Observation of multiple mechanisms for stimulating ion waves in ignition scale plasmas

Abstract

The laser and plasma conditions expected in ignition experiments using indirect drive inertial confinement have been studied experimentally. It has been shown that there are at least three ways in which ion waves can be stimulated in these plasmas and have a significant effect on the energy balance and distribution in the target. First ion waves can be stimulated by a single laser beam by the process of stimulated Brillouin scattering (SBS) in which an ion acoustic and a scattered electromagnetic wave grow from noise. Second, in a plasma where more than one beam intersects, ion waves can be excited at the “beat” frequency and wave number of the intersecting beams, causing the sidescatter instability to be seeded, and substantial energy to be transferred between the beams [R. K. Kirkwood et al., Phys. Rev. Lett. 76, 2065 (1996)]. And third, ion waves may be stimulated by the decay of electron plasma waves produced by stimulated Raman scattering (SRS), thereby inhibiting the SRS process [R. K. Kirkwood et al., Phys. Rev. Lett. 77, 2706 (1996)].

A plasma amplifier to combine multiple beams at NIF

“…These measurements were done with 1 micron wavelength beams in plasmas created in a gas-jet target and for which simulated profiles were confirmed by Thomson scattering measurements of electron temperature and interferometric measurements of electron density. Most of the normalized parameters in this experiment were similar to what was used in the past CBET applications with 351 nm wavelength beams, [9][10][11][12][13][14][15][16][17][18] and the comparison with the predicted linear response of the plasma to the beams was justified by reducing the seed beam intensity to Շ1% of the pump beam intensity so that the total power scattered from the pump beam was only $1%. This data set demonstrated the validity of the model of the polarization dependence of the amplification of a single linearly polarized pump beam interacting with a weak linearly polarized seed beam in the linear limit to the highest accuracy yet achieved.…”

“…28,29 The close agreement with observations in cases where the ion wave response was linear confirmed the Vlasov model of the plasma in predicting a linear plasma response that is applicable in a wider range of situations. Still, there have been numerous observations of the amplification of beams by CBET in conditions where the plasma response was either inferred to be non-linear by comparison of seed amplification with linear models 10,11,18,[20][21][22] or directly demonstrated to be non-linear via the scaling of transmitted amplified seed power with incident seed power. 12,13 Further, the predictions using linear Vlasov or fluid models of the plasma response overestimate the amplification observed in the seed in these cases, as well as in a case in which observations showed a linear scaling of scattering with seed beam power 9 indicating other effects reduced the scattering from the pump.…”

“…Efforts to produce controlled power deposition profiles in laser fusion experiments have already taken advantage of the wavelength tuning capability at the National Ignition Facility (NIF) 11,19,20 to control the symmetry of imploded capsules of fusion fuel. The need for accurate prediction of the effects of wavelength tuning on implosion symmetry has built on early models of the linear and non-linear CBET processes observed in experiments specifically designed to study CBET [20][21][22] and produced new models used describe the effect in the fusion target experiments at the NIF.…”

Multi-beam effects on backscatter and its saturation in experiments with conditions relevant to ignition

“…The first of these multi-beam processes was identified as being important for controlling radiation symmetry in hohlraum targets both because the high amplitude seed consisting of one or more co-propagating beams could cause significant power and energy transfer from other beams even when the overall gain exponents were <1, and because the plasma conditions in the beam crossing volume in ignition targets have the sonic flows needed to match the SBS amplification resonance when the beams have the same wavelength. 13 The demonstration of a strong wavelength dependence of seeded SBS forward scatter, or power transfer between crossing beams as it is now called, led to the concept of wavelength tuning of beams that enter ignition target at different angles 13 in order to control forward scatter and produce the laser power deposition profile needed on the interior of the hohlraum wall for symmetric implosions. As a result NIF now has wavelength tuning capability, 14 and an analysis of the SBS amplification of the forward going power of all entering beams by all other beams has been implemented and used to estimate the optimal wavelengths for the beams to produce symmetric x-ray drive in all ignition target designs.…”

Observation of resonant energy transfer between identical-frequency laser beams

“…In a supersonic plasma (|v d | ≥ c s ) the resonant ion wave can have zero frequency in the laboratory frame (ω ia = 0), and the ion wave can therefore transfer energy between two identical frequency beams over many acoustic periods [14][15][16].…”