Ion Temperature, Toroidal Rotation, and Low-Z Impurity Density Profiles

NSTX-U employs charge-exchange recombination spectroscopy (CHERS)–based diagnostics to measure ion temperature (Ti), toroidal rotation velocity (Vφ), and low-Z impurity density profiles with high spatial resolution and millisecond time response. A key strength of this diagnostic set is its radial coverage and temporal resolution: Toroidal CHERS (T-CHERS) provides core-to-edge profiles of Ti and Vφ with ~10 ms time resolution, while the Edge Rotation Diagnostic (ERD) focuses on the pedestal and edge region to resolve rotation gradients important for stability and transport.

Together, these measurements constrain the ion pressure profile, momentum transport, and flow shear that influence turbulence suppression, MHD stability, and confinement transitions. When combined with electron profiles and equilibrium reconstruction, the CHERS systems enable determination of total pressure, radial electric field (via force balance), and inputs for stability and transport modeling in high-β spherical tokamak plasmas.

Toroidal CHERS (T-CHERS)

The T-CHERS system measures Ti(R) and Vφ(R) by observing Doppler-broadened and Doppler-shifted emission from fully stripped carbon (C6+) ions following charge exchange with injected neutral beam atoms. The diagnostic views the neutral beam along toroidally oriented sightlines, enabling direct inference of the toroidal rotation velocity from the wavelength shift and ion temperature from spectral broadening.

On NSTX-U, T-CHERS provides profile measurements across ~50 spatial channels (commonly cited as ~51) with ~10 ms time resolution in standard operating modes. This coverage enables detailed characterization of core rotation, ion temperature gradients, and momentum redistribution during confinement transitions, MHD activity, and changes in neutral beam torque.

T-CHERS data are widely used to evaluate ion thermal transport, momentum confinement, rotation shear, and to compute the radial electric field through force-balance analysis when combined with density and magnetic equilibrium information.

References:

  • B. P. LeBlanc et al., “Charge-exchange recombination spectroscopy diagnostic on NSTX,” Review of Scientific Instruments 74, 1659 (2003).
  • S. P. Gerhardt et al., “Rotation and momentum transport studies in NSTX,” Plasma Physics and Controlled Fusion 52, 124025 (2010).
  • J. E. Menard et al., “Overview of NSTX Upgrade results,” Nuclear Fusion 52, 083015 (2012).
Edge Rotation Diagnostic (ERD)

The ERD complements core CHERS measurements by focusing on the edge and pedestal region, where strong gradients in temperature and rotation influence stability and turbulence suppression. Using charge-exchange spectroscopy techniques optimized for the edge geometry, ERD provides edge toroidal rotation profiles with ~10 ms time resolution.

Edge rotation measurements are essential for understanding E×B shear at the pedestal, rotation braking during resonant magnetic perturbations (RMPs), and the evolution of flow during L-H transitions and edge-localized modes (ELMs). ERD data, when combined with T-CHERS, provide a continuous picture of toroidal rotation from core to edge, enabling improved constraints on momentum transport and stability analyses.

References:

  • S. P. Gerhardt et al., “Edge rotation and pedestal stability studies in NSTX,” Physics of Plasmas 18, 056102 (2011).
  • R. Maingi et al., “Edge rotation and confinement studies in spherical tokamaks,” Nuclear Fusion 50, 064010 (2010).