Resistive Stability and Shape Optimization Research in NSTX-U

Carlos Paz-Soldan, Columbia University in the City of New York

This project aims to enable the robust exploitation of the National Spherical Tokamak Experiment –
Upgrade (NSTX-U) via targeted studies on resistive stability and shape optimization effects. Work in the
resistive stability area will enable high performance operation in NSTX-U free from the deleterious error
field or profile driven tearing instabilities, enabling NSTX-U milestone #2: “Develop operation at large
bootstrap fraction and advance the physics basis required for non-inductive, high-performance and lowdisruptivity
operation of steady-state compact fusion devices”. This will be achieved by developing a robust
error field source model for NSTX-U using as-built metrology data, and deploying it in an adaptive feedforward
algorithm for routine use by the NSTX-U team. If the intrinsic EF source is found to be of little
importance, project focus in later years would plan to shift towards validating emerging first-principles
resistive stability models against NSTX-U data and using this model to improve discharge trajectory design.
Work in the shape optimization area will explore the access to high performance L-mode scenarios in
NSTX-U via developing negative triangularity plasma shapes (up-down symmetric and/or asymmetric).
Negative triangularity scenarios offer an unexpected path to high-performance with favorable particle
exhaust properties, enabling an alternate path to achieve NSTX-U milestone #3 “Develop and evaluate
conventional and innovative power and particle handling techniques to optimize plasma exhaust in high
performance scenarios” that is furthermore robustly stable to the edge-localized mode. Program
deliverables in this area will be simulated control scenarios to realize various negative triangularity
scenarios, followed later by an experimental realization in low-power experiments after assuring low
machine risk via targeted calculations. Incremental program options will be later proposed if this scenario
proves feasible and offers the promised benefits to mainstream NSTX-U experiments. The project naturally
provides well-rounded research opportunities for doctoral students in the NSTX-U program, with
mentorship from the PI facilitated by close geographic proximity to Columbia University.