Proposal
Introduction and Context
The majority of modern accelerators use superconducting radiofrequency (SRF) resonators
(cavities) made of high purity niobium for the acceleration of particle beams. One of the
important limitations in the performance of SRF cavities is the occurrence of field-dependent
losses in the walls of the cavity, which manifests itself as a degradation of the resonator quality
factor (Q) as a function of the applied RF field. These losses occur at different RF field levels,
both in the medium and high field range, and therefore impose severe limitations in the SRF
cavity performance both for pulsed and CW (continuous wave) applications, where highest
possible gradients and maximizing Q respectively are the primary goals.
A possible explanation for the high field “Q-slope” is that magnetic flux penetrates into the
niobium surface prematurely and the fluxoids in the Nb cause the degradation of the quality
factor. Recent experiments spearheaded by one of the co-applicants of this proposal (R. Laxdal)
at the TRIUMF SR facility with Nb samples cut-out from RF cavities that exhibit losses
establish a strong correlation between the static magnetic response of Nb and RF losses, in the
perpendicular field geometry [A. Grassellino et al. “Muon spin rotation studies of niobium for
superconducting rf applications,” Phys. Rev. ST Accel. Beams 16, 062002 (2013)].
To further investigate premature magnetic flux penetration, we are presently pursuing a threepronged approach designed to probe the magnetic response of the Nb on different length scales.
This includes SQUID magnetometry, SR, and -NMR measurements on Nb samples that have
undergone different surface treatments, in perpendicular and parallel (to the face of the sample)
applied magnetic field configurations. The data will allow us to establish correlations between
the magnetic response of the Nb and RF properties of the samples.
First, using a SQUID magnetometer, that is available at the UBC campus, we measure AC and
DC magnetization on Nb used in cavities, and bulk Nb superconductivity properties. Excellent
earlier work [S. Casalbuoni et al., "Surface superconductivity in niobium for superconducting RF
cavities," Nuclear Instruments and Methods in Physics Research A, vol. 538, pp. 45-64, 2005]
has proved the usefulness of this instrument in understanding properties of Nb but no
dependence on orientation was studied. This is one of the goals of our studies.
Second, we use TRIUMF’s SR spectrometers with "surface muons," as they are best suited for
thin samples, to apply a field of up to 200 mT parallel and perpendicular to the Nb surface of
samples from SRF cavities that have undergone different surface treatments, including no
treatment, to determine the field of first magnetic flux entry in the samples, and correlate it with
measurements of RF losses in the cavities. SQUID and SR fundamentally measure the bulk
magnetic response.
Since only the first ~100nm layer of the Nb cavity inner surface determines its superconducting
properties, our third approach is to use TRIUMF's -NMR facility, to probe depth-dependent
magnetic properties on nm length scale, and thus obtain direct information on the penetration of
magnetic flux near the surface of a superconductor. This probes the magnetic response of the
near surface region which is the most relevant part of the sample for SRF development.
TRIUMF's -NMR facility has the capability to apply a field parallel to the surface of the
sample, and thus offers a sensitive new way to probe the near surface properties of Nb used in
SRF cavities. However, the maximum field on the transverse field spectrometer is presently
limited to ~24 mT. To reach fields up to 200 mT, needed to understand high field Q-slope, an
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upgrade of the -NQR spectrometer is necessary. Bringing this capability into existence is the
main objective of this RTI.
To overcome the severe limitation vortex penetration has on high field performance in bulk Nb
RF cavities for accelerators, and to reach higher fields, Gurevich in 2006 [Appl. Phys. Lett. 88,
012511 (2006)] proposed to deposit nanoscale coating on a Nb cavity to partially shield the bulk
Nb from the applied rf surface field. This method, if successful, is bound to bring a breakthrough
in the technology of superconducting RF cavities. The topic is presently under vigorous
investigation and debate in the SRF community. In particular, a theoretical model for the selfconsistent calculation of the vortex penetration field has been derived [Kubo, et al], and predicts
optimum parameters for the multilayer coating, such as thickness of layers and choice of
materials. The new high transverse field spectrometer will be a powerful tool for the
experimental validation of this model, and this will be a tremendous contribution to this
controversial and highly promising topic.
Objectives
The short term objective of this RTI is to design and construct a high field upgrade of the NMR spectrometer that will enable unprecedented measurements of Nb samples within the first
~100 nm layer of the inner surface, and allow us to systematically evaluate the effects of surface
treatments on the cavity performance in terms of the static magnetic response of Nb.
