Modifications produced on a large magnetized plasma column by a floating end-plate that is partially emissive: Experiment and theory

Abstract

An experiment is performed on a large plasma device operated by the Basic Plasma Science Facility at the University of California, Los Angeles, in which an electrically floating structure is placed near the end of the 18-m magnetized plasma column. The structure consists of a flat carbon plate that acts as a mask for a smaller, ring-shaped LaB6 emissive surface whose temperature can be externally controlled. This configuration has been previously used to study electron heat transport and pressure-driven avalanches [B. Van Compernolle and G. J. Morales, Phys. Plasmas 24, 112302 (2017)] by biasing the LaB6 ring-cathode with respect to a distant anode in a cold afterglow plasma. In contrast, the present study is performed during the active portion of the steady-state discharge in which the nominal plasma parameters are determined by the injection of an electron beam from a BaO cathode at the opposite end. It is found that, even without an applied bias on the LaB6 cathode, the self-consistent potential and current profiles are modified near the end plate as the LaB6 temperature is increased, resulting in density increases on the field lines in contact with the ring-cathode. In the absence of enhanced ionization, at the largest cathode temperatures, the ambient density can be doubled. A theoretical model is presented that provides a quantitative explanation for the experimental observations.

Trade-off in perpendicular electric field control using negatively biased emissive end-electrodes

“…obtained from the combination of Ohm's law for a static background neutral j = σE and charged conservation ∇ • j = 0 allowed for analytical solutions when imposing Dirichlet conditions [36], the use of more physical flux conditions requires numerical modeling. Equation ( 16) is thus solved here using finite differences in the interior of the domain shown in figure 7 and implementing flux conditions at the electrodes in a way very similar to that employed by Von Compernolle et al [46]. Specifically, noting z 0 = −L/2 the axial position of the left boundary of the domain, the ion-sheath in front of the electrode is modeled via the non-linear Neumann condition…”

Trade-off in perpendicular electric field control using negatively biased emissive end-electrodes

“…Eq. ( 16) is thus solved here using finite differences in the interior of the domain shown in figure 7 and implementing flux conditions at the electrodes in a way very similar to that employed by Von Compernolle et al [43]. Specifically, noting z 0 = −L/2 the axial position of the left boundary of the domain, the ion-sheath in front of the electrode is modeled via the non-linear Neumann condition ∂ψ(r, z) ∂z…”