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

Recent accomplishments and Progress
accurate measurements of hot, magnetically confined plasmas have had a profound effect upon
our ability to understand and manipulate those plasmas. Great progress has been made during
the past 20 years in measuring crucial quantities of such a complex and strongly interacting system.
We point out only a few examples here, selected for their relevance to this Theme.
• Plasma shape has been shown to have a profound effect upon stability and confinement.
We have developed the capability to measure the shape of the flux surfaces with an
accuracy of a few percent of the plasma minor radius, i.e., roughly 1 cm.
• knowledge of the profiles of fundamental quantities like density, temperature, and
current density is crucial for evaluating fusion power production, transport, and heating
efficiency. The gradients of these quantities also provide crucial input to comparisons
with theory. We now make profile measurements of (electron) temperature and density
with a spatial resolution in the plasma edge approaching the ion gyroradius, roughly 1
mm. (The importance of current density profile measurements is also discussed in the
“accomplishments” section of the Theme 2 chapter.)
• The diagnosis of fast ion instabilities is especially important for burning plasmas, since
alpha heating is crucial for achieving the burning plasma state. Great progress has been
made in measuring the confined-alpha particles, and the signatures and effects of a
number of fast ion instabilities with exotic names like “fishbones,” “alfvén eigenmodes,”
and “energetic particle modes.”
• Plasma rotation and rotation shear play an important, though not yet fully understood,
role in plasma confinement. Plasmas exhibit significant “intrinsic” rotation, i.e., they rotate
even in the absence of any known external momentum input. measurements of plasma
rotation and rotation shear have demonstrated the importance of these phenomena and
stimulated theory to understand them.
SCiEnCE CHaLLEngES, OPPORtunitiES, anD RESEaRCH nEEDS
Development of the measurement capabilities needed for physics understanding of issues
crucial for the achievement of burning plasmas
one opportunity in this area is to provide targeted measurement capability sufficient for physics
understanding in a number of critical areas, with the ultimate goal being to develop a predictive
capability that can be confidently extrapolated to future burning plasmas. The testing of
comprehensive models of plasma behavior comprises a significant component of modern experimental
programs. to date, model validation efforts in magnetic fusion plasmas have necessarily
been limited in scope and have not been held to the rigorous standards employed in fields such as
fluid dynamics and aerodynamics. This situation is evolving as more sophisticated and comprehensive
approaches to validation are developed, including methodologies and metrics by which to
evaluate progress and quantify the agreement of models with experiments. 3 models of transport,
macro-stability, energetic particle-driven modes, pedestal dynamics, edge physics, and scrape-off
layer phenomena have advanced greatly in recent years, as computational capabilities, advanced
numerical techniques, and understanding of the physics have matured. more comprehensive di-
3 P. Terry et al., Phys. Plasmas 15 (2008) 062503.
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