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

Development of predictive capability for anomalous electron transport at high b and
low n e : This capability is needed to optimize the performance of future st devices, including the
determination of optimal aspect ratio, and to control plasma profiles. Research needs to develop
this capability include: (i) development and validation of theoretical models for electromagnetic
turbulence at high b and low n e , including modeled diagnostic output for comparison with fluctuation
measurements, and (ii) improved transport measurements. There seem to be multiple
physical effects driving stochastic heating and energy loss in the st. measurements of the energy
distribution function for all particles are therefore needed to study the physical mechanisms.
in addition, tools that will enable perturbative electron transport measurements in the st, such
as ebW heating and fast electron temperature diagnostics, are highly desirable, and (iii) development
of electron transport control tools, such as localized electron heating, magnetic field line
pitch (q-profile) reversal, flow shear, and control of fast ion instabilities.
ion scale tRansPoRt
in strongly rotating high-confinement st plasmas, ion thermal and particle transport is near
neoclassical levels. There is insufficient understanding of long-wavelength turbulence suppression
mechanisms in the st to allow predictions for ctF or demo.
research requirements
most needs overlap with those for electron transport. ion specific requirements are:
Study of the role of flow shear, magnetic shear, pressure gradient, zonal flows and poloidal
rotation over an extended parameter range: While exb shear is believed to be responsible
for the neoclassical ion transport observed in strongly rotating st plasmas, a direct experimental
link has yet to be made. The amount of exb turbulence suppression is unknown. observed
anomalous momentum transport suggests “remnants” of long-wavelength turbulence. There is a
need to investigate if zonal flows can be generated by electron-scale turbulence, their inherent qdependence,
and their relation to scrape-off layer flows.
Study of transport regimes with low rotation input: to inform demo, ion transport needs
to be investigated in regimes where pressure-driven flows and/or pressure gradients suppress
high-wavelength turbulence. The heating power required to produce high confinement needs
study.
Study of the role of neoclassical impurity transport at low magnetic field: While neoclassical
particle transport is desirable for an st-ctF or demo, there could be deleterious effects of
neoclassical high atomic number impurity transport at large gyroradius, such as collisional penetration
of edge density gradients, or strong impurity accumulation.
Study of the role of anomalous ion heating: Recent experimental results suggest that fast ion
driven instabilities may stochastically heat ions. intense micro-tearing might also have a similar
effect through small-scale magnetic reconnections.
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