Particle Deceleration for Collective QED Signatures

TL;DR

This paper investigates how re-accelerated electron-positron pairs in a laser field influence frequency upshifts in QED cascades, highlighting the significance of particle dynamics and laser mode structure for experimental signatures.

Contribution

It demonstrates that re-accelerated particles significantly impact laser frequency upshifts and explores the use of Laguerre-Gaussian modes to enhance pair confinement and collective effects.

Findings

Re-accelerated particles affect laser upshift more than stopped pairs.

Laguerre-Gaussian modes can sustain particle contributions longer.

Structured beams can enhance pair density and reduce laser requirements.

Abstract

Frequency upshifts have been proposed as a first experimental signature of collective effects in QED cascade generated electron-positron pair plasmas. Since the high effective masses of generated pairs will reduce any frequency change, stopped pairs at minimal Lorentz factor in the lab frame were thought to be the dominant contribution to the the laser upshift. However, we demonstrate that only considering stopped particles unduly neglects the contributions of particles re-accelerated in the laser propagation direction. Re-accelerated particles should, on a per particle basis, affect the laser more strongly, and over a much longer timescale. To maximize particle contributions to the laser upshift, we consider a Laguerre-Gaussian (LG) mode to better reflect generated pairs. The LG mode doesn't have an advantage in particle deceleration and re-acceleration when compared against a Gaussian beam, but the LG mode can maintain particle contributions for a longer duration, allowing for more pair density accumulation. Deceleration with a structured beam to keep pairs within the laser should create a larger upshift, thereby lowering the demands on the driving laser.