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To make operation at such a high repetition rate economically viable, it is critical to minimize the amount of input RF power that is wasted as heat loss. Cavity performance is characterized by the quality factor, Q0, a ratio of energy stored insiide inside the cavity to power lost through cavity walls. Because the nitrogen-doped niobium cavities of the SC-Linac are designed to dissipate far less heat through the walls than the copper cavities of the original LCLS linac, they have a higher Q0 so can transfer more of the power generated by the RF to the electron beam. Due to the superfluid 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, three orders of magnitude lower than power lost through copper walls!
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