Anomalies in Universal Intensity Scaling in Ultrarelativistic Laser-Plasma Interactions

TL;DR

This paper investigates the harmonic spectrum decay in ultrarelativistic laser-plasma interactions, revealing deviations from the predicted universal decay law due to plasma effects, which weaken the harmonic decay rate.

Contribution

It demonstrates that plasma effects can alter the expected universal harmonic decay law, showing a range of decay indices different from the theoretical prediction.

Findings

Harmonic decay index ranges from 5/3 to 7/3 under plasma influence

Universal decay law p=8/3 is invalidated when plasma effects are significant

Spectrum extends to high orders with a slower decay than predicted

Abstract

Laser light incident on targets at intensities such that the electron dynamics is ultrarelativistic gives rise to a harmonic power spectrum extending to high orders and characterized by a relatively slow decay with the harmonic number m that follows a power law dependence, m^{-p}. Relativistic similarity theory predicts a universal value for p = 8/3 up to some cut-off m = m*. The results presented in this work suggest that under conditions in which plasma effects contribute to the emission spectrum, the extent of this contribution may invalidate the concept of universal decay. We report a decay with harmonic number in the ultrarelativistic range characterised by an index 5/3 < p < 7/3, significantly weaker than that predicted by the similarity model.