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approvedelectronloss_sherwood2016_.pdf2016-04-06 21:34:37Allen Boozer


Author: Allen H Boozer
Requested Type: Poster
Submitted: 2016-02-14 15:35:09

Co-authors: A. Punjabi

Contact Info:
Columbia University
287B Engineering Terrace
New York, NY   10027
United States

Abstract Text:
When an evolving tokamak plasma has broad annuli of stochastic lines separated by annuli of confining magnetic surfaces, the loss of relativistic electrons can occur in short pulses of length τ_L in localized regions on the walls. Relativistic electrons that result from a runaway process on ITER move in close alignment with the magnetic field and have essentially the same trajectories. Evolution can cause the confining magnetic surfaces outside a broad stochastic region to be lost. Questions are how many toroidal transits of the lines in the stochastic regions are required before they strike the surrounding wall and what is the area they strike. These are subtle questions in Hamiltonian mechanics, which implies the outermost confining magnetic surface breaks by the opening of gaps across the lies that have widths that increase linearly with time. Along the field-line trajectories, the gaps may narrow. Flux conservation implies the lines make proportionately larger radial excursions. The length along a field line while in a gap of significant width is comparable to the system size, so the fraction of the magnetic flux that can escape through a gap scales as f~(t/τ_ev)^1/2, where the evolution time, τ_ev~100ms. Electrons following the lines take many toroidal transits, each requiring τ_t=2πR/c~0.1µs, to cross the stochastic region and approach the breaking outer surface. An evolving area-conserving map was used for simulations, which find that the time to cross the stochastic region is τ_s~τ_t/f when the field structure is evolving but τ_s~τ_t/f^2 when a fixed field structure drifts into a wall. The loss time is τ_L=τ_s/f. That is, the simulations show that for an evolving field structure τ_L ~(τ_t τ_ev)^1/2 and f~(τ_t/τ_ev)^1/4. The simulations show that for a fixed field structure drifting into a wall τ_L~τ_t^2/5 τ_ev^3/5 and f~(τ_t/τ_ev)^1/5. Support DE-FG02-03ER54696, DE-FG02-04ER54793, and NERSC support DE-AC02-05CH11231.