A higher harmonic cavity (HHC) is popularly employed in synchrotron light storage rings to enhance the machine performance. which requires its fundamental mode resonant frequency to be tuned above the radio-frequency harmonic. However. this detuning is likely to cause Robinson instability. https://www.roneverhart.com/100-Percent-Sunglasses-SPEEDCRAFT-LE-Money-Mike-HiPER-Silver-Mirror/