Accounting for speckle-scale beam bending in classical ray tracing schemes for propagating realistic pulses in indirect drive ignition conditions

Abstract

We propose a semi-analytical modeling of smoothed laser beam deviation induced by plasma flows. Based on a Gaussian description of speckles, the model includes spatial, temporal, and polarization smoothing techniques, through fits coming from hydrodynamic simulations with a paraxial description of electromagnetic waves. This beam bending model is then incorporated into a ray tracing algorithm and carefully validated. When applied as a post-process to the propagation of the inner cone in a full-scale simulation of a National Ignition Facility (NIF) experiment, the beam bending along the path of the laser affects the refraction conditions inside the hohlraum and the energy deposition, and could explain some anomalous refraction measurements, namely, the so-called glint observed in some NIF experiments.

Shock formation in flowing plasmas by temporally and spatially smoothed laser beams

“…(11)], indicate an average over a speckle ensemble, ℓ RPP = 45 64 Fλ denotes the speckle correlation length of an RPP beam 13 , and n c the critical density. Similar expressions in the context of beam bending 8,11,20 have been derived, with the dependence on the plasma density n, the average ponderomotive potential ⟨U⟩, and the speckle (or beam) width ∼ Fλ , with F denoting the beam optics Fnumber. Note that the spatially averaged plasma density ⟨n⟩ and the average ponderomotive potential ⟨U⟩/T e normalized to the electron temperature can be assumed to vary slowly on the speckle's width Fλ and response time Fλ /c s .…”

“…In absence of flow, a stationary equilibrium can be reached via balance between the local density and the ponderomotive potential, namely ρ 0 = (n e /n 0 ) v ⊥ =0 ≡ exp(−U/T e ). It has, however, been shown that flow can considerably modify the response of the plasma fluid 8,9,[18][19][20] .…”

“…The effect of SSD on beam deflection and consequently on the bow shock formation is modified by the non-stationary features of the beam speckle pattern. 12,20,28 SSD is a method of using bandwidth in combination with a grating to produce speckle movement. This method was originally proposed 29 for temporally smoothing laser focal spots.…”