Compact toroid experiments and theory

Two types of compact toroids have been studied: spheromaks and field-reversed configurations (FRC). Spheromaks, which contain both toroidal and poloidal fields, have been formed with a magnetized co-axial injector and trapped in both prolate and oblate flux conservers. As expected from theory, the prolate configuration always tilts, but the oblate configuration can be made stable even in the presence of a guide field. Observations include 150 μs life-times, average value of n approximately 1014cm-3, and a decrease of field fluctuations by a factor of 100 at the time of complete reconnection. - Theoretical studies of the FRC (no toroidal field) have been compared with the results of two field-reversed theta pinches, FRX-A and FRX-B. Typical FRC parameters achieved on FRX-B are Tsub(i)=200 eV, Tsub(e)=110 eV, nsub(max)=3.5x1015cm-3, separatrix radius=5.4 cm, plasmoid length=50 cm, and particle confinement time=40 μs. An increase in the trapped bias flux appears desirable and improved pre-ionization techniques in the presence of increased bias fields (approximately 3 kG) have been developed on FRX-A. In an elongated FRC equilibrium, xsub(s) (ratio of separatrix radius to wall radius) constrains the average β, the closed flux, and the elongation during adiabatic compression. The most dangerous ideal modes are fluting and internal tilting, but they are not observed experimentally. The prediction that approximately 0.5 of the confined plasma must be lost to produce the instability agrees with the FRX-B results. Both 0-D and 1-D calculations, including analytic 2-D effects and lower-hybrid-drift turbulence, reproduce the experimentally observed confinement times