Focal spot smoothing experiments on the Vulcan glass laser system

Abstract

Laser/plasma interaction experiments have generally suffered from poor focal spot quality. This problem is a direct result of the coherence of the laser. A number of different methods1,2 have been developed to modify the coherence properties of the laser in an attempt to produce a smooth focal spot profile. In designing a scheme suitable for Vulcan a number of constraints have to be considered: the laser system must remain flexible to meet the needs of a broad based user community; the focal spot requirements differ significantly between the different groups and all have to be catered to; and the cost of any scheme has to be limited. The method chosen was a combination of induced spatial incoherence (ISI) and random phase plates (RPP). To produce ISI a short coherence length oscillator is used. To generate spatial incoherence transparent echelons are placed at the output of the rod amplifier chain before the final disk amplifier stages. This method as used on Vulcan has a limit on the size of focal spots available. A second smoothing method is therefore used: RPPs are inserted into each beam to control the divergence of the laser. This hybrid approach to focal smoothing has proved extremely successful.