No abstract

A simple model based on non-ambipolar radial transport and planar sheath physics is used to describe the generation of radial electric fields and currents in the scape-off layer of the Tokamak de Varennes (TdeV) during divertor plate biasing. In general, the calculated predictions compare favourably with TdeV results over a variety of plasma conditions and divertor magnetic configurations. Validated by the experiment, the model is used to study the scaling laws of perpendicular ion mobility and to test existing related theories. Finally, the model is proposed as a useful tool for the design and upgrade of biased divertors through optimization of the plate and throat geometry.

The most promising concept for deep fuelling a reactor is by the injection of compact toroid (CT) plasmoids. The first results showing CT fuelling of a tokamak plasma, without any adverse perturbation t o the tokamak discharge, are reported. The Compact Toroid Fueller (CTF) device was used to inject a CT-spheromak plasmoid into the TdeV tokamak. Following the CT penetration, the tokamak particle inventory increased by 16%, the loop voltage and the plasma current did not change, and there was no increase in magnetohydrodynamic (MHD) activity. The number of injected impurities was low and dominated by non-metallic elements. The plasma diamagnetic energy and the energy confinement time increased by more than 35%.

Electrically insulated divertor plates are used on TdeV (Tokamak de Varennes) [18th EPS Conference on Controlled Fusion and Plasma Physics Berlin (European Physical Society, Petit-Lancy, 1991), Vol. 15C, Part I, pp. 1–141] to produce various biasing configurations, which can be decomposed into two basic modes. Plasma biasing, with a radial electric field Er in the scrape-off layer (SOL), is most promising for divertor applications. The Er field is produced with a particular divertor plate geometry, causing a nonambipolar radial current and a particle flow in the Er×BT direction, toward one of the divertors (the active divertor). The pressure and impurity retention in the active divertor are shown, in the Ohmic regime, to be strongly increased by biasing. He exhaust through this divertor is increased by a factor of almost 3 with modest biasing voltages and currents scalable to larger devices. Biasing also modifies the power repartition between the divertors, with the active divertor also receiving a larger fraction of the power.

The Electric Tokamak, a low field ITER sized device with R = 5 m, has been operating with well equilibrated clean plasmas since January 2000. Short, 0.9 s, discharges with a central energy confinement time τE(0) = 150 ms are now routinely obtained at a toroidal field B = 0.1 T with kTe, kTi≃120 eV. The discharges are feedback controlled in up/down position and in plasma current. Biased electrode driven H modes have been obtained that compare well with the results obtained on CCT by R.J. Taylor and align with the `neoclassical bifurcation' theory of K.C. Shaing. Very successful second harmonic ion heating has been demonstrated with the ICRF antenna outside the vacuum system and with 50% single pass absorption. ICRF heated discharges indicate that poloidal rotation sufficient for edge bifurcation (H mode) may soon be achieved by ICRF induced fast ion losses. The threshold electrode biasing current required for bifurcated poloidal rotation has so far been reduced by 70% owing to ICRH driven ion orbit loss. The remaining critical ICRF item needed for the exploration of high beta plasma equilibria is the demonstration of the required current profile shaping. It is expected that mode conversion in the ion-ion hybrid regime, with high field side launch, will allow the current drive required to approach and exceed the Troyon beta limit. In 1-D full wave calculations, high harmonic current drive appears most promising at higher beta. Achieving the goal of plasma equilibration near unity beta will require 10 s long discharges (at kT = 3 keV, ne = 3×1019 m-3, B = 0.25 T) because of current profile shaping requirements.