Plans for Future High Energy Electron Positron Colliders
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Lindau Nobel Laureate Meetings191 / 340
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WednesdayChandrasekhar limit
Transcript: English(auto-generated)
00:16
I will tell you something this morning about some work that is not directly my own work,
00:22
but in which I have a little bit participated, and that's work that is going on in many places over the world, in the United States, especially at Stanford, at CERN, in Geneva, in Russia, Novosibirsk, in Japan, and various other places. And this is work
00:42
that's trying to find the solution for the problem of making very large linear electron-positon colliders. Now, why do we want to make those? As you know, with... With old-fashioned accelerators, one accelerates a particle, and this doesn't seem to work either.
01:08
Ah, yes, it is. Okay, you accelerate the particle and shoot it against this fixed particle, and now you want to look at the interaction, but you have to conserve the momentum,
01:23
and since this particle has momentum to start with, the final product must have the same momentum, and therefore a lot of this energy is going to sit in the kinetic energy of the products, of the interaction, and what you have left for, for instance, producing new particles, or changing states, or so, is the energy in the center of mass, and that is much smaller
01:44
than this energy you put in. It only increases with the square root. Now, in classical physics, this energy in the center of mass is just one half of this, but if you take into account special relativity, you'll find that it works like this, and especially for electrons which have a low mass, this is very small. If you accelerate an electron to a thousand GeV,
02:03
and you shoot it on a fixed electron, the energy in the center of mass is only one GeV. So, that's why modern accelerators work like as colliders, where you shoot two particles against each other, and you get twice the energy of each particle in the center of mass,
02:21
which is very much more interesting. Now, why electrons and positrons? You can also accelerate and collide protons and protons, or protons and antiprotons. The point is that you really want to study not proton-proton collisions, but you want to study quark-quark
02:41
collisions at very high energy. Of course, these protons consist of quarks and gluons, keeping them together, and if you have a quark-quark collision with a certain energy, you need at least ten times as much energy in your proton-proton system to get the comparable quark-quark collision energy as you would get here. So, if you built a hard-run collider,
03:03
you must have about ten times more energy to get comparable physics. Also, hard-run colliders give much more background, because the actual quark-quark interactions at high energy you want to study have a very low cross-section that goes with one over the energy squared, whereas all kinds of other peripheral processes in this collision have a cross-section that is
03:24
maybe ten orders of magnitude bigger, and that gives you a lot of background. Now, you might say, why doesn't everyone build electron-positon colliders then? That is because it's very much more difficult than to build hard-run colliders. The reason it is more difficult is, of course, that you are used to building these circular machines with protons and protons colliding
03:43
against each other, and you can bend them in a circle, which makes that they meet each other many, many times, whereas in a linear collider, they can only meet once, and you cannot use electrons in circular colliders because they radiate when you deflect them, and especially at high energies, they radiate very much. It depends very strongly on energy.
04:03
So, we are building an electron-positon collider that is circular at CERN, and that will have 50 or 100 GV energy, and that's about the highest that we can ever build an electron-positon collider for, because it just gets, you get too much radiation if you make higher energy.
04:21
So, if you use higher energy, you have to make linear colliders, and then the only example we have at present is this machine in Stanford, the so-called SLC, and it works as follows. You have an electron gun. You accelerate electrons and accelerate them in this linac and shoot them on a target and make positrons here. You make a shower, you collect the
04:42
positrons, put them back and accelerate them again and put them in a little damping ring here, and this damping ring is to make the beam more intense, to damp the oscillations and the energy spread. So, it circulates you for some time, and you make other electrons and accelerate them and also put them in a little damping ring, and when both electrons and positrons
05:04
have been damped sufficiently, you eject them from these rings and accelerate them together. They are very near together in this common linear accelerator, and then electrons go one way and positrons the other way, and there they interact. Now, this trick of using only one accelerator to accelerate both the electrons and the positrons, you can use because here we have
05:24
50 GeV, and you can still bend them around, but if you want to go to higher energies like we would like, one TeV, then you cannot do this trick at all. They would radiate far too much, so you have to make two linear colliders that are really collinear or maybe at a very, very small angle. What we would like would be one TeV, 20 times more, and since the interesting
05:46
sections go with one over e squared, we also need a very much higher luminosity, and that is really what the problem is. It's not difficult to make a linac that gives a very high energy, but what makes the difficulty is this high luminosity. Of course, these machines
06:01
are very much bigger than what Gever told us about in his lecture, but they are very much less big than the machines that Bloomberg told us about, so that is a consolation. Now, there is the luminosity of these machines, which you can very simply describe with this equation.
