22 research outputs found
Ray-based calculations of backscatter in laser fusion targets
A 1D, steady-state model for Brillouin and Raman backscatter from an
inhomogeneous plasma is presented. The daughter plasma waves are treated in the
strong damping limit, and have amplitudes given by the (linear) kinetic
response to the ponderomotive drive. Pump depletion, inverse-bremsstrahlung
damping, bremsstrahlung emission, Thomson scattering off density fluctuations,
and whole-beam focusing are included. The numerical code DEPLETE, which
implements this model, is described. The model is compared with traditional
linear gain calculations, as well as "plane-wave" simulations with the paraxial
propagation code pF3D. Comparisons with Brillouin-scattering experiments at the
OMEGA Laser Facility [T. R. Boehly et al., Opt. Commun. 133, p. 495 (1997)]
show that laser speckles greatly enhance the reflectivity over the DEPLETE
results. An approximate upper bound on this enhancement, motivated by phase
conjugation, is given by doubling the DEPLETE coupling coefficient. Analysis
with DEPLETE of an ignition design for the National Ignition Facility (NIF) [J.
A. Paisner, E. M. Campbell, and W. J. Hogan, Fusion Technol. 26, p. 755
(1994)], with a peak radiation temperature of 285 eV, shows encouragingly low
reflectivity. Re-absorption of Raman light is seen to be significant in this
design.Comment: 16 pages, 19 figure
Assessing the effects of data compression in simulations using physically motivated metrics
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An MPP hydrocode to study laser-plasma interactions
Because of the increased size and power inherent in a laser-AGEX on NIF, laser-plasma interactions (LPI) observed in NOVA AGEX play an increasingly important role. The process by which filamentation and stimulated backscatter grow is complex. Furthermore, there is a competition among the instabilities so that lessening one can increase another. Therefore, simulating them is an integral part to successful experiments on NIF. In this paper, we present a massively parallel hydrocode to simulate laser-plasma interactions in NIF-relevant AGEX regimes
Threshold for Electron Trapping Nonlinearity in Langmuir Waves
We assess when electron trapping nonlinearity is expected to be important in
Langmuir waves. The basic criterion is that the inverse of the detrapping rate
nu_d of electrons in the trapping region of velocity space must exceed the
bounce period of deeply-trapped electrons, tau_B = (n_e/delta n)^{1/2}
2pi/omega_pe. A unitless figure of merit, the "bounce number" N_B = 1/(nu_d
tau_B), encapsulates this condition and defines a trapping threshold amplitude
for which N_B=1. The detrapping rate is found for convective loss (transverse
and longitudinal) out of a spatially finite Langmuir wave. Simulations of
driven waves with a finite transverse profile, using the 2D-2V Vlasov code
Loki, show trapping nonlinearity increases continuously with N_B for transverse
loss, and is significant for N_B ~ 1. The detrapping rate due to Coulomb
collisions (both electron-electron and electron-ion) is also found, with
pitch-angle scattering and parallel drag and diffusion treated in a unified
manner. A simple way to combine convective and collisional detrapping is given.
Application to underdense plasma conditions in inertial confinement fusion
targets is presented. The results show that convective transverse loss is
usually the most potent detrapping process in a single f/8 laser speckle. For
typical plasma and laser conditions on the inner laser cones of the National
Ignition Facility, local reflectivities ~3% are estimated to produce
significant trapping effects.Comment: 16 pages, 15 figures, accepted for publication in Phys. Plasma