NSTX-U employs a broad suite of spectroscopic diagnostics to measure impurity emission, recycling, and plasma-material interaction (PMI) signatures across the main plasma and divertor. These systems provide line-integrated and spatially resolved spectra spanning visible, UV, and EUV bands, enabling identification of impurity species, monitoring of fueling and recycling (H/D), and constraints on radiative losses. A key strength of this diagnostic set is its spectral coverage and viewing flexibility: filter-based monitors provide robust line tracking for common species; survey spectrometers provide broad impurity inventories; and imaging spectrometers resolve spatial profiles and divertor plasma conditions.
Together, these diagnostics support impurity source identification, evaluation of wall conditioning (e.g., lithium coatings), interpretation of divertor regime evolution, and validation of impurity transport and radiation modeling. When combined with bolometry, IR thermography, and edge profiles, spectroscopy provides key constraints on power balance and plasma-surface interaction in high-β spherical tokamak plasmas.
EIES provides robust, filter-based monitoring of selected emission lines associated with common edge and divertor species such as H/D, Li, B, and C. By tracking line brightness versus time, EIES enables rapid identification of impurity influx events, recycling changes, and the response of edge emission to gas fueling, RF heating, and wall conditioning.
Filter-based monitors are particularly useful for scenario development and operations because they provide high signal-to-noise tracking of specific lines without requiring full spectral fitting. EIES signals are commonly correlated with bolometry, divertor diagnostics, and PFC heat-load measurements to interpret impurity-driven radiative behavior.
References:
- R. K. Janev, D. Reiter, and U. Samm, “Collision processes in low-temperature hydrogen plasmas,” Forschungszentrum Jülich Report JUEL-4105 (2003).
- H. R. Griem, Plasma Spectroscopy, McGraw-Hill (1964).
NSTX-U uses multiple EUV spectrometers to measure impurity spectra over approximately 1-40 nm wavelength coverage. EUV spectroscopy provides access to emission from a wide range of charge states, enabling identification and monitoring of all impurity species present in the plasma, including metallic contaminants and wall-related species.
EUV spectra support impurity inventory tracking, radiative loss interpretation, and constraints on impurity transport modeling. Time-resolved EUV measurements are especially valuable during impurity injection experiments, wall conditioning campaigns, and radiative transient events.
References:
- R. Barnsley et al., “EUV spectroscopy and impurity measurements in fusion plasmas,” Plasma Physics and Controlled Fusion 46, B165 (2004).
- E. Hinnov and H. H. Huberman, “Ultraviolet spectroscopy of impurities in tokamak plasmas,” Physics of Fluids 23, 479 (1980).
VIPS provides visible spectroscopy for impurity emission surveys and monitoring of H/D ratio through Balmer-series line analysis. The system supports multiple viewing configurations and can be coupled to various lines of sight targeting the main plasma or divertor.
Visible spectroscopy complements EUV measurements by providing strong signals for recycling and low-charge-state impurities, enabling routine monitoring of wall conditions, fueling response, and impurity source behavior.
References:
- A. J. H. Donné et al., “Diagnostics for impurity and radiation measurements,” Plasma Physics and Controlled Fusion 49, S83 (2007).
- H. R. Griem, Plasma Spectroscopy, McGraw-Hill (1964).
The DIMS diagnostic is a high-resolution UV-visible-NIR divertor imaging spectrometer with approximately 18 spatial channels. It measures spatially resolved impurity emission profiles in the divertor region and can provide ion temperature estimates in divertor plasmas from spectral line broadening when appropriate lines and conditions are available.
Imaging spectroscopy is central to divertor physics studies because it localizes emission to specific divertor regions and supports interpretation of strike-point motion, detachment front dynamics, and impurity source localization.
References:
- A. Kallenbach et al., “Divertor spectroscopy and impurity radiation studies,” Plasma Physics and Controlled Fusion 55, 124041 (2013).
- H. R. Griem, Spectroscopy of Low Temperature Plasma, Cambridge University Press (1997).
DIBS provides imaging spectroscopy of Balmer-series emission in the divertor. Balmer lines are sensitive to recycling, excitation, and recombination processes, enabling inference of divertor plasma conditions and impurity behavior. Under appropriate modeling assumptions and calibration, Balmer spectroscopy can constrain electron temperature and provide divertor impurity profile information.
DIBS is particularly useful for studying divertor regime evolution (attached to detached), strike-point physics, and the response of divertor emission to fueling and impurity sources.
References:
- D. Reiter, “The EIRENE code user manual,” Forschungszentrum Jülich Report JUEL-4103 (2001).
- R. K. Janev, D. Reiter, and U. Samm, Collision processes in low-temperature hydrogen plasmas, JUEL-4105 (2003).
Spatially resolved UV-VIS-NIR spectroscopy provides line emission measurements of divertor and center-stack regions, enabling monitoring of impurity species and recycling behavior across different PFC locations. Multi-chord views support interpretation of spatial localization and asymmetries in emission, which are important for PMI and divertor studies.
References:
- A. Kallenbach et al., “Divertor spectroscopy and impurity radiation studies,” Plasma Physics and Controlled Fusion 55, 124041 (2013).
- H. R. Griem, Plasma Spectroscopy, McGraw-Hill (1964).
The Transmission Grating Imaging Spectrometer (TGIS) is an imaging EUV/soft X-ray spectrometer covering approximately 5-70 nm. Transmission grating spectroscopy provides broad spectral access with imaging capability, enabling spatially resolved measurements of impurity radiation and emissivity structure.
Imaging EUV/SXR spectroscopy is useful for identifying impurity species, tracking impurity transport, and constraining radiative power distribution and emissivity structure when combined with bolometry and soft X-ray arrays.
References:
- J. L. Schwob et al., “High-resolution grating spectroscopy of impurities in tokamak plasmas,” Review of Scientific Instruments 58, 1601 (1987).
- R. Barnsley et al., “EUV spectroscopy and impurity measurements in fusion plasmas,” Plasma Physics and Controlled Fusion 46, B165 (2004).