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

esearch requirements
There are two principal approaches for dealing with ntms in tokamaks. The first is ntm avoidance
by operating with q profiles that prevent low order resonances, created at specific depths
in the plasma based on the magnetic field line pitch. The second is to use active feedback by employing
localized eccd to stabilize the slowly growing ntms. in present sts, ntm avoidance
by operating with an elevated value of the minimum q, q min is the preferred method. however,
present experiments are largely unable to sustain high-performance discharges with elevated
q min and hence, ntm instabilities remain a prominent issue. current drive mechanisms that will
maintain q min greater than two are highly desired in future sts. active feedback with eccd is
not available in present-day sts due to wave penetration issues related to the relatively low magnetic
field strength. success in mitigating or eliminating ntms has been shown in nstX experiments
during the past two years, related to the application of lithium to the inside wall of the device.
The physical mechanism responsible for this effect should be conclusively determined and
understood to extrapolate this result to future sts.
Theoretical models do not yet exist to accurately predict all of the trends in the experimental data.
of particular note to the st is the scaling with normalized ion gyroradius, r*, and with large plasma
rotation and shear. as such, enhanced emphasis is needed on st-relevant theory and computation
in this area. coupled with this study is the need to develop diagnostics that can probe the
detailed structure of magnetic island physics in sts.
tools to address ntm physics and stabilization need to be developed. Upgrading present sts to
higher magnetic field and current may provide a mechanism to viable scenarios that avoid ntms.
Present methods to control plasma current and rotation profiles need to be further expanded. The
use of localized current drive sources for active control of ntms in sts needs to be assessed. in
particular, localized eccd should be explored as a possibility at higher magnetic field. other options
should be explored for localized current drive in sts that may provide a mechanism to stabilize
ntms.
ST AREA 5: TECHNOLOgy FOR STEADy STATE
maGnets
The tight space and inability to fully neutron-shield the center section of an st motivates elimination
or simplification of electromagnets in this region.
research requirements
The simplest approach to the toroidal field magnet has a single-turn, resistive, intensely cooled
copper centerpost. no single-turn centerpost st has been operated anywhere in the world since
the staRt device. spherical torus fusion reactor design has focused on the single-turn centerpost
because of the difficulty in finding an insulator, which will not become conductive due to neutron
damage in the presence of an electric field. very low impedance power supplies, possibly homopolar
generators, will be required to power the single-turn toroidal field magnet system, but have
not been demonstrated within an order of magnitude of the currents required. any maintainable
system requires demountable joints between the centerpost and the outer legs of the toroidal field
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