Devendra Sharma

Physical Aspects and Design of the Wendelstein 7-X Divertor

Abstract For the Wendelstein 7-X stellarator, an “open divertor” was chosen as a first step in divertor development for the expected extended magnetic and plasma parameter range. Particularly, the three-dimensional (3-D) geometry of the boundary and the provided stationary operation are challenges for the design. So far, simplified models have been used to specify the geometry of the divertor and the performance of the high-heat-load surfaces. By applying the 3-D codes that are now available, the results concerning local heat load and particle exhaust can have more detailed evaluation and can be confirmed generally. Together with the development of improved high-heat-load components, a significant reduction of the target area in comparison with the previous design is possible. The new specifications will be characterized.

Three-dimensional Monte Carlo simulations of W7-X plasma transport: density control and particle balance in steady-state operations

This paper presents self-consistent three-dimensional (3D) plasma transport simulations in the boundary of stellarator W7-X obtained with the Monte Carlo code EMC3-EIRENE for three typical island divertor configurations. The chosen 3D grid consists of relatively simple nested finite toroidal surfaces defined on a toroidal field period and covering the whole edge topology, which includes closed surfaces, islands and ergodic regions. Local grid refinements account for the required high resolution in the divertor region. The distribution of plasma density and temperature in the divertor region, as well as the power deposition profiles on the divertor plates, are shown to strongly depend on the island geometry, i.e. on the position and size of the dominant island chain. Configurations with strike-point positions closer to the gap of the divertor chamber generally favour the neutral compression in the divertor chamber and hence the pumping efficiency. The ratio of pumping to recycling fluxes is found to be roughly independent of the separatrix density and is thus a figure of merit for the quality of the configuration and of the divertor system in terms of density control. Lower limits for the achievable separatrix density, which determine the particle exhaust capabilities in stationary conditions, are compared for the three W7-X configurations.

Generation of multiple toroidal dust vortices by a non-monotonic density gradient in a direct current glow discharge plasma

Observation of two well-separated dust vortices in an unmagnetized parallel plate DC glow discharge plasma is reported in this paper. A non-monotonic radial density profile, achieved by an especially designed cathode structure using a concentric metallic disk and ring of different radii, is observed to produce double dust tori between cathode and anode. PIV analysis of the still images of the double tori shows oppositely rotating dust structures between the central disk and the ring. Langmuir probe measurements of background plasma shows a non-uniform plasma density profile between the disk and the ring. Location and sense of rotation of the dust vortices coincides with the location and direction of the radial gradient in the ion drag force caused by the radial density gradient. The experimentally observed dust vorticity matches well with the calculated one using hydrodynamic formulations with shear in ion drag dominating over the dust charge gradient. These results corroborate that a radial gradient in the ion drag force directed towards cathode is the principal cause of dust rotation.

Dynamics of a confined dusty fluid in a sheared ion flow

Dynamics of an isothermally driven dust fluid is analyzed which is confined in an azimuthally symmetric cylindrical setup by an effective potential and is in equilibrium with an unconfined sheared flow of a streaming plasma. Cases are analyzed where the confining potential constitutes a barrier for the driven fluid, limiting its spatial extension and boundary velocity. The boundary effects entering the formulation are characterized by applying the appropriate boundary conditions and a range of solutions exhibiting single and multiple vortex are obtained. The equilibrium solutions considered in the cylindrical setup feature a transition from single to multiple vortex state of the driven flow. Effects of (i) the variation in dust viscosity, (ii) coupling between the driving and the driven fluid, and (iii) a friction determining the equilibrium dynamics of the driven system are characterized.

A 3D Monte-Carlo study of the W7-X island divertor transport for different magnetic configurations

With a magnetic structure depending on the rotational transform, the control coil current and the plasma beta, the low shear stellarator W7-X provides a high flexibility in modifying the edge magnetic islands to optimize the island divertor transport. The magnetic field around the islands is usually weakly stochastic and the degree of the stochasticity is affected by the shear, the control-coil field and the plasma current including the beta effect. In this paper, we analyse and compare the plasma and neutral transport for different divertor configurations using the 3D Monte Carlo code EMC3-EIRENE. Three typical divertor configurations covering a wide topology range have been studied. The effect of change in magnetic topology on the recycling-related island divertor operation is studied and the transport characteristics are obtained. An optimization is done with respect to the density control and particle balance in steady-state operations and the effects of stochasticity on the energy transport are characterized.

