A novel light source
How does it work ?
The production of the polarized gamma-ray beam at the HIGS facility is accomplished by scattering photons (few eV in energy) from high energy electrons (200 - 1200 MeV). To achieve this we employ a polarized photon beam (a laser) and a circulating electron beam. At the HIGS facility, the electrons which are used as the scattering target are also responsible for producing the laser photons, i.e, a Free-Electron Laser (FEL) system is used. This type of scattering in which the FEL photons and electrons share the same beam pipe is called intra-cavity scattering. The advantage of this setup over systems where an external laser is used is higher flux. The FEL photons are trapped in an optical cavity, which is part of the electron storage ring. The trapped photons are then scattered by high energy electrons to boost the photons from a few eV to MeV energies.
What makes HIGS beam unique ?
Intra-Cavity Scattering
At HIgS, the FEL optical cavity is part of the Duke electron storage ring (DSR). This implies that there is no external laser (unlike other backscattering sources). Since the electron beam and the photon bem must be colinear to achieve lasing, the injected second electron bunch is guaranteed to hit the reflected photons. This means that the system is self-aligning with respect to producing gamma rays. Since the electrons backscatter from the lased photons inside the lasing cavity, there is a gain of a factor of about 1000 compared to that obtained when using an external laser, since the lased light does not need to be extracted from the cavity.
100 % Polarization Transfer
HIGS FEL photons are produced in a polarized state by virtue of the electron motion in the optical klystron (OK-4). Upon Compton scattering this polarization state is completely preserved resulting in ~100 % polarized gamma-rays. The new OK-5 system will be capable of delivering both linear and circular polarizations, with beam polarizations of ~100% in both cases.
Background-Free Beam
Due to excellent vacuum conditions, continuous bremsstrahlung spectra is nearly absent at the HIGS facility. At a given energy, the ratio of signal ( peak) to background (continuous) is more than a million to one! The only source of beam related background is that generated by collimation, which is eliminated by post-collimation shielding.
Energy Resolution and Photon Density
Due to Compton scattering production of the gamma-ray beam, the energy spread and the angular acceptance as determined by collimation are correlated. This aids in selection of energy resolution of the beam. For experiments with reltively low energy resolution requirements (5-10%), relatively large collimators are used. Higher resolution (~1%) requires smaller collimators (0.5 cm) at the expense of the flux on target.
