  We report Alfv\'en-wave experiments with liquid rubidium at the Dresden High
Magnetic Field Laboratory (HLD). Reaching up to 63 T, the pulsed magnetic field
exceeds the critical value of 54 T at which the Alfv\'en speed becomes equal to
the sound speed (plasma-$\beta$ unity). At this threshold we observe a period
doubling of an applied 8 kHz CW excitation, a clear footprint for a parametric
resonance between magnetosonic waves and Alfv\'en waves.
