Long Discharge Particle Balance and Fuel Retention in Tore Supra

Abstract

In the new CIEL configuration of Tore Supra, all the plasma facing components are actively cooled. The surface area of the wall that is covered with carbon measures about 15 m2 (2 to 4 m2 of which in close interaction with the plasma). Steady-state plasma conditions up to 4 min 25 s have been maintained in this configuration. In these experiments, the required gas injection to maintain the prescribed density remains constant during the whole discharge. The exhausted flux is also constant and equal to 40 ÷ 50% of the injected flux. Therefore, 50 ÷ 60% of the injected particles remain trapped in the vessel, the total retention being proportional to the plasma duration. Since the amount of gas recovered between shots or by He-glow discharges does not always balance the injected gas, it follows that a quantity of deuterium remains indefinitely trapped in the vessel, which appears as an infinite reservoir. This reservoir is believed to be dominated by co-deposited layers, as observed in several places of the vessel. The thickest deposits (up to 800 µm) are observed on the leading edge of the neutralizers of the pump limiter. They display a column-like shape (typical growth rate ~20 nm/s) and have a graphite-like structure. Their deuterium concentration is D/C ~ 1%. Conversely, in regions that are shadowed from the direct plasma flux, the deposits show a smoother shape and their deuterium content is typically ~10 ÷ 15%.