell you that the next stage for this 112 00:06:31,390 --> 00:06:36,580 is in about five years to complete the project, 113 00:06:36,580 --> 00:06:38,020 or complete the production of-- 114 00:06:38,020 --> 00:06:41,500 construction of the project, and produce energy 115 00:06:41,500 --> 00:06:43,480 for the first time with this project 116 00:06:43,480 --> 00:06:44,800 in this controlled environment. 117 00:06:44,800 --> 00:06:48,190 It will take another 10, 15 years on this road 118 00:06:48,190 --> 00:06:52,480 map of research in order to produce, or be able to produce, 119 00:06:52,480 --> 00:06:55,210 nuclear power reactors-- 120 00:06:55,210 --> 00:06:57,190 nuclear fusion power reactors which 121 00:06:57,190 --> 00:06:59,920 can be used in some sort of commercial way. 122 00:06:59,920 --> 00:07:01,870 There's a few other interesting projects 123 00:07:01,870 --> 00:07:04,510 which use different magnet technologies which 124 00:07:04,510 --> 00:07:10,510 might have a more faster pathway to success. 125 00:07:10,510 --> 00:07:13,210 But coming back to the story here, 126 00:07:13,210 --> 00:07:17,920 so you could start with protons again 127 00:07:17,920 --> 00:07:20,390 in a proton-proton reaction. 128 00:07:20,390 --> 00:07:23,410 But it turns out that this is a rather slow process, 129 00:07:23,410 --> 00:07:28,390 and not very promising for a controlled reaction. 130 00:07:28,390 --> 00:07:32,320 But deuterium or tritium are also very promising. 131 00:07:32,320 --> 00:07:35,020 Note that, for deuterium here, you 132 00:07:35,020 --> 00:07:37,450 have to overcome the same Coulomb barrier, right? 133 00:07:37,450 --> 00:07:40,600 The charges involved are the very same. 134 00:07:40,600 --> 00:07:42,250 But the cross sections are higher. 135 00:07:42,250 --> 00:07:45,560 And therefore, the likelihood for the process to occur 136 00:07:45,560 --> 00:07:46,060 is higher. 137 00:07:46,060 --> 00:07:49,270 And therefore, this can happen much faster. 138 00:07:51,940 --> 00:07:54,940 Deuterium is-- again, as I was saying before-- very abundant. 139 00:07:54,940 --> 00:07:56,830 You can just extract it from water. 140 00:07:56,830 --> 00:07:59,980 Tritium is a little bit more difficult to deal with-- 141 00:07:59,980 --> 00:08:02,440 to produce and to control because it's 142 00:08:02,440 --> 00:08:07,920 radioactive and has some really not so good features. 143 00:08:07,920 --> 00:08:10,090 But this is a model picture of ITER. 144 00:08:10,090 --> 00:08:13,490 Again, this is an international project. 145 00:08:13,490 --> 00:08:17,310 It has a rather bad reputation these days. 146 00:08:17,310 --> 00:08:20,685 It's by far the most expensive scientific endeavor. 147 00:08:23,710 --> 00:08:26,330 But again, I mean, I think this is investment 148 00:08:26,330 --> 00:08:29,250 to the future of this planet. 149 00:08:29,250 --> 00:08:31,680 And hopefully it will succeed in the next years 150 00:08:31,680 --> 00:08:33,360 with this project, and in the long run 151 00:08:33,360 --> 00:08:36,809 with having nuclear fission-- 152 00:08:36,809 --> 00:08:38,820 fusion avail-- sorry, nuclear fusion 153 00:08:38,820 --> 00:08:41,490 available for energy creation. 154 00:08:41,490 --> 00:08:43,740 What you see here is-- 155 00:08:43,740 --> 00:08:50,460 the key feature of this reactor is toroidal magnets 156 00:08:50,460 --> 00:08:53,370 which confine the plasma. 157 00:08:53,370 --> 00:08:55,800 And also, electric fields are used in order 158 00:08:55,800 --> 00:08:57,360 to heat the plasma up first. 159 00:08:57,360 --> 00:09:00,210 So you have to provide heat to a point 160 00:09:00,210 --> 00:09:03,660 that the heat being produced in the fusion process 161 00:09:03,660 --> 00:09:06,630 is sufficient to self-sustain. 162 00:09:06,630 --> 00:09:12,990 And so confining the plasma and providing enough energy, 163 00:09:12,990 --> 00:09:15,570 and the radiation in this and so on, this 164 00:09:15,570 --> 00:09:19,910 is all very difficult problems to solve.