A new experiment has been constructed to study the sustainment of a field-reversed configuration (FRC) with a rotating magnetic field (RMF). FRCs were formed with cold, unmagnetized ions and thus without a kinetic ion component that was believed to provide stability to internal tilt modes. No destructive instabilities were observed for the RMF FRC. Only peripheral radial penetration of the RMF was observed. The radially inward flow arising from axial screening currents at the FRC edge reduced convective and conductive losses to the measurement limit of the diagnostics.
After extensive experimentation on the Translation, Confinement, and Sustainment rotating magnetic-field (RMF)-driven field reversed configuration (FRC) device [A. L. Hoffman et al., Fusion Sci. Technol. 41, 92 (2002)], the principal physics of RMF formation and sustainment of standard prolate FRCs inside a flux conserver is reasonably well understood. If the RMF magnitude Bω at a given frequency ω is high enough compared to other experimental parameters, it will drive the outer electrons of a plasma column into near synchronous rotation, allowing the RMF to penetrate into the plasma. If the resultant azimuthal current is strong enough to reverse an initial axial bias field Bo a FRC will be formed. A balance between the RMF applied torque and electron-ion friction will determine the peak plasma density nm∝Bω∕η1∕2ω1∕2rs, where rs is the FRC separatrix radius and η is an effective weighted plasma resistivity. The plasma total temperature Tt is free to be any value allowed by power balance as long as the ratio of FRC diamagnetic current, I′dia≈2Be∕μo, is less than the maximum possible synchronous current, I′sync=⟨ne⟩eωrs2∕2. The RMF will self-consistently penetrate a distance δ* governed by the ratio ζ=I′dia∕I′sync. Since the FRC is a diamagnetic entity, its peak pressure pm=nmkTt determines its external magnetic field Be≈(2μopm)1∕2. Higher FRC currents, magnetic fields, and poloidal fluxes can thus be obtained, with the same RMF parameters, simply by raising the plasma temperature. Higher temperatures have also been noted to reduce the effective plasma resistivity, so that these higher currents can be supported with surprisingly little increase in absorbed RMF power.
High-beta plasmoids can survive the violent dynamics of supersonic reflection off mirror structures, producing a stable high-beta field-reversed configuration (FRC). This shows both the robustness of FRCs and their tendency to assume a preferred plasma state, possibly conforming to a relaxation principle. The key observations are (1) approximate preservation of the magnetic helicity, (2) substantial conversion from toroidal to poloidal magnetic flux, (3) substantial toroidal flow, and (4) a high-beta quiescent final state. These results are from the Translation, Confinement, and Sustainment experiment where a disorganized plasmoid is injected at super-Alfvenic speed into a confinement chamber. After successive reflections from end mirrors, the plasmoid settled into a near-FRC state with high beta and low toroidal magnetic field. The flux conversion and helicity preservation are inferred by an interpretive model.
A new experimental device has been constructed to study the flux build-up and sustainment of a field reversed configuration (FRC) with a rotating magnetic field (RMF). Even though complete penetration was expected from RMF theory, the RMF field was observed to penetrate only a few centimeters inside the FRC separatrix. Despite the limited penetration, significantly larger toroidal currents (40 kA) were driven than in previous experiments (∼2 kA) with the same RMF field. The high currents and lack of deep penetration allowed the axial field to be the dominant field throughout the FRC. The radially inward pondermotive force arising from axial screening currents at the FRC edge had a significant influence on energy and particle confinement, reducing convective losses to the limit of observability. With only ohmic heating, the measured low ion temperatures (2 eV) left the ions unmagnetized while the electrons (∼40 eV) were well magnetized. No destructive instability was observed for the RMF driven FRC despite the lack of a strong kinetic ion component.
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