Dynamic Formation of a Hot Field Reversed Configuration with Improved Confinement by Supersonic Merging of Two Colliding High-$\beta$ Compact Toroids

A hot stable field-reversed configuration (FRC) has been produced in the C-2 experiment by colliding and merging two high-$\beta$ plasmoids preformed by the dynamic version of field-reversed $\theta$-pinch technology. The merging process exhibits the highest poloidal flux amplification obtained in a magnetic confinement system (over tenfold increase). Most of the kinetic energy is converted into thermal energy with total temperature ($T_i+T_e$) exceeding 0.5 keV. The final FRC state exhibits a record FRC lifetime with flux confinement approaching classical values. These findings should have significant implications for fusion research and the physics of magnetic reconnection.

Figure 1

Schematic of the C-2 device.

Figure 2

Evolution of the excluded flux radius in C-2 obtained from a series of external diamagnetic loops at the two $\theta$-pinch formation sections and magnetic probes embedded inside the central metal confinement chamber. Time is measured from the instant of synchronized field reversal in the $\theta$ pinch sources, and distance $z$ is given relative to the center of the confinement chamber.

Figure 3

Comparison of the initial translated CTs and the resultant merged FRC, showing time traces of the external poloidal field $B_e$, the excluded flux radius $r_{\Delta \varphi }$, and the poloidal flux ${\varphi }_p$ for individual CTs produced by $\theta$-pinches on the north and south side, respectively, and the result from merging individual CTs formed under identical conditions. Flux decay time, ${\tau }_{\varphi }$, for the merged FRC is also shown. Note that the two single CTs were not captured by the end mirrors due to their high translation speeds. Merging occurs around $30\mu \mathrm{s}$, as indicated.

Figure 4

Effect of dynamic translation speed. The data are obtained right after merging ($\sim 40\mu \mathrm{s}$) of the FRC from a series of reproducible and well-controlled discharges with the same magnetic configurations and gas fuelling rates. The translation speed is varied by energizing the $\theta$-pinch coils at various different times during the initial formation process. Conservation of particle number is assumed.

Figure 5

$T_e$ determined by Thomson scattering at different radii and $T_t$ derived from the radial pressure balance for typical merged FRC conditions.

Figure 6

Time evolution of the excluded flux radius $r_{\Delta \varphi }$ and the line-integrated density $\int n_{e}d\ell$ at $r=3.3\mathrm{cm}$ obtained from a six-channel integrated ${\mathrm{CO}}_2/\mathrm{HeNe}$ interferometer system (top), and contour of the electron density $n_e(r)$ deduced from the interferometer measurements using the Abel inversion technique (bottom).

Figure 7

(a) Magnetic flux decay time, ${\tau }_{\varphi }$ , versus the LSX scaling for conventional $\theta$-pinch formed FRCs, ${\tau }_{\varphi }^{\mathrm{LSX}}$, and results from recent FRC translation experiments, TCS [5] and FIX [3]; (b) diffusivities, $D_{\perp }$, derived from ${\tau }_{\varphi }$, versus classical diffusivities, $D_{\perp }^{\mathrm{cl}}$, for the well-centered merging FRCs in C-2.