Three-dimensional superconducting resonators at T<20T < 20 mK with the photon lifetime up to τ=2\tau=2 seconds

Very high quality factor superconducting radio frequency cavities developed for accelerators can enable fundamental physics searches with orders of magnitude higher sensitivity, as well as offer a path to a 1000-fold increase in the achievable coherence times for cavity-stored quantum states in the 3D circuit QED architecture. Here we report the first measurements of multiple accelerator cavities of $f_0=$1.3, 2.6, 5 GHz resonant frequencies down to temperatures of about 10~mK and field levels down to a few photons, which reveal record high photon lifetimes up to 2 seconds, while also further exposing the role of the two level systems (TLS) in the niobium oxide. We also demonstrate how the TLS contribution can be greatly suppressed by the vacuum heat treatments at 340-450$^\circ$C.Comment: revised versio

The Effect of Mechanical Cold Work on the Magnetic Flux Expulsion of Niobium

Expulsion of ambient flux has been shown to be crucial to obtain high quality factors in bulk niobium SRF cavities. However, there remain many questions as to what properties of the niobium material determine its flux expulsion behavior. In this paper, we present first results from a new study of two cavities that were specially fabricated to study flux expulsion. Both cavities were made from large grain ingot niobium slices, one of which had its slices rolled prior to fabrication, and none these slices were annealed prior to measurement. Expulsion measurements indicate that a dense network of grain boundaries is not necessary for a cavity to have near-complete flux trapping behavior up to large thermal gradients. The results also contribute to a body of evidence that cold work is a strong determinant of flux expulsion behavior in SRF-grade niobium.Comment: 7 pages, 10 figure

Advancement in the understanding of the field and frequency dependent microwave surface resistance of niobium

The radio-frequency surface resistance of niobium resonators is incredibly reduced when nitrogen impurities are dissolved as interstitial in the material, conferring ultra-high Q-factors at medium values of accelerating field. This effect has been observed in both high and low temperature nitrogen treatments. As a matter of fact, the peculiar anti Q-slope observed in nitrogen doped cavities, i.e. the decreasing of the Q-factor with the increasing of the radio-frequency field, come from the decreasing of the BCS surface resistance component as a function of the field. Such peculiar behavior has been considered consequence of the interstitial nitrogen present in the niobium lattice after the doping treatment. The study here presented show the field dependence of the BCS surface resistance of cavities with different resonant frequencies, such as: 650 MHz, 1.3 GHz, 2.6 GHz and 3.9 GHz, and processed with different state-of-the-art surface treatments. These findings show for the first time that the anti Q-slope might be seen at high frequency even for clean Niobium cavities, revealing useful suggestion on the physics underneath the anti Q-slope effect.Comment: To be published in Proceedings of SRF 201

Impact of high Q on ILC250 upgrade for record luminosities and path toward ILC380

In this paper, we address the possibility of upgrading the ILC250 luminosity to $8.1 \times 10^{34}$, so that with the polarization feature, the effective luminosity will be $2.0 \times 10^{35}$ to compete with the FCC-ee luminosity and two detectors. The additional cost of the higher luminosity option will be about 2.2 B ILCU. The total cost for the ILC high luminosity machine will therefore be about 7.7 B versus FCC-ee 10.5 B. The AC power (267 MW) to operate the ILC luminosity upgrade will also be less than the AC power for FCC-ee (300 MW). Even with a modest quality factor Q of $1 \times 10^{10}$ for SRF cavities, the total cost of the upgrade will be 2.5 B ILCU additional over ILC250 baseline. We expect that, if approved, ILC250 will first be built at the baseline luminosity, operated for many years at this luminosity, and later upgraded to the high luminosity option. A significant part (RF power and cryo-power) of the additional cost for the luminosity upgrade overlaps with the expected additional costs for anticipated energy upgrade paths. A second ILC upgrade discussed in this paper will be to the higher energy Top Factory at 380 GeV. We also estimate the additional cost of this upgrade (1.5 B ILCU)

Efficient expulsion of magnetic flux in superconducting RF cavities for high Q0Q_0 applications

Even when cooled through its transition temperature in the presence of an external magnetic field, a superconductor can expel nearly all external magnetic flux. This Letter presents an experimental study to identify the parameters that most strongly influence flux trapping in high purity niobium during cooldown. This is critical to the operation of superconducting radiofrequency cavities, in which trapped flux degrades the quality factor and therefore cryogenic efficiency. Flux expulsion was measured on a large survey of 1.3 GHz cavities prepared in various ways. It is shown that both spatial thermal gradient and high temperature treatment are critical to expelling external magnetic fields, while surface treatment has minimal effect. For the first time, it is shown that a cavity can be converted from poor expulsion behavior to strong expulsion behavior after furnace treatment, resulting in a substantial improvement in quality factor. Future plans are described to build on this result in order to optimize treatment for future cavities.Comment: 5 page

