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The interference of waves in a plasma helped to capture and accelerate electrons by an “optical rocket”

Physicists from China and the USA for the first time realized in the experiment a method for obtaining high-energy electron beams from a plasma, based on the use of two coherent laser pulses directed at an angle to each other. Due to the interference of electromagnetic waves and ponderomotive drift, the electrons are first captured from the plasma, and then accelerated, forming a beam with an energy of up to 300 megaelectronvolts. This approach was called an “optical rocket” by the authors and suggests using it to improve modern installations for accelerating electrons, as well as to study the interaction of electromagnetic waves in a plasma, scientists in Physical Review Letters have  reported .

Since all the particles in the plasma are in the ionized state, its properties are primarily determined by collective electromagnetic interactions. For example, the electrodynamic properties of a plasma are determined by the presence of electromagnetic waves , which can propagate along it. One way of obtaining these waves in the laboratory is to irradiate the plasma with high-energy laser pulses or accelerated charged particles. This is the approach that physicists propose to use, in particular, for the wakefield acceleration of electrons : with simultaneous excitation of an electromagnetic wave in a plasma and irradiation with an electron beam, electrons from a beam are captured by a wave and accelerated to an energy of the order of a gigaelectronvolta.

A group of physicists from the US and China, led by Donald Umstadter of the University of Nebraska-Lincoln, showed that the same phenomena that are used to accelerate electrons can also be used to capture an electron directly from the plasma and generate high-energy electron beams. To this end, scientists have used effects theoretically described more than twenty years ago – ponderomotive drift (motion of charged particles in an inhomogeneous oscillating electromagnetic field) and interference of several electromagnetic waves in a plasma. The main idea of ​​the proposed experiment was that the plasma was irradiated not by one but simultaneously by two very powerful coherent laser pulses, which were focused at a given point and directed at an angle to each other.

In the physics experiment, femtosecond laser pulses with an intensity of more than 10 20 watts per square centimeter, with a wavelength of 800 nanometers and a duration of about 30-40 femtoseconds, were used. The angle between two beams with horizontal polarization was 155 degrees, as a result of which a very strong gradient of the wave intensity appeared after interference, which led to the capture of electrons. The scientists varied the time delay between the two pulses and found that, depending on which of the pulses goes before (co-directed with the resulting electron beam-into which the electrons from the plasma are sucked, which are then investigated at the output, or the second-the injector pulse ), the number of electrons captured from the plasma and their energy after acceleration depends.

It turned out that when interacting with both electromagnetic waves, electrons are captured by a wave in three stages under the influence of an intensity gradient and ponderomotive forces on the side of each of the waves. In this case, if the first wave is co-directed with the resulting electron beam, then the effect of wave interference dominates in the capture of electrons, and in the case when the injector wave is earlier, both the interference and the ponderomotive drift. At the output, electron beams with an energy of more than 300 megaelectronvolts and a charge of up to several picoculons are formed – both quantities are almost two orders of magnitude greater than when using a single wave.The results of the physics experiment were confirmed and studied in more detail by numerical simulation. According to the scientists, the experimental realization of such a mechanism of electron capture with the help of two high-energy laser pulses can be used in the future to improve modern compact installations for accelerating electrons, as well as for studying the dynamics of waves in a plasma. In addition, the authors of the work describe the effect described by them as an “optical rocket” – using only light pulses they succeeded in obtaining a beam of massive particles with a velocity close to the speed of light.

The wakeful acceleration of electrons by means of plasma is interesting first of all in that by using this approach it is possible to accelerate electrons in very compact installations. For example, it was with the help of wake acceleration that CERN physicists recently succeeded in accelerating electrons to 2 gigaelectronvolts in an installation only 10 meters in length, which is approximately twice as high as for accelerators on radio-frequency superconducting resonators.

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