Summary Report of Working Group 1: Laser-Plasma Acceleration Page: 2 of 10
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push the energy frontier and to develop new techniques fw control wee also discussed This summary paper presents
highlights from each of these discussions, outlining the proceedings of die worlng group In many cases, a paper
presented results relevant to many of the topics, and may appear more than once in the following Detailed results and
references may be found in the respective papers i these proceedings
INJECTION CONTROL
The quality of the initial injected particle bunch, and control over the trapping process, are vital to obtaining high
qual[ty beams, and are a topic of intense research covered by the working group This is because, in the absence of
damping rngs or other special techniques used in colliders, the acceleramng structure at best preserves bunch emittance
such that a low emrioance injector is required Similarly, the injector must be controlled to inject the appropnaie
longitudinal bunch shape at the correct phase in the structure such that acceleration. including beam loading and
dephasing, will produce narrow energy spread Progress was presented on cornrolhng injection using a variety of
techniques, including refinement of the controlled self trapping which first produced monoenergetic beams in such
acceleratura, and controlLed trapping via colliding laser pulses, ioruzation. and shaping of plasma or applied magnetic
fields
Self trapping, which occurs when the wake riches an arnpl tude sulfficient to trap electrons &orm the plasma through
which it propagates, has been used since early experiments, and refinement in understanding and control of this
process continues While broad energy spread beams were produced in early expenMers ([12] and others) which
drove the wake far above the trapping threshold and/or did not control acceleration length, it has been demonstrated
that appropriate control of laser and plasma parameters (amplitude just above the trapping threshold, and extracting the
bunch as it is momentum compressed by dephasig from the accelerating field) can generate bunches with few percent
energy spread and mrad divergence [2, 3, 4, 13] Stable operation at up to GeV energies has been observed (5, 8]
Several presentations addressed the theory of this injection process, iluding trapping threshold and beam quality
General trapping conditons for non-evolving wakes were presented by Wei Lu of UCLA, giving a physical picture
of m]ecton Simulations were used for venficalion of the physical picture by utilizing non-evolving beam drivers
This method was then used to characterize the effects of driver shape and plasma channels Analysis of the trapping
condition in a bubble was also denved by Alec Thomas and presented by Stuart Mangles by analyzing trajectories in a
comoving frame with the bubble, which is assumed non-evolving on the time scale of injecnon (though evolution of the
laser up to the injection point is taken into account) Good agreement with experiments from Impenal and LLNL was
shown Two talks addressed the effects on injection of evolution in the bubble structure dnven by intentional laser spot
size variation. Serge Kalmykov of U Nebraska Lincoln and Vladimur Khudik of lI Texas Austin Kalmykov presented
simulations showing that an initially 'over focused' laser pulse diffracts to a stable self guided radius at which it then
propagates During diffraction, the bubble radius and length expand, trapping particles that would otherwise not be
trapped (such an effect is also produced when the plasma density decreases (14, 7]) Simulations using the quasi-static
code WAKE with add-on con-quasistatc 'test' electrons to simulate injection (preferred by some for noise reasons)
gave reasonable agreement with full PIC under these conditions, neglecting beam loading of the structure which was
not severe Kalmykov and Khudik each presented a Hamirltonian model of trapping in such evolving bubbles Khudik
presented test-particle models of particle tmjectones illustrating the role of expansion, and showing which particles are
trapped The difference between accelerated and trapped particles was analyzed He found that the 'bump' in electron
density caused by re-convergence of particles at the end of the bubble had a signilicani effect in lowenng trapping
threshold relative to idea zed circular bubbles The working group hosted discussion and comparison of each of these
theones While the first two talks found trapping cnteria without laser evolution, the latter two found that bubble
evolution was critical to rnjecton, and this remains a topic of discussion as does the relative mert of quasistatic with
add-on particles versus full explicit PIC (which self consistently handles injecton) for these problems
Improved understanding is facilitating control over self trapping Results on production of stable 20D MeV electron
bunches from self trapping using a gas cell were presented by Stefan Karsch of LIlMU/MPQ A variable length gas cell
was also used to charactenze dephasing effects A new laser system at NPQ is producing high power pulses at 8 fs,
allowing resonant excitation of plasma wakes at high densiles, and has demonstrated production of 25 MeV bunches
with 3% energy spread using only 70 mJ, as presented by Laszlo Veisz The self trapping threshold was characterized
as a function of plasma density in experiments at LLNL presented by Bradley Pollock, and in a related plenary by
Froula which also presented an overview of other experiments Several other groups presented data on self trapping
and its relation to laser spot evolution and symmetry, which are covered in the sections below
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Geddes, C.G.R.; Clayton, C.; Lu, W. & Thomas, A.G.R. Summary Report of Working Group 1: Laser-Plasma Acceleration, article, June 1, 2010; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc842016/m1/2/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.