Diagnostic options for radiative divertor feedback control on NSTX-U

Area of Science:

• Nuclear Fusion Engineering
• Plasma Physics
• Materials Science

Background:

• Tokamak fusion devices utilize radiative divertor techniques to manage high heat fluxes on plasma-facing components (PFCs).
• Previous experiments in the National Spherical Torus Experiment (NSTX) demonstrated heat flux reduction using pre-programmed gas injections.
• The upcoming NSTX-Upgrade (NSTX-U) faces projected heat fluxes of 20-30 MW/m(2), necessitating advanced control strategies.

Purpose of the Study:

• To discuss diagnostic options for a real-time feedback control system for active radiative divertor detachment control in NSTX-U.
• To ensure sustained core H-mode confinement while managing high heat loads in future tokamak operations.
• To prevent material erosion and thermal damage to PFCs under extreme heat flux conditions.

Main Methods:

• Analysis of NSTX divertor detachment measurements to identify suitable control signals.
• Proposal of diagnostic signals including divertor radiated power, neutral pressure, and spectroscopic signatures.
• Integration of these signals into a digital plasma control system for active management of divertor detachment via impurity gas seeding.

Main Results:

• Identified key diagnostics for NSTX-U: divertor radiated power, neutral pressure, spectroscopic deuterium recombination, infrared thermography, and thermoelectric scrape-off layer current.
• Recommended spectroscopic monitoring for confinement and pedestal degradation as a safety measure.
• Established a framework for real-time feedback control to actively manage divertor detachment.

Conclusions:

• The proposed diagnostic suite and control system are essential for managing high heat loads in NSTX-U.
• Active radiative divertor control is crucial for protecting PFCs and maintaining plasma performance in future fusion devices.
• This approach supports the development of ITER and other next-generation tokamaks.