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

Experiments on the edge plasma of tokamaks have discovered magnetic fluctuations which are highly correlated along the magnetic field, and are correlated with scale size of the electron inertial length (δ = c/ω pe ) across the field. They are, in all probability, shear Alfvén waves. The FREJA, FAST, and Interball satellites have frequently encountered density striations in the auroral ionosphere. These can be narrow, also of the order of δ. Intense wave activity has been measured within these structures and tentatively identified as shear inertial ( V A > V the ) Alfvén waves. These waves have been studied in great detail in the Large Plasma Device at UCLA. The plasma, which is 10 m in length and 500 ion Larmor radii in diameter (He (λ ∥ ≈2 m), Ar (λ ∥ ≈10 m, 1.5 kG, 40 cm plasma diameter, n = 1.0-4.0×10 12 cm -3 , fully ionized) supports Alfvén waves. Our initial investigations, which will be briefly reviewed, involved low-amplitude (δ B wave / B 0 ≈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 higher-power waves (δ B wave / B 0 ⩾10 -3 ) waves with the use of a helical antenna has extended these studies. Both shear Alfvén waves (ω<ω ci ) and compressional Alfvén waves have been investigated. Below f ci 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 δ B wave / B 0 exceeds 10 -3 even when the wave propagates below the cyclotron frequency. The perturbation is measured using Langmuir probes as well as laser-induced fluorescence (LIF) signal from Ar II ions. We present data of the wave propagation in which the temporal history of the vector magnetic field 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 wave pattern is more complex than in argon. We also present the space and time 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 electric field of the wave, ion polarization currents and the motion of the ions as they form the density non-uniformities.