Bright ultrashort X-rays from intense subpicosecond laser-plasma interactions

Summary form only given. Short-pulse, high-intensity lasers interacting with solid targets make possible the study of a new class of laser-plasma interactions. They are unique because during the ultrashort laser pulse relatively little expansion occurs, and the density scale length remains much less than the laser wavelength. This makes possible the direct deposition of a significant amount of the laser energy at close to solid density. Steep plasma temperature and density gradients subsequently cause rapid cooling, resulting in highly non-equilibrium conditions and the concurrent emission of extremely bright ultrashort X-ray pulses. In this study, the latter are investigated experimentally with temporally and spectrally resolved soft X-ray diagnostics. The emitted X-ray spectra from solid targets with various atomic numbers are characterized for a laser pulse width /spl tau//sub l//spl sim/400 fs. With low contrast (10/sup 5/), the X-ray spectrum in the /spl lambda/=40-100 /spl Aring/ spectral region is dominated by line emission, and the X-ray pulse duration is found to be characteristic of a long-scale-length, low-density plasma. Bright, picosecond, broadband emission, characteristic of a short-scale-length, high-density plasma, is produced only when a high laser contrast (10/sup 10/) is used. It is demonstrated experimentally that the pulse width of laser-produced X-ray radiation may be varied down to the picosecond time scale by adjusting the incident ultrashort-pulse laser flux.