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This paper reports on the interchange stability of a realistic analytical equilibrium solution of a field-reversed configuration (FRC) contained within a tight fitting cylindrical flux conserver with end walls, no external flux, and arbitrary elongation E. For E≫1, interchange stability is possible with ps/p0>7/12, where p0 is the peak pressure at the magnetic axis, and ps is the pressure at the separatrix/wall, a condition that appears to be compatible with high-pressure wall-confined, pulsed FRC implosions. Oblate configurations require somewhat less ps/p0 for stability. Analytical equilibria have been found for FRCs with two magnetic axes (doublets) with an internal figure-of-eight separatrix. A striking improvement in stability with respect to interchange modes was found in doublets: the private flux region (core) can be completely stabilized with zero separatrix pressure. This new finding suggests that doublet FRCs may be more relevant for steady-state fusion applications, which for good performance ...

The field-reversed configuration (FRC) is a high-beta compact toroidal plasma confinement scheme in which the external poloidal field is reversed on the geometric axis by azimuthal (toroidal) plasma current. A quasineutral, hybrid, particle-in-cell (PIC) approach [Y. A. Omelchenko and R. N. Sudan, Phys. Plasmas 2, 2773 (1995)] is applied to study long-term nonlinear stability of computational FRC equilibria to a number of toroidal modes, including the most disruptive tilt mode. In particular, a self-generated toroidal magnetic field is found to be an important factor in mitigating the instability and preventing the confinement disruption. This is shown to be a unique FRC property resulting from the Hall effect in the regions of vanishing poloidal magnetic field. The instability-driven toroidal field stabilizes kink formation by increasing the magnetic field energy without destabilizing curvature-driven plasma motion. Finally, the tilt instability saturates due to nonlinear, finite Larmor radius (FLR) effects and plasma relaxation to a quasisteady kinetic state. During this transition the FRC is shown to dissipate a substantial amount of initially trapped flux and plasma energy. These effects are demonstrated for kinetic and fluid-like, spherical and prolate FRCs.

Results from recent experiments demonstrate that quiescent H-mode (QH-mode) sustained by magnetic torque from non-axisymmetric magnetic fields is a promising operating mode for future burning plasmas. Using magnetic torque from n=3 fields to replace counter-Ip torque from neutral beam injection (NBI), we have achieved long duration, counter-rotating QH-mode operation with NBI torque ranging from counter-Ip to up to co-Ip values of 1-1.3 Nm. This co-Ip torque is 3 to 4 times the scaled torque that ITER will have. These experiments utilized an ITER-relevant lower single-null plasma shape and were done with ITER-relevant values of νped* and βNped. These discharges exhibited confinement quality H98y2=1.3, in the range required for ITER. In preliminary experiments using n=3 fields only from a coil outside the toroidal coil, QH-mode plasmas with low q95=3.4 have reached fusion gain values of G=βNH89/q952=0.4, which is the desired value for ITER. Shots with the same coil configuration also operated with net zero...