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When the RF drive frequency matches the resonant frequency of an accelerating cavity, standing waves can form inside the cavity storing the electromagnetic energy needed used to accelerate a particle beam. Due to thermal expansion, a cavity's volume will change with temperature, shifting the resonant frequency of the cavity. Cavities also experience Lorentz force detuning which is a distortion of the cavity walls due to the pressure from the electromagnetic fields of the RF.
Frequency tuners are used to change the shape of the cavity to get it back on resonance. There are two types of tuners: stepper motors that can make slow, coarse changes to the cavity frequency and piezo tuners that make fast, incremental adjustments to maintain the resonant frequency during normal running.In section 1 of this paperis a brief description of Lorentz force detuning.
Cryo
Inside each cryomodule, the cavities are welded into titanium "helium vessels" which contain the volume of liquid helium used to cool cavities to their 2K operating temperature. There are three stages of cooling down from ambient temperatures to 40K, 5k, and finally 2K. (See Introduction to LCLS-SC Cryo Systems article for more details.)
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