31 research outputs found
Shielding superconductors with thin films
Determining the optimal arrangement of superconducting layers to withstand
large amplitude AC magnetic fields is important for certain applications such
as superconducting radiofrequency cavities. In this paper, we evaluate the
shielding potential of the superconducting film/insulating film/superconductor
(SIS') structure, a configuration that could provide benefits in screening
large AC magnetic fields. After establishing that for high frequency magnetic
fields, flux penetration must be avoided, the superheating field of the
structure is calculated in the London limit both numerically and, for thin
films, analytically. For intermediate film thicknesses and realistic material
parameters we also solve numerically the Ginzburg-Landau equations. It is shown
that a small enhancement of the superheating field is possible, on the order of
a few percent, for the SIS' structure relative to a bulk superconductor of the
film material, if the materials and thicknesses are chosen appropriately.Comment: 7 pages, 5 figure
Evaluation of predictive correlation between flux expulsion and grain growth for superconducting radio frequency cavities
A series of experiments were carried out in an effort to develop a simple
method for predicting magnetic flux expulsion behavior of high purity niobium
used to fabricate superconducting radio frequency (SRF) cavities. Using
conventional metallographic characterizations in conjunction with high spatial
resolution electron backscattered diffraction-orientation imaging microscopy
(EBSD-OIM), we found that the flux expulsion behavior of 1.3 GHz single cell
SRF Nb cavities is significantly associated with the grain growth of the Nb
material during heat treatment. Most of Nb grains rapidly grew during 900C heat
treatment, and likely full-recrystallized with 1000C HT. With comparison of the
magnetic flux expulsion ratio (Bsc/Bnc) at dT = 5 K, the flux expulsion
efficiency of the cavities increases along with increasing of grain size. Most
interestingly, 900C HT shows a roughly linear trend that suggests this
criterion could be used to predict appropriate heat treatment temperature for
sufficient flux expulsion behavior in SRF-grade Nb. This result would be used
to see if flux expulsion can be predicted by examining the materials coming
from the Nb vendor, prior to cavity fabrication
Selective thermal evolution of native oxide layer in Nb and Nb3Sn-coated SRF grade Nb: An in-situ angular XPS study
This contribution discusses the results of an in-situ angular XPS study on
the thermal evolution of the native oxide layer on Nb3Sn and pure Nb. XPS data
were recorded with conventional spectrometers using an AlK(alpha) X-ray source
for spectra collected up to 600 C, and an MgK(Alpha) X-rays source for
temperatures above 600 C. The effect of the thickness, composition, and thermal
stability of that oxide layer is relevant to understanding the functional
properties of superconducting radiofrequency (SRF) cavities used in particle
accelerators. There is a consensus that oxide plays a role in surface
resistance (Rs). The focus of this study is Nb3Sn, which is a promising
material that is used in the manufacturing of superconducting radiofrequency
(SRF) cavities as well as in quantum sensing, and pure Nb, which was included
in the study for comparison. The thermal evolution of the oxide layer in these
two materials is found to be quite different, which is ascribed to the
influence of the Sn atom on the reactivity of the Nb atom in Nb3Sn films. Nb
and Sn atoms in this intermetallic solid have different electronegativity, and
the Sn atom can reduce electron density around neighbouring Nb atoms in the
solid, thus reducing their reactivity for oxygen. This is shown in the
thickness, composition, and thermal stability of the oxide layer formed on
Nb3Sn. The XPS spectra were complemented by grazing incident XRD patterns
collected using the ESRF synchrotron radiation facility. The results discussed
herein shed light on oxide evolution in the Nb3Sn compound and guide its
processing for potential applications of the Nb3Sn-based SRF cavities in
accelerators and other superconducting devices
Plasma Cleaning of LCLS-II-HE verification cryomodule cavities
Plasma cleaning is a technique that can be applied in superconducting
radio-frequency (SRF) cavities in situ in cryomodules in order to decrease
their level of field emission. We developed the technique for the Linac
Coherent Light Source II (LCLS-II) cavities and we present in this paper the
full development and application of plasma processing to the LCLS-II High
Energy (HE) verification cryomodule (vCM). We validated our plasma processing
procedure on the vCM, fully processing four out of eight cavities of this CM,
demonstrating that cavities performance were preserved in terms of both
accelerating field and quality factor. Applying plasma processing to this
clean, record breaking cryomodule also showed that no contaminants were
introduced in the string, maintaining the vCM field emission-free up to the
maximum field reached by each cavity. We also found that plasma processing
eliminates multipacting (MP) induced quenches that are typically observed
frequently within the MP band field range. This suggests that plasma processing
could be employed in situ in CMs to mitigate both field emission and
multipacting, significantly decreasing the testing time of cryomodules, the
linac commissioning time and cost and increasing the accelerator reliability.Comment: 11 pages, 10 figure
Mechanical optimization of superconducting cavities in continuous wave operation
Several planned accelerator facilities call for hundreds of elliptical cavities operating cw with low effective beam loading, and therefore require cavities that have been mechanically optimized to operate at high Q_{L} by minimizing df/dp, the sensitivity to microphonics detuning from fluctuations in helium pressure. Without such an optimization, the facilities would suffer either power costs driven up by millions of dollars or an extremely high per-cavity trip rate. ANSYS simulations used to predict df/dp are presented as well as a model that illustrates factors that contribute to this parameter in elliptical cavities. For the Cornell Energy Recovery Linac (ERL) main linac cavity, df/dp is found to range from 2.5 to 17.4  Hz/mbar, depending on the radius of the stiffening rings, with minimal df/dp for very small or very large radii. For the Cornell ERL injector cavity, simulations predict a df/dp of 124  Hz/mbar, which fits well within the range of measurements performed with the injector cryomodule. Several methods for reducing df/dp are proposed, including decreasing the diameter of the tuner bellows and increasing the stiffness of the enddishes and the tuner. Using measurements from a Tesla Test Facility cavity as the baseline, if both of these measures were implemented and the stiffening rings were optimized, simulations indicate that df/dp would be reduced from ∼30  Hz/mbar to just 2.9  Hz/mbar, and the power required to maintain the accelerating field would be reduced by an order of magnitude. Finally, other consequences of optimizing the stiffening ring radius are investigated. It is found that stiffening rings larger than 70% of the iris-equator distance make the cavity impossible to tune. Small rings, on the other hand, leave the cavity susceptible to plastic deformation during handling and have lower frequency mechanical resonances, which is undesirable for active compensation of microphonics. Additional simulations of Lorentz force detuning are discussed, and the results are compared to measurements on the ERL injector cavities