Experimental observation of Alfvén wave cones

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Abstract
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)].