NSTX-U employs a comprehensive set of diagnostics to monitor plasma-facing components (PFCs), including divertor tiles and first-wall surfaces. These systems measure surface temperature, heat flux, and edge plasma conditions to ensure machine protection and to characterize plasma–material interaction (PMI). A key strength of this diagnostic set is its spatial and temporal coverage: embedded thermocouples provide direct temperature measurements; Langmuir probes measure local edge plasma parameters; and multiple infrared (IR) thermography systems provide high-resolution imaging of divertor heat loads and strike-point motion.
Together, these diagnostics enable quantitative heat-flux reconstruction, assessment of transient events (e.g., ELMs, disruptions), validation of divertor models, and evaluation of operational limits in high-β spherical tokamak plasmas.
Embedded thermocouples provide direct measurements of PFC surface and sub-surface temperatures. These sensors are installed within divertor tiles and other plasma-facing components to monitor temperature evolution during steady-state and transient plasma conditions.
Thermocouple data are used to infer deposited heat flux, validate IR-based heat-flux reconstructions, and monitor cumulative thermal loading over multiple shots. Their absolute calibration and robustness make them important machine-protection diagnostics.
References:
- A. Loarte et al., “Plasma–wall interaction in ITER and future devices,” Nuclear Fusion 47, S203 (2007).
Langmuir probes embedded in divertor and PFC surfaces measure local electron temperature (Te), electron density (ne), and plasma potential in the edge region. By sweeping probe voltage and analyzing I–V characteristics, these probes provide localized edge plasma parameters that determine sheath conditions and surface heat flux.
Langmuir probe data are essential for studying edge transport, divertor detachment, strike-point physics, and plasma–surface interaction. When combined with IR thermography, probe data enable comparison between conductive and convective heat-flux channels at the divertor.
References:
- I. H. Hutchinson, Principles of Plasma Diagnostics, Cambridge University Press (Langmuir probe theory).
Multiple infrared (IR) thermography systems provide high-resolution imaging of divertor and first-wall surfaces, enabling reconstruction of surface temperature and heat-flux distribution. IR cameras monitor strike-point location, heat-flux profiles, and transient loading during ELMs and disruptions.
Upper Outer Divertor IRView from Bay G lower. Provides IR imaging of the upper divertor region and strike-point motion.
Lower Outer Divertor IRView from Bay H upper. Provides IR imaging of the lower divertor region and strike-point dynamics.
Wide-Angle IR (Midplane View)View from Bay G midplane. Provides wide-angle imaging of plasma-facing components, enabling global heat-load visualization.
References:
- D. L. Rudakov et al., “Infrared thermography measurements of divertor heat flux,” Review of Scientific Instruments 72, 453 (2001).
- A. Herrmann et al., “Heat load studies in divertor tokamaks,” Plasma Physics and Controlled Fusion 44, 883 (2002).