Laser
Plasma Acceleration - Plasma Acceleration Subgroup |
Vitaly
Yakimenko: ACCELERATION AND FOCUSING OF
RELATIVISTIC ELECTRONS IN OVER-DENSE PLASMA |
We describe our studies
on the generation of plasma wake fields by a relativistic
electron bunch, and on phasing between the longitudinal-
and transverse-fields in the wake. The leading edge
of the electron bunch excites a high-amplitude plasma
wake inside the over-dense plasma column, and the
acceleration and focusing wake fields are probed
by the bunch tail. By monitoring the dependence
of the acceleration upon the plasma’s density,
we approached the beam-matching condition and achieved
an energy gain of 0.6 MeV over the 17-mm plasma
length, corresponding to an average acceleration
gradient of 35 MeV/m. Wake-induced modulation in
energy and angular divergence of the electron bunch
are mapped within a wide range of plasma density.
We confirm a theoretical prediction about the phase
offset between the accelerating and focusing components
of plasma wake. |
F.
Grigsby & M. Fomotskyi : LWFA with Low
Energy Raman Seeded Pulses |
Analytical and numerical
calculations of plasma wakefield excitation and
particle acceleration by Raman seeded laser pulse
in self-modulation regime are presented. We derive
energy threshold for self-modulation of diffraction-limited
pulses. The parameter range where the Raman seeded
amplitude plays an important role is investigated.
We show that the seeded amplitude provides a coherent
control mechanism for the phase of the wakefield
wave. We show that with the use of Raman seed self-modulated
wakefield acceleration is achievable for the pulses
of intensities much lower than those typically used
in the experiments. In particular, our 2D particle-in-cell
simulations show that 30mJ pulse combined with Raman
seeded pulse, which is 1% in intensity of the main
pulse is capable of generating ~1nC of relativistic
electrons. |
Albert
Reitsma: Laser wakefield acceleration -
a fully self-consistent analysis |
In this talk, the
interplay between pump depletion, electron dephasing
and beam loading in the nonlinear regime of the
resonant laser wakefield accelerator is discussed.
A fully self-consistent one-dimensional model that
includes laser pulse evolution and electron beam
loading effects is presented. It is found that the
laser pulse length is an important parameter that
controls the efficiency. The energy spread can be
minimized by exploiting the combined effects of
beam loading and phase slippage. Simulation results
of efficient acceleration up to 1 GeV in a single
stage are shown.
|
Wayne
Kimura: Laser Wakefield Acceleration Driven
by ATF CO2 Laser |
A new experiment
has begun that builds upon the successful STELLA
experiment, which demonstrated high-trapping efficiency
and narrow energy spread in a staged laser accelerator.
STELLA was based upon inverse free electron lasers
(IFELs); the new experiment, called STELLA-LW, is
based upon laser wakefield acceleration (LWFA).
The first phase of STELLA-LW will be to demonstrate
LWFA in a capillary discharge driven by the BNL
ATF terawatt CO2 laser beam. This will be the first
time LWFA is conducted at 10.6-micron laser wavelength.
It will also be operating in an interesting pseudo-resonant
regime where the laser pulse length is too long
for resonant LWFA, but too short for self-modulated
LWFA. Analysis [1] has shown that in pseudo-resonant
LWFA, pulse-steepening effects occur on the laser
pulse that permits generation of strong wakefields.
Various approaches are being explored for the capillary
discharge tube including a polypropylene one [2]
already demonstrated at the ATF and a hydrogen-filled
one demonstrated at the University of Oxford [3].
Planned diagnostics for the experiment include usage
of coherent Thomson scattering (CTS) to detect the
wakefield generation. This will be one of the first
times CTS is used on a capillary discharge. Details
of the planned experiment and its model predictions
will be presented.
[1] N. E. Andreev, et al., Phys. Rev. ST Accel.
Beams 6, 041301 (2003).[2] D. Kaganovich, et al.,
Appl. Phys. Lett. 71, 2925 (1997).[3] A. Butler,
et al., Phys. Rev. Lett. 89, 185003 (2002).
|
Serguei
Kalmykov: Application of detuned plasma
beatwave for generation of few-cycle electromagnetic
pulses |
Two external laser
beams with a near-resonant frequency detuning ($\Omega\approx\omega_{pe}$)
drive a high-amplitude electron plasma wave (EPW)
which modifies the refractive index of plasma so
as to produce a periodic phase modulation of the
pump field with a beatwave period. The pump then
breaks up into a series of chirped pulses of a duration
$2\pi/\Omega$. The chirp is positive (the longer-wavelength
sidebands go ahead) when $\Omega<\omega_{pe}$
and negative otherwise. Finite group velocity dispersion
(GVD) of radiation in a plasma can compress these
positively chirped laser beamlets to a few-laser-cycle
duration thus creating a chain of sharp electromagnetic
spikes separated in time by $2\pi/\Omega$. The resonant
interaction via the driven EPW can strongly reduce
the GVD in the system of coupled laser sidebands.
In this case, a separate plasma of higher density,
where the laser spectral components become uncoupled,
can be used for the pulse compression. |
Kazuhisa
nakajima: Ultrahigh current electron acceleration
in relativistic laser-plasma interactions |
Recently there is
a great interest growing in advanced accelerator
technologies based on laser and plasma acceleration
mechanisms, which have a tremendous potential for
applications to a wide range of sciences, not only
basic sciences but also medical and industrial sciences.
The novel particle acceleration technologies make
it possible to produce unique high quality beams
in a compact system, which are crucial for specific
applications. In particular extraordinary evolution
of ultraintense ultrashort pulse lasers can generate
ultrahigh gradient accelerating fields of the order
of 1 TeV/m in a plasma to accelerate electron beams
well collimated with small emittance and femtosecond
buch length up to 1 GeV in a 1 mm plasma. It is
known that this mechanism of electron acceleration
in plasma plays a role of accelerating protons and
ions with tens of MeV energy as well as production
of bright radiations. In the relativistic highly
nonlinear regime, relativistic electron beams can
be accelerated by more complex mechanisms based
not only on wakefields and also on direct ponderomotive
fields. We report the recent results of electron
beam acceleration experiments made by using 20 TW,
20 fs laser pulses at JAERI-APRC. The experiments
result in ultrahigh current relativistic electron
beam acceleration of the order of Mega Ampere with
energy spectrum characterized by a power law up
to 40 MeV rather than a Maxwellian distribution.
The talk will focus on the acceleration mechanism
of ultrahigh current electron beams in plasma and
their applications to femtosecond X-ray beam generation.
|
Sergey
Gordienko: Electron acceleration in the
Bubble regime: analytical theory and numerical simulations |
We present analytical
theory and numerical simulations of laser-plasma
electron acceleration in the Bubble regime that
is able to generate a nearly monochromatic electron
bunch [1]. This regime is achieved in underdense
plasma provided that the laser pulse has an ultra-relativistic
amplitude and its duration is shorter than the plasma
period. We study equations of cold ultrarelativistic
plasma hydrodynamics coupled with the Maxwell equations
and find 3D solutions of these equations. Surprizingly,
the efficient electron acceleration works in an
almost linear regime that is free of a chaotic behavior
inherent in non-linear dynamic systems. Particularly,
there exists a solution in the form of a spherical
cavity with radius R almost free from plasma electrons.
The resonance interaction between the wave and the
particles expels background plasma electrons from
the cavity and we can relate the cavity radius R
with the relativistic factor $\gamma_p$ of the cavity
propagation: (2\pi R/\lambda_p)^2=2.6\gamma_p\sqrt{\ln(2\gamma_p)}$.
