Cross Section of Accelerator Cavity

This pictures shows a cross section of the electron accelerator that will be installed.

The cavity consists of two cells: one 0.6 cell and one full cell. The design frequency is 2998.5 MHz (about 10 cm wavelength in vacuum). The RF waves are coupled in axial by the coaxial line. The RF waves are converted from rectangular to coaxial waveguide by the RF input coupler. The inner counductor is hollow to pass the UV excitation pulse and electron bunch.

An ultrashort UV pulse illuminates the cathode surface. By the photolectrical effect electrons are emitted from the copper cathode (up to 1 nC). These electrons will experience the very high acceleration field (about 100 MV/m) inside the cavity. Due to the very high field strength the bunch leving the cavity will be still very short, about 1 ps. The final electron energy is little less than 5 MeV. The required RF power is roughly 8.5 MW peak power during a few microseconds.

Special feature of this cavity (designed by the Eindhoven University of Technology) is the fact that the cells are no longer brassed together but pressed by screws. Therefore, a stainless steel container is needed for vacuum purpose. However, due to the fact that the faces of the copper parts are turned by single diamand turning machines a very large compression rate is expected between the inner and outer vacuum sections.

On the outside of the copper cavity cooling and heating facilities are designed. There are in total three seperate cooling channels: 1- integrated in the cathode part, 2- integrated in the center iris part, 3- on the RF input coupler. There are two integrated heating elements (Thermocoax): 1- on the cathode part, 2- on the iris part. Two thermocouples will measure as good as possible the local temperature of the cells. Through the back flange of the stainless steel container several feedthrough can be seen to support all those features.

The back flange of the stainless steel container contains also a bucking coil. This magnet compensates locally on the cathode surface the magnetic field generated by a big magnet around the cavity. For electron beam optical reasons the magnetic field has to be zero at the point where the electrons are generated.

Due to the cilindrical symmetrical design a magnet can be placed around the cavity to focus the electron beam exitting the cavity.