Post on 18-Dec-2015
Flux-surface closureDRAFT
Bick HooperJuly 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 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
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
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