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

• stellarators designed with quasi-symmetric magnetic fields have demonstrated reduced
neoclassical transport in low collisionality plasma regimes.
• a peak plasma pressure of 1.5 atmospheres and (separately) a volume-averaged normalized
plasma pressure of 〈b〉 >5% have been achieved. (b is the ratio of plasma pressure to
magnetic pressure.) in contrast to tokamaks, the limiting behavior for plasma pressure is
benign and does not lead to an abnormal termination of the plasma.
• maximum plasma densities achieved in stellarators are well above those in tokamaks at
comparable magnetic field strengths. The density limit is understood in terms of a simple
radiative power balance. exceeding the density limit does not result in abrupt plasma
termination, but a relatively slow radiative collapse.
• near steady-state operation has been attained, with record energy throughput for any
toroidal plasma.
• application of 3-d magnetic fields has been shown to control edge localized modes (elms)
in tokamaks, and to induce plasma rotation through neoclassical transport effects.
Spherical torus
The spherical torus (st) is a low aspect ratio tokamak, i.e., the ratio of the plasma’s major radius
to minor radius is relatively small (< 2). a principal advantage of the st is stability provided by enhanced
magnetic field line curvature. This stability at high plasma pressure, along with the more
spherical-like geometry, could lead to a compact fusion system with simple magnet design and
good maintainability. For these reasons, the st is considered a strong candidate as the basis for a
nuclear fusion component testing device. highlights of st research include:
• noninductive plasma initiation and growth: startup plasma currents have reached 25%
of the required level (160ka using coaxial helicity injection [chi], and 100ka with pointsource
helicity injection). Post-formation current buildup (to 350 ka) was shown in steadystate
chi discharges. Robust transition to inductive current drive was demonstrated.
• Plasma-material interface in compact geometry: a significant increase in energy
confinement and an st-record plasma thermal energy (0.48 mJ) were achieved with the
application of lithium to plasma facing components. a four-fold reduction of the peak
divertor heat flux was shown via gas injection while maintaining high-energy confinement.
• confinement at low aspect ratio: a strong dependence of energy confinement on the
toroidal field strength was found, in contrast to conventional tokamak behavior. an
onset of short wavelength turbulence is consistent with drift wave theory for the electron
temperature gradient instability. a 40-50% reduction in global recycling and a five-fold
increase in electron energy confinement were demonstrated in a small device with liquid
lithium walls.
• stability of high beta st plasmas with very broad current profiles: volume-averaged
plasma pressure, 〈b〉, up to 20% was produced. (This corresponds to a toroidal beta value
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