Electron Temperature and Density Profile Measurements

NSTX-U employs Thomson-scattering-based diagnostics to measure electron temperature (Te) and electron density (ne) profiles with high spatial resolution and a range of temporal resolutions. A key strength of this diagnostic set is its time-scale coverage: the Multi-Pulse Thomson Scattering (MPTS) system provides routine profile measurements across the discharge and real-time MPTS (rt-MPTS) produces rapid, processed profiles for the Plasma Control System (PCS).

Together, these measurements support equilibrium and transport analysis, pedestal and edge-gradient studies, validation of kinetic profile models, and characterization of rapid events such as confinement transitions, ELMs, MHD activity, and transient heating/current-drive changes. When combined with magnetics and charge-exchange spectroscopy, the electron profiles enable constraints on pressure, collisionality, and derived quantities used throughout NSTX-U analysis workflows.

Multi-Pulse Thomson Scattering (MPTS)

The MPTS diagnostic provides routine measurements of Te(R) and ne(R) by detecting Thomson-scattered light from a high-power Nd:YAG laser as it traverses the plasma. The scattered spectrum is collected by a multi-channel optical system and analyzed to infer the local electron temperature and density at each spatial point along the measurement chord. On NSTX-U, the MPTS system is designed to deliver profile coverage with dozens of spatial channels (commonly cited as ~42 channels) and multi-pulse operation (often described as two 30 Hz lasers, enabling up to ~60 Hz effective sampling in standard modes).

MPTS is a primary electron-kinetic diagnostic used to characterize core and edge profiles, quantify gradients relevant to transport, and provide inputs to equilibrium reconstruction and integrated modeling. Its profile capability is particularly important in spherical tokamaks, where strong shaping and high-β conditions produce rapidly evolving pressure profiles.

References:

  • A. Diallo et al., “Prospects for the Thomson scattering system on NSTX-Upgrade,” Review of Scientific Instruments 83, 10D532 (2012).
  • B. P. LeBlanc et al., “ Thomson scattering density calibration by Rayleigh and Raman scattering,” Review of Scientific Instruments 79, 10E737 (2008).
  • R. F. Parsells et al., “Multipulse Thomson scattering system for the National Spherical Torus Experiment,” Review of Scientific Instruments 70, 776 (1999).
Real-Time Multi-Pulse Thomson Scattering (rt-MPTS)

The rt-MPTS capability provides near-real-time analysis of Thomson scattering measurements and delivers processed electron temperature and density information to the Plasma Control System (PCS). In this workflow, data acquisition electronics and analysis software are configured to rapidly digitize key channels, compute Te and ne (often on the millisecond time scale), and output control-relevant quantities within the discharge timing cycle. This enables feedback-oriented use of electron profile information, including applications such as density control and scenario development where real-time kinetic constraints are valuable.

rt-MPTS is especially useful when profile information is needed with minimal latency, complementing high-fidelity offline analysis products used for detailed transport and profile-physics studies.

References:

  • F. M. Laggner et al., “A scalable real-time framework for Thomson scattering analysis: Application to NSTX-U,” Review of Scientific Instruments 90, 043501 (2019).
  • R. Rozenblat et al., “Development of real-time software for Thomson scattering diagnostics at NSTX-U,” Fusion Science and Technology (2019).