Nordsieck length measurements of an intense relativistic electron beam propagating in high pressure neutral gas

An intense relativistic electron beam (IREB) injected into neutral gas in the high pressure regime characteristically propagates in a self-pinched mode but is subject to the resistive hose instability. The effects of the instability on the beam range from rapid radial expansion to large amplitude hosing and disruption of propagation. Typically, beams are conditioned for propagation experiments using techniques which reduce perturbations that may excite resistive hose and which tailor the emittance profile of the beam such that the convective growth of the instability is decreased. Ideally, a well conditioned IREB would propagate many betatron wavelengths, gradually expanding in radius due to transverse temperature growth from electron-neutral scattering and radiative losses only. This stable beam expansion can be characterized by the Nordsieck length, LN (the distance over which the beam radius e-folds). In practice, however, anomalous radial expansion measurements may indicate the presence of resistive hose. The extent of the deviation of these measurements from the ideal Nordsieck expansion rate provides a basis for assessing the influence of the instability on the propagating beam. A conditioned 5 MeV, 20 kA, 40 ns FWHM IREB produced by the SuperIBEX accelerator is injected into a 2 meter diameter propagation tank at pressures in the 400--760 Torr range for these studies. The radial profile of the beam is measured using time resolved optical diagnostics and a segmented concentric Faraday cup; beam and net currents are recorded using beambugs and Rogowski monitors. These diagnostics are deployed at various positions along the axis of propagation providing a time resolved history of beam profile evolution. The expansion rate as a function of background pressure is then determined numerically and compared with calculations of the Nordsieck length based on input beam parameters and a given pressure