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

These requirements are applicable to both stellarators and tokamaks.
LaRgE ELM HEat FLux
The heat flux due to large elms is a potentially more serious issue in demo than in iteR. For
demo, the fusion power will increase by a factor of ~5 and the combined alpha and auxiliary
heating power ~3-4. Though the size of the reactor remains to be determined, it is likely to be
about that of iteR. if the incident energy impulse were limited to ~1mJ, and the fraction of energy
lost by elms relative to power transported through the last closed flux surface were similar
in iteR and demo, then the frequency of mitigated elms will be 50 to 150hz. assuming a 50%
availability in demo and operation for two years prior to replacement of the divertor targets, this
implies that 1.5 x 10 9 to 5 x 10 9 elms will take place.
There is significant variability in elm behavior and the mitigation techniques are not fully reliable.
This imposes another consideration. occasional elms of the order of 1.5 mJm -2 would have
to be limited to ~5000 before 10 mm of erosion takes place if carbon fiber composite (cFc) target
plates were used. For tungsten targets, incident heat fluxes between 0.5-1.0 mJm -2 are predicted
to result in edge melting, while surface melting will occur at higher fluxes. This will not only have
an adverse impact on impurity influx, but additional fatigue considerations. an elm mitigation
system for both cFc and tungsten targets has to be highly reliable to avoid damage. This motivates
research on techniques that fully suppress elms with high reliability. The research needs
for elm suppression, elm-free operational regimes, and operation with small elms identified in
Theme 1 are applicable for Theme 2, but with greater emphasis on complete suppression. one additional
specific research need is:
• evaluate whether close fitting coils are viable in demo for suppression by means of
resonant magnetic perturbations.
Stellarator ELMs
edge localized modes are often observed in stellarator h-mode discharges as well, but the available
data and understanding are more limited at this stage. as in tokamaks, stellarator elms are
associated with strong pressure gradients at the plasma edge. in W7-as, the estimated energy
loss per elm is estimated as DW< 4%. accordingly, the research requirement for controlling elms
in stellarator configurations is similar to the research requirements for tokamaks described in
Theme 1. nonetheless, the stellarator may offer different scenarios for elm mitigation and suppression
in that the appearance of elms depends sensitively on the edge rotational transform.
moreover, an elm-free high-density h-mode (hdh) regime attained in stellarators exhibits good
thermal confinement with low impurity confinement times. The high beta regimes in W7-as and
the large helical device (lhd) did not have elms.
The stellarator may also differ from the tokamak due to the apparent absence of a critical temperature
gradient determining transport, such as the critical gradient expected for ion-temperature
gradient mode (itG) microturbulence in tokamaks. in high-performance tokamaks the ratio of
the central to the edge temperature is three to six, but in stellarators it appears a larger ratio of
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