06:21
It is the square of number of particles per bunch times the frequency at which you accelerate these bunches divided by, well, the cross-section area. This is for round beams. Now, if you want higher luminosity, you can increase N or F, but that also increases the powers in the beam, and the power in the beams is what limits these machines. It gets very high,
06:41
and it's very difficult to make, so what you want to do is make a very small cross-section at the interaction point. You focus your beams to very small points. For instance, in the slack machine, the design value for the focus is about a micron or so. What they have actually achieved, I think, is four or five microns, but they are still busy
07:00
running it in. It's not yet finished. What we want to do in our new machines of one TV is something in the order of 15 nanometers or something like that. It's very much smaller. Now, the limits on making very small spot sizes are technological limits. It's difficult to make strong enough focusing lenses. It's difficult to have a chromatic aberration that's small enough
07:22
and so on. And then there are two other limits, which are disruption and what is called, with a not very beautiful word, beamstralum. And what's that? That's if you have a bunch of electrons, positrons, and you shoot a particle in the opposite direction. This green thing is a
07:42
bunch. That is a particle. It's deflected by the field from this bunch, and it pinches. And the other one is also deflected by the field from this bunch, and it also pinches. Now, you might think that is nice because the beam cross-section got smaller. You got higher luminosity. And in fact, up to a certain point, it's nice. But as this effect gets
08:01
stronger and stronger, the focusing gets so strong that the beam more or less explodes before it has had any chance to interact. It's over-focused. And, well, I will not go into this because I haven't got much time. But that is the beam disruption. And what also happens during this process is that the particles being deflected, they radiate. And that's called
08:21
beamstralum, as analogy to bremsstralum. And that depends in a different way from the parameters of the beam, like the number of particles in the cross-section and so on. And therefore, these two things together limit the parameters very strongly. And in fact, if you specify what luminosity you want to have and what beam power you allow and
08:41
what disruption factor you allow and what beamstralum factor you allow, then most of the parameters are pretty well fixed. There are a few that you can still choose, like the frequency of your accelerator. But there isn't very much choice. Now, one of the tricks you can do is to make not round beams, but to make flat beams in the collision point. Because if you keep the same area, you will have the same luminosity.
09:03
But obviously, in this case, the – well, you can see the magnetic lines of force will be longer than here, so you will have less field and less disruption, less beamstorm. But the problem with that is, of course, that if you want to keep the same area and still make a flat beam, it means that the beam height will have to be smaller, and even smaller than it would already be. And that makes it even more difficult.
09:25
Now, to make a very small beam spot, of course, one thing you can do is to start out with a very small beam, a very low-emittance beam, very dense. And that's what these damping rings are for. The particles turn round in these damping rings, and being deflected, they radiate.
09:40
Particles with higher energy radiate more than particles with lower energy, and that reduces the energy spread. Of course, they are kept at the same energy by a radio frequency accelerating system. And also, when the particle makes oscillations like this and it radiates while it's going up, then some of the transverse momentum is also reduced, and therefore, it also gives damping of beta-ton oscillations, if you arrange the
10:02
focusing properly. These damping rings are quite an art in themselves to construct. Usually, the radiation is intensified by using wiggles, like you do in free electron lasers. By all the bending, you increase the radiation. But I cannot go into it in any detail in this talk. Now, how does a linear accelerator work, like the one in Stanford, for instance?
10:28
It's built up out of accelerating sections, which are kind of radio frequency structures. They are tubes divided by little irises, which are plates with a hole in the middle. And you fill this structure with RF energy made in transmitting tubes, klystrons,
10:46
which give a short pulse, typically a microsecond or a few microseconds. And this RF frequency is something in the gigahertz region. These are like little cavities that are all coupled. So the cavities are filled one by one. It propagates slowly here with group
11:02
velocity, which is lower than the light velocity. And by the time the structure has filled, that's what this length of pulse corresponds to, you shoot through a particle, which goes, of course, much faster. It goes at light velocity. And then you shoot through a bunch of particles. And they take part of the energy out of the structure and are accelerated.
11:21
Now, it would be very nice for the sake of efficiency to take all of the energy out of the structure with your particles. But that you cannot do, because if you do that, then the last particle will see no field at all. And the front particle will see the full field. So you get a very large energy spread. And to reduce the energy spread is very necessary, because a large energy spread makes it very difficult to make your final focusing. You get
11:44
a chromatic aberration. So you can at most take out something like 5 percent of the energy of the structures. And the rest is going into the termination resistor and is lost, because you have to pulse these accelerators. If you would do this DC at the powers you need here, it would be terrific. It would heat very much, too much. Now, so the problem is that
12:05
if we want 20 times more energy than SLC, and this thing is three kilometers long, we would then get two Linux of 60 kilometers. And that's far too expensive, far too long. So we want to have a higher accelerating gradient. That, of course, increases, again,
12:21
the power we need. Then the power is increased because we need a higher energy. And the power is increased because we need a higher luminosity. So we are really limited by power very much. Now, one way to solve this would, of course, be to use a superconducting Linux, which you could operate DC. You could have a very high bunch frequency.
