Electron pulses — all with nearly the same tunable energy — have been produced from a laser accelerator. The accomplishment makes electrons, which travel at a velocity close to the speed of light, easier to control as a tool for ultrafast physics experiments. A team at the Laboratory for Attosecond Physics, led by Dr. Laszlo Veisz and professor Stefan Karsch of Max Planck Institute of Quantum Optics, produced the femtosecond-scale pulses using a laser. Until now, bunches of electrons were mostly created using large and costly radio-frequency (RF) accelerators, but the process was complicated and resulted in substantial particle loss. Lasers have been seen as a viable alternative, except for the difficulty in giving all particles in the bunch the same energy, which creates "cool" bunches that are easier to control for experiments such as probing ultrafast biological processes. The advantage of the RF accelerator is that it always contains a particle source that defines the number of particles in the bunch, the pulse duration and energy width. Such a defined particle source had been missing from laser accelerators, until the team demonstrated how to integrate such a source and use it to create bunches, the individual particles in which all have nearly the same energy. A laser pulse (red) hits helium atoms (blue), streaming from a nozzle with supersonic velocity. A very compact and controlled difference in density (dark-blue ray) develops as a result of the partial obscuring of the nozzle by a razor blade. Precisely at this difference in density, the laser pulse hits the helium atoms, separates the electrons and accelerates them to nearly the speed of light. Because the electrons are all separated at the same location and the same time from the atoms, they gain nearly the same energy. Courtesy of Thorsten Naeser. To do this, the researchers released helium atoms from a small nozzle at supersonic velocity. Directly above the nozzle, they placed a razor blade that obscured part of its orifice. After the supersonic helium stream is released from the nozzle and hits the razor blade edge, it forms a shock front, or density step, in the gas. At precisely that position, the investigators focused an extremely strong, 28-fs laser pulse. The pulse separates electrons from their atoms, forming a plasma channel and accelerating them close to the speed of light within a few hundred microns, and gives them all approximately the same energy. All electrons start their journey at the shock position, traveling exactly the same acceleration distance until the end of the gas jet and gaining the same energy. Without the shock front, various electrons would start at a random position and gain different energy. "By changing the position of the razor blade above the nozzle, we can determine where the density step is formed and, hence, how long the acceleration distance is and what energy the electrons gain," Veisz said. The result is perfectly controlled, ultrashort electron pulses that can be used to generate femtosecond light pulses down to the x-ray regime, the researchers say. Such high-beam-quality pulses produced by compact, cheap and well-controlled laser accelerators could have medical applications, such as serving as a lower-dose x-ray source for patients. The work, "Shock-Front Injector for High-Quality Laser-Plasma Acceleration," recently appeared in Physical Review Letters (DOI 10.1103/PhysRevLett.110.185006). For more information, visit: www.mpq.mpg.de