Thermal Feedback in Coaxial Superconducting RF Cavities




McMullin, Mattias

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Superconducting RF (SRF) cavities are used in particle accelerators to efficiently transfer energy from an RF power source to a beam of charged particles. The power losses associated with this process are inversely proportional to the cavity quality factor Q0, which decreases with increasing RF field strength in a phenomenon known as Q-slope. Q-slope limits the achievable RF field strength in SRF cavities because the increased power dissipation drives up the cost and complexity of an accelerator’s cryogenics infrastructure. Two effects contribute to Q-slope in SRF cavities: thermal feedback (TFB), an extrinsic effect in which Q0 decreases due to heating of the cavity walls, and field-dependent surface resistance, which is intrinsic to the RF surface. Q-slope is believed to be an unimportant effect in elliptical SRF cavities, but its effect in coaxial cavities is unknown. Additionally, the question of how much Q-slope should be attributed to TFB as opposed to field-dependent surface resistance has not been answered quantitatively for any cavity geometry because of a lack of data on boiling from niobium surfaces in liquid helium at the relevant scales of power dissipation. Without knowing these thermal parameters, computational models of heating in SRF cavities are incomplete. In the present study, direct measurements of liquid helium boiling from niobium surfaces were performed. Using these measured thermal parameters, a novel finite element code was developed to calculate the impact of TFB on Q-slope in coaxial cavities. This code removes TFB effects from measurements of Q-slope to reveal the underlying field-dependent surface resistance. Results are presented showing the impact of TFB on data from TRIUMF’s coaxial test cavity program at a wide range of RF frequencies. TFB was found to be a weak effect on Q-slope in coaxial cavities in the operating regimes relevant to acceleration.