Simulation of electron-beam transport in low-pressure gas conditioning cells

The resistive hose instability can disrupt propagation of self-pinched beams in dense gas. To reduce growth of the instability, beams can be conditioned prior to propagation. The objectives of beam conditioning are to center the beam in order to reduce initial transverse perturbations which seed the hose instability, and to ‘‘tailor’’ the beam emittance in order to detune the head-to-tail coherence of the instability. Emittance tailoring can be performed by transporting the beam through a ‘‘passive ion-focused regime’’ (IFR) cell, a low-pressure gas cell that induces a head-to-tail taper of the beam radius. The radius taper is then converted to an emittance taper by passing the beam through a thick exit foil which scatters the beam. Beam centering can be accomplished by transporting the beam through either: (i) a passive IFR cell that is narrow enough to provide wall guiding, or (ii) a laser-ionized ‘‘active’’ IFR cell, or (iii) a wire cell in which the centering is provided by a current-carrying wire. Axisymmetric particle simulation studies of IFR tailoring cells, alone and in tandem with each of these types of centering cells, and also the effect of supplementary focusing lenses and conducting foils are reported. The parameter choices that are conducive to effective beam conditioning are discussed. The emphasis is on conditioning configurations and beam parameters that have actually been tested in experiments with the Advanced Test Accelerator and SuperIBEX accelerator.