A model for the steady-state operation of an emissive cathode is presented. The cathode, biased negative with respect to a cold anode, emits electrons thermionically and is embedded within a large magnetized-plasma column. The model provides formulas for the spatial shape of the global current system, the partition of potential across the plasma-sheath system, and the effective plasma resistance. The formation of a virtual cathode is explored, and an analytical expression for the critical operating conditions is derived. The model is further developed to include the self-consistent increase in plasma temperature which results from thermionic injection. In a companion paper [S. Jin et al., Phys. Plasmas 26, 022105 (2019)], results from transport experiments in the Large Plasma Device at the University of California Los Angeles are compared with this model, and excellent quantitative agreement is achieved.

A LaB 6 thermionic emitter of annular shape is used in the Large Plasma Device at the University of California, Los Angeles to create off-axis heating conditions for various transport studies. Since the emitter is biased relative to a distant anode, which is many collision lengths away, the entire magnetized plasma develops a self-consistent, potential structure that simultaneously generates transverse and axial flows with shear. This study uses swept Langmuir probe techniques and Mach probes to map the flow patterns and their dependence on bias and plasma parameters. By implementing additional biasing configurations, it is possible to control the magnitude of the flows and their shear strength. The experimental measurements, including the selfconsistent currents, are compared to predictions of a model that incorporates the boundary conditions associated with thermionic injection, combined with a Braginskii transport code for the electron temperature.

A theoretical and numerical modeling study is made of a novel heating configuration recently implemented in the Large Plasma Device at the University of California, Los Angeles. The injection of an electron beam from a masked LaB 6 cathode into a magnetized plasma results in a hollow, cylindrical filament of elevated temperature. The hot cylindrical ring has an axial extent that is about one-thousand times larger than its thickness, and the peak temperature can be ten times larger than that of the surrounding plasma. The simultaneous positive and negative radial pressure gradients provide an ideal platform for the investigation of transport phenomena of contemporary interest, including avalanches [Van Compernolle et al., Phys. Rev. E 91, 031102 (2015)] and nonlocal transport. The present study delineates both the parameter regimes achievable by classical transport and the linear stability of the self-consistent profiles, including temperature and density gradients. An avalanche model is developed based on the self-consistent evolution of drift-wave eigenfunctions in nonlinearly modified profiles of electron temperature and plasma density.

This paper reports a new numerical scheme to simulate the RF induced radio-frequency sheath, which is suitable for a large 3D simulation. In the RF sheath boundary model, the tangential component of the electric field ($E_{\rm t}$) is given by the gradient of a scalar electric field potential. We introduce additional two scalar potentials for the tangential components of the magnetic field, which effectively impose the normal electric displacement ($D_n$) on the plasma sheath BC via in-homogeneous Neumann boundary condition and constrain the tangential electric field on the surface as curl-free ($\nabla \times E_{\rm t} = 0$). In our approach, the non-linear sheath impedance is formulated as a natural extension of the large thickness (or asymptotic) sheath limit ($D_{\rm n}=0$), allowing for handling both asymptotic and non-linear regimes seamlessly. The new scheme is implemented using the Petra-M FEM analysis framework and is verified with simulations in the literature. The significance of non-linearity is discussed in various plasma conditions. An application of this scheme to asymptotic RF sheath simulation on the WEST ICRF antenna side limiters is also discussed.

Results are presented from a basic heat transport experiment using a magnetized electron temperature filament that behaves as a thermal resonator. A small, crystal cathode injects low energy electrons along the magnetic field into the afterglow of a large pre-existing plasma forming a hot electron filament embedded in a colder plasma. A series of low amplitude, sinusoidal perturbations are added to the cathode discharge bias that create an oscillating heat source capable of driving thermal waves. Langmuir probe measurements demonstrate driven thermal oscillations and allow for the determination of the amplitude and parallel phase velocity of the thermal waves over a range of driver frequencies. The results conclusively show the presence of a thermal resonance and are used to verify the parallel thermal wave dispersion relation based on classical transport theory. A nonlinear transport code is used to verify the analysis procedure. This technique provides a novel measure of the density normalized thermal conductivity, independent of the electron temperature.