A TUNER FOR A SUPERCONDUCTING CH-PROTOTYPE CAVITY A. Bechtold, M. Busch, H. Liebermann, H. Podlech, U. Ratzinger, IAP, J. W. Goethe-Universitaet, Frankfurt, Germany. Abstract The superconducting CH multi-cell prototype cavity will be equipped with a frequency tuning system. 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 effects on rf-frequency and field distribution have been measured and were compared with previous calculations. The tuning system implies two stages, a slow mechanical device and a fast piezo system, all parts were manufactured already. Additionally, the mechanical resonances of the cavity have been investigated experimentally in the environment of an acoustical laboratory at room temperature and recently 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. INTRODUCTION Multi-cell CH-structures could improve the efficiency of DTL-structures for protons and light ions in the low and medium energy range [1,2]. A superconducting CH- prototype (fig. 1) has been developed at IAP in Frankfurt to investigate the potential of that structure experimentally. It has 19 accelerating gaps, the resonance frequency is 360 MHz and the achievable accelerating gradient was expected to be E acc = 6 MV/m. First cryogenic tests performed in July and September 2005 and in January 2006 ended up with only E acc = 4.7 MV/m and an increasingly intensive X-radiation towards higher gradients [3]. Detailed investigations with thermo luminescence detectors (TLDs) allowed a precise localization of field emitter [4]. It was then decided to repeat the chemical surface treatment (Buffered Chemical Polishing, BCP) removing 50 μm material from the inner surface followed by High Pressure water Racing (HPR). A recent cryogenic test ended up very successfully with E acc = 7 MV/m. The rf-oscillator just followed the frequency changes of the resonator in earlier tests. Recently, active periodic tuning by piezo actors was successfully included in the cold test. Within the frame of the HIPPI project, an appropriate tuning system for the multi gap CH-structure, then operated at a horizontal cryostat, is under development. Figure 1: CH-prototype and investigated piezo actuator. Operating frequency 360 MHz Relativistic β 0.1 Total length of tank 1048 mm Active length (βλ-definition) 810 mm Tank diameter 280 mm # accelerating gaps 19 Accelerating field gradient E acc. (meas.) 7 MV/m Surface peak field gradient E peak 36 MV/m Maximum magnetic flux B peak 40 mT Accelerating voltage U acc. 5.6 MV Table 1: CH-cavity parameters and measurements. RF RESPONSE ON APPLIED EXTERNAL TUNING FORCES An elastic deformation of the cavity by applying an pushing or pulling force at the end flanges of the tank changes the width of the outer most accelerating gaps and thus gives an efficient possibility to tune the rf-frequency during operation [4]. The rf-response of the cavity was simulated with MWS [5] and by multi-physics program COMSOL [6] (fig. 2). These calculations can now be compared with a measurement which had been executed at room temperature. The force was applied by the outer corset that allows a variation of the cavity length. From simulation results a force of F def. = 6.5 kN must be applied to have a deformation of Δx = 1 mm at both ends of the Proceedings of SRF2007, Peking Univ., Beijing, China 618 WEP54 WEP: Poster Session II