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1. Diocotron modes in pure electron plasmas in the APEX levitating dipole trap

   https://doi.org/10.1088/1361-6587/ad9e70  

   Deller, A; Bayer, V C; Steinbrunner, P; Card, A; Danielson, J R; Stoneking, M R; Stenson, E V (December 2024, Plasma Physics and Controlled Fusion)

   Abstract Observations of persistent toroidal modes in dipole-trapped pure electron plasmas are presented. Non-neutral electron plasmas were confined by the poloidal magnetic field of the APEX (A Positron Electron eXperiment) levitating dipole trap. The negative bias of the electron emitter was gated to control the filling time. Large amplitude, narrow-band oscillations were consistently observed in wall probe measurements after the fill. The oscillatory image-charge currents induced by the trapped electron plasma typically persisted for many seconds with fundamental frequencies,f₀ = 50 to 500 kHz, that scale with the $\overrightarrow{E}\times \overrightarrow{B}$ drift velocity. We attribute this feature to the dipole-equivalent of the diocotron mode that is commonly observed in non-neutral plasmas in linear traps. 

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2. Inviscid damping of an elliptical vortex subject to an external strain flow

   https://doi.org/10.1063/5.0086227  

   Wongwaitayakornkul, P.; Danielson, J. R.; Hurst, N. C.; Dubin, D. H.; Surko, C. M. (May 2022, Physics of Plasmas)

   Inviscid spatial Landau damping is studied experimentally for the case of oscillatory motion of a two-dimensional vortex about its elliptical equilibrium in the presence of an applied strain flow. The experiments are performed using electron plasmas in a Penning–Malmberg trap. They exploit the isomorphism between the two-dimensional Euler equations for an ideal fluid and the drift-Poisson equations for the plasma, where plasma density is the analog of vorticity. Perturbed elliptical vortex states are created using [Formula: see text] strain flows, which are generated by applying voltages to electrodes surrounding the plasma. Measurements of spatial Landau damping (also called critical-layer damping) are in agreement with previous studies in the absence of an applied strain, where the damping is due to a resonance between the local fluid motion and the vortex oscillations. Interestingly, the damping rate does not change significantly over a wide range of applied strain rates. This can be accurately predicted from the initial vorticity profile, even though the resonant frequency is reduced substantially due to the applied strain. For higher amplitude perturbations, nonlinear trapping oscillations also exhibit behavior similar to the strain-free case. In principle, higher-order effects of the applied strain, such as separatrix crossing of peripheral vorticity and interactions with harmonics of the fundamental resonance, are expected to change the damping rate. However, this occurs only for conditions that are not realized in the experiments described here. Vortex-in-cell simulations are used to investigate the possible roles of these effects. 

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3. Design and operation of APEX-LD: A compact levitated dipole for a positron–electron experiment

   https://doi.org/10.1063/5.0311702  

   Card, A; Stoneking, M R; Deller, A; Stenson, E V (February 2202, Review of Scientific Instruments)

   The objective of the APEX (A Positron–Electron eXperiment) project is to magnetically confine and study positron–electron pair plasmas. For this purpose, a levitated dipole trap (APEX-LD) has been constructed. The magnetically levitated, compact (7.5-cm radius), closed-loop, high-temperature superconducting floating (F-)coil consists exclusively of a no-insulation rare-earth barium copper oxide winding pack, solder-potted in a gold-plated-copper case. A resealable in-vacuum cryostat facilitates cooling (via helium gas) and inductive charging of the F-coil. The 70-min preparation cycle reliably generates persistent currents of ∼60 kA-turns and an axial magnetic flux density of B0 ≈ 0.5 T. We demonstrate levitation times in excess of 3 h with a vertical stability of σz < 20 μm. Despite being subjected to routine quenches (and occasional mechanical shocks), the F-coil has proven remarkably robust. We present the results of preliminary experiments with electrons and outline the next steps for injecting positron bunches into the device. 

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4. Vortex splitting in two-dimensional fluids and non-neutral electron plasmas with smooth vorticity profiles

   https://doi.org/10.1063/5.0201712  

   Hurst, N C; Tran, A; Wongwaitayakornkul, P; Danielson, J R; Dubin, D_H E; Surko, C M (May 2024, Physics of Plasmas)

   Initially, elliptical, quasi-two-dimensional (2D) fluid vortices can split into multiple pieces if the aspect ratio is sufficiently large due to the growth and saturation of perturbations known as Love modes on the vortex edge. Presented here are experiments and numerical simulations, showing that the aspect ratio threshold for vortex splitting is significantly higher for vortices with realistic, smooth edges than that predicted by a simple “vortex patch” model, where the vorticity is treated as piecewise constant inside a deformable boundary. The experiments are conducted by exploiting the E × B drift dynamics of collisionless, pure electron plasmas in a Penning–Malmberg trap, which closely model 2D vortex dynamics due to an isomorphism between the Drift–Poisson equations describing the plasmas and the Euler equations describing ideal fluids. The simulations use a particle-in-cell method to model the evolution of a set of point vortices. The aspect ratio splitting threshold ranges up to about twice as large as the vortex patch prediction and depends on the edge vorticity gradient. This is thought to be due to spatial Landau damping, which decreases the vortex aspect ratio over time and, thus, stabilizes the Love modes. Near the threshold, asymmetric splitting events are observed in which one of the split products contains much less circulation than the other. These results are relevant to a wide range of quasi-2D fluid systems, including geophysical fluids, astrophysical disks, and drift-wave eddies in tokamak plasmas. 

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5. Scaling Up of Non-Thermal Gliding Arc Plasma Systems for Industrial Applications

   https://doi.org/https://doi.org/10.1007/s11090-021-10203-5  

   Alexander Rabinovich, Gary Nirenberg (January 2022, Plasma chemistry and plasma processing)

   null (Ed.)

   Scaling up of transitional “warm” plasmas to industrial level gives possibility to develop plasma systems that combine advantages of thermal and non thermal discharges such as low temperature and high process selectivity (compare to thermal plasma) at high pressure and average power density. Non-equilibrium “cold” gliding arcs (with observation of equilibrium to non equilibrium transition) has been demonstrated at power level 2–3 kW and proved to be a highly efficient plasma stimulators of several plasma chemical and plasma catalytic processes, including hydrogen/syngas generation from biomass, coal and organic wastes, exhaust gas cleaning, fuel desulfurization and water cleaning from emerging contaminants. The gliding arc evolution includes initial micro-arc phase with fast transition to transient non-equilibrium phase with elevated electric field, low gas and high electron temperatures, as well as selective generation of active species typical for cold plasmas. The paper will describe experimentally achieved scaling up of the non-equilibrium gliding arc discharges to the level of 10–15 kW, as well as theoretical scaling up limitations of this powerful non-equilibrium plasma systems. 

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