A DTL with short tanks and external focusing for high power cw linacs

Recently, major projects, for the transmutation of nuclear waste, the production of tritium or more generally for the production of neutrons by spallation, have been proposed using high power linacs and a CW operation. In this context, the manufacturing of the low or medium energy part of the linac with a standard DTL, appears to be a tricky job, particularly the insertion of high gradient quadrupoles into the drift tubes, with a specific cooling system needed for CW operation, and many brazing or welding processes. Moreover the reliability and the maintenance of the DTL could be affected sooner or later. It is proposed in this paper to use a DTL with short tanks (let say 2 to 5 cells) with a focusing system outside the tanks. The mechanical design is in that way very simplified, the drift tube geometry can be optimised for a better shunt impedance, the cooling system is easy and efficient and the alignment problem is greatly reduced. This concept is applied to a 5 MeV to 100 MeV CW proton linac operating at 352 MHz, with a 40 mA beam current, and shows moreover that the linac global efficiency is increased compared with a classical approach. Preliminary beam dynamics studies are included