Globally coherent magnetic fluctuations often observed during the driven phase after spheromak formation in the Sustained Spheromak Physics Experiment 1 (SSPX) can be reduced to small amplitude by programming the magnetic flux=ψ gun and the discharge current=I gun in the formation gun. Scanning the edge normalized current=λ edge =λ gun =µ 0 I gun /ψ gun above and below the minimum energy eigenvalue 2 =λ FC of the flux conserver provides a variation in the internal q=safety factor profile producing the expected q=m/n=poloidal/toroidal mode spectrum. By driving the edge with the proper λ gun , the system can be operated with the poloidal/toroidal mode spectrum between the m/n=1/2 and 2/3 modes producing low magnetic fluctuation amplitudes and high electron temperature=T e > 350 eV. Transport and confinement parameters calculated using Thomson scattering-measured T e and n e profiles coupled with the equilibrium code internal current profiles show a reduction in electron thermal diffusivity as T e increases. This scaling behavior is more classical-like than Bohm or open field line transport models 3 where thermal diffusivity increases with T e . Electron diffusivity is calculated to be less than 10 m 2 /s, approaching levels seen in tokamaks.
The Sustained Spheromak Physics Experiment (SSPX) [E.B. Hooper, et. al., Nuclear Fusion, Vol. 39,No. 7] explores the physics of efficient magnetic field buildup and energy confinement,
A 60-element magnetic probe array has been constructed using miniature commercial chip inductors. The array consists of twenty clusters of three coils each mounted on a linear fixture. The coils are oriented in orthogonal directions to yield three-dimensional information. The array has been used to investigate magnetic properties of spheromaks.
Interferometric density measurements in plasmas rotating in shaped, open magnetic fields demonstrate strong confinement of plasma parallel to the magnetic field, with density drops of more than a factor of 10. Taken together with spectroscopic measurements of supersonic E × B rotation of sonic Mach 2, these measurements are in agreement with ideal MHD theory which predicts large parallel pressure drops balanced by centrifugal forces in supersonically rotating plasmas.
The Sustained Spheromak Physics Experiment (SSPX) was a high-temperature (T e up to 0.5 keV) spheromak formed by coaxial helicity injection (CHI) and with plasma duration of a few milliseconds following the high-current formation stage. Clean walls and low impurity operation were obtained by a combination of baking, discharge cleaning and titanium deposition on the walls, allowing the generation of high-quality plasmas. Resistive-magnetohydrodynamic simulations, benchmarked to the experiment, were used to elucidate the physics. The detailed characteristics of the n φ = 1 toroidal mode associated with CHI were determined as was the physics of the nonlinear current drive and magnetic reconnection that formed and sustained the spheromak. If the helicity injection rate was reduced following formation the plasma became relatively quiescent and magnetic surfaces formed. The measured thermal diffusivity in the core was as low as ∼1 m 2 s −1 . However, reconnection events during buildup or sustainment of the plasma current by CHI were found to open magnetic surfaces throughout the plasma allowing rapid energy loss to the walls. As a result, experiments and simulations in SSPX found no path to simultaneous sustainment by CHI and good energy confinement. Additional physics results are also presented in this review.
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