Physics design studies of 10 MeV, 325 MHz drift tube linac for the Indian spallation neutron source

A 1 GeV H^- injector linac is being designed at RRCAT for the proposed Indian Spallation Neutron Source. The front end of the injector linac will use Radio Frequency Quadrupole which will accelerate the H^- beam from 50 keV to 3 MeV. In order to accelerate the beam from 3 MeV to 10 MeV, two design choices are available: Superconducting Single Spoke Resonators (SSRs) or Normal Conducting Drift Tube Linac (DTL). Earlier, the electromagnetic design of SSRs was performed in this energy range. However, for the pulsed operation, superconducting structure appears to be expensive compared to DTL. We have therefore performed the physics design studies of DTL to explore this option, which we describe in this paper. We have optimized the geometry of individual DTL cells using DTLFISH for maximum effective shunt impedance per unit length. Beam dynamics studies have been performed using the code PARMILA. A suitable lattice of quadrupole magnets housed in the drift tubes has been evolved to keep the beam focused in transverse plane, while ensuring that the quadrupole gradients are practical. The phase and amplitude of electric field in the RF gaps have been fixed to accelerate the beam from 3 MeV to 10 MeV, while keeping the beam focused in longitudinal plane. Various stringent design criteria have to be satisfied in order to avoid the onset of instabilities that can lead to beam loss. Evolved lattice ensures maximum beam transmission with minimum emittance growth. MDTFISH has been used to tune the cells for the desired electric field profile in the DTL tank. (author)