Page 6 - Rotational stability a long field-reversed configuration

032507-6
Rahman et al.
Phys. Plasmas 21, 032507 (2014)
FIG. 8. Mid-plane current densities, at t 1⁄4 50, 100, and 150 ls.
total current predicted from the above simulation (shown as a green curve). As indicated, the total-plasma current magni- tudes and time-profiles are well-matched. Also shown is the visible light produced by the plasma, which is recorded by a optical photodiode.
IV. DISCUSSION
The dynamics of a finite-laboratory plasma, embedded in a background magnetic field and subject to an externally applied electric field, cannot be adequately described by standard MHD or reduced MHD models, since the electric field and the effects of the finite particle gyro-orbits are
FIG. 9. Current components vs. time, ion, electron, and total integrated over the cross-section of the FRC, r1⁄410 30cm and z1⁄4650cm.
absent in the momentum equation. The electric field in these descriptions arises through Ohm’s Law, which assumes that the total current is due only to electrons.
The force applied by the electric field in a weakly magne- tized plasma must accelerate electrons and ions differently, due to the large difference in orbit size. In the present example, the ion-Larmor radius is qi 1.6 cm, and the electron-Larmor radius is qe 1⁄4 0.04 cm. The azimuthal-electric field produced by the flux coil accelerates electrons in the first half of their gyro-period and decelerates them in the second half of their gyro-period. Thus, their net acceleration is zero, averaged over the entire gyro-period. In the presence of crossed electric and magnetic fields, the electrons can only drift radially, causing a charge imbalance that is compensated by the flow of electrons along the magnetic-field lines.
Ions, on the other hand, continue to accelerate in the azi- muthal direction as long as the azimuthal-electric field is
FIG. 10. (Simulated) total-plasma current, integrated over the full cross- section of the FRC, r1⁄410 30cm and z1⁄4650cm, superimposed on the experimentally determined current, measured using a Rogowski probe.