Abstract Details
Abstracts
Author: Alexandre P Sainterme
Requested Type: Consider for Invited
Submitted: 2024-03-27 19:16:18
Co-authors: C.R. Sovinec
Contact Info:
University of Wisconsin - Madison
318 Norris Ct. Apt 5
Madison, WI 53703
United States
Abstract Text:
A beam of runaway electrons (REs) within a cold, resistive background plasma can be unstable to resistive hose modes. Helical deflections of the current centroid of the beam away from its initial position are imperfectly canceled by eddy currents induced in the cold background plasma. The resulting macro-scale, overstable oscillations grow on a time scale determined by the resistivity of the cold background plasma. This scenario is relevant to post-disruption tokamak plasmas during the current plateau phase, where it is observed that a beam-like population of REs carries a majority of the current. In this work, an RE beam is modeled using a fluid approximation coupled to MHD equations for the background plasma. The RE beam provides a source of resistance-free current density whose direction depends on the time-evolving magnetic field. Numerical solutions of the linearized set of equations for small perturbations about an MHD equilibrium supported by RE beam current are presented in both cylindrical and toroidal geometry. For equilibria that are unstable to tearing modes, we recover the RE-modified growth rate scaling of [Liu, et al., PoP 27, 092507(2020)] for large Lundquist number. We additionally find that high-frequency m=1,n=1, resistive hose modes grow faster than tearing modes at low Lundquist numbers [PoP 31, 010701 (2024)]. For RE beams in DIII-D, we estimate that this transition will occur at a background plasma temperature of approximately 1-2 eV for the equilibrium current profile under consideration. The growth rate also depends on the gradient of the RE parallel current profile. Additional calculations for current profiles representative of RE beam experiments in DIII-D show the possibility of hose modes. Nonlinear evolution of the instability flattens the parallel current profile near the magnetic axis. In some cases, the flattening of the current profile extends to the plasma boundary.
Work supported by the US DOE through grant DE-SC00180001
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