12:42
No problem with efficiency. It would be very efficient, apart from the cryogenic losses. Then the problem is that with these superconducting cavities, you can only make very low gradients. This is about the highest that has been made for practically operating cavities. And what we need is at least three or four times more than that. Otherwise,
13:00
these accelerators get too long. Now, the problem here is not a fundamental one, but the problem is that these superconducting cavities are limited by field emission effects, by little impurities on the surfaces. They have to be cleaned very carefully. Then there are always some hot spots which you have to detect and to clean again. And to do that for a 10-kilometer length accelerator is unthinkable at the moment.
13:23
So this is completely out. It may be that the high-temperature superconductivity will give a solution, but we are still very, very far from the parameters we need. We need high current densities and very high Qs, and that is still not solved. Now, I propose to tell you various ways that have been thought of to improve the
13:42
construction of these accelerators, various new methods that have been thought of, most of which are at present, as far as we can see, not immediately useful. But I wanted to tell you about this because it's fun. It's nice physics. Now, one of the things that have been proposed is the so-called switched power linac, which is like this. You have a lot of disks, parallel disks,
14:04
the hole in the middle, and around there are rings here that go around like this. And this is a blown-up image of this. This is a ring which you charge at, say, 100 kilovolts or so. And this is a photocathode surface on which a laser pulse is directed. This charges your ring, and there's an electrical pulse running to the center.
14:24
And because here, of course, the radius gets smaller, the field strength gets bigger, and you get very high fields here in the center, and you start to accelerate your beam. Now, this is a very nice idea, and it is being worked on very hard, mainly on paper, but also some models are being built. One of the big problems are these photocathodes to make them reliable,
14:43
and it's much too early yet to be sure that that will work. Also, the efficiency of these things, it looks at first sight as if it may be better than a radio frequency because the pulse gets in only once and not for a microsecond. And it should be more efficient in principle, but it hasn't yet been proved that this is so. Now, one other class of accelerators proposes to use wake fields. What is a wake field?
15:07
If you send a bunch of particles through one of these accelerating structures, suppose you have no voltage in the structure to start with, then these particles will have lines of force associated with them. I've tried to draw the electrical lines of force here,
15:21
and they will fill these cavities with energy, and they will stay behind because this particle goes at light velocity, and this field can never overtake it anymore. Now, it looks as if this is a small technical problem, but it is very fundamental because this is the way the particles are accelerated. After all, they have to take energy out of the electromagnetic field, and to do that, the electromagnetic field has to be reduced,
15:44
and it's the wake field that does that. This wake field is very important. Now, you can also think of using that. You take an empty structure and you shoot a bunch that makes wake fields behind it, and you use these wake fields to accelerate other bunches. Now, the problem is, of course, that these other bunches are accelerated,
16:01
but the bunch you shoot in first is decelerated, and you would like to have a large transformation ratio to accelerate the bunches that come behind much more than you decelerate your first bunch. Now, that is not so easy. There is a theorem about it which I will skip, which says that you cannot do that very easily unless you use special tricks, and one of the tricks is being tried out in Hamburg at DESI,
16:25
and it is like this. You, again, have these disks with holes, and now this primary beam, this driving beam, is a ring here, a ring-shaped beam, and it makes wake fields that go outside and are reflected here, and this is what represents the wake field. It goes inward,
16:41
and the voltage is amplified because here the characteristic impedance gets higher and higher, and here is then the beam that is accelerated and which sees a very high gradient. Now, this is, of course, very nice. It's a bit complicated because you have to make these ring-shaped beams. You have to be very careful that there are no transfer field components here
17:01
that would spoil your accelerated beam, and this is still being studied in great detail, and maybe it will work. It will lead to some solution. It's, however, rather complicated, and it does not solve the problem of efficiency that is there like in all other accelerators. Now, one other way of using wake fields is by using plasma. You use plasma oscillations.
