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 measuremen t of the energy stored in a magnetized plasma. Two concentric diamagnetic l oops enclose a plasma column. The advantage of this method is the capabilit y to eliminate to a high extent the background magnetic field and the effec t of eddy currents in the measurement of relatively small diamagnetic signa ls. 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 res olution 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 estim ate are presented. (C) 2001 American Institute of Physics.