Carbon Dioxide Laser Scattering on Radio Frequency Waves in ACT-1

A radially scanning small-angle CO(,2) laser scattering system was designed and built to study driven radio frequency (RF) waves in the steady-state Advanced Concepts Torus (ACT-1). The well characterized multispecies hydrogen plasma (H(,1)('+), H(,2)('+), H(,3)('+)), with low neutral pressure and warm ions, is a convenient medium for the study of finite ion Larmor radius (FLR) effects. Two types of waves, the ion Berstein wave (IBW) and lower hybrid wave (LHW), are investigated. Ion Berstein waves, existing because of the inclusion of FLR terms in the wave dispersion relation, are launched at frequencies below the n('th) ion cyclotron harmonic (f = n(omega)(,ci)/2(pi), n = 2,3,4) and studied through their density (laser) and potential (probe) fluctuations. Detailed dispersion curves are mapped out, showing the highest harmonic FLR effects yet observed. A new nonintrusive method for measuring hydrogen ion temperature, using externally launched IBW as test waves detected by laser scattering, is demonstrated. This technique does not require knowledge of T(,e) or n(,e) to obtain T(,i). Application of this technique to large, hot tokamak plasmas is discussed. Lower hybrid waves are studied near the lower hybrid resonance, i.e., in the frequency range 1(, )<(, )(omega)/(omega)(,LH) < 3. Resonance cones are detected with the laser system, and wave K(,(PARLL))-spectra are measured both by scanning the scattering k-vector (k(,(PERP))) with fixed plasma conditions, and by sweeping the density while holding k(,(PERP)) constant. In this manner, the effects of antenna phasing and electron Landau damping on the LHW k(,(PARLL))-spectrum in the plasma are observed. High levels of low frequency turbulence are seen to destroy the laser resonance cone signal. Extensive two-dimensional plasma parameter profiling shows the importance of planar wave-fronts to aid detection of the low (absolute) level RF waves with laser scattering. Power scaling shows nonlinear effects associated with background neutral ionization, and buckling of plasma profiles. Finally, FLR effects are observed on LHW trajectories, both by probe and laser, with periodicity (omega)(,ci) in the range 4 < (omega)/(omega)(,ci) < 8, when 1(, )<(, )(omega)/(omega)(,LH) < 2. These effects are highly sensitive to plasma and antenna parameters, and can be qualitatively understood with a wave trajectory code including magnetized warm ion terms.