Active Pointing Control of Pulsed, High-Power Laser Beam After 65-Meter Transport

Laser-Assisted Charge Exchange (LACE) requires the position and angle of a high-power laser beam to be stable at the sub-millimeter and sub-milliradian level at a point 65 meters from the laser. Due to the high laser power and improvised nature of the laser transport line, the laser beam suffers from pointing instabilities in the form of drift and pulse-to-pulse jitter. A closed-loop Laser Pointing Control System (LPCS) has been developed in the lab and deployed to the field to control and stabilize both the position and angle of the laser beam. The LPCS relies on feedback between CMOS cameras and beam relay mirrors on kinematic mounts controlled by a PC running custom LabVIEW software. The system has been demonstrated to reduce the standard deviation of beam position to 200 μm and of pointing angle to 300 μrad at the interaction point, corresponding to a reduction of ~2x.

I. Introduction

The Laser-Assisted Charge Exchange (LACE) experiment at the Spallation Neutron Source (SNS) particle accelerator in Oak Ridge National Laboratory aims to overcome long-standing limitations associated with the foil-based charge stripping technique to convert a pulsed beam of H^- ions into protons [1],[2]. The intermediate step in the LACE process is the interaction of the particle beam with the high-power laser beam. The laser produces 50 ns pulses of 355 nm UV light at a rate of 10 Hz, with 2 MW peak-power [3]. To convert the beam of H^- ions into protons with high efficiency, the laser beam at the laser-particle interaction point (IP) must fulfil stringent stability requirements. To achieve >90% charge-exchange efficiency, the laser beam must be centered on the IP within a range of ±0.5 mm of the design, while the laser-particle crossing angle cannot deviate by more than ±400 μrad.