Phase-controlled direct laser acceleration enabled by longitudinal variation of the laser-driven quasi-static plasma magnetic field

TL;DR

This paper demonstrates that a slow longitudinal variation of the plasma magnetic field can control phase interactions in direct laser acceleration, enabling sustained electron energy gain by suppressing reversibility.

Contribution

It introduces a novel phase control mechanism in DLA through longitudinal magnetic field variation, enhancing net electron energy gain.

Findings

Hysteresis in betatron to laser frequency ratio enables phase control.

Longitudinal magnetic field variation suppresses reversible energy exchange.

Electrons can retain energy and sustain acceleration without losses.

Abstract

Direct laser acceleration (DLA) enables energy transfer from an ultra-high-intensity laser to plasma electrons and underpins many laser-driven particle and radiation-source concepts. A laser-driven azimuthal plasma magnetic field is a key player in this process: it confines energetic electrons, induces betatron oscillations, and makes possible a resonant interaction between the betatron motion and the laser field. While this betatron resonance can enhance electron energy gain, the gain itself generally drives frequency detuning and promotes largely reversible energy exchange that limits net acceleration. Here we show, using a test-electron model with prescribed fields, that a slow longitudinal increase of the quasi-static plasma magnetic field qualitatively changes DLA by introducing hysteresis in the ratio of the betatron frequency to the laser frequency experienced by the electron, so that this ratio depends on the prior evolution of the electron even at the same energy. This hysteresis enables phase control of the electron-laser energy exchange and suppresses the usual reversibility of DLA, allowing electrons to retain the acquired energy and sustain energy gain without intermittent losses.