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The basic principle of laser acceleration seems quite simple: A bundled, ultra-strong laser beam hits a trace of gas, which instantly creates plasma--an ionized state of matter or, in other words, a whirling mix of charged particles. The power of the light pulse pushes electrons away from their parent ions, creating a sort of bubble-like structure with a strong electric field in the plasma. This field, which the laser pulse drags behind itself like a stern wave, traps the electrons, accelerating them to nearly the speed of light. "These speedy particles allow us to generate x-rays," says Arie Irman from the HZDR Institute of Radiation Physics. "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."
 

The strength of the secondary radiation greatly depends on the particles' electrical current. The current, in turn, is mostly determined by the number of electrons fed into the process. Laser-powered acceleration therefore holds great potential, because it reaches significantly higher peak currents in comparison with the conventional method. However, as physicist Jurjen Pieter Couperus points out, 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." But in order to use them as a tool for other experiments, each beam must have the same parameters. "The electrons have to be in the right place at the right time," summarizes Couperus, who is a PhD candidate in Irman's team.

Together with other colleagues at the HZDR, the two researchers were the first to demonstrate how the beam loading effect can be exploited for improved beam quality. They add a bit of nitrogen to the helium at which the laser beam is usually directed. "We can control the number of electrons we feed into the process by changing the concentration of the nitrogen," Irman explains. "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."