A hybrid ion source concept for a proton driver front end

A novel concept for creating intense beams of negative hydrogen ion beams has been devised, and first steps towards its realization have been taken. In this approach, an ECR plasma generator operating at 2.45 GHz frequency is utilized as a plasma cathode, and electrons are extracted instead of ions and injected at moderate energy into an SNS-type multi-cusp H^- ion source. This secondary source is then driven by chopped d. c. power, rather than rf power, but does not need filaments which are the cause for the rather short lifetime of conventional H^- sources. The development of this ion source is primarily aimed at the future beam-power goal of 3 MW for the Spallation Neutron Source (SNS) that will be pursued after the start of SNS operations. The first two phases of this development effort have been successfully passed: assembly of a test stand and verification of the performance of an rf-driven SNS ion-source prototype and extraction of electrons with more than 200 mA current from a 2.45-GHz ECR ion source obtained on loan from Argonne National Laboratory. An electron-extraction chamber that joins these ECR and H^- sources has been fabricated, and the next goal is the demonstration of actual H^- ion production by this novel, hybrid ion source. This paper describes the source principle and design in detail, reports on the current status of the development work, and gives an outlook on future lines of development.