Mitigating parametric instabilities in plasmas by sunlight-like lasers

H H Ma,X F Li,S H Yew,J Zhang,S Kawata,S M Weng,P Gibbon,Z M Sheng

doi:10.1063/5.0054653

Abstract

Sunlight-like lasers that have a continuous broad frequency spectrum, random phase spectrum, and random polarization are formulated theoretically. With a sunlight-like laser beam consisting of a sequence of temporal speckles, the resonant three-wave coupling that underlies parametric instabilities in laser–plasma interactions can be greatly degraded owing to the limited duration of each speckle and the frequency shift between two adjacent speckles. The wave coupling can be further weakened by the random polarization of such beams. Numerical simulations demonstrate that the intensity threshold of stimulated Raman scattering in homogeneous plasmas can be doubled by using a sunlight-like laser beam with a relative bandwidth of ∼1% as compared with a monochromatic laser beam. Consequently, the hot-electron generation harmful to inertial confinement fusion can be effectively controlled by using sunlight-like laser drivers. Such drivers may be realized in the next generation of broadband lasers by combining two or more broadband beams with independent phase spectra or by applying polarization smoothing to a single broadband beam.

Highlights

The above 1D and 2D simulations verify that a sunlight-like laser with a relatively low bandwidth is effective in mitigating stimulated Raman scattering (SRS) in a homogeneous plasma
Fluid-type simulations have shown that the enlargement of the SRS resonant region in the interactions of broadband lasers with inhomogeneous plasmas may increase the convective SRS gain,45,46 even though broadband lasers decrease the SRS growth rate
Wen et al.47 studied the development of SRS in the interactions of broadband lasers with inhomogeneous plasmas in both the fluid and kinetic regimes

Summary

INTRODUCTION

As one of the most critical and fundamental problems that arise in attempts to achieve inertial confinement fusion (ICF), parametric instabilities have attracted significant attention for many years. With the construction of high-power lasers at higher energy for either direct- or indirect-drive approaches to ICF, it becomes more urgent to control parametric instabilities in laser–plasma interactions. In the indirect-drive approach, for example, laser beams have to propagate through a large-scale relatively uniform underdense plasma before arriving at the hohlraum wall and converting their energy into soft x rays that drive the final implosion of the fusion capsule. The parametric instabilities involved here include for example, stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), and cross-beam energy transfer (CBET). These parametric instabilities scatter away a considerable proportion of the laser energy, but may produce harmful hot electrons that affect compression and implosion efficiency.. The parametric instabilities involved here include for example, stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), and cross-beam energy transfer (CBET).. Theoretical analyses and simulations have long predicted that parametric instabilities can be significantly suppressed if the laser bandwidth is larger than the instability growth rates.. The bandwidths required to adequately suppress parametric instabilities (especially SRS) are usually too large and beyond the capabilities of contemporary high-power laser technology. We propose a novel scheme for mitigating parametric instabilities using a sunlight-like laser, which can dramatically raise the intensity thresholds for instabilities with a much smaller bandwidth in comparison with conventional broadband lasers. Sunlight-like laser pulses have obvious superiority in mitigating parametric instabilities in comparison with conventional broadband lasers. We focus on the mitigation of SRS, since this type of instability usually has a higher growth rate, and its mitigation requires a larger bandwidth and is more challenging than other instabilities

SPECIFIC MODELS FOR BROADBAND LASERS AND SUNLIGHT-LIKE LASERS

DISCUSSION AND CONCLUSION