Paramagnetic spin-up of a field reversed configuration with rotating magnetic field current drive
Peter, Andrew Maxwell
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A transverse Rotating Magnetic Field (RMF) can drive toroidal current and sustain the poloidal flux of a Field Reversed Configuration (FRC) through the application of a 〈vz x Br〉 Lorentz force on the electrons, where vz is the axial screening current and Br is the radial component of the RMF. The torque applied by the RMF will eventually be transferred to the ions through resistive collisions. In the absence of any drag force, the plasma will rapidly spin up in the ion paramagnetic direction, negating the current drive and possibly becoming rotationally unstable. A multi-chord Intensified Charge-Coupled Device (ICCD) spectrometer has measured the ion rotation profile via the Doppler shift of impurity line radiation in the Translation, Confinement, and Sustainment (TCS) experiment. The plasma is observed to rapidly spin up in the ion paramagnetic direction to a rigid rotation frequency of about oi ≈ 7 x 104 s-1, less than 15% of the typical RMF frequency o ≈ 0.5 x 106 s-1 . Neutral deuterium is observed to have no rotational velocity, and had been proposed as a mechanism for preventing synchronous spin-up of the ions. The neutral density and resulting charge-exchange and ionization rates have been calculated from an array of absolutely calibrated Dalpha detectors. The typical neutral fraction of about 2% of the plasma density is several times too low for ion-neutral collisions to balance the applied torque. Other possible braking mechanisms are shorting of the radial electric field needed to confine paramagnetic ions, and viscous drag. Assuming axial and azimuthal symmetry and pure deuterium, viscous wall drag is found to be insufficient to slow the plasma as well. Viscous drag could be significant if the edge plasma has high impurity content or is spatially non-uniform.