1. ON THE HIGH GRADIENT Nb CAVITIES
A COMMON R&D
ON THE HIGH GRADIENT Nb CAVITIES
Kenji Saito
Takayuki Saeki
Yasuo Higashi
Fumio Furuta
Bernard Visentin
Fabien Eozénou
Mélanie Bruchon
16/05/2008
- Fabien Eozénou FJPPL A_RD_04

2. R&D ON HIGH GRADIENT Nb CAVITIES
A CRUCIAL
R&D ON HIGH GRADIENT Nb CAVITIES
16/05/2008
- Fabien Eozénou FJPPL A_RD_04

3. TWO MAIN ROJECTS RELATED TO THE “COLD” TECHNOLOGY
International Linear Collider
~17000 Superconductive Nb cavities > 31.5 MV/m
1 Tev
XFEL (X-Ray Free-Electron Laser)
17.5 GeV λ=0.1nm
808 cavities E=23.5 MV/m assembled in 101 cryomodules
16/05/2008
- Fabien Eozénou FJPPL A_RD_04

4. Electro-Polishing (EP) And Baking: Reminder
‘Standard’ preparation to reach high gradients:
Barrel Polishing + EP + Anneal + Final EP + HPR + Rinse + Baking
Electro-Polishing:
Internal Surface Treatment of the Cavity filled with acid in presence of Current
Standard Baking:
Cavity is baked under vacuum to remove high field Q-Slope whose origin remains mysterious
‘Standard’ parameters: 120°C: 48h
16/05/2008
- Fabien Eozénou FJPPL A_RD_04

5. Electro-Polishing (EP): More details
Process
- Nb polished internally with an electrolyte
(concentrated H2SO4 - HF mixture) under voltage
- Cavity = Anode & Cathode is an Al pipe inside the cavity
Chemical reactions
- Anode: Niobium oxidation
2 Nb + 5 H2O  Nb2O H e-
- Cathode: H2 generation
2 H+ + 2 e-  H2 (Teflon net to prevent H contamination)
- HF: dissolution of oxide Nb2O5 layer
- H2SO4: forming of a viscous layer
16/05/2008
- Fabien Eozénou FJPPL A_RD_04

6. Electro-Polishing (EP) and Baking: Main Breakthroughs
Nb Cavity Performances ( )
Continuous improvements with Heat 800 and 1400°C,
High Pressure Rinse, Baking & ElectroPolishing.
42 MV/m hard barrier (  2004 )
16/05/2008
- Fabien Eozénou FJPPL A_RD_04

7. GOALS OF THE COMMON R&D PROGRAM
Improvement of the Baking Procedure:
Facilitate the baking procedure
Decrease the baking time
Improvement of Electro-Polishing:
Obtaining of high reproducible gradients
Better understanding of the process (use of modelling)
16/05/2008
- Fabien Eozénou FJPPL A_RD_04

8. IMPROVING THE BAKING PARAMETERS
16/05/2008
- Fabien Eozénou FJPPL A_RD_04

9. FAST ARGON BAKING B.Visentin Use of Argon Instead of high Vacuum
IR baking system in Clean Room at Saclay.
Use of Argon Instead of high Vacuum
120°C 48h
145°C 3h
In terms of Oxygen diffusion in the bulk niobium
16/05/2008
- Fabien Eozénou FJPPL A_RD_04

10. BAKING RESULTS ON ICHIRO CAVITIES
“Ichiro” (or Low Loss) Geometry makes it possible to reach higher gradients
→ Two cavities received from KEK
IS8 Baked under argon during 1 hour
at 145°C
→ + 9MV/m
+ 1hour additional baking
: removal of residual slope.
2 hours = optimal duration for “fast argon” baking
Will be presented at EPAC 2008: MoPP154 (B. Visentin et al.)
An additional test will be done to reach the quench
16/05/2008
- Fabien Eozénou FJPPL A_RD_04

11. IMPROVING THE EP PARAMETERS
16/05/2008
- Fabien Eozénou FJPPL A_RD_04

12. LOW REPEATITIBILITY
TESTS ON 9-CELL CAVITIES SHOW A LOW REPEATITIBLITY OF THE GRADIENT COMPARED TO THE CHEMICAL POLISHING.
MANY CAVITIES ARE LIMITED BY FIELD EMISSION.
IT IS NECESSARY TO IMPROVE RINSING PROCEDURES.
16/05/2008
- Fabien Eozénou FJPPL A_RD_04

13. ILC GDE S0 PROGRAM ON SINGLE CELL
ILC S0 International Program on single-cell cavity:
Different Rinsing procedures are evaluated: 6 tests for each “recipe” to have statistics:
1) EP(20 µm) + HPR + Test
2) EP (20) + H2O2 Rinsing + HPR + Baking + Test RF
3)EP (20) + Degreasing + HPR + Baking + Test RF
4) EP (20) + Ethanol Rinsing + HPR + Baking + Test RF
5) EP (20) + HF Rinsing + HPR + Baking + Test RF
6) EP (20) + Boiling Water Rinsing + HPR + Baking + Test RF
7) EP (20) + EP (3μm fresh acid) + HPR + Baking + Test RF
8) EP (20) + Oxipolishing + HPR + Baking + Test RF
16/05/2008
- Fabien Eozénou FJPPL A_RD_04

14. EXAMPLE: EVALUATION OF FRESH EP AT KEK
16/05/2008
- Fabien Eozénou FJPPL A_RD_04

15. Light EP + 30’ Ethanol Rinsing + HPR
RESULTS WITH ETHANOL RINSING
WITHIN GDE S0 PROGRAM ON 1-CELL
Test Within S0 program on single cell:
Recipe:
Light EP + 30’ Ethanol Rinsing + HPR
Mixture: HF(40%) - H2SO4 (95%) with ratio 1 – 9
3 Cavities (Tesla Shape):
Before EP at Saclay:
1DE1: 37 MV/m
1DE3: 30 MV/m
1C03: 40 MV/m
16/05/2008
- Fabien Eozénou FJPPL A_RD_04

16. RESULTS FOR 1DE1 AND 1DE3 CAVITIES
16/05/2008
- Fabien Eozénou FJPPL A_RD_04

17. SUMMARY FOR THE 6 TESTS FOR S0
16/05/2008
- Fabien Eozénou FJPPL A_RD_04

18. FINAL TABLE AND AVERAGE GRADIENT
*Additional HPR At Saclay before Test #2
For the 5 EP on Desy Cavities
Eacc Max = /- 1.27
Eacc Max= 1.29E10 +/- 5.27E9
ETHANOL RINSING + HPR SHOULD BE CONSIDERED AS A RELIABLE RECIPE!
16/05/2008
- Fabien Eozénou FJPPL A_RD_04

19. SEARCH FOR NEW PARAMETERS: EP at LOWER VOLTAGE (5V)
PROCESS EASY TO CONTROL!
= Current Oscillation Regime at any Temperature and stirring conditions.
+ It does not seem to depend on bath aging.
+ Low heating of the Bath
Surface after 130 µm EP at 5V
Tests With 1C21 Cavity: 2 sequences: 80 µm + 50 µm +Ethanol Rinsing. EP at low temperature first (~20°C)
Very Bright and little grainy surface
Ratio Iris / Equator seems promising
110 µm / 90 µm in the half cell were acid is better renewed
Lower than for Standard EP (17V) ? → To be confirmed
What is the result in terms of RF performance?...
16/05/2008
- Fabien Eozénou FJPPL A_RD_04

20. RF RESULTS AFTER LOW VOLTAGE EP
Promising Result :1C21 cavity belong to a batch of cavities that show poor results after BCP (Baking Resistant cavities! Eacc<20 MV/m)
OUTLOOK:
→ EP at low Voltage with ICHIRO/1DE1 Cavities + Standard EP on 1C21
→ EP at higher temperature to increase the removal rate
16/05/2008
- Fabien Eozénou FJPPL A_RD_04

21. IMPROVING THE EP PROCESS
16/05/2008
- Fabien Eozénou FJPPL A_RD_04

22. IMPORTANCE OF THE FLUID DISTRIBUTION
Example of study of the process by E. Kako (KEK)
16/05/2008
- Fabien Eozénou FJPPL A_RD_04

23. → Modelling of fluid dynamics and chemical reactions in 2D
HOW CAN MODELLING HELP US? STUDY WITH COMSOL SOFTWARE
M. Bruchon
Is the standard EP responsible for the dissymmetric attack of the surface?
→ Modelling of fluid dynamics and chemical reactions in 2D
Composition of the bath:
In practice: Mixture of hydrofluoric and sulfuric acids (1:9 in volumes)
Modelling: Density and viscosity of sulfuric acid
Nb oxydation:
2 Nb + 5 H2O → 5 H2 + Nb2O (1)
Nb2O HF → 2 H2NbOF5 + 3 H2O (2)
Modelling: Only equation 2, Nb2O5 is continuously formed
Kinetic equation:
x is the production of H2NbOF5 at time t, α and β the initial concentration of Nb2O5 and HF
16/05/2008
- Fabien Eozénou FJPPL A_RD_04

24. Phenomenon transport :
GENERAL EQUATIONS
Mass balance equation:
Phenomenon transport :
Diffusion
Migration
Convection
Navier Stockes equation (incompressible fluid)
Electroneutrality
Gravity effect
HF concentration versus time and fluid velocity
High concentrated electrolyte
16/05/2008
- Fabien Eozénou FJPPL A_RD_04

25. ASSYMETRIC EP FOR SINGLE-CELL CAVITIES
Modelling of Saclay EP Set-up for different shapes of single cell
Velocity LL
TESLA HF
Left beam tube is badly swept by the acid
Acid “rebounds” on the right iris HF
Velocity
16/05/2008
- Fabien Eozénou FJPPL A_RD_04

26. IMPROVEMENT FOR SINGLE-CELL CAVITIES
Inflow and outflow like 9-cells cavities
Velocity field
HF at s
New symmetric acid distribution
→ more uniform attack (be careful for the irises)
Will be experimentally tested at Saclay
16/05/2008
- Fabien Eozénou FJPPL A_RD_04

27. 9-CELL CAVITIES MODELLING
Specifications from KEK (size of holes, inflow and outflow of acid)
ICHIRO Cavities
Fluid velocity
[HF] at s
[HF] at s
TESLA Cavities HF
Good electropolishing of central cell #5
Electropolishing needs to be improved in the other cells
Velocity
Explanation to the field flatness degradation experimentally observed at KEK after electropolishing ?
→Outlook: 3D Modelling, Alternative cathode shape
16/05/2008
- Fabien Eozénou FJPPL A_RD_04

28. IMPROVING THE INFRASTRUCTURES
16/05/2008
- Fabien Eozénou FJPPL A_RD_04

29. NEW EP FACILITY AT KEK 9-Cell Cavity EP at Nomura Plating
Building of a new set-up for 9-Cell cavities EP at KEK
16/05/2008
- Fabien Eozénou FJPPL A_RD_04

30. STUDY FOR 9-CELL EP SET-UP IMPLEMENTATION AT SACLAY
16/05/2008
- Fabien Eozénou FJPPL A_RD_04

31. Preparation and Assembly Facilities for SRF Activities at Saclay
SPIRAL2, SOLEIL, … cryomodule assembly 10 m X 13 m
XFEL cryomodule assembly
4.4 m X 35 m
XFEL cold mass assembly
6.5 m X 14 m
NEW EQUIPMENT
EXISTING EQUIPMENT
Surface characterization
(optical microscope, MEB, SIMS, …)
chemical treatment
of multicell cavities
(+US and UP water rinsing)
Ultra Pure water plant
CLEAN ROOMS
ISO 7 (class 10000)
6 x 7.5 m
ISO 5 (class 100)
6 x 6 m
ISO 4 (class 10)
6 x 14.4 m
EP multi-cells
Chemistry storage area
Gas cleaning plant
EP monocell
R&D on samples
CP monocell
16/05/2008
- Fabien Eozénou FJPPL A_RD_04