Precise and Ultra-stable Laser Polarization Control for Polarized Electron Beam Generation

Parity-violating scattering of helicity-flipping polarized electron beam has developed over the past decades into a key tool to study both the structure of electroweak interaction and the structure of nucleons. It has been used in the success of many nuclear physics programs such as HAPPEX in Jefferson Lab’s Continuous Electron Beam Accelerator Facility (CEBAF). The helicity-flipping polarized electrons are generated by using circular polarized laser whose polarization is flipped by a polarization controller. Raytum Photonics will develop a precise and ultra-stable circular laser polarization flipping system. This system uses our state-of-art transverse dual DKDP crystal modulator driven by our innovative digital voltage switching driver. It will help to achieve the objectives specified in the topic 29e to develop polarized electron beam sources with high flipping frequency (2 kHz repetition rate with 95% duty cycle and high flipping rate desirable for MOLLER experiment. We will collaborate with Jefferson Lab to minimize the system’s helicity-correlation asymmetries. First, we will optimize Pockels cell’s alignment and voltage with laser-table beam evaluation setup built by Jefferson Lab; secondly, we will test our close loop control circuitry with simulated signals of beam position and current monitors provided by Jefferson Lab. The system specs will be targeted as following. The laser beam position difference will be ~ 200nm without analyzer and ~ 400nm with S1 or S2 analyzer; the spot size asymmetry shall be < 10-4 (RMS); the helicity-correlated beam intensity asymmetry will be <5000ppm for 4 peak separation with S1 analyzer and <500 ppm with S2 analyzer. The average asymmetry shall drift no more than 20000 ppm per 30 minutes with S1 analyzer.