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

Mitigation of disruptions
in the event that normal operation cannot be maintained or recovered by these techniques , suitable
actions must be available to avoid the worst effects of a disruption. The most desirable solution
is a controlled shutdown of the plasma, in which the thermal energy and plasma current are
brought smoothly to zero. as a last resort, a rapid shutdown must be available, for example, by gas
or pellet injection. massive gas injection has successfully mitigated the electromagnetic and thermal
loads on internal components in current experiments. The primary unsolved issue is the possible
generation of a runaway electron avalanche during a disruption or rapid shutdown, although
issues also remain for the extrapolated electromagnetic and thermal loads on iteR and demo.
Reliability requirements motivate the development of a broad portfolio of mitigation strategies,
and redundancy of hardware systems. detailed, validated models are also crucial for extrapolation
of mitigation techniques to iteR, where timely progress in the research program depends on
having only a small number of unmitigated disruptions.
key research steps include:
• Rapid and reliable disruption prediction, enabling a control decision to abandon disruption
avoidance and initiate shutdown.
• development of gas, solid, or liquid injection systems that deliver a sufficient quantity of
electrons to the plasma core rapidly enough for collisional suppression of runaways.
• development of alternate solutions for runaway electron suppression (e.g., stochastic
magnetic fields, or control of the runaway beam long enough to decelerate it).
• development and benchmarking of 2-d and 3-d models for the entire shutdown process:
mass delivery and transport to the plasma core, the resulting shutdown of the discharge,
and accompanying generation, confinement, and loss of runaway electrons.
avoidance of ELM-induced impulsive heat loads
The most desirable solution for avoidance of elms is one that maintains a high h-mode pedestal
pressure at the edge of the plasma, without the edge localized modes that are often driven by the
strong edge gradients, but with sufficient transport to avoid buildup of energy and particles. several
operating regimes with no elms or very small elms have been identified, and non-axisymmetric
(3-d) magnetic fields have been demonstrated to suppress elms. however, the physics of
these processes is not yet well understood. Uncertainties in extrapolation to burning plasma regimes
and the possibility that close-fitting 3-d magnet coils may not be credible in a demo device
motivate the development of multiple methods of elm control.
key research steps include:
• develop predictive capability for the effects of 3-d stochastic magnetic fields on particle
transport and thermal transport in the plasma edge.
• determine requirements for the 3-d magnetic spectrum, including radial localization,
elm suppression over a range of edge q values, and minimization of deleterious
nonresonant fields.
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