6.3 Optical and Spectroscopic Techniques

Optical diagnostics are crucial for understanding plasma behavior in fusion experiments. These techniques use light interactions to measure key plasma parameters like temperature, density, and impurities, providing vital data for optimizing fusion performance.

Advanced spectroscopic methods take plasma analysis further. By exploiting charge exchange processes and soft X-ray emissions, researchers can probe ion temperatures, plasma rotation, and instabilities, offering deeper insights into fusion plasma dynamics and transport phenomena.

Optical Diagnostics

Principles of Thomson scattering

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• Powerful diagnostic technique measures plasma electron temperature and density
  • Incident laser light scatters off electrons in the plasma
  • Scattered light spectrum depends on electron velocity distribution (Maxwell-Boltzmann distribution)
• Determines electron temperature
  • Spectral width of scattered light is proportional to electron temperature due to Doppler broadening from thermal motion of electrons
  • $\Delta \lambda \propto \sqrt{T_e}$, where $\Delta \lambda$ is the spectral width and $T_e$ is the electron temperature
• Measures electron density
  • Intensity of scattered light is proportional to electron density
  • Calibration with known density reference allows absolute density determination (Rayleigh scattering from neutral gas)
• Achieves spatial resolution by focusing laser beam and collecting scattered light from a specific volume (scattering volume)
• Temporal resolution limited by laser pulse duration and detector response time (nanosecond to picosecond range)

Spectroscopy for plasma analysis

• Visible and UV spectroscopy study plasma impurities and ion temperature
• Monitors impurities
  • Plasma impurities emit characteristic line radiation in the visible and UV range (hydrogen Balmer series, carbon and oxygen lines)
  • Identifies impurity species and their concentration
  • Monitors impurity influx and transport (edge plasma to core plasma)
• Measures ion temperature
  • Doppler broadening of impurity emission lines relates to ion temperature
  • $\Delta \lambda \propto \sqrt{T_i/m_i}$, where $\Delta \lambda$ is the spectral width, $T_i$ is the ion temperature, and $m_i$ is the ion mass
  • Requires high-resolution spectrometers for accurate temperature determination (echelle spectrometers)
• Achieves spatial resolution by line-of-sight integration or tomographic reconstruction (multiple viewing angles)
• Temporal resolution limited by detector exposure time and readout speed (millisecond to second range)

Advanced Spectroscopic Techniques

Charge exchange recombination spectroscopy

• Measures ion temperature and rotation velocity
• Utilizes charge exchange process
  1. Inject neutral atoms into the plasma (hydrogen or helium beam)
  2. Ions undergo charge exchange with neutrals, capturing an electron into an excited state
  3. Excited ions emit characteristic line radiation (C VI, He II)
• Measures ion temperature
  • Doppler broadening of CXRS emission lines relates to ion temperature
  • Uses high-resolution spectrometers to resolve the spectral shape
• Measures rotation velocity
  • Doppler shift of CXRS emission lines is proportional to the rotation velocity
  • Viewing the plasma from different angles determines toroidal and poloidal rotation (tangential and radial views)
• Achieves spatial resolution by focusing the viewing optics on specific plasma regions (core plasma)
• Temporal resolution limited by the integration time required for sufficient signal-to-noise ratio (millisecond range)

Soft X-ray diagnostics in plasmas

• Studies plasma instabilities and energy transport
• Detects plasma instabilities
  • Soft X-ray cameras detect fluctuations in the plasma emission (10-1000 eV range)
  • Identifies instability modes (sawteeth oscillations, tearing modes)
  • Characterizes instability frequency, amplitude, and spatial structure (poloidal and toroidal mode numbers)
• Investigates energy transport
  • Soft X-ray emission is sensitive to electron temperature and density
  • Radial profiles of soft X-ray emission provide information on energy transport (heat pulse propagation)
  • Comparison with transport models helps understand confinement and heat dissipation mechanisms (anomalous transport)
• Achieves spatial resolution by pinhole cameras or collimated detector arrays (silicon drift detectors, gas electron multipliers)
• Temporal resolution limited by detector response time and data acquisition rate (microsecond to millisecond range)