Target positioning and alignment on the Astra-Gemini facility

The drive to ever higher intensities and the move to shorter focal length reflective optics for focussing in solid target interactions are increasingly important for studies into high intensity secondary source generation, QED and high field studies. To ensure reproducible optimum interaction conditions, presents a significant problem for accurate target positioning. Commercial optical systems exist to aid the imaging and positioning of targets. However, these are often expensive and difficult to situate within the limited space usually available inside the interaction chamber. At the Astra-Gemini system of the Central Laser Facility, the push for intensities above I = 10 21 Wcm -2 with f/2 and f/1 focussing optics means positioning targets within the Rayleigh range of < few microns. Here, we present details of two systems to be implemented on the Astra-Gemini system to cheaply and accurately position targets with ≈ micron accuracy. These involve:- (i) a multi-wavelength interferometer to enable sub-micron accuracy in the positioning of the front surface at the interaction point within the Rayleigh range and (ii) a small, low cost near field/far field microscope with illumination at 800nm (the same as the Gemini IR beam) for imaging the rear of the target and the focal plane with high resolution. The combination of these two systems significantly improves our accuracy in target positioning and also results in a decrease in the time required to align targets between shots.