Evaluation of soft x-ray average recombination coefficient and average charge for metallic impurities in beam-heated plasmas

The soft x-ray continuum radiation in tokamak fusion test reactor (TFTR) low-density neutral-beam discharges can be much lower than its theoretical value obtained by assuming a corona equilibrium. This reduced continuum radiation is caused by an ionization equilibrium shift toward lower states, which strongly changes the value of the average recombination coefficient of metallic impurities gamma-bar, even for only slight changes in the average charge Z-bar. The primary agent for this shift is the charge exchange between the highly ionized impurity ions and the neutral hydrogen, rather than impurity transport, because the central density of the neutral hydrogen is strongly enhanced at lower plasma densities with intense beam injection. In the extreme case of low-density, high neutral-beam power TFTR operation (energetic ion mode) the reduction in gamma-bar can be as much as one-half to two-thirds. We calculate the parametric dependence of gamma-bar and Z-bar for Ti, Cr, Fe, and Ni impurities on neutral density (equivalent to beam power), electron temperature, and electron density. These values are obtained by using either a one-dimensional impurity transport code or a zero-dimensional code with a finite particle confinement time. As an example, we show the variation of gamma-bar and Z-bar in different TFTR discharges