  The free-electron laser FLASH at DESY and the European XFEL are operated with
superconducting radio frequency cavities and supply beam to several user
experiments. The switching time between experiments is limited to dozens of
microseconds. This contribution will show a regulation with a frequency shifted
superconducting cavity to manipulate and change the accelerating properties of
electron bunches with 250 kHz.The main challenge of the concept presented in
this contribution can be summarized in this statement: finding a way to
modulate the energy of individual bunches in a single-source multiple-cavity
scheme for a potential CW upgrade of the EuXFEL.
