Diamagnetic flux loop

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The diamagnetic loop is a type of flux-loop designed to pick up net toroidal flux. Nonzero plasma pressure leads to some amount of diamagnetism (i.e. a quantity of excluded flux), so that plasma pressure (specifically βp AKA βθ) can be measured by comparing a vacuum measurement of toroidal flux against toroidal flux through a poloidal cross-section which includes the plasma. Since the proportion of flux excluded by plasma diamagnetism is generally <<1%, this vacuum toroidal flux compensation is ideally performed in hardware by wiring the diamagnetic loop together with a compensation coil in opposite polarity. As a relatively direct measure of plasma pressure, this is especially useful for constraining equilibrium reconstructions.

Plasma Diamagnetic Loops

In-Vessel Loop

The
Fig. 1: In-vessel diagmagnetic loop (red), according to documentation from Sept. 2008, plotted with all in-vessel boundaries (black) and occupied center-stack space (solid black).
in-vessel diamagnetic loop is located near magnet N, just inside the right-hand midplane port in section M-N. The loop is not square, having unevenly chamfered corners (Fig. 1).

Key parameters:

  • Coil area (A=LxH): 756.0 sq. in. (4,801 cm2)
  • Estimated flux: 40.67 μWb/A in TF coils (uncompensated shot 100678)
  • Number of windings (N): 1 (2 exist but one is shorted)

As of Dec. 20th 2018 it appears that one of the in-vessel windings is somehow shorted to the vessel. Since both windings have completely independent pinouts at the feedthrough, the plan is currently to simply not use the shorted winding. A new, configurable compensation coil has been wound to match Weber-turns of a single in-vessel winding. As of Jan. 3rd 2020, the machine-side D-sub connector still has its plastic shell, which cannot be replaced until a flange with screwlocks is available for the digitizer-side connector.

Ex-Vessel Loop

A new, ex-vessel diamagnetic loop was installed in September 2019. The new loop passes through an unused TF center stack cooling channel and through one of the two unused windings in the TF case of Magnet E. Each end of each winding connects to a pin of a 9-pin D-sub connector, and the winding-to-winding connections and center-tap grounding are established by jumpers in the matching D-sub/differential-LEMO adapter.

Key parameters:

  • Coil area (A=LxH): 802.4 sq. in. (5,095 cm2)
  • Estimated flux: approx. 102 μWb/A in TF coils (from uncompensated shot 101414)
  • Number of windings (N): 4

The compensation coil for the ex-vessel loop is mounted inside the Diamagnetic Flux Loop#Configurable compensation coil.

Old In-Magnet Loop

A loop once existed which passed through a TF center stack cooling channel, but was removed sometime prior to operations resuming in 2018. This loop was not well documented, but may have provided data for LTX.

Compensation Coils

Two old compensation coils currently exist: a shielded compensation coil, and an unshielded double compensation coil.

Shielded compensation coil

Wrapped
Fig. 2: Shielded compensation coil.
on G10, with a copper tape electrostatic shield and 2-pin differential LEMO output. Due to the low measured resistance, the coil is probably shorted, although it is not shorted to the shield.

Key parameters:

  • Coil length (L): 17.3" (44.0 cm)
  • Coil height (H): 3.0" (7.6 cm)
  • Coil perimeter (P): 40.6" (103.12 cm)
  • Coil resistance (R): 0.547Ω (NB: probably internal short)
  • Coil area (A=LxH): 51.9 sq. in. (334.4 cm2)
  • Number of windings (N): ?
  • Target flux: ? μWb/A in TF coils (from prior diamagnetic loop calibration)
  • Nominal coil position (Rcent): ?

Unshielded double compensation coil

Wrapped
Fig. 3: Unshielded compensation coil.
on transparent plastic, with a triax connector output. Center-tap between two sets of windings seems to connect to triax connector shield.

Key parameters:

  • Coil length (L): 14.4" (36.5 cm)
  • Coil height (H): 3.5" (9.0 cm)
  • Coil perimeter (P): 35.8" (90.93 cm)
  • Coil resistance (R): 28Ω (14Ω each)
  • Coil area (A=LxH): 50.4 sq. in. (328.5 cm2)
  • Number of windings (N): 34.2 (17.1 each)
  • Target flux: 1.2226 μWb/A in TF coils (from prior diamagnetic loop calibration)
  • Calculated coil position (Rcent): ~50cm

Configurable compensation coil

Wrapped
Fig. 4: Configurable compensation coil.
on a G10 frame, with a triax connector output. Center-tap between two sets of windings and shimstock coil shield both connect to triax connector body. The coil mounts into a slot which allows two-axis (machine-radial sliding and rotation in the R-φ plane) configuration of the coil's position.

Key parameters:

  • Coil length (L): 8" (20.3 cm)
  • Coil height (H): 3" (7.6 cm)
  • Coil perimeter (P): 22" (55.9 cm)
  • Coil resistance (R): 3.0Ω (2x1.5Ω)
  • Coil area (A=LxH): 24 sq. in. (154.8 cm2)
  • Number of windings (N): 32 (2x16)
  • Target flux: 1.271 μWb/A in TF coils
    • Measured flux: approx. 1.232 μWb/A in TF coils (comparing compensated shot 100679 to uncompensated shot 100678)
    • Flux error: 1.3Wb/A in TF coils (~3%)
  • Estimated coil position (Rcent): 35.5cm
  • Anticipated alignment angle (θalign): -1.5°<θalign<1.5°

The compensation coil for the Ex-Vessel Loop is wound on a one-piece G10 frame with a differential LEMO connector output, and mounted inside the configurable compensation coil. This coil can slide machine-radially relative to the configurable compensation coil.

Key parameters:

  • Coil length (L): 4.5" (11.43 cm)
  • Coil height (H): 1.4" (3.56 cm)
  • Coil perimeter (P): 11.8" (30.0 cm)
  • Coil resistance (R): ?Ω (2x?Ω)
  • Coil area (A=LxH): 6.3 sq. in. (40.64 cm2)
  • Number of windings (N): 52 (2x26)
  • Target flux: 0.26 μWb/A in TF coils w/ 30x amplification
    • Measured flux: 0.3155 μWb/A in TF coils (measuring separately)
    • Amplification target: x25.067
      • Division set: x26.8 ( 26.075*(102.1/99.39) w/ 2 integrator channels )
      • Flux error: -7%
    • Amplification target: x25.067
      • Amplification set: x???? Not done yet
      • Flux error: -7%
  • Estimated coil position (Rcent): 35.5cm

Work Remaining

As of Dec. 20th 2018:

  • Calibrate compensation coil
    • Requires calibration procedure
    • Will need to fire TF magnets to test compensation
    • Will need plasma to verify cancellation of plasma current poloidal field

Acquisition

The diamagnetic loop's feedthrough is a 6-pin Mil-C on a T off of the lower above-midplane port at section M-N. The two windings are independent, and are connected to the compensation circuit box (see photodocumentation here), whose output connects to integrator 3 channel 32, from which it passes through a voltage divider switch box (see photodocumentation here), and is then digitized on Neos input 93. The compensation coil is mounted below section H-I.

Neos input 94 is reserved for a second diamagnetic loop, but this would need to be characterized and connected.

Related Pages

Photo galleries: Old Documentation | Compensation and Acquisition

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

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