Next-Generation Superconducting RF Technology based on Advanced Thin Film Technologies and Innovative Materials for Accelerator Enhanced Performance and Energy Reach

Abstract

Superconducting radio frequency (SRF) systems, essentially based on bulk niobium (Nb), are the workhorse of most particle accelerators and achieve high levels of performance and reliability. Today's exceptional performance of bulk Nb SRF cavities is the fruit of more than five decades of intensive research, essentially aimed at optimizing the material for thermal stabilization of defects, and significant funding efforts. Last incremental advances with several novel surface treatments are allowing Nb cavities to exceed previous record Q factors and avoiding degradation with increasing gradients. These include nitrogen infusion and doping, oxygen alloying with a two-step baking process. The next generation of particle accelerators will require operational parameters beyond the state-of-the-art Nb capabilities. Superconducting RF has then to rely on superior materials and advanced surfaces beyond bulk Nb for a leap in performance and efficiency. The SRF thin film development strategy aims at transforming the current SRF technology into a technology using highly functional materials, where all the necessary functions are addressed. The community is deploying efforts in three research thrusts to develop the next-generation thin-film based cavities. The first line of developments aims at investigating Nb/Cu coated cavities that perform as good as or better than bulk niobium at reduced cost and with better thermal stability. Recent results with greatly improved accelerating field and dramatically reduced Q-slope show the potential of this technology for many applications. In particular high current storage ring colliders such as FCC, EIC and CEPC, where the frequency is typically lower and the gradients are modest, could benefit greatly from the cost savings and operational advantages of this technology. The second research thrust is to develop cavities coated with materials that can operate at higher temperatures or sustain higher fields. Proof-of-principle has been established for the merit of Nb 3 Sn for SRF applications. Research is now needed to further exploit the material and reach its full potential with novel deposition techniques. The third line of research is to push SRF performance beyond the capabilities of the superconductors alone with multi-layer coatings. In parallel, developments are needed to provide quality substrates, innovative cooling schemes and cryomodule design tailored to SRF thin film cavities.Recent results in these three research thrusts suggest that SRF thin film technologies are at the eve of a technological revolution. However, in order for them to mature, active community support and sustained funding are needed and should address fundamental developments supporting material deposition techniques, surface and RF research, and technical challenges associated with scaling, automation and industrialization. With dedicated and sustained investment in SRF thin film R&D, next-generation thin-film based cavities will become a reality with high performance and efficiency, facilitating energy su...

RF Accelerator Technology R&D: Report of AF7-rf Topical Group to Snowmass 2021

“…Three White Papers discuss new SRF materials beyond niobium [27,28,29]. Research on advancing performance of superconducting cavities beyond the capabilities of bulk niobium follows the three thrusts focused on developing the next-generation thin-film based cavities.…”

RF Technologies for Future Colliders

“…There are two main reasons: 1) Nb has the highest critical temperature and critical magnetic field of all elemental superconductors, and 2) it is a ductile metal that can easily be formed into complex geometries of SRF cavities. With niobium cavity gradients approaching the fundamental limit (determined by the Nb superheating magnetic field H s ≈ 2, 200 Oe), R&D on alternative superconductors becomes more and more important [36]. There are several materials with higher critical temperature and critical field that could potentially surpass the niobium technology for SRF applications.…”

“…They must be deposited on a suitable substrate-often Nb or Cu-as either thick or thin films. As application of most alternative superconductors to the SRF is still far in the future, it is suitable for the readers to be referred to review articles [36,37]. Here we only briefly address Nb 3 Sn as an alternative superconductor closest to practical use [38].…”

Quasiparticle spectroscopy, transport, and magnetic properties of Nb films used in superconducting transmon qubits

“…Another recently discovered source of potential decoherence in transmons is nano-sized niobium hydrides [7] (large hydrides in superconducting RF cavities are believed to be the cause of the so-called Q-disease (a dramatic reduction of the quality factor above some amplitude of electromagnetic field inside) and have been known for a long time [18][19][20][21][22][23]). While bulk niobium used in SRF cavities was extensively characterized both in normal and the superconducting states, the niobium films used in QIS technologies have mostly been studied in the normal state and, in the superconducting state, specifically to examine the quality factor behavior and losses at GHz frequencies [24]. Bulk and film states of niobium are very different in terms of morphology, purity, and relative length scales involved, and more conventional studies in the superconducting state are needed.…”