Suppression of autoresonant stimulated Raman scattering in transversely weakly magnetized plasmas

Abstract

We investigate the behaviour of backward stimulated Raman scattering (BSRS) in inhomogeneous plasma with a transverse external magnetic field via 1D and 2D particle-in-cell simulations and theoretical analysis. The external magnetic field B e x t causes nonlinear damping of large-amplitude electron plasma waves (EPWs), and significantly limits the reflectivity of stimulated Raman scattering (SRS) even if B e x t is very weak. For a single resonance point, B e x t limits the kinetic nonlinear frequency shift, impedes the autoresonance behaviour of SRS, and narrows the resonance region. We also exhibit B e x t distorts the distribution function of electrons evidently, and alters the wavefronts of EPWs in high dimensional cases. The simulation results indicate a moderate B e x t can significantly reduce the trapped electron induced gain of SRS reflectivity, and make the BSRS reflectivity stay at its linear convective saturation level steadily.

Suppression of stimulated Brillouin and Raman scatterings using an alternating frequency laser and transverse magnetic fields

“…Eventually the particle detraps after accelerating to the escape speed and then continues executing cyclotron motion in p x -p y space. [19] In addition, the forward SRS is slightly enhanced in the external magnetic field, as shown in Fig. 5(b1).…”

“…5 is B z = 60 T, the electron cyclotron frequency ω c = 1.96 × 10 −3 ω 0 (ω c /ω 0 1) and the influence of an external magnetic field on the dispersion relations of electromagnetic waves and EPWs is negligible. [19] The spectrum signal of SRS with B z = 0 T is shown in Fig. 5(a1).…”

“…In recent years, more and more attention has been paid to the effect of magnetic fields on LPIs. [10][11][12][13][14][15][16][17][18][19][20][21] Montgomery et al pointed out that an external magnetic field of 7.5 T can significantly increase the electron temperature and improve the laser coupling in hohlraum plasmas. [13] Li et al presented a new staged hot-electron acceleration mechanism involving the two-plasmon decay (TPD) instability in the transverse magnetic field and showed that the hot electrons generated by the forward EPW of TPD can be trapped and accelerated again by the backward EPW of TPD, eventually obtaining higher energy.…”

“…[17,18] The research results of Zhou et al indicated that since the nonlinear damping of EPWs is caused by the external transverse magnetic field, the magnetic field can reduce the trapped electrons, limit the kinetic nonlinear frequency shift and narrow the autoresonance region of SRS; thereby, the reflectivity of SRS in inhomogeneous plasma can be effectively controlled. [19] In the early stages of laser development, a number of strategies for incident laser processing were proposed. They include various laser smoothing technologies, such as random phase plates, [22] continuous phase plates, [23,24] smoothing by spectral dispersion, [25] and polarization smoothing.…”

Weibel instability induced by kinetic stimulated Raman scattering in unmagnetized and magnetized plasmas

“…Furthermore, the longitudinal magnetic field makes hot electrons move along the magnetic lines of force, which reduces the growth rate of Weibel instability [25][26][27]. Many works [28,29] have indicated that the kinetic BSRS can be suppressed when the magnetic field (≳20 T) is perpendicular to the laser propagation direction. Since the amplitude of EPW is suppressed and the BSRS is suppressed accordingly, the Weibel instability will no longer be important.…”