116 research outputs found
Muon spin rotation studies of niobium for superconducting RF applications
In this work we investigate superconducting properties of niobium samples via
application of the muon spin rotation/relaxation (muSR) technique. We employ
for the first time the muSR technique to study samples that are cutout from
large and small grain 1.5 GHz radio frequency (RF) single cell niobium
cavities. The RF test of these cavities was accompanied by full temperature
mapping to characterize the RF losses in each of the samples. Results of the
muSR measurements show that standard cavity surface treatments like mild baking
and buffered chemical polishing (BCP) performed on the studied samples affect
their surface pinning strength. We find an interesting correlation between high
field RF losses and field dependence of the sample magnetic volume fraction
measured via muSR. The muSR line width observed in ZF-muSR measurements matches
the behavior of Nb samples doped with minute amounts of Ta or N impurities. An
upper bound for the upper critical field Hc2 of these cutouts is found.Comment: 20 pages, 14 figure
Superheating in coated niobium
Using muon spin rotation it is shown that the field of first flux penetration Hentry in Nb is enhanced by about 30% if coated with an overlayer of Nb3Sn or MgB2. This is consistent with an increase from the lower critical magnetic field Hc1 up to the superheating field Hsh of the Nb substrate. In the experiments presented here coatings of Nb3Sn and MgB2 with a thickness between 50 and 2000 nm have been tested. Hentry does not depend on material or thickness. This suggests that the energy barrier at the boundary between the two materials prevents flux entry up to Hsh of the substrate. A mechanism consistent with these findings is that the proximity effect recovers the stability of the energy barrier for flux penetration, which is suppressed by defects for uncoated samples. Additionally, a low temperature baked Nb sample has been tested. Here a 6% increase of Hentry was found, also pushing Hentry beyond Hc1
Coaxial multi-mode cavities for fundamental SRF research in an unprecedented parameter space
Recent developments in superconducting radio-frequency (SRF) research have
focused primarily on high frequency elliptical cavities for electron
accelerators. Advances have been made in both reducing RF surface resistance
and pushing the readily achievable accelerating gradient by using novel SRF
cavity treatments including surface processing, custom heat treatments, and
flux expulsion. Despite the global demand for SRF based hadron accelerators,
the advancement of TEM mode cavities has lagged behind. To address this, two
purpose-built research cavities, one quarter-wave and one half-wave resonator,
have been designed and built to allow characterization of TEM-mode cavities
with standard and novel surface treatments. The cavities are intended as the
TEM mode equivalent to the 1.3GHz single cell cavity, which is the essential
tool for high frequency cavity research. Given their coaxial structure, the
cavities allow testing at the fundamental mode and higher harmonics, giving
unique insight into the role of RF frequency on fundamental loss mechanisms
from intrinsic and extrinsic sources. In this paper, the cavities and testing
infrastructure are described and the first performance measurements of both
cavities are presented
Depth-resolved measurement of the Meissner screening profile in a niobium thin film from spin-lattice relaxation of the implanted -emitter Li
We report measurements of the Meissner screening profile in a Nb(300
nm)/AlO thin film using Li -detected nuclear magnetic
resonance (-NMR). The NMR probe Li was ion-implanted into the Nb
film at energies 20 keV, corresponding to mean stopping depths
comparable to Nb's magnetic penetration depth . Li's strong
dipole-dipole coupling with the host Nb nuclei provided a
"cross-relaxation" channel that dominated in low magnetic fields, which
conferred indirect sensitivity to the local magnetic field via the spin-lattice
relaxation (SLR) rate . From a fit of the data to a model
accounting for its dependence on temperature, magnetic field, and
Li implantation energy, we obtained a magnetic penetration depth
= 51.5(22) nm, consistent with a relatively short carrier
mean-free-path = 18.7(29) nm typical of similarly prepared Nb films. The
results presented here constitute an important step towards using Li
-NMR to characterize bulk Nb samples with engineered surfaces, which are
often used in the fabrication of particle accelerators.Comment: 16 pages, 4 figure
Key directions for research and development of superconducting radio frequency cavities
Radio frequency superconductivity is a cornerstone technology for many future
HEP particle accelerators and experiments from colliders to proton drivers for
neutrino facilities to searches for dark matter. While the performance of
superconducting RF (SRF) cavities has improved significantly over the last
decades, and the SRF technology has enabled new applications, the proposed HEP
facilities and experiments pose new challenges. To address these challenges,
the field continues to generate new ideas and there seems to be a vast room for
improvements. In this paper we discuss the key research directions that are
aligned with and address the future HEP needs.Comment: contribution to Snowmass 202
Shape coexistence and mixing of low-lying 0+ states in 96Sr
The low energy excited 02,3 + states in 96Sr are amongst the most prominent examples of shape coexistence across the nuclear landscape. In this work, the neutron [2s1/2]2 content of the 01,2,3 + states in 96Sr was determined by means of the d(95Sr, p) transfer reaction at the TRIUMF-ISAC2 facility using the SHARC and TIGRESS arrays. Spectroscopic factors of 0.19(3) and 0.22(3) were extracted for the 96Sr ground and 1229 keV 0+ states, respectively, by fitting the experimental angular distributions to DWBA reaction model calculations. A detailed analysis of the γ-decay of the isomeric 03 + state was used to determine a spectroscopic factor of 0.33(13). The experimental results are compared to shell model calculations, which predict negligible spectroscopic strength for the excited 0+ states in 96Sr. The strengths of the excited 02,3 + states were also analyzed within a two-level mixing model and are consistent with a mixing strength of a2=0.40(14) and a difference in intrinsic deformations of |Δβ|=0.31(3). These results suggest coexistence of three different configurations in 96Sr and strong shape mixing of the two excited 0+ states
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