The long-term objective is to contribute towards the fundamental understanding of limitations of
SRF cavity performance, and ultimately devise better treatments for Nb that will improve the
performance, and elucidate the controversy over the promise of multi-layered superconductors
for higher performance SRF cavities.
An important and overarching objective of this research is to expand the expertise and
knowledge base in SRF science and technology in Canada by significantly contributing to
training of students and postdocs in this ever expanding field of research.
Methodology
Beta-detected NMR (-NMR) is a powerful experimental technique for probing electronic and
magnetic properties of materials. It is similar to SR in that both techniques provide local
information via the radioactive decay of the probe. In both cases a beam of highly polarized
radioactive probe spins (i.e. 8Li, or muons), is generated and then implanted into the material of
interest.
At ISAC a low energy (1- 30 keV) beam of hyperpolarized 8Li has been developed and shown
to be a sensitive new way to probe depth-dependent properties on nm length scale [R. Kiefl, et
al. “Low-energy spin polarized radioactive beams as a nano-scale probe of matter,” Physica B
326 (2003) 189-195]. Electrostatic deceleration is used to control the depth of the implanted ions,
typically in the range of 2-300 nm. Further, -NMR is a highly sensitive probe, roughly ten
orders of magnitude more sensitive than conventional NMR, therefore it can be used to perform
novel NMR experiments on films as thin as a few nm and within the near surface region of bulk
materials where there are too few nuclei to produce a useful signal by conventional techniques.
There are two spectrometers associated with the -NMR facility at ISAC [G. Morris “-NMR,”
to be published in Special Edition Hyperfine Interactions “Laboratory Portrait: ISAC at
TRIUMF”], designed to cover different ranges of the applied magnetic field: the high field NMR and the zero- and low-field -NMR/NQR spectrometer. Both spectrometers are equipped
with helium cryostats in the temperature range of 4 – 300 K. Each spectrometer is mounted on a
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high voltage platform which is electrically isolated from ground. The ion beam energy, and thus
the depth of ion implantation, is controlled by adjusting the platform bias voltage.
The high field -NMR spectrometer is constructed around a 9T superconducting solenoid. The
solenoid field, beam momentum, and initial polarization are all normal to the face of the sample.
The zero and low field -NMR/NQR spectrometer is optimized for low magnetic fields (0-24
mT). Since the spin of the 8Li ions exiting the polarizer is preserved (because of the use of
electrostatic elements) but the beam momentum is bent by 90 degrees, the spin of the 8Li ions at
the low field spectrometer is transverse to the beam momentum, therefore parallel to the face of
the sample. Since the external magnetic field is oriented parallel to the initial polarization, it is
also parallel to the sample face. This arrangement is ideal for measuring the depth dependence of
magnetic field in the Meissner state of superconductors, which is precisely the focus of our
studies. The applied DC magnetic field is presently limited to 24 mT, whereas fields up to 200
mT (~ critical field Hc1 of Nb) are needed to understand SRF Nb cavity limitations.
The current RTI proposal is to build a high-field extension of the -NQR spectrometer, up to 2
kG, along with a second access port onto the existing -NQR platform. The second port would
hold an existing 3He cryostat which is capable of temperatures in the range 300 mK to 100K,
thus allowing us to carry out measurements within the full operating temperature range of SRF
cavities (1.8 to 4.5K), which is presently not accessible. The cryostat already exists and was
funded by a 2008 NSERC RTI grant of $93k. The project includes the detail design, fabrication
and installation of a short ~2m extension of the -NQR spectrometer beamline.
The beam optics requirements for the -NMR facility are generally challenging [R. Baartman,
“ISAC LEBT” to be published in Special Edition Hyperfine Interactions “Laboratory Portrait:
ISAC at TRIUMF”]. Radioactive isotopes (usually 8Li) must reach the sample at energies down
to 500 eV or up to 30 keV. On the low field spectrometer, needed for SRF development, the
polarization and external magnetic field are transverse to the beam. The Lorentz force and
resulting transverse deflection is significant at an applied field of 2 kG. Compensating for this
deflection is a key feature of the optics design of the new spectrometer, which was recently
completed and meets all the requirements specifications. Xxxx Suresh paragraph...
