Transport of long-pulse relativistic electron beams in preformed plasma channels in the ion focus regime.

Experiments have been performed demonstrating efficient transport (up to 76% instantaneous current transport and up to 56% charge transport efficiency over a distance of 1 m within a 2.54 cm radius) of long-pulse (380 ns), high-current (200 A), relativistic electron beams (REBs) in preformed plasma channels in the ion focus regime (IFR). Plasma channels were created by low-energy ($<$300 eV) electron beam ionization of low pressure monatomic gases (Ne, Ar, Kr, and Xe). Experimental results show development of transverse current centroid oscillations with real frequencies and qualitative scaling with the space-charge neutralization fraction, f$\sb{\rm e}$, and channel ion mass, in agreement with theoretical values predicted for the ion hose instability. Microwave emission has also been observed indicative of REB-plasma electron two-stream instability. Plasma channel density measurements indicate that the two-stream instability can become dominant for measured f$\sb{\rm e}$ values slightly above unity. We have introduced a theoretical analysis for high-current REB transport and modulation in axially periodic IFR plasma channels. Analytic expression for the electric field are found for the case of a cosine modulation of the channel ion density. Two different types of channels are considered: (i) periodic beam-induced ionization channels, and (ii) periodic plasma slab channels created by an external source. Analytical conditions are derived for the matched radius of the electron beam and for approximate beam envelope motion using a smooth approximation. Numerical solutions to the envelope equation show that by changing the wavelength or the amplitude of the space-charge neutralization fraction of the ion channel density modulation, the beam can be made to focus and diverge, or to undergo stable, modulated transport.Ph.D.Plasma physicsPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/128381/2/9001684.pd