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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.
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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 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:
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