Comparison of the influence of 2D and 3D geometry of the main chamber on plasma parameters in the SOL of ASDEX Upgrade

In this paper the influence of toroidally asymmetric wall features on plasma solutions for ASDEX Upgrade is investigated by using the 3D scrape-off-layer simulation code EMC3-EIRENE. A comparison of simulation results in a 2D case with a toroidally symmetric first wall and divertor and a 3D case that differs from the 2D setup by including the 3D structure of the poloidal rib-limiters on the low field side of ASDEX Upgrade, highlights notable differences in the main chamber neutral particle distributions, ionisation sources and plasma flow patterns. Both neutral particle distribution and ionisation sources extend poloidally further upwards at the outer mid-plane in the 3D case and the plasma flow is globally influenced by the 3D wall features. Both simulations are conducted with identical input parameters to isolate the influence of wall geometry from other factors. By analysing the transport of neutrals from different poloidal locations it was possible to explain the observed discrepancies by different transport paths for recycled neutrals from the divertor region, only accessible in the 3D version of the wall geometry. Together with observed differences in fall-off lengths for plasma flow and electron temperature at the outer mid-plane, presented results are of key importance for interpreting global impurity migration experiments.

Evolution of Kelvin–Helmholtz Instability in the Fan-spine Topology

We use multiwavelength imaging observations from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory to study the evolution of the Kelvin–Helmholtz (K–H) instability in a fan-spine magnetic field configuration. This magnetic topology exists near an active region AR12297 and is rooted in a nearby sunspot. In this magnetic configuration, two layers of cool plasma flow in parallel and interact with each other inside an elongated spine. The slower plasma flow (5 km s−1) is the reflected stream along the spine’s field lines from the top, which interacts with the impulsive plasma upflows (114–144 km s−1) from below. This process generates a shear motion and subsequent evolution of the K–H instability. The amplitude and characteristic wavelength of the K–H unstable vortices increase, satisfying the criterion of the fastest-growing mode of this instability. We also describe how the velocity difference between two layers and the velocity of K–H unstable vortices are greater than the Alfvén speed in the second denser layer, which also satisfies the criterion of the growth of the K–H instability. In the presence of the magnetic field and sheared counterstreaming plasma as observed in the fan-spine topology, we estimate the parametric constant Λ ≥ 1, which confirms the dominance of velocity shear and the evolution of the linear phase of the K–H instability. This observation indicates that in the presence of complex magnetic field structuring and flows, the fan-spine configuration may evolve into rapid heating, while the connectivity changes due to the fragmentation via the K–H instability.

Plasma Flow Generation due to the Nonlinear Alfvén Wave Propagation around a 3D Magnetic Null Point

The behavior of current density accumulation around the sharp gradient of magnetic field structure or a 3D magnetic null point and with the presence of finite plasma pressure is investigated. It has to be stated that in this setup, the fan plane locates at the xy plane and the spine axis aligns along the z-axis. Current density generation in presence of the plasma pressure that acts as a barrier for developing current density is less well understood. The shock-capturing Godunov-type PLUTO code is used to solve the magnetohydrodynamic set of equations in the context of wave-plasma energy transfer. It is shown that propagation of Alfvén waves in the vicinity of a 3D magnetic null point leads to current density excitations along the spine axis and also around the magnetic null point. Besides, it is pointed out the x component of current density has oscillatory behavior while the y and z components do not show this property. It is plausible that it happens because the fan plane encompasses separating unique topological regions, while the spine axis does not have this characteristic and is just a line without separate topological regions. Besides, current density generation results in plasma flow. It is found that the y component of the current density defines the x component of the plasma flow behavior, and the x component of the current density prescribes the behavior of the y component of the plasma flow.

Impact of the plasma flow of a low-pressure high-frequency capacitive discharge on the adhesive and physical and mechanical characteristics of aramid and carbon fibers

The changes in the adhesive and physico-mechanical characteristics of aramid and carbon fibers treated with plasma of a highfrequency capacitive gas discharge of reduced pressure are considered. An increase of ~ 30% in the wettability of the surface of aramid and carbon fibers is shown. It was found that the strength of plastics made on the basis of aramid and carbon fibers treated with a plasma flow increases by ~ 26%.

On hybrid type of cathode attachment in high current vacuum arcs

This paper discusses the issues of a possible change of the type of cathode attachment of high-current vacuum arcs (HCVA) with an average cathode current density of more than 105 A/cm2. This type of HCVA is used as pumping plasma gun in experiments with plasma puff z-pinches. These experiments showed that the measured linear mass of the HCVA plasma jet is much higher (by a factor of 10 or more) than the expected mass, which can be obtained from the assumption that cathode attachment occurs only through a multitude of cathode spots emitting supersonic plasma jets. It is shown that in HCVA of the type under consideration, at some time instant there are two types of cathode attachments - cathode spots and thermionic erosion attachment (TEA). It can be said that HCVA of this type have a hybrid cathodic attachment. Unlike cathode spots, TEA produces a subsonic plasma flow, which contributes to an increase in the linear mass of the HCVA plasma jet.

Determination of the voltage drop on a high-current vacuum arc discharge under conditions of a limited cross-section of the plasma flow

The work is devoted to the study of the high-current vacuum arc discharge characteristics under conditions of a limited cross-section of the plasma flow. The experiments were carried out on the IMRI-5 setup with a sinusoidal arc current amplitude of 300–350 kA and a rise time of 500 ns. Aluminum rods with diameters from 3 to 7 mm were used as a cathode. The plasma flow was formed in a channel whose diameter was equal to that of the cathode. The features of the formation of a plasma jet with various configurations of the used plasma gun are described. The electrophysical parameters of the arc discharge are presented. Theoretical estimates of the voltage drop across the high-current arc during the outflow of a plasma flow through holes with a limited diameter are provided.

Electric arc in plasma flow of gas discharge with a liquid electrolyte

The properties of an electric arc ignited in a gas discharge plasma flow with a liquid electrolyte cathode are experimentally investigated. Aqueous solutions of sodium chloride with a specific electrical conductivity of 10-15 mS/cm were used as a liquid electrolyte. The distance between the anode and the liquid electrolyte cathode was set in the range of 5-60 mm, and the distance between the anode and the metal cathode was varied in the range of 2-30 mm. The current of gas discharge with a liquid electrolyte cathode was set in the range of 5-10 A, and the arc current varied in the range of 1-10 A. The conditions under which the arc burns with the formation of a contracted channel are revealed.