Proposal
A Programme of Research and Development on the Improvement of SRF Cavities for Future Accelerators
Introduction
Owing to the tremendous advances in RF superconductivity in the recent years and to the
successful operation of large scale SRF accelerator installations, superconducting RF (SRF) has
become the technology of choice for modern particle accelerators. SRF accelerating structures
are chosen for high energy as well as high intensity, continuous wave (CW) applications, where
maximizing the accelerating gradient and maximizing the quality factor respectively are the
primary goals. SRF structures are also considered for deflecting/separator and crabbing cavities
used to deliver beam to a number of experimental stations simultaneously, and to increase the
luminosity of colliders and to generate sub-picosecond x-ray pulses in light sources.
This proposal comprises a set of interlinked but complementary projects aiming towards
improving the performance of SRF cavities for future accelerators, and more broadly towards
advancing the field of RF superconductivity. This application requests funds to allow
undergraduate and graduate students and postdoctoral researchers to participate in SRF research
and development, thereby receiving valuable training in accelerator physics and engineering.
This proposal is accompanied by five RTIs that request funds for the associated experimental
equipment.
The various elements of this proposal and the associated RTIs are:
- Building on recent successful experiments at TRIUMF’s muSR facility, perform
fundamental studies of RF superconductivity using magnetometry, muSR and the
proposed high-field expansion of the betaNMR spectrometer to obtain information on the
penetration of magnetic flux near the surface of the Nb superconductor, and to explore
correlations between magnetic response of Nb and RF measurements of SRF Nb cavities.
- Develop an advanced diagnostic system based on temperature mapping that gives a
detailed and precise picture of the RF characteristics of cavities under test.
- Develop a UHV rf induction heat treatment oven for processing both samples of niobium
as well as a single cell 1.3GHz cavity, with the aim to reduce the concentration of
hydrogen near the niobium surface, which would increase the Q value thereby improving
the performance of the ARIEL e-linac cavities.
- Design, construct and test a novel SRF separator cavity capable of high transverse
deflecting fields and CW (continuous wave) mode of operation. This development will
enable the simultaneous operation of the ARIEL e-linac for RIB delivery and the
production of ERL-driven coherent IR/THz radiation.
- Develop instrumentation, techniques and infrastructure to perform RF measurements of
both fundamental and higher order modes in accelerating and deflecting cavities, towards
improving the performance of the ARIEL cavities and in support of all other activities in
this proposal.
Objectives
The broad objective of this research program is the improvement of SRF cavities for future
accelerators. In the near term we will focus on developing instruments and tools that will help us
improve cavity performance. These include: a temperature mapping apparatus, an rf induction
oven, HOM and SRF lab instrumentation, and the high field upgrade of the betaNMR
spectrometer.
In the long term, the objectives are two-fold:
a) to develop state-of-the-art SRF resonators for ARIEL and more broadly for the international
community. This includes improvements in the performance of accelerating cavities, specifically
raising the quality factor by identifying appropriate and cost-effective heat treatments, and by
providing sufficient damping of the HOMs, and the development of a novel SRF separator cavity
that will be used to deflect interleaved bunches of electrons to the ARIEL target stations for RIB
production, and to the recirculation ring to drive a FEL.
b) to contribute towards the understanding of limitations of SRF cavity performance at a
fundamental level, and ultimately devise better treatments for Nb that will improve cavity
performance. Towards this end, in the first phase of the project, SQUID magnetometry and
muSR measurements on Nb samples that have undergone different surface treatments will be
conducted, and correlations established. Samples cutout of cavities that exhibit lossy behavior
and have been characterized by temperature mapping will also be tested. In parallel, a new
extension of the betaNMR spectrometer will be implemented capable of high field external
magnetic field parallel to the face of the sample, in conjunction with a new cryostat. This new
capability will allow unprecedented measurements of the previously tested Nb samples within
the first 100 nm layer of the inner surface, and gain insights into the effectiveness of surface
treatments in terms of static magnetic response of Nb. Ultimately, using the correlations
established in the first phases of this program, we will explore new Nb samples and treatments,
e.g. pristine Nb coated with insulating or metallic films, with the goal to devise better treatments
for Nb that will lead to improved RF cavity performance.
An important 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.
A broad set of milestones would be:
Year 1:
Complete Temperature mapping system??
Design rf induction oven??
Design betaNMR upgrade
Design SRF separator
Year 2:
Treat Nb samples and single cell 1.3 GHz in rf oven
Construct betaNMR upgrade
Build SRF separator
Year 3:
Carry out tests on Nb samples with high-file beta NMR
Test SRF separator
Literature Review
Discuss the literature pertinent to the proposal, placing the proposed research in the context of
the state-of-the-art.
