Advancements in High-Q Development for Novel Medium-velocity 644 MHz 5-cell Elliptical Superconducting Rf Cavities for Continuous-wave Operation in Heavy-ion Linacs
- Michigan State U.
The Facility for Rare Isotope Beams is a world-leading center for experimental nuclear physics research, and relies on a first-in-kind superconducting RF driver linac to supply 400 kW beams of a uniquely large range of particles, from protons through the heaviest uranium ions. The FRIB400 project proposes to double the end energy of the current superconducting FRIB linac for the heaviest uranium ions from 200 MeV per nucleon (MeV/u) to 400 MeV/u, which equates to approximately 1 GeV for protons. The increased rare isotope production from higher-energy drive beams would deliver new and exciting capabilities to FRIB users, which include extending the facility's reach along the neutron drip-line, greatly increasing the facility's rare isotope yield, and significantly improving the precision of parameter measurements for the nuclear matter equation of state, which is a particularly timely complement to the recent advent of multi-messenger astronomy, and the study of neutron star mergers. The unique set of design parameters defined by the 400 MeV/u (uranium) energy goal, the physical size, continuous-wave operation, and cryogenic capacity of the extant facilities at FRIB, requires development of a first-in-kind $$\beta = v/c =$$ 0.65, 644 MHz 5-cell elliptical superconducting RF (SRF) cavity, with field-leading efficiency, measured by the cavity's intrinsic quality factor, $$Q_0$$. The field of SRF has recently experienced a very active period of development, which demonstrated the potential to raise the $$Q_0$$ of 1.3 GHz cavities by factors of two to three or more. In addition to uncovering fascinating new properties of the fundamental physics of superconducting RF phenomena, these improvements suggest substantial improvements could be made over the minimum $$Q_0$$ of the proposed FRIB400 upgrade SRF cavities, if these new techniques can be adapted to the $$\beta =$$ 0.65, 644 MHz regime. This work presents the first validation of the the novel $$\beta =$$ 0.65, 644 MHz 5-cel l elliptical superconducting RF cavity design, and the first results of both conventional and advanced RF surface preparation techniques. In particular, this work focuses on two recently-developed high-$$Q_0$$ recipes: N-doping, and furnace baking, setting the current world-record $$Q_0$$ for this type of resonant cavity at $$3.8\times10^{10}$$ at the FRIB400 operating gradient of 17.5 MV/m. We also find evidence that these results may be further improved upon, and make the case for further RF surface preparation recipe optimization. We then pair these high-power RF investigations with techniques borrowed from material science to try to better understand the ways in which specific properties of the niobium material, such as grain structure and superconducting flux pinning force, can be related to its superconducting RF performance. These experiments better elucidate the effects of high-temperature annealing on niobium samples, which suggest methods of further optimizing cavity flux expuls ion and thus SRF performance.
- Research Organization:
- Fermi National Accelerator Laboratory (FNAL), Batavia, IL (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC), High Energy Physics (HEP)
- DOE Contract Number:
- AC02-07CH11359
- OSTI ID:
- 2318664
- Report Number(s):
- FERMILAB-THESIS-2023-18; oai:inspirehep.net:2761065
- Country of Publication:
- United States
- Language:
- English
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