Enhanced energy gain through higher-order resonances during direct laser acceleration with superluminal phase velocity

TL;DR

This paper shows that higher-order resonances during direct laser acceleration with superluminal phase velocity significantly enhance energy gain by maintaining frequency matching over a broad energy range.

Contribution

It demonstrates that superluminal phase velocity enables sustained higher-order resonances, leading to improved energy transfer in laser-plasma interactions.

Findings

Higher-order resonances become dominant at superluminal phase velocities.

Superluminal phase velocity creates a non-monotonic frequency ratio facilitating sustained resonance.

Energy gain is significantly enhanced through these higher-order resonances.

Abstract

Ultra-high intensity laser-plasma interactions can produce ultra-relativistic electrons via direct laser acceleration, assisted by quasi-static plasma magnetic and electric fields. These fields transversely confine electron motion and induce betatron oscillations. The net energy gain is strongly influenced by the interplay between two frequencies: the betatron frequency and the frequency of laser field oscillations experienced by the electron. Prior work has shown that energy gain is enabled by a resonance between the betatron oscillations and the oscillations of the laser field. In particular, higher-order resonances occur when the laser field completes multiple cycles during one betatron oscillation, allowing additional regimes of energy transfer beyond the fundamental (betatron) resonance. In this work, we demonstrate that such resonances become particularly effective when the laser's phase velocity is superluminal. Although the two frequencies generally evolve differently with increasing electron energy, which leads to detuning, a superluminal phase velocity introduces a non-monotonic frequency ratio with a global minimum. This minimum allows sustained frequency matching over a broad energy range, thereby enabling enhanced energy gain. As the phase velocity increases, the betatron resonance becomes ineffective due to premature frequency detuning. At the same time, higher-order resonances become increasingly effective, emerging as the dominant mechanisms for enhanced energy gain in this regime of direct laser acceleration.