A multimode resonator resonating at two or more frequencies is operated at
cryogenic temperatures and composed of a superconducting material or a
normal metal with a superconducting section serving as a RF
superconducting switch. The multimode resonator is coupled to a NMR
spectrometer and a RF switch power source, wherein its one frequency is
selected to correspond to the operating frequency of the NMR spectrometer
and at least a second frequency is tuned to a frequency of RF switch
power source, unrelated to the spectrometer frequency, therefore power at
this frequency does not perturb the operation of the spectrometer. When
activated, the RF switch power source induces a current sufficient to
approach or exceeds the critical current in one or more sections of the
superconducting material of the multimode resonator, thereby increasing
its resistance and reducing the Q factor of the multimode resonator.