Development of a Superconducting Rf Module for Acceleration of Protons and Deuterons at Very Low Energy

A prototype superconducting accelerator module housing six 176 MHz half wave resonators and three superconducting solenoids is currently under production at ACCEL as part of a 40 MeV linear accelerator at the SOREQ NRC [1]. The module will accelerate protons and deuterons from energy of 1.5 MeV/u up to 6.5 MeV. The design is based on a peak electric field of 25 MV/m and maximum 10 W of power dissipated into the helium bath by each cavity. Main design considerations of the cavities, solenoids, tuners and couplers as well as for the module especially in view of assembly and alignment will be presented. Cold test results of the cavities obtained in ACCEL’s new cold RF test facility will be presented. All components like tuners, helium vessel, solenoids and couplers as well as thermal shield, magnetic shield and vacuum vessel have been produced in the meantime and the assembly of the module is almost finished. The module will be RF tested at 4.2 K at ACCEL before shipment to Israel in order to demonstrate cavity performance after assembly. MODULE DESIGN CONSIDERATIONS The prototype superconducting module has to accelerate protons and deuterons at very low energy. Beam dynamic calculations show, that the space between adjacent cavities must be then minimized in order to preserve the good beam quality from the gun and RFQ and achieve a good emittance at the exit of the module. Therefore it was decided to integrate superconducting solenoids into the SRF module as it is also done at other institutes like Argone and Triumf for modules housing quarter wave cavities. Backing coils ensure that the stray field of the solenoids are well below the critical magnetic field of niobium at the location of the halfwave resonators to avoid performance degradation of the cavities. Also from beam dynamic calculations the required alignment tolerance of cavities and solenoids was determined to be 0.3 mm in transverse direction. In order to achieve this alignment also after cooldown, it was decided to use a stainless steel reference frame produced out of 316LN with intermediate annealing during the manufacturing process to remove stresses thus avoiding deformation during cooldown. All cavities and solenoids are aligned in respect to this reference frame which can be machined very precisely with todays large CNC milling machines. Table 1: Main Design Parameters for the Prototype Superconducting Module Dimensions and weight Height 3 m Width: 1.2 m length: 2.6 m weight: 6 tons Thermal mass 80 MJ @ 4 K 20 MJ @ 70 K Volume of liquid helium 200 l Crogenic losses 10 W static @ 4 K 70 W total @ 4 K 120 W @ 70 K Operating pressure (temperature) 1.2 bar (4.4 K) Beam pipe diameter 30 mm Solenoid field < 6 T Cavity design gradient Epeak = 25 MV/m Bpeak@Epeak=25 MV/m 53 mT Uacc@Epeak=25 MV/m 0.85 MV Beam current 2 (4) mA Coupler power 2 (4) kW cw