11 Search Results

In dark-matter searches using axion haloscopes, the search sensitivity depends on the quality factors (Q₀) of radio-frequency cavities immersed in multitesla magnetic fields. Increasing Q₀ would increase the scan rate through the parameter space of interest. Researchers developing superconducting radio-frequency cavities for particle accelerators have developed methods for obtaining extremely high Q₀ of approximately 10¹¹ in microteslascale magnetic fields. In this paper, we describe efforts to develop high-Q cavities made from Nb₃Sn films using a technique developed for particle-accelerator cavities. Geometry optimization for this application is explored, and two cavities are tested: an existing particle-accelerator-style cavity and a geometry developedmore » and fabricated for use in high fields. A quality factor of (5.3 ± 0.3) × 10⁵ is obtained at 3.9 GHz and 6 T at 4.2 K.« less

High-quality cavities are crucial for various fundamental physical studies and applications. In this report we find that by coupling two cavities directly or via a phase-tunable coupling channel, the photon lifetime of the local field can exceed that of the bare cavities. The cavity photon lifetime is modified by the phases of the initial states and the phase accumulation on the coupling channel, which affect the interference between cavities. In experiments, by coupling superconducting radio-frequency cavities via phase-tunable cables, we realize a factor of 2 improvement in the cavity photon lifetime. The results can bring rich revenue to quantum informationmore » science, sensing, and high-energy physics.« less

Electropolishing (EP) of 1.3 GHz niobium (Nb) superconducting RF cavities is conducted to achieve a desired smooth and contaminant-free surface that yields good RF performance. Achieving a smooth surface of a large-sized elliptical cavity with the standard EP conditions was found to be challenging. This work aimed to conduct a systematic parametric EP study to understand the effects of various EP parameters on the surface of 650 MHz cavities used in PIP-II linac. Parameters optimized in this study provided a smooth surface of the cavities. The electropolished cavities met the baseline requirement of field gradient and qualified for further surfacemore » treatment to improve the cavity quality factor.« less

Field emission is one of the main factors that can limit the performance of superconducting radio frequency cavities. To reduce possible field emission in the Linac Coherent Light Source II (LCLS-II), we are developing plasma processing for 1.3 GHz nine-cell cavities. The ultimate goal of plasma processing will be to apply the technique in situ in the cryomodules in order to mitigate hydrocarbon-related field emission without disassembling them. Herein is presented the first systematic study of plasma processing applied to LCLS-II superconducting radio frequency cavities. Having developed a new method of plasma ignition for LCLS-II cavities, we applied plasma processing to 1.3 GHzmore » cavities starting with a clean nitrogen doped cavity and proceeding with studying natural field emission and artificially contaminated cavities. All the cavities were cold tested before and after plasma cleaning in order to compare their performances. It was proved that this technique successfully removes carbon-based contamination from the cavity iris and that it is able to eliminate field emission in a naturally field emitting cavity. The effect of plasma processing on cavities exposed to vacuum failures was also investigated, showing positive results in some cases. This work shows how successful plasma processing is in removing hydrocarbon related contamination from the cavity surface without affecting the high Q-factors and quench fields characteristic of nitrogen doped cavities.« less

We repormore » t on an effort to improve the performance of superconducting radiofrequency cavities by using heat treatment in a temperature range sufficient to dissociate the natural surface oxide. We find that the residual resistance is decreased significantly, and we find an unexpected reduction in the Bardeen-Cooper-Schrieffer (BCS) resistance. Together these result in extremely high-quality factor values at relatively large accelerating fields $E_{\mathrm{acc}}$~20 MV/m: $Q_0$=3–4× ${10}^{11}$ at <1.5 K and $Q_0$~5× ${10}^{10}$ 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 nΩ at 16 MV/m. Secondary ion mass spectrometry measurements confirm that the oxide is dissociated significantly, 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 study is performed on 1.3 GHz cavities, but the effect may be extendable to other frequencies as well. The possibilities for applications and the planned future development are discussed.« less

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 ismore » 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.« less

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 finemore » 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.« less

In this paper, we present the frequency dependence of the vortex surface resistance of bulk niobium accelerating cavities as a function of different state-of-the-art surface treatments. Higher flux surface resistance per amount of trapped magnetic field - sensitivity - is observed for higher frequencies, in agreement with our theoretical model. Higher sensitivity is observed for N-doped cavities, which possess an intermediate value of electron mean-free-path, compared to 120° C and EP/BCP cavities. Experimental results from our study showed that the sensitivity has a non-monotonic trend as a function of the mean-free-path, including at frequencies other than 1.3 GHz, and thatmore » the vortex response to the rf field can be tuned from the pinning regime to flux-flow regime by manipulating the frequency and/or the mean-free-path of the resonator, as reported in our previous studies. The frequency dependence of the trapped flux sensitivity to the amplitude of the accelerating gradient is also highlighted.« less

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 niobiummore » 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. Lastly, 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.« less

Previous studies on magnetic flux expulsion as a function of cooldown procedures for elliptical superconducting radio frequency (SRF) niobium cavities showed that when the cavity beam axis is placed parallel to the helium cooling flow and sufficiently large thermal gradients are achieved, all magnetic flux could be expelled and very low residual resistance could be achieved. In this paper, we investigate flux trapping for the case of resonators positioned perpendicularly to the helium cooling flow, which is more representative of how SRF cavities are cooled in accelerators and for different directions of the applied magnetic field surrounding the resonator. Wemore » show that different field components have a different impact on the surface resistance, and several parameters have to be considered to fully understand the flux dynamics. A newly discovered phenomenon of concentration of flux lines at the cavity top leading to temperature rise at the cavity equator is presented.« less