An innovative accelerator project at DESY has produced its first electron beam. The experimental facility goes by the name of LUX and is being operated in collaboration with the University of Hamburg, DESY and ELI Beamlines. It is based on the promising technology of plasma wakefield acceleration which will hopefully one day give rise to smaller and more powerful particle accelerators. During a first test run, LUX was able to accelerate electrons to about 400 mega-electronvolts, using a plasma cell that is just a few millimetres long. This corresponds very nearly to the energy produced by DESY’s 70-metre, linear pre-accelerator LINAC II.
“The result is a first important milestone on the path to developing compact laser/plasma-based accelerators in Hamburg,” explains Reinhard Brinkmann, Director of the Accelerator Division at DESY. However, the new technology still has to overcome a number of hurdles before it can be routinely used.
In plasma wakefield acceleration, a wave is produced in an electrically charged gas, known as a plasma, inside a narrow capillary tube. There are several different ways of doing this, which are being tried out in various projects on the DESY Campus in Hamburg as well as at ELI Beamlines. “LUX uses a laser with a power of 200 tera-watts, which fires ultra-short pulses of laser light into the hydrogen gas,” says the LUX project leader in Hamburg, Andreas R. Maier from the University of Hamburg. Each pulse lasts a mere 30 quadrillionth of a second (30 femto-seconds) and ploughs its way through the gas in the shape of a narrow disk: the light pulses are just 0.01 millimetres long and 0.035 millimetres high. “They snatch the electrons from the hydrogen molecules, just as a snowplough sweeps aside snow,” the physicist explains. “The electrons collect in the wake of the light pulse and are accelerated by the positively charged plasma wave in front of them – much like a wakeboarder riding the stern wave of a boat.”
The physicists participating on the project are hoping to use this technology to accelerate particles to up to 1000 mega-electronvolts. “The technology is still in the very early stages of development,” says Maier, “but as it is able to produce up to 1000 times the acceleration of conventional accelerators, it will allow us to build far more compact accelerators for future applications in fundamental research and in medicine.”
“Once developed, the LUX beamline will be moved to ELI Beamlines and integrated with its laser beams, enabling time resolved studies of processes in complex materials or biological molecules with applications ranging from studying impact of land use on soil quality, via development of new generation of photovoltaic cells and accumulators to studies of molecular ices interesting for astrobiology,” adds Lukáš Přibyl, the leader of LUX team at ELI Beamlines.
Over the coming months, the physicists will examine and further optimise the as yet “untidy” electron beam produced by LUX. To this end, the apparatus will be extended by adding further measuring equipment for so-called beam and plasma diagnostics. Furthermore, the researchers are planning to install a short magnetic slalom course, a so-called undulator, in which the fast electrons from the plasma accelerator will produce x-rays, which will later also serve the users of ELI Beamlines.
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