This paper provides perspectives on recent progress in understanding the physics of devices in which the external magnetic field is applied perpendicular to the discharge current. This configuration generates a strong electric field that acts to accelerate ions. The many applications of this set up include generation of thrust for spacecraft propulsion and separation of species in plasma mass separation devices. These “E × B” plasmas are subject to plasma–wall interaction effects and to various micro- and macroinstabilities. In many devices we also observe the emergence of anomalous transport. This perspective presents the current understanding of the physics of these phenomena and state-of-the-art computational results, identifies critical questions, and suggests directions for future research.

We experimentally demonstrate tunable guided resonance in twisted bilayer photonic crystals. Both the numerically and the experimentally obtained transmission spectra feature resonances with frequencies strongly dependent on the twist angle, as well as resonances with frequencies that are largely independent of the twist angle. These resonant features can be well understood with a simple analytic theory based on band folding. Our work illustrates the rich tunable resonance physics in twisted bilayer systems.

Coaxial Hall discharges (also known as Hall thrusters, stationary plasma thrusters, and closed-drift accelerators) are cross-field plasma sources under development for space propulsion applications. The importance of the electron-wall interaction to the Hall discharge operation is studied the through analysis of experimental data and simulation of the electron energy distribution function (EEDF) inside the discharge channel. Experimental time-average plasma property data from a laboratory Hall discharge are used to calculate the electron conductivity and to estimate the rate of wall-loss collisions. The electron Boltzmann equation is then solved in the local field limit, using the experimental results as inputs. The equation takes into account ionization and wall collisions, including secondary electrons produced at the wall. Local electron balances are used to calculate the sheath potential at the insulator walls. Results show an EEDF depleted at high energy due to electron loss to the walls. The calculated EEDFs agree well with experimental electron temperature data when the experimentally determined effective collision frequency is used for electron momentum transport. The electron wall-loss and wall-return frequencies are extremely low compared to those predicted by a Maxwellian of equal average energy. The very low frequency of wall collisions suggests that secondary electrons do not contribute to cross-field transport. This conclusion holds despite significant experimental uncertainty.

Several techniques have been developed recently for performing time-resolved laser-induced fluorescence (LIF) measurements in oscillating plasmas. One of the primary applications is characterizing plasma fluctuations in devices like Hall thrusters used for space propulsion. Optical measurements such as LIF are nonintrusive and can resolve properties like ion velocity distribution functions with high resolution in velocity and physical space. The goals of this paper are twofold. First, the various methods proposed by the community for introducing time resolution into the standard LIF measurement of electric propulsion devices are reviewed and compared in detail. Second, one of the methods, the sample-hold technique, is enhanced by parallelizing the measurement hardware into several signal processing channels that vastly increases the data acquisition rate. The new system is applied to study the dynamics of ionization and ion acceleration in a commercial BHT-600 Hall thruster undergoing unforced breathing mode oscillations in the 44–49 kHz range. A very detailed experimental picture of the common breathing mode ionization instability emerges, in close agreement with established theory and numerical simulations.

A three-dimensional woodpile structure tunable plasma photonic crystal is designed, simulated, and experimentally characterized over the S – X band of the electromagnetic (EM) spectrum. The measurements confirm that the EM response is rich in dynamics. The photonic crystal’s reconfigurability achieved through individual discharge control of the properties of the woodpile plasma columns offers an unprecedented opportunity to better resolve the interactions between both the Bragg and localized surface plasmon modes which dominate the spectrum at lower frequencies. Both the experiments and simulations reveal evidence for the coupling of Bragg and surface plasmon modes through a Fano resonance.

Photon-enhanced thermionic emission from semiconducting cathodes is a promising means of increasing cathode current density in low temperature vacuum thermionic energy devices [J. W. Schwede et al., Nature Mater. 9, 762 (2010)]. However, the space charge resulting from high emission current densities prevents emitted electrons from reaching the collector, even with gaps as small as 100 μm and at voltages as high as 1 V. We demonstrate by particle-in-cell simulations that one possible solution to overcoming the space charge is to add cesium filling and generation of a space charge-neutralizing plasma by continuous laser excitation of the cesium resonance level.