An Approach to Chemical Free Surface Processing for High Gradient Superconducting

RF Cavities

Frederick Mako1, Ph.D., Bing Xiao1, Ph.D. and

Larry Phillips2, Ph.D., William Clemens2

1FM Technologies, Inc., Chantilly, VA, USA, 2Thomas Jefferson Laboratory, Newport News, VA, USA

Work Supported by US DOE under SBIR

Seven-Cell Nb SRF Cavity at Thomas Jefferson National Accelerator Facility.

Pictures Show Typical Defects Occurring Inside The Nb SRF Cavity Cells Around The Equator

EBW Overlaps And Remaining After The Chemical Treatment

Outstanding irregularity (step) near equatorEBW overlap of cell#7 from waveguide.

Many “bubbles” sporadically present inside the weld.

Two other cells have less pronounced feature.Four other cells have no recognizable feature.

Many apparent “deep pits” in heat affected zone.

Electron beam weld

1 mm

Edge of electron beam weld

1 mm

1 mm

1 mm

step

Study of Beam Processing for Cavities

Objectives:• Achieve a Smooth Surface with Minimal

Defects and Impurities• Achieve a Low Strain Surface to Reduce

Corrosion• Final Goal is to Attain Reproducible High Q

(>1010) and High Field (40MV/m) Cavities

Electron Beam Melted Nb Samples Using J-lab SCIAKY Welder

25 mm

Each single pass melt region is about 6 mm x 74 mm x 0.1-0.2mm deep. A 10 kHz circular to elliptical raster with 0.5 mm diameter 50 keV beam was used.

Beam current and translation rates varied from 20-250mA and 5-20 inches/minute.

28 plates of Nb with dimensions 3 mm thick x 25.4 mm wide x 88.9 mm long.

Magnification of Melt Zone

Melted Region

Un-Melted Region

HIROX digital microscope view of sample #6.

The bottom half of the image shows the smooth melted region that highlights the grain size of about 300-400 µm, while the upper half of the image shows the rough un-melted small grain region.

Grain Reference Orientation Deviation (GROD) Map for Nb Flat Sample #7

Both samples #6 and #7 show GROD in the range of 0 – 3° over a distance of 100 – 300 μm.

40mA and 10in. /min.

No chemical etching has occurred.

Grain Reference Orientation Deviation (GROD) Map for Nb Flat Sample #15

Samples #14 and #15 show less lattice distortion compared to samples #6 and #7 as measured by GROD from 0 – 2° with the majority of the distortion less than 1°over 100 – 300 μm.

40mA and 18in./min.

No chemical etching has occurred.

Grain Reference Orientation Deviation (GROD) Map for Nb Pipe Sample #16

The pipe sample #16 that utilized a double low current (24 mA & 18in./min.) e-beam pass also shows compared to samples #6 and #7with very little lattice distortion as indicated by its GROD with the majority of the distortion less than 1° over 100 – 300 μm.

No chemical etching has occurred.

Grain Reference Orientation Deviation (GROD) Map for Deep Drawn Half Cell

Courtesy of Dr. Roy Crooks of Black Laboratories

The section is taken from 3 mm from the equator.

The GROD map shows extreme lattice distortion up to an angular rotation of about 20 degrees.

No chemical etching has occurred.

Comparison of Grain Deviation for E-Beam Processed Nb vs. Deep Drawn Nb Cell

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 200

10

20

30

40

50

60

70

80

90

100

Deep Drawn

Sample 6

Sample 7

Sample 14

Sample 15

Double Pass

Maximum Grain Orientation Deviation Angle (deg.)

Frac

tion

of G

rain

s at

Max

imum

Dev

iatio

n A

ngle

(%)

Comparison of Grain Deviation for Various E-Beam Processed Nb Samples

1 2 30

10

20

30

40

50

60

70

80

90

100

Sample 6

Sample 7

Sample 14

Sample 15

Double Pass

Maximum Grain Orientation Deviation Angle (deg.)

Frac

tion

of G

rain

s at

Max

imum

Dev

iatio

n A

ngle

(%)

HIROX Imaging for Sample #15

Sample #15 shows very smooth HIROX and AFM images in the melt zone.

AFM of Sample #15

AFM RMS value of less than 3 nm, which is 1/20 of what can be accomplished by electro-polishing.

Magnification of Melt Zone

Melted Region

Un-Melted Region

Grain Boundary Step

HIROX digital microscope view of sample #6.

Grain boundary steps show up in higher heating rate samples.

AFM of Sample #6 at Grain Boundary

Chemical Free Half-Cell Processed in the J-lab E-beam SCIAKY Welder

Electron Gun

Half Cell

Finished E-Beam Processed Half-Cell

The beam parameters were: 40 mA, 0.5mm diameter beam, travelling at 18 inches per minute, the melting diameter is about 6 mm with a circular pattern at 10 kHz.

Summary of Beam Parameters for A Smooth Low Strain Surface

• Both HIROX & AFM suggest that a smoother surface is attained with lower heating rates

• EBSD (GROD) maps suggest a lower strain surface is attained at lower heating rates

• Desired Beam Parameters are: 50keV, 40mA, 46cm/min. in single pass or 50keV, 24mA,46cm/min. in double pass

Electron Gun and Beam Transport Design

• Two Strategies: Ballistic Focusing and Magnetized Beam

CRITERIA:– Can process from iris to equator and the circumference– Prevent Nb vapor arcing– Can tolerate beam induced thermal radiation & filament

heat load– Beam Parameters ~50kV, up to 200mA, spot ~1mm– Focal Length 30-100cm.

Ballistic Focusing Gun-For In or Out of Cavity Processing

High Voltage Insulator

Grid Provides 1st Focus, V=-50 to -51kV, Current Not Effected

Anode, V=0

Cathode & Filament,

V=-50kV

Magnetic & Radiative Heat Shield, Water Cooled,V=0

Dipole w Soft Iron Case provides 2nd Focus, V=0

Quadrapole Provides Rastering, Circle or Ellipse, V=0

46mm

Diameter

Ballistic Focusing Gun-Electron Beam Trajectory

Helmholtz Coils Provide R-Z Beam Scanning from Iris to Equator.

q Scanning Provided by Rotating either Gun or Cavity or Helmholtz coils on Axis.

Magnetized E-Beam Gun

Simple Gun Design Only Needs Cathode, Grid & Anode

Magnetized E-Beam Gun-Electron Beam Trajectory

Beam location is dependent on bucking coil position.

50 keV, 50 mA, 3 mm diameter.

Also, beam position can be adjusted by changing the bucking coil current.

Summary and Conclusions

• Beam Parameters Have Been Determined to Give a Smooth Low Strain Surface using a Conventional Rastered Beam

• Two Gun Designs Examined (Ballistic & Magnetized Beams) to Meet the Above Beam Conditions With & Without Rastering for Both Internal & External Gun Operation

• Maintaining a Constant Beam Power Density on the Cavity Surface Will Require Computer Controls

• Further Sample Studies are Required Before Gun Selection