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

influence on plasma performance, and an improved understanding of this area will be necessary
to optimize the configuration. The current profile and the transport of particles and energy are
strongly coupled, particularly when the current and heating source are dominantly from the plasma
itself. The longest time scale for core plasma physics is the current profile redistribution time
(t J ). The transport of the energetic alpha-particles and impurities (intentional and unintentional)
must be understood to control the distribution of power onto the various material surfaces in the
tokamak. all of these issues contribute to the critical need for experimental demonstrations of a
high-performance plasma core with the many simultaneous processes associated with a burning
plasma as it approaches the self-organized regime.
Magnetohydrodynamic stability
The magnetohydrodynamic (MHD) stability properties of a high-performance core plasma will determine
the maximum fusion power density achievable. The understanding of stabilizing a plasma above the nowall
beta limit, as a function of the plasma rotation, background error fields, and feedback control, is
critical to accessing the highest possible performance. In the burning plasma state with low rotation,
fast alpha particles, and strong pedestals, what maximum stability properties will
the plasma access?
experimental results indicate that in the presence of a conducting wall, even slow plasma rotation
– less than 0.5% of the alfvén wave velocity – is sufficient to maintain plasma stability up to b n
(normalized plasma pressure) values close to the maximum with-wall limit. although this is encouraging
for future large devices where the plasma rotation is expected to be small, a reliable extrapolation
to demo requires a deeper understanding of the stabilization physics. it is expected
that the remaining error fields in the plasma are affecting the stability threshold. although projections
to high fusion power plasmas consider feedback control as the sole stabilization method, it
should be recognized that sustained, direct feedback-stabilization without the stabilizing effect of
rotation or fast particles has not been demonstrated yet in a high-beta tokamak. The range of plasma
betas, in the demo regime, is between the value requiring no conducting wall (b n no wall ) and
that requiring a conducting wall (b n with wall ). on a time scale longer than a resistive time, feedback
is complicated by the nonideal response of the plasma. The benefits of operating above the no-wall
beta limit are clear, and reliable stabilization methods to access this regime are necessary.
The self-heating of the plasma comes from the 3.5 mev alpha particles slowing down and heating
the ions and electrons. Fast ions can drive instabilities, causing enhanced transport of the energetic
particles necessary for heating, and possibly damaging the first wall through losses from the
plasma. These fast particles can play an important role in the global stability of the plasma, contributing
to the access to high beta.
The region near the plasma edge, featuring a transport barrier referred to as the pedestal, strongly
determines the core plasma properties. high pressure at the top of the pedestal can support
high overall performance in a fusion plasma. The magnitude of this pressure is a key parameter
requiring more accurate prediction for demo. a key additional issue for the pedestal is that its
density and temperature must be consistent with the high-performance core plasma, fueling, and
the divertor where particles and power are received.
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