title: Observation of Critical-Gradient Behavior in Alfvén-Eigenmode-Induced Fast-Ion Transport.
abstract: Experiments in the DIII-D tokamak show that fast-ion transport suddenly becomes stiff above a critical threshold in the presence of many overlapping small-amplitude Alfvén eigenmodes (AEs). The threshold is phase-space dependent and occurs when particle orbits become stochastic due to resonances with AEs. Above threshold, equilibrium fast-ion density profiles are unchanged despite increased drive, and intermittent fast-ion losses are observed. Fast-ion Dα spectroscopy indicates radially localized transport of the copassing population at radii that correspond to the location of midcore AEs. The observation of stiff fast-ion transport suggests that reduced models can be used to effectively predict alpha profiles, beam ion profiles, and losses to aid in the design of optimized scenarios for future burning plasma devices.
authors.affiliation: University of California-Irvine, Irvine, California 92697, USA.
authors.forename: C S
authors.initials: CS
authors.lastname: Collins
authors.affiliation: University of California-Irvine, Irvine, California 92697, USA.
authors.forename: W W
authors.initials: WW
authors.lastname: Heidbrink
authors.affiliation: University of Texas-Austin, Austin, Texas 78712, USA.
authors.forename: M E
authors.initials: ME
authors.lastname: Austin
authors.affiliation: Princeton Plasma Physics Laboratory, P.O. Box 451, Princeton, New Jersey 08543, USA.
authors.forename: G J
authors.initials: GJ
authors.lastname: Kramer
authors.affiliation: General Atomics, P.O. Box 85608, San Diego, California 92186-5608, USA.
authors.forename: D C
authors.initials: DC
authors.lastname: Pace
authors.affiliation: General Atomics, P.O. Box 85608, San Diego, California 92186-5608, USA.
authors.forename: C C
authors.initials: CC
authors.lastname: Petty
authors.affiliation: University of California-Irvine, Irvine, California 92697, USA.
authors.forename: L
authors.initials: L
authors.lastname: Stagner
authors.affiliation: General Atomics, P.O. Box 85608, San Diego, California 92186-5608, USA.
authors.forename: M A
authors.initials: MA
authors.lastname: Van Zeeland
authors.affiliation: Princeton Plasma Physics Laboratory, P.O. Box 451, Princeton, New Jersey 08543, USA.
authors.forename: R B
authors.initials: RB
authors.lastname: White
authors.affiliation: University of California-Irvine, Irvine, California 92697, USA.
authors.forename: Y B
authors.initials: YB
authors.lastname: Zhu