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The SRF cavities used for LCLS-SC are 1.038m long and made of nitrogen-doped niobium. Each cavity contains nine cells. Strings of eight cavities are bolted together with a button design BPM and a multifunction magnet inside a cryomodule.
RF Source
The 1.3 GHz CW RF is produced by the low level RF (LLRF) system then amplified by a solid state amplifier (SSA). Each cavity has its own SSA. The RF generated by the SSA is directed into the tunnel through rectangular waveguides and fed into the downstream end of the cavity through a fundamental power coupler (FPC) .
HOMs
Each cavity also has two higher-order-mode couplers (HOM couplers), small cylinderical cans (a few cm in diameter and ~10cm long) attached to each end of the cavity that absorb frequencies above 1.3 GHz. They also contain antenna for signal monitoring, but these aren't connected in the housing.
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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.)
Frequency Tuning
When the RF drive frequency matches the resonant frequency of an accelerating cavity, standing waves can form inside the cavity storing the electromagnetic energy which can be used needed 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.
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