The cavity rear side is unstable due to the electron
density perturbations leading to electron trapping
and acceleration. We find that the electrons can
be accelerated in the Bubble up to the $\gamma_m$-factor
$\gamma_m= 11\gamma_p^3\sqrt{ln(2\gamma_p)}$.p The
development of the jet and cavity formation have
also been investigated by 3D PIC simulations. [1]
A. Pukhov, J. Meyer-ter-Vehn, Appl. Phys. B 74,
355 (2002).
|
Shouyuan
Chen: Experimental
Evidence of Ionization Blue Shift Seeded Forward
Raman Scattering |
We report the results
of spectroscopic experiments that wereconducted
by focusing an intense ultra-short laser pulse into
a heliumgas target. The scattered light from the
interaction region wasmeasured spectrally and spatially
from various directions as afunction of laser intensity
and plasma density and correlated withthe accelerated
electron beam. The experiment data showed thatforward
Stimulated Raman Scattering (SRS) was sensitive
to thefocus position of laser relative to the nozzle.
Together with theplasma channel imaged by a CCD
camera, the measurements indicatethat SRS is seeded
by the ionization blue shifted light. The cross-phase
modulation between the SRS and laser beam was also
observedin the experiment.
|
James
Cooley: Broad-Energy Electron Beam Injection
and Loading in Laser Wakefield Accelerators |
Laser Wakefield Accelerators
(LWFA), in the resonant regime, require use of an
initial electron beam that can be injected into
the accelerating phase of the wake. Ideally, the
injection electron bunch would be mono-energetic
with femto-second timing. Generating an injection
electron bunch with this timing requirement will
likely lead to use of an optical method to generate
the electrons. Several methods for generating this
electron bunch exist e.g., Laser Ionization and
Ponderomotive Acceleration (LIPA) and Self-Modulated
LWFA among others. Each of these schemes produces
an electron bunch with a characteristic energy distribution.
We examine the trapping characteristics for this
type of injection source for resonant LWFA by modeling
the distributions to have a broad energy spread
that can be characterized using a Boltzmann distribution
with an "effective temperature". We will
present results of both analytic calculations and
simulations which provide a methodology for optimizing
the resulting accelerated electron bunch characteristics
i.e., energy and energy spread, for a given LWFA
configuration. We will discuss the required trade-offs
between total energy gain and overall beam quality.
These results will also include beam-loading effects
and plasma wake disruption due to high charge density
in the initial electron bunch. |
Mike
Downer: Femtosecond
pump-probe studies of preformed plasma channels
and clustered plasmas |
I will survey our
recent pump-probe experiments in He plasma waveguides
using sub-relativistic pump pulses of 0.2x10^18
W/cm^2. These experiments develop fs-time- and µm-space-resolved
diagnostic techniques based on frequency-domain
interferometry and photon acceleration that will
be required to characterize channeled LWFA. I will
update our current efforts to guide fully relativistic
pulses. I will also report on our recent pump-probe
experiments in clustered gases, an attractive medium
for plasma channels and accelerators. We observe
sharp, delayed resonant enhancements in third harmonic
production from the expanding clusters. PIC simulations
suggest that collective bulk electron oscillations
are responsible for the enhanced nonlinear susceptibility. |
Eric
Esarey: Nonlinear group velocity, pump depletion,
and electron dephasing inlaser wakefield accelerators |
The nonlinear evolution
of sub-ps laser pulses in underdense plasmas is
analyzed for arbitrary laser intensity. Some of
the new results derived include expressions for
the nonlinear group velocity of the laser pulse,
the nonlinear phase velocity of the wakefield, thenonlinear
frequency shift, and the laser pulse envelope distortion.
New scalings are presented for the pump depletion
length and the electron dephasing length. Analytical
results are compared to numerical calculations based
on fluid models. Implications for anoptimized design
of a 1 GeV accelerator stage are discussed.
This work supported by DoE, DE-AC03-76SF0098.
|
Gwenael
Fubiani: Beat wave injection of electrons
into plasma waves using two interferinglaser pulses |
An electron injector
concept that uses a single injection laser pulsecolliding
with a pump laser pulse in a plasma is analyzed.
