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

in stellarator discharges with ohmically-driven current. disruption avoidance in stellarators is
thus generally not a critical challenge. it is not yet clear, however, what level and distribution of
current can be tolerated in a high-b stellarator while remaining robustly stable.
it is possible that the application of 3-d quasi-axisymmetric shaping to tokamak-like plasmas
may confer some disruption immunity and reduce the requirements for rapid control. it is not
known what level and type of 3-d shaping might suppress disruptions in tokamak plasmas without
adverse effect on confinement. experimentally, information could potentially be obtained
through 3-d shaping experiments/modifications to smaller tokamaks, which could hopefully be
flexible to provide a wide range of data.
stellarators do not have stringent control requirements for maintaining the density and temperature
profiles for optimal bootstrap currents for stable, sustained operation. Thus, the allowable
plasma profiles in stellarators are flexible and may be used to advantage, e.g., high-density
edge plasmas for radiative divertor operation or smaller edge gradients for elm suppression.
Peaked density profiles due to reduced thermodiffusion (associated with the good confinement)
may, however, result in impurity accumulation.
There are presently no experiments with strong 3-d shaping also capable of large ohmic or bootstrap
currents. a university-scale stellarator experiment is studying disruption suppression with
stellarator fields, but has only a modest ohmic heating capability. The extent to which non-axisymmetric
3-d fields can be applied to suppress disruptions, as well as for general concept improvement
in tokamaks, remains to be fully examined and is an important area for investigation.
because stellarators already demonstrate the passive avoidance of disruptions, it is clearly a requirement
to maintain this property through validated understanding. any high-performance
stellarator designs, including Qa stellarators with comparatively high levels of bootstrap current,
must be evaluated for susceptibility to disruption and the related need for profile control.
research requirements
• ensuring that higher performance quasi-symmetric stellarator plasmas with significant
bootstrap current continue to passively avoid disruptions or, if not, determining the
disruptive boundaries.
• Understanding the transition of stellarators away from disruption immunity with
application of significant current.
• extension of variable levels of 3-d shaping to tokamaks to avoid density and pressuredriven
disruptions.
• The benefit of 3-d shaping to high-performance tokamaks would most likely require an
experimental test on a high-performance tokamak once the appropriate factors have been
identified from smaller experiments, theory and modeling.
• Possible control of the thermally driven (or the self-organized alpha pressure driven)
bootstrap current needs to be evaluated.
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