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

• a sufficiently high confining magnetic field to reduce the ratio of electron drive velocity
to ion thermal speed, g d = v de /v ti , below unity.
it can be shown that s ~ Rb/t 1/2 ~ Rn 1/2 in a high beta FRc (b=2μ o n[t e +t i ]/b e 2 ≈1), where R is the
plasma radius, b the magnetic field and b e the field outside the plasma, t the average temperature,
and n the average density. Rb is thus a critical engineering parameter and needs to be increased
by a factor of several from recent experiments to study high-s stability. to keep sustainment
powers reasonable, densities should be in the low 10 19 m –3 regime. to achieve high s values
in steady-state FRcs, the magnetic-field separatrix radius should be of order 0.5-1 m; a poloidal
flux ~ 20-50 mWb will be needed to confine energetic (10-20 kev) ions from nbi to study their
stabilizing effects. improved diagnostic capabilities, especially for density, temperature and magnetic
field profiles, and for s, are urgently needed to resolve internal physics.
There are three ways of forming such FRcs, and one or more can be used in experiments to close
this gap. (1) FRc formation employing a field reversed theta pinch, together with translation, has
been successful in forming hot stable FRcs at densities appropriate for neutral injection. confinement
scaling for these FRcs indicates that an lsX-scale translation experiment could provide
both the flux and confinement required for beam trapping and sustainment experiments. (2)
merging pairs of 5 mWb spheromaks have formed FRcs with over 5 mWb of flux. much larger (up
to 75 mWb) spheromaks have been formed, suggesting that it is possible to employ this method
to create large flux FRcs. transformer drive could be added to sustain flux in a physics experiment.
(3) The scaling of rotating magnetic field current drive (RmF) to form and sustain FRcs is
now reasonably well understood, yielding FRcs with diameters and fluxes ranging from ~5 cm
and ~10 μWb to ~ 75 cm and 5 mWb. scaling up another factor of 2.5 in diameter would be relatively
straightforward, although new issues may arise as the plasma size and density increase.
if the resistivity scaling with g d continues as predicted, higher fluxes and temperatures might be
sustained with mW-level sustainment powers similar to those used in present experiments.
besides tangentially-injected nbi, it would be useful to have the capability to study the physics
of toroidal flow shear, perhaps through a combination of RmF and nbi, which produce momentum
in opposite directions; and to study both oblate and prolate FRcs, including passive stabilizers
needed for the oblate FRcs.
an expanded simulation effort closely coupled to the experiment is needed to guide, understand
and optimize experiments. simulations need to be three-dimensional; for example, the RmFdriven
FRc is 3-d in nature. There is a need to develop 3-d codes, e.g., hybrid (mhd+kinetic), kinetic,
Pic, and mhd, to explain and predict experimental results. such code development is already
underway at a low manpower level, showing promising agreement with experiments.
tRanSPORt: REDuCE tRanSPORt in KEV FRC PLaSMaS anD unDERStanD
tRanSPORt SCaLing.
Present experiments observe anomalous resistivity and associated anomalous transport. There is
no present capability to study transport at high flux and large radius where the anomalous transport
is anticipated to be reduced. This lack of capability is an important gap.
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