Posted in | Quantum Physics

Optimum Conditions for Laser Plasma Acceleration Discovered

In modern research, conventional electron accelerators have become a crucial tool. The majorly bright radiation produced by synchrotrons, or free electron lasers, supplies unique insights into matter at the atomic level. However, even the smallest versions of these super microscopes resemble the size of a soccer field.

In a so-called target chamber, the light pulse of the high-performance laser DRACO hits a gas-jet. The aim is to accelerate electrons to almost the speed of light on a distance shorter than a pencil's width. Foto: HZDR / F. Bierstedt

An alternative could be offered by laser plasma acceleration: with a much smaller footprint and extremely higher peak currents it could be the basis for the modern generation of compact light sources. Until now, the challenge with laser accelerators has been to develop a stable and reliable electron beam, which is considered to be the prerequisite for potential applications.  A method to increase both beam quality and stability has now been developed by Physicists at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR).

The fundamental principle of laser acceleration seems to be quite simple: A bundled, ultra-strong laser beam hits a trace of gas, which immediately develops plasma – an ionized state of matter also referring to a whirling mix of charged particles. Electrons are pushed away from their parent ions by the power of the light pulse, thus developing a kind of bubble-like structure with a powerful electric field in the plasma.

This field is dragged behind by the laser pulse like a stern wave, and it traps the electrons, accelerating them to practically the speed of light.

These speedy particles allow us to generate x-rays. For instance, when we make these electron bundles collide with another laser beam, the impact generates bright, ultra-short x-ray flashes – an immensely valuable research tool for examining extreme states of matter.

Dr. Arie Irman, the HZDR Institute of Radiation Physics

Right Time + Right Place = Perfect Acceleration

The strength of the secondary radiation majorly relies on the electrical current of the particles. In turn, the current is regularly determined by the number of electrons fed into the process. Thus, laser-powered acceleration holds immense potential since it reaches considerably higher peak currents when compared with the standard method.

However, as Physicist Jurjen Pieter Couperus highlights, the so-called beam loading effect kicks in, These higher currents create an electric self-field strong enough to superimpose and disturb the laser-driven wave, distorting thereby the beam. The bundle is stretched out and not accelerated properly. The electrons therefore have different energies and quality levels.” However, in order to use them as a tool for several other experiments, it is necessary for each beam to have the same parameters. “The electrons have to be in the right place at the right time,” summarizes Couperus, who is a Ph.D. candidate in Irman’s team.

The two Researchers, in partnership with other colleagues at the HZDR, were the first to show how the beam loading effect can be exploited for enhanced beam quality. A bit of nitrogen is added to the helium at which the laser beam is generally directed.

We can control the number of electrons we feed into the process by changing the concentration of the nitrogen. In our experiments, we found out that conditions are ideal at a charge of about 300 picocoulomb. Any deviation from it – if we add more or fewer electrons to the wave – results in a broader spread of energy, which impairs beam quality.

Dr. Arie Irman, the HZDR Institute of Radiation Physics

The Physicists’ calculations demonstrate that experiments under perfect conditions yield peak currents of about 50 kA. “To put this in context, only about 0.6 kiloamperes flow through the standard overhead line for a German high-speed train,” Jurjen Pieter Couperus explains. He is indeed confident that they will be able to beat their own record, “Using our findings and a laser pulse in the petawatt range, which our high-intensity laser DRACO can achieve, we should be able to generate a high-quality electron beam with peak currents of 150 kiloamperes. That would exceed modern large-scale research accelerators by about two orders of magnitude.” An achievement which the Researchers from Dresden believe would make way for the modern generation of compact radiation sources.

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