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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). 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.
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.)
HOMs
Each cavity also has two higher-order-mode couplers (HOM couplers) which are small cylinders (a few cm in diameter and ~10cm 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.
Frequency Tuning
When the drive frequency matches the resonant frequency of an accelerating cavity, standing waves form inside the cavity storing electromagnetic energy which can be 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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RF Amplitude is given in units of volts. A electron after passing through a 1MV potential field An electron will have 1 MeV of energy (E=qV) after passing through a 1MV potential field. 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.
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