A team around DESY’s plasma acceleration researcher Jens Osterhoff has developed a new concept to improve electron-beam-powered plasma accelerators. Simulations suggest that their idea should enable the acceleration of very high-quality particle bunches. The DESY researchers, who are planning experiments at SLAC in the USA and FLASH in Hamburg, have now presented the results of their studies in the scientific journal Physical Review Letters.
Simulated propagation of a compact and ultra-relativistic electron beam through a hydrogen plasma. The top frames show a sequence of the process of injection of the helium electrons by means of the strong accelerating wakefields only. Main frame shows the final position within the wake of the injected electron bunch. The accelerating field surpasses 100 GV/m there.
Plasma wakefield acceleration is a relatively new technology aiming to accelerate particle beams to highest energies over very short distances. Its fundamental idea is to create a plasma, a highly excited state of ionised matter in which electrons and atomic nuclei can move freely, from a gas using either a particle beam or an intense laser flash, and utilize this plasma as the acceleration medium. The particle bunch that is shot into the gas excites an electron density wake – a so-called wakefield – travelling behind it, in which very high electric fields form. The scientists’ goal is to accelerate electrons using these fields, with the electrons either originating directly from the plasma or being sent into the plasma from an external source at a suitable time. This promising technology is currently the subject of intensive studies to investigate whether it is suitable for use in future accelerators. This is done, among others, within the ARD (Accelerator Research & Development) portfolio programme of the Helmholtz Association. In the long term, this research might lead to compact accelerators for particle physics or for operating free-electron lasers.
A main issue in plasma acceleration is the capturing of particle beams into the wakefield and their further acceleration with the quality required for the intended applications. This problem is considerable in beam-driven plasma accelerators, i.e. when the plasma wave is driven by a particle beam. Here, the wakefield propagates at almost the speed of light and is simply too fast to carry and accelerate the free electrons in the plasma “from rest” and form them into a usable bunch.
In order to solve this issue, the DESY physicists are now proposing an unconventional method: they mix small quantities of helium locally into the hydrogen gas inside the plasma cell. If a particle bunch is now injected into the cell, two distinct areas are created in which the hydrogen gas is completely ionised. However, the helium atoms added to the short, frontmost part of the plasma stay neutral during the passage of the electron beam and are only ionized by the following plasma wakefield. Thus, the electrons from the helium are released only in a well-defined area of the plasma wave and are immediately taken along by the high electric fields. “This arrangement means that these electrons can optimally follow the strong acceleration in the wakefield area without use of any other instrumental assistance”, says Alberto Martinez de la Ossa, who had the idea for this study. “Because the injection of the electrons into the plasma wake only depends on the own wakefields, this technology seems to be more stable than any other previous technique.” In addition, the helium doping allows for a simple control of the number of captured electrons.
To validate these ideas, the researchers carried out computationally intensive 3D simulations of their experiment on the DESY-HPC (high-performance computing) cluster and the high-performance computer JUQUEEN at the Jülich research centre. They simulated a particle bunch with an energy of 23 gigaelectronvolts (GeV) and characteristics as provided by the FACET accelerator at the Californian research centre SLAC, shot into a 46-centimetre-long plasma cell. The plasma cell in the simulation was set up such that it was mostly filled with pure hydrogen. Only at the entrance of the cell was a helium–hydrogen mixture injected through a thin nozzle. “The field profiles predicted by the simulations are very promising”, says Jens Osterhoff. “In the helium area, the wakefield features a very high field strength, which can ionise the helium and accelerate its electrons at more than 100 gigavolts per metre; the field strength is maintained when the beam propagates in the area of the pure hydrogen, so that the high-quality electron bunch is constantly accelerated further”. The simulations, which were carried out over an acceleration distance of twenty millimetres, showed that the electron bunch that was created and accelerated inside the plasma not only acquired an energy of 3 GeV over this short distance and had a pulse length of approximately one micrometre, but also displayed high quality in properties that are important in accelerator physics, such as intensity, emittance and energy distribution. “We were able to stop the computationally intensive simulation at 20 millimetres because the critical starting phase of the bunch had ended, while the further acceleration can be scaled up linearly with good approximation”, says Osterhoff. “The established theory predicts that we can reach twice the input energy of the beam that initiated the plasma wave. The bunch of accelerated electrons would feature an energy of 46 GeV at FACET, and its emittance is improved by an order of magnitude in comparison with the driver beam”.
As the next step, the researchers want to carry out experiments at the FACET facility at SLAC in order to test their gas cell under actual conditions. The “FLASHForward” experiment is then set to go into operation at DESY’s FLASH accelerator in 2016; using a 10-centimetre-long plasma cell, the researchers want to both accelerate particles from the plasma itself and push particle bunches from FLASH to higher energies.