MODELING OF THE TOROIDAL ASYMMETRY OF POLOIDAL HALO CURRENTS IN CONDUCTING STRUCTURES

During plasma disruptions, substantial toroidal and poloidal eddy curr ents are generated in the vacuum vessel and other plasma facing conduc ting structures. Eddy currents that conduct charge through paths which close through the plasma periphery are called halo currents, and thes e can be of substantial magnitude. Of particular concern for tokamak d esign and operation is the observed toroidal asymmetry of the halo cur rent distribution: such an asymmetric distribution leads to problemati c non-uniform forces on the conducting structures. The premise is adop ted that the source of toroidal asymmetry is the plasma deformation re sulting from the non-linear external kink instability that develops du ring the current quench phase of a disruption. A simple model is prese nted of the kinked plasma that allows an analytic calculation of the d ependence of the toroidal peaking factor (TPF) on the ratio of the hal o current to the total toroidal plasma current, I-h/I-p. Expressions f or the TPF as a function of I-h/I-p are derived for m/n = 2/1 and m/n = 1/1 helical instabilities. The expressions depend on a single parame ter, which measures the amplitude of the saturated state of the kink i nstability. A comparison with disruption data from experiments shows g ood agreement. Numerical experiments that simulate non-linear external kinks provide guidance on the values expected for the saturated ampli tude. It is proposed that a simple plasma halo model is adequate for a ssessing the engineering impact of asymmetric halo currents, since the force distribution on the conducting structures depends mainly on the 'resistive distribution' of the eddy currents. A brief description is given of an electromagnetics code that calculates the time developmen t of eddy currents in conducting structures, and the code is applied t o two halo current disruption scenarios. These are used to emphasize t he importance of having an accurate eddy current calculation to correc tly estimate the engineering impact of tokamak disruptions.