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

tRanSPORt: DEtERMinE unDERLying tRanSPORt MECHaniSMS anD
COnFinEMEnt SCaLing in LOW COLLiSiOnaLity SPHEROMaK PLaSMaS
core electron energy confinement in quiescent spheromaks has been demonstrated to be comparable
to tokamak l-mode values, but the mechanisms are not understood and there are no measurements
of ion transport. There are presently no experiments capable of spheromak transport
studies. This is an important research gap.
research requirements
new experiments having long pulse lengths and high magnetic flux are needed. an experiment at
least the size of ssPX (R0 > 0.3 m, iplasma > 500 ka) will be needed so that charge exchange is not
a dominant loss and so there is enough power to burn through impurities. supporting theory and
simulations are also important. diagnostic capability is essential, e.g., to differentiate between
transport driven by magnetic and electrostatic fluctuations. This research will broaden our scientific
knowledge by extending confinement studies to the safety factor range ~0.2 < q < 1 lying between
the RFP and the tokamak.
energy confinement experiments in slowly decaying plasmas yield electron thermal conductivities
in the l-mode range, although the database is limited. Power-balance calculations with 0.1
kev < t e,peak < 0.5 kev and densities ~ 10 20 m –3 yield electron thermal conductivities ~1 m 2 /s - 10
m 2 /s in the core of the spheromak. because the experiments were at the concept exploration level,
there were no measurements of ion temperatures or ion thermal conductivity; future work will
need ion confinement diagnostics. The electron transport mechanism is unknown, although it
was found that the best confinement occurred on ssPX when the q-profile did not cross low-order
resonances. Resistive mhd simulations support this result: in slowly decaying plasmas, closed
magnetic surfaces form and the electron temperature rises to a few hundred ev. if the q-profile
evolves to cross low-order surfaces, islands form; when they overlap they result in stochastic magnetic
field lines and high thermal losses throughout much of the spheromak. on ctX the best
confinement occurred soon after helicity injection was stopped and the equilibrium decayed slowly.
The spheromaks ohmically heated to b e,peak > 20% before a strong pressure-driven interchange
occurred, demonstrating ohmic heating to the beta limit. in the limit of macroscopic (mhd-like)
dynamics being eliminated, microturbulence may govern cross-field transport. how this occurs
in high-b, low-q equilibria will be a new area of research.
Rotation and flow shear seem to have a very positive effect on the stability and confinement of
tokamak and st plasmas, and should be applied to a high-temperature spheromak, perhaps by
using neutral beam injection. Flow shear may also be used to control turbulence caused by relaxation
or drift modes.
bEta LiMitS: unDERStanD bEta-LiMiting MECHaniSMS
Past experiments have demonstrated high peak and volume-averaged betas (5-20%), but the limiting
mechanisms are not well understood. no present experimental facility is capable of addressing
this issue.
220