Cairo University

MTPR Journal

 

A HIGH-FINESSE FABRY–PEROT CAVITY WITH A FREQUENCY-DOUBLED GREEN LASER FOR PRECISION COMPTON POLARIMETRY at JEFFERSON LAB

2016-12-17
Mohamed A. Hafez, Abdurahim Rakhman, Sirish Nanda, Al W. Tobias, Gordon D. Cates
Thomas Jefferson National Accelerator Facility, Newport News,
VA 23606, USA

Vol./Issue: 16 , id: 200

In Jefferson Laboratory, Hall A experiments using a polarized electrons beam require measurement of the beam polarization accurately. The Hall A Compton Polarimeter (CP) utilizing the principle of Compton scattering of electrons from a circularly polarized photons measures the polarization of the electron beam from cross section asymmetry of scattered photons and electrons. CP is a superior technical solution since it is non-destructive for the electron beam. A high-finesse Fabry–Perot cavity with a frequency-doubled continuous wave green laser (532 nm) has been built and installed in Hall A of Jefferson Lab for high precision Compton Polarimetry. The infrared (1064 nm) beam from a ytterbium-doped fiber amplifier seeded by a Nd:YAG nonplanar ring oscillator laser is frequency doubled in a single-pass periodically poled MgO:LiNbO3 crystal. The maximum achieved green power at 5 W infrared pump power is 1.74 W with a total conversion efficiency of 34.8%. The green beam is injected into the optical resonant cavity and enhanced up to 3.7 k W with a corresponding enhancement of 3800. The scattered electrons and photons are detected using an electron and photon detectors. All the devices of the setup are installed on an optical table which is located between magnetic dipoles (Polarimeter Chicane). The polarization transfer function has been measured in order to determine the intracavity circular laser polarization within a measurement uncertainty of 0.7 %. The PREx experiment at Jefferson Lab used this system for the first time and achieved 1.0 % precision in polarization measurements of an electron beam with energy and current of 1.06 GeV and 50 μA.