17:26
Now, what happens in a plasma, you have electrons and you have ions, and these electrons can oscillate. They have mass, and they repel each other. It's a little bit like an elastic medium, but it is different. If you have an acoustic wave in air, the particles collide, and that's when they feel each other, but electrons feel each other from a distance
17:42
because of their electromagnetic fields. So if you write down the equations, the Maxwell equations and the equations of movement, you find that there is a group velocity corresponding to these oscillations that this happens to be zero in the plasma. That's very nice because it means that if you now shoot a particle, a bunch of particles through a plasma, this green thing here,
18:05
then in the wake of that, you will get these oscillations. This is trying to represent the density of fluctuations, and so this is kind of wave that follows this bunch. So you have a phase velocity that is the same as the bunch velocity, but you have a group velocity zero, which means that this modulation of the density, it doesn't spread out. It
18:23
remains confined to the wake of this bunch. Of course, the field, the electric fields, which is supposed to be these lines here, go a little bit outside, but not very much. Now, there's a plasma wavelength, which is a characteristic of the plasma. It only depends on the density of the gas and on some fundamental constants. It can be very small. It
18:43
can be in the order of tens of millimeters, and the maximum fields you can make are easy to calculate, and they are in the order of GeV per meter, which is extremely high. However, that hasn't yet been done. It works very nicely on paper, but nobody has yet made fields of GeV per meter. The idea then is to use a driving bunch and to let it be followed by
19:08
a smaller bunch that gets accelerated by the wake field. Now, again, this wake field, you want to have a high transformation ratio. Now, one of the tricks you can do is to have, this is the bunch direction of movement. This is the density of the driving bunch,
19:23
and you see it's like a sawtooth. It's like a motor board that goes through water, and it has a shape like a triangle, and it softly pushes the water away, and behind it, it suddenly ends, and the water makes enormous waves, and you see these waves here. This is the wake field, and you can accelerate this bunch
19:41
of particles if you put it here at the maximum of that wave, and that is much higher than this, and in fact, the ratio of the two is 2 pi N, where N is the number of wavelengths, of plasma wavelengths. This bunch extends, though. Of course, this driving bunch must be much more intense than that for conservation of energy. Now, this all looks very nice,
20:02
and a lot of work has been done on that on paper, but the problem is that apart from accelerating fields, there are also focusing fields, and you might think that's very nice because it focuses your beam, but the focusing fields tend to be very strong, and it is 90 degrees out of phase with the accelerating field, so if you put your particles here, they are not focused in
20:23
principle, and you could put them slightly in front, then they would be focused, but if you calculate it, you find that for practical solutions, you can only tolerate bunches that are of the order of angstroms long, and that is too short, and this focusing is much too strong unless you make this driving beam very wide, and if you make it
20:42
very wide, then the efficiency gets very bad because then the wake field extends over a wide region, whereas your driven beam has a very low emittance, and this is very, very narrow, so this focusing problem is really the big problem to be solved. Also, the focusing of the driving beam is much too strong because the driving beam, of course, has low energy,
21:01
and that is still not solved. There is one other way to make waves in plasma that do very similar things, which is the plasma beat wave principle. Here, you use two light waves from lasers which beat with each other, and if you have a single wave, then the electrons of the plasma
21:21
will move like this in figures of eight because the electric field is at right angles, and because of the magnetic field, it will do this, and they will not be accelerated, but if you have a beat like this, they tend to go down this slope. These figures of eight deform, and particles move in towards here, and you get the density fluctuations of your electrons that again move in this
21:42
direction with a phase velocity, and they can then accelerate other particles. Now, this principle even has more problems than the plasma wick field accelerator, and although people are still working on it here and there, it doesn't promise very good results very soon. Now,
22:02
other possibilities. The stored energy in one of these radio frequency structures of Linux is inversely proportional to the square of the frequency because if you increase your frequency, you can reduce the size, and for the same field, you have less energy. So, the solution is to use very high frequencies, and in fact, people have proposed to use laser frequencies, but then you
22:23
have to have your accelerator beam very, very near to the accelerator structure, and in fact, it turns out that this is not practical, but what you can do is increase the presently used frequencies of, say, three gigahertz to ten times that value, where you have a wavelength of a centimeter, and you may still be just able to make suitable structures. So, that is one
22:45
approach that is, at present, looks the most likely thing that we could possibly use. The problem is that high frequency sources are not available at these wavelengths, at least not of sufficient power. So, one idea is to, instead of using klystrons, which we don't have at
23:03
these frequencies, to use a single beam that runs parallel to the beam you want to accelerate. So, in fact, you combine all the klystrons, which have their own electron beams, into one single beam, and one of the ways to do that is what they have been doing at Berkeley Livermore.
