Laboratory experiments on phase mixing of Alfvén waves under conditions similar to solar coronal hole

We have studied Alfvén wave propagation through a density gradient transverse to the ambient magnetic field under conditions similar to coronal holes. The experiments were performed in the Large Plasma Device located at the University of California, Los Angeles. The Alfvén waves are observed to phase-mix as they propagate along the cylindrical plasma column in the presence of a transverse density gradient. We present two-dimensional measurements of the wave magnetic field across the plasma column at multiple axial positions, mapping the evolution of the phase-mixed wave as it travels away from the antenna. Spatial Fourier analysis of the wave magnetic field reveals the generation of higher perpendicular wavenumber, k_⊥, compared to the initial value as the wave propagates through a transverse density gradient. No such increase in k_⊥ is observed in a uniform plasma. The higher k_⊥ generation is accompanied by a significant reduction in wave energy. A comparison between experiment and theory shows that wave damping in the laboratory is significantly faster than predicted. Further analysis of the laboratory results suggests that phase mixing may play a more prominent role in affecting Alfvén wave processes in coronal holes than previously thought.