Paul welander cryogenic rf characterization of sc materials at slac with cu and nb-coated cavities

Cryogenic RF Characterization of
Superconducting Materials at SLAC
with Cu and Nb-Coated Cavities
Paul B. Welander, Matt Franzi, Jiquan Guo*, Sami Tantawi
SLAC National Accelerator Laboratory, Menlo Park, CA 94025
6 October 2014
* now at JLab

TFSRF 2014 – P. Welander – welander@slac.stanford.edu 2
Outline
• Motivation
• Review of Previous Work at SLAC
• New Cryostat and New Cavity
• Initial Measurements on Bulk Nb
• Thin Film Growth at Stanford

TFSRF 2014 – P. Welander – welander@slac.stanford.edu 3
The Takeaway
• Cryogenic RF testing underway again at SLAC
• New cryostat dedicated to SRF materials evaluation
• Two cavities (Cu & Nb) for sample characterization
- Hquench up to 360 mT, Rs with sub-nΩ resolution
• Low-power Q vs. T takes less than 24 hrs.
• We’re eager to test your samples!

TFSRF 2014 – P. Welander – welander@slac.stanford.edu
Motivation
Test bed for SRF materials
• Characterize a variety of samples, bulk and thin-film
• Magnetic quenching field measurements up to high fields
- Possibly higher than Nb’s 170-180 mT
• Quick testing cycles with small (easy to coat) samples
• Able to explore higher Tc materials (e.g. MgB2, A15s)
• Surface resistance characterization (no longer Cu-limited)
Non-superconducting materials
• RRR of copper in different forms
• Other materials, such as complex oxides

System Capabilities
• Characterize surface impedance by measuring the quality
factor, Q0, of a cavity at 11.4 GHz, down to 4 K
• Capable of low power (PNA) and high power (Klystron)
measurements
• Compact design thanks to X-band design (5.5” diameter)
• Interchangeable flat cavity bottom, fits 2” diameter samples
up to 0.25” thick.
• Cavity design maximizes H-field and minimizes E-field on
the sample surface
• Can achieve Hpeak ~ 360 mT with 50 MW Klystron running
1.6 μs flat pulses and Qe ~ 3.2e5, Q0 ~ 3.2e5
• New Nb-coated cavity designed for Qe ~ 3.2e7, Q0 ~ 3.2e7

Cryomech Pulse-Tube Cryocooler
Our cavity cryostat utilizes a
Cryomech cryorefrigerator.
• Two-stage pulse-tube operation
• Base temperature of 2.8 K with
cooling power of 1.5 W at 4.2 K
In our system, the practical base
temperature is about 3.6 K.
One shortcoming of this model is
the motor vibrations  large
fluctuations in resonant peak
• Solved in new cryostat by using
remote motor version
6

Cu Cavity Design
High-Q hemispheric cavity
under a TE013-like mode
• Zero E-field on sample
• Maximize H-field on the sample,
Hpeak on bottom is 2.5 times of
peak on dome
• Maximize loss on the sample,
36% of cavity total
• No radial current on bottom
Copper cavity body
• No temperature transition or
quenching
• Higher surface impedance
• Coupling sensitive to iris radius
HFSS modeling of high-Q Cu
cavity under TE013-like mode
H E
Sample
R = 0.95”

Cu Cavity Design
HFSS modeling of high-Q Cu
cavity under TE013-like mode
H E
훼퐶푢 = 퐺푡표푡푎푙 /퐺퐶푢 = 0.655
Sample
R = 0.95”
f0 = 11.40 GHz
Qtotal = 1.60e5
Gtotal = 1289 Ω
GCu = 1967 Ω
Gsample = 3742 Ω
1
푄0
=
푅푡표푡푎푙
퐺푡표푡푎푙
=
(훼퐶푢푅퐶푢 + 훼푠푎푚푝푙푒푅푠푎푚푝푙푒)
퐺푡표푡푎푙
훼푠푎푚푝푙푒 = 퐺푡표푡푎푙 /퐺푠푎푚푝푙푒 = 0.345

Cavity Cryostat Assembly – Model View
9
Cryocooler
2nd Stage
Sample
Under
Test
RF Feed
40 K Shield
Diode
Temp
Sensors
Sample
Plate
Cavity Iris

