Efficient Way to Prevent Reflection from Stimulated Brillouin Backscattering

Abstract

The same physical mechanism which causes stimulated Brillouin backscattering of a laser pulse when interacting with the underdense plasma (in laser fusion systems) can be used to reverse the scattering direction in order to drastically reduce the reflectivity. Forced stimulated rescattering may be obtained by additional radiation pulses or by a multihumped spectrum with each peak down-shifted by twice the acoustic frequency. Analytical four-mode and numerical six-mode models show strong reduction of the reflectivity for the same input intensity.PACS numbers: 52.25.Ps, 52.35.Mw, 52.40.Db, 52.50.Jm In laser-plasma interaction experiments on inertial fusion devices, a problem of importance is the stimulated backscattering of the incident radiation since this effect is capable of reflecting a large fraction of the laser energy incident on a fusion target and may reduce the coupling efficiency. Theoretical and numerical calculations suggest that intense laser radiation interacting with ion-acoustic waves in the underdense coronal plasma surrounding the target may lead to strong stimulated Brillouin (back-)scattering (SBS). 1 " 4 The physical mechanism may be interpreted as a parametric instability where the incident laser wave of frequency oo 0 and wave vector k 0 resonantly drives a backscattered electromagnetic wave at oo R = <JO 0 -co ia and k R = -k 0 c^-oo 0 /c and a forward-traveling ion-acoustic wave of frequency k ia c s and wave vector k ia = 2k 0 c-2a> 0 /c, whereThe instability occurs because the ponderomotive force generated by the superposition of the ingoing and backscattered waves satisfies the matching conditions for frequency (energy) and wave vector (momentum). 4 In the heavy sound wave damping limit (y ia >^ia; ZT e ~T;), for which I shall present my results, the acoustic intensity remains approximately proportional to the product of the ingoing-and backscattered-wave intensities.Since these intensities decay in the forward direction because of pump depletion, the acoustic intensity builds up from its boundary value to a peak value of the order of the attenuation length l s = y ia /c s and then decays in the forward direction for the remaining part of the interaction region. The instability is convective, 1,3 SBS merges to stimulated-ion-Compton scattering 3 (SICS), and the backscattered radiation which was downshifted by the acoustic frequency o; ia becomes now downshifted by the ion-Doppler width Aco D . = 2(KT { /m i c 2 ) l/2 u) 0 . I shall call Aco BS the effective backscattered downshift which can be modified by plasma motion and light reflection from the critical surface. 5 Experiments with Ndrglass lasers show that SBS can reflect large amounts of laser light. 6 It may attain up to 60% in longpulse C0 2 lasers 7 interacting with an underdense millimeter-length hydrogen plasma at intensities of 10 13 W/cm 2 . Widely varying conditions are involved for the saturation mechanism, 4 but reflectivity from SBS remains important when the threshold for the backscatter instability is well overcome....

Ion acoustic wave generation by a standing electromagnetic field in a subcritical plasma

“…Because of the nonlinear response of the plasma to the electromagnetic pulse, a zero frequency electron density perturbation is created with a wave number twice the electromagnetic one. Then, after the pulse, the plasma relaxes towards equilibrium, emitting ion acoustic waves which verify the usual dispersion relation [2].…”

Brillouin mirror with an inverted acoustic profile in the presence of strong acoustic dispersion

“…The now standard 3-wave model for SBS optical fibers [1,2,35,36] and, more recently, for some optomechanical cavities [37] was originally derived from the coherent model of SBS in laser-plasma interactions [38][39][40][41]. It is usually assumed that the slowly varying envelopes approximation holds for both optical waves (pump and Brillouin, of respective frequencies and ), as well as for the acoustic wave, of frequency , as long as nanosecond or longer pulsed regimes, or a fortiori stationary or quasi-stationary regimes, are considered.…”