NSTX-U employs a suite of bolometric and soft X-ray diagnostics to measure total radiated power and its spatial distribution across the main plasma and divertor. These measurements are essential for understanding power balance, impurity radiation, divertor regime evolution (attached / partially detached / detached), and transient radiative events. A key strength of this diagnostic set is its spectral and geometric complementarity: tangential AXUV bolometers provide midplane radiated power profiles; resistive bolometers provide multi-chord measurements in the main plasma and divertor; multi-energy soft X-ray (ME-SXR) systems provide high-time-resolution emissivity structure and (with filtering) information related to electron-temperature contours; and Lyman-alpha diode arrays (LADA) provide fast broadband photodiode views of divertor emission.
Together, these diagnostics support reconstruction of radiated power profiles, identification of impurity-driven radiation events, characterization of divertor detachment behavior, and validation of modeling of impurity transport and radiation. When combined with kinetic profiles and equilibrium reconstruction, radiated power measurements provide key constraints on power exhaust and plasma–material interaction regimes in high-β spherical tokamak plasmas.
The tangential AXUV bolometer system measures line-integrated radiation along tangential chords through the toroidal midplane, enabling inference of radiated power profiles in the main plasma. AXUV photodiode bolometers are sensitive to a broad band of soft X-ray and extreme ultraviolet radiation, providing an effective measure of total emissivity along each chord. With appropriate inversion (and geometry knowledge), the chord set supports reconstruction of midplane emissivity and radiated power distribution.
Midplane radiated power profiles are used to evaluate impurity accumulation, core radiation peaking, and the role of radiation in confinement transitions and MHD activity. Tangential geometry provides good sensitivity to core emissivity structure while maintaining robust signal levels across a wide range of operating conditions.
References:
- K. F. Mast et al., “Bolometry for fusion plasmas,” Review of Scientific Instruments 62, 744 (1991).
- A. W. Leonard, “Edge and divertor diagnostics for radiated power and detachment studies,” Plasma Physics and Controlled Fusion 60, 044001 (2018).
Resistive bolometers measure radiated power by sensing the temperature rise of an absorbing element whose resistance changes with heating from incident radiation. Multi-chord arrays provide line-integrated measurements through the main plasma and divertor, enabling reconstruction of radiated power distribution and time evolution. Compared with photodiode bolometers, resistive bolometers can offer excellent absolute calibration stability and broad spectral sensitivity.
These measurements are critical for power balance and detachment studies, enabling estimation of total radiated power fraction, localization of radiation to core vs edge/divertor regions, and tracking of radiative transients. When combined with IR thermography and Langmuir probes, resistive bolometers help quantify the partition of power between radiation and surface heat loads.
References:
- K. F. Mast et al., “Bolometry for fusion plasmas,” Review of Scientific Instruments 62, 744 (1991).
- A. Loarte et al., “Plasma–wall interaction in ITER and future devices,” Nuclear Fusion 47, S203 (2007).
The ME-SXR diagnostic measures soft X-ray emissivity along a set of chords in the toroidal midplane. In an unfiltered configuration, the system serves as a fast radiated-emission monitor sensitive to changes in impurity content and core emissivity. With energy-selective filtering, ME-SXR can provide enhanced sensitivity to specific photon energy bands, enabling high-time-resolution tracking of emissivity structure that is often used to infer iso-Te contour evolution and to identify rapid MHD activity (e.g., sawteeth, tearing modes) through emissivity fluctuations.
Because soft X-ray emissivity depends on both electron temperature and impurity radiation, ME-SXR data are interpreted in conjunction with electron profile diagnostics, equilibrium reconstruction, and spectroscopy. High bandwidth makes the system especially useful for correlating core emissivity dynamics with magnetic activity and confinement transitions.
References:
- R. J. Fonck et al., “Soft X-ray diagnostics for tokamak plasmas,” Review of Scientific Instruments 61, 3487 (1990).
- K. K. Kappatou et al., “Soft X-ray tomography and impurity radiation studies in tokamaks,” Plasma Physics and Controlled Fusion 57, 014017 (2015).
The Lyman-Alpha Diode Array (LADA) is a broadband one-dimensional photodiode array viewing the lower divertor. The system provides fast measurements of divertor ultraviolet/visible emission that are commonly used as proxies for recycling and divertor plasma conditions. While LADA is not a bolometer, its high-speed diode measurements complement radiated power diagnostics by providing localized divertor emission signals with excellent time resolution.
LADA measurements support studies of divertor regime evolution, strike-point motion, and transient divertor phenomena. When combined with bolometry, IR thermography, and Langmuir probes, LADA helps connect divertor emission dynamics to heat loads and plasma parameters at the target.
References:
- A. Loarte et al., “Plasma–wall interaction in ITER and future devices,” Nuclear Fusion 47, S203 (2007).
- J. P. Gunn et al., “Divertor detachment physics and diagnostics,” Plasma Physics and Controlled Fusion 59, 014001 (2017).