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

Disruption characterization and modeling
Greater use should also be made of present facilities, in iteR and demo-relevant plasma configurations
and operating modes, to compile and interpret the “statistical” aspects of disruption
causes and characteristics. The characteristics and consequences of “worst-case” disruptions in
iteR and reactor tokamaks are already reasonably well known, but further data is needed on current
quench and electromagnetic loads, attributes of thermal quench and effect on plasma facing
components, runaway electron generation, confinement and loss, and effect of mitigation methods.
as part of this effort, there is a need to develop theory-based, integrated, 2-d and 3-d models
that can accurately predict thermal and electromagnetic loadings and the potential for runaway
electron conversion during disruptions in iteR. The same models are also essential for interpreting
how disruption mitigation actions will affect these quantities. These models must be validated
with data from existing facilities.
Large eLM heat flux
edge instabilities known as elms are a nearly ubiquitous feature of h-mode discharges. These instabilities
are associated with the development of a large edge pressure gradient and cause very
rapid release of significant amounts of energy into the open-field line region. Recent studies for
iteR have concluded that an incident energy impulse of more than ~0.3% of the total thermal
plasma energy, which corresponds to a loss of ~1 mJ per elms event, can cause tile fatigue and
cracking as well as erosion, and larger energy losses can ablate or melt divertor materials, potentially
degrading the purity of iteR plasmas and greatly reducing the lifetime of the iteR divertor.
These results imply a need to reliably reduce the energy impulse by a factor of ~20 for the level
expected for unmitigated elms in iteR.
The iteR team has adopted two strategies toward this goal: (1) elm suppression by means of
non-axisymmetric coils and (2) elm size reduction with the use of pellet injection to induce more
frequent elms. because the core confinement in h-mode plasmas is strongly correlated with the
parameters in the pedestal, this must be done without significantly degrading the pedestal conditions.
The impact of unmitigated elms on solid divertor targets is so severe in both iteR and future
higher-power long-duration fusion facilities that multiple approaches are being investigated
to address this issue.
eLMs suppression
The application of non-axisymmetric fields in the edge region is a promising technique to suppress
elms, as shown in experiments on diii-d, which demonstrated conditions in which the
elms were fully suppressed (see Figure 12). Further experimental and theoretical research is required
to provide a firm scientific and technical basis to support the iteR program and extrapolate
to demo by addressing the following topics:
• determine the requirements for radial localization of the perturbed field and the magnetic
spectrum, which appears to be important for the attainment of elms suppression.
• Understand the transport processes associated with these fields in the edge and pedestal
region.
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