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

magnetic diffusion time, t a is the transit time for alfvén waves over the minor radius, t e is the
electron temperature, n i is the ion density, and r i is ion gyroradius, the size of the orbits of ions
about magnetic field lines. in the limit where stochastic magnetic transport is dominant, the scaling
of magnetic turbulence with S is crucial. in the limit where stochastic magnetic transport is
minimized, electrostatic transport may be most important. This second limit has begun to be accessed
using current profile control, which reduces the current-gradient drive for tearing modes,
but such a limit may also be reached spontaneously at high S. if electrostatic transport is dominant,
then as in configurations with a larger winding number, the main controlling parameters
will be r* and the collisionality, the rate at which particles collide with other particles.
The spontaneous reduction in magnetic turbulence resulting from a natural transition from a
multi-helicity state (multiple tearing modes) to a single-helicity state (single dominant mode) is
an emerging theme with potentially large impact on RFP confinement. operation at higher current
(and higher S) in RFX-mod indicates a natural preference (self-organization) for quasi-singlehelicity
conditions. application of 3-d shaping or modifications of the plasma surface via external
control coils might cause these states to be even more robust. The control of resistive wall modes
in next-step RFP experiments requires a flexible set of active control coils, and these coils might
also serve to control single-helicity transitions using 3-d shaping.
Presently mst is capable of i p ~ 0.5 ma, with maximum S of about 10 7 (without current profile
control) and r* = 0.013, both quantities measured at the plasma center. RFX-mod is presently capable
of i p ≥ 1.5 ma, with S of about 4×10 7 and r* = 0.007. a burning RFP plasma, with an estimated
i p ~ 20 ma and t e = 10 kev, will have much larger S of about 6×10 9 and much smaller r* ~
0.002. There also exists a large gap in computation and theory. single-fluid computation has been
used to study RFP dynamics with S ≤ 10 6 . at substantially higher S, two-fluid (electron and ion)
physics is expected to be more important. Gyrokinetic calculations are only now beginning to be
applied to the RFP. The capability for significant 3-d shaping will be important to understand single-helicity
physics, both theoretically and experimentally.
research requirements
Understanding transport mechanisms and confinement scaling in the RFP requires a substantial
extension in the lundquist number and normalized ion gyroradius, both experimentally and
in theoretical modeling. an upgrade to mst’s power supplies may allow for modestly higher ip (e.g., 0.8 ma), with S up to 4×107 and r* down to 0.009. an upgrade already underway at RFXmod
will allow ip of at least 2.2 ma, with S up to 3×108 and r* down to 0.005. but to achieve the
iteR-era goal, at least one RFP facility will be needed with capabilities well beyond that of mst
and RFX-mod. one possible embodiment of such a facility in the Us is envisioned as a two-stage
experiment, ultimately capable of ip ≥ 4 ma, S ~ 109 , and r* ≤ 0.007. This single facility could first
be operated as an advanced proof-of-principle experiment with initially lower ip and lower S. but
with a substantial upgrade to its power supplies, it would then transition to a full-fledged performance
extension facility to establish the basis for an RFP burning plasma in the iteR era. basing
two next steps on a single facility can hasten progress in RFP research and may prove less costly.
The critical physics that governs robust transition to a steady-state single-helicity state is not yet
known, although high plasma current and good control of the magnetic boundary are observed
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