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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 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.
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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.)
Magnets
Due to spatial constraints, there is one multifunction conductively-cooled magnet in each cryomodule. The magnet has three concentric coils at each pole wired as quadrupole, x corrector, and y corrector.
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From Andy's 22May2020 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.
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:
