"This is fantastic!": New particle acceleration method works

"This is fantastic!": New particle acceleration method works
"This is fantastic!": New particle acceleration method works
Anonim

For the first time in the world, CERN proved experimentally that acceleration of an electron beam in a plasma using a proton driver is possible. In the future, scientists hope, the new method will significantly reduce the size of colliders, because now accelerators have reached their limit in terms of this parameter. The results have already been published in Nature. The experiment was carried out by the AWAKE collaboration; the Institute of Nuclear Physics named after V.I. G.I.Budker SB RAS (INP SB RAS). Project manager Edda Gschwenter talks about the next stages of work and the Russian contribution.

Edda Gschwenter

In the future, scientists hope, the new method will significantly reduce the size of colliders, because now accelerators have reached their limit in terms of this parameter. The results have already been published in Nature. The experiment was carried out by the AWAKE collaboration; the Institute of Nuclear Physics named after V.I. G.I.Budker SB RAS (INP SB RAS). Project manager Edda Gschwenter talks about the next stages of work and the Russian contribution.

- Edda, what is the essence of this experiment?

- In the classical scheme, the electron beam in the collider is accelerated under the influence of the electromagnetic field. In our experiment, a proton beam flies in the plasma, it creates a wave and thereby provides the acceleration of the electron beam flying in the wake, this is called wake acceleration. An electron beam with an energy of 19 MeV flew ten meters in the plasma and increased the energy to 2 GeV, that is, more than 100 times. This means that the average acceleration rate was reached - 200 MeV / m.

- How did the AWAKE project start? When did Russia join?

- I became one of the leaders of the AWAKE project in 2012, and in August 2013 it was approved, that is, received funding. First, we wrote justifications in order to achieve approval of the project in various committees, in 2012 we got a preliminary draft, we found a place where we want to build a building for an experiment. At this stage, I had my first contacts with the Budker Institute. But he has been participating in the project since 2008.

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We have several teams. One is engaged in physics, the other - in accelerators, the third, INP SB RAS - in theory, that is, calculations and modeling. The participation of the Budker Institute is less applied, but extremely important, because it allows us to understand what we are spending our money on. This is a very large part of the work, and INP SB RAS is engaged in it, since it has its own computer codes. In particular, his staff wrote 2D code that proved to be quite useful. In general, there are several 3D programs, and all of them allow you to do excellent calculations, but they require a lot of computers and time. If you need to quickly deal with some issue or get data in order to start designing an experiment, then the INP SB RAS code turns out to be irreplaceable. It does not require many computers to use it, and most importantly, it has been tested many times and you can trust it.

I believe that computer simulations are the main contribution of the Institute of Nuclear Physics. GI Budker in this project. In addition to modeling, INP SB RAS also took part in the experiment. For me, the interaction with the staff of the institute was extremely useful.Based on their calculations, we were able to make an initial design of the facility, study the features of electron acceleration and understand how the proton beam self-modulation occurs. It was a real challenge! This is especially true for modeling the behavior of beams of electrons and protons in plasma, because this is terra incognita, in contrast, for example, to the processes occurring in the LHC. With the help of these calculations, we understood how to build the installation, what parameters it will have, and what beams to use. This helped us justify the project in the CERN leadership. It seems to me that AWAKE is one of the fastest approved CERN experiments: it took only six months from recommendation to agreement.

- What is the project budget?

- Currently, CERN's contribution is 15 million Swiss francs, but it does not include salaries for CERN permanent employees, only the cost of materials and payments for students, graduate students and temporary positions. Funds are needed, for example, for the creation of magnets, beam extraction channels and other high-tech equipment. The total cost, together with contributions from other institutions participating in the collaboration, is 20-25 million Swiss francs.

So, so far we have conceptually proven that we can accelerate an electron beam in a plasma.

- Those very successful ten meters, about which it is written in the article Nature.

