Spatiotemporal Evolution of Runaway Electron Momentum Distributions in Tokamaks

C. Paz-Soldan, C. M. Cooper, P. Aleynikov, D. C. Pace, N. W. Eidietis, D. P. Brennan, R. S. Granetz, E. M. Hollmann, C. Liu, A. Lvovskiy, R. A. Moyer, and D. Shiraki
Phys. Rev. Lett. 118, 255002 – Published 22 June 2017

Abstract

Novel spatial, temporal, and energetically resolved measurements of bremsstrahlung hard-x-ray (HXR) emission from runaway electron (RE) populations in tokamaks reveal nonmonotonic RE distribution functions whose properties depend on the interplay of electric field acceleration with collisional and synchrotron damping. Measurements are consistent with theoretical predictions of momentum-space attractors that accumulate runaway electrons. RE distribution functions are measured to shift to a higher energy when the synchrotron force is reduced by decreasing the toroidal magnetic field strength. Increasing the collisional damping by increasing the electron density (at a fixed magnetic and electric field) reduces the energy of the nonmonotonic feature and reduces the HXR growth rate at all energies. Higher-energy HXR growth rates extrapolate to zero at the expected threshold electric field for RE sustainment, while low-energy REs are anomalously lost. The compilation of HXR emission from different sight lines into the plasma yields energy and pitch-angle-resolved RE distributions and demonstrates increasing pitch-angle and radial gradients with energy.

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  • Received 31 January 2017

DOI:https://doi.org/10.1103/PhysRevLett.118.255002

© 2017 American Physical Society

Physics Subject Headings (PhySH)

Plasma Physics

Authors & Affiliations

C. Paz-Soldan1,*, C. M. Cooper2, P. Aleynikov3, D. C. Pace1, N. W. Eidietis1, D. P. Brennan4, R. S. Granetz5, E. M. Hollmann6, C. Liu4, A. Lvovskiy2, R. A. Moyer6, and D. Shiraki7

  • 1General Atomics, San Diego, California 92186, USA
  • 2Oak Ridge Associated Universities, Oak Ridge, Tennessee 37831, USA
  • 3Max-Planck Institute for Plasma Physics, Greifswald, Germany
  • 4Princeton University, Princeton, New Jersey 05764, USA
  • 5Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
  • 6University of California–San Diego, La Jolla, California 92093, USA
  • 7Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

  • *paz-soldan@fusion.gat.com

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Issue

Vol. 118, Iss. 25 — 23 June 2017

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