Stabilization of Interchange Modes by Rotating Magnetic Fields

Interchange modes have been a key limiting instability for many magnetic confinement fusion configurations. In previous studies intended to deal with these ubiquitous instabilities, complex, transport enhancing, minimum-$B$ producing coils were added to the otherwise simple linear mirror plasma. Possible solutions for returning to a simple symmetric mirror configuration, such as ponderomotive fields, are weak and difficult to apply. A new method is demonstrated here for the first time, utilizing rotating magnetic fields that are simple to apply and highly effective. A simple and easily comprehensible theory has also been developed to explain the remarkable stabilizing properties. Although this work has been performed on field reversed configurations, it should have a wide application to other confinement schemes, and could become a cornerstone for high-$\beta$ plasma stability.

Figure 1

Calculated RMF lines for TCS antenna geometry without and with conducting plasma column. Antenna locations are shown on the vacuum field plot, with field lines shown at the time when only the vertical field producing coils are carrying current.

Figure 2

Total integrated stabilizing inward RMF force $\underset{}{\overset{}{\int }}⟨j_z{B}_{\theta }⟩$ as a function of azimuthal angle for an elliptical distortion with ${\xi }_{\max }=0.1r_s$, normalized to the normal steady confinement pressure $B_e{}^2/2{\mu }_0$. An end-on sketch of the FRC and the resultant RMF lines are also shown.

Figure 3

Time traces of the line-integrated density $\underset{}{\overset{}{\int }}nd\ell$ and excluded flux radius $r_{\Delta \varphi }$ for two comparable FRCs produced using separated RMF antennas with different gaps in the center. Shown as insets are the tomographic reconstructions of bremsstralung radiation (between 510 and 580 nm) monitored by a 5-fan (6–12 chords per fan) visible tomography system.

Figure 4

(a) Internal probe measurements of axial magnetic fields $B_z$ at several different radial locations showing $n=2$ oscillations (at frequency $\Omega /\pi$) towards the end of the discharge. (b) Contour of the frequency spectrum of $B_z$ for the period highlighted in (a). RMF frequency is also indicated.

Figure 5

Radial profiles of normalized magnetic oscillation $\Delta B_{z,\mathrm{rms}}/B_e$ and RMF field $B_{\theta }/B_e$, as well as the normalized pressure profile $p/p_{\max }$.

Figure 6

Stability diagram for the FRCs formed and sustained by the RMF at different frequencies based on Eq. (7).