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Investigation of Microscopic Materials Limitations of Superconducting RF Cavities

Steven M. Anlage

Department of PhysicsCenter for Nanophysics and Advanced MaterialsUniversity of Maryland29 October, 2008

SRF Materials Workshop

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Questions to Address

What new experiments shed light on limits played by topology, surface phasesand structures, chemical makeup, or physics?

How can we separate intrinsic behavior from that which is dependent on history orprocessing pathway?

Are there new studies … telling us about the intrinsic qualities of the niobium surface?

Do we still need to focus on heat transfer effects?

My Answers:

New Microscopic Techniques to link RF Properties to Local Structure

1) Near-Field Microwave Microscope

2) Laser Scanning Microscope

Use these to establish connectionsbetween surface structure and RFperformance on a microscopic scale

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Near-Field Microwave Microscope

1Create Strong ( BRF ~ 200 mT ) Highly Localized ( < 1 m ) RF ( 1 - 2 GHz )

Magnetic Fields on Nb Surfaces at Low Temperatures ( < 2 K )

2 Measure Local Response:Nonlinearity (Harmonics, Intermodulation)Sensitive to RF breakdown, flux generation, …

3 Correlate Local RF Properties to:TopographyDefects (welds, grain boundaries, …)Surface TreatmentProcessing

Microwave Microscope Probe

Intense BRF

Drive

Nb SampleLocalizedExcitationon Surface

Fundamental Response

Harmonic / Intermod Response

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-2 .0 -1 .5 -1 .0 -0 .5 0 .0 0 .5 1 .0 1 .5 2 .0-2 .5

-2 .0

-1 .5

-1 .0

-0 .5

0 .0

0 .5

X ( m m )Y

(m

m)

- 1 0 5 .0- 1 0 0 .0- 9 5 .0 0- 9 0 .0 0- 8 5 .0 0- 8 0 .0 0- 7 5 .0 0- 7 0 .0 0- 6 5 .0 0- 6 0 .0 0- 5 5 .0 0- 5 0 .0 0

GB

-2 .0 -1 .5 -1 .0 -0 .5 0 .0 0 .5 1 .0 1 .5 2 .0-2 .5

-2 .0

-1 .5

-1 .0

-0 .5

0 .0

0 .5

X ( m m )Y

(m

m)

- 1 0 5 .0- 1 0 0 .0- 9 5 .0 0- 9 0 .0 0- 8 5 .0 0- 8 0 .0 0- 7 5 .0 0- 7 0 .0 0- 6 5 .0 0- 6 0 .0 0- 5 5 .0 0- 5 0 .0 0

GB Third harmonicpower (dBm)

Near-Field Microwave MicroscopeExample Result

Localized Harmonic Generation from a single bi-crystal grain boundaryin a high-Tc (YBa2Cu3O7-) thin film

200m loop probe500Å

YBCO

STO

J

30° misorientation Bi-crystal grain boundary

P = +8 dBm

X

FundamentalTone In

HarmonicTones Out

<< Josephson junctions are strongly nonlinear >>

Superconductors have an intrinsic nonlinearity, clearly visible as J → Jc

The superfluid density is suppressed and very sensitive to perturbations

2

1)0,(),(c

ss J

JTJT

s becomes time-dependent, giving rise to harmonics and intermods

Phys. Rev. B 72, 024527 (2005)

fin = 6.5 GHzT = 60 K

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Near-Field Microwave Microscope► Next Generation for SRF Applications ◄

Magnetic Write Heads createi) Strong RF magnetic fields: ~ 1 T in existing write headsii) Highly localized RF magnetic fields (|| to surface!): 100’s of nm

Schematic of a longitudinal write head

RFDrive

Superconductors show nonlinearity due to both intrinsic and extrinsic effects

Noise Level

Temperature (K)

Example ResultSeagate Longitudinal Recording Head

3rd-Harmonic Response of a Homogeneous high-Tc film

3rd-H

arm

onic

Pow

er P

3f (

dBm

)

fin = 6.4 GHz

Goals: Achieve ~200 mT surface RF fields on Nb at microwave frequencies at 2 KImage RF breakdown fields and correlate with surface properties…

