MODELING THE EDGE OF A TOKAMAK PLASMA

We find, using a time-dependent coronal model, that radiation from stationary (i.e., non-diffusing) low Z impurities cannot account for the experimentally observed radiation losses in tokamaks, whereas recycling low Z impurities might. Thus, we infer from this and other circumstantial evidence that impurities recycle from the wall into the plasma and out again, on time scales not much longer than the energy containment time. The (neo)classical influx of impurities provides an inward transport mechanism; the outward transport process is unknown. However, a recent and more exact (no mass ratio expansion) impurity transport calculation1has shown that a “temperature-screening” effect may inhibit or reverse at least the influx of low charge states of low mass impurities. Another quandry occurs in gas puffing experiments where the electron density at the plasma center rises rapidly. We have investigated neutral transport in the plasma edge in a number of different models, including an ANISN-type (NUTRLSN) transport code. The effects considered include geometry, energy-cascading, reflection and a proper accounting of the (small) power lost via charge-exchange. We find that the observed density rise cannot be explained by a combination of neutral influx and particle diffusion. In order to resolve these quandries relating to the plasma edge, we hypothesize a “new” transport model based on the developing, generalized theory of dissipative trapped-electron instabilities. In this model, the anomalous transport adds to neoclassical transport and affects only the “cross-field” processes. In the resultant model the electron temperature is determined by the anomalous electron heat transport, as usual. However, the particle density is determined by balancing the inward Ware pinch with the outward anomalous particle diffusion; the impurity density results from balancing the inward classical diffusion against the outward anomalous diffusion. The inward Ware pinch is found to be sufficiently rapid to explain the density increases observed in ORMAK gas puffing experiments and in ALCATOR up to 3 × 1014 cm−3.