Sample
Plate Cavity Iris
40 K Shield
Cavity Assembly
Sample
Under
Test
RF Feed

System Overview
Measurement ports:
Forward Power: 2 or 5
Reflected power: 4 or 3
Waveform measured by either a Peak
Power Meter or a scope with mixers
Low-power PNA measurement: 6, 7, or 3
Cryostat
Waveguide to
Klystron/NWA
1
2
3
4
Cavity
Klystron
10dB
5
45dB 45dB
6
55dB
7
Cryostat
Mode
converter Bend
Load
System Diagram

Gradual Quenching Theory

Measurement Results: Bulk Cu
SLAC Cu sample
Low power test result
Q0
2.5 105
2 105
1.5 105
1 105
5 104
0 50 100 150 200 250 300
Q0
Temperature(K)
This Cu reference sample
is used to estimate the
surface impedance, Rs, of
the cavity body. It uses
similar material as the
body and underwent the
same annealing process.

Measurement Results: Bulk Nb, low power test
0.014
0.012
0.01
0.008
0.006
0.004
0.002
0
0 5 10 15 20 25 30
FNAL Nb S15-1
measured Q
0
Before baking, w/o shielding
Before baking, w/ shielding
After baking, w/ shielding
FNAL bulk large grain Nb sample
3.5 105
3 105
2.5 105
2 105
1.5 105
1 105
5 104
• Sample surface impedance is estimated from the measured Q0 of the
cavity with Nb sample and the measured copper surface impedance.
• Without magnetic shielding, Rs is high. After adding a magnetic shielding
and 800 °C vacuum bake, surface impedance reduced by a factor of 3.
FNAL-Nb S15-1
Estimated Rs
W/o shielding, before baking
W/ shielding, before baking
W/ shielding, after baking
Rs (Ohm)
Temperature(K)
0
0 20 40 60 80 100
Q0
Temperature (K)

Measurement Results: Bulk Nb, high power test
FNAL bulk large grain Nb sample
• The residual resistivity causes
pulse heating and degrades the
quenching field.
• Without magnetic shielding and
baking, the sample quench onset
is ~ 65 mT and temp rises ~ 5 K.
• After shielding and baking,
quenching onset is ~120 mT and
1.8 105
1.7 105
1.6 105
1.5 105
1.4 105
1.3 105
FNAL Nb S15-1
Q vs H, T=3K
w/ and w/o shielding/baking
Ql 04012010, no shielding, no baking
Ql 09216010 with shielding, after baking
1.1 10 temperature rises only ~ 3 K. 5
1.2 105
0 20 40 60 80 100 120 140 160
loaded
Q
Hpeak (mT)

Measurement results: 300nm MgB2 on Sapphire
300nm MgB2 thinfilm on Sapphire
H=10mT vs low power
Q0, H=10mT
Q0, network analyzer
4 105
3.5 105
3 105
2.5 105
2 105
1.5 105
1 105
MgB2 thinfilm on Sapphire
Q0
4 105
3.5 105
3 105
2.5 105
2 105
1.5 105
1 105
5 104
0
Q vs T
0 5 10 15 20 25 30 35 40
Q0
Temperature(K)
5 104
QvsH
T=3K, 04082010
10 15 20 25 30
Q0
Hpeak (mT)
300 nm MgB2 thin film on sapphire substrate,
provided by LANL and deposited at STI.

Measurement results: MgB2/Al2O3/Nb
3.5 105
3 105
2.5 105
2 105
1.5 105
1 105
Q vs H
MgB
2
/Al
2
O
3
/Nb
T=3K, June 11, 2010
Q0
Q vs T for MgB
2
/Al
2
O
3
/Nb
Low power test(NWA) vs high power test(12mT)
Q0(NWA, 06112010)
Q0(NWA, 06042010)
Q0(H=12mT, 06102010)
Q0(H=12mT, 06112010)
200 nm MgB2/300 nm Al2O3/Nb sample provided by
LANL, Al2O3 coated at ANL, MgB2 coated at STI.
1.6 105
1.4 105
1.2 105
1 105
8 104
Qloaded
10 20 30 40 50 60 70
Q0
Qloaded
Hpeak (mT)
3.5 105
3 105
2.5 105
2 105
1.5 105
1 105
5 104
0 10 20 30 40 50
Q0
Temperature(K)

