Experiments on the edge plasma of tokamaks have discovered magneticfluctuations which are highly correlated along the magnetic field, and arecorrelated with scale size of the electron inertial length (δ = c/ωpe) across the field. They are, in all probability, shearAlfvén waves. The FREJA, FAST, and Interball satellites have frequentlyencountered density striations in the auroral ionosphere. These can be narrow,also of the order of δ. Intense wave activity has been measured withinthese structures and tentatively identified as shear inertial (VA>Vthe) Alfvén waves. These waves have been studied in great detailin the Large Plasma Device at UCLA. The plasma, which is 10 m in length and500 ion Larmor radii in diameter (He (λ∥≈2 m),Ar (λ∥≈10 m, 1.5 kG, 40 cm plasma diameter,n = 1.0-4.0×1012 cm-3, fully ionized) supports Alfvénwaves. Our initial investigations, which will be briefly reviewed, involvedlow-amplitude (δBwave/B0≈10-4) shear waves launchedby modulating a skin depth size current channel, and have examined the wavecharacteristics in the kinetic (VA<Vthe) and inertial regimesand in magnetic field gradients. Launching higher-power waves (δBwave/B0⩾10-3) waves with the use of a helical antenna has extendedthese studies. Both shear Alfvén waves (ω<ωci) andcompressional Alfvén waves have been investigated. Below fci thewave fields slowly spread across the background magnetic field and the currentassociated with it forms a rotating spiral. The higher-power wave causes alocalized density perturbation when δBwave/B0 exceeds10-3 even when the wave propagates below the cyclotron frequency. Theperturbation is measured using Langmuir probes as well as laser-inducedfluorescence (LIF) signal from Ar II ions. We present data of the wavepropagation in which the temporal history of the vector magnetic field wasacquired at 20 000 spatial locations. The data is used to calculate 3D wavecurrents, wave phase fronts and energy propagation. In helium the wave patternis more complex than in argon. We also present the space and time evolution ofthe density perturbations associated with the wave in an Ar plasma. LIF datawas 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 densitynon-uniformities.
Excitation and propagation of low frequency wave in a FRC plasma