Formation of a Long-Lived Hot Field Reversed Configuration by Merging Two Colliding High-$\beta$ Compact Toroids

A new compact toroid (CT) device, C-2, has been built to form and sustain fusion-relevant field reversed configurations (FRC), one of the simplest magnetic confinement entities with average $\beta $ (ratio of average plasma to magnetic pressure inside the separatrix) $\sim $10. High temperature FRCs are produced in C-2 by dynamically merging two oppositely directed, highly supersonic high-$\beta $ deuterium plasmoids preformed by the conventional $\theta $-pinch technology, achieving record lifetimes of over 2 ms based on external diamagnetic measurements, with plasma diameter $\sim $ 1 m, poloidal flux \textit{$\phi $}$_{p} \quad \sim $ 15 mWb, electron density $n_{e} \quad \sim $ 10$^{20}$ m$^{-3}$, and $T_{i}+T_{e} \quad >$ 0.5 keV. Most of the kinetic energy is converted into thermal energy upon collision, predominantly going into the ion channel: $T_{i} \quad \sim \quad T_{e} \quad \sim $ 30 eV before merging, while $T_{i} \quad \sim $ 4.5$T_{e}$ with $T_{e} \quad \sim $ 100 eV after merging, as derived from radial pressure balance and multi-chord, muli-pulse Thomson scattering measurements. Such high ion temperatures are also consistent with Doppler spectroscopy and neutron measurements. Strong poloidal flux amplification occurs during the merging process with a flux amplification factor exceeding 10, the highest ever obtained in a magnetic confinement system. Both temperatures and poloidal fluxes of the merged FRCs depend strongly on the speed of the initial individual plasmoids, favoring fast translation. The dynamics of the merging/reconnection process of the translated CTs are reproduced, for the first time, by a newly developed 2-D resistive magnetohydrodynamic code, LamyRidge. What is even more remarkable is that the final merged FRC state exhibits a dramatic improvement in transport with flux confinement times approaching classical values. The formation of such a well-confined, long-lived, high-$\beta $ plasma state via collisional merging and magnetic reconnection should be of wide interest to fusion energy sciences and basic plasma physics research.