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

small, concept exploration experiments and resistive mhd simulations are testing innovative
ideas. exploratory research is important and should continue, but cannot test ideas in a fusionquality
environment. lack of such an experiment and supporting theory is a major gap in the
spheromak program.
research requirements
advancing the spheromak program requires an experiment with the high-quality vacuum environment,
long-pulse capability, flexible power systems, comprehensive diagnostics, and closely
coupled theory needed for high-quality research in fusion plasmas. it is a high priority to determine
the detailed features of this experiment and to construct it to address the issue of sustainment
and other important spheromak research.
magnetic relaxation has sustained spheromaks, and spheromak research has contributed significantly
to self-organized plasma research. however, the accompanying magnetic fluctuations have
an impact on confinement, as shown in laboratory and resistive mhd simulations. The rotating
n=1 “column” mode (with nonlinear harmonics) that is excited by dc, coaxial helicity injection
has been shown to be detrimental to closed magnetic flux surfaces. scaling of fluctuations and
confinement with plasma temperature and magnetic field strength is an active area of research,
so the possibility of relaxation-based current drive remains open. nonetheless, the iteR-era goal
compels investigation of new sustainment methods that reduce or avoid relaxation over at least
part of a discharge cycle.
two approaches have been proposed for addressing this issue: (a) developing better helicity injection
or other steady-state, efficient current drive techniques that are compatible with good confinement,
and (b) periodically rebuilding the plasma flux and current separated in time from a
“coasting” phase, which experiments have already shown to have good confinement. Research
needs for these are discussed below.
(a) Plasma currents in the spheromak can be generated (and sustained) by a variety of
schemes. important issues are minimal perturbation of confinement, efficient transfer of
electrical energy to stored plasma energy, and favorable scaling to the next step. known
spheromak attributes could be exploited in different ways: for example, different steadystate
helicity-injection techniques, pulsing, merging spheromaks, use of solenoids, and
auxiliary current drive using neutral beams or radiofrequency. it may be useful to harness
higher order modes and have more control over reconnection, or to use auxiliary
current profile control, heating or externally applied magnetic fields to influence the
conductivity profile. non-axisymmetric methods such as the inductively driven concept
need further study to evaluate their promise fully. schemes more commonly considered
for other concepts, such as oscillating field current drive, could also be considered.
(b) an alternate approach is to separate the plasma sustainment and confinement phases
in time. This was demonstrated for coaxial helicity injection on ssPX where it was termed
refluxing, and in principle could be applied to inductive helicity injection techniques. in
refluxing, periodic current pulses build spheromak flux and are followed by slowly decaying
flux and current with closed magnetic surfaces that confine plasma well. in a reactor,
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