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.
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.
The origin of intermittent fluctuations in an experiment involving several interacting electron plasma pressure filaments in close proximity, embedded in a large linear magnetized plasma device, is investigated. The probability density functions of the fluctuations on the inner and outer gradient of the filament bundle are non-Gaussian and the time series contain uncorrelated Lorentzian pulses that give the frequency power spectral densities an exponential shape. A cross-conditionally averaged spatial reconstruction of a temporal event reveals that the intermittent character is caused by radially and azimuthally propagating turbulent structures with transverse spatial scales on the order of the electron skin depth. These eruption events originate from interacting pressure gradient-driven drift-Alfvén instabilities on the outer gradient and edge of the filament bundle. The temporal Lorentzian shape of the intermittent structures and exponential spectra are suggestive of deterministic chaos in the underlying dynamics; this conclusion is supported by the complexity–entropy analysis (CH-plane) that shows the experimental time series are located in the chaotic regime.
The results of a basic electron heat transport experiment using multiple localized heat sources in close proximity and embedded in a large magnetized plasma are presented. The set-up consists of three biased probe-mounted crystal cathodes, arranged in a triangular spatial pattern, that inject low energy electrons along a strong magnetic field into a pre-existing, cold afterglow plasma, forming electron temperature filaments. When the three sources are activated and placed within a few collisionless electron skin depths of each other, a non-azimuthally symmetric wave pattern emerges due to interference of the drift-Alfvén modes that form on each filament’s temperature gradient. Enhanced cross-field transport from chaotic ( $\boldsymbol{E}\times \boldsymbol{B}$ , where $\boldsymbol{E}$ is the electric field and $\boldsymbol{B}$ the magnetic field) mixing rapidly relaxes the gradients in the inner triangular region of the filaments and leads to growth of a global nonlinear drift-Alfvén mode that is driven by the thermal gradient in the outer region of the triangle. Azimuthal flow shear arising from the emissive cathode sources modifies the linear eigenmode stability and convective pattern. A steady-current model with emissive sheath boundary predicts the plasma potential and shear flow contribution from the sources.
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