In the current -NQR apparatus, focusing and steering of the beam on the sample are diagnosed
by replacing the sample with a scintillator and imaging with a CCD camera. The new
spectrometer would have a similar system. The spectrometer will be UHV compatible in order to
avoid a buildup of residual gases on the surface of the sample. Differential pumping is required
to reduce the pressure from 10-7 Torr upstream of the spectrometer to 10-10 Torr in the main
chamber. Details of all the elements of the new beamline are presented in the Budget
Justification section.
Impact
Particle accelerators find ever expanding applications both as instruments of scientific research
in subatomic physics, x-ray and neutron science, but also for societal applications in medicine,
energy, environment, security, and industry. Nearly all future accelerators will utilize
superconducting technology to improve operational efficiency and performance. The impact on
improving SRF cavity gradient and quality factor is tremendous in terms of cost and
performance. Higher gradients make smaller accelerator layouts possible and higher Q’s require
smaller cryo-plants for a given energy reach. Both factors result in substantial reduction of the
capital and operating costs of the facility.
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Further, the proposed high field extension of the b-NQR spectrometer will result in a facility
with unmatched capabilities anywhere in the world: a depth-dependent probe of superconducting
materials at the relevant length scale, combined with a high magnetic field (~Hc1 of Nb) parallel
to the surface of the sample. This new capability is uniquely suited to probe multi-layered
structures. Given the broad interest in the SRF community in multi-layered superconductors, the
TRIUMF high field b-NQR spectrometer will serve as a test facility to experimentally
investigate samples produced in laboratories around the world, including KEK, Saclay, and Los
Alamos.
With the ARIEL e-linac completion in the fall of 2014, and first beam to b-NMR from the elinac (via 8Li photo-production in 9Be targets) expected in 2016 -2017, the beamtime allocation
to b-NMR will increase substantially (from 5 to ~16 weeks), allowing the full exploitation of this
new spectrometer.
The fundamental nature of these investigations places them at the forefront of scientific research,
and establishes TRIUMF and Canada as one of the leaders in SRF science, being already a leader
in SRF technology, when combined with PAVAC, a Canadian company.
Since this RTI is essentially being developed by a graduate student and a postodoc, this research
strongly supports HQP training on a broad range of skills. It also further strengthens the
technology transfer and commercialization aspects of the SRF program in Canada.
Figure 1
Figure 2
Figure 1: xxxx
Figure 2: xxxx
Excellence of participants
Lia Merminga is head of the Accelerator Division at TRIUMF, and co-leader of the ARIEL
project. She has made significant contributions to R&D on SRF linacs, ERLs and electron-ion
colliders, and beam physics and applications of these accelerators.
Bob Laxdal is an expert in RF superconductivity. He made critical contributions to establishing
a world-class SRF facility and building ISAC–II SRF linac at TRIUMF.
Bob Orr is a well known experimental particle physicist who has been involved in the ILC
program, testing high gradient cavities and investigating different surface treatments. He and his
team developed diagnostics for SRF cavity tests, in particular second sound transducers.
Collaborators
R. Kiefl, Professor at UBC, Department Physics and Astronomy, xxxxxxxxxx
Rick Baartman is a Beam Physicist with expertise in space charge effects, beam optics,
electrostatic beamlines, xxx
H. Hayano, T. Saeki, T. Kubo, KEK ......
Y. Iwashita, Univ. of Kyoto
Training of Highly Qualified Personnel
One postdoctoral fellow, Suresh Saminathan, is a member of the TRIUMF Beam Physics
Group. He comes from xxxx and his expertise range from beam optics, ion sources. On this
project he led the beam optics design and simulations, and he will participate in the construction,
installation and commissioning of the new spectrometer.
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Terry Buck is a MSc student in Physics at the University of British Columbia (UBC)
participating in this research. Terry arrived in the fall of 2012 on NSERC Alexander Graham
Bell CGS M award to work on fundamental studies of RF superconductivity. His academic
supervisor is Prof. Rob Kiefl who pioneered the use of -NMR as a probe of matter.
Terry is learning SQUID, uSR and -NMR experimental techniques and data analysis, and
contributes to the design and construction of the beamline, thereby being trained in a broad range
of skills including beam optics, diagnostics, and UHV techniques, as well as analytical methods,
numerical simulations and modeling, and experimental techniques. This year Terry is supported
via the PI’s Subatomic Physics Project grant entitled “A Programme of Research and
Development on the Improvement of SRF Cavities for Future Accelerators” that was funded last
year.
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