Precision current measurements on Pegasus II using Faraday rotation

The authors measure the current on the Los Alamos pulsed power machine, Pegasus II, using the Faraday rotation technique in a twisted, low-birefringence optical fiber. This technique yields results which are reproducible to within about 1%. When comparing their results with data from a Rogowski loop and from B-dot loop detectors, they find discrepancies larger than the uncertainties in the measurements. They have calibrated their system in three different ways: (1) the Pegasus II experiment was driven into a shorted load in a ring-down test to measure the load inductance. The measured Faraday data were fitted to a damped sinusoidal equation and compared with current calculated from the measured voltage and capacitance; (2) a single capacitor drove about 3 kA of current into a 403 turn solenoid coil. A Pearson transformer calibrated to about 1% measured the current supplied to the coil and the Faraday data were compared with the Pearson data; and (3) on a separate machine, a calibrated Rogowski coil provided direct comparison with fiber optic Faraday measurements. The Verdet constant has been measured for bulk silica glass at a wavelength of 633 nm by several researchers. The authors extrapolated these averaged results of 4.61 radians/MA to their wavelength of 830 nm and corrected it for the 4% germania dopant in the glass from which their optical fiber was fabricated. They obtained results from all three methods consistent with a Verdet constant 6% smaller than the extrapolated value. They are continuing to investigate this discrepancy and are working with NIST to measure the Verdet constant in their glass at 830 nm.