Gemini: A high-repetition rate, ultra-intense dual beam laser facility for plasma physics experiments in extreme conditions

We report the latest developments at the Gemini laser system at the Central Laser Facility of the STFC Rutherford Appleton Laboratory. This dual beam facility can deliver 15 J, 30 fs pulses per beam with a shot in every 20 seconds. Gemini recently completed a series of electron acceleration experiments using gas jets and gas-filled capillaries with the first beamline. The second beamline is expected to be online in 2009. Developments are being made in the target area to incorporate complex solid targets at the maximum repetition rate. With a F/2 focusing geometry, Gemini will have access to a range of intensities up to 10²² W cm^-2 at high repetition rates. With a 10⁷-10⁸ contrast level, the ASE and pedestal preceding the main pulse can cause a preplasma and damage the targets, especially the complex ones, before the main pulse arrives. At these levels, the routine use of a minimum of two plasma mirrors is required to prevent this. We have designed a fully motorized plasma mirror system. The geometry requires a synchronised raster scan of both substrates and the reflectivity of the first plasma mirror defines the distance to the second. The raster routine has been set to incorporate this so that the plasma mirror system takes the maximum number of shots between optic changes.