Most slide images are from Dan's, Sebastian's, and Janice's talks on 31 Aug 2021. Some are from Andy Benwell's talk 22 May 2020.
Also highly recommended is the Cryomodule Internal Photo Tourwhich details the components of a cryomodule and their functions.
Motivation
If you want a very high repetition rate machine, you need superconducting RF (SRF) cavities to manage the heat from the RF. Because of the superconducting nature of low temperature liquid helium, it conducts the heat to the surface of the liquid, away from the cavities. From Dan Gonnella's talk 31 Aug 2021, if the cavities were made of copper and you fed in 20 MV of CW RF, the dissipated power in the cavity walls would be 15 MW. With niobium at 2K, the dissipated power is 15W. The cryogenic costs increase the wall power to 15 kW.
Features
Cavity Description
The SRF cavities used for LCLS-SC contain 9 cells, are 1.038m long, and are made of niobium. The cavities are welded into titanium "helium vessels" which create a space around them to hold liquid helium. (See Introduction to LCLS-SC Cryo Systems article for more details.)
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). 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 is fed 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:
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. Each cavity also has two higher-order-mode couplers (HOM couplers) which are small cylinders (a few cm in diameter and ~10 cm long) attached to each end of the cavity. These cans are tuned to absorb frequencies above 1.3 GHz. They also contain antenna for signal monitoring, but these aren't connected in the housing.
LLRF Mode for Operation
The cavities are typically run in CW mode called self-excited loop (SEL) with LLRF feedbacks on amplitude and phase to keep the RF stable. This mode is called SELAP = SEL + Amplitude feedback + Phase feedback. The phase feedback syncs to an external phase reference such that all cavities are aligned allowing for beam acceleration.
From Andy's 22May2020 talk:
Amplitude vs Gradient
RF Amplitude is given in units of volts. A electron after passing through a 1MV potential field will have 1 MeV of energy (E=qV). In our system, the LLRF is given a desired RF amplitude in MV (ADES) which is the total energy imparted to the beam if the cavity is phased correctly. Gradient (in MV/m) is amplitude divided by the cavity length. The LLRF also takes as input the desired phase (PDES). After initially phasing the cavities with the beam and calibrating the amplitude, a PDES of 0 should be the phase that imparts the most energy to the beam (fully forward phased). If PDES=0, the energy imparted to the beam by the cavity should be ADES.
From Andy's 22May2020 talk. Amplitude and phase control is on the left side of the screen in the middle.
Quench
If there is a problem that causes part of the cavity to increase in temperature such that it is no longer superconducting, the RF will no longer resonate and all the power will reflect back from the cavity. This is called a quench. This is typically caused by impurities in the cavity or on the surface and each cavity is measured to ascertain where it quenches. When the CM were tested at the partner lab before shipping, the cavities were processed up to 21 MV/m or other limiting factor. For LCLS-SC the plan is to run the cavities at a maximum at 16 MV/m (16.6 MV). If there is a known problem, the cavity max PV (ADES_MAX) will be set to the operation limit.
From Andy's 22May2020 talk:
