Diamagnetic diagnostics of rapid events in the T-15MD tokamak

Abstract

The resistive wall effect on the diamagnetic signal is analysed. The calculations are performed by the DINA code for the T-15MD tokamak. The accompanying analytics reveals the main dependencies allowing easy extrapolation of the results to any other tokamak. It is explicitly shown that the key parameter in the task is the decay time of the poloidal current induced in the wall. This current can be neglected for the events with transient times longer by factor of ten. If the plasma evolves faster, the difference between the measured signal and the plasma-produced variation of the toroidal magnetic flux will be larger than 10%, up to 100% for typically rapid thermal quenches. Incorporation of the induction equation for the wall current greatly improves the diagnostic capabilities.

Measurements and modelling of diamagnetic flux in ASDEX Upgrade

“…Correction terms related to tilting and expansions of coils are not found to make significant contributions on ASDEX Upgrade. Eddy current contributions are expected to be important only for events with transient times smaller than the resistive wall time in the poloidal direction (about 10 ms on ASDEX Upgrade) [30]. Contributions to the diamagnetic flux measurement by wall currents in the vacuum vessel are expected to be negligible compared to the plasma current correction term in the steady state phase of the discharge.…”