Helicity conservation and energy confinement in CTX spheromaks

Results are reported for the CTX spheromak experiment with a flux conserver of 0.67 m radius. The measured currents in the mesh allow a non-perturbing determination of the equilibrium current profile. Analysis of magnetic data shows that injected magnetic helicity is conserved. A mode of operation has been encountered in which periodic decreases in the helicity content of sustained spheromaks are correlated with the quiescence of current-driven instabilities. Increase of the spheromak radius from 0.40 m to 0.67 m has not led to an increase in energy confinement. Attempts to resolve this issue, including the use of a conducting core and applied bias fields, are discussed. (author)