PRELIMINARY DESIGN OF A 352 MHZ DRIFT TUBE LINAC WITH EXTERNAL FOCUSING

For a Superconducting Proton Linac (SPL) injecting into the CERN PS, as well as for most of high energy SC linac proposals, an optimum transition energy between low energy Room Temperature (RT) and high energy Super Conducting (SC) cavities lies somewhere between 100 and 200 MeV [1].For the aims of the preliminary SPL feasibility study the transition energy does not need to be optimised, and it has been somehow arbitrarily fixed to 150 MeV.The optimum output energy of the RFQ injector being around 5 MeV, a specific RT structure is needed to cover the range between 5 and 150 MeV.RF standardisation with the SC section advises to operate also the RT part at a frequency of 352.2 MHz.This study first compares the features of four different structures suitable for this energy range, and then concentrates on a design based on a Drift Tube Linac (DTL) made up of short (8 βλ long) tanks without quadrupoles inside the drift tubes.In this design, doublets or triplets between the tanks can provide the focusing.The RF structure has been optimised, and then a complete linac layout between 5 and 150 MeV has been outlined.The multiparticle beam dynamics is studied by means of the code PARMILA and of a specially written interface program, to allow the analysis with PARMILA of strings of DTL tanks with external focusing.