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

Operation with small eLMs
While the suppression of elms may be the preferred strategy for iteR and future machines that
use an h-mode edge, it may be necessary to develop operating regimes with frequent small elms
or adopt a strategy to make frequent small elms reliably. to reliably operate in such small-elm regimes,
it would be necessary to achieve reduction of the elm energy loss fraction to DW elm /W < 0.3%
in iteR for tungsten or carbon fiber composite (cFc) divertor targets. experimentally, the magnitude
of the heat loss per elm event scales roughly with the elm period. Thus, increasing the elm
frequency enables the production of smaller elms. The research needs in this area include:
• develop regimes of operation such as type ii, Grassy, or type iv elms, which reliably
meet the requirements for small elms in conditions that extrapolate to iteR.
• assess whether techniques such as pellet pacing, vertical jogs, or application of n > 0
oscillating magnetic fields can reliably stimulate small elms.
The experiments described to control elms can be done on existing facilities with modest upgrades
in the tools to suppress elms by the application of non-axisymmetric fields and the tools
to modify the edge conditions.
Alpha particles ejected by plasma instabilities
The standard iteR Q~10 h-mode is expected to have up to ≈15 mJ of stored energy in the fusionalpha
population; lower density, higher temperature operating points may have more. Plasma instabilities
can enhance the loss of the fusion alphas beyond the relatively small “first orbit” losses.
a loss of 5% to 10% of the ≈15 mJ of confined alphas — comparable to loss fractions for fishbones
or toroidal alfvén eigenmode (tae) avalanches in present devices — would correspond to 0.75
mJ to 1.5 mJ per event. This would be at the margin for reaching the surface-melting threshold
if the area for heat loss is comparable to that of the background plasma.
Further, the lost alphas, with energies of up to 3.5 mev, may present a danger to plasma facing
components beyond that of transient heating. laboratory experiments have found that the 3.5
mev alphas can severely degrade metal target plates by implanting helium in the tungsten divertor
targets. The deposition pattern for alpha particles lost during toroidal alfvén eigenmode avalanches,
sawteeth, edge localized modes, fishbones, or neoclassical tearing modes in the divertor
or elsewhere has not been calculated for iteR, so neither the magnitude of the fluence nor the
deposition area is known. Rough estimates of the alpha-particle loss due to instabilities in iteR
indicate that blistering might degrade the lifetime of first wall and divertor components and contribute
to dust production. note that loss of the entire fusion alpha population in a single event,
such as a disruption, is well below the blistering threshold.
enhanced losses of energetic particles in present-day experiments seem to correlate both with instabilities
driven by the background plasma such as tearing modes, elms, disruptions, sawteeth,
and kinetic ballooning modes (kbm) as well as with energetic particle-driven instabilities such
as taes and other related alfvén eigenmodes, so-called energetic particle modes (ePm), and fishbones.
losses of up to 20% per event have been observed, with a timescale of ~1 ms. While substantial
progress has been made in identifying the onset conditions for the occurrence of the fast
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