Transport simulations of a density limit in radiation-dominated tokamak discharges—profile effects

Abstract

The density limit observed in tokamak experiments is thought to be due to a radiative collapse of the current channel. A transport code coupled with an MHD equilibrium routine is used to determine the detailed, self-consistent evolution of the plasma profiles in tokamak discharges with radiated power close to or equalling the input power. The present work is confined to ohmic discharges in steady state. It is found that the shape of the density profile can have a significant impact on the variation of the maximum electron density with plasma current. Analytic calculations confirm this result.

Transport Simulations of a Density Limit in Radiation-Dominated Tokamak Discharges II

“…The similarity of the behavior in these runs to those using coronal equilibrium leads us to conjecture that the use of flat density profiles would again give rise to a maximum density which is nearly independent of plasma current. 7 Although we have not made attempts to model specific experimental dis charges in detail and have not incorporated a great deal of physics (beyond impurity radiation) into our simulations, the trends we are finding are largely consistent with existing density limit data. We conclude that the evidence for the density limit being precipitated by excessive impurity radiation is very strong indeed.…”

“…There is experimental evidence supporting the P*£ scaling, U2A although some devices have reported a weaker dependence of the maximum density on auxiliary input power. [3][4][5][6][7][8][9][10][11][12][13] In fact, JET has proposed a scaling for the edge density 1 which is proportional to P^H-Note that the present theory as well can be more easily phrased in terms of the edge rather than the average density. That is, only the edge density appears in Eq.…”

“…Here, we use the model developed in Ref. 7 (referred to hereafter as I) to once again focus on the density limit set by radiation. The previous work dealt only with Ohmic discharges in a TFTR geometry.…”

On threshold of radial detachment in tokamaks

“…These results contradict, however, the observations of radial poloidally symmetric detachments in experiments and numerical simulations of this process. 11,12 The above said demonstrates the necessity for further reexamining of the detachment mechanism. This is indispensable not only from the academic point of view, in order to resolve the contradictions outlined, but also can bring a new insight into the physics of phenomena caused by impurity radiation.…”