Diamagnetic double-loop method for a highly sensitive measurement of energy stored in a Stellarator plasma

In this article we describe a diamagnetic double-loop method for measurement of the energy stored in a magnetized plasma. Two concentric diamagnetic loops enclose a plasma column. The advantage of this method is the capability to eliminate to a high extent the background magnetic field and the effect of eddy currents in the measurement of relatively small diamagnetic signals. This method was applied to initial plasmas produced in the helical axis stellarator Heliotron J. A stored energy of 0.7 kJ was measured with a resolution of ±0.02 kJ at rippling magnetic field of 1 T in a plasma heated by rf discharges up to 400 kW. This diamagnetic double-loop method improved the signal to noise ratio of the conventional single loop method sixfold. The magnetic diagnostics hardware, experimental results, and an error estimate are presented.