iallfrey_APS2019_final

Development of a Low Frequency Magnetic Field Sensor Array for the C-2W Experiment
I. ALLFREY, T. ROCHE, J. ROMERO, D. MADURA, G. SNITCHLER AND THE TAE TEAM TAE Technologies, Inc., 19631 Pauling, Foothill Ranch, CA 92610
Abstract:
In TAE Technologies’ current experimental device, C-2W (also called “Norman”) [1], record breaking, advanced beam-driven field reversed configuration (FRC) plasmas are produced and sustained in steady state utilizing variable energy neutral beams, expander divertors, end bias electrodes, and an active plasma control system. Fast magnetic fields in C-2W are thoroughly diagnosed using B-dot probes and Rogowski coils. However, these sensors have limited low-frequency response and may introduce long-timescale errors in integrators and ancillary electronics. It is beneficial to have an independent and absolute validation of these fields, which are quasi-DC on the plasma timescales. As such, an array of high-precision Hall sensor-based low frequency probes, has been developed. Details of the diagnostic, as well as preliminary data will be presented.
C-2W Overview[1]:
n C-2W is a magnetically confined fusion experiment.
n The experiment consists of quasi-DC magnetic fields, with flux conserver timescales shorter than plasma lifetimes. As such, accurate measurements of low-frequency field profiles are necessary.
Fig. 1: C-2W overview cartoon.
Fig. 2: View of probe location mounted on a DC magnet.
Hall Effect
n The Hall effect is causes a potential in a conductor orthogonal to both the current through the conductor and an externally applied magnetic field. This is due to the Lorentz force, !=#(%+'×)).
n The measured potential is proportional to the magnetic field, where polarity is in the direction of the applied field.
Magnetic Field Profiles:
n Magnetic field measurements correspond to expected fields, based on DC magnet power supply current measurement.
Data Analysis/Processing:
n Data is automatically processed after each shot.
n This allows us to programmatically verify that magnetic field measurements correctly correspond to experimental settings to validate C-2W’s engineering controls.
n This data can also be used for physics analysis by an end user or as part of an automated analysis.
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Magnet Current vs B-field
Magnetic Field
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Signal(A.U.)
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Hall Effect Sensor, Electronics and Cabling:
MDSPlus
Machine State Database
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Raw data
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Post-shot Processing
Fig. 3: Graphic of the Hall effect.
Fig. 6: Current in Formation DC coil compared to magnetic field.
n The magnetic fields vary during plasma lifetime. As plasma acts as a flux conserver, it perturbs the external magnetic fields.
Fig. 8: Schematic view of the data processing pipeline.
Plasma Feedback and Control:
n Eventually, the signals will be fed into our plasma feedback system [2], which provides real-time plasma control. For instance, this diagnostic can be used in the feedback loop for magnet current control.
References:
n [1] H. Gota et al., Nucl. Fusion 59, 112009 (2019) Formation of hot, stable, long-lived field-reversed configuration plasmas on the C- 2W device.
n [2] J. Romero et al., UP10.00127 The C-2W plasma control system: Overview and experimental results.
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Presented at the 2019 APS DPP meeting in Ft Lauderdale, FL October 24, 2019
Contact: ian@tae.com, 949-830-2117
n The sensor, Honeywell SS39ET, is sensitive to +/-1kG with a frequency response from DC to ~30kHz.
n Power and data are both transmitted over ethernet cable.
n The circuit consists of filtered power, the Hall sensor and a
unity-gain op amp to drive a low-impedance transmission line.
Fig. 4: Picture of the probe circuit board on its mounting fixture.
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Plasma creation
B-field vs Plasma, Shot 117406
plasma decay
Magnetic Field
Current decay
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Fig. 7: Magnetic field perturbation due to plasma.
Magnetic Field Model Validation:
n The probes are located such that they measure the return magnetic field lines, so as to not saturate the sensors. As we are not directly measuring the field of interest the signals must be compared to a magnetic field model.
n Once the array is fully fielded we will have a probe on every DC magnet, as well as certain areas of interest on the experiment.
n In the near future we will build a temporary array of sensors, used with a vacuum (plasma free) shot to verify that field lines internal to our vacuum boundary, which consists of flux conservers, are in agreement with our eddy current model.
5V
SS39ET Hall Sensor
Fig. 5: Simplified circuit schematic of the diagnostic electronics.