Active Pointing Stabilization of MW Peak-Power UV Laser Beam for Laser-Assisted Charge Exchange

The Laser-Assisted Charge Exchange (LACE) experiment at the Spallation Neutron Source (SNS) accelerator in Oak Ridge National Lab aims to overcome long-standing limitations associated with the foil-based charge stripping technique to produce high-intensity proton beams. Pointing stability of the high energy (3 MW peak-power at 10 Hz) UV laser (355 nm) beam at the sub-mm level is critical to maintain high efficiency and high reliability of the experiments. An un-evacuated laser transport line (LTL) was retrofitted into the accelerator tunnel to transport the laser beam 65 m from a separate building above ground to the laser-particle interaction point (IP). Due to the high laser power and improvised nature of the LTL, the laser-LTL system suffers from pointing instabilities at the IP in the form of drift and pulse-to-pulse jitter. Pointing instabilities are caused by thermal effects on the optical components, mechanical vibrations and temperature fluctuations along the LTL. A closed-loop beam drift stabilization system was developed using a CMOS camera, LabVIEW based computer image processing, and a piezo-driven steering mirror. The system is capable of making corrections to the pointing at the optimal rate of 10 Hz with high reliability and high position detection accuracy in high radiation environments. A laser beam pointing stability of 3.2 μrad (rms) with a corresponding beam drift of 200 μm (rms) has been achieved at the IP located 65 m away from the laser.