Biasing and improved divertor performance on the TdeV tokamak

Electrical biasing of the plasma is used on the TdeV tokamak (R = 0.86 m, a = 0.26 m, Ip = 250 kA) to improve the divertor impurity confinement and exhaust pumping capability, two important issues for fusion research. Plasma biasing, applied through the divertor neutralization plates, strongly affects the plasma properties in the scrape off layer (SOL), modifying the plasma potential, the edge density profile and ion flow in the SOL. The ErxB flow, associated with the radial electric field established near the separatrix, increases impurity transport into the divertor. Biasing greatly increases the efficiency of the divertor. There is a linear dependence of divertor pressure on the bias voltage; negative biasing having the strongest dependence. For negative biasing, divertor pressure is a non linear function of plasma density. Negative biasing also increases impurity retention in the divertor. The high pressure and increased impurity retention in the divertor produced by negative plasma biasing can greatly increase the efficiency of a pumped divertor for plasma impurity control and He ash removal in a reactor. External fueling will compensate for the plasma transport to the divertor to maintain a constant plasma density