Stability Research for Disruption Prediction and Avoidance in MAST-U Spherical
Tokamak Plasmas
S.A. Sabbagh (Principal Investigator), J.W. Berkery, Y.S. Park, Columbia University
Disruption prediction and avoidance in tokamaks to enable long-pulse plasma operation are present day
“grand challenge” topics in high performance tokamak fusion plasma science. They are top priority,
critical issues raised to the level of a dedicated Strategic Objective for Fusion Science and Technology as
reviewed in “A Community Plan for Fusion Energy and Discovery Plasma Sciences”, the Report of the
2019–2020 American Physical Society Division of Plasma Physics Community Planning Process (CPP).
Table A1 in this report shows this as the highest ranked physics Strategic Objective based on polling at
the meeting that provided report input (it ranked third, with the first and second-ranked Objectives being
technology objectives). The CPP report “forms the basis for the strategy detailed” as stated in the recent
FESAC Long Range Plan (LRP) report which first and foremost calls to “build the science and
technology required to confine and sustain a burning plasma”. The recognition of disruption prediction
and avoidance as highest priority DOE Fusion Energy Sciences strategic priority elements was founded in
the DOE report “Fusion Energy Sciences Workshop on Transients Report” (2016).
This research directly addresses this objective by aiming at a largely untapped capability of the
MAST-U spherical tokamak (ST), uniquely leveraging the low aspect ratio, high elongation, high beta,
flexible heating, and 3D coil capabilities of the device to test stability calculations and modeling that
support disruption prediction and avoidance. This renewal proposal highly leverages the extensive work
already performed to create software interfaces and models that produced required analysis capability that
will be of immediate use in this newly-proposed research. This proposal is a coordinated experimental
and physics analysis research program to be conducted by a small collaborative U.S. team with extensive
experience in the unique aspects and challenges of equilibrium reconstruction, stability analysis, and
disruption forecasting of high performance ST plasmas. The overall goal is to produce world-class
analysis regarding forecasting and avoidance of disruptions by leveraging the physics understanding over
a wide range of MAST-U plasma parameters using direct, quantifiable figures of merit. Several physics
analyses are planned using codes and control algorithms either written by or available to the PI and
proposal researchers (including our unique and powerful Disruption Event Characterization and
Forecasting (DECAF) capability, MISK (kinetic global magnetohydrodynamic (MHD) stability),
PHOENIX (automated equilibrium reconstruction), ideal DCON, resistive DCON, M3D-C1, TRANSP,
and VALEN-3D. MAST/-U will challenge our existing models for disruption characterization and
forecasting, yielding improved physics understanding and modeling to produce a unified disruption
forecasting algorithm needed for ITER and future devices. The ITER Control group recently requested to
have DECAF installed as part of the ITER control system. Dedicated experiments will also be proposed
and conducted on MAST-U for these studies, following our successful experiments from last year
including active assessment techniques to measure plasma stability and to test the disruption forecasting
research elements. The proposed research has access to the entire MAST-U database, and to the existing
MAST plasma database of over 30,000 discharges.
The MAST-U device is the only ST device in the world that allows the proposed high stability
performance research to demonstrate consistency with the plasma shaping of the most advanced tokamak
divertor configurations. MAST-U in its continued operation will run some of the highest stability
parameters to date in a fully-diagnosed, mega-amp plasma current-class ST device. Neutral beam
injection (NBI) power will range from 3.5 – 5 MW for pulse lengths up to 2s. Plasmas created in the first
operational campaign exceeded one second – longer than the key physics timescales of interest needed for
the proposed physics studies. Present plasmas have approached the ideal MHD n = 1 no-wall stability
limit. Experiments are planned for the 2022 run of MAST-U to exceed this limit. Significant plasma
response by resonant field amplification and entry into the regime of global MHD instability
(kink/ballooning/resistive wall modes (RWM)) will be further examined. Pulse lengths will extend to 5s
in the MAST-U Core Scope period and a plan for up to 10 MW total NBI heating is stated as a further
enhancement to the device. The move toward a highly non-inductive plasma operational regime planned
for MAST-U in later years will also be especially informative for stability forecasting studies.