S. Casalbuoni et al., "Surface superconductivity in niobium for superconducting RF
cavities," Nuclear Instruments and Methods in Physics Research A, vol. 538, pp. 4564, 2005
A. Grassellino et al. “Muon spin rotation studies of niobium for superconducting RF
applications,” what’s the best reference?
http://accelconf.web.cern.ch/AccelConf/SRF2011/talks/tuiob04_talk.pdf
A.Grassellino, “Field-dependent losses in superconducting niobium cavities” PhD. Thesis, U. of
Pennsylvania (2011)
R. Kiefl et al. “Low-energy spin polarized radioactive beams as a nano-scale probe of matter,”
Physica B 326 (2003) 189-195
J. R. Delayen, H. Wang, “New compact TEM-type deflecting and crabbing rf structure”
PHYSICAL REVIEW SPECIAL TOPICS - ACCELERATORS AND BEAMS 12, 062002
(2009)
Sergey Belomestnykh, Ivan Bazarov,ValeryShemelin, John Sikora,Karl Smolenski, Vadim
Veshcherevich, “Deflecting cavity for beam diagnostics at Cornell ERL injector,” Nuclear
Instruments and Methods in Physics Research A, 614 (2010) 179-183
G. Ciovati, et al., "High field Q slope and the baking effect: Review of recent experimental
results and new data on Nb heat treatments", Phys. Rev. ST Accel. Beams 13, 022002 (2010)
R. E. Ricker, et al., “Evaluation of the Propensity of Niobium to Absorb Hydrogen During
Fabrication of Superconducting Radio Frequency Cavities for Particle Accelerators” J. Res. Natl.
Inst. Stand. Technol. 115, 353-371 (2010)
G. Ciovati, et al., "High-Temperature Heat Treatment Study on a Large-Grain Niobium Cavity",
Proc. of the 15th Int. Conf. on RF Superconductivity, Chicago, July 25-29, 2011, paper
TUPO051, 2011.
P. Maheshwari, et al., “Surface Analysis of Nb Materials for SRF Cavities”, Surf. Int. Analysis
43, 151-153 (2011)
J.P. Wallace, “Proton in SRF Niobium”, Proceedings of the Symposium on the Superconducting
Science and Technology of Ingot Niobium", Newport News, Virginia, September 22-24, 2010,
AIP Conference Proceedings 1352 (2011), p. 205
P. Dhakal, et al, A Path to Higher Q0 with Large Grain Niobium Cavities, Proc. IPAC12, New
Orleans, 2012
Methodology
Describe the methods and proposed approach, providing sufficient details to allow the reviewers
to assess the feasibility of the research activities.
The methodology for each research activity is described:
1) Temperature Mapping
In the RTI request last year we proposed building two different temperature mapping
systems; one using thin film resistors, and one using precision carbon resistors. Given the
amount of funding awarded we had to reassess our approach. In order to understand the
current state of the art, one of us (R.S. Orr) is spending six months as a visitor in the
Fermilab SRF group. The Fermilab group operates a carbon resistor system designed and
constructed by Alexandr Romanenko. This is a simple system based on the design of
Padamsee at Cornell. After taking part in tests and discussing with Romanenko, we
concluded that the most direct approach was for us to duplicate, at TRIUMF, the reliable
and well tested carbon resistor system. A typical test result at the Fermilab Vertical test
System is shown in Fig. 2. The top left hand plot shows the measured Q value as a
function of accelerating field in the cavity. The temperature maps allow one to correlate
the heating pattern in the cavity with features in the Q slope. At low field the heating is
rather generalized, then becomes concentrated in the high magnetic field region at the
equator of the cavity, and finally concentrated at the region of the quench. These
particular temperature maps are for a cavity which was only processed with centrifugal
barrel polishing, HF rinse, and high pressure rinsing. This is part of an investigation of
whether high Q can be achieved without extensive electro-chemical processing. This
research on the production of high Q in an economical fashion is clearly very relevant to the
cryoplant capital and operating costs at ARIEL.
2) Oven
In this proposal we plan to develop a UHV degassing treatment using rf induction heaters. We
propose to develop a test chamber to be used to heat both samples of niobium as well as a single
cell 1.3GHz cavity. After the original study we would expand the size to enable a degassing
treatment for a wide variety of cavity sizes including the ARIEL cavities, ISAC-II quarter wave
cavities and other cavities that are being fabricated in a collaboration between PAVAC and
TRIUMF. The samples heated in the oven will also be tested in the sample cavity being funded n
the discovery grant of the PI, in the muSR facility and in condensed matter test facilities at the
University of British Columbia (UBC). The heat treatment of 1.3GHz single cell cavities will be
characterized in cold tests in the ISAC-II clean room test area
3) ARIEL e-Linac Test Equipment for HOM Measurement and Clean Room Monitoring
4) Surface superconductivity in Nb cavities using beta-NMR
Magnetic measurements on Nb samples are a useful tool to explore the surface treatments
which improve cavity performance. The samples will be cut out from remainders of Nb
sheets used in cavity production and will be subjected to the same chemical, electrochemical, and thermal treatments as the TESLA cavities. The sample magnetization is
determined with a SQUID magnetometer at temperatures ranging from 2 to 300 K in external
DC fields between zero and 1 Tesla.
