Particle and Wave Scattering from Density Fluctuations
Particle and wave scattering due to density fluctuations is investigated in the Irvine Torus. This plasma resembles that found in edge plasmas, divertors, and also in the scrape-off layers of large tokamaks. A Laser-Induced-Fluorescence system suitable for large tokamaks obtains ion velocity distributions, temperatures and absolute convective velocities along all three axes. Neutral density's effect on ion temperature is studied. Background light intensity at 461 nanometers is lowered after depletion of the argon ion metastable state. The background light intensity's return to the steady state level provides evidence of tagged ions. The process, called Darkon tagging, allows the measurement of ion diffusion. Argon metastable lifetime and test particle diffusion coefficient are determined. The application of radio frequency waves increases low frequency ion density fluctuations and diffusion. A linear fit to this enhanced diffusion as a function of density fluctuation level gives a slope of 196 meters squared per second for fluctuations below 0.88%. No single power law explains the growth of turbulent diffusion over the range measured. Above 0.88% a different linear fit can be made to the data. This implies the presence of a transition in the growth of turbulent diffusion. Both slow and fast lower hybrid waves are observed in a toroidal plasma using a waveguide array designed to excite only the fast wave polarization. Direct measurements of wavelengths and phase velocities confirm the specific role of fast and slow waves. No waves are observed for densities below the fast-wave cutoff density, indicating the waveguide array does not excite slow waves directly in the plasma. Just above the fast-wave cutoff density, slow waves alone are seen. At more than three times the cutoff density, the fast waves are finally observed and have a substantial shift in measured index of refraction along the magnetic field. We speculate that density fluctuations contribute to both the presence of the slow wave and the change in fast wave wavenumber spectra.
- Pub Date:
- TURBULENT DIFFUSION;
- Physics: Fluid and Plasma