Studies of Inductively Sustained Compact Toroids in MRX

A central solenoid has been installed in the Magnetic Reconnection Experiment, in order to study the inductive sustainment of compact toroids (FRCs and spheromaks) formed from spheromak merging. Inductive sustainment applied to Argon FRCs extends the lifetime from $\sim $35$\mu $s to 350$\mu $s. The sustainment manifests itself as a balance between an inward pinch and resistive diffusion of flux and particles. In the configuration for these experiments, with neither strong plasma shaping nor nearby stabilizers, FRC sustainment in lighter gasses is difficult due the growth of co-interchange instabilities. The stability in Argon results from limited equilibrium field shaping, resistive diffusion, and finite-Larmour radius effects. When induction is applied to spheromaks, terminal tilt (Helium) or n=2 modes (Neon) typically develop. Induction applied to an Argon spheromak results in conversion to an FRC: the toroidal flux resistively decays while the poloidal flux is sustained by induction. The stability throughout the conversion is provided by resistive diffusion. These results will be related to the SPIRIT oblate FRC concept. Work supported by DOE.