A ug 2 00 0 STATUS OF THE RFD LINAC STRUCTURE DEVELOPMENT

The Proof-of-Principle (POP) prototype of the Rf-Focused Drift tube (RFD) linac structure is currently under test at Linac Systems, after years of delay due to a variety of technical problems. A discussion of these technical problems and their solutions will be presented. The status of these tests will be reported. Plans for future development of this linac structure will be revealed. Potential uses of this linac structure for a variety of scientific, medical, and industrial applications will be described, including: proton linac injectors for proton synchrotrons, compact proton linacs for PET isotope production, epithermal neutron sources for the BNCT application, energy boosters for proton therapy, compact portable neutron sources for thermal neutron radiography, and pulsed cold neutron sources for cold neutron physics and related applications. The POP prototype [4,6,9] came into operation on June 19, 2000 in the Linac Systems laboratory in Waxahachie, TX. We had about 9 mA of 25-kV beam entering the RFQ and about 9 mA of beam transmitted through the RFQ and RFD linac structures. That beam impinged on a 1.25-MeV-thick absorber foil and Faraday cup assembly. The beam that passed through the absorber foil and into the Faraday cup (the accelerated beam) showed the expected threshold for both rf fields and injection energy. Because we suffer from an inadequate amount of rf power (180 kW) and are operating very close to the linac excitation threshold, the accelerated beam corresponded to only about 4% of the transmitted beam. Nevertheless, we now have conditions that produce a steady 0.3-mA beam of protons at 1.636-MeV for hours at a time. The initial operation was sporadic as a result of several problems (in addition to the near threshold operation), which have now been rectified. We have always had a " breathing " phenomenon, which we attributed to some mechanical vibrations in the drift tube stems. A description of the solution to this problem is given below. We often witness a 10% decline in RFQ fields after 15 seconds of operation. A description of the solution to this problem is also given below. We are confident that, with adequate rf power and the improvements that are identified below, the performance of the RFD linac structure will come up to our expectations. The RFD linac structure [1-4] resembles a drift tube linac (DTL) with radio frequency quadrupole (RFQ) focusing incorporated into each drift tube. The RFD drift tubes comprise two separate …