Flux-surface closure DRAFT Bick Hooper July 11, 2012

2 Flux-surface closure –– new simulation results NSTX–– forms a significant volume of closed flux after the injection voltage is shorted –Flux-surface closure was supported by oPhotographs showing the boundaries of a substantial volume of flux with the geometry of a closed surface oEFIT reconstructions oPersistence of toroidal current for a long time with no current drive oThe ability to transition to an Ohmically-driven plasma –A similar result was observed in HIT-II Simulation –– forms a closed volume with an X-point near the bottom of NSTX –Closure occurs due to resistive effects in axisymmetric simulations –Conditions on the boundary need to be adjusted correctly: oThe injector bias-flux drops to zero on a few ms time scale oThe injector slot is narrow (4 cm as in the experiment) (Small closed flux forms with wide slot.) –Volume is smaller than observed in the experiment and does not persist as long –Transition to an Ohmically-driven plasma has not been attempted

3 Flux-surface closure –– after crowbarring injector voltage Comments The injector current is measured by RB  above the injector slot. Closure verified by field-line tracing X-point in poloidal flux occurs after the (measured) injector current reaches zero X-point does not form if the bias poloidal magnetic field is constant As the toroidal current decays, the closed flux region decouples from the X-point and the closed-flux region shrinks in volume

4 Current flow near bottom plate depends on slot width Current flow in flux bubble Current flow along bottom during injection – plots of RB  Narrow slot (4 cm)Wide slot (11 cm) The narrow slot generates a narrower distribution of current and a stronger flow of return current along the bottom plate to the slot

5 Turning-off the injector bias coils drives flux-surface closure Bias field turned off Narrow slot (4 cm) Constant bias field Narrow slot (4 cm) Almost no current flows at >10 ms if the bias field is held constant