Q-factor optimization for high-beta 650 MHz cavities for PIP-II

Abstract

High Q-factors are of utmost importance to minimize losses of superconducting radio-frequency cavities deployed in continuous wave particle accelerators. This study elucidates the surface treatment that can maximize the Q-factors in high-beta 650 MHz elliptical niobium cavities. State-of-the-art surface treatments are applied in many single-cell cavities, and surface resistance studies are performed to understand the microwave dissipation at this unexplored frequency. The nitrogen doping treatment is confirmed to be necessary to maximize the Q-factors at medium RF fields. We applied this treatment in five-cell high-beta 650 MHz cavities and demonstrated that extremely high Q-factors were obtained at medium RF fields with this treatment. We also demonstrated that adding a cold electropolishing step after N-doping is crucial to push the quench field of multicell cavities to higher gradients.

Quality Factor Enhancement of 650 MHz Superconducting Radio-Frequency Cavity for CEPC

“…Therefore, the anti-Q-slope behavior cannot be observed at 650 MHz cavity mid-T furnace baked. Similarly, nitrogen doping could not reverse R BCS of 650 MHz cavity, either [7,15].…”

“…The radiation began to increase at 33 MV/m, so field emission was low during the vertical test. R S of the 650 MHz cavity (blue icon) increased exponentially along with the accelerating gradient in Figure 2, which is typical for cavity EP processed [7,9]. Hence, the cavity was considered qualified to receive mid-T furnace baking.…”

“…For low-frequency (<1 GHz) elliptical cavities used in colliders and proton/neutron sources [7][8][9], Q 0 usually decreases more quickly along with accelerating gradient than that of 1.3 GHz cavities. Nitrogen doping could increase Q 0 of 650 MHz single-cell β = 0.9 cavity for the Proton Improvement Plan II (PIP-II) project to 7 × 10 10 at medium fields of approximately 15-25 MV/m [7]. However, there are very few mid-T baking attempts of low-frequency cavities, which may demonstrate differences with 1.3 GHz as well as nitrogen doping [7,10].…”

“…Nitrogen doping could increase Q 0 of 650 MHz single-cell β = 0.9 cavity for the Proton Improvement Plan II (PIP-II) project to 7 × 10 10 at medium fields of approximately 15-25 MV/m [7]. However, there are very few mid-T baking attempts of low-frequency cavities, which may demonstrate differences with 1.3 GHz as well as nitrogen doping [7,10]. Thus, the mid-T furnace baking of 650 MHz cavity for CEPC was carried out at IHEP, which obtained high Q 0 and low R S successfully.…”

RF Technologies for Future Colliders

“…There was a rapid progress in developing advanced surface treatments over the last decade [23]. Most notably, a nitrogen doping technology allowed reaching unprecedented high-quality factors greater than 3 • 10 10 at 2 K and medium gradients for freeelectron laser linacs LCLS-II and LCLS-II-HE [26] (1.3 GHz cavities) and proton SRF linac PIP-II (650 MHz cavities) [27].…”