Research Needs for Magnetic Fusion Energy Sciences - US Burning ...

ed. many ideal and resistive mhd stability codes exist, and efforts are needed to adapt them for
real-time analysis. disruption prediction on current devices has not achieved the accuracy needed
for iteR or demo.
Disruption avoidance is also in use at many facilities, typically in the form of a soft shutdown
or retreat from high performance in case of loss of the desired operating state or onset of a tearing
mode. Feedback-controlled stabilization of neoclassical tearing modes and resistive wall modes
and feedback-controlled correction of error fields have been successfully demonstrated, but are
not yet in routine use. control strategies to take appropriate action based on calculated or measured
approach to stability limits also are needed. techniques to locally modify the pressure and
current profile to avoid disruptions with small auxiliary power requirements have yet to be established.
Disruption mitigation by means of gas or pellet injection has received significant attention as
a research topic, but is not routinely used in existing facilities. techniques to suppress runaway
electrons during a disruption have not yet been assured, but several approaches are under investigation.
decision processes that determine when and how to initiate a soft shutdown or a rapid
shutdown — reliably but without a high rate of “false positives” — need to be developed.
several approaches to ELM control are presently under investigation. The use of Resonant magnetic
Perturbation (RmP) coils for elm control was pioneered by the Us and has now been accepted
as part of the iteR design. Work is also in progress to investigate other approaches such as pellet
pacing and operating regimes without elms. however, the physics of elm suppression is not
yet well understood, and much additional research is needed to assure its extrapolation to iteR.
a research thrust on control of transient events is well suited to the Us fusion program. key features
of the existing Us facilities include:
actuators for equilibrium profile control, including heating, current drive, and torque from neutral
beams, and several types of radiofrequency systems for heating and current drive.
• versatile sets of non-axisymmetric coils (internal and external).
• other actuators for direct suppression of instabilities, including localized current drive.
• Gas injectors and pellet injectors for rapid shutdown.
• extensive and mature diagnostic systems for equilibrium and stability measurements,
model validation, and real-time control.
• sophisticated, extensible digital control systems.
integration of research elements
The multiple research elements associated with prediction, avoidance, and mitigation of disruptions
form part of a single effort. off-line predictive modeling of stability limits and of disruption
dynamics provide the theoretical foundation for this effort. diagnostic measurements and analysis
are necessary for scientific understanding as well as prediction of disruptions and detection of
instabilities. disruption avoidance requires hardware systems to modify the plasma equilibrium
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