Negative halogen ions for fusion applications (invited)

Over the past quarter century, advances in hydrogen negative-ion sources have extended the usable range of hydrogen-isotope neutral beams to energies suitable for large magnetic confinement fusion devices. Recently negative halogen ions have been proposed as an alternative to positive ions for heavy-ion fusion drivers in inertial confinement fusion, because electron accumulation would be prevented in negative-ion beams, and if desired, the beams could be photodetached to neutrals. This article reports an experiment comparing the current density and beam emittance of Cl+ and Cl- extracted from substantially ion-ion plasmas with that of Ar+ extracted from an ordinary electron-ion plasma, all using the same source, extractor, and emittance scanner. At similar discharge conditions, the Cl- current was typically 85%-90% of the positive chlorine current, with an e(-)/Cl- ratio as low as 7 without grid magnets. The Cl- current was as much as 76% of the Ar+ current from a discharge with the same rf drive. The minimum normalized beam emittance and infer-red ion temperatures of Cl+, Cl-, and Ar+ were similar, so the current density and optical quality of Cl- appear as suitable for heavy-ion fusion applications as a positive noble gas ion of similar mass. Since F, 1, and Br should all behave similarly in an ion source, they should also be suitable as driver beams. (c) 2006 American Institute of Physics.