23:23
You take a low energy beam, you accelerate it from time to time by so-called induction accelerator gaps, and in between you have wiggles, where the electrons are made to wiggle, and they then emit photons, and you get a frequency which depends on the wiggler frequency and on the energy of these particles, and you make, for instance, 30, I think they use 35 gigahertz,
23:43
and use that for your main linac. Now, they have made, actually, of the order of one gigawatt per meter or so, which is more or less in the order of magnitude you want, and this may well be the way to make high energy accelerators, although there are serious problems with this approach. One of the problems is that these electrons are not really
24:03
very relativistic. They are a few MeV, and that's because, otherwise, you don't get the right frequency, and if these electrons are not quite relativistic, their speed varies. Here they are decelerated, there they are accelerated, and so on. Then this frequency will also, the phase of the signal will also vary a little bit,
24:21
and it is extremely difficult to keep the phase of all these generators constant over the whole 10-kilometer length of the accelerator, and that is one of the big problems. There are some other problems. Now, one of the approaches is this one. That is what we are studying at CERN, where we have, again, two beams. The drive linac is accelerated from time to time at distances
24:46
by superconducting cavities at the lowest frequency, and it is then bunched at this frequency, but there are also little bunchlets. The bunches of 350 megahertz are subdivided in little bunches at 30 gigahertz, and they pass through accelerating structures here where the
25:01
wake field produces a wave that fills these structures, and then you can get very high gradient here, whereas the gradient here is very low. It turns out that the ratio of gradients here and there scales with the ratio of these frequencies, so you can use very low frequency here like we do here compared with this one, and that is just what we want because these
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superconducting cavities at these frequencies exist. They are being built for the lab machine. So this is an approach which on paper seems at least to give a good solution from the RF point of view, power point of view. That all seems to work very nicely. We have made some models,
25:41
but it is getting a bit too late, so I will skip that. I would like to say something about one of the other problems in these accelerators. That problem is the transverse wake fields. I have told you about the longitudinal wake fields, but a particle that is not exactly on the central axis of the accelerator will also cause a transverse electric fields
26:02
which will deflect particles coming after these particles. Now, if you have a particle that is offset, it will make oscillations because it is a focusing structure, and these wake fields will also oscillate in the same way, and the particles at the tail of the bunch will then be deflected by these wake fields also in an oscillatory way, and if they have the
26:24
same frequency as the particles in front of the bunch, then there will be a resonance, and these bunches will break up, and that is in fact one of the things that limits the Stanford Linux. And this problem has been really a play for people who designed these high-energy machines, but we think that we have now found the solution to that.
26:42
One of the solutions that people propose is to have very, very strong focusing. Then you can reduce this effect, but it is very expensive, and it makes the alignment of the machine very, very critical. The other possibility is to have slightly different focusing frequency between the front and the tail of the bunch. And one way to do that, which we have resurrected,
27:04
you might say at CERN, is to use in these accelerating structures instead of little round holes to use slits, which cause radio frequency focusing, and the focusing then depends on the place of the particle in the bunch, and you make quite a big difference between the focusing
27:22
of the front of the bunch and the end of the bunch. And if you adjust the sign properly, in fact these wake fields may even damp the offsets that you have initially. So the result will then be that initial beam offsets will be reduced by these wake fields, and they will make the alignment of the whole machine much easier. Now, this is something I cannot
27:44
go into much detail about, but I think it is one of the big advances that we have made in the last year at CERN. Now, one of the problems is, of course, the final focus, but I see that I'm nearly running out of time. So I'm only going to mention that the problem is to focus
28:01
the beam to a very small point for all different energies in the beam, so the chromatic aberration is the big problem. You somehow have to compensate that, and that is done with very complicated systems where you deflect the beams a little bit, and you then have a dispersion, and the particles of different energy are at different places, and you use non-linear lenses
28:21
to give them different focusing. That problem is not yet quite solved on paper. At CERN, we have designed a machine with certain parameters, but we have not yet completely obtained a focusing system on paper that fits these parameters, and it is fair to say that nobody anywhere has yet made a completely coherent design, so that even if we would get the money today, we would not
28:43
know how to build such a machine. But we are getting near. There is quite clear progress visible, and that is the thing I would like to end with. The problem is to be solved at ease, to make a very intense bench for the driving beams, to design a proper final focus, to work out
29:03
how we can get sufficiently small tolerance in alignment, and you see the transverse wakefield I have crossed out, because since I gave this talk last time, I think this has been solved. Now, I wouldn't like to leave you with the impression that this is all very, very difficult. It is true that it is very, very difficult, but there are many interesting problems for
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young physicists to work on, and I can all recommend you, if you have some time, to think about these problems and to help us solve these propositions. Thank you very much.