JASRI/SPring-8, Hyogo-ken
At the SPring-8 1 GeV linac, a beam current or charge is measured by means of an integrator circuit. A signal from a current transformer is processed into an integrated voltage. The Fast Gated Integrator and Boxcar Averager Module (Stanford Research Systems) is presently used as the integrator. However we plan to expand a dynamic range and an integration time of the integrator. Because the noise level of the present integrator becomes too large for the expansion, we developed a low-noise and high-resolution integrator. Both the present and developed integrators have the same functions such as signal gating, accumulation of analog signal and sample hold. The principal noise of the integrator was found to be a switching noise of the gate switch. To reduce the switching noise a GaAs transfer switch SW-283-PIN (M/A-COM) was adopted as the gate switch. The experimental data of the developed integrator showed 1/10 of the noise level of the present integrator.
JASRI/SPring-8, Hyogo-ken
The acceleration gradient is limited by breakdown in an accelerating rf structure, including its surface condition of the inner wall. The surface treatment is an important technique to achieve the maximal acceleration gradient of an accelerating structure. We chose chemical etching as a method of surface treatment for accelerating rf structures made of copper. To study rf breakdown and effect of surface treatments, we used a pillbox-type single cell rf gun cavity. The highest cathode surface field (190 MV/m) of rf gun cavity was accomplished with this surface treatment under rf-conditioning elapsed time (21 days) in 2004. SPring-8 rf gun has been operating with the highest gradient in the world. This indicates that our treatment is considerably effective to improve the inner cavity surface made of copper. Further, we developed the high-pressure chemical etching for more complicated inner structures in 2006. Using a cartridge-type photocathode rf gun, high-field experiments were performed with cathode plugs chemical etching treated under deferent pressure condition. We report these results on highest gradient, using test copper samples treated with high-pressure chemical etching.
JASRI/SPring-8, Hyogo-ken
I developed a 3-D pulse shaping system in UV as an ideal laser for yearlong stable photoinjector. At SPring-8, the laser's pulse-energy stability has been improved to 0.7~1.4% at the UV (263 nm) under the laser environmental control included humidity. In addition, the ideal spatial and temporal profiles of an UV-laser pulse are essential to suppress emittance growth in an rf gun. I apply a deformable mirror that automatically shapes the spatial profile with a feedback routine, based on a genetic algorithm, and a pulse stacking system consisting of three birefringence Alpha-BBO crystal rods for temporal shaping at the same time. The 3D shape of the laser pulse is spatially top-hat (flattop) and temporally a square stacked chirped pulse. Using a 3D-shaped laser pulse with diameter of 0.8 mm on the cathode and pulse duration of 10 ps (FWHM), we obtain a normalized emittance of 1.4 pi mm mrad with a beam energy of 26 MeV. To keep the mirror away from beam axis, I developed a new hollow laser incidence with an axicon final focusing. Furthermore, I am developing a laser-induced Schottky-effect-gated photocathode gun using Z-polarization of the laser source with the hollow incidence.
