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 < Eh < 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 driftresonant fluctuations, w = w d h , 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, p, 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.

Observation of Nonlinear Frequency-Sweeping Suppression with rf Diffusion

“…Previous reports have described the complex, time evolution of the P H Y S I C A L R E V I E W L E T T E R S HEI frequency spectrum as well as (i) the use of ECRH to create plasma confined by a strong magnetic dipole, (ii) the structure of the unstable eigenmodes, and (iii) the effect of the resonant electrostatic potential fluctuations on energetic electron radial transport [9,10,12,13,19]. Reference [12] interpreted the strong modulations of energetic electron flux reported in Refs. [9,10] as a consequence of phase-space structures.…”

“…The observations were made in the Collisionless Terrella Experiment (CTX) [9,10], where spontaneous frequency sweeping occurs during the nonlinear evolution of the hot electron interchange (HEI) instability [11][12][13]. The HEI instability creates intense electrostatic potential perturbations that propagate in the direction of the magnetic drifts of energetic trapped electrons.…”

Observation of Centrifugally Driven Interchange Instabilities in a Plasma Confined by a Magnetic Dipole

“…For all three modes, the radial structure extends smoothly from the inner region, L 0:27 m, where hot electrons are created at the microwave cyclotron resonance, out to a radius near, but just within, the outer vacuum vessel wall, L 0:67 m. The radial phase, not shown here, has a significant =2 radial phase shift for the dominant m 1 mode. This In order to interpret the observed centrifugal instability, we have modified a nonlinear, self-consistent numerical simulation [17,18]. Since the plasma undergoes rigid rotation, the modification required to model the centrifugal interchange instability is easily implemented when the evolution equations are expressed in the plasma's rotating frame.…”

“…In this frame, only the equation for cold ion dynamics changes. Using previous notation [17,18], the number of ions within a tube of unit flux, N i ; ; t, evolves in time due to the net flux of ions caused by density-weighted, field line averaged particle drifts,…”

“…Additionally, a small nonresonant dissipation is added to damp potential fluctuations. Dissipation is needed to explain the nonlinear phenomena of ''frequency sweeping'' that has been observed in CTX [17][18][19].…”