SQUID will be used for bulk superconductivity measurements to determine Tc. SQUID an
also provide surface superconductivity measurements of the critical surface field, and
coherent surface superconductivity.
a) We will use SQUID to perform the following:
1) Determine Tc for all different samples. Scan around 9.25 K (Tc for high purity Nb
RRR=1600+/-400)
2) Determine Ohmic resistivity to confirm the purity of the samples and the manufacturers’
specs of RRR~300.
3) Plot thermodynamic and upper critical field as a function of temperature.
4) Determine the GL parameter at zero temperature, the GL coherence length, London
penetration depth.
5) Does EP or Low Temperature Bakeout change the superconductor parameters of bulk
Nb?
6) Do you observe hysteresis loops in the magnetization measurements? (mu0M[T] vs.
B[T], fig. 2). They are a clear proof of magnetic flux pinning.
7) Magnetic impurities?
b) MuSR is a method .......
In the muSR LAMPF spectrometer
c) Upgrade of beta-NQR spectrometer
In the first phase of the project, SQUID magnetometry and muSR measurements on Nb
samples that have undergone different surface treatments will be conducted, and
correlations established. Samples cut-out from cavities that exhibit lossy behavior and
have been characterized by temperature mapping will also be tested. The temperature
maps allow one to correlate the heating pattern in the cavity with features in the Q slope.
In parallel, a new extension of the betaNMR spectrometer will be implemented capable
of up to 3 kG external magnetic field parallel to the face of the sample, in conjunction
with a new cryostat. This new capability will allow unprecedented measurements of the
superconducting state of the previously tested Nb samples within the first 100 nm layer of
the inner surface, and gain insights into the effectiveness of surface treatments in terms of
static magnetic response of Nb. Ultimately, using the correlations established in the first
phases of the program, we will explore new Nb samples and treatments e.g. pristine Nb
coated with insulating or metallic films, with the goal to devise better treatments for Nb
that will improve RF cavity performance.
Figure 1: xxxxxxxxxx
Figure 2: xxxxxxxxxxxxxx
5) SRF separator cavity
We propose to design, fabricate, and test with beam a novel superconducting RF separator cavity
capable of high transverse deflecting fields (Pperp ~ 0.2MeV/c) in CW (continuous wave) mode
of operation. Design challenges of the ARIEL SRF separator include achieving the required
transverse deflecting field while avoiding multipacting (spurious x-ray emission), and ensuring
the required phase and amplitude stability of the high intensity beams, with stringent
requirements on beam quality driven primarily by the FEL operation. This development will be
an important step to realizing the first linac-based coherent THz or IR source in Canada, and a
unique scientific resource in North America. It will also be of interest to the broader accelerator
community for multiple applications, including in linac-based X-ray sources for fanning out the
beam to multiple users at MHz rates.
One of the major goals of this project would be the training of highly qualified personnel in the
areas of SRF cavity design and RF measurements. This project is an excellent student training
ground on a broad and comprehensive range of skills: technical specification of the cavity, cavity
design using analytical approach, electromagnetic modeling using codes such as ANSYS, HFSS,
COMSOL, structural simulations to determine pressure sensitivity, frequency sensitivity to
microphonics, He pressure fluctuations, Lorentz force detuning, multipacting simulations, design
of tuner and coupler, cavity fabrication, test cryostat and test coupler for low power cavity tests,
beam loading simulations, bead-pull measurements of modes and field distribution, cavity
preparation and processing (surface chemical etching, BCP (buffered chemical polishing), high
pressure water rinsing), cavity assembly for low power SRF tests, design of cryomodule, cavity
assembly, high power commissioning, and beam tests.
Impact
Explain the anticipated significance of the work.
Training of HQP
Three co-op and two summer students will be involved in this research, and three PhD degrees in
accelerator physics will be granted on the basis of this proposal. Further, one postdoctoral fellow
will receive training on designing, building and installing, commissioning of the upgraded betaNMR spectrometer and in using it to perform measurements on Nb samples.
One of the graduate students participating in this research, Terry Buck, a MS student in Physics
at the University of British Columbia (UBC) who 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 betaNMR as a probe of matter.
The other graduate student, Douglas Storey is a PhD student in the University of Victoria. He
has a xxxxxxxxxxx fellowship and his research topic is the design and development of the SRF
separator cavity. He applied for the NSERC PGS fellowship this year. His academic supervisor
is Prof. Dean Karlen, PI of the ARIEL e-linac CFI grant.
We request a co-op student to be the “link person” on building and operating the temperature
mapping system. Our preferred candidate would be Syed Haider Abidi who would be on a
sixteen month co-op leave from the University of Toronto Engineering Science programme.
Haider has already spent xxx months at TRIUMF working on ..... He is interested in pursuing an
advanced degree in accelerator physics.