Frequency compensation of a diamagnetic loop using a digital data acquisition system

A method is presented for frequency compensation of a diamagnetic loop that is magnetically coupled to a concentric stainless steel vacuum vessel. In contrast to previous approaches employing active RC filter circuits, this technique can be done entirely with digital computer signal processing. Absolute signal calibration and empirical measurement of the system transfer function are accomplished with a pulsed simulation solenoid. Maximum high frequency response is determined by transfer function noise limitations. Measurements made on the Maryland MIX-1 device show that although the plasma includes hot and cold ions and cold electrons, the diamagnetic loop signal is dominated by the hot-ion kinetic pressure. In particular, hot-ion temperatures deduced from a comparison of diamagnetic loop and ion density (probe) signals compare favourably with independent measurements made with an end loss analyser. Similarities between hot-ion energy and density confinement times indicate that energy confinement is dominated by hot-ion streaming losses.