Ion flows in the scrape-off layer with biased limiter: implications for Tore Supra toroidal pumped limiter design

Abstract

A simple analytic fluid model based on particle and momentum conservation describes the response of the scrape-off layer to an applied radial electric field. The model explains why the variation of plenum pressure during limiter biasing in TEXTOR shows the opposite behaviour as divertor biasing in Tokamak de Varennes: the mechanical baffles that define the throats of the pump limiter are aligned along the magnetic field, thus preventing collection of the induced perpendicular flux when the drift is directed towards the scoop. Rather, the optimal pumping is expected when the drift is directed away from the scoop due to the increase of the parallel flux needed to satisfy the Bohm-Chodura criterion. Qualitative agreement is obtained with measurements of the parallel flow near the limiter throat, and with measurements of the plenum pressure during negative biasing for both directions of toroidal magnetic field. The results, extrapolated to Tore Supra, predict that the actual design with throats should provide close to optimal pumping with minor modification of the natural radial electric field, but greater performance could be obtained with negative biasing if a toroidally symmetric neutralizer geometry were adopted.

Measurement and simulation of edge plasma flow in the Tore Supra tokamak

“…The model has had great success in explaining some of the basic features of poloidal asymmetries that result from the action of a poloidal E × B drift, especially during SOL biasing experiments [8]. A strong imposed radial electric field driving a near-sonic poloidal drift towards the low-field side (LFS) of the ALT-II limiter in TEXTOR was observed to slow the parallel flow to stagnation, in good quantitative agreement with the simple model [9]. The same model can predict the angular distribution of ion current around a cylindrical electrodein the presence of drifts [10], in excellent agreement with a 2D kinetic calculation [1].…”

Local Effects of Biased Electrodes in the Divertor of NSTX

“…Therefore we can not assume a simple linear relationship between the LiI light level and the local electron density, and so use the LiI light only as a qualitative indication of where in space and time the electrodes were changing the local plasma. Figure 3 shows the way in which the electrode biasing was applied for [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] successive modulations cycles during the plasma current flat-top period for each shot in the database. Each modulation cycle consisted of 10 msec with the biasing on followed by 10 msec with the biasing off.…”

“…[6][7][8][9][10][11][12][13][14][15][16][17]. However, most of these aimed to create axisymmetric radial or poloidal electric fields in the plasma edge using electrodes or large-area wall biasing to control the edge plasma transport.…”

The Toroidal Pump Limiter ALT-II in TEXTOR

“…ALT-II is electrically isolated from the liner and the wall; this allows the application of an electrical voltagẽ biasing! 62,63 between the belt limiter and the liner or the inner bumper limiter, which is electrically connected with the liner. Essentially, two configurations of the plasma positioning for biasing have been studied:~a!…”