Emittance Reduction in Photo-Cathode RF Gun for the PAL XFEL

Abstract

Photo-cathode RF gun, a high-brightness electron source with extremely low emittance, is highly required for future light sources such as X-ray free electron lasers (XFEL). A BNL Gun IV type photocathode RF gun has been fabricated and tested at Pohang Accelerator Laboratory (PAL). The gun test stand for the photoinjector consists of the BNL 1.6 cell photocathode RF gun, a single emittance compensation magnet, and the beam diagnostics. The beam energy has maximum at the laser injection phase around 50°. The relative energy spreads at the laser injection phase around 50° is about 0.5% rms. Best normalized transverse rms emittance attained 1.5 mm mrad at the 0.1 nC beam charge. A coupling hole between the waveguide and the cavity of a photo-cathode RF gun causes asymmetries in the RF fields at the coupler cell. The dipole and quadruple fields are the dominant sources of the transverse RF emittance growth. In the BNL Gun-III, the dipole field is reduced by adding a symmetric pumping hole at the opposite side of the waveguide coupling hole. The dipole field can be reduced further by adjusting the size of the pumping hole. However, the quadruple field cannot be suppressed by the single pumping hole. We have designed new RF cavity in which the quadruple field as well as the dipole one is suppressed. In this design, two additional pumping ports are placed at 90° positions with respect to the coupling hole and the pumping holes. Beam dynamics simulation for newly designed RF gun shows that the vertical transverse emittance is reduced by about 60% compared to the old one.