Principal physics of rotating magnetic-field current drive of field reversed configurations

Abstract

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.

Stochastic Ion Heating in a Field-reversed Configuration Geometry by Rotating Magnetic Fields

“…The TCS parameters in Table I, taken from a publication, 18 are for one of its best performing RM F e discharges, having achieved a steadystate electron temperature near 50 eV atn e = 1.3 × 10 13 cm −3 . Measurements showed that the RM F e only penetrated ∼ 10 cm into the plasma, to about the O-point line.…”

Inductive Sustainment of Oblate FRCs with the Assistance of Magnetic Diffusion, Shaping and Finite-Lamor Radius Stabilization

“…RMF current drive 10,23 is the most heavily studied and successful method for sustaining a prolate FRC ͑prolate FRCs have elongation E = Z S / R S Ͼ 1, where Z S and R S are the maximum axial and radial locations on the separatrix͒. This technique was developed in a series of rotomak experiments at Flinders University, 12 and has subsequently been applied to the prolate FRC with good success.…”

“…A first experiment in STX ͑Star Thrust Experiment͒ was able to generate axial field reversal significantly larger than the magnitude of the rotating field, 14 and to maintain the field-reversed configuration for as long as the RMF was applied, and subsequent experiments have expanded the operational regime and physics basis of this technique. It has been applied both to sustain a plasma formed by theta-pinch initiation 14 and to form an FRC from a uniform axial field with a pre-ionized fill; 17,20 it has sustained the configuration when the antennas were located outside of an insulating vacuum chamber, 23 or inside an all-metal chamber. 22 RMF applies a rotating transverse magnetic field to the plasma, leading to a time-dependent axial electric field.…”

“…Historically, most FRCs have been allowed to resistively decay after formation. More recently, techniques have been proposed and/or utilized to sustain the plasma, including neutral beam injection ͑NBI͒ 8,9 and rotating magnetic fields ͑RMF͒ [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] in axially elongated prolate FRCs. For the more spherical oblate FRC, inductive sustainment [27][28][29] has been utilized with some success.…”

Field-reversed configuration formation scheme utilizing a spheromak and solenoid induction

“…The formation of spheromak plasmas has been demonstrated via many different methods, 4 including flux-cores, 16 coaxial plasma guns, 17 combined theta-and Z pinches, 18 conical theta-pinches, 19 kinked Z pinches, 20 and steady inductive helicity injection ͑SIHI͒. 21 The formation of axially elongated FRC plasmas has, however, been limited to rotating magnetic fields, 22 fast formation by theta-pinch coils, 5 and in a single case, a scheme known as the Coaxial Slow-Source. 23,24 Formation of oblate ͑more spherical͒ FRC plasmas has generally been limited to the merging of spheromaks, [25][26][27][28] or in a single case, a laser produced plasma coupled to fast coil ramps.…”