Cathode Tuning and Cooling System
The cathode is geometrically adjusted by the cathode alignment and cooling system. This system allows the cathode to be centered in the cathode channel as well as shifted in the longitudinal direction in the range of a few millimeters.
Inside the cryostat the cathode rod is fixed to the cathode cooler by a bayonet socket system. A spring affords the force between the touched cone areas of the photocathode body and the cooling body. The normal conducting cathode is cooled by liquid N2.
Care must be taken not to deteriorate the performance of the superconducting cavity by introducing the cathode. According to estimations, the maximum power introduced into the cathode by shining with the laser is in the order of 1 W. The power induced by the cavity RF field is in the order of 10 W. Due to the 1 mm gap between the cathode stem and the cathode channel the heat transfer from the cathode to the cavity walls can take place only by radiation. According to the Stefan-Boltzmann law for black body radiation for a given temperature difference between the cathode and the cavity walls, the power irradiated onto the cavity is in the order of 10 mW. This is much less than the heat generated in the cavity itself by the RF which is in the order of 10 W.
Staufenbiel, F. et al., Test of the photocathode cooling system of the Formula Not Shown cell SRF gun, Physica C 441(2006)1-2, 216-219