Experimental measurements of the propagation of large-amplitude shear Alfven waves

Experiments on the edge plasma of tokamaks have discovered magnetic fluctua tions which are highly correlated along the magnetic field, and are correla ted with scale size of the electron inertial length (delta = c/omega (pe)) across the field. They are, in all probability, shear Alfven waves. The FRE JA, FAST, and Interball satellites have frequently encountered density stri ations in the auroral ionosphere. These can be narrow, also of the order of delta. Intense wave activity has been measured within these structures and tentatively identified as shear inertial (V-A > V-the) Alfven waves. These waves have been studied in great derail in the Large Plasma Device at UCLA . The plasma, which is 10 m in length and 500 ion Larmor radii in diameter (He (lambda (parallel to) approximate to 2 m), Ar (lambda (parallel to) app roximate to 10 m, 1.5 kG, 40 cm plasma diameter, n = 1.0-4.0 x 10(12) cm(-3 ), fully ionized) supports Alfven waves. Our initial investigations, which will be briefly reviewed, involved low-amplitude (deltaB(wave)/B-0 approxim ate to 10(-4)) shear waves launched by modulating a skin depth size current channel, and have examined the wave characteristics in the kinetic (V-A < V-the) and inertial regimes and in magnetic field gradients. Launching high er-power waves (<delta>B-wave/B-0 greater than or equal to 10(-3)) waves wi th the use of a helical antenna has extended these studies. Both shear Alfv en waves (omega < <omega>(ci)) and compressional Alfven waves have been inv estigated. Below f(cl) the wave fields slowly spread across the background magnetic field and the current associated with it forms a rotating spiral. The higher-power wave causes a localized density perturbation when deltaB(w ave)/B-0 exceeds 10(-3) even when the wave propagates below the cyclotron f requency. The perturbation is measured using Langmuir probes as well as las er-induced fluorescence (LIF) signal from Ar II ions. We present data of th e wave propagation in which the temporal history of the vector magnetic fie ld was acquired at 20 000 spatial locations. The data is used to calculate 3D wave currents, wave phase fronts and energy propagation. In helium the w ave pattern is more complex than in argon. We also present the space and ti me evolution of the density perturbations associated with the wave in an Ar plasma. LIF data was used to directly measure the ion motion in the electr ic field of the wave, ion polarization currents and the motion of the ions as they form the density non-uniformities.