Observations of tilt instabilities in field-reversed configurations of a confined plasma

We report the first consistent observations of internal tilt instabilities in field-reversed configurations. Detailed comparisons with numerical calculations establish that data from an array of external magnetic probes are signatures of these destructive plasma instabilities. As suggested by finite-Larmor-radius theory, field-reversed configurations appear grossly stable when s/e [(average number of ion gyroradii)/(separatrix elongation)] is less than 0.2--0.3 and show MHD-like tilt instabilities when s/e{~}1.