Transport and divertor properties of the dynamic ergodic divertor

Abstract

The concept of the dynamic ergodic divertor (DED) is based on plasma edge ergodization by a resonant perturbation. Such a divertor concept is closely related to helical or island divertors in stellerators. The base mode of the DED perturbation field can be m/n = 12/4, 6/2 or 3/1. The 3/1 base mode with its deep penetration of the perturbation field provides the excitation of tearing modes. This topic was presented elsewhere. In this contribution we concentrate on the divertor properties of the DED. We report on the characterization of the topology, transport properties in ergodic fields, impurity transport and density limit behaviour.The 12/4 base where the perturbation is restricted to the plasma edge is suitable for divertor operation. With increasing perturbation field island chains are built up at the resonance layers. Overlapping islands lead to ergodization. The plasma is guided in the laminar region via open field lines of short connection length to the divertor target. The magnetic topology is not only controlled by the coil current but especially by the edge safety factor. For appropriate edge safety factor we observe a strong temperature drop in the plasma edge, indicating an expansion of the laminar region, which is necessary to decouple the divertor plasma from the core plasma. The modifications of

Numerical analysis of particle recycling in the TEXTOR helical divertor

“…A significant impact of this magnetic structure on plasma transport has been observed experimentally [11,12,13,14,15]. However, the modification of edge plasma DED target Figure 1.…”

Helical modulation of the electrostatic plasma potential due to edge magnetic islands induced by resonant magnetic perturbation fields at TEXTOR

“…First, formation of a so-called laminar zone where interleaved short and long connection length magnetic field lines in the stochastic layer form a set of correlated magnetic flux bundles of different wall to wall connection length [31,32,33,34,35]. This configuration represents a helical scrape-off layer (SOL) and features reduced particle confinement when the dominant resonance layer is located inside of the ionization source [36] but can also yield improved particle confinement when the resonant surface is moderately perturbed [36,37,38].…”

Application of advanced edge diagnostics for transport studies in the stochastic boundary of TEXTOR-DED

“…The studies related to this topic and published in several papers (see for example [24,29,23,12,30,8,31,13,15,26,14]) revealed the basic transport mechanisms in close correlation to the modeled magnetic topology. These investigations gave rise to more detailed analysis of the generic transport processes in the stochastic boundary of TEXTOR-DED.…”

“…This again emphasizes the convective loss imposed by the short field lines and the diffusive characteristics of the ergodic domain. This findings could be elaborated in great detail due to the diagnostic capabilities at TEXTOR as described for example in [12,13,14,15,24]. The ergodic domains represent a diffusive transport domain and they show an increase in the radial particle transport by 20% for the m/n = 12/4 base mode.…”

“…The picture on the left shows results from Langmuir probe measurements (particle flux Γ (ϕ) along toroidal angle in upper plot and T e (ϕ) along toroidal angle in the lower plot) on the DED target where the flux tubes end, i.e. the downstream region (adapted from [24]). The middle figures sketches a projection of the toroidal evolution of a laminar flux tube in the poloidal plane.…”