Azimuthal structures emerging in beam-generated partially magnetized plasmas are investigated using three-dimensional particle in cell-Monte Carlo collision simulations. Two distinct instability regimes are identified at low pressures. When the gas pressure is sufficiently high, quasineutrality is attained and 2D spiral-arm structures form as a result of the development of a lower-hybrid instability, resulting in enhanced cross-field transport. At lower pressures, quasineutrality is not achieved and a 3D helical-rotating plasma structure forms due to development of the diocotron instability. Analytical formulas are proposed for the critical threshold pressure between these regimes and for the rotation frequency of the helical structures. Preliminary experimental verification is provided.

Direct-current ionization breakdown in argon at very high voltage on the low-pressure branch of the Paschen curve is studied by means of kinetic simulation. The employed particle-in-cell/Monte Carlo collision model tracks the motion of electrons, ions and fast neutral atoms, as well as their collisions with the background gas. Secondary electron emission due to the bombardment by ions and fast atoms, as well as backscattering of different species on the electrode surfaces are also consistently accounted for. To validate the model, we compare the Paschen curve predicted by kinetic simulations against published experimental results as well against the prediction of a reduced (fluid) model. The model is then applied to predict, for the first time, the low-pressure branch of the Paschen curve for argon gas in the 10–300 kiloVolt range of applied voltage. One of the findings is the existence of a turning point on the Paschen curve at V b r ≈ 20 k V and p d ≈ 0.08 T o r r ⋅ c m , where V b r is the breakdown voltage and the reduced pressure p d is the product of the gas pressure with the inter-electrode distance. According to our analysis, the presence of the turning point is due to the phenomena of electron and ion runaway in high electric field. We further investigate the role of the heavy species (i.e. ions and fast neutral atoms) in sustaining the Townsend discharge at high voltage and thus setting the breakdown threshold. Specifically, the role of fast atoms is significant over the entire low-pressure branch of the Paschen curve due to their contribution to both ionization and secondary electron emission. Also, the ions, as the precursor for the fast atoms, show an important influence on the latter process. The newly predicted low-pressure branch of the Paschen curve for argon at high voltage should provide an essential reference for the design of high-voltage devices, e.g. circuit breakers in electrical power transmission.

A new drift module has been developed in the DIVIMP code, enabling two-dimensional simulations of tungsten (W) transport in the edge plasma with full drifts included. By using the SOLPS-DIVIMP code package, the impact of drifts on W transport and screening has been investigated for various levels of dissipative divertor conditions and different divertor geometric configurations in EAST. Simulation results reveal that E ⃗ × B ⃗ drifts can enhance the W leakage by more than one order of magnitude. Under the favorable B_t direction, W erosion mainly occurs on the outer divertor target, making the W leakage from the outer divertor region the dominator. A leakage path from the near-scrape-off layer (SOL) region is revealed by the modeling results. In the leakage path, both the ion temperature gradient force and the reversed poloidal E ⃗ × B ⃗ drift are pointing upstream. With the radial E ⃗ × B ⃗ drift pushing W ions from the well-screened far-SOL region to the near-SOL region, the leakage from the near-SOL region becomes significant. As the divertor condition varies from the low-recycling regime to the deep detachment regime, the decrease of the ion temperature gradient velocity and poloidal E ⃗ × B ⃗ drift velocity narrows the width of the near-SOL leakage tunnel and thus enhances W screening. While under the unfavorable B_t , W erosion and leakage from the inner divertor target matters, the leakage mechanism especially the leakage path from the near-SOL region is similar as the favorable B_t cases. Furthermore, the effect of different divertor geometries on the W screening has been investigated. The configuration with the outer strike point (OSP) on the horizontal divertor plate is proved to narrow the near-SOL leakage tunnel, and thus the unreversed poloidal E ⃗ × B ⃗ drift pointing to the divertor target dominates and helps to enhance the divertor W screening. For the same D ₂ puffing rate, the W leakage ability of cases with the OSP on the horizontal target can be more than 10 times weaker than the cases with the OSP on the vertical target, especially when the divertor is detached.

In a recent discovery (Wen et al 2022 Phys. Rev. Lett. 129 045001), streaming waves were found in multipactor-induced plasma discharges. However, due to the limitations of a 1D simulation setup, these waves displayed only transverse dynamics. In this letter, an extended 2D particle-in-cell/Monte Carlo model is used to simulate multipactor-induced plasma discharge above a dielectric surface. The results reveal that the streaming waves are not solely transverse but oblique, featuring both transverse and longitudinal components of the wave vector. Furthermore, it is identified that the sheath-accelerated field-emission electrons, rather than the previously reported secondary emission electrons, predominantly cause the excitation of streaming waves. The simulated wave spectrum achieves an excellent agreement with the theoretical dispersion relation. The identification of oblique streaming waves provides new insights into multipactor physics and is anticipated to inspire novel mitigation strategies for multipactor-induced breakdown processes.