Experiments and modeling of intense charged-particle beam transport cells

Summary form only given. The intense electron beam generated by a paraxial diode can deposit, in under 50 ns, on the order of 1 Joule/cm/sup 3/ in the downstream 10 Torr air transport cell. Such large energy densities and short time scales require a theoretical and computational physics model which can accurately handle molecular dissociation, multiply ionized atoms, and non-equilibrium populations. Correctly modeling the conductivity of the background plasma is critical for determining beam stopping and the plasma return current, J/sub p/. This plasma return current, which partially neutralizes the self-magnetic fields of the beam, in turn affects the spot size, transverse temperature, optimal location of a bremsstrahlung converter, and, finally, the radiation output. The gas-physics method described here extends an earlier scalar conductivity swarm model which was compared to experiment through theory and computation. In the present model, at early times in the experiment the relaxation toward equilibrium of the molecular and atomic degrees of freedom is taken to control the level of dissociation of the primary diatomic components of air, N/sub 2/ and O/sub 2/. Later, for moderate background electron densities (/spl sim/10/sup 17//cm/sup 3/) and high temperatures (/spl sim/5 eV), a time-dependent coronal model, balancing collisional excitation with radiative decay, is appropriate. The combination of dissociation physics and coronal modeling allows the extension of the conductivity model to severely-disturbed gases characteristic of paraxial diodes. An NRL Gamble II paraxial-diode experiment has conducted laser interferometry, time-resolved spot size, target energy-deposition calorimetry, and net-current measurements of a 1.2 MV, 40 kA electron beam in the gas transport cell. Comparisons of these results both with theory and an extended gas-physics model in the computational code LSP are discussed. The computer code also presents opportunities to study pre-ionizat- on of the transport gas and advanced paraxial-diode geometries. The physics understanding gained by the present campaign should prove valuable for optimizing the performance of a diode which has previously been studied primarily empirically, as well as exploring various parameter regimes of severely-disturbed plasmas.