New Cryostat for Cavity Testing
• Recently completed assembly of a 2nd
cryostat dedicated to cavity testing.
• Improvements on old design:
 Remote-motor cryocooler – to reduce
cavity vibrations and fluctuations in
resonant frequency.
 Increased pumping – to improve
cryostat base pressure (1e-9 torr vs.
1e-6 torr prior).
 Improved thermal isolation – to
increase 4 K cooling power reserved
for cavity dissipation.

Nb-Coated Cavity Design
Hemisphere Surface Sample Surface
훼푁푏 = 퐺푡표푡푎푙 /퐺푁푏 = 0.672
f0 = 11.33 GHz
Qtotal = 1.59e7
Gtotal = 1389 Ω
GNb = 2067 Ω
Gsample = 4233 Ω
1
푄0
=
푅푡표푡푎푙
퐺푡표푡푎푙
=
(훼푁푏푅푁푏 + 훼푠푎푚푝푙푒푅푠푎푚푝푙푒 )
퐺푡표푡푎푙
훼푠푎푚푝푙푒 = 퐺푡표푡푎푙 /퐺푠푎푚푝푙푒 = 0.328

Two Cavities
20
Coated w/ 5 μm Nb film at CERN (S. Calatroni)

Single-Crystal Nb in Both Cavities
• Single-crystal bulk Nb from
DESY
- Received January 2008
- Baked in 2010, untreated since
• Cavity comparison shown
- 2010 measurement in Cu cavity
after baking.
- 2014 measurement in Nb cavity
after solvent cleaning.
• In Cu cavity, low-temperature
Q0 is limited by Cu surface
resistance.
S001-C1 in Both Cavities
Nb Cavity
Cu Cavity
21
10
5
10
6
10
7
10
8
Q0
4 6 8 10 12 14
Temperature (K)

Single-Crystal Nb in Nb Cavity
S001-C1 in Nb Cavity
6
4
6
4
6
4
0.1
2
1
2
10
2
Rs (m
4 5 6 7 8 9
10
2 3
Temperature (K)
• Single-crystal bulk Nb from
DESY
- Received January 2008
- Baked in 2010, untreated since
• At 4.1 K and 11.42 GHz, Rs =
48 μΩ
- Assumes Rs,sample = Rs,cavity
- Standard deviation of 1%
- Assuming f 2 and (T/Tc)4
dependence,  Rs = 35 nΩ at
2.0 K and 1.3 GHz
22

Thin Film Deposition
• Collaborative effort with Mac Beasley
(Applied Physics) & Bruce Clemens
(Materials Science) at Stanford
• 3-target sputtering system being
utilized for deposition on 2” wafers.
• Elemental targets and nitrogen gas
used for reactive DC sputtering of
nitride films.
• Recently started thin-film growth
development with Nb films, followed
by NbN and NbTiN.
Characterize materials using analysis
tools at the Stanford Nano Center
(e.g. XRD, XPS, TEM)

Cryogenic DC Characterization
• Utilizing cryogenic DC transport measurements to
optimize growth for maximum critical temp, Tc.
• Use Design of Experiments to model process
outcome (Tc) versus controllable process
parameters (e.g. gas flow, pressure, gun power)
14
12
10
8
6
Modeled Tc (K)
Model Variables:
- N2 Flow
- Pressure
- Gun Power
2
= 0.341
6 8 10 12 14
Measured Tc (K)

Quantum
Design
PPMS

Summary
• Cryogenic RF testing underway again at SLAC
• New cryostat dedicated to SRF materials evaluation
• Working toward NbN and NbTiN film growth
• Two cavities (Cu & Nb) for sample characterization
- Hquench up to 360 mT, Rs with sub-nΩ resolution
• Low-power Q vs. T takes less than 24 hrs.
• Send us your samples!

Paul welander cryogenic rf characterization of sc materials at slac with cu and nb-coated cavities

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