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Flux-surface closure DRAFT

Flux-surface closure DRAFT. Bick Hooper July 11, 2012. 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

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Flux-surface closure DRAFT

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  1. Flux-surface closureDRAFT 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 o Photographs showing the boundaries of a substantial volume of flux with the geometry of a closed surface o EFIT reconstructions o Persistence of toroidal current for a long time with no current drive o The 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: o The injector bias-flux drops to zero on a few ms time scale o The 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 RBf 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 RBf Wide slot (11 cm) Narrow slot (4 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 Constant bias field Narrow slot (4 cm) Bias field turned off Narrow slot (4 cm) Almost no current flows at >10 ms if the bias field is held constant

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