Energy gain by laser-accelerated electrons in a strong magnetic field

TL;DR

This paper demonstrates how a strong magnetic field can significantly enhance laser-driven electron acceleration in plasma, enabling higher energy gains over shorter distances than in vacuum conditions.

Contribution

It introduces a novel mechanism where a static magnetic field enables energy gain for electrons initially unable to accelerate effectively in laser-plasma interactions.

Findings

Magnetic field induces electron energy increase by rotating electron momentum.

Energy gain continues beyond the dephasing point with small dephasing values.

Maximum energy gain depends on magnetic field strength and wave phase velocity.

Abstract

The manuscript deals with electron acceleration by a laser pulse in a plasma with a static uniform magnetic field $B_*$. The laser pulse propagates perpendicular to the magnetic field lines with the polarization chosen such that $({\bf{E}}_{laser} \cdot {\bf{B}}_*) = 0$. The focus of the work is on the electrons with an appreciable initial transverse momentum that are unable to gain significant energy from the laser in the absence of the magnetic field due to strong dephasing. It is shown that the magnetic field can initiate an energy increase by rotating such an electron, so that its momentum becomes directed forward. The energy gain continues well beyond this turning point where the dephasing drops to a very small value. In contrast to the case of purely vacuum acceleration, the electron experiences a rapid energy increases with the analytically derived maximum energy gain dependent on the strength of the magnetic field and the phase velocity of the wave. The energy enhancement by the magnetic field can be useful at high laser amplitudes, $a_0 \gg 1$, where the acceleration similar to that in the vacuum is unable to produce energetic electrons over just tens of microns. A strong magnetic field helps leverage an increase in $a_0$ without a significant increase in the interaction length.