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

dated in the corresponding structural designs. however, the impact on designs for demo, which
needs to breed tritium, has not been fully assessed. if currently available first-wall materials were
utilized and the same design assumptions used for iteR characterizing disruptions were applied
to demo, then either the blanket would fail to breed tritium or the availability goals would not
be met due to component failure.
The time-weighted energy deposition in iteR and demo will be an order of magnitude greater
than in present tokamaks, and may approach or exceed thresholds for onset of tungsten surface
melting (or carbon vaporization). even preemptive mitigation will result in uniform-deposition
loadings on the first-wall surfaces that approach the respective wall melting thresholds for iteR,
and this problem will become greater in demo due to its likely higher stored energy and comparable
wall area. avalanche multiplication of runaway electron content may occur during disruption,
vde, or fast-shutdown current decays (the latter effected by gas or pellet injection). The
current decay integrated avalanche gain, G aval @ exp[2.5I(ma)], is sufficiently high that even very
minute levels of “seed” runaway current can convert most of the initial plasma current to runaway
electron current. advanced tokamak modes of operation offer the possibility of increased
performance at the same or lower plasma current. Thus, if this problem were successfully solved
for iteR, then it would likely be solved for demo as well.
disruption avoidance provides an additional challenge for demo relative to iteR. estimates demonstrate
that large improvements in two metrics for “disruptivity” are needed relative to present
experiments. (see Figure 11 in chapter 1.) For iteR, 10-fold reduction in per-pulse disruption
rate (discharge setup reliability) and 1000-fold reduction in per-second disruption rate (flattop/
burn sustenance reliability) are needed. demo requires an additional 100-fold and 1000-fold reduction
beyond iteR.
as noted earlier in Theme 1, accurate prediction of an impending disruption is a key element of
both avoidance and mitigation of disruptions. This will rely on detailed plasma measurements,
which are especially challenging in demo, as discussed in the section on measurements. For future
advanced tokamak burning plasmas it will be essential to develop plasma operation and control
procedures that avoid, wherever possible, occurrence of disruption onset, to identify means
to reliably predict pending onset of disruption, and to have means available to mitigate the consequences
of disruptions that cannot otherwise be avoided. an additional consideration is that
even the techniques used in present machines to avoid disruptions, by modifying the current
and pressure profile, are challenging to incorporate in demo, as discussed later in this Theme in
greater detail. new techniques, which modify the profiles but require little auxiliary power, may
have to be developed. 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. substantial open issues exist in all four categories — prediction,
avoidance, mitigation and characterization and modeling — and these gaps are larger for
demo than for iteR, which are described in Theme 1. The research requirements in this area can
be summarized by:
• determine the level of steady-state performance that tokamaks and sts can achieve and
remain disruption free.
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