SUPERCONDUCTIVE TEST CAVITY FOR THE ESS

Measurements at the prototype cavity were performed at the FZJ Jülich in order to support the design work of the SC accelerator part for the European Spallation Source (ESS) [1]. A shielded test facility has been constructed providing 25kW peak RF power operating at 500MHz [2]. The test cavity has been fabricated at ACCEL Instruments and was installed into the shielded area. This 5-cell cavity has been designed to a β value of 0.75 at an accelerating field of E acc> 5MV/m with Q0=2×109, operating at 4.2K. We have developed a cavity control circuit based on a fast analogue phase-lock loop allowing automated measurements. The first quasi static measurements using a 500W broadband amplifier almost verified the design values of Q0 and Eacc. The piezoelectric tuning system was used to record the spectrum of mechanical resonances of the cavity. A comparison of several computer simulations with measured results is presented. Furthermore, we have taken dynamical data of the Lorentz force detuning during pulsed-mode operation at a repetition rate of 50Hz. First results achieved with an active compensation of the Lorentz force detuning using the fast piezo frequency control system are also presented. 1 OPERATIONAL CHARACTERISTICS The first RF measurements were done during the acceptance tests of the cavity. We took the Q o-E acc curve (Fig. 1) at nearly critical coupling in order to measure Q0 as accurate as possible