Measurements of buried layer heating with the VULCAN Petawatt laser

Abstract

Summary form only given. Petawatt lasers have several important applications including fast ignition of fusion targets and the development of compact X-ray and charged particle sources. Understanding the transport and energy deposition of the relativistic electron beam produced by the absorption from ultra-intense lasers is a crucial part of these applications. We have used the thermal X-ray emission of aluminium tracer layers in a CH substrate to diagnose the temperature as a function of depth. With laser intensities around 10/sup 20/ W cm/sup -2/ and a pulse length of /spl sim/800 fsec we find temperatures of 650 eV-450 eV at depths of 8 micron to 17 micron. Deeper layers show strongly non-thermal emission characteristic of 'hollow atoms' with multiple inner shell vacancies. We have used the hybrid plasma simulation code LSP to model the transport and energy deposition from the relativistic electron beam and find broad agreement with our measurements which is improved if some laser pre-pulse is added to the model which has the effect of inhibiting the penetration of the relativistic electrons. The LSP simulations also show that the aluminium tracer layer should be significantly hotter than the CH substrate due to enhanced resistive heating, but with very thin tracer layers the electrons responsible for exciting the X-ray emission are not in equilibrium at the higher temperature due to finite mean free path effects (non-local transport).