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

exist between the spheromak and FRc. in the spheromak, the profile of magnetic field-line winding
(safety factor, q) lies between the profiles found in tokamaks and RFPs, and can have large
beneficial shear. compatibility with divertors is well established, and formation can be achieved
without a central solenoid. The FRc can be formed by fast transients and by low-frequency field
rotation. moreover, it is the highest-b confinement configuration; ideally, it has no toroidal field.
energetic large-orbit particles can compensate for the absence of stabilizing toroidal field, and
FRcs have proven remarkably resilient to translation. low total field within the confined plasma
makes the FRc a leading candidate for advanced fuels, another potentially transformative area,
where synchrotron radiation is prohibitive for other concepts.
Elements of the Thrust
all three of the minimal external field configurations need a range of activities to achieve their
iteR-era goals. here, we describe the activities and how they contribute to the Thrust.
• Facility upgrade and construction — whether an experimental discharge behaves
qualitatively like a reactor-grade plasma depends on wave propagation relative to
diffusion (lundquist number, S) and on system size relative to particle gyroradius
(s = 1/r*). achieving representative values is critical for understanding transport, current
drive, and stability in all three configurations. both parameters increase with increasing
plasma current and with system size. For larger current, power supply upgrades can
provide a first step for scaling information. With added flexibility, upgrades will also
improve current drive and formation studies. There are limits to what can be achieved in
existing facilities, however. substantially greater capability than presently exists will be
essential to demonstrate integrated solutions associated with the iteR-era goals for each
of the low external-field configurations. With improved understanding from upgrades,
new diagnostics, and validated modeling, new construction will provide the necessary
advances through a combination of size, power, and optimization of factors such as
shaping and boundary control.
• Theory and simulation — modeling efforts to understand nonlinear effects will continue
to be important in all three configurations. collaborative modeling and experimental
studies will explore nonlinear interactions between macroscopic dynamics, energetic
particles, and transport on scaling and b-limits. integrated simulations will also be used
as predictive tools to explore and optimize current drive systems, plasma shaping, and
divertor configurations. The modeling will need 3-d macroscopic two-fluid dynamics,
parallel kinetic effects from free streaming particles, and large-orbit energetic particle
effects. algorithms applied to the RFP, spheromak, and FRc presently stress different
physical requirements, and modeling efforts will benefit from collaboration to make the
code features compatible. Research to extend the theory of micro-turbulent transport to
low-q conditions is important for scaling in magnetically quiescent plasmas. analytical
developments must address relatively high plasma-b and the more significant role of
fluctuations with finite parallel wavelength.
• diagnostic development — new diagnostics and analysis techniques are essential for the
Thrust and should be integral parts of facility planning. major cross cutting developments
include: 1) diagnostics suitable for low-field plasma measurements, 2) structure and
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