Tc

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Laser Scanning Microscope

1 Create a Microwave ( ~ GHz ) Resonance at Low Temperatures ( < 2 K )

2 Perturb the Surface with a Modulated Laser Spot to cause Local Heating

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Image:JRF(x, y) Local RF Current DensityLocal sources of Nonlinear ResponseRF vortex Entry and FlowThermal Healing Length

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Measure the change in f0 and Q as the laser spot is scanned over the surface

“Short-Sample” RF / Materials Science of Nb Surfaces

Co-planarWaveguideResonator

f0 , Q

MicrowaveInput

Ground Plane

Material of interest

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1 x 8 mm scan

Grain Boundaries

LAO

YBCO

Large Grain position

Laser Scanning Microscope“Short-Sample” RF / Materials Science of Nb Surfaces

Reflectivity Image

RF Image

100 m x 200 m

IRF

BRF

RF contrast developed from grain bdes,cracks, scratches, etch features, corners,etc.Low Temperature Physics 32, 592 (2006)

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Nb foil(160 m thickness)

RF input

Nb foil(160 m thickness)

RF input

Copper finger

Sapphire rod

Sapphire disc

RFIN

RFOUT

ground

Nb stripScanned area Pb

Nbsapp

hire

reflectivity

inductive resistive

1 m

m

Laser Scanning MicroscopePreliminary Results on Bulk Nb Surfaces

Nb foil(160 m thickness)

RF input

Nb foil(160 m thickness)

RF inputPb

Nbsapp

hire

reflectivity

inductive resistive

1 m

m

NbNb

Pb Pb

defect defect

LineScan

PbNb

7.2 K

9.2 K

5.6 K

10.6 KT

empe

ratu

re

1 mm

Line scan

LSM PR

0

+ peak

- peak

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Determination of the Thermal Healing Length

T = 79 KP = - 10 dBmf = 5.285 GHzfmod = 99.9 kHz

YBCO/LaAlO3

CPW Resonator

1 x 8 mm scan

Wstrip = 500 m

ModmThermal fc

k

k Thermal Conductivity

c Specific heat

m Mass Density

Modf Modulation Freq.

Intensity modulatedfocused laser beam

Bulk Metal

Coatingheatsource

x

z

Intensity modulatedfocused laser beam

Bulk Metal

Coatingheatsource

x

z

Fit gives

m 4 Thermal

J. Supercond. 19, 625 (2006)

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Conclusions

A proposal to DOE/HEP on SRF Materials Issues is in preparation

I am looking for materials collaborators who need to solve specificmaterials / low-T RF property problems and think that amicroscopic approach is fruitful

Some topics of interest:RF properties of grain boundaries and step edgesRF properties of etch pitsUnderstanding the microscopic physics of intrinsic RF breakdownCan coatings (S / I / S /…, or novel superconductor) prolong RF breakdown?

I believe the Near-Field and Scanning Laser Microwave Microscopes can helpto solve vexing SRF materials problems

http://www.cnam.umd.edu/anlage/AnlageHome.htm

Existing Collaborators:Dragos Mircea [Seagate → Hitachi]Alexander Zhuravel [Kharkov, Ukraine]Alexey Ustinov [Karlsruhe, Germany]

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Agilent E8362MicrowaveSynthesizer

LakeShore340Temperature Controller

Low-Pass Filters

High-Pass Filters

MicrowaveAmplifiers

Gain ~ 60dB

Directional Coupler (-6dB)

sampleWriter

Cryogenicchamber

Agilent E4407BSpectrum Analyzer

f

f, 2f, 3f,..

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f

f, 2f, 3f,…

f

MW source

Low pass filter

Directional coupler

High pass filter

sample

Cryogenic environment

2f, 3f,…

Spectrum Analyzer

Amp

f

3f

f, 2f, 3f,…

f

Port 1 Ref Out Ref In Port 2

VNA in FOM

Comb.

Gen.

f, 2f, 3f, …

Band pass filter

Low pass filter

Directional coupler

High pass filter

sample

Cryogenic environment

2f, 3f,…

13 www.answers.com/topic/read-write-head

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New Approach: inductive writer from a HDD

B ~ 1 T !!!