The pumppulse generates a large amplitude laser
wakefield (plasma wave). Thecounterpropagating injection
pulse collides with the pump laser pulseto generate
a beat wave with a slow phase velocity. The ponderomotiveforce
of the slow beat wave is responsible for injecting
plasmaelectrons into the wakefield near the back
of the pump pulse. Testparticle simulations indicate
that significant amounts of charge canbe trapped
and accelerated (~ 10 pC). For higher charge, beamloading
limits the validity of the simulations. The acceleratedbunches
are ultrashort (~ 1 fs) with good beam quality (relativeenergy
spread of a few percent at a mean energy of ~ 10
MeV and anormalized rms emittance on the order 0.4
mm.mrad). The effects ofinteraction angle and polarization
are also explored, e.g., efficienttrapping can occur
for near-collinear geometries. Beat wave injectionusing
a single injection pulse has the advantages of simplicity,
easeof experimental implementation, and requires
modest laser intensityI ~ 8.8e17 W/cm2. |
Julien
FUCHS: Ultra-high brightness laser-accelerated
proton source & novel acceleration schemes |
Laser-accelerated
high energy protons have a number of remarkable
characteristics that have triggered interest for
potential applications of this source. Among these
unique characteristics are its high degree of laminarity
and extremely low source size, the capacity to manipulate
the beam by playing with the laser intensity distribution
or the target shape. We will present results on
the measurement of the beam’s transverse emittance.
We have used a technique that allows to directly
image the proton-emitting surface by imprinting
grooves on the target rear surface, and to fully
reconstruct the transverse phase space. With this
technique, we have determined experimentally that
for protons of up to 10 MeV, the transverse emittance
is as low as 0.004 mm.mrad (normalized, rms), i.e.
100-fold better than typical RF accelerators and
at a substantially higher ion current (kA range).
We have also shown that the removal of the co-moving
electrons after 1 cm of the quasi-neutral (protons
and electrons) beam expansion did not increase significantly
the measured proton transverse emittance. The exceptionally
low measured emittance stems from the extremely
strong, transient acceleration that takes place
from a cold, initially unperturbed surface and from
the fact that during much of the acceleration the
proton space charge is neutralized by the co-moving
hot electrons. We will also present plans to inject
the laser-accelerated protons into a simple induction
linac cavity in order to post-accelerate them while
keeping their transverse emittance. The cavity is
planned to be driven by a pulsed power machine at
U. of Nevada, Reno (1 MA, 2 MV, 1.9 ohm driver,
with 80 ns rise time) that is being coupled to a
100 TW short-pulse laser that will be used for generating
the protons. Our aim is to demonstrate the feasibility
of injection and longitudinal phase space rotation
of the laser source. |
Nasr
Hafz: Experiment on the Thomson Backscattering
from LWFA e-beams |
In a collaboration
between KERI and GIST in Korea, we are planning
to use a 20 TW 40 fs Ti: sapphire laser system for
generating ultrashort x-ray pulses by the Thomson
backscattering from plasma-accelerated electron
beams. A beam splitter will be used to split the
main beam into two beams; one (high-power) will
be focused on a gas jet for generating e-beam. The
second (low power) beam will be focused on the e-beam
after it is emitted from the plasma. The synchronization
between the two beams can be achieved via an optical
delay. The x-ray spectrum is expected to be of wide
range form soft to hard x-rays as a result of the
naturally large energy spread of the electron beam.
Some details about this experiment are presented. |
Tomonao
Hosokai: Experimental study of fs electron
bunch generation by laser plasma cathode |
Plasmas irradiated
by intense short laser pulses can be an efficient,
compact, and flexible source of multi-MeV electrons
constituting a bunch with its duration in the femtosecond
range. So we have studied a laser plasma cathode
based on Laser Wake Field Acceleration (LWFA) with
electron injection by wave-breaking, which is driven
by a femtosecond intense laser pulse (Ti:Sapphire,
max 12TW, lambda = 800nm , 50fs, 10Hz) tightly focused
in supersonic helium gas-jet. The wave-breaking
is the simplest way to inject energetic electrons
into the wakes produced by an intense laser pulse.
