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

estal structure need to be coupled to core models of heat and momentum transport to arrive at a
comprehensive, predictive, and validated confinement model.
Figure 2. Example of an H-mode pedestal (shaded region)
near the plasma boundary (dashed line) from Alcator C-
Mod: (a) electron density, (b) electron temperature, and
(c) electron thermal pressure. (Figure courtesy of Jerry
Hughes.)
other outstanding issues that need to be studied include the effect on the quality of the h-mode
pedestal with (1) helium or hydrogen operation, (2) near-unity ratio of input power to h-mode
threshold power, (3) relatively small separation between the primary and secondary separatrices,
and (4) high opacity to edge neutrals, since iteR will not likely be dominated by edge fueling as
are tokamaks today.
Scenarios for extended pulse length. two operational scenarios are foreseen in iteR for extending
the pulse length: the hybrid scenario and the advanced tokamak (at) scenario. The hybrid
mode is characterized by a broad current profile with a central safety factor ≥1. improved
stability and confinement occurs, allowing the current to be reduced below the value nominally
needed for standard h-mode without loss of performance. While promising for iteR, the physics
of the hybrid scenario is not completely understood. The task is to: Develop predictive physics understanding
of the hybrid scenario, including whether current drive tools are needed to maintain a broad current
profile with central safety factor ≥1 and to determine if performance extrapolates favorably to ITERrelevant
regimes. in particular, the role of mhd activity in sustaining the current profile in today’s
hybrid regimes needs to be better understood.
steady-state at modes for iteR require even further development. The preferred mode has nearly
zero or slightly reversed magnetic shear, with the central safety factor ~ 2. as in the hybrid scenario,
enhanced stability and confinement are key features with the additional benefit of a large
bootstrap current fraction. such at modes have been obtained in existing tokamaks, but only for
durations of a few resistive skin times (see Figure 3). The research required is to develop a predictive
physics understanding of steady-state AT scenarios so that they can be confidently extrapolated to ITER.
an important issue is the degree to which the duration for such at regimes can be extended for
times long compared with the resistive diffusion time.
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