An international team of researchers led by DESY scientists has demonstrated for the first time in the FLASHForward experiment that it is in principle possible to run plasma accelerators at the repetition rates that particle physicists and photon scientists desire. This opens the possibility of using high-gradient accelerators such as boosting stages in existing high-repetition rate facilities, such as the European FLASH and XFEL wide-band X-ray lasers, to dramatically increase the energy of long trains of particles. short distances. The team presents the results of its studies in the journal nature Today.
Plasma acceleration is an innovative technology that is being applied to the next generation of particle accelerators due to both its compactness and versatility, the aim being to use the accelerated electrons in various fields of application in the scientific, industrial and medical fields. The acceleration occurs in a very thin channel—usually only a few centimeters—filled with an ionized gas, the plasma. A high-energy laser or particle beam fired through the plasma can excite a strong electromagnetic field – a kind of “wake up” – which can be used to accelerate charged particles. In this way, plasma accelerators can achieve acceleration gradients up to a thousand times higher than the most powerful accelerators in use today. Thus it can significantly reduce the size of kilometer structures such as particle colliders or free electron lasers.
Modern accelerators of cutting-edge science must also meet high requirements in terms of efficiency, beam quality and the number of accelerating points per second. In order to generate a large number of light flashes or particle collisions in the shortest possible time, thousands or even millions of dense particle beams must be pushed through accelerators in one second. Therefore, plasma accelerators must achieve a similar repetition rate to be competitive with the latest particle accelerator technology. Current plasma acceleration testing facilities typically operate at much slower repetition rates in the range of one to ten accelerations per second. The team led by DESY researcher Jens Osterhoff has demonstrated that it is possible to achieve much higher rates. “At FLASHForward, we were able to show for the first time that repetition rates in the megahertz range are in principle supported by plasma accelerations,” says Osterhoff.
In FLASHForward, the accelerating wave – the so-called waking field in the plasma – is generated by a group of electrons from the FLASH accelerator that travels through the plasma at nearly the speed of light. The electrons in this ‘push beam’ cause the free electrons in the plasma to oscillate in their wake, thus generating very strong electric fields. These fields accelerate the electrons of a group of particles that fly directly behind the conducting group. “Unlike conventional accelerators, where long-lived electromagnetic waves stored in a resonant cavity can accelerate multiple beams of particles in rapid succession, electromagnetic fields generated in plasmas decay very rapidly after each acceleration,” explains Richard Darcy, first author of the study. “To initiate a new, similar acceleration process, the electrons and ions in the plasma must have nearly ‘recovered’ to their initial state so that the acceleration of the next pair of particle beams is not affected by the acceleration of the first. In their experiments, the scientists made use of the highly flexible superconducting FLASH accelerator to generate beams of particles Very short time lapse.
The first generated beam was passed through the plasma, driving a high-density field and thus disabling the plasma in its wake. At varying intervals thereafter, pairs of particle beams were sent through the plasma cell; The first drives the second wake field and the second is accelerated by the resultant fields. The properties of these subsequent beams were precisely measured by the experimenters and compared with the properties of the beams that underwent this process in unperturbed plasma. The result: After about 70 billionths of a second (70 nanoseconds), it is no longer possible to distinguish whether the second acceleration occurred in previously disordered or unperturbed plasma. “We were able to accurately observe the turbulence decay, which was completed within the first 70 nanoseconds, and explain it exactly in the simulations,” says Darcy. “In subsequent measurements, we want to investigate how different frame conditions in the setup affect the recovery time of a plasma wave. For example, heating a plasma medium due to high-frequency operation can affect how quickly it takes the plasma to recover.
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