Control of amplified spontaneous emission in the laser system for intense laser plasma acceleration research

One of the emerging trends in new accelerator techniques is the laser based acceleration of particles, known as laser - plasma acceleration. Interaction of intense ultra short laser pulse with matter generates oscillations in hot plasma and the subsequent acceleration of electrons in a wakefield .Electrons can be accelerated to around 100 MeV within 1 mm distance. Ion beams can also be produced by laser plasma acceleration, though with a broader energy spectrum than achieved with electrons. Since laser- plasma acceleration offers the potential of compact accelerator technology, at BARC work has begun on the development of 20 terawatt pulsed laser system based on Nd:glass. Whether the laser plasma accelerator can produce high quality particle beams , depends on the characteristics of the laser pulse itself. These laser systems generally use the technique of chirped pulse amplification, where, after amplification the laser pulses are recompressed to obtain the high intensity required for laser plasma acceleration. The recompressed laser pulse temporal profile is very crucial for the best particle acceleration. The pulse should be free of a pedestal and pre-pulse, giving a high intensity contrast ratio (ratio of intensity of the short pulse to the intensity of the pedestal) for the recompressed pulse. The pedestal and pre pulse contains enough energy to create a plasma when the laser pulse is focused on target, before the arrival of the powerful femto-second range pulse. Amplified spontaneous emission (ASE) in the amplifier stages give the main contribution to the harmful pedestal. The optical scheme used in the front end amplifier stage that has been designed and developed to reduce the ASE is described. A detailed numerical analysis using Frantz and Nodvik rate equation model was done for the amplifier. We have found that by introducing an angular misalignment , inducing a differential loss for the main pulse to be amplified and the spontaneous emission, it is possible to increase the intensity contrast ratio by two orders of magnitude in the recompressed pulse. In the experimental set up too , for the amplification of the laser pulse, we have incorporated this method of misalignment to reduce amplified spontaneous emission . This should improve the temporal profile of the final compressed pulse. (author)