Plasma effects in high harmonic spectra from ultrarelativistic laser–plasma interactions

A single particle model has been applied to examine the competition between radiation from a strong Langmuir source and coherent bremsstrahlung in ultra-relativistic laser–plasma interactions. Output from this model shows generally satisfactory agreement with that from particle-in-cell (PIC) simulations, in particular reproducing spectra characterised by a decay index p=5/3p=5/3. At the highest intensities considered, both model and PIC spectra show the competing contribution from relativistic electron bremsstrahlung, characterised by a further relaxation in the decay index to p=2/3p=2/3. Combinations of laser and plasma parameters have been identified that define regions of parameter space where one or other emission is dominant.