However, the wave-breaking requires a steep density
plasma interface with order of the plasma wavelength.
According to our previous experiment, this condition
can be produced via generation of a shock-wave in
the gas jet by proper laser prepulse irradiation.
It shows ejection of a narrow-cone MeV electron
beam from a gas jet depend strongly on the prepulse
contrast ratio. Furthermore, in case of higher gas
density, the effect of laser prepulse become more
sophisticate.The laser pulse is refracted at the
edge of the cavity produced by the prepulse and
this effect limit the narrow-cone MeV electron beam
production of LWFA. In addition, I will also mention
a narrow energy spread e-spectrum obtained in our
recent experiment, development of a shockwave-free
supersonic gas jet and gas filled capillary discharge
wave-guide.
|
Richard
Hubbard: Trapping
and Acceleration of Nonideal Injected Electron Bunches
in Channel-Guided LWFAs |
The standard regime
for the laser wakefield accelerator (LWFA) usually
requires external injection of MeV electrons. Ideally,
the injected electron bunch should be injected into
the proper phase of the accelerating wake, have
a bunch length that is small compared with the plasma
wavelength, and a low emittance and energy spread.
This paper reports simulation studies of several
‘nonideal’ injection schemes that demonstrate
strong phase bunching and good accelerated beam
quality in a channel-guided laser wakefield accelerator.
For the case of monoenergetic, unphased (long bunch)
injection, there is an optimum range of injection
energies for which the LWFA can trap a significant
fraction of the injected pulse while producing an
ultrashort, high-quality accelerated pulse. These
favorable results are due to a combination of pruning
of particles at unfavorable phases, rapid acceleration,
and strong phase bunching. In addition, the required
minimum energy for injection may be substantially
lower in a channel than in a uniform plasma because
of a favorable shift in the size and phase of the
portion of the wake that is both accelerating and
focusing. Simulation agree well with the predictions
of a simple Hamiltonian model using an ideal sinusoidal
wake moving at the group velocity of the laser pulse.
This long bunch case may apply to injection with
a conventional or photocathode RF gun. Phased and
unphased injection in a channel-guided LWFA with
a broad injected energy spread has also been simulated.
Although the trapping fraction is generally much
smaller than in the monoenergetic case, some simulations
exhibit final accelerated bunches with remarkably
small energy spread. These results suggest that
relatively poor quality injection pulses may still
be useful in LWFA demonstration experiments. Implications
for planned LWFA experiments at NRL are discussed.
Supported by the Department of Energy and Office
of Naval Research.
|
Dino
Jaroszynski: Progress on the UK Advanced
Laser Plasma High-energy Accelerator ALPHA-X project |
One of the goals
of developing laser-driven accelerator technology
is to produce a compact coherent radiation source.
The project, Advanced Laser Plasma High-energy Accelerators
towards X-rays, ALPHA-X, aims to develop a laser
driven wakefield (LWF) accelerator and establish
the feasibility, in the longer term, of creating
a compact free-electron laser. A consortium consisting
of several UK universities and facilities, and collaborators
from Europe and the US, has been set up to meet
this challenge. To meet the stringent injection
requirements, two different electron injection methods
are being investigated. The first is a compact all
optical method, which involves extracting electrons
from plasma using an intense laser pulse and injecting
them into the LWF accelerator. Encouraging recent
results to create mono-energetic electron bunches
for injection will be presented. In parallel we
are developing a 2.5 cell 3 GHz RF photoinjector
to produce electron bunches with durations a fraction
of the plasma wake period. To characterise the ultra-short
electron bunches new techniques based on electro-optic
sampling of the Coulomb field have been developed.
Recent results using these THz time domain techniques
to measure a 300 fs electron bunch and the output
from a THz free-electron laser will be presented.
These demonstrate the feasibility of single shot
measurements. A fully ionised hydrogen filled capillary
has the dual purpose of acting as a waveguide for
the driving pulse while providing the medium for
the LWF accelerator. Recent results of the characterisation
of a suitable plasma channel will be presented.
