Evaluation of plasma current quench time using a discrete Rogowski coil in the KSTAR tokamak

Abstract

The plasma current was earlier measured by a Rogowski coil on the wall inside the vacuum vessel of the KSTAR tokamak. After installing in-vessel components, this measurement included toroidal eddy currents induced on the in-vessel components between plasma boundary and the wall in the transient phases. The plasma current is now evaluated by using the sum of local poloidal magnetic field measurements, as a discrete Rogowki coil measurement, at the plasma facing components along a poloidal circumference that does not encompass the eddy currents. These two measurements agree to within a few percent in the flat-top phase of the discharge. The current quench time previously overestimated by the Rogowski coil measurement can be now evaluated from the linear fit on the wave-form of the plasma current excluding the eddy current during the disruption, so it is more accurately estimated as 2.7 ± 1.0 ms for the plasma current of 0.3–1.1 MA. In addition, an estimate of the eddy currents is obtained by subtracting the discrete Rogowski coil measurement from the Rogowski coil measurement in the transient phases, and its magnitude is validated by comparing with the toroidal current directly measured at the gap resistors connecting two adjacent passive stabilizers.