Measurement of Ion Motion in a Large Amplitude Shear Alfvén Wave

An antenna is used to launch a large amplitude (δB/B_0≈ 10^-3 > √(m_e/M_I) shear Alfvén wave in a magnetoplasma (Argon, n_0=1.0-4.0X10^12cm^-3, B_0=1.5 kG, D = 40 cm). The wave produces a density perturbation (δn/n₀ ≈ 0.15 ) several meters away from the antenna ( L < λ) (Gekelman et. al., Plasma Phys. Cont. Fusion, 42, B15 (2000)). The density and magnetic field on several planes parallel to the background magnetic field was measured with Langmuir and dotB probes. The ion distribution function was also measured on several planes using LIF, with a narrowband dye laser tuned to the ArII 611.492 nm spectral line. The large plasma column dictated the development of a fiber based exciter/receiver probe. The ion measurements enabled identification of the EXB and polarization drifts. This, in turn provides a way to derive E_allel from the measurement of E_⊥ of the kinetic shear wave. The experiment suggests the density perturbation is due to local electron heating rather than the pondermotive force of the wave. Computer simulations at UCLA show that plasma heating can generate similar density channels