On the nonlinear trapping nature of undamped, coherent structures in collisionless plasmas and its impact on stability

An outstanding notion for collisionless plasmas is the essential nonlinear character of their coherent structures, which in the stationary, weak amplitude limit are described by a continuum of cnoidal electron and ion hole modes governed by a multiparametric nonlinear dispersion relation. The well-known discrete structure of undamped linear plasma modes is seamlessly embedded in this nonlinear continuum as the microscopic texture of plasma begins to reveal itself in the high temperature collisionless plasma limit. This transforms the linear-threshold-based operating mechanism of plasma turbulence into a fundamental nonlinear, multifaceted one. Based on a comprehensive three-level description of increasing profundity, a proof of this novel dictum is presented, which makes use of the joint properties of such structures, their coherency and stationarity, and uses in succession a fluid, linear Vlasov and a full Vlasov description. It unifies discrete and continuum limits by resolving the inevitable resonant r...

Observation of dust torus with poloidal rotation in direct current glow discharge plasma

Observation of dust cloud rotation in parallel-plate DC glow discharge plasma is reported here. The experiments are carried out at high pressures (∼130 Pa) with a metallic ring placed on the lower electrode (cathode). The dust cloud rotates poloidally in the vertical plane near the cathode surface. This structure is continuous toroidally. Absence of magnetic field rules out the possibility of E × B induced ion flow as the cause of dust rotation. The dust rotational structures exist even with water cooled cathode. Therefore, temperature gradient driven mechanisms, such as thermophoretic force, thermal creep flow, and free convection cannot be causing the observed dust rotation. Langmuir probe measurement reveals the existence of a sharp density gradient near the location of the rotating dust cloud. The gradient in the density, giving rise to a gradient in the ion drag force, has been identified as the principal cause behind the rotation of dust particles.

Kinetic Lagrange simulation of a source-driven magnetized oblique presheath

A source driven magnetized oblique presheath is analyzed by means of a kinetic model and a detailed kinetic simulation using a backward-Lagrangian characteristics based simulation procedure. The four-dimensional ion phase-space distribution function and its associated moments, namely, density, flow velocity, and parallel and perpendicular temperatures are simulated. Extra simplification in the moment derivation results from the simulation procedure which facilitates the use of a structured distribution of nodes in the three-dimensional velocity space. Strong kinetic effects appear in the oblique geometry as the two temperatures are observed to drop preferentially in the limits of normal and grazing incidences. The effects are reflected also in the detailed behavior of flow velocity profiles which show a finite dependence on the angle of magnetic field’s incidence in the region of magnetized presheath. A comparison of the kinetic solutions with those obtained from the existing fluid models is also presented.

Nonlinearly interacting trapped particle solitons in collisionless plasmas

The formulation of collective waves in collisionless plasmas is complicated by the kinetic effects produced by the resonant particles, capable of responding to the smallest of the amplitude disturbance. The dispersive plasma manifests this response by generating coherentnonlinear structures associated with phase-space vortices, or holes, at very small amplitudes. The nonlinear interaction between solitary electron phase-space holes is studied in the electron acoustic regime of a collisionless plasma using Vlasov simulations. Evolution of the analytic trapped particle solitary solutions is examined, observing them propagate stably, preserve their identity across strong mutual interactions in adiabatic processes, and display close correspondence with observable processes in nature.

Predictive two-dimensional scrape-off layer plasma transport modeling of phase-I operations of tokamak SST-1 using SOLPS5

Computational analysis of coupled plasma and neutral transport in the Scrape-Off Layer (SOL) region of the Steady-State Superconducting Tokamak (SST-1) is done using SOLPS for Phase-I of double-null divertor plasma operations. An optimum set of plasma parameters is explored computationally for the first phase operations with the central objective of achieving an effective control over particle and power exhaust. While the transport of plasma species is treated using a fluid model in the B2.5 code, a full kinetic description is provided by the EIRENE code for the neutral particle transport in a realistic geometry. Cases with and without external gas puffing are analyzed for finding regimes where an effective control of plasma operations can be exercised by controlling the SOL plasma conditions over a range of heating powers. In the desired parameter range, a reasonable neutral penetration across the SOL is observed, capable of causing a variation of up to 15% of the total input power, in the power deposite...