1. Field emission in SRF Cavities
With the European XFEL + FLASH cavities (beta=1) as reference
Svem Lederer, Detlef Reschke
Berlin, Dec 18, 2012

2. Outline Introduction Cavity surface preparation Handling and assembly
Practical limitations of SRF cavities
Field emission
Cavity surface preparation
Handling and assembly
Some results of RF tests

3. Niobium Cavities: 1.3 GHz XFEL- Cavity (β = 1)

4. Practical limitations of SC cavities
text
Q-slope
Hydrogen Q-disease
Courtesy R. Geng

5. Practical limitations
Local thermal (or magnetic) breakdown => “Quench”
Q-drop (without field emission) + Q-slope (at medium field)
Field emission
Multipacting
Hydrogen Q-disease
Increased residual resistance

6. Practical limitations: Field emission
Field induced emission of electrons: - Metallic (conducting) particles of irregular shape; => typical size: 0, µm - Only 5% - 10% of the particles emit - Modified Fowler-Nordheim’s law : I  AFN·(FNE)2/ · exp ( ) - AFN (FN emission area) not directly correlated to physical size of emitter
Measures against Field emission: - Lots of “CLEAN” => Clean room, pure media, clean vacuum, clean handling, High pressure ultra pure water rinsing (HPR), etc.
C 3/2
FN E

7. Some general statements
Critical for multi-cell cavities, especially in beam operation:
Field Emission!!
P. Kneisel + B. Lewis, SRF Workshop1995: „Progress towards routinely achieving higher gradients for future applications of rf-superconductivity goes hand in hand with shifting the onset of field emission loading towards higher fields.” “It is generally accepted that the field emission behavior of a niobium cavity reflects the level of cleanliness of the superconducting surfaces subject to the rf-fields.”
As you all know by your experience field emission is the major limitation of srf cavities of the last 5-10 years. It results in rapidly increasing losses limiting the operating gradient of the cavity and in accelaerator operation in dark currents limiting the machine performance.
The sentences of Peter given in his paper at the Saclay Workshop 10 years ago are absolutely valid:

8. Field emission: Cavity limitation
Improved clean preparation techniques allow an increased field emission onset
Typical (good) onset of field emission at 1.3 GHz (beta = 1) - single-cell cavities: Eacc,onset > 30 MV/m - multi-cell cavities (vertical + horizontal): Eacc,onset  25 MV/m
BUT: Multi-cell cavities with no field emission above Eacc,onset > 40 MV/m
At 1.3 GHZ state-of-the-art are onset gradients often well above 30MV/m (60MV/m Epeak) for single-cell protoypes. Due to the complex sahe and larger surface area in multi-cell cavities typically FE starts at 20-25MV/m. This is achieved both for vertical and horizontal operation.
These are the acc. gradients NOT surface fields Epeak, which are ususally in beta=1 cavities a factor of 2 – 2.5 higher.
For lower frequencies the larger cavity surface results in a higher probability of emission sites and the onset gradients are lower.
In addition cleanliness becomes more and more important for both the cavity fabrication process and of course all components of the beam vacuum.

9. Present picture of field emission: observations
Metallic (conducting) particles or “scratches” of irregular shape; typical size: 0, µm
Only 5% - 10% of the particles emit
Hydrocarbon contamination of the vacuum system
Sulfur contamination after electropolishing process
Modified Fowler-Nordheim’s law : I  AFN·(FNE)2/ · exp ( )
typical -values between 50 and 500 for srf cavities
AFN (FN emission area) not directly correlated to physical size of emitter
No substantial difference in rf and dc behaviour
C 3/2
FN E
A short summary of field emission:
FE originates mainly from irregular shaped, electrical conducting particles. The typical size is in the order of µm. Regular shaped metallic particles show much less FE. Isolating particles show no or only weak FE. Typically only 5% - 10% of the particles emit.
Ebenfalls wichtig: hydrocarbon Verunreinigungen des Vakuumsystems
Both, dc and rf FE is well described by the modified FN-equation, which gives the exponential relation between current and local electrical surface field. Parameters are A and .  is the so-called field-enhancement factor. It varies between 50 and 500. A is the emission area, which is not identical with the physical emitter size.  is the Nb work function and C a constant.
For the major effects there is no substantial difference between rf and dc behaviour.

10. Cavity surface preparation
Standard process for FLASH and European XFEL cavities: - US: Ultrasonic cleaning (very important for pre-cleaning and components) - BCP/EP: etching/electropolishing - HPR: High pressure ultra pure water rinse - pumping systems, leak check and venting installations - tooling for handling and assembly - furnaces: 120 C “bake” (=> Q-drop cure), C firing (=> hydrogen degassing, mechanical stress release (>1200 C postpurification with getter material)
Alternative process approaches: - Tumbling (Centrifugal barrel polishing) - Megasonic - Dry-ice cleaning

11. Cavity surface preparation
European XFEL surface preparation schemes:
EP scheme (similar to ILC):
110 µm + 40 µm removal
He-tank welding after final surface treatment
“BCP Flash” scheme:
140 µm EP + 10 µm BCP
He-tank welding before final surface treatment
Results in less handling + preparation steps
BCP Flash
Final EP

12. BCP + EP
Chemical etching (BCP) or electro polishing (EP) HF : HNO3 : H3PO4 HF : H2SO4 volume ratio: 1 : 1 : : 9 removal rate: app. 1 µm/min app. 0.4 µm/min
BCP Surface
(1µm roughness)
0.5 mm
EP Surface
(0.1µm roughness)
0.5 mm

13. BCP + EP (ctd.) no cleaning, but surface removal
Typically: “main” treatment: 100 – 140 µm final removal µm
no (weak) removal of e.g. grease, plastics
closed system with integrated DI-/pure water rinsing
acid quality: “pro analysi” or better
Systems are well established, but environmental and safety critical
EP requires ethanol (or special detergent) rinse in order to remove sulphur contamination

14. BCP + EP systems

15. Ethanol rinse Motivation: Field emission can be caused by sulphur
Crystalline sulphur segregates out of the acid as a reaction with the alumina electrode
Sulphur is insoluble in water
Either ethanol rinse or cleaning with special detergent + US necessary
PVDF tube before and after ethanol cleaning
Sulphur removed from a PVDF tube

16. High pressure ultra pure water rinsing
High pressure ultra pure water rinsing (cleanroom cl )
repeated inside rinsing i) after final surface treatment (1x) ii) after final assembly of cavity (> 3x)
ultra pure water with p = ( ) bar
no moving parts inside the cavity
well adjusted amount of water for high cleaning efficiency
outside rinsing maybe helpful to avoid transport of contamination into the assembly area
check of particles (+ TOC) of HPR supply water check of drain water as QC of rinsing effect ?

17. High pressure ultra pure water rinsing (HPR)
HPR is the final cleaning step of the cavity preparