Impurity transport studies on MAST

Henderson, Stuart Scott and Summers, Hugh (2014) Impurity transport studies on MAST. PhD thesis, University Of Strathclyde.

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    Abstract

    Impurity transport is a subject of fundamental importance in plasma physics in general and in tokamak physics in particular. The behaviour of the various impurity species and the evolution of their concentration determines, among other things, the fuel dilution and the fusion reaction rate, the plasma radia tion pattern and the local energy balance, the plasma effective charge and resistivity and the neutral beam particle and power deposition profile. It is therefore impor tant to develop both a sound experimental base and reliable models to interpret the experimental results and to predict the transport properties of impurities. Time-dependent helium and methane gas puff experiments have been performed on the Mega Ampere Spherical Tokamak (MAST) during a two point plasma current, I p , scan in L-mode and a confinement scan at constant I p . For the I p scan, a dimensionless safety factor, q , scan was attempted by using a constant toroidal magnetic field and by moderating the beam power to match the plasma temperature. The temperature and magnetic field was also kept constant during the confinement scan to probe the effects of the electron density gradient. An evaluation of the He II ( n = 4 → 3) and C VI ( n = 8 → 7) spectral lines, induced by active charge exchange emission and measured using the RGB 2D camera on MAST, indicate that carbon experiences moderately higher rates of diffusion and inward convection than helium in the L-mode high I p plasma. Lowering I p in L- mode caused a moderate increase in the helium diffusion and co nvection coefficients near the plasma edge. Neoclassical simulations were carrie d out which indicate anomalous rates of helium and carbon diffusion and inward convection in the outer regions of both L-mode plasmas. Similar rates of helium diffusion are found in the H-mode plasma, however these rates are consistent with neoclassical predictions. The anomalous inward pinch found for helium in the L-mode plasmas is also not apparent in H-mode. An outward flux of helium and carbon is found at mid-radius in H-mode, corresponding the region of positive electron density gradient. Linear gyrokinetic simulations of one flux surface in L-mode using the gs2 and gkw codes were performed which show that equilibrium flow shear is sufficient to stabilise ion temperature gradient (ITG) modes, consistent with BES observations, and suggest that collisionless trapped electron modes (TEMs) may dominate the anomalous helium particle transport. A quasilinear estimate of the dimensionless peaking factor associated with TEMs is in good agreement with experiment. Collisionless TEMs are more stable in H-mode because the electron density gradient is flatter. The steepness of this gradient is therefore pivotal in determining the inward neoclassical particle pinch and the particle flux associated with TEM turbulence.