Improved magnetic diagnostics on General Fusion Plasma Injector 3

General Fusion (GF) recently completed a campaign to improve the accuracy of the Mirnov coils (B-probes) in their largest plasma injector machine, Pi3. This is part of a broader project aimed at reaching the 1% relative uncertainty required for the magnetic diagnostics on GF's Fusion Demonstration Plant (FDP).

The main sources of uncertainty were identified across the full diagnostic stack (sensor, data acquisition, calibration, data processing) through bench tests and simulations. The single largest source of error for most sensors was found to be temperature-dependent drifts of the DAQ gain, corrected with hardened components and software corrections based on predictive models of the system response. A robust calibration strategy was also devised to better constrain and quantify residual errors from the frequency calibration required by GF’s diagnostic setup.

The resulting improvements brought the uncertainty on Pi3's B-probes from an estimated relative error of ~15% to a measured ~5% on most sensors. No significant offsets are detected for B-probes at the same axisymmetric locations, whose measurements fall within the typical sensor’s instrumental noise (<~5 mT), confirming the accuracy of the improved sensors.