Energy and impurity transport in the Alcator C tokamak

Energy and impurity confinement times, tausub(E) and tausub(I), have been investigated in Ohmically heated Alcator C discharges in the following parameter range: density n-bar<=8x1020 m-3, current Isub(p)<=0.75 MA and toroidal field Bsub(T)<=13 T. Geometrical dependences have been studied by operation with full circular apertures of various major and minor radii, R, asub(l). In all cases the dependence of energy confinement time on n-bar is characterized by a low-density regime, in which electron conduction dominates and tausub(E) increases linearly with n-bar, and a high-density regime, in which ion conduction dominates and tausub(E) increases only slowly or not at all with n-bar. At all densities, the dependence of tausub(E) on asub(l) is weak. At n-bar=2x1020 m-3, the results of size-scaling experiments are summarized by the relation tausub(E) is proportional to asup(0.8) Rsup(2.3). At high density, the inferred ion thermal conductivity chisub(i) apparently exceeds the neoclassical value of Hinton and Hazeltine by a factor which varies from 2 to about 4 depending on geometry and plasma parameters. Other interpretations of the data are, however, possible. Impurity confinement has also been studied in these discharges using the laser blow-off technique. The principal new results concerning tausub(I) are (1) independence of the charge and mass of the injected impurity ion; (2) a linear dependence on asub(l); (3) a rapid deterioration as a function of the amplitude of the m=2,3 tearing modes; and (4) an approximately linear dependence on the effective charge. (author)