Laser energy deposition in relativistic interactions with underdense plasmas

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