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

2MOrC-05 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

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

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 HAZ HAZ Weld Weld CW + weld + 12 hrs @ 600 °C CW + weld + 2 hrs @ 800 °C 1 cm x 1 cm 1 cm x 1 cm 3 Lance Cooley et al., CEC-ICMC 2013, 2MOrC-05

Verification of dislocation clustering? EP + 800°C 3h A B ½ cell A B cut-out C D F E C D After 100 µm EP Sample E 2.8 mm (all pix) E F RF side (all pictures) A. Dzyuba 4 Lance Cooley et al., CEC-ICMC 2013, 2MOrC-05

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

Vertical Test after BULK EP ONLY • NO VACUUM ANNEAL; yes - 120°C / 48 hr vac. bake 1 E+11 TE1AES012 TE1AES013 TE1AES014 Quality Factor 1 E+10 Q-disease test passed for #12, #14 Q-disease test failed for #13, Fixed after standard de-gassing anneal (data shown here) 1 E+09 0 5 10 15 20 25 30 35 40 45 50 Accelerating Gradient, MV/m 6 Lance Cooley et al., CEC-ICMC 2013, 2MOrC-05

Meanwhile… Study of hydrogen uptake • Experiment: Annealed niobium bars were strained in our 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 7 Lance Cooley et al., CEC-ICMC 2013, 2MOrC-05

Resistivity at 11 K after strain + EP 8 Lance Cooley et al., CEC-ICMC 2013, 2MOrC-05

Strain – EP – Hydrogen absorption surface Threshold for accelerated hydrogen uptake Annealed pieces resist hydrogen uptake Isagawa 1980 J. Appl. Phys . 51 , 4166 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? 11 Lance Cooley et al., CEC-ICMC 2013, 2MOrC-05