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

• identification and stabilization of neoclassical tearing modes by electron cyclotron
current drive.
• identification and stabilization of resistive wall mode instabilities by the application of
non-axisymmetric magnetic fields.
• suppression of elms by the application of non-axisymmetric magnetic fields.
• identification of various types of elms and the discovery of tokamak operational regimes
that are elm-free.
• observation of alpha particle loss due to instabilities in d-t plasmas.
SCiEnCE CHaLLEngES, OPPORtunitiES, anD RESEaRCH nEEDS
Disruptions
disruptions and disruption-like loss-of-vertical-stability events, called vertical displacement
events (vdes), pose significant design and operational challenges for iteR and for subsequent
reactor tokamaks in general. in iteR, the consequences of disruptions are anticipated to be more
severe than in present-day devices. electromagnetic loadings and area/size-normalized body forces
increase modestly (~3 times) from present tokamaks to iteR and can be accommodated in the
corresponding structural designs. however, the time-weighted energy deposition onto plasma
facing components in iteR and demo will be an order of magnitude greater than in present tokamaks
and could approach or exceed thresholds for the onset of tungsten surface melting (or
carbon vaporization). avalanche multiplication of a highly energetic electron population (known
as runaway electrons) may occur during disruption, vertical displacement events, or fast-shutdown
current decays (the latter effected by gas or pellet injection). The avalanche gain for this process,
G aval @ exp[2.5I(ma)], is sufficiently high that even very minute levels of “seed” runaway current
can convert to a very large population (> 5 ma) of ~10 mev electrons. subsequent deposition
of these electrons on plasma facing components could cause considerable damage.
Relative to present-day devices, disruption avoidance in iteR is significantly more challenging
(see Figure 11). estimates indicate that large improvements in two metrics for “disruptivity” relative
to present experiments are needed: viz., a 10-fold reduction in per-pulse disruption rate (discharge
setup reliability) and a 1000-fold reduction in per-second disruption rate (flattop/burn
sustenance reliability).
For future burning plasmas, accurate prediction of an impending disruption is a key element of
both avoidance and mitigation of disruptions. it will be essential (1) to develop plasma operation
and control procedures that avoid, wherever possible, occurrence of disruption onset, (2) to identify
means to reliably predict pending onset of disruption, and (3) to have means available to mitigate
the consequences of disruptions that cannot otherwise be avoided. in developing strategies
for disruption prediction, avoidance, and mitigation, it will also be essential to develop quantitative
metrics to characterize and model disruptions and disruption consequences. in all four categories,
there are substantial “gaps” or open issues.
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