Measurement of the Local Current Density Inside a Current Sheet Using a Rogowski Coil Array on UTST Merging Plasmas

Abstract

Plasma merging is a unique method of plasma start-up, and magnetic reconnection is a key process for plasma heating in this method. On the University of Tokyo Spherical Tokamak (UTST), we use a pickup coil array to obtain the macroscopic behavior of magnetic reconnection, but direct measurement of the localized current is still necessary to study the details of the magnetic reconnection process. A small Rogowski coil is a useful tool for measuring small scale currents, and we used five such coils to measure the radial structure near the X-points of merging plasmas on UTST. As a result, we have found current densities up to 1.5 MA/m², with a typical time scale of several micro seconds. The current density increases with the plasma current, and the average radial width is about 30 mm.