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

Recent nstX and mast results indicate that the parametric dependencies of energy confinement
in the st differ from conventional tokamak scaling, creating the possibility of better than expected
confinement at low-a. in particular, the energy confinement improves faster with increasing
magnetic field and decreasing plasma collisionality in the st than at high-a. The differences appear
related to strong flow-shear suppression of turbulent ion transport in the st to near neoclassical
(collisional) levels, together with increased electron thermal transport. since neoclassical
ion transport and some of the instabilities thought to drive rapid electron transport, such as
micro-tearing modes, have a strong collisionality and field dependence, it is important to understand
how this transport scales to higher field and lower collisionality. The latter strongly affects
the large trapped particle population in sts and is the dimensionless variable of largest extrapolation
from present to future sts. Furthermore, with its relatively low viscosity and high flow
rates, the low ion transport regime accessible in the st offers a unique test of our understanding
of ion transport in toroidal plasmas. in addition, a large gap exists between experimental observations
and our understanding of energetic particle (eP) effects on plasma heating, current drive
and transport. importantly, the energetic particle populations of present sts have ratios of fast
ion velocity to alfvén velocity similar to those of the alpha populations in tokamak reactors. Thus,
st eP research is highly relevant for burning plasmas and iteR.
actions in the area of thermal transport:
• determine confinement trends in the st over an extended range of collisionality,
current, field, power input, aspect ratio, and wall recycling conditions. studies would
begin in planned upgraded sts (near-term) and could require new st experiments with
engineering parameters several times those of present devices for confident extrapolation
to next-step devices.
• Understand the causes for anomalous electron transport utilizing advanced diagnostics
for high and low wavelength, electrostatic and magnetic turbulence in the planned
upgraded sts, including diagnosis of electron temperature gradient streamers and microtearing
instabilities.
• measure the ion and electron velocity distribution function in the st to supplement
power-balance transport assessments.
• develop a predictive understanding of anomalous transport in the st utilizing numerical
codes (including synthetic diagnostics) and transport control tools to optimize the
performance of future reactors — in particular, optimizing the aspect ratio.
actions in the area of energetic particle transport and their interaction with the background
plasma:
• develop numerical and theoretical models for energetic particle transport in the presence
of multiple instabilities arising from the large normalized gyroradius, r*, of fast ions in the
st. This key parameter will differ from present-day sts by a factor of 2 in a component test
facility (ctF) and 4 in burning plasma sts, challenging predictive models. experiments
at intermediate r* (e.g., ctF) are required to extrapolate to burning plasma demo-class
sts.
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