Using controlled magnetic perturbations to study the plasma confinement in a spheromak

Some regimes of the plasma confinement in spheromaks are governed by breaking magnetic surfaces by non-axisymmetric perturbations. To get some insights into this effect, we designed an experiment based on the use of an external coil for creating static non-axisymmetric perturbations (see preliminary analysis in M.R. Terry et al, BAPS, 47, 40, 2002). To get a more quantitative estimate of the required level of perturbation, we have performed detailed line-tracing numerical study for the model where the external field is simply added to the magnetic field of an axisymmetric spheromak equilibrium. The required amplitude of the perturbation was found to be a few percent of the equilibrium field. We describe the design of the perturbing coil for SSPX and discuss issues of its optimum orientation and positioning with respect to various diagnostics. By the time of the Conference, we expect to perform first experiments on SSPX using a single circular perturbation coil. Work performed for U.S. DOE by UC LLNL under contract W-7405-Eng-48.