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

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

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

Henoch-Schönlein purpura, patients admitted to Landspítali-University Hospital 1984-2000

Neðst á síðunni er hægt að nálgast greinina í heild sinni með því að smella á hlekkinn View/OpenObjective: Henoch-Schönlein purpura is a disease of small vessels which primarily affects children. The epidemiology of Henoch-Schönlein purpura in Iceland is not known. The main purpose of the study was to find out the incidence of the disease in Iceland and gather further epidemiological information. Patients and methods: All patients diagnosed with Henoch-Schönlein purpura during the years 1984-2000 and were admitted to the Children's Hospital Iceland, Landspítali Hringbraut as well as the paediatric department at the Landspítali Fossvogi were included in the study cohort. Information was gathered from medical records. The temporal relationship between the incidende of Henoch-Schönlein purpura and epidemics of three common infectious diseases was explored. Results: A total of 101 patients were found to have Henoch-Schönlein purpura, 51 females and 50 males. The incidence of Henoch-Schönlein purpura in Iceland is therefore approximately six cases annually. The mean age at diagnosis was 5.4 years. In addition to the rash, 72.3% had arthralgia and 45.5% abdominalia, 36.6% had hematuria or proteinuria during the hospitalization and 5.9% had severe renal involvement. The disease was more common in the winter months (Nov. - April) compared to the summer months (May - Oct.) (p=0.045). Conclusions: The epidemiology of Henoch-Schönlein purpura in Iceland is comparable to other countries. The disease was most common in children aged 2-5 years, which could reflect a common infection in the community as a part of the pathogenesis. Older children may have antibodies against these infections resulting in a decreased incidence of the disease. Our results support the theory that infection may play an important role in the pathogenesis of Henoch-Schönlein purpura. However, some abnormalities or variations in the immune response are probably also involved.Tilgangur: Henoch-Schönlein purpura er sjúkdómur sem einkennist af æðabólgu og kemur fyrst og fremst fram hjá börnum. Faraldsfræði Henoch-Schönlein purpura á Íslandi er ekki þekkt. Tilgangur rannsóknarinnar var því að kanna nýgengi sjúkdómsins á Íslandi auk þess að afla annarra faraldsfræðilegra upplýsinga. Sjúklingar og aðferðir: Allir sjúklingar 16 ára og yngri sem fengið höfðu greininguna Henoch-Schönlein purpura á tímabilinu 1984-2000 og voru lagðir inn á Barnaspítala Hringsins eða barnadeild Landspítala Fossvogi (áður SHR og Landakotsspítali) mynduðu rannsóknarhópinn. Safnað var upplýsingum úr sjúkraskrám um einkenni og rannsóknarniðurstöður. Sveiflur í tíðni sjúkdómsins voru bornar saman við sveiflur nokkurra þekktra smitsjúkdóma. Niðurstöður: Alls greindist 101 sjúklingur með Henoch-Schönlein purpura á tímabilinu, 51 stúlka og 50 drengir. Nýgengi Henoch-Schönlein purpura virðist svipað á Íslandi og víðast annars staðar, eða um sex tilfelli á ári. Meðalaldur var 5,4 ár. Auk útbrotanna reyndust 72,3% hafa liðverki og 45,5% kviðverki, 36,6% höfðu blóðmigu eða prótínmigu á meðan á sjúkrahúsvist stóð og 5,9% fengu alvarlegan nýrnasjúkdóm. Sjúkdómurinn var algengari á vetrarmánuðum (nóvember-apríl) en á sumarmánuðum (maí-október) (p= 0,045). Ályktanir: Faraldsfræði Henoch-Schönlein purpura hjá börnum á Íslandi virðist vera svipuð og í öðrum löndum. Áhugavert er að sjúkdómurinn er algengastur meðal barna tveggja til fimm ára og gæti það endurspeglað að algeng sýking eða sýkingar séu einn af orsakavöldum hans. Þegar börnin eldast og öðlast mótefni gegn slíkum sýkingum dregur úr tíðni sjúkdómsins. Mögulegt er að sýkingar séu einn þáttur í ferli sem ræsir sjúkdóminn en að auki þurfi ákveðin samsetning ónæmiskerfis að vera til staðar

Parity Violation in Proton-Proton Scattering

Measurements of parity-violating longitudinal analyzing powers (normalized asymmetries) in polarized proton-proton scattering provide a unique window on the interplay between the weak and strong interactions between and within hadrons. Several new proton-proton parity violation experiments are presently either being performed or are being prepared for execution in the near future: at TRIUMF at 221 MeV and 450 MeV and at COSY (Kernforschungsanlage Juelich) at 230 MeV and near 1.3 GeV. These experiments are intended to provide stringent constraints on the set of six effective weak meson-nucleon coupling constants, which characterize the weak interaction between hadrons in the energy domain where meson exchange models provide an appropriate description. The 221 MeV is unique in that it selects a single transition amplitude (3P2-1D2) and consequently constrains the weak meson-nucleon coupling constant h_rho{pp}. The TRIUMF 221 MeV proton-proton parity violation experiment is described in some detail. A preliminary result for the longitudinal analyzing power is Az = (1.1 +/-0.4 +/-0.4) x 10^-7. Further proton-proton parity violation experiments are commented on. The anomaly at 6 GeV/c requires that a new multi-GeV proton-proton parity violation experiment be performed.Comment: 13 Pages LaTeX, 5 PostScript figures, uses espcrc1.sty. Invited talk at QULEN97, International Conference on Quark Lepton Nuclear Physics -- Nonperturbative QCD Hadron Physics & Electroweak Nuclear Processes --, Osaka, Japan May 20--23, 199

Depth-resolved measurement of the Meissner screening profile in a niobium thin film from spin-lattice relaxation of the implanted β\beta-emitter 8^{8}Li

We report measurements of the Meissner screening profile in a Nb(300 nm)/Al$_{2}$O$_{3}$ thin film using $^{8}$Li $\beta$-detected nuclear magnetic resonance ($\beta$-NMR). The NMR probe $^{8}$Li was ion-implanted into the Nb film at energies $\leq$ 20 keV, corresponding to mean stopping depths comparable to Nb's magnetic penetration depth $\lambda$. $^{8}$Li's strong dipole-dipole coupling with the host $^{93}$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 $1/T_{1}$. From a fit of the $1/T_{1}$ data to a model accounting for its dependence on temperature, magnetic field, and $^{8}$Li$^{+}$ implantation energy, we obtained a magnetic penetration depth $\lambda_{0}$ = 51.5(22) nm, consistent with a relatively short carrier mean-free-path $\ell$ = 18.7(29) nm typical of similarly prepared Nb films. The results presented here constitute an important step towards using $^{8}$Li $\beta$-NMR to characterize bulk Nb samples with engineered surfaces, which are often used in the fabrication of particle accelerators.Comment: 16 pages, 4 figure

Development of SRF Cavity Tuners for CERN

Superconducting RF cavity developments are currently on-going for new accelerator projects at CERN such as HIE ISOLDE and HL-LHC. Mechanical RF tuning systems are required to compensate cavity frequency shifts of the cavities due to temperature, mechanical, pressure and RF effects on the cavity geometry. A rich history and experience is available for such mechanical tuners developed for existing RF cavities. Design constraints in the context of HIE ISOLDE and HL-LHC such as required resolution, space limitation, reliability and maintainability have led to new concepts in the tuning mechanisms. This paper will discuss such new approaches, their performances and planned developments