Pointing Control and Stabilization of the High-Energy UV Laser for Laser-Assisted Charge Exchange

Laser-Assisted Charge Exchange (LACE) is an experimental method of charge exchange injection into a proton accumulator ring that is being developed at the Spallation Neutron Source (SNS) in Oak Ridge National Laboratory (ORNL) as an alternative to hazardous injection foils. The current scheme of LACE requires a high-energy, low-repetition-rate UV (355 nm) laser beam (140 mJ pulses at 10 Hz) to be transported over 65 meters to the laser-particle interaction point (IP) in a high-radiation area of the accelerator. Thermal effects and other disturbances along the free-space laser transport line cause the beam to slowly drift away from the IP and jitter at a frequency comparable to the pulse repetition rate. A control system was designed, simulated, and constructed to stabilize the pointing of the laser beam to allow stable operation of the experiment. The laser pointing stabilization system is based on feedback between Complementary Metal-Oxide-Semiconductor (CMOS) cameras and a steering mirror with piezoelectric actuators. A PC running custom-made LabVIEW software acts a controller in open- and closed-loop modes, as well as a diagnostic tool. An analytical model of the system was used for optimization of the control law, and the system performs as well in the field as it did in laboratory tests. The laser pointing stabilization system eliminates the slow drift by keeping the beam aligned at the IP for an indefinite amount of time, and the jitter is reduced to the level of the pulse-to-pulse fluctuations.