Shell eddy sensors
An array of seven two-axis Mirnov probes, totaling 14 coils. These sensors will be useful for characterizing eddy currents flowing through the limiting shells by direct measurement.
Design
The shell eddy sensor array uses two-axis Mirnov probes scavenged from the old Gap Mirnov array. Each orthogonal pair of Mirnov coils is wound onto a magnesia (MgO) ceramic form with ceramic-clad wire.<
Coil Parameters:
- Length (L): 0.90" (2.3cm)
- Width (W): 0.45" (1.15cm)
- Flux Area (A=WxD): 0.410 sq. in. (2.64cm2)
- Number of windings (N): 24 (P) or 22 (T)
- Sensitivity:
- Predicted:
- Perpendicular: 6.348 mVs/T
- Tangent: 5.819 mVs/T
- Measured:
- Perpendicular: 6.348 ± 0 mVs/T
- Tangent: 5.819 ± 0 mVs/T
- Predicted:
Each probe form is mounted by #2-56 screws to a 1/16" thick 316 stainless steel plate bent to shape for its location on the shell. Drawings of the mounting plates can be found here. The array is mounted entirely to the upper north shell, and consists of two probes mounted at the toroidal shell edges near the top of the shell, two probes attached just below the to the shell support "boomerang," and three probes approximately 2" above the midplane shell edge (Fig. 1).
Sensor Locations
Sensor
- "East Edge Perpendicular&Tangent " (EEP/EET) mounted near the top of the upper shell edge near magnet I.
Routs through east D-sub to pins X and Y (P) and W and Z (T) of the 32-pin connector at the upper-top port of bay M-N. - "East Upper Perpendicular&Tangent " (EUP/EUT) mounted below the shell boomerang at port J-K.
Routs through east D-sub to pins X and Y (P) and W and Z (T) of the 32-pin connector at the upper-top port of bay M-N. - "East Midplane Perpendicular&Tangent " (EMP/EMT) mounted near the midplane at port J-K.
Routs through east D-sub to pins X and Y (P) and W and Z (T) of the 32-pin connector at the upper-top port of bay M-N. - "North Midplane Perpendicular&Tangent " (NMP/NMT) mounted near the midplane at port L-M.
Routs directly to pins X and Y (P) and W and Z (T) of the 32-pin connector at the upper-top port of bay M-N. - "West Midplane Perpendicular&Tangent " (WMP/WMT) mounted near the midplane at port N-O.
Routs through west D-sub to pins X and Y (P) and W and Z (T) of the 32-pin connector at the upper-top port of bay M-N. - "West Upper Perpendicular&Tangent " (WUP/WUT) mounted below the shell boomerang at port N-O.
Routs through west D-sub to pins X and Y (P) and W and Z (T) of the 32-pin connector at the upper-top port of bay M-N. - "West Edge Perpendicular&Tangent " (WEP/WET) mounted near the top of the upper shell edge near magnet P.
Routs through west D-sub to pins X and Y (P) and W and Z (T) of the 32-pin connector at the upper-top port of bay M-N.
Calibration
Calibration on the Helmholtz coil yielded considerably higher sensitivities (~60 mVs/T) than expected (~6mVs/T), but with fairly high precision (~1-2%) at 100Hz, 300Hz, 1kHz, and 3kHz, suggesting that the discrepancy is not in the experimental setup. 11 of 14 sensors have good sensitivity; all three faulty (i.e. very low signal) sensors appear to be shell-tangential (T-axis) coils. There is noticeable frequency dependence on many but not all sensors. Sensitivity dropped by 10-20% at 10kHz on 6 of 11 good sensors (8 of 14 total); all shell-perpendicular (P-axis) sensors have some non-zero drop in sensitivity at 10kHz, but none of the four good shell-tangential coils do. Mean sensitivity of the 11 good coils is ~59.4±6.3mVs/T, with the remaining six having sensitivities of approximately 26.4 (WET), 7.0 (NMT), and 2.3 (WUT) mVs/T.
Individual coil calibration results and LEMO breakout polarities can be found in this Google spreadsheet.
Installation
During installation of the shell eddy sensors, it was found that some damage had been sustained on the leads, in particular a broken lead from EUT (probably due to insufficient strain relief and excessively rough handling during installation) and shorting between NMP and NMT, and intermittently from NMT to the braided shield (and therefore to the shell and vessel).
The broken lead on EUT was repaired by trimming and rebuilding that pin of the east D-sub.
The shorting on the NMx sensor pair was repaired by adding individual fiberglass sleeves to the two twisted pairs, where they had previously shared a single fiberglass sleeve; it's suspected that one twisted lead pair rubbed against the other and they abraded through one another's ceramic insulation. The sleeves were propagated down into the pass-through hole in the mounting plate where it was suspected NMT shorted to the plate. Later, similar coil-to-coil shorting was found on EUx as well, and the same repairs were conducted on that assembly.
Acquisition
The shell eddy sensor array uses 14 channels exiting the vessel from a 32-pin Mil-C feedthrough at the upper-top port of bay M-N (Fig. 2). The twisted-pair shielded cable terminates in a 37-pin female D-sub, which mates to a D-sub/differential-LEMO breakout adapter. Each LEMO connector is individually labelled with its shell eddy sensor name. The breakout adapter shell is tied to the magnetics rack ground bus.
Sensors 01-07 (EEP, EUP, EMP, NMP, WMP, WUP, and WEP) connect to ~10,000x-gain integrator 5 channels 1-7 and Helios channels 1-7. Sensors 06-14 (EET, EUT, EMT, NMT, WMT, WUT, and WET) connect to ~10,000x-gain integrator 5 channels 17-23 and Helios channels 17-23.
Cross-reference this Google Doc for integrator and digitizer usage information.
Related Pages
Shell eddy sensor photo documentation gallery
Diagnostics
Magnetics (Local): Reentrant array | Toroidal array | Saddle coils | Shell eddy sensors
Magnetics (Areal): Poloidal flux loops | Loop voltage | Diamagnetic loop | IP Rogowski
Spectroscopy: Thomson scattering | ChERS | X-ray spectroscopy | Spectrometry | Vacuum UV spectroscopy
Microwaves: Reflectometry | Interferometry
Physical Probes: HFS edge probes | LFS SOL probes | RFEA
Operational Diagnostics: Ion gauges | Coil current monitors | Thermocouples | RTDs
