Enhanced collisionless laser absorption in strongly magnetized plasmas

TL;DR

This paper demonstrates that strong magnetic fields in plasmas significantly enhance laser energy absorption through resonant wave decay processes, with implications for fusion and x-ray source development.

Contribution

It reveals how magnetic fields enable new wave interactions and instabilities that increase laser energy absorption in plasmas, a novel insight for magnetized plasma physics.

Findings

Resonant decay of laser light into magnetized plasma waves occurs at specific frequencies.

Magnetic fields introduce phase mixing, broadening wave damping and energy transfer.

Enhanced absorption explains higher temperatures in magnetized laser experiments.

Abstract

Strongly magnetizing a plasma adds a range of waves that do not exist in unmagnetized plasmas and enlarges the laser-plasma interaction (LPI) landscape. In this paper, we use particle-in-cell (PIC) simulations to investigate strongly magnetized LPI in one dimension under conditions relevant for magneto-inertial fusion experiments, focusing on a regime where the electron-cyclotron frequency is greater than the plasma frequency and the magnetic field is at an oblique angle with respect to the wave vectors. We show that when electron-cyclotron-like hybrid wave frequency is about half the laser frequency, the laser light resonantly decays to magnetized plasma waves via primary and secondary instabilities with large growth rates. These distinct magnetic-field-controlled instabilities, which we collectively call two-magnon decays, are analogous to two-plasmon decays in unmagnetized plasmas. Since additional phase mixing mechanisms are introduced by the oblique magnetic field, collisionless damping of large-amplitude magnetized waves substantially broadens the electron distribution function, especially along the direction of the magnetic field. During this process, energy is transferred efficiently from the laser to plasma waves and then to electrons, leading to a large overall absorptivity when strong resonances are present. The enhanced laser energy absorption may explain hotter-than-expected temperatures observed in magnetized laser implosion experiments and may also be exploited to develop more efficient laser-driven x-ray sources.