Modeling of eddy current distribution in the SST-1 tokamak

TL;DR

This paper models the distribution of eddy currents in the SST-1 tokamak's passive structures, highlighting their effects on magnetic flux, plasma startup, and equilibrium, using a toroidal-filament model validated by measurements.

Contribution

It introduces a toroidal-filament model to simulate eddy currents in SST-1 and compares the results with experimental magnetic diagnostics.

Findings

Eddy currents cause flux shielding and delay plasma startup.

The model accurately predicts eddy current distribution based on measurements.

Eddy currents influence magnetic null location and plasma equilibrium.

Abstract

The time varying currents in the Ohmic transformer in the SST-1 tokamak induce voltages that drive large eddy currents in the passive structures like the vacuum vessel and cryostat. Since the vacuum vessel and the cryostat are toroidally continuous without electrical breaks in SST- 1, this leads to a shielding effect on the flux penetrating the vacuum vessel. This reduces the magnitude of the loop voltage seen by the plasma as also delays its buildup. Also the induced currents alter the null location of magnetic field. This will have serious implications on the plasma breakdown and startup and corrective measures may be required in case of an insufficient loop voltage or an improper null. Further, the eddy currents distribution will be vital for the plasma equilibrium and need to be considered while reconstructing the equilibrium. Evolution of the toroidal eddy currents in SST-1 passive structures has been studied using a toroidal-filament model. The model calculations are compared with the measured signals in the magnetic diagnostics like the toroidal flux loops and magnetic pick-up coils installed on the SST-1.