Impurity studies on ZT-40M, ZTH and RFX using a zero-dimensional model for the reversed field pinch

Abstract

A time dependent zero-dimensional model of plasma transport for reversed field pinch devices is developed to perform parametric studies on next generation experiments. The model includes energy balance equations for electrons and ions. Particle density and current are specified quantities. Impurities are treated using a coronal nonequilibrium model. Particle leakage and recycling are included for plasma and impurities. The particle lifetime is a specified parameter. The energy confinement times for electrons and ions are specified as multiplicative coefficients times the Ohmic energy confinement time. Ohmic heating is enhanced by an anomaly factor, and a direct ion heating term is included via anomalous resistive heating represented by a multiplicative coefficient times Ohmic heating. A single set of coefficients is found to simulate a variety of ZT-40M runs with different currents and wall conditions. The resulting model is employed to perform parametric analysis on the behaviour of ZTH at 2 MA operation. A confinement law consistent with the Connor-Taylor scaling is used. Three parameter variations are evaluated: I/N from 3 x 10' 14 A-m to 6 x 10" 14 A-m, the impurity recycle time from 5 ms to 1000 ms, and the carbon percentage from 1% to 10%. It is found that the temperature at the end of the flat-top phase of the current profile is proportional to I/N, and the proportionality constant increases with the particle confinement time r p . For fixed I/N, the temperature increases with r p largely because a reduction of the impurity recycle reduces the radiation loss. When the model is applied to RFX, the temperatures are almost identical to those in ZTH if r p scales as the square of the plasma radius.