Breakdown of Universal Intensity Scaling in Relativistic Laser-Plasma Interactions

Abstract

Laser light incident on targets at intensities such that the electron dynamics is ultra‐relativistic, gives rise to a harmonic spectrum extending to high orders and characterized by relatively slow decay with harmonic number. The decay with harmonic number m follows a power law dependence m−p and arguments based on relativistic similarity theory predict a universal value for the decay index p = 8/3 up to some cut‐off m = m*. Results presented in this paper suggest that under conditions in which plasma effects lead to modulated spectra, the extent of the modulation may be such as to effectively localize the scaling across the spectrum and so invalidate the concept of universal decay. We find moreover that under favourable conditions, the decay with harmonic number may be significantly weaker than that predicted by the similarity limit.