Spheromak merging and field reversed configuration formation at the Swarthmore Spheromak Experiment

The initial results of coaxial co- and counter-helicity spheromak merging studies at the Swarthmore Spheromak Experiment (SSX) [M. R. Brown, Phys. Plasmas 6, 1717 (1999)] are reported. In its new configuration, SSX is optimized to study field reversed configuration (FRC) formation and stability by counter-helicity spheromak merging. A pair of midplane coils magnetically restricts the merging process to determine how the stability of the resulting magnetic configuration depends upon the quantity of toroidal flux remaining from the initial spheromaks. The diagnostic set at SSX, featuring the capability of measuring up to 600 magnetic field components at 800 ns time resolution, permits detailed studies of the dynamic three-dimensional magnetic structures produced during these experiments. A compact array of magnetic probes examines the local reconnection process, while a distributed array of probes examines global magnetic structure. Counter-helicity merging produces a FRC that persists for several Alfvén times, although the oppositely directed toroidal field of the initial spheromaks does not completely annihilate. The m=1 (toroidal) mode dominates late in the evolution, and is consistent with the tilt instability. Co-helicity merging produces a single elongated spheromak that rapidly tilts.