199 research outputs found
Nonlinear evolution of the plasma beatwave: Compressing the laser beatnotes via electromagnetic cascading
The near-resonant beatwave excitation of an electron plasma wave (EPW) can be
employed for generating the trains of few-femtosecond electromagnetic (EM)
pulses in rarefied plasmas. The EPW produces a co-moving index grating that
induces a laser phase modulation at the difference frequency. The bandwidth of
the phase-modulated laser is proportional to the product of the plasma length,
laser wavelength, and amplitude of the electron density perturbation. The laser
spectrum is composed of a cascade of red and blue sidebands shifted by integer
multiples of the beat frequency. When the beat frequency is lower than the
electron plasma frequency, the red-shifted spectral components are advanced in
time with respect to the blue-shifted ones near the center of each laser
beatnote. The group velocity dispersion of plasma compresses so chirped
beatnotes to a few-laser-cycle duration thus creating a train of sharp EM
spikes with the beat periodicity. Depending on the plasma and laser parameters,
chirping and compression can be implemented either concurrently in the same, or
sequentially in different plasmas. Evolution of the laser beatwave end electron
density perturbations is described in time and one spatial dimension in a
weakly relativistic approximation. Using the compression effect, we demonstrate
that the relativistic bi-stability regime of the EPW excitation [G. Shvets,
Phys. Rev. Lett. 93, 195004 (2004)] can be achieved with the initially
sub-threshold beatwave pulse.Comment: 13 pages, 11 figures, submitted to Physical Review
Ion acceleration from laser-driven electrostatic shocks
Multi-dimensional particle-in-cell simulations are used to study the
generation of electrostatic shocks in plasma and the reflection of background
ions to produce high-quality and high-energy ion beams. Electrostatic shocks
are driven by the interaction of two plasmas with different density and/or
relative drift velocity. The energy and number of ions reflected by the shock
increase with increasing density ratio and relative drift velocity between the
two interacting plasmas. It is shown that the interaction of intense lasers
with tailored near-critical density plasmas allows for the efficient heating of
the plasma electrons and steepening of the plasma profile at the critical
density interface, leading to the generation of high-velocity shock structures
and high-energy ion beams. Our results indicate that high-quality 200 MeV
shock-accelerated ion beams required for medical applications may be obtained
with current laser systems.Comment: 33 pages, 12 figures, accepted for publication in Physics of Plasma
Laser-driven shock acceleration of monoenergetic ion beams
We show that monoenergetic ion beams can be accelerated by moderate Mach
number collisionless, electrostatic shocks propagating in a long scale-length
exponentially decaying plasma profile. Strong plasma heating and density
steepening produced by an intense laser pulse near the critical density can
launch such shocks that propagate in the extended plasma at high velocities.
The generation of a monoenergetic ion beam is possible due to the small and
constant sheath electric field associated with the slowly decreasing density
profile. The conditions for the acceleration of high-quality, energetic ion
beams are identified through theory and multidimensional particle-in-cell
simulations. The scaling of the ion energy with laser intensity shows that it
is possible to generate MeV proton beams with state-of-the-art 100
TW class laser systems.Comment: 13 pages, 4 figures, accepted for publication in Physical Review
Letter
DEVELOPMENT OF PICOSECOND CO 2 LASER DRIVER FOR AN MEV ION SOURCE
Abstract Laser-Driven Ion Acceleration in thin foils has demonstrated high-charge, low-emittance MeV ion beams with a picosecond duration. Such high-brightness beams are very attractive for a compact ion source or an injector for RF accelerators. However in the case of foils, scaling of the pulse repetition rate and improving shot-to-shot reproducibility is a serious challenge. CO 2 laser-plasma interactions provide a possibility for using a debris free gas jet for target normal sheath acceleration of ions. Gas jets have the advantage of precise density control around the critical plasma density for 10 μm pulses (10 19 cm -3 ) and can be run at 1-10 Hz. The master oscillator-power amplifier CO 2 laser system at the UCLA Neptune Laboratory is being upgraded to generate 1 J, 3 ps pulses at 1Hz. For this purpose, a new 8 atm CO 2 module is used to amplify a picosecond pulse to ~10 GW level. Final amplification is realized in a 1-m long TEA CO 2 amplifier, for which the field broadening mechanism provides the bandwidth necessary for short pulses. Modeling of the pulse amplification shows that ~0.3 TW power is achievable that should be sufficient for producing 1-3 MeV H + protons from the gas plasma
Collisionless shock acceleration of narrow energy spread ion beams from mixed species plasmas using 1 m lasers
Collisionless shock acceleration of protons and C ions has been
achieved by the interaction of a 10 W/cm, 1 m laser with a
near-critical density plasma. Ablation of the initially solid density target by
a secondary laser allowed for systematic control of the plasma profile. This
enabled the production of beams with peaked spectra with energies of 10-18
MeV/a.m.u. and energy spreads of 10-20 with up to 3x10 particles within
these narrow spectral features. The narrow energy spread and similar velocity
of ion species with different charge-to-mass ratio are consistent with
acceleration by the moving potential of a shock wave. Particle-in-cell
simulations show shock accelerated beams of protons and C ions with
energy distributions consistent with the experiments. Simulations further
indicate the plasma profile determines the trade-off between the beam charge
and energy and that with additional target optimization narrow energy spread
beams exceeding 100 MeV/a.m.u. can be produced using the same laser conditions.Comment: Accepted for publication in Physical Review Accelerators and Beam
AWAKE-related benchmarking tests for simulation codes
Two tests are described that were developed for benchmarking and comparison
of numerical codes in the context of AWAKE experiment.Comment: 4 pages, 4 figures, 1 tabl
Double resonant plasma wakefields
Present work in Laser Plasma Accelerators focuses on a single laser pulse driving a non-linear wake in a plasma. Such single pulse regimes require ever increasing laser power in order to excite ever increasing wake amplitudes. Such high intensity pulses can be limited by instabilities as well engineering restrictions and experimental constraints on optics. Alternatively we present a look at resonantly driving plasmas using a laser pulse train. In particular we compare analytic, numerical and VORPAL simulation results to characterize a proposed experiment to measure the wake resonantly driven by four Gaussian laser pulses. The current progress depicts the interaction of 4 CO2 laser pulses, λlaser = 10.6μm, of 3 ps full width at half maximum (FWHM) length separated peak-to-peak by 18 ps, each of normalized vector potential a0 0.7. Results confirm previous discourse and show, for a given laser profile, an accelerating field on the order of 900 MV/m, for a plasma satisfying the resonant condition ωp = π tFWHM
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