Density and confinement time limits of Ohmically heated tokamaks

The maximum electron density and confinement time of Ohmically heated tokamaks can be limited by radiation losses. This has been investigated with a 1-D electron energy balance, used to derive current density distributions. Application of ideal-MHD stability criteria then leads to closed regions of stability in (I, ne)-space. Subsequent application of resistive-MHD theory indicates many possible causes of the disruptive instability and extends the ideal-MHD density limit by about 1.4 times. Murakami scaling, modified by a term involving the plasma effective charge , arises naturally from the model and suggests that results should be plotted in (q−1(a), (neR/Bt) )-space. Further, for heavy impurities, large divergences from Alcator scaling of τe are obtained within the stable region, leading to a peaking of τe versus ne at a critical density which scales as q−1 (a) Bt/R. The nτ-limit imposed by stability is found to further restrict the ignition region when referred to axial radiation losses only.