Optimal conditions for the most efficient operation of laser plasma accelerators 17.09.2017

Traditional electron accelerators have long become one of the main types of scientific instruments, extremely intense and short radiation pulses produced by synchrotrons and free electron lasers allow scientists to study matter and processes occurring on an atomic scale. But even the smallest electron accelerators now occupy an area comparable to the area of ​​a football field.

An alternative to traditional electron acceleration technologies is the laser-plasma acceleration method, which, with a small size of the accelerator, makes it possible to obtain a beam of accelerated high-intensity electrons. But accelerators of this type have one drawback - with their help it is very difficult to obtain a stable beam of electrons with a stable brightness. And this problem was solved by physicists from the research center HZDR (Helmholtz-Zentrum Dresden-Rossendorf), Germany, who managed to determine a number of parameters for creating optimal operating conditions for a laser-plasma electron accelerator.

The principle underlying the technology of laser-plasma acceleration is quite simple, a powerful laser beam is focused in a gas medium, which under its influence turns into a plasma, into an ionized state of matter. The energy of the laser beam causes the electrons to leave their "native" atoms, which creates a kind of "bubble" of a strong electric field in the volume of the plasma. This region of the electric field, which follows the pulse of laser light, is a wave that travels almost at the speed of light. And the electrons trapped on the crest of this wave are also accelerated almost to the speed of light. Exposure of these electrons to an additional pulse of laser light produces bright and ultrashort X-ray pulses, with the help of which scientists "see through" the studied samples of various materials.

The strength of secondary X-ray radiation directly depends on the number of high-energy electrons involved in this process. However, when a large number of electrons are accelerated, the plasma wave decays due to the influence of effects associated with these electrons and their electric field, which, moreover, adversely affects the shape of the beam. The distorted shape of the beam and the instability of the plasma wave lead to the fact that the beam contains electrons with different levels of their energy and other parameters.

"But in order to be able to use an electron beam for high-precision experiments, a stable beam consisting of electrons with the same parameters is required," says physicist Jurjen Pieter Couperus, "All the electrons in the beam must be in the correct place at the right time."

Scientists from HZDR carried out a number of works aimed at improving the quality of the electron beam produced by laser-plasma accelerators. They found that adding a small amount of nitrogen to helium, which is used to create plasma, greatly improved the situation. "We can control the number of electrons riding the plasma wave by changing the concentration of nitrogen," explains Jurien Peter Kuperus, "In our experiments, we found that the ideal case is when the plasma wave carries electrons with a total charge of exactly 300 picocoulombs. Even the smallest deviation from this value in any direction leads to energy dissipation, which reduces the quality of the generated beam. "

The performed calculations have shown that in order to generate a high-quality one, it is still required that the peak current of electron movement on the plasma wave crest be at least 50 kiloamperes.

“Using the ultrashort pulses of DRACO's petawatt laser, we will be able to generate a high-quality electron beam at a peak current of 150 kiloamperes,” says Jurien Peter Kuperus, “This will exceed the capabilities of all modern large-scale electron accelerators by as much as two orders of magnitude. And this will allow us to create very compact next generation X-ray sources".