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A thin boron film was applied to the DIII-D tokamak plasma facing surfaces to reduce impurity influx, particularly oxygen and carbon. A direct result of this surface modification was the observation of a regime of very high energy confinement, VH-mode, with confinement times from 1.5 to 2 times greater than predicted by the H-mode scaling relation for the same set of parameters. VH-mode discharges are characterised by low ohmic target densities, low edge neutral pressure, and reduced recycling. These conditions have reduced the collisionality (v(*)) in the edge region, producing a higher edge pressure gradient and a significant bootstrap current, up to 30% of the total current. The edge plasma properties after boronisation are described, including reductions in recycling inferred from measurements of tau(p)(*). In particular the edge plasma conditions necessary for access to VH-mode, including the boronisation process and properties of the deposited film, are discussed.

Additional information

Authors: JACKSON G L ET AL., General Atomics, San Diego (US);WINTER J, Kernforschungsanlage Jülich GmbH, Institut für Plasmaphysik, Jülich (DE);MOYER R, University of California, Los Angeles (US);WATKINS J G, Sandia National Laboratories, Livermore (US)
Bibliographic Reference: Report: GA-A20904 EN (1992) 17 pp.
Availability: Available from General Atomics, P.O. Box 85608, San Diego, CA 92186-9784 (US)
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