A 475.76 MHz Linac Chopper

The Cambridge Electron Accelerator uses an 'S' band (2854.56 MHz) linac as an injector, and a UHF (475.70 MHz) synchrotron rf acceleration system. A chopper is used to eliminate five out of the six linac bunches, and the sixth bunch can be injected at the optimum phase with respect to the synchrotron rf voltage. This reduces synchrotron oscillation, and reduces beam loading on the rf, thus allowing for much larger accelerated currents. The linac uses constant phase velocity waveguides (vp=c). The energy of the electrons introduced into the linac determines the trapping efficiency and the phase angle of the electron bunch with respect to the linac traveling wave. The chopper, situated between the gun and the linac waveguide, is a cylindrical 475.76 MHz cavity operating in the TM010 mode. It provides one half of the injection voltage(70 kv) for the selected electrons. The other half is provided by the gun pulse. Besides increasing the synchrotron intensity, the chopper system produced many other additional benefits.