These include measuring the plasma channel waveguide
density profile using transverse interferometry
and new techniques to measure the longitudinal density
using Raman amplification. The latter has allowed
measurements of the plasma density transition at
the entrance to the channel, which is important
for coupling the laser and electron beams into the
channel. |
Dmitri
Kaganovich: Experimental Demonstration of
a Staged Optical Injection and Laser Wakefield Acceleration |
A two stage experiment
on optically injected Laser Wakefield Acceleration
(LWFA) was performed at the Naval Research Laboratory.
Two temporally and spatially synchronized laser
beams at 2 TW and 10 TW respectively are each focused
collinearly into two adjacent gas jets. Electrons
at <0.5 MeV generated from the 2 TW laser beam
are injected into the wakefields generated by the
10 TW laser beam. Accelerated electrons >10 MeV
are observed, implying an acceleration gradient
of 10 GeV/m for the ~1 mm acceleration distance.
Apparent peaks and structures are observed in the
energy spectrum. This is a clear demonstration of
substantial laser wakefield acceleration of externally
and optically produced injection electrons. Details
of the experiment will be presented. |
Kazuyoshi
Koyama: Generation
of Quasi-monoenergetic High-energy Electron Beam
by Plasma Wave |
We have demonstrated
an acceleration of a quasi-monoenergetic electron
beam by trapping electrons in a plasma wave. Experiments
were performed by focusing 2-TW (50 fs) laser pulses
on supersonic gas jets of nitrogen or helium. An
intensity in a focal spot diameter of $5\mu m$ was
$5 \times 10^{18} W/cm^2$($a_0=1.5$). An electron
density was estimated to be $1.3 \times 10^{20}
cm^{-3}$ by the measured neutral gas density and
the barrier suppression ionization model. The 1-D
electron detuning length was around $60 \mu m$.Electron
energy as well as an angular spread of the electron
beam was measured by using an electron-spectrometer
(ESM) with a 2-D sensor of the Imaging Plate (Fuji;
BAS-SR). Besides a spectrum of a forward scattering,
a side-scattering image of the pump laser pulse
and an ultra short pulse shadowgraph of the plasma
were simultaneously observed to study the laser
interaction with plasma.The quasi-monoenergetic
electron beam at 7 MeV was recorded at a high-contrast
of 10:1. An energy spread of the peak of 2.5 MeV
was limited by an enrgy resolution of the ESM. Background
electrons had a Boltzmann-like energy spectrum with
an effective temperature and a maximum energy of
6.5 MeV and 30MeV, respectively. The angular spread
of the monoenergetic electron beam was $\pm 1.1degree$.
An appearance of a Stokes Raman satellite in the
forward scattering, which was a clear evidence of
a plasma-wave excitation, well correlated with the
quasi-monoenergetic electron beam. A frequency shift
of the satellite coincided with a plasma frequency
at the measured plasma density. Appearance of the
Raman satellite coincided with appearances of a
fishbone structure in a side-scattering image. Supposing
the fishbone structure originated from the plasma
wave, an acceleration length was estimated to be
a few-hundred microns.\This work was supported by
the Budget for Nuclear Research of the MEXT and
the Advanced Compact Accelerator Department Program
of the MEXT. |
Carl
Schroeder: Trapping, wavebreaking, and dark
current in nonlinear plasma waves |
The trapping of thermal
plasma electrons in a nonlinear plasma wave of arbitrary
phase velocity is investigated. The relation betweentrapping
and wavebreaking is discussed. The threshold plasma
wave amplitude for trapping is presented, thereby
determining the fractiontrapped and the expected
dark current in a plasma-based accelerator. Reduction
of dark current in plasma-based accelerators is
critical for producing high-quality electron beams.
