Recycling and particle control in DIII–D

Abstract

Particle control of both hydrogen and impurity atoms is important in obtaining reproducible discharges with a low fraction of radiated power in the DIII–D tokamak. The main DIII–D plasma facing components are graphite tiles (40% of the total area and covering regions exposed to the highest heat fluxes) and Inconel. Hydrogenic species desorbed from graphite during a tokamak discharge can be a major fueling source, especially in unconditioned graphite where these species can saturate the surface regions. In this case the recycling coefficient can exceed unity, leading to an uncontrolled density rise. In addition to removing volatile hydrocarbons and oxygen, DIII–D vessel conditioning efforts have been directed at the reduction of particle fueling from the graphite tiles. Conditioning techniques include: baking to ≤400 °C, low power pulsed discharge cleaning, and glow discharges in deuterium, helium, neon, or argon. Helium glow wall conditioning, is now routinely performed before every tokamak discharge. The effects of these techniques on hydrogen recycling and impurity influxes are presented. The Inconel walls, while not generally exposed to high heat fluxes, nevertheless represent a source of metal impurities which can lead to impurity accumulation in the discharge and a high fraction of radiated power, particularly in H-mode discharges at higher plasma currents, Ip ≳ 1.5 MA. To reduce metal influx a thin (∼100 nm) low Z film (carbon or boron) has been applied on all plasma facing surfaces in DIII–D. The application of the boron film, referred to as boronization, has the additional benefit over a carbon film (carbonization) of further reducing the oxygen influx. Following the first boronization in DIII–D a regime of very high confinement (VH-mode) was observed, characterized by low ohmic target density, low Zeff, and low radiated power.