Laser energy deposition in relativistic interactions with underdense plasmas

It is well established that, at sub-relativistic intensities, the absorptio n of laser light by underdense plasmas decreases with increasing pulse inte nsity as interaction enters a non-linear regime. On the other hand, as the relativistic interaction regime is reached, further absorption mechanisms c an be activated which can account for a substantial energy transfer. Using the particle code WAKE, we performed numerical simulations of the relativis tic interaction of intense laser pulses with underdense plasmas in conditio ns that can be experimentally tested. Our simulations show that, while the relativistic laser intensity generates a population of fast electrons, a co nsiderable fraction of the pulse energy goes into a population of thermal e lectrons. These findings open new possibilities for a direct observation of relativistic interaction processes using high resolution soft X-ray techni ques.