Electron density measurements during transport of intense, MeV proton beams through low-pressure gas

Summary form only given. Light-ion-beam-driven inertial confinement fusion may require the transport of current-neutralized intense ion beams along several meters through low-pressure gas. Recent experiments have studied such transport of a collimated, 1-Mev, 1-kA/cm/sup 2/ proton beam produced by the Gamble II generator at the Naval Research Laboratory. The electron density was measured to diagnose beam-induced gas ionization and the net current was measured to diagnose subsequent conductivity growth. Gases included air, He, and Ne at pressures of 0.3-4 Torr. Net current fractions of 2 to 8% were measured outside the beam channel, and measurable ionization was confined to the beam channel with ionization fractions of a few percent. As expected, the ionization fraction was inversely correlated with the net current. A HeNe interferometer using heterodyne phase detection was used to determined the electron density.