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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.
Cryo
The 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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The 1.3 GHz CW RF is produced by the low level RF (LLRF) system then amplified by a solid state amplifier (SSA). The output is directly through rectangular waveguide from the SSA in the gallery to the cavity in the accelerator housing. 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) to the cavity . The coupler feeds the RF into the downstream end of the cavity.
From Andy's talk 22May2020: 
From Janice's 31Aug2021 talk:
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Due to spatial constraints, there is one multifunction conductively-cooled magnet in each cryomodule. The four quadrants of the magnet are each magnet has three concentric coils at each pole wired as a quadrupole, x corrector, and y corrector.
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From Andy's 22May2020 talk:
Diagnostics
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Signals to Monitor
Each cavity has several antennae and couplers for monitoring the RF. Inside each cavity is a pickup antenna used to monitor the RF in the cavity. On the waveguide to the FPC is a coupler that allows monitoring of the forward and reverse RF signals.
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