It is shown that the presence of a laser field (e.g.,
trapping in the self-modulated regime of the laser
wakefield accelerator) increases the threshold for
trapping plasma electrons. The threshold field for
trapping reduces to previous wavebreaking calculations
in the relevant limits. Implications for experimental
and numerical laser-plasma studies are discussed.
This work supported by DoE, DE-AC03-76SF0098.
|
Hyyong
Suk Suk: Generation of MeV-level high-energy
electrons from the interaction of a 2 TW laser beam
and a gas target |
We have
an experimental program for laser plasma acceleration
research at KERI (Korea Electrotechnology Research
Institute). For the laser plasma accelerator research,
we recently built and tested a table-top terawatt
laser system, which can provide a 2 TW (1.4 J/700
fs) from the Ti:sapphire/Nd:glass hybrid-type system.
As a first step in the laser accelerator research,
we focused a 2 TW laser beam onto a He gas target,
in which the density was on the order of 10^18 cm^-3.
Its was observed that several MeV-level high-energy
electrons with a charge of ~ 2 nC per bunch were
produced from the self-modulated laser wakefield
acceleration. The electron beam (especially outer
part of the beam) is highly space-charge-dominated,
so the beam was observed to expand very rapidly.
In this talk, various beam measurement results are
presented. |
Antonio
Ting: NRL Laser Injection Laser Wakefield
Accelerator |
Optically
generated injection electrons have the advantage
of being tightly bunched and synchronized to the
laser wakefield for phased injection and acceleration
to final high energies. A high-density LIPA (Laser
Ionization and Pondermotive Acceleration) injection
mechanism is studied using the 10 TW laser beam
of the NRL T3 laser and a gas jet of high density
nitrogen. The highest energy electrons up to 7 MeV
were produced in the 0 degree forward direction.
The angular distribution peaked at the 30 degrees
forward direction. Both observations agreed quite
well with NRL simulations. These electrons are very
desirable for injection into a second stage of channel-guided
LWFA leading to ultra-high final energies. Setup
and results of the experiments will be presented. |
Mikhail
Tushentsov: A study of undulator induced
transparency of magnetized plasma in the linear
propagation regime |
The
propagation and mode structure of waves in magnetized
plasma is investigated both numerically and analytically
in the magnetically induced transparency regime
at a linear intensity level.The magnetic undulator
renders possible coupling of the transverse andlongitudinal
electromagnetic waves in the plasma. This coupling
yields ultra-slow hybrid electromagnetic waves with
a group velocity which is substantially less than
the speed of light in vacuum, and leads to an extreme
energy compression in the plasma. The mode conversion
problem is examined for different configurations
of inhomogeneous external magnetic field and plasma
density. Fluid and particle-in-cell codes are used
for modelling this effect and a detailed comparison
of the results with theory is performed.The proper
selection of undulator wavelength combined with
judicious choice of axial magnetic field profile
enables microwaves to penetrate plasma with realistic
(smooth) density profiles at moderate levels of
the undulator field, suppressing the strong resonant
absorption at the cyclotron frequency.
|
Jonathan
Wurtele: Robust
autoresonant excitation in the plasma beat-wave
accelerator |
A modified version of the Plasma Beat-Wave Accelerator
scheme is proposed,based on autoresonant phase-locking
of the Langmuir wave to the slowlychirped beat frequency
of the driving lasers by passage through resonance.Peak
electric fields above standard detuning limits seem
readilyattainable, and the plasma wave excitation
is robust to large variationsin plasma density or
chirp rate. This scheme might be implemented inexisting
Chirped Pulse Amplification or $\mbox{CO}_{2}$ laser
systems witha little technological effort. |
Csaba
Toth: A multi-beam, multi-terawatt Ti:sapphire
laser system for laser wake-field acceleration of
electrons and for bright THz radiation production
|
The Lasers, Optical Accelerator Systems Integrated
Studies (L’OASIS) Lab of LBNL operates a highly
automated and remotely controlled Ti:sapphire chirped
pulse amplification (CPA) laser system that provides
synchronized beams of 2x1.0 TW, 12 TW, and 100 TW
peak-power, in a unique, radiation shielded facility.
