Transport of Carbon Ion Test Particles and Hydrogen Recycling in the Plasma of the Columbia Tokamak "hbt".

Carbon impurity ion transport is studied in the Columbia High Beta Tokamak (HBT), using a carbon tipped probe which is inserted into the plasma (n_ {e}~ 1 - 5 times 10^{14 } (cm^{-3}), T_{e}~ 4 - 10 (eV), B_{t }~ 0.2 - 0.4 (T )). Carbon impurity light, mainly the strong lines of C_{II} (4,267A , emitted by the C^+ ions) and C_{III} (4,647A , emitted by the C^{++} ions), is formed by the ablation or sputtering of plasma ions and by the discharge of the carbon probe itself. The diffusion transport of the carbon ions is modeled by measuring the space-and-time dependent spectral light emission of the carbon ions with a collimated optical beam and photomultiplier. The point of emission can be observed in such a way as to sample regions along and transverse to the toroidal magnetic field. The carbon ion diffusion coefficients are obtained by fitting the data to a diffusion transport model. It is found that the diffusion of the carbon ions is "classical" and is controlled by the high collisionality of the HBT plasma; the diffusion is a two -dimensional problem and the expected dependence on the charge of the impurity ion is observed. The measurement of the spatial distribution of the H_{alpha} emissivity was obtained by inverting the light signals from a 4-channel polychromator; the data were used to calculate the minor -radial influx, the density, and the recycling time of neutral hydrogen atoms or molecules. The calculation shows that the particle recycling time tau_ p is comparable with the plasma energy confinement time tau_ E; therefore, the recycling of the hot plasma ions with the cold neutrals from the walls is one of the main mechanisms for loss of plasma energy.