Abstract
By comparing the impact of established laser smoothing techniques like Random Phase Plates (RPP) and Smoothing by Spectral Dispersion (SSD) to the concept of "Spike Trains of Uneven Duration and Delay" (STUD pulses) on the amplification of parametric instabilities in laser-produced plasmas, we show with the help of numerical simulations, that STUD pulses can drastically reduce instability growth by orders of magnitude. The simulation results, obtained with the code {\sc Harmony} in a nonuniformly flowing mm-size plasma for the Stimulated Brillouin Scattering (SBS) instability, show that the efficiency of the STUD pulse technique is due to the fact that successive re-amplification in space and time of parametrically excited plasma waves inside laser hot spots is minimized. An overall mean fluctuation level of ion acoustic waves at low amplitude is established because of the frequent change of the speckle pattern in successive spikes. This level stays orders of magnitude below the levels of ion acoustic waves excited in hot spots of RPP and SSD laser beams.
Highlights
By comparing the impact of established laser smoothing techniques like Random Phase Plates (RPP) and Smoothing by Spectral Dispersion (SSD) to the concept of “Spike Trains of Uneven Duration and Delay” (STUD pulses) on the amplification of parametric instabilities in laser-produced plasmas, we show with the help of numerical simulations, that STUD pulses can drastically reduce instability growth by orders of magnitude
An overall mean fluctuation level of ion acoustic waves at low amplitude is established because of the frequent change of the speckle pattern in successive spikes. This level stays orders of magnitude below the levels of ion acoustic waves excited in hot spots of RPP and SSD laser beams
The concept of “Spike Trains of Uneven Duration and Delay” (STUD pulses [1]) is a novel approach that aims to overcome the problem of strongly growing parametric instabilities in laser-produced plasmas in the context of inertial confinement fusion (ICF) and high energy density physics (HEDP) more generally
Summary
The concept of “Spike Trains of Uneven Duration and Delay” (STUD pulses [1]) is a novel approach that aims to overcome the problem of strongly growing parametric instabilities in laser-produced plasmas in the context of inertial confinement fusion (ICF) and high energy density physics (HEDP) more generally. -called optical smoothing techniques, like Random Phase Plates (RPP), Smoothing by Spectral Dispersion (SSD), and Induced Spatial Incoherence (ISI) have been developed in the last three decades to diminish the risk of hydrodynamic instabilities in fusion capsules, as well as with the hopes that residually, they may limit the growth of laser-plasma instabilities The impact of these smoothing techniques on parametric instabilities, like stimulated Brillouin scattering, is the subject of the work presented here. Neither RPP nor SSD can efficiently inhibit growth of parametric instabilities inside potentially cooperative regimes between thousands of laser-intensity “hot spots” that form the typical speckle patterns of smoothed laser beams
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