Observation of electric currents in diverted tokamak scrape-off layers

Abstract

A prototype tile current array, consisting of resistors to sense current flowing from six divertor tiles into the metallic vacuum vessel, was installed in DIII-D. Data show that typically 1-2 kA circulates poloidally around the single-null divertor scrapeoff layer (SOL). A current of similar magnitude flows vertically in the outer SOL of double-null plasmas and a much smaller current in the inner SOL. Little or no current flows across the short SOL connecting the inner and outer divertor strike points. The SOL current increases with core plasma current and edge localized modes (ELMs); it decreases at the onset of the H-mode and with heavy gas puffing. The SOL current jumps rapidly to 5-10 kA during giant ELM events. Three possible mechanisms to drive the SOL current are examined qualitatively with respect to the data, but the evidence is insufficient to identify the dominant one. During disruptions, poloidal current in excess of 100 kA flows to the wall.

Observation of SOL current correlated with MHD activity in NBI heated DIII-D tokamak discharges

“…5 imply that the SOLC flows toroidally in the same direction as the plasma current in this discharge. Assuming that T e was higher in the outboard divertor than in the inboard divertor in this discharge, as is often the case in this magnetic configuration (ion -B drift toward the X-point) in DIII-D, the current flowed in the same direction as the thermoelectric potential gradient, and is in agreement with earlier findings [4][5][6]. Figure 6 shows spiky features in a tile current signal (magneta) from the inboard ring #10B in the same discharge shown in the previous figure (but a different tile).…”

“…A swept strike point method may be used to overcome this limitation [5]. The strike point is moved in such a way to "sweep" a radial profile of SOLC over the edge of a tile ring (boundary between two adjacent rings), thereby varying with time the radial range of integration.…”

“…6.3). Nevertheless, a "total quantitative accounting" of the SOLC, i.e., total current measured on the outboard rings equaling the negative of total current measured on the inboard rings, is found difficult to accomplish in this and other [5] discharges in DIII-D as well as discharges in other devices [11]. Insufficient spatial resolution of the measurement is one reason for the difficulty.…”

“…The current density was 5 A/cm 2 in an L-mode LSN discharge in DIII-D in which the measured outboard (hotter side) divertor T e was 30-40 eV and an estimated T e difference was 20-30 eV [5]. A peak (spatial) parallel current density of ~ 9 A/cm 2 was reported for L-mode attached divertor discharges in JT60-U in which T e ~ 55 eV in the outboard divertor and 2 5 eV in the inboard divertor [11].…”

DIII-D Diagnostic Systems

“…In order to study the large vessel currents and forces that appear during vertical instability of diverted plasmas a tile current array (TCA) containing three tile current monitors (TCMs) was installed [84], each monitor consisting of a compensated Rogowski loop wrapped around the base of a selected divertor target tile. A set of six, more-sensitive TCMs was installed poloidally across the lower divertor target in 1990 [85]. These and all subsequent DIII-D TCMs have each of the instrumented tiles GENERAL ATOMICS REPORT A24642 electrically insulated from the vessel, and the tile current is conducted via a thin metal sheet electrode in pressure contact under the tile base to a sensing resistor, whose far end is securely bolted to the vessel wall.…”