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 10(16)-10(17) 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 x 10(19) W cm(-2). Laser absorption, plasma heating and energy transport data are supported throughout with analytical and numerical modelling.

The Atomic Weapons Establishment has proposed building a new laser facility in the United Kingdom. This will use 10 ns-class beams in conjunction with two, subpicosecond, petawatt-class beams to access plasma conditions inaccessible to even the largest megajoule-class facilities. Diagnostic techniques for the long pulse regime are fairly mature, whereas techniques in the short pulse regime are still evolving. This article describes the development of a suite of target diagnostics to exploit the high temperature, high density plasma conditions that will be achievable on the Orion laser, and discusses some of the opportunities and problems that will be encountered in attempting to combine the two sets of techniques.