Experimental observation of breakdowns in the Fermilab RF Gun G 4 ∗

Fermilab has developed and delivered to DESY Hamburg two RF guns for the operation of the phase I of the TESLA Test Facility (TTF) accelerator. The first RF gun (designated G3 in the following) has been operated on TTF from October 1998 to March 2002. G3 presented a reliable behaviour with short RF pulses (< 300 μs ). For longer RF pulses, G3 suffered from breakdowns occurring in the coupling slot located between the waveguide and the full-cell of the gun [1]. These breakdowns were followed by photo-multiplier or vacuum interlocks and for RF pulses longer than 500 μs the breakdowns rate increased to an unacceptable value. Therefore, a second RF gun (G4) has been installed in June 2002 on TTF. G4 was in operation at the A0 photo-injector at Fermilab since January 1999 where it has been successfully conditioned at 1 Hz repetition rate for RF pulses up to 550 μs. G4 has been operated at DESY with long RF pulses (up to 900 μs), high forward power (up to 2.8 MW) and a repetition rate ranging from 0.1 to 2.0 Hz. Similar to G3, breakdowns were observed at DESY during the operation of G4 as well. This paper describes the dependency of the breakdowns measured at DESY on G4 with respect to the RF pulse length, forward power and repetition rate. A part of the work presented in this paper is also discussed in the reference [2].