A Tuner for the 350 MHz Superconducting CH-Structure* A. Bechtold 1 , M.Busch 1 , H. Liebermann 1 , H. Podlech 1 , U. Ratzinger 1 1 IAP, Universität Frankfurt, Germany. Abstract The superconducting CH multi-cell prototype cavity, which has passed a very succesfull power test recently leading to an accelerating gradient of E a = 7 MV/m, will be equipped with a frequency tuning system [1]. The rf- tuning during operation bases on the principle of a slight elastic deformation at both ends of the tank. This is causing a change in the gap width of the first and last accelerating cell and the accompanying variation of capacity finally results in a frequency shift. The mechanical resonances of the cavity have been investigated experimentally in the environment of an acoustical laboratory at room temperature and within the vertical cryostat at 4 K. Moreover, an active periodic cavity detuning provided by the piezo tuners was implied, while stable superconducting cavity operation was kept by a frequency control loop acting on the rf-frequency oscillator. Tuner Design The frequency tuning device comprises two stages: A slow mechanical tuner with a tuning range of Δf mech. = ±1 MHz and a fast piezo tuner operating in an expected range of Δf piezo = ±1 kHz. The piezos will be inserted into the beam pipe, between the inner cold mass containing the helium and the outer room temperature vacuum vessel (fig. 1). Figure 1: Scheme of the cryostat with tuner positioning on top left. Three piezo elements will be used for the fine tuning. Microphonics To avoid instabilities in the control system it is impor- tant to care about mechanical resonances of the cavity and their impact on the resonance frequency. Very low reso- nances can then be damped or pushed to higher frequen- cies if necessary. We have measured the resonances by using one of the piezos as an actuator stimulating the cav- ity with either a sinusoidal signal from an acoustic wave generator or with white noise comprising all frequencies between 0 and 100 kHz, alternatively. The response of the cavity was then detected by a microphone at room tem- perature or by a second piezo used as a detector in the cryostat and was digitally recorded (fig. 2). These wave data were Fourier analyzed subsequently. Figure 2: Microphonics at cryogenic temperatures and the impact on the rf-resonance. The VCO (Voltage Controlled Oscillator) signal indicates the action of the control loop to provide stable operation. Power Test In former tests the cavity performance was limited. Af- ter surface preparation at ACCEL the performance in- creased significantly (fig. 3) [2]. Figure 3: Measured Q-value as a function of accelerating gradient E a . References [1] A. Bechtold, M. Busch, H. Liebermann, H. Podlech, U. Ratzinger, "A tuner for a Superconducting CH-Prototype Cavity", SRF2007, Peking. [2] H. Podlech, A. Bechtold, M. Busch, H. Klein, H. Liebermann, U. Ratzinger, “Development of the su- perconducting CH-cavity and Applications to Proton and Ion Acceleration”, SRF2007, Peking. piezo *project supported by by GSI, BMBF contr. No. 06F134I and EU contr. No. 516520-FI6W INSTRUMENTS-METHODS-46 252