44 research outputs found
Modeling Vacuum Arcs
We are developing a model of vacuum arcs. This model assumes that arcs
develop as a result of mechanical failure of the surface due to Coulomb
explosions, followed by ionization of fragments by field emission and the
development of a small, dense plasma that interacts with the surface primarily
through self sputtering and terminates as a unipolar arc capable of producing
breakdown sites with high enhancement factors. We have attempted to produce a
self consistent picture of triggering, arc evolution and surface damage. We are
modeling these mechanisms using Molecular Dynamics (mechanical failure, Coulomb
explosions, self sputtering), Particle-In-Cell (PIC) codes (plasma evolution),
mesoscale surface thermodynamics (surface evolution), and finite element
electrostatic modeling (field enhancements). We can present a variety of
numerical results. We identify where our model differs from other descriptions
of this phenomenon.Comment: 4 pages, 5 figure
Rebuttal to "Comment by V.M. Krasnov on 'Counterintuitive consequence of heating in strongly-driven intrinsic junctions of Bi2Sr2CaCu2O8+d Mesas' "
In our article [1], we found that with increasing dissipation there is a
clear, systematic shift and sharpening of the conductance peak along with the
disappearance of the higher-bias dip/hump features (DHF), for a stack of
intrinsic Josephson junctions (IJJs) of intercalated Bi2Sr2CaCu2O8+{\delta}
(Bi2212). Our work agrees with Zhu et al [2] on unintercalated, pristine
Bi2212, as both studies show the same systematic changes with dissipation. The
broader peaks found with reduced dissipation [1,2] are consistent with broad
peaks in the density-of-states (DOS) found among scanning tunneling
spectroscopy [3] (STS), mechanical contact tunneling [4] (MCT) and inferred
from angle (momentum) resolved photoemission spectroscopy [5] (ARPES); results
that could not be ignored. Thus, sharp peaks are extrinsic and cannot
correspond to the superconducting DOS. We suggested that the commonality of the
sharp peaks in our conductance data, which is demonstrably shown to be
heating-dominated, and the peaks of previous intrinsic tunneling spectroscopy
(ITS) data implies that these ITS reports might need reinterpretation.Comment: Rebuttal to Comment of Krasnov arXiv:1007.451
Tunneling study of cavity grade Nb: possible magnetic scattering at the surface
Tunneling spectroscopy was performed on Nb pieces prepared by the same
processes used to etch and clean superconducting radio frequency (SRF)
cavities. Air exposed, electropolished Nb exhibited a surface superconducting
gap delta=1.55 meV, characteristic of clean, bulk Nb. However the tunneling
density of states (DOS) was broadened significantly. The Nb pieces treated with
the same mild baking used to improve the Q-slope in SRF cavities, reveal a
sharper DOS. Good fits to the DOS were obtained using Shiba theory, suggesting
that magnetic scattering of quasiparticles is the origin of the gapless surface
superconductivity and a heretofore unrecognized contributor to the Q-slope
problem of Nb SRF cavities.Comment: 3 pages, 3 figure
Probing the superconducting condensate on a nanometer scale
Superconductivity is a rare example of a quantum system in which the
wavefunction has a macroscopic quantum effect, due to the unique condensate of
electron pairs. The amplitude of the wavefunction is directly related to the
pair density, but both amplitude and phase enter the Josephson current : the
coherent tunneling of pairs between superconductors. Very sensitive devices
exploit the superconducting state, however properties of the {\it condensate}
on the {\it local scale} are largely unknown, for instance, in unconventional
high-T cuprate, multiple gap, and gapless superconductors.
The technique of choice would be Josephson STS, based on Scanning Tunneling
Spectroscopy (STS), where the condensate is {\it directly} probed by measuring
the local Josephson current (JC) between a superconducting tip and sample.
