Laboratory Observations of Wave-Induced Radial Transport within an "Artificial Radiation Belt"

Abstract
Wave-induced radial transport of energetic electrons has been observed in a laboratory terrella. In the experiment, electron-cyclotron-resonance heating (ECRH) is used to create a localized population of trapped energetic electrons (1 keV < E_h < 50 keV) within a low-density discharge which we refer to as an "artificial radiation belt." As the intensity of the radiation belt increases, quasiperiodic bursts of drift-resonant fluctuations, ω ≈ ω_dh, are excited. The frequency spectrum of this instability is time-varying and complex, and global chaotic radial transport is induced whenever the frequency spectrum is both intense and compact. High-speed measurements of the energetic electron transport are made with particle detectors, and these measurements can be directly compared with nonlinear and self-consistent simulations. We find quasilinear transport simulations do not reproduce the experimental measurements. In contrast, simulations which retain the electron's guiding-center Hamiltonian dynamics and which preserve the first, µ, and second, J, adiabatic invariants reproduce key temporal characteristics of the experimental measurements. The resemblance between simulation and experiment suggests that persistent phase-space structures strongly modulate the energetic electron transport and contribute to the growth and saturation of the instability.