Free electron laser optical axis measurement system

The polarization of the gamma-ray beam plays a critical role in experimental photonuclear research by probing angular momentum. For example, the multipolarities of the 80Se(g,n)79Se reaction can be assigned by measuring cross-sections relative to the plane of polarization*. Dynamic control over gamma beam polarization will open new opportunities in nuclear research, particularly by allowing relative asymmetries to be calculated without the uncertainty introduced by relative detector efficiency. A gamma-ray beam with rotational linear poarization and high polarization purity (Plin ~ .99) has been demonstrated at the High Intensity Gamma-ray Source (HIGS)**. Without active tuning by an accelerator physicist, polarization quality is degraded due to decoupling of the free-electron laser (FEL) axis and the electron beam orbit. The FOAMS is an active feedback system that is sensitive to the small centroid motions of the FEL optical axis. Measurement uncertainty characterization has been conducted. Ongoing work will utilize this feedback system to automatically sustain controllable gamma-ray polarization for nuclear physics experiments.