An experiment on ultrafast acceleration of electrons in carbon nanotubes is being prepared

An experiment on ultrafast acceleration of electrons in carbon nanotubes is being prepared

Fig. one. A holder for a target in the ASTA facility in Fermilab, which will be used to test the method of electron wake acceleration in carbon nanotubes. Photo from the website fast.fnal.gov

Particle accelerators are needed both for new fundamental discoveries and for numerous practical applications. Radical progress here will be possible only after the introduction of new particle acceleration technologies. Thus, wakefield acceleration in plasma has long been developed, where physicists expect to obtain accelerating fields of tens of gigavolts per meter. But this is not the limit. At Fermilab, preparations are underway for an experiment in which wake acceleration will be realized inside carbon nanotubes, and the accelerating gradient, according to estimates, will reach transcendental teravolts per meter.

When it comes to future colliders of elementary particles, designed for even higher energies, the main stumbling block is their large size – tens of kilometers! – and, as a result, high cost. High-energy ring accelerators cannot be made smaller, because this will either require superstrong magnetic fields that are not yet available, or, in the case of electrons, too much energy will be spent on each revolution of the beam. Linear electron-positron colliders are free of these drawbacks, but they also run into the technological limit – the currently available acceleration rate of particles. The accelerating electric field in the currently used accelerating sections reaches several megavolts per meter. The project of the international linear collider ILC is designed for a higher accelerating gradient – 31.5 MV / m. But even with such values, almost 10 km of the direct accelerating section will be required to accelerate the electrons to an energy of at least 250 GeV. And if ILC or other similar projects can still be implemented, then further – a financial dead end.

Accelerators also find numerous practical applications. There are now over 30,000 accelerators in the world, and almost all of them are used specifically for applied research. Their energies are small, but such installations still occupy entire buildings. Such accelerators will become much cheaper and more affordable if, with the same energy, they can be turned into desktop installations.

All these requests of fundamental and applied research can be satisfied only through the implementation of a new technology for accelerating particles with a gradient of the order of GV / m or more. There are no fundamental physical obstacles here. If, for example, a short and dense proton bunch-driver is launched into the plasma, then on its way it will generate a strong oscillation, a kind of bubble flying forward at a near-light speed, inside which the electric field will reach tens of GV / m. An electron bunch launched after it will constantly be in the region of a superstrong electric field and will quickly accelerate to high energies. A similar bubble can be generated in plasma and a super-powerful laser pulse.

This technology, called wake acceleration, is now being actively developed in many laboratories, including CERN, where a special AWAKE experiment is working on this issue. Details can be found in our news (see, for example, Scientists have increased the efficiency of the plasma accelerator by increasing the beam density, “Elements”, 11/28/2014 and links to earlier news), in the popular lecture by Artem Korzhimanov On the crest of a plasma wave for clear bioimaging and in a lecture by Konstantin Lotov Wake acceleration of particles in plasma.

Is it possible to make wake acceleration even more efficient? For a long time, physicists drew attention to the fact that inside a solid, this process can be much more efficient. The density of electrons in a continuous medium is hundreds and thousands of times higher than the electron density in a rarefied plasma. Therefore, if such a bubble is created inside a crystal, one can swing at an accelerating gradient of the order of tens of TV / m – millions of times stronger than what is available now! As in a plasma, an electron bubble in a crystal can be created very quickly, so that the nuclei of the crystal lattice will not have time to move from their places. As a result, the electrons will fly between the rows of almost naked nuclei and will be accelerated almost without scattering. The crystal will most likely be destroyed during such a procedure, but it will allow the electrons to be accelerated to energies that are currently unattainable. Theoretically, this can be done in a crystal using a super-powerful (gigawatt) hard X-ray pulse with photon energies of tens of keV. Unfortunately, sufficiently powerful pulsed sources of such hard radiation do not yet exist. One could try, by analogy with plasma, to use a short driver clot to create a bubble, rather than a hard X-ray flash, but, alas, this clot will not be sufficiently stable inside the substance.

And then the material comes to the rescue, which has long been heard, but for completely different reasons – carbon nanotubes (see just one recent example of their remarkable properties in the news White phosphorus, enclosed in a nanotube, turned into “pink”, “Elements”, 14.06.2017). They are graphite planes rolled into long and even tubes. They can be multi-walled and quite wide, with a diameter of a thousand interatomic distances, while maintaining their amazing structural strength. If a compact bunch-driver is passed through such a nanotube, it will trigger a strong plasmon oscillation in it – oscillations of the electron density and electromagnetic field that support each other (Fig. 2). A region of a strong accelerating field of the order of TV / m will appear on the nanotube axis. Due to the fact that the gap in the nanotube is much wider than the interatomic distance, the requirements for the bunch-driver are much less stringent than for channeling in the crystal. As a result, the nanotube itself will not collapse and will be ready to receive new bunches.

Fig.  2. Simulation of electron density in a multi-walled carbon nanotube

Fig. 2. Simulation of the electron density in a multi-walled carbon nanotube as a pair of electron bunches pass through it. Image from the article in question

The idea described above was proposed several years ago by a team of authors from the University of Northern Illinois and the National Accelerator Laboratory. E. Fermi – Fermilabe (see the short description in the preprint arXiv: 1502.02073 and a more detailed analysis in the article arXiv: 1504.00387). Their assessments then showed that current technologies, at a minimum, make it possible to carry out a test experiment to validate this idea. In a recent publication TeV / m Nano-Accelerator: Current Status of CNT-Channeling Acceleration Experiment, the researchers outlined the current state of the planned experiment.

The experiment is planned to be carried out at Fermilab. The FAST accelerator complex is under construction there, which will allow researchers to test various acceleration schemes with an eye on next-generation accelerators. In particular, the ASTA accelerator line will emit an electron beam with a modest energy of 50–300 MeV, with which, however, many manipulations can be performed. In particular, an electron bunch in it can be prepared in such a way that when it passes through the nanotube, its “head” starts a strong vibration, and the “tail” located at a controlled distance is accelerated in the resulting superstrong field.

The array of nanotubes for this experiment will be manufactured by NanoLab Inc. by the well-known technology of vapor deposition onto a porous alumina substrate. The company has already presented the first test sample with nanotubes of the required dimensions (diameter 200 nm, length 0.1 mm), see Fig. 3. All other infrastructure of the FAST accelerator line, including sensors and detectors, is in principle ready.

Fig.  3. Technology for manufacturing an array of nanotubes and the first sample

Fig. 3. Left: technology for manufacturing an array of multi-walled carbon nanotubes on a substrate, on right: the first sample manufactured by NanoLab Inc. Images from the article in question

Finally, a detailed numerical simulation of the process was performed for various parameters of the setup (the energy of the initial beam, the charge of the bunch, the radius of the nanotube in comparison with the wavelength of the plasmon oscillation). His results confirm that a sufficiently dense bunch-driver is capable of creating an accelerating gradient up to 1 TV / m. With the beam parameters that the installation can produce so far, the effect will be weaker, and even at submillimeter distances, the energy gain will be very modest. But now the main task is not records, but a demonstration of the technology itself. As soon as it works and the expected patterns are confirmed, it will be possible to purposefully work to enhance the effect.

A source: Y. M. Shin, A. H. Lumpkin, J. C. Thangaraj, R. M. Thurman-Keup, V. Shiltsev. TeV / m Nano-Accelerator: Current Status of CNT-Channeling Acceleration Experiment // arXiv preprint: 1705.01983 [physics.acc-ph]…

Igor Ivanov

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