  A shear Alfv\'en wave parametric instability is observed for the first time
in the laboratory. When a single finite $\omega/\Omega_i$ kinetic Alfv\'en wave
(KAW) is launched in the Large Plasma Device above a threshold amplitude, three
daughter modes are produced. These daughter modes have frequencies and parallel
wave numbers that are consistent with copropagating KAW sidebands and a low
frequency nonresonant mode. The observed process is parametric in nature, with
the frequency of the daughter modes varying as a function of pump wave
amplitude. The daughter modes are spatially localized on a gradient of the pump
wave magnetic field amplitude in the plane perpendicular to the background
field, suggesting that perpendicular nonlinear forces (and therefore
$k_{\perp}$ of the pump wave) play an important role in the instability
process. Despite this, modulational instability theory with $k_{\perp}=0$ has
several features in common with the observed nonresonant mode and Alfv\'en wave
sidebands.
