Session 7IB - Alternative Concepts.
INVITED session, Thursday morning, November 14
Grand Ballroom, Adam's Mark
The world's first high power auxiliary heating experiments in a tight aspect ratio (or spherical) tokamak have been performed on START at R/a\sim1.25, using the 40keV, 0.5MW Neutral Beam Injector loaned by the Oak Ridge National Laboratory. Beam performance is being increased in stages. At present 30keV hydrogen neutrals are injected into hydrogen plasmas, and this provides a combination of higher density and effective heating of both ions and electrons, with no adverse MHD instabilities seen so far. Theoretically, the beta limit diagram (\beta_T vs. I/aB_T) for START is large in comparison to that for a conventional tokamak, because much higher values of normalised plasma current can be achieved whilst retaining q_\psi(a) sufficiently high for kink stability. For example, calculations show(T.C.Hender et al, Proc.22nd EPS, Bournemouth III) p233 (1995) that in START, \beta_T = 28% (with \beta_N = 4.5) should be stable to both low-n kinks and n\rightarrow\infty ballooning modes. The highest \beta value achieved to date in START is in shot 30234, with \beta_T=8% and central beta \beta_0 = 48%, maintained for about 2 energy confinement times. Already high, these values are expected to further increase with use of higher beam power and with longer plasma discharges. Electron thermal pressure is measured by a 30-point Thomson scattering diagnostic, ion pressure by measuring the central (thermal) ion temperature by a neutral particle analyser and by charge exchange spectroscopy and assuming the same radial profile as the electrons, with dilution factor of 0.8. The \underlinetotal plasma pressure, including the fast ion component, is estimated both by equilibrium reconstruction and by kinetic measurement supplemented by calculation of the fast ion population with a Fokker-Planck code. These early results give a convincing demonstration of the effectiveness of beam heating in spherical tokamak plasmas which will be further exploited in START and its successor machine, MAST, which is now under construction at Culham.
\parAcknowledgement This work is presented on behalf of the START and NBI teams. We wish to acknowledge the enthusiasm and practical assistance of ORNL personnel, in particular Martin Peng, Jim Tsai and Dick Colchin. This work is funded by the UK Department of Trade and Industry, Euratom and the US DoE.