- Exactly. We have demonstrated this capability on a ten meter long plasma cell. The energy of the proton beam is very high - it is kilojoule - so it is very profitable to use it as a driver. In other experiments, for example BELLA, a laser is used as a driver, its energy is much lower, only a few joules. The main goal of our experiment is applications in the field of high energy physics. If we want to accelerate electrons to high energies, we need to use a very long plasma cell. In this case, the driver must travel the entire length without being destroyed by interaction with the plasma. There is no such problem with a proton beam, because its energy is so great that it is not completely decelerated in plasma and does not lose all of its energy. It gives only part of the energy to the plasma, and the plasma, in turn, gives it to the electrons - this is the beauty of our experiment.

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- Do you believe that colliders will be based on this principle of operation?

- Yes, of course, we strive for this.

- When?

- There is the International Committee for Future Accelerators, this is an international organization, I am a member of it. We are thinking about how to build a multi-TeV accelerator based on plasma. We hope that a detailed project will be prepared by 2035.

- What's the next step? Need to build another 10, 20, 100 meters? Or should the energy be increased?

- Both of these parameters are important. AWAKE has demonstrated the fundamental possibility of such acceleration, the first step has been taken. The difficulty is that such an experiment requires a very short beam of protons, which do not exist in nature. Why did the experiment succeed anyway? The point is that self-modulation of the beam occurs during the passage of protons in the plasma. This means that it breaks up into microbeams, due to which the wake acceleration works. We have demonstrated this, and this was the goal for 2016-2017. This is a very non-trivial physical problem, and in fact it is fantastic that we managed to solve it.

The proton beam is 12 centimeters long; it passes through the plasma and decays into hundreds of microbeams with an interval of about one millimeter. This distance depends on the plasma density: if the density is higher, then it is shorter.

Thus, we have shown for the first time in the world that a proton beam creates a wake wave in plasma, in which electrons are accelerated.

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- Incredible!

- Exactly. It is also incredible that our project was being implemented strictly according to plan throughout the entire time. We have four experimental runs this year and it was not easy to keep to the schedule.

AWAKE's first task for the future is to change the setup and set up the experiment in such a way as to show that we can get a good quality electron beam and that it is suitable for physical applications. The second challenge is to demonstrate its scalability, which is what you asked about. This program has not yet been approved, and we have to go the same way as in 2013.

- Hope you get it done as quickly as the first time.

- Yes, and the Budker Institute can help with this, since we need to understand what the basic design of this experiment should be, how to get a beam of good quality. Today we have done a lot of calculations, because the development of the installation is completely based on simulation. Now we understand that it is better to make two plasma cells. The length of the first will be five to ten meters. Self-modulation of the proton beam will take place in it. Then the protons will fly into another cell, into which we also inject a beam of electrons. This is where the wake acceleration will take place. The Institute has performed a lot of calculations of the injection scheme, and now we are continuing to work with it on the choice of the installation concept.

- What kind of car will it be?

- From a technical point of view, the easiest way would be to make two cells of the plasma, with a gap of two meters, in which the injection of electrons will take place. But calculations have shown that this can degrade the quality of the beam. Now we know that the gap should be small enough, but there are still a lot of questions for engineers.

- What are the possible physical applications of this acceleration method?

- There are two main applications. One of them: a fixed target experiment for searching for dark matter at CERN, similar to the NA64 experiment. We figured that if we applied our technology, we could produce a lot more electrons, the result would be much more interesting. Of course, the experiment needs to be adapted - and the INP SB RAS is also engaged in this. It is necessary to establish how long the installation will be, how large the plasma cell will be, what the loss of electrons will be, what physical processes will take place there. There are already the first calculations of the plant design. This information cannot be obtained from experiment, so modeling is important.

The second application is an experiment called Plasma Electron-Proton / Ion Collider. The point is to collide the electrons accelerated in the plasma section with the proton beam from the Proton Supersynchrotron (SPS). According to calculations, for this experiment it is necessary to build a plasma installation 130 meters in size, which will produce electrons with an energy of 70 GeV: they, in turn, will collide with protons from the LHC. For this experiment, we need to determine whether new tunnels need to be built. Calculations will help determine what energy we can get, what quality of the beam will be in this experiment, what parameters the proton driver should have.

Potentially, in the distant future, other applications of this method are also possible - for example, those related to medicine. Indeed, the use of accelerators in medicine is often limited by their size. But for now, our priority is high-energy physics.

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