Modeling the Effects of (lambda)-gun on SSPX Operation: Mode Spectra, Internal Magnetic Field Structure, and Energy Confinement

摘要

The Sustained Spheromak Physics Experiment (SSPX) shows considerable sensitivity to the value of the injected ('gun') current, I(sub gun), parameterized by the relative values of (lambda)(sub gun) = (mu)(sub 0)I(sub gun)/(Psi)(sub gun) (with (Psi)(sub gun) the bias poloidal magnetic flux) to the lowest eigenvalue of (del) x B = (lambda)(sub FC)B in the flux conserver geometry. This report discusses modeling calculations using the NIMROD resistive-MHD code in the SSPX geometry. The behavior is found to be very sensitive to the profile of the safety factor, q, with the excitation of interior MHD modes at low-order resonant surfaces significantly affecting the evolution. Their evolution affects the fieldline topology (closed flux, islands, stochastic fieldlines confined by KAM surfaces, and open fieldlines), and thus electron temperature and other parameters. Because of this sensitivity, a major effect is the modification of the q-profile by the current on the open fieldlines in the flux core along the geometric axis. The time-history of a discharge can thus vary considerably for relatively small changes in I(sub gun). The possibility of using this sensitivity for feedback control of the discharge evolution is discussed, but modeling of the process is left for future work.