However, Josephson STS is an experimental challenge since it requires stable
superconducting tips, and tunneling conditions close to atomic contact. We
demonstrate how these difficulties can be overcome and present the first
spatial mapping of the JC on the nanometer scale. The case of an MgB film,
subject to a normal magnetic field, is considered.Comment: 7 pages, 6 figure
Probing the superfluid velocity with a superconducting tip: the Doppler shift effect
We address the question of probing the supercurrents in superconducting (SC)
samples on a local scale by performing Scanning Tunneling Spectroscopy (STS)
experiments with a SC tip. In this configuration, we show that the tunneling
conductance is highly sensitive to the Doppler shift term in the SC
quasiparticle spectrum of the sample, thus allowing the local study of the
superfluid velocity. Intrinsic screening currents, such as those surrounding
the vortex cores in a type II SC in a magnetic field, are directly probed. With
Nb tips, the STS mapping of the vortices, in single crystal 2H-NbSe_2, reveals
both the vortex cores, on the scale of the SC coherence length , and the
supercurrents, on the scale of the London penetration length . A
subtle interplay between the SC pair potential and the supercurrents at the
vortex edge is observed. Our results open interesting prospects for the study
of screening currents in any superconductor.Comment: 4 pages, 5 figure
Modeling RF breakdown arcs
We describe breakdown in 805 MHz rf accelerator cavities in terms of a number of self-consistent mechanisms. We divide the breakdown process into three stages: (1) we model surface failure using molecular dynamics of fracture caused by electrostatic tensile stress, (2) we model the ionization of neutrals responsible for plasma initiation and plasma growth using a particle-in-cell code, and (3) we model surface damage by assuming a process similar to unipolar arcing. We nd that the cold, dense plasma in contact with the surface produces very small Debye lengths and very high electric fields over a large area, consistent with unipolar arc behavior, although unipolar arcs are strictly de ned with equipotential boundaries. These high elds produce strong erosion mechanisms, primarily self-sputtering, compatible with the crater formation that we see. We use the OOPIC model to estimate very high surface electric elds in the dense plasma and measure these elds using electrohydrodynamic arguments to relate the dimensions of surface damage with the applied electric eld. We also present a geometrical explanation of the large enhancement factors of field emitters.This is consistent with the apparent absence of whiskers on surfaces exposed to high elds. The enhancement factors we derive, when combined with Fowler-Nordheim analysis, produce a consistent picture of breakdown and eld emission from surfaces at local elds of 7{10 GV/m. We show that the plasma growth rates we obtain from OOPIC are consistent with growth rates of the cavity shorting currents using x-ray measurements. We believe the general picture presented here for rf breakdown arcs should be directly applicable to a larger class of vacuum arcs. Results from the plasma simulation are included as a guide to experimental veri cation of this model
Effect of high temperature heat treatments on the quality factor of a large-grain superconducting radio-frequency niobium cavity
Large-grain Nb has become a viable alternative to fine-grain Nb for the
fabrication of superconducting radio-frequency cavities. In this contribution
we report the results from a heat treatment study of a large-grain 1.5 GHz
single-cell cavity made of "medium purity" Nb. The baseline surface preparation
prior to heat treatment consisted of standard buffered chemical polishing. The
heat treatment in the range 800 - 1400 C was done in a newly designed vacuum
induction furnace. Q0 values of the order of 2x1010 at 2.0 K and peak surface
magnetic field (Bp) of 90 mT were achieved reproducibly. A Q0-value of
(5+-1)1010 at 2.0 K and Bp = 90 mT was obtained after heat treatment at 1400 C.
This is the highest value ever reported at this temperature, frequency and
field. Samples heat treated with the cavity at 1400 C were analyzed by
secondary ion mass spectrometry, secondary electron microscopy, energy
dispersive X-ray, point contact tunneling and X-ray diffraction and revealed a
complex surface composition which includes titanium oxide, increased carbon and
nitrogen content but reduced hydrogen concentration compared to a non
heat-treated sample
Giant two-phonon Raman scattering from nanoscale NbC precipitates in Nb
High purity niobium (Nb), subjected to the processing methods used in the
fabrication of superconducting RF cavities, displays micron-sized surface
patches containing excess carbon. High-resolution transmission electron
microscopy and electron energy-loss spectroscopy measurements are presented
which reveal the presence of nanoscale NbC coherent precipitates in such
regions. Raman backscatter spectroscopy on similar surface regions exhibit
spectra consistent with the literature results on bulk NbC but with
significantly enhanced two-phonon scattering. The unprecedented strength and
sharpness of the two-phonon signal has prompted a theoretical analysis, using
density functional theory (DFT), of phonon modes in NbC for two different
interface models of the coherent precipitate. One model leads to overall
compressive strain and a comparison to ab-initio calculations of phonon
dispersion curves under uniform compression of the NbC shows that the measured
two-phonon peaks are linked directly to phonon anomalies arising from strong
electron-phonon interaction. Another model of the extended interface between Nb
and NbC, studied by DFT, gives insight into the frequency shifts of the
acoustic and optical mode density of states measured by first order Raman. The
exact origin of the stronger two-phonon response is not known at present but it
suggests the possibility of enhanced electron-phonon coupling in transition
metal carbides under strain found either in the bulk NbC inclusions or at their
interfaces with Nb metal. Preliminary tunneling studies using a point contact
method show some energy gaps larger than expected for bulk NbC.Comment: Phys. Rev. B, accepte
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