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Pinput

loop

coaxial probe

sample surface

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The inductive reader/writer : general concept

Attractive features :Attractive features :

• ~ T magnetic field~ T magnetic field

• sub-micron pole tipssub-micron pole tips

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An inductive magnetic writer/reader : Example

Pins for reader/writerPins for reader/writer

magnetic disk / superconducting samplemagnetic disk / superconducting sample

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An inductive magnetic writer/reader : Detail

writerwriter

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An inductive magnetic writer/reader : Detail

microcoilsmicrocoils

high-high- magnetic magnetic corecore

wiring of the wiring of the microcoils to the microcoils to the

pinspins

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An inductive magnetic writer/reader : Schematics

R. Hsiao R. Hsiao IBM J. Res. Develop.IBM J. Res. Develop., vol. 43, no.1/2, , vol. 43, no.1/2, Jan/March 1999Jan/March 1999

SAMPLESAMPLE

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2 2(2 / )'2

( , , ) (1 )L Mr r i tzLL

L

PP r z t e e e

Laser-induced signal generation model

The power distribution induced by a focused modulated laser beam can be described as:

temporalspatial

x-y z t

focused laser beam(lLAS = 670 nm, PL = 1 mW)

substrate

HTS film

d

heatsource

x

z

The thermally induced changes of S21(f) in the probe are understood as LSM photo-response (PR) that can be expressed as:

2 2 2 22 12 12 120 12

12 20 12

( ) ( ) ( )1 (1/ 2 )( )

2 (1/ 2 )

S f S f S ff SQPR S f T

f T Q T TS

inductive PR + resistive PR + insertion loss PR 2

2 1212 2 2

0

( )1 4 ( / 1)

SS f

Q f f

where

A.P. Zhuravel, S. M. Anlage and A.V. Ustinov

~2121 21

21

2121

21

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Scanning Laser Microscopy of Superconducting Microwave Devices

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Results: Power dependence of PRR(x,y)

LAO

YBCO10 mm

0 dBm

+6 dBm+4 dBm

+2 dBmLAO

Images of resistive LSM PR penetrating into HTS film (area B) at the different input HF power indicated in the images. White dotted boxes show the YBCO/LAO patterned edge. Brighter regions correspond to larger amplitude of PRR(x,y).

3D plot of resistive LSM PR at +6 dBm

LAO

YBCO

PRR(x,y)

A.P. Zhuravel, S. M. Anlage and A.V. Ustinov Scanning Laser Microscopy of Superconducting Microwave Devices

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Microwave Microscope Probe

Intense BRF

Drive

Nb SampleLocalizedExcitationon Surface

Fundamental Response

Harmonic / Intermod Response

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- YBCO film- LAO substrate

1 mm

1x1 mm

XY

XY

JRF

0

max

Frequency

Pow

er [

dBm

]

f1 f2

- 42

-14 -14

- 43

2f1 -f2 2f2 –f1

- 43- 42

- 49- 55

1x1 mm

IMD PRJrf

x

y

1x1 mm

0 max

(a) (b) (c)

RFIN

RFOUT

XY

JRF

XY

JIMD

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TM010 Resonant ModeCurrent Distribution

FiducialMaterials

Nb Strip withTwo differentTreatment

26

27

Pb

Nbsapp

hire

reflectivity

inductive resistive

1 m

m

NbNb

Pb Pb

Sapp

hire

Sapp

hire

defect defect

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Pb

Nb

reflectivity

inductive resistive

1 m

m

NbNb

Pb Pb

defect defect

29

reflectivity RF = -3dBm

Nb stripGap1 mm

F1

30

LineScan

PbNb

7.2 K

9.2 K

5.6 K

10.6 K

Tem

pera

ture

1 mm

Line scan

LSM PR

0

+ peak

- peak

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Nb foil(160 m thickness)

RF input

Pb foil(50 m) Pb foil(50 m)

Areascan

Linescan

Nb foil(160 m thickness)

RF input

Pb foil(50 m) Pb foil(50 m)

Areascan

LinescanPb foil(50 m) Pb foil(50 m)

Areascan

Linescan

Nb foil(160 m thickness)

RF input

Nb foil(160 m thickness)

RF input