Important points to take away Excellent Q and gradient were obtained - - PowerPoint PPT Presentation

Single-Pass SRF Cavity Processing?

Lance Cooley, Anna Grassellino, A. Curtis Crawford, Alexander Dzyuba, Damon Bice, Allan Rowe Technical Division, Fermilab Thanks also to: Mark Champion (ORNL) and John Rathke (Advanced Energy Systems) Supported by DOE-HEP General Accelerator R&D

2MOrC-05

Important points to take away

• Excellent Q and gradient were obtained for only a

single polishing step and without 600-800 °C vacuum annealing

• Key step: Half cells were annealed as a batch prior to welding
• r chemical polishing
• When Q-disease was encountered, a vacuum anneal (see

Grassellino talk) was effective without need for a subsequent chemical polish

• Materials R&D may explain why
• A threshold of cold work + polishing time defines a point

where hydrogen absorption becomes accelerated

• Annealed metal is below this threshold – NO de-gassing
• Proposal: weld  anneal  light EP
• A very simple, low-cost cavity process?
• Materials R&D should tweak the anneal to balance strength,

recovery of “damage”, and EP requirement

Lance Cooley et al., CEC-ICMC 2013, 2MOrC-05 2

Flashback to 2011

• “Annealing to mitigate pitting in SRF cavities”
• Selected for best structural materials paper!
• Lesson: annealing helps to reduce pits, but does not

eliminate them entirely

Lance Cooley et al., CEC-ICMC 2013, 2MOrC-05 3

CW + weld + 2 hrs @ 800 °C 1 cm x 1 cm

Weld HAZ

CW + weld + 12 hrs @ 600 °C 1 cm x 1 cm

Weld HAZ

Verification of dislocation clustering?

Lance Cooley et al., CEC-ICMC 2013, 2MOrC-05 4

½ cell cut-out

A B F C E RF side (all pictures) 2.8 mm (all pix)

After 100 µm EP

D

EP + 800°C 3h

A B F C E D

• A. Dzyuba

Sample E

Cavity fabrication details

• 10 single-cell cavities were commissioned from AES in

August 2011

• 1 cavity was large grain material
• Special treatment: After deep-drawing half cells, AES

shipped them to Fermilab for annealing in our cavity furnace used for de-gassing

• A titanium alloy rack was constructed to support the half cells
• Batch heat treatment, 800 °C, 2 hours
• Cleaning procedures were applied before annealing
• The annealed half cells were welded and finished

cavities delivered to Fermilab in 2012

• Pre-weld typically requires a 25 µm etch, done at 12 °C

Lance Cooley et al., CEC-ICMC 2013, 2MOrC-05 5

Vertical Test after BULK EP ONLY

• NO VACUUM ANNEAL; yes - 120°C / 48 hr vac. bake

Lance Cooley et al., CEC-ICMC 2013, 2MOrC-05 6

1 E+09 1 E+10 1 E+11 5 10 15 20 25 30 35 40 45 50

Quality Factor

Accelerating Gradient, MV/m

TE1AES012 TE1AES013 TE1AES014

Q-disease test passed for #12, #14 Q-disease test failed for #13, Fixed after standard de-gassing anneal (data shown here)

Meanwhile… Study of hydrogen uptake

• Experiment: Annealed niobium bars were strained in
• ur tensile tester, 12%, 27%, 35%, and 50% elongation
• Control and strained bars were electropolished, 50 to

250 µm material removed

• Standard temperature, voltage, etc. – see our earlier coupon

EP literature

• Resistivity measurements were used to infer hydrogen

content

• R (11K) was especially sensitive to hydrogen uptake, on top of

the contribution from cold work.

• Vacuum annealing at 800 °C for 2 hours degassed much

hydrogen as well as recovered cold work

• Resistance jumps were observed as temperature was swept

through the range associated with hydride precipitate formation

 Jumps were absent after the vacuum anneal

Lance Cooley et al., CEC-ICMC 2013, 2MOrC-05 7

Resistivity at 11 K after strain + EP

8 Lance Cooley et al., CEC-ICMC 2013, 2MOrC-05

Strain – EP – Hydrogen absorption surface

Isagawa 1980 J. Appl.

• Phys. 51, 4166

Threshold for accelerated hydrogen uptake Annealed pieces resist hydrogen uptake

9 Lance Cooley et al., CEC-ICMC 2013, 2MOrC-05

Resistivity reveals hydride formation

10 Lance Cooley et al., CEC-ICMC 2013, 2MOrC-05

Conclusion – Anneal first, EP only once

• i.e. INVERT the standard sequence…
• The success of the annealed single-cell cavity

experiments is explained by the material R&D study

• Annealed niobium does not take up hydrogen as quickly as

cold-worked niobium

• EP was also conducted with external water cooling
• Evidently, annealing to mitigate pitting also is annealing to

mitigate hydrogen uptake – No need for post EP de-gassing

• Annealing should reduce the material “damage layer”
• “Damage layer” consists of various structural and chemical

defects that are introduced by rolling, forming, machining

 We have never found clear evidence for this layer  Many past cavity results show improved properties with

increased material removal

• How much chemical polishing is then needed if the annealing

produces a full recrystallization? Will a “light EP” suffice?

Lance Cooley et al., CEC-ICMC 2013, 2MOrC-05 11