SCALE ROOM TEMPERATURE MODEL OF THE SUPERCONDUCTING RFQ1 FOR THE PIAVE LINAC V. Andreev, G. Bisoffi, M. Comunian, A. Lombardi, A. Pisent, A.M. Porcellato INFN-LNL, Padova, Italy T. Shirai Nuclear Science Research Facility, Kyoto University, Kyoto, Japan Abstract The new injector PIAVE (Positive Ion Accelerator for Very low Energy) will use two bulk niobium (Nb) superconducting RFQs (SRFQ1 and SRFQ2), which will be operated at 80 MHz. The electrode length of SRFQ1 is about twice as long as the being built SRFQ2 thus requiring an EBW vacuum chamber larger than the previously available one. A principal decision was hence taken to split the electrodes and the tank into two parts. A half-scale room temperature model was built at LNL in order to test RF characteristics of the structure and to determine its exact dimensions. Results of the model measurements and M.A.F.I.A. code simulations are presented. 1 INTRODUCTION PIAVE injector is now under construction at LNL for the superconducting (SC) linac ALPI. The beam, generated by an ECR source, will be bunched by a three harmonic buncher and accelerated by two SC RFQs, followed by eight SC QWRs, for an equivalent voltage of 8 MV (U +28 beam) [1]. The cavity design of both SRFQ1 and SRFQ2 is based on the 4-rod structure with 90 o -apart- stem arrangement [2]. The components of the cavities will be manufactured from 3 mm thick niobium (Nb) sheet and will be assembled by electron beam welding (EBW). SRFQ2 is being already manufactured in LNL [3]. A design of SRFQ1 was done based on the M.A.F.I.A. code simulation for the case of unmodulated electrodes [4]. Since SRFQ1 electrodes are about twice as long as those of SRFQ2, it is impossible to weld them in the EBW vacuum chamber used for SRFQ2. We decided to split the vanes and the tank into two parts. Finally the two tanks will be welded by EBW into a single cavity. The cutting point of the unmodulated electrodes should be at the center between the stems ( the point of zero longitudinal current ) in order to minimize the field disturbance of the gaps. However, for modulated electrodes, the point should be slightly shifted to the position of electric field minimum closest to the position of zero longitudinal current. Since the welding of the electrode tips is technically impossible, a 1 mm gap between electrode parts is foreseen. RF contact between them will be provided by welding only proper Nb strips to avoid mode disturbance. A half scale aluminum model of SRFQ1 with modulated electrodes was constructed having the goal: 1) to determine the tank diameter for the required resonant frequency; 2) to optimize the stem position in order to obtain proper field distribution; 3) to test the effect of the vane cutting and the strip connecting the gap; 4) to determine alignment error tolerance. 2 ALUMINUM MODEL DESIGN The model ( Fig.1 ) allowed easy access to the central part of the structure and hence proper alignment of the electrodes. Figure 1: View of the half scale model of SRFQ1 (end flanges and one of the shells are not installed) It is first assembled in 4 parts by means of longitudinal and transverse aluminum bars. The assembling procedure of the structure was the following. At first each 1/4 structure was assembled independently: four stems, supporting the split (with 0.5 mm gap) electrode were installed onto the longitudinal bar and fixed. Then four such parts were assembled with respect to an external frame and aligned. After that four copper cavity shells were slided into proper slits with good RF contact. Finally two end flanges closed the structure. The first alignment procedure was the longitudinal positioning of the electrodes. Transverse positioning of the electrodes was obtained with respect to reference planes manufactured on each electrode. Positioning of the electrodes was provided with an accuracy of ± 25 μm. 965