Superconductive test cavity for the ESS

Abstract

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 Q 0 =2×10 9 , 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 Q 0 and E acc . 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. OPERATIONAL CHARACTERISTICSThe 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 Q 0 as accurate as possible. Using the 500W amplifier we can achieve accelerating fields up to nearly 8MV/m without detecting a quench, but the measured Q 0 degradation at increasing RF field strength and the monitored x-ray levels point to active field emitters inside the cavity. At higher field levels we also saw moving light spots on the vacuum window (glass) which seem to be generated by strong electron loading. Even the mirror installed outside the cavity to allow an indirect view with a camera was tarnished by a brown film. The estimated dose of the mirror is about 30Gy during one minute. We can switch from the 500W amplifier to a SIEMENS 25kW transmitter on loan from ACCEL. We checked that transmitter and incorporated it into a high power system with a circulator and a high power switch. That allows tests of the SIEMENS transmitter and simultaneously use the 500W amplifier for cavity tests or vice versa [3]. The input coupler of the cavity is adjustable from nearly critical coupling to a loaded Q of the order of 1E6. We can reach a filling time of about 2ms with this coupling and 30kW RF power. After filling the cavity and maintaining the flattop by switching back to 7kW to simulate the beam, the RF power is switched off 1ms later (Fig. 2). Even at a repetition rate of 50Hz, we reached a field-level of about 5MV/m in this mode as guaranteed by ACCEL applying a forward power of about 30kW. We can only operate without the Phase-locked-loop if the microphonic noise is minimized by switching off the cryopump and if the system is appropriately detuned. We have conditioned the coupler and the cavity at this pulse scheme...

Beam steering in superconducting quarter-wave resonators: An analytical approach

“…This steering effect strongly depends on synchronous phase, beam velocity, accelerating gradient, and resonator geometry [1][2][3][4][5][6]. This is usually not critical in normal conducting resonators; in superconducting ones, due to the very high gradient and to special constraints in the cavity design, the magnetic component can be very strong and steering could become intolerable, especially for acceleration of high q=A beams [7]. We want to derive a formula, valid for any realistic E a , , and , which describes QWR steering as a function of calculable geometrical constants and transit time factors in the form of simple analytical expressions.…”