Effect of interstitial impurities on the field dependent microwave surface resistance of niobium

Previous work has demonstrated that the radio frequency surface resistance of niobium resonators is dramatically reduced when nitrogen impurities are dissolved as interstitial in the material. The origin of this effect is attributed to the lowering of the Mattis and Bardeen surface resistance contribution with increasing accelerating field. Meanwhile, an enhancement of the sensitivity to trapped magnetic field is typically observed for such cavities. In this paper we conduct the first systematic study on these different components contributing to the total surface resistance as a function of different levels of dissolved nitrogen, in comparison with standard surface treatments for niobium resonators. Adding these results together we are able to show for the first time which is the optimum surface treatment that maximizes the Q-factor of superconducting niobium resonators as a function of expected trapped magnetic field in the cavity walls. These results also provide new insights on the physics behind the change in the field dependence of the Mattis and Bardeen surface resistance, and of the trapped magnetic vortex induced losses in superconducting niobium resonators

Performance-defining properties of Nb3_3Sn coating in SRF cavities

Nb$_3$Sn has great potential to become the material of choice for fabrication of SRF cavities. The higher critical temperature of Nb$_3$Sn potentially allows for an increased operational temperature in SRF cavities, which could enable tremendous simplification of cryogenic system, leading to significant cost reduction. We present extended characterization of a Nb$_3$Sn coated Nb cavity prepared at Cornell University. Using combination of thermometry during cavity RF measurements, and structural and analytical characterization of the cavity cutouts, we discover Nb$_3$Sn coating flaws responsible for the poor cavity performance. Our results clearly show degraded material quality in the cavity cutouts which exhibit significant heating during the RF testing. Results of extended comparison of cavity cutouts with different dissipation profiles not only point out the cause of significant Q-slope but also establish figures of merit for material qualities in relation to the quality of SRF performance

The Role of Magnetic Flux Expulsion to Reach Q0>3x10^10 in SRF Cryomodules

When a superconducting radiofrequency cavity is cooled through its critical temperature, ambient magnetic flux can become "frozen in" to the superconductor, resulting in degradation of the quality factor. This is especially problematic in applications where quality factor is a cost driver, such as in the CW linac for LCLS-II. Previously, it had been unknown how to prevent flux from being trapped during cooldown in bulk niobium cavities, but recent R&D studies showed near-full flux expulsion can be achieved through high temperature heat treatment and cooling cavities through the superconducting transition with a spatial thermal gradient over the surface. In this paper, we describe the first accelerator implementation of these procedures, in cryomodules that are currently being produced for LCLS-II. We compare the performance of cavities under different conditions of heat treatment and thermal gradient during cooldown, showing a substantial improvement in performance when both are applied, enabling cryomodules to reach and, in many cases, exceed a Q0 of ~3x10^10.Comment: 11 page

Ultra-Low Surface Resistance via Vacuum Heat Treatment of Superconducting Radiofrequency Cavities

We report on an effort to improve the performance of superconducting radiofrequency cavities by the use of heat treatment in a temperature range sufficient to dissociate the natural surface oxide. We find that the residual resistance is significantly decreased, and we find an unexpected reduction in the BCS resistance. Together these result in extremely high quality factor values at relatively large accelerating fields Eacc ~20 MV/m: Q0 of 3-4x10^11 at <1.5 K and Q0 ~5x10^10 at 2.0 K. In one cavity, measurements of surface resistance versus temperature showed an extremely small residual resistance of just 0.63+/-0.06 nOhms at 16 MV/m. SIMS measurements confirm that the oxide was significantly dissociated, but they also show the presence of nitrogen after heat treatment. We also present studies of surface oxidation via exposure to air and to water, as well as the effects of very light surface removal via HF rinse. The possibilities for applications and the planned future development are discussed.Comment: 9 pages, 9 figure

Accelerating fields up to 49 MV/m in TESLA-shape superconducting RF niobium cavities via 75C vacuum bake

In this paper we present the discovery of a new surface treatment applied to superconducting radio frequency (SRF) niobium cavities, leading to unprecedented accelerating fields of 49 MV/m in TESLA-shaped cavities, in continuous wave (CW); the corresponding peak magnetic fields are the highest ever measured in CW, about 210 mT. For TESLA-shape cavities the maximum quench field ever achieved was ~45 MV/m - reached very rarely- with most typical values being below 40 MV/m. These values are reached for niobium surfaces treated with electropolishing followed by the so called mild bake, a 120C vacuum bake (for 48 hours for fine grain and 24 hours for large grain surfaces). We discover that the addition during the mild bake of a step at 75C for few hours, before the 120C, increases systematically the quench fields up to unprecedented values of 49 MV/m. The significance of the result lays not only in the relative improvement, but in the proof that niobium surfaces can sustain and exceed CW radio frequency magnetic fields much larger than Hc1, pointing to an extrinsic nature of the current field limitations, and therefore to the potential to reach accelerating fields well beyond the current state of the art