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

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/cm2 proton beam produced by the Gamble 11 generator at NRL. 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. The ionization fraction was higher for Ne than for He and was lower at higher pressure for all gases. Results of theoretical calculations of ionization fractions and net currents in He, obtained using the DYNAPROP2 and IPROP3 codes, are in rough agreement with the measured results. A HeNe interferometer using heterodyne phase detection was used to determine the electron density. In the above experiment the practical time resolution, 25 ns or greater, prevented the ionization fraction from being measured on the time scale of the ion current front. A 200-MHz interferometer is now being fielded to allow time-resolved measurements of the ion-beam-induced gas ionization, Gases include He, Ne, and Ar at pressures of 1--4 Torr. Results from these measurements, and comparisons with the measured net currents and with theoretical predictions, will be presented