The system has been specially designed for studying
high field laser–plasma interactions and particularly
aimed for the investigations of laser wake-field
particle acceleration. It generates and recombines
multiple beams having different pulse durations
wavelengths, and pulse energies for various stages
of plasma preparation, excitation, and diagnostics.
The amplifier system is characterized and continuously
monitored via local area network (LAN) from a radiation
shielded control room by an array of diagnostics,
including beam profile monitoring cameras, remote
controlled alignment options, self-correcting beam-pointing
stabilization loops, pulse measurement tools (single-shot
autocorrelator for pulse duration and third-order
correlator for contrast measurements, FROG for pulse
shape studies). Detailed results of these diagnostics
and experiments on laser wake-field acceleration
will be presented.As an example application of the
system, coherent THz radiation was produced from
relativistic electrons accelerated via self-modulated
laser wakefield acceleration (SM-LWFA) in a high
density (10^19 cm^-3) pulsed gas jet. As the electrons
exit the plasma, coherent transition radiation is
generated at the plasma-vacuum boundary for wavelengths
long compared to the bunch length. Radiation yield
in the 0.3 to 19 THz range and at 94 GHz has been
measured and found to depend quadratically on the
bunch charge. Modeling indicates that optimization
of this table-top source could provide more than
100 microJ/pulse THz energy. Together with intrinsic
synchronization to the laser pulse, this will enable
numerous applications requiring intense terahertz
radiation.
|
Mori
Warren: Near gev electron acceleration of
self-injected electrons in cm scale plasma channels |
The -self-injection and laser wakefield acceleration
of electrons is studied in three-dimensions using
the particle-in-cell (PIC) code OSIRIS. The simulations
model a 50fs, 13 TW, .8m m laser propagating through
a leaky plasma channel with a minimum density of
3x1018 cm-3. The initial wake is not large enough
to trap background electrons. However, the self-consistent
evolution of the laser due to photon acceleration
and deceleration, group velocity dispersion, and
transverse self-focusing results in larger wake
generated in the blowout regime. When the blowout
is severe enough that electrons from the axis overtake
those at the channel edge then those electrons on
the wall are attracted to the axis ahead of the
main group leading to self-injection. The resulting
acceleration leads to a mono-energetic beam with
central energy of 260 MeV and a ~20% energy spread.
This first group of particles beam loads and eventually
stretches the wake, leading to the trapping of a
second bunch of electrons. The peak energy of the
second bunch approaches 840MeV(with a more continuous
distribution). We will discuss the evolution of
the laser, the self-injection mechanism, the acceleration
mechanism, the generation of mono-energetic bunch,
and the difference between 2D and 3D simulations.
The simulations followed 2.5x108 particles on 3600
x 256 x 256 grids for a distance of .94 centimeters. |
Gennady
Shvets: Plasma Beatwave Accelerator Based
on a Nonlinear Bistability of Relativistic Plasma
Waves |
Plasma beatwave is one of the originally proposed
schemes for making a plasma-based accelerator. Its
traditional implementation assumes that the plasma
wave is a linear oscillator driven by the ponderomotive
force of the two lasers detuned by the plasma frequency.
In this talk we demonstrate how the off-resonance
excitation of the plasma wave and the relativistic
dependence of the plasma frequency on its amplitude
can be used to observe the nonlinear bistability
of the plasma wave. The relativistic threshold for
observing the nonlinear bistability is derived and
confirmed using particle in cell simulations. One
of the surprising results is the realization that
a very long laser beatwave pulse slightly detuned
from the nonrelativistic plasma wave resonance can
excite plasma waves behind the pulse if the peak
pulse amplitude exceeds the threshold. Specific
examples of laser and plasma parameters relevant
to high-gradient plasma accelerators will given.
Also, excitation of plasma wakes by a strongly modulated
(for example, by an FEL interaction) electron beam
is considered, and the possibility of developing
a combined accelerator/fs-injector will be discussed.
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