Abstract
In the context of inertial confinement fusion, optical smoothing is a technique used to provide laser irradiation as homogeneous as possible. It is required for limiting the development of laser-plasma instabilities, in particular, stimulated Brillouin backscattering (SBS). Here, we carefully compare longitudinal and transverse smoothing by spectral dispersion in an ideal smoothing configuration for each case. With 3D codes, we have simulated SBS in a gold plasma. We show that, as opposed to common belief, the time evolution of the SBS reflectivity exhibits some differences between both smoothing schemes. First, the asymptotic values of the saturation levels are not quite the same. With a simple ray description and computing the SBS gain for each ray, we were able to explain this difference. Moreover, the dynamics of SBS are also somewhat different. We have shown that SBS dynamics are driven by the time evolution of speckle properties and in particular by the effective interaction length between the Brillouin backscattered light and the hot-spots. This effective interaction length depends both on the longitudinal velocity and on the length of the hot-spots. As a matter of fact, synchronizing the effective interaction lengths of both smoothing schemes also synchronizes the backscattering curves before reaching the saturation level.
Highlights
Large laser facilities such as the Laser Megajoule (LMJ)1 in France and the National Ignition Facility (NIF)2 in the USA are designed to achieve, among many other experiments, Inertial Confinement Fusion (ICF).3,4 This consists in compressing a capsule filled with hydrogen isotopes using radiation, such as powerful laser beams
Since the size of an ICF designed laser is directly related to the maximum intensity and fluence it can withstand, and since we have shown that the laser should be top-hat in both cases, the laser should have the same surface for Transverse Spectral Dispersion (TSSD) and Longitudinal smoothing by spectral dispersion (LSSD) and rffiffiffiffiffiffiffi w 1⁄4 pL2 1⁄4 1:03 m: (11)
We have conducted a thorough comparison between the TSSD and LSSD effects on stimulated Brillouin backscattering (SBS) in the context of ICF experiments
Summary
Large laser facilities such as the Laser Megajoule (LMJ) in France and the National Ignition Facility (NIF) in the USA are designed to achieve, among many other experiments, Inertial Confinement Fusion (ICF). This consists in compressing a capsule filled with hydrogen isotopes using radiation, such as powerful laser beams. For all these reasons, controlling the laser focal spot to ensure uniform illumination on the target by reducing high intensities in space and in time is still of fundamental interest in ICF For this purpose, laser smoothing schemes have been developed on many facilities world-wide, for example, Smoothing by Longitudinal Spectral Dispersion (LSSD) implemented on LMJ and Ligne d’Integration Laser (LIL) and Transverse Spectral Dispersion (TSSD) implemented on the NIF and OMEGA facility. By highlighting the speckle properties involved in SBS: hot-spot shapes and dynamics, we show that SBS is mainly governed by the product of the longitudinal velocity and the length of hot-spots referred to as the effective interaction length We conclude that both schemes are equivalent if we only consider a shorttime scale of approximately 15 ps when their hot-spot effective interaction length evolutions are synchronized but differ slightly from each other by the saturation level of SBS induced (Sec. V)
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