An international team of researchers led by DESY scientists has demonstrated
for the first time at the FLASHForward experiment that in principle it is
possible to operate plasma accelerators at the repetition rates desired by
particle physicists and photon scientists. This opens the opportunity to
utilise such high-gradient accelerators as booster stages in existing
high-repetition-rate facilities, such as the large-scale X-ray free-electron
lasers FLASH and European XFEL, in order to significantly increase the
energy of long trains of particles in short distances. The team presents the
results of their studies in the journal Nature today.
Plasma acceleration is an innovative technology for application to the next
generation of particle accelerators due to both its compactness and
versatility, with the aim being to utilise the accelerated electrons for
various fields of application in science, industry, and medicine. The
acceleration takes place in an extremely thin channel—typically only a few
centimetres long—which is filled with an ionised gas, the plasma. A
high-energy laser or particle beam fired through the plasma can excite a
strong electromagnetic field—a kind of ‘wake’—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. They could thus drastically reduce the size of
kilometre-scale facilities such as particle colliders or free-electron
lasers.
Modern accelerators for cutting-edge science must also meet high
requirements in terms of efficiency, beam quality, and number of bunches
accelerated per second. In order to generate a particularly large number of
light flashes or particle collisions in the shortest possible time,
thousands or even millions of densely packed particle bunches must be
propelled through accelerators in a single second. Plasma accelerators
would, therefore, have to achieve a similar repetition rate in order to be
competitive with state-of-the-art particle-accelerator technology. Current
test facilities for plasma acceleration are usually operated at much slower
repetition rates in the range of one to ten accelerations per second. The
team led by DESY researcher Jens Osterhoff has now proven that much higher
rates are possible. “At FLASHForward we were able to show for the first time
that, in principle, repetition rates in the megahertz range are supported by
the plasma acceleration processes”, says Osterhoff.
At FLASHForward the accelerating wave – the so-called wakefield in the
plasma – is generated by an electron bunch from the FLASH accelerator that
ploughs through the plasma at almost the speed of light. The electrons of
this ‘drive beam’ cause the freely moving electrons of the plasma to
oscillate in its wake and thus generate very strong electric fields. These
fields accelerate the electrons of a particle packet flying directly behind
the driver bunch. “Unlike in conventional accelerators, where long-living
electromagnetic waves stored in a resonating cavity can accelerate several
particle bunches in quick succession, the electromagnetic fields generated
in plasma decay very quickly after each acceleration process”, explains
Richard D’Arcy, first author of the study. “To start a new similar
acceleration process, the plasma electrons and ions must then have
‘recovered’ to approximately their initial state such that the acceleration
of the next pair of particle bunches is not modified by that of the previous
one.”In their experiments, the scientists took advantage of the highly
flexible superconducting FLASH accelerator to generate particle bunches with
extremely short temporal spacings. The first bunch generated ploughed
through the plasma, driving a high-strength wakefield and thus perturbing
the plasma in its wake. At variable intervals thereafter, pairs of particle
bunches were sent through the plasma cell; the first driving a second
wakefield and the second being accelerated by the resulting fields. The
properties of these subsequent bunches were precisely measured by the
experimenters and compared with those of bunches that had experienced this
process in an undisturbed plasma. The result: after about 70 billionths of a
second (70 nanoseconds), it was no longer possible to distinguish whether
the second acceleration had taken place in a previously disturbed or
undisturbed plasma. “We were able to precisely observe the decay of the
perturbation, which reached completion within the first 70 nanoseconds, and
to explain it exactly in simulations,” says D’Arcy. “In subsequent
measurements, we want to check how different framework conditions in the
setup influence the recovery time of the plasma wave.” For example, the
heating of the plasma medium due to high-frequency operation may have an
influence on how quickly the plasma takes to replenish.
The current findings,which involved scientists from DESY, University College
London, and the Universities of Oxford and Hamburg, open the door for
equipping today´s particle accelerators, which are operated at repetition
rates in the kilohertz-to-megahertz regime, with plasma accelerator modules
acting as booster stages to significantly increase the particle energy over
the shortest distance. “The findings have a profound impact on the potential
for implementation of the plasma technology towards future high repetition
rate facilities for which DESY is world renowned”, concludes Wim Leemans,
Director of the Accelerator Division at DESY.
Reference:
Recovery time of a plasma-wakefield accelerator; R. D’Arcy, J. Chappell, J.
Beinortaite, S. Diederichs, G. Boyle, B. Foster, M.J. Garland, P. Gonzalez
Caminal, C.A. Lindstrøm, G. Loisch, S. Schreiber, S. Schröder, R.J. Shalloo,
M. Thévenet, S. Wesch, M. Wing, and J. Osterhoff; „Nature“, 2022;
DOI: 10.1038/s41586-021-04348-8
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Physics