Laser heating of large noble gas clusters: from the resonant to the relativistic interaction regimes^*

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 pre-focus 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 self-emission data, the optical probing also indicates laser pre-pulse effects at peak vacuum irradiance of 5 × 10¹⁹ W cm^-2. Laser absorption, plasma heating and energy transport data are supported throughout with analytical and numerical modelling.