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

in a tokamak d-t burning plasma experiment the actual sustainment of a high P alpha /P input plasma
near the Greenwald density limit with high self-driven current would be possible for the first
time. The resulting nonlinear interaction of the alpha power source with the transport, the current
profile and mhd stability could be investigated, a highly critical demonstration for the viability
of fusion power production. having the sustained burning plasma, which relies on the hydrogenic
ions for its power source, provides many constraints to issues ranging from fueling to
alpha ash to impurity generation, which would not be present without a fusion burn. The strongly
coupled mhd behavior associated with fast alpha particles, the global stability, the pedestal, and
error fields, and feedback control requirements would be accessed in a d-t regime.
The longest core plasma time constant is the plasma current profile redistribution time (t J ). The
pulse lengths proposed for the asian tokamaks, and for iteR in its steady-state mode, should exceed
their plasma current profile redistribution times by a factor of at least five. however, plasma-material
interaction processes can require longer times to come into equilibrium, and they
may generate effects on the core over these longer time scales. The pulse length, therefore, can
provide some separation of missions. to demonstrate a high-performance fusion core plasma,
several issues related to the long-term plasma-material interactions may be mitigated by choosing
the pulse length sufficiently long to address the core plasma processes, and short enough to
reduce plasma facing component (PFc) and materials-related issues at a given step. considerable
information on handling of demo-level heat loads is possible on devices accessing several
current profile redistribution times for integrated core demonstrations, since such a core plasma
would likely deliver large power and particle loads to the divertor. to demonstrate a core plasma,
it must be self-consistent with its edge plasma and plasma-material interfaces for the time scales
of the core plasma experiments. scrape-off layer plasma time scales are also shorter than the core
plasma duration, allowing the observation of these physics processes. Therefore, consideration of
issues related to edge plasma and material interfaces that affect the core plasma on these time
scales would be both possible and necessary. Fully integrated effects of demo-level power densities
with PFc material evolution would require longer time scales. This is an important consideration
in the phasing of activities toward the fully integrated high-performance steady-state burning
plasma regime.
Activity 1a: Examine potential ITER advanced tokamak (AT) scenarios in detail, with focus
on making ITER more flexible in heating and current drive sources, extending to larger
alpha power relative to input power, and significantly extending above the no-wall beta
limit with MHD feedback control.
iteR targets are to first demonstrate Q = 10 (P alpha ~ 2P input ), in plasmas sustained by an inductive
transformer (non-steady-state), and later to explore noninductively sustained plasmas with Q =
5 (P alpha ~ P input ) for longer durations. The later phase on advanced tokamaks, relevant to demo,
would demonstrate a d-t plasma with a P alpha /P input of approximately 1, for a baseline pulse of
3000 s (≈ 7-8t J ). The normalized beta value is expected to be about 3.0. here the plasma current
would be steady state (100% noninductively sustained) with a self-driven current fraction (bootstrap)
of about 50-65%. This beta and bootstrap current fraction are similar to the lower end of
the projected power plant range, although at much lower P alpha /P input . a comparison of the iteR
and power plant target parameters is shown in table 2. Presently the heating and current drive
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