EXPERIMENTAL-OBSERVATION OF ALFVEN-WAVE CONES

The spatial evolution of the radial profile of the magnetic field of a shear Alfvén wave launched by a disk exciter with radius on the order of the electron skin depth has been measured. The waves are launched using wire mesh disk exciters of 4 mm and 8 mm radius into a helium plasma of density about 1.0×1012 cm-3 and magnetic field 1.1 kG. The electron skin depth δ=c/ωpe is about 5 mm. The current channel associated with the shear Alfvén wave is observed to spread with distance away from the exciter. The spreading follows a cone-like pattern whose angle is given by tan θ=kAδ, where kA is the Alfvén wave number. The dependence of the magnetic profiles on wave frequency and disk size are presented. The effects of dissipation by electron-neutral collisions and Landau damping are observed. The observations are in excellent agreement with theoretical predictions [Morales et al., Phys. Plasmas 1, 3765 (1994)]. © 1994 American Institute of Physics.