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Plasma Diagnostics

AI/ML FOR FUSION SUMMER SCHOOL

DR. EVDOKIYA (EVA) KOSTADINOVA

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Outline

  • Classification of plasma diagnostics
  • Magnetic field diagnostics
  • Electron Cyclotron Emission
  • Density Interferometer
  • Thomson Scattering

KOSTADINOVA, EGK0033@AUBURN.EDU

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6/4/2025

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Outline

  • Classification of plasma diagnostics
  • Magnetic field diagnostics
  • Electron Cyclotron Emission
  • Density Interferometer
  • Thomson Scattering

KOSTADINOVA, EGK0033@AUBURN.EDU

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Magnetic Filed Diagnostics

  • Magnetics – measurements made by sensing directly the magnetic fields in various places inside and outside the plasma using coils and probes

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KOSTADINOVA, EGK0033@AUBURN.EDU

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Lenz law: insert a wire loop in plasma, measure induced current

Image credit: CADANCE

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Plasma Density & Temperature

  • Plasma particle flux – measurements based on directly sensing the flux of plasma particles using probes in contact with the plasma

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KOSTADINOVA, EGK0033@AUBURN.EDU

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Langmuir probe: insert a biased wire in the plasma, collect electrons and ions

Image credit: IMPEDANS.COM

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Plasma Density & Temperature

  • Electromagnetic wave scattering – radiation scattered by plasma particles when subjected to incident radiation.
  • Plasma refractive index – measurements of the plasma's refractive index by transmission of EM waves through plasma

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KOSTADINOVA, EGK0033@AUBURN.EDU

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Thomson scattering: shoot laser into plasma, capture spectrum of scattered light�Image credit: DIII-D Thomson Scattering

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Emission Diagnostics – Free Electrons

  • EM emission from free electrons – detection of radiation emitted by free electrons, e.g., cyclotron and synchrotron emission (electrons accelerating), bremsstrahlung (electrons hitting stuff).

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KOSTADINOVA, EGK0033@AUBURN.EDU

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Gamma Ray Imager: crystals produce a pulse of visible light when hit by a gamma ray. Light pulse detected by PIN diode that converts it to voltage spike.

Image credit: DIII-D GRI

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Emission Diagnostics – Boud Electrons

  • EM emission from bound electrons – observation of the line radiation from atoms and ions that are not fully ionized

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KOSTADINOVA, EGK0033@AUBURN.EDU

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Spectrometer: collects photons emitted from electron transitions from one bound energy state to another inside an atom. Image credit: US Army Research Laboratory

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Fusion Products Diagnostics

  • Neutral atom diagnostics – measurements of escaping neutrals, e.g., from charge-exchange reactions, and injection of neutral beams and pellets
  • Fast ions and fusion products – measurements based on nuclear reaction products or injected energetic ions

KOSTADINOVA, EGK0033@AUBURN.EDU

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Image credit: CPEP

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Outline

  • Classification of plasma diagnostics
  • Magnetic field diagnostics
  • Electron Cyclotron Emission
  • Density Interferometer
  • Thomson Scattering

KOSTADINOVA, EGK0033@AUBURN.EDU

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Mirnov Coils for B-Field Reconstruction

  • Mirnov coils: cylindrical coils used to detect fluctuations in the magnetic field in plasma.
  • In a tokamak, Mirnov coils can be placed in a poloidal or toroidal configuration.

KOSTADINOVA, EGK0033@AUBURN.EDU

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Image source: S. Zurek, Encyclopedia Magnetica

Wang, A., et al. "Magnetic pickup coil systems on the HL-3 tokamak." AIP Advances 13.10 (2023).

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Mirnov Coils: How do they work?

  • When plasma becomes unstable, the plasma current fluctuates, which makes the poloidal B-flux fluctuate
  • Faraday’s Law of Induction says that changing magnetic flux induces voltage in a nearby coil
  • Strategically placed coils can pick up fluctuations in the B-field �🡪 relevant to disruptions!
  • In Alcator C-Mod, 4 poloidal arrays of 26 coils each were installed to measure the local poloidal B-field parallel to the wall

KOSTADINOVA, EGK0033@AUBURN.EDU

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Granetz, R. S., et al. "Magnetic diagnostics in Alcator C‐MOD." Review of scientific instruments 61.10 (1990): 2967-2969.

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Outline

  • Classification of plasma diagnostics
  • Magnetic field diagnostics
  • Electron Cyclotron Emission
  • Density Interferometer
  • Thomson Scattering

KOSTADINOVA, EGK0033@AUBURN.EDU

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Electron Cyclotron Emission Diagnostics

  • Electron Cyclotron Emission: radiation emitted by electrons gyrating in a magnetic field in a plasma
  • In a tokamak, ECE is typically used to measure electron temperature, but can also be combined with other techniques to study plasma fluctuations and turbulence 🡪 relevant to disruptions!
  • Related techniques include ECE Imager (2D temperature maps) and Correlation ECE (for temperature fluctuations)

KOSTADINOVA, EGK0033@AUBURN.EDU

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ECE: How does it work?

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KOSTADINOVA, EGK0033@AUBURN.EDU

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Chatterjee, R., et al. "High resolution ECE radiometer for electron temperature profile and fluctuation measurements on Alcator C-Mod." Fusion engineering and design 53.1-4 (2001): 113-121.

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Outline

  • Classification of plasma diagnostics
  • Magnetic field diagnostics
  • Electron Cyclotron Emission
  • Density Interferometer
  • Thomson Scattering

KOSTADINOVA, EGK0033@AUBURN.EDU

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Density Interferometer Diagnostics

  • Laser Interferometer: measures phase shift of a laser beam passing through the plasma
  • Interferometers can measure:
    • Total line-integrated density by comparing a chord's phase shift to a local oscillator
    • Density fluctuations by comparing the phase shift between adjacent chords

KOSTADINOVA, EGK0033@AUBURN.EDU

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Kasten, Cale Phillip. Two-Color interferometry as a fluctuation diagnostic on Alcator C-Mod. Diss. Massachusetts Institute of Technology, 2013.

Schematics of Michaelson Interferometer

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Impurities & Density Fluctuations

  • Impurity transport in the plasma can cause plasma disruptions
  • Injecting impurities can be used to mitigate plasma disruptions
  • Ex: material from the wall enters the plasma:
    • material heats up and disassociates into atoms
    • atoms ionize (loose all their electrons)
    • interferometer picks up the excess electron density

KOSTADINOVA, EGK0033@AUBURN.EDU

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0.2 grams/sec

Electron density is measures with CO2 laser interferometer at DIII-D

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Outline

  • Classification of plasma diagnostics
  • Magnetic field diagnostics
  • Electron Cyclotron Emission
  • Density Interferometer
  • Thomson Scattering

KOSTADINOVA, EGK0033@AUBURN.EDU

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Thomson Scattering Diagnostic

  • Thomson scattering: interaction between an electromagnetic wave (photon) and a charged particle, primarily electrons, resulting in the scattering of the electromagnetic radiation.
  • The E-field of the incident wave accelerates the electron, causing it to oscillate. This oscillation then generates radiation, which is the scattered photon.

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KOSTADINOVA, EGK0033@AUBURN.EDU

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Schematics of Thomson scattering �Source: Wikipedia

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Thomson Scattering: How does it work?

  • In tokamaks, laser pulse is shone into the plasma, and the scattered light provides information about the plasma temperature and density
  • The plasma temperature is derived from the Doppler shift in the scattered light, while the density is derived from the total scattered intensity.

KOSTADINOVA, EGK0033@AUBURN.EDU

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Thomson scattering system in Alcator C-Mod

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What Units to expect for Alcator C-Mod

KOSTADINOVA, EGK0033@AUBURN.EDU

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Kasten, Cale Phillip. Two-Color interferometry as a fluctuation diagnostic on Alcator C-Mod. Diss. Massachusetts Institute of Technology, 2013.

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Takeaways

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