51 research outputs found
On the field dependent surface resistance of niobium on copper cavities
The surface resistance Rs of superconducting cavities prepared by sputter
coating a thin niobium film on a copper substrate increases significantly
stronger with the applied RF field compared to cavities of bulk material. A
possible cause is that due to the thermal boundary resistance between the
copper substrate and the niobium film Rs is enhanced due to global heating of
the inner cavity wall. Introducing helium gas in the cavity and measuring its
pressure as a function of applied field allowed to conclude that the inner
surface of the cavity is heated up by only 60+/-60 mK when Rs increases with
Eacc by 100 nOhm. This is more than one order of magnitude less than what one
would expect from global heating. Additionally the effect of cooldown speed and
low temperature baking have been investigated in the framework of these
experiments. It is shown that for current state of the art niobium on copper
cavities there is only a detrimental effect of low temperature baking. A fast
cooldown results in a lowered Rs.Comment: 7 pages, 8 figure
Investigations of the surface resistance of superconducting materials
In particle accelerators superconducting RF cavities are widely used to achieve high accelerating gradients and low losses. Power consumption is proportional to the surface resistance RS which depends on a number of external parameters, including frequency, temperature, magnetic and electric field. Presently, there is no widely accepted model describing the increase of Rs with applied field. In the frame of this project the 400 MHz Quadrupole Resonator has been extended to 800 and 1200 MHz to study surface resistance and intrinsic critical RF magnetic field of superconducting samples over a wide parameter range, establishing it as a world-wide unique test facility for superconducting materials. Different samples were studied and it was shown that Rs is mainly caused by the RF electric field in the case of strongly oxidized surfaces. This can be explained by interface tunnel exchange of electrons between the superconductor and localized states in adjacent oxides. For well prepared surfaces, however, the majority of the dissipation is caused by the magnetic field and Rs factorizes into field and temperature dependent parts. These different loss mechanisms were correlated to surface topography of the samples and distribution of oxides by using ultrasonic force microscopy and X-ray photon spectroscopy
Depth-resolved measurements of the Meissner screening profile in surface-treated Nb
We report depth-resolved measurements of the Meissner screening profile in
several surface-treated Nb samples using low-energy muon spin rotation
(LE-SR). In these experiments, implanted positive muons, whose stopping
depths below Nb's surface were adjusted between ~10 nm to ~150 nm, reveal the
field distribution inside the superconducting element via their spin-precession
(communicated through their radioactive decay products). We compare how the
field screening is modified by different surface treatments commonly employed
to prepare superconducting radio frequency (SRF) cavities used in accelerator
beamlines. In contrast to an earlier report [A. Romanenko et al., Appl. Phys.
Lett. 104 072601 (2014)], we find no evidence for any "anomalous" modifications
to the Meissner profiles, with all data being well-described by a London model.
Differences in screening properties between surface treatments can be explained
by changes to the carrier mean-free-paths resulting from dopant profiles near
the material's surface.Comment: 15 pages, 5 figures, 2 table
Experimental Evidence for Electric Surface Resistance in Niobium
Identifying the loss mechanisms of niobium cavities enables an accurate determination of applications for future accelerator projects and points to research topics required to mitigate current limitations. For several cavities an increasing surface resistance above a threshold field, saturating at higher field has been observed. Measurements on samples give evidence that this effect is caused by the surface electric field. The measured temperature and frequency dependence is consistent with a model that accounts for these losses by interface tunnel exchange between localized states in dielectric oxides and the adjacent superconductor
Mid-T Heat Treatments on BCPed Coaxial Cavities at TRIUMF
Mid-T heat treatments in the range from 250 to 400 C on superconducting
radio-frequency (SRF) cavities have been shown to provide high quality factors
that rise with applied rf field strength in high frequency, electro-polished
(EP), elliptical cavities operating at 2K, similar to nitrogen doped cavities.
The rise in quality factor is attributed to a decrease in the temperature
dependent part of the surface resistance . Until now, no results have
been reported for these new treatments on quarter-wave resonators (QWR) and
half-wave resonators (HWR). The TRIUMF multi-mode coaxial cavities are
dedicated test cavities that allow frequency and temperature resolved
performance characterization of treatments without changing environments,
therefore providing an excellent test vehicle to test these new treatments with
rf frequencies ranging from 200 to 1200 MHz. In this paper, performance
measurements from both QWR and HWR cavities are reported and their performance
compared with four different treatments: baseline, a conventional 120C low
temperature bake for 48 hours, and two mid-T bakes at 300 and 400C for 3 hours.
In addition, sample analysis using SEM, EDX and SIMS of witness samples is also
shown. It is found that the mid-T bakes are not directly transferable to low
frequency cavities. In the fundamental modes of the two test cavities, no
performance gain over the baseline treatment nor a decreasing temperature
dependent component with rising rf amplitude was observed. At frequencies above
1GHz and low temperatures, the mid-T bakes show a reduced field dependence of
compared to both the baseline and 120C treatments
Direct measurement of the Meissner screening profile in superconductor-superconductor bilayers using low-energy muon spin rotation
Superconducting radio frequency (SRF) cavities, which are critical components
in many particle accelerators, need to be operated in the Meissner state to
avoid strong dissipation from magnetic vortices. For a defect-free
superconductor, the maximum attainable magnetic field for operation is set by
the superheating field, , which directly depends on the
surface current. In heterostructures composed of different superconductors, the
current in each layer depends not only on the properties of the individual
material, but also on the electromagnetic response of the adjacent layers
through boundary conditions at the interfaces. Three prototypical bilayers
[(50 nm)/Nb, (80 nm)/Nb, and
(160 nm)/Nb] are investigated here by depth-resolved
measurements of their Meissner screening profiles using low-energy muon spin
rotation (LE-SR). From fits to a model based on London theory (with
appropriate boundary and continuity conditions), a magnetic penetration depth
for the thin layers of 182.5(31) nm is found, in good agreement with literature values for the bulk
alloy. In contrast, a simple London model without appropriate boundary
conditions overestimates by more than a factor
of two, suggesting that it is inappropriate for quantifying
here. Using the measured
, the maximum vortex-free field,
, of the superconductor-superconductor (SS) bilayer structure
was estimated to be 610(40) mT. The strong suppression of the surface current
in the layer suggests an optimal thickness of 261(14) nm.Comment: 13 pages and 8 figure
Initial Studies of Electron Beams as a means of Modifying Collagen
We present the initial design studies and specifications for an accelerator and conveyor system to irradiate collagen samples, modifying properties such as the putrescibility and mechanical behaviours in a paradigm shift from existing, widely used technology. We show the integrated design requirements for a magnetic rastering scheme to move the beam position in order to ensure a uniform dose distribution over the full surface of the hide and discuss its dependence on factors such as the size of the hide, the beam current and conveyor speed. We also present initial energy deposition studies using beam particle interaction simulation program G4beamline, in order to determine the numerical beam parameters and angle of incidence needed to ensure a uniform depth-dose distribution throughout the hide thickness
A low energy muon spin rotation and point contact tunneling study of niobium films prepared for superconducting cavities
Point contact tunneling and low energy muon spin rotation are used to probe, on the same samples, the surface superconducting properties of micrometer thick niobium films deposited onto copper substrates using different sputtering techniques: diode, dc magnetron and HIPIMS. The combined results are compared to radio-frequency tests performances of RF cavities made with the same processes. Degraded surface superconducting properties are found to correlate to lower quality factors and stronger Q-slope. In addition, both techniques find evidence for surface paramagnetism on all samples and particularly on Nb films prepared by HIPIMS
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
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