27 research outputs found
Longitudinal instabilities affecting the moving critical layer laser-plasma ion accelerators
In this work we analyze the longitudinal instabilities of propagating
acceleration structures that are driven by a relativistically intense laser at
the moving plasma critical layer [1]. These instabilities affect the
energy-spectra of the accelerated ion-beams in propagating critical layer
acceleration schemes [2][3]. Specifically, using analytical theory and PIC
simulations we look into three fundamental physical processes and their
interplay that are crucial to the understanding of energy spectral control by
making the laser-plasma ion accelerators stable. The interacting processes are
(i) Doppler-shifted ponderomotive bunching [1][4] (ii) potential quenching by
beam-loading [2] and (iii) two-stream instabilities. These phenomenon have been
observed in simulations analyzing these acceleration processes [5][6][7]. From
the preliminary models and results we present in this work, we can infer
measures by which these instabilities can be controlled [8] for improving the
energy-spread of the beams.Comment: submitted to the proceeding of the Advanced Accelerator Concepts
workshop July 2014, San Jose, CA, US
Optimal positron-beam excited plasma wakefields in Hollow and Ion-Wake channels
A positron-beam interacting with the plasma electrons drives radial suck-in,
in contrast to an electron-beam driven blow-out in the over-dense regime,
. In a homogeneous plasma, the electrons are radially sucked-in from
all the different radii. The electrons collapsing from different radii do not
simultaneously compress on-axis driving weak fields. A hollow-channel allows
electrons from its channel-radius to collapse simultaneously exciting coherent
fields. We analyze the optimal channel radius. Additionally, the low ion
density in the hollow allows a larger region with focusing phase which we show
is linearly focusing. We have shown the formation of an ion-wake channel behind
a blow-out electron bubble-wake. Here we explore positron acceleration in the
over-dense regime comparing an optimal hollow-plasma channel to the ion-wake
channel. The condition for optimal hollow-channel radius is also compared. We
also address the effects of a non-ideal ion-wake channel on positron-beam
excited fields.Comment: Proceedings of IPAC2015, Richmond, VA, USA 3: Alternative Particle
Sources and Acceleration Techniques A22 - Plasma Wake eld Acceleration
http://accelconf.web.cern.ch/AccelConf/IPAC2015/papers/wepje001.pdf, 2015
(ISBN 978-3-95450-168-7) pp 2674-267
Quasi-monoenergetic Laser-Plasma Positron Accelerator using Particle-Shower Plasma-Wave interactions
An all-optical centimeter-scale laser-plasma positron accelerator is modeled
to produce quasi-monoenergetic beams with tunable ultra-relativistic energies.
A new principle elucidated here describes the trapping of divergent positrons
that are part of a laser-driven electromagnetic shower with a large energy
spread and their acceleration into a quasi-monoenergetic positron beam in a
laser-driven plasma wave. Proof of this principle using analysis and
Particle-In-Cell simulations demonstrates that, under limits defined here,
existing lasers can accelerate hundreds of MeV pC quasi-monoenergetic positron
bunches. By providing an affordable alternative to kilometer-scale
radio-frequency accelerators, this compact positron accelerator opens up new
avenues of research.Comment: submitted to Physical Review Letters, January 201
Motion of the Plasma Critical Layer During Relativistic-electron Laser Interaction with Immobile and Comoving Ion Plasma for Ion Acceleration
We analyze the motion of the plasma critical layer by two different processes
in the relativistic-electron laser-plasma interaction regime (). The
differences are highlighted when the critical layer ions are stationary in
contrast to when they move with it. Controlling the speed of the plasma
critical layer in this regime is essential for creating low- traveling
acceleration structures of sufficient laser-excited potential for laser ion
accelerators (LIA). In Relativistically Induced Transparency Acceleration
(RITA) scheme the heavy plasma-ions are fixed and only trace-density light-ions
are accelerated. The relativistic critical layer and the acceleration structure
move longitudinally forward by laser inducing transparency through apparent
relativistic increase in electron mass. In the Radiation Pressure Acceleration
(RPA) scheme the whole plasma is longitudinally pushed forward under the action
of the laser radiation pressure, possible only when plasma ions co-propagate
with the laser front. In RPA the acceleration structure velocity critically
depends upon plasma-ion mass in addition to the laser intensity and plasma
density. In RITA, mass of the heavy immobile plasma-ions does not affect the
speed of the critical layer. Inertia of the bared immobile ions in RITA excites
the charge separation potential whereas RPA is not possible when ions are
stationary.Comment: Invited paper (submitted), Division of Plasma Physics, American
Physical Society, Nov 2013, Denver, C
Self-injection by trapping of plasma electrons oscillating in rising density gradient at the vacuum-plasma interface
We model the trapping of plasma within the density structures excited
by a propagating energy source () in a rising plasma density
gradient. Rising density gradient leads to spatially contiguous coupled
up-chirped plasmons (). Therefore phase mixing
between plasmons can lead to trapping until the plasmon field is high enough
such that trajectories returning towards a longer wavelength see a
trapping potential. Rising plasma density gradients are ubiquitous for
confining the plasma within sources at the vacuum-plasma interfaces. Therefore
trapping of plasma- in a rising ramp is important for acceleration
diagnostics and to understand the energy dissipation from the excited plasmon
train \cite{LTE-2013}. Down-ramp in density \cite{density-transition-2001} has
been used for plasma- trapping within the first bucket behind the driver.
Here, in rising density gradient the trapping does not occur in the first
plasmon bucket but in subsequent plasmon buckets behind the driver. Trapping
reduces the Hamiltonian of each bucket where are trapped, so it is a
wakefield-decay probe. Preliminary computational results for beam and
laser-driven wakefield are shown.Comment: Proceedings of International Particle Accelerator Conference, IPAC
2014, Dresden, Germany, June 2014,
http://accelconf.web.cern.ch/AccelConf/IPAC2014/papers/tupme051.pd