Flux-surface closureDRAFT

Bick HooperJuly 11, 2012

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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 byo Photographs showing the boundaries of a substantial volume of flux

with the geometry of a closed surfaceo EFIT reconstructionso Persistence of toroidal current for a long time with no current driveo 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 scaleo 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

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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

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Current flow near bottom plate depends on slot width

Current flow in flux bubble Current flow along bottom

during injection – plots of RBf

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

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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