1. ANL Proposal to Perform Electropolishing for the ILC
6/24/2018
ANL Proposal to Perform Electropolishing for the ILC
Speaker: Mike Kelly
December 5-7, 2005
Test

2. Joint ANL/FNAL Cavity Processing Facility

3. Electropolishing at Argonne
Argonne has electropolished ~150 SC niobium cavities
Cavity performance at ANL using EP is directly relevant to ILC
Experience with various geometries is directly applicable to ILC
Major infrastructure for EP of large SC cavities is in place

4. Cavity Processing at ANL Cavity Performance
Outline
Cavity Processing at ANL
Cavity Performance
An ANL Proposal for the ILC
Technical Issues with 9-cell EP

5. I. Cavity Processing: High Performance SC Structures for RIA
6/24/2018
I. Cavity Processing: High Performance SC Structures for RIA
345 MHz =0.40 Double-spoke
115 MHz =0.15
Steering-Corrected QWR
172.5 MHz =0.26 HWR
345 MHz = Triple-spoke
57.5 MHz QWR-based structures 0.03<  <0.14
Recent work focused on set of drift-tube cavities for RIA
Quarter-, half-, multispoke cavities.
Similar to elliptical cell structures- large cavities, high RRR niobium, geometry optimized in 3D to minimize B field
Large extended region of high magnetic field
We are used to thinking about processing of new shapes
345 MHz =0.5 Triple-spoke
1 m
Test

6. I. Cavity Processing: High-Pressure Water Rinsing
6/24/2018
I. Cavity Processing: High-Pressure Water Rinsing
Horizontal HPR Development
Manual HPR
Full cavity processing capability
In close touch with developments at DESY, KEK and JLab
Optimize flow patterns for relatively complicated spoke geometry
Automated HPR: RIA Triple-Spoke
Automated HPR: RIA Half-wave
Test

7. I. Cavity Processing: Clean Room Assembly
Goal: Minimize or eliminate field emission from particulates using a simple, practical and effective curtained clean area
Coupler & pumping lines mounted in class 100 (or better) area
Simplicity is crucial; cavity connections ideally made in seconds
Work performed below & downstream from open cavity
Hardware must be compatible with clean conditions
Half-wave for RIA

8. Cavity Processing: Clean room assembly

9. I. Cavity Processing: ANL Recipe for EP with TEM Cavities
Electropolishing has been a mainstay at ANL
Based on the Siemens process
Cavity (anode) - observed no polishing rate sensitivity to electrical connection point even for large cavities
High purity Al cathode (3003 series) tailored to the cavity
Acid composition 85:10 mixture of 96% H2SO4, 40% HF, reagent grade
Temperature 28-32o C (chilled water through the cathode)
Average anode current density ~40 mA/cm2, acid replaced when value drops below 30 mA/cm2

10. I. Cavity Processing: Niobium Geometries Electropolished at ANL
6/24/2018
I. Cavity Processing: Niobium Geometries Electropolished at ANL
Quarter-wave
We believe that electropolishing is a key step in our fabrication process.
Smaller parts use same technique as for elliptical cavity half cells
Long center conductors are rotated in the electrolyte bath with the aluminum cathode around outside
Most challenging spoke geometry – large surface area housing – critical spoke region required good polishing with no stagnant areas in the electrolyte
Devised a washing machine action together with a set of impellers – dealt with issue of moving electrical connections
Co-axial half-wave
Important EP Technical Issues:
Temperature gradients and stability
Acid flow patterns
Double spoke
Test

11. I. Cavity Processing:
ANL b=0.63 Triple-Spoke Cavity, Area ~1.5 m2

12. I. Cavity Processing: High RRR & Hydrogen Q-disease
Test #2 after K
Issue: Hydrogen is introduced during fabrication
Has been shown that in high RRR cavities hydrides form preferentially at surface, grain boundaries, lattice imperfections
Data clearly indicate the presence of hydrogen Q-disease

13. I. Cavity Processing: Hydrogen Degassing
FY06 program to bake out all high RRR cavities for RIA
600 oC bake for 10 hours to degas hydrogen has been performed
10 mm chemical polish, high-pressure rinse, clean assembly
(Furnace is also suitable for 9-cell cavities)

14. II. Cavity Performance: Beta=0.63 Multi-Spoke Cavities
6/24/2018
II. Cavity Performance: Beta=0.63 Multi-Spoke Cavities
Q-disease was observed; hydrogen degassing at 600 oC was performed at ANL
2 K surface resistance decreased substantially after 600 0C bake.
No X-rays
T = 4.2 K (unchanged after bake)
Test results for the three multi-spoke cavities developed for RIA are shown here. The two spoke cavity results shown here at the left easily exceed the initial RIA design goal. Recent tests of the beta=0.5 and beta=0.63 triple spoke resonators at 4.2 K shown here in red also exceed the RIA design goal even though the development is not yet complete. In particular, the first time we looked for hydrogen Q-disease in one of the cavities by hold in the hydride formation temperature region we strong effect with the beta=0.63 cavity. Performance may yet increase after hydrogen degassing which will be performed.
Test

15. II. Cavity Performance: Residual Surface Resistance vs. BPEAK
6/24/2018
II. Cavity Performance: Residual Surface Resistance vs. BPEAK
(Best b=0.61 Cavity)
Residual RF Surface Resistance (Ohms)
Lower is better
Test results for the three multi-spoke cavities developed for RIA are shown here. The two spoke cavity results shown here at the left easily exceed the initial RIA design goal. Recent tests of the beta=0.5 and beta=0.63 triple spoke resonators at 4.2 K shown here in red also exceed the RIA design goal even though the development is not yet complete. In particular, the first time we looked for hydrogen Q-disease in one of the cavities by hold in the hydride formation temperature region we strong effect with the beta=0.63 cavity. Performance may yet increase after hydrogen degassing which will be performed.
Peak Surface Magnetic Field (Gauss)
Test

16. III. An ANL Proposal for the ILC: Electropolishing
Motivation
It has been generally agreed that a U.S.-based technical capability to build, process and operate high-performance 9-cell elliptical cavities is required for the proposed International Linear Collider
Proposal
To leverage the existing infrastructure and expertise at Argonne to build a complete electropolish apparatus for ILC-type 9-cell elliptical cavities and perform electropolishing on 9-cell cavities
Deliverables
FY06: An operational electropolishing facility for ILC 9-cell cavities
Resources Required
FY06: FTE (0.75 Scientist, 0.75 Designer/Technician); $135K M&S

17. III. An ANL Proposal for the ILC: Electropolishing
FY07 Activities
Electropolish cavities obtained through the ILC collaboration
Study technical issues and optimize operating parameters
Document procedures and train personnel
Near term plan
Generate at ANL together with Tajima (LANL) a technical plan and drawings (2 months)
Take this plan out for design review by technical experts at Fermilab, JLab, KEK and DESY
Fabricate and test the apparatus using a test cavity obtained though ILC collaboration
Pre-rinse possibly at ANL, seal cavities in the joint clean room facility. Final HPR at Fermilab

18. III. An ANL Proposal for the ILC: Capability at Argonne
Comprehensive chemical processing capability suitable for several cavities per week
Primary infrastructure:
A pair of large chemistry rooms
A large air scrubber
Three clean room areas for post-processing (high-pressure rinsing)
A large volume de-ionized water system
Procedures for procurement, storage, handling, disposal
Hardware: power supplies, acid pumps, water chiller

19. III. An ANL Proposal for the ILC: Resources for ILC EP
60 feet
EP Power Supplies
10 kW Chiller
32 l/m DI Water

20. III. An ANL Proposal for the ILC: A “Closed Loop” EP Flow Diagram
AIR/ N2 INTAKE

21. III. An ANL Proposal for the ILC: A 9-cell Cavity in the ANL Chemistry Room
16 feet
Floor Plan
16 feet
Side View

22. IV. Technical Issues: Horizontal and Vertical Polishing
Direct ANL experience indicates the following issues are important:
Orientation of the niobium surface: Horizontal downward facing Nb surfaces polish faster than horizontal upward facing surfaces.
Hydrogen bubbles: Streaming from the cathode can cause grooves, streaks in the cavity. Increasing bubble density raises resistivity of the electrolyte bath
Uneven polishing: Differential polishing in elliptical cavities is not a cathode proximity effect; however is likely due to flow rate/temperature effects

23. IV. Technical Issues: Orientation of the Niobium Surface
Horizontal-downward facing surfaces polish faster than horizontal upward surfaces
Uneven polishing will be difficult to avoid in with a 9-cell cavity oriented vertically
Polishing Rate at 3 Different Points on a Cavity
Cavity flipped after every 50 cycles
Surface facing down
Surface facing up

24. IV. Technical Issues: Differential Iris/Equator Polishing
Cathode Distance vs. Bath Potential
Relevant niobium EP information contained in the literature
Hydrogen bubbles shown to lead to an increase in the electrolyte resistivity
Reference: Diepers et al, Research Laboratories of Siemens (1971)
Data implies that the cathode distance is not critical
Horizontal elliptical cavities should polish uniformly, however not so in practice

25. IV. Technical Issues: Differential Iris/Equator Polishing
Test: Cathode distance vs. polishing rate
Impeller
Anode
Cathode
Anode – 12”x7/8”x1/8” RRR=250 Nb (Wah Chang)
Cathode – 3 Turns 3/8” 3003 Series Aluminum
Anode/Cathode voltage = 16 V
Temperature = oC
Time = 1 hour, continuous
Acid flow rate ~ 1 cm/s at the cathode
13 inches

26. IV. Technical Issues: Differential Iris/Equator Polishing
Cathode Distance vs. Polishing Rate
Sample 1
EP removal rate mm/minute
Sample 2
Distance to Cathode (cm)
NO RATE DEPENDENCE ON CATHODE PROXIMITY
NO MASKING OR COMPLITCATED SHAPES NEEDED FOR 1.3 GHz CAVITIES

27. IV. Technical Issues: The ANL Proposal Would Address…
Uneven polishing 4 flow rate effect? 4 separate flow rate from temp. stability by direct (water) cooling of cavity
The perceived need to polish without a He vessel
Temperature stability 4 use high-purity Al heat exchanger coil rather than Teflon

28. Concluding Statement
ANL is eager to contribute to the ILC collaboration in an area of strong technical expertise and capability at this laboratory, namely, electropolishing