Formation of free-boundary flux-core spheromaks

Free-boundary flux-core spheromaks are stably produced in the external magnetic field in the TS-3 facility. In the first experiment, a ζ-θ-discharge is initiated with the reversed bias magnetic field, and the axial current is supplied to amplify the magnetic flux. The flux-core spheromak can be sustained by this axial current. Its lifetime is about 120 μs. In the second experiment, only by the axial current, the flux-core spheromak is successfully produced in the reversed in the bias magnetic field. Its lifetime is larger than 100 μs. The force-free parameter μ is estimated. It is almost time invariant, while the discharge current is changing sinusoidally. It is 11 m-1 in the first experiment and 12.6 m-1 in the second experiment, and is smaller than the eigenvalue of the spheromak with no external magnetic flux. (author)