First operation of a superconducting rf photoelectron gun

In the last year at the Forschungszentrum Rossendorf, in cooperation with the Budker Institute Novosibirsk, DESY, the Max-Born-Institut Berlin, and ACCEL, a superconducting rf photoelectron gun (SRF gun) has been developed [1]. On March 5, 2002 this gun is going into successful operation. A half-cell niobium cavity has been cooled down to 4 K. In this cavity a CsTe2 photocathode is installed, isolated electrically and thermally by a vacuum gap. The Q value and the accelerating field strength in the cavity have been measured. For 4 K, Q=2x108 and an accelerating field of 14 MV/m have been obtained. This value did not change during four weeks of operation time. The energy of the electron beam is 0.62 MeV and the bunch charge is 26 pC. The laser works with a duty factor of 25% and with a repetition rate of 26 MHz inside the macropulse. The average laser power in the macropulse is 1.5 W (l=260 nm), and the length of the micropulse is 5 ps FWHM. This corresponds to an average electron current of 170 µA and 680 µA in the macropulse. The isolated cathode allows the separate measurement of the cathode current and the current inside the beam dump as a function of the laser phase. The next measurements are planned for late May.

[1] E. Barthels et al., Nucl. Instr. and Meth. A 445 (2000) 408