Laser heating of large noble gas clusters: From the resonant to the relativistic interaction regimes

Abstract

Wide-ranging measurements of sub-picosecond laser interactions with large noble gas cluster targets have been conducted in order to help clarify the nature and extent of the underlying laser-plasma heating. Within the sub-relativistic vacuum irradiance range of 1016-1017 W cm-2, we find that electron temperatures measured with continuum x-ray spectroscopy exhibit a pronounced multi-keV enhancement. Analysis indicates this behaviour to be consistent with collisional or collisionless resonant heating mechanisms. We also present the first measurements of laser-to-cluster energy deposition at relativistic vacuum irradiances, our data demonstrating absorption fractions of 90% or more. Optical probing was used to resolve the onset of a supersonic ionization front resulting from this very high absorption, and shows that despite significant prefocus heating, the greatest plasma energy densities can be generated about the vacuum focus position. Electron energy spectra measurements confirm that laser-plasma super-heating occurs, and together with ion data establish that relativistic laser-plasma coupling in atomic clusters can take place without significant MeV particle beam production. In conjunction with optical selfemission data, the optical probing also indicates laser pre-pulse effects at peak vacuum irradiance of 5 × 1019W cm-2. Laser absorption, plasma heating and energy transport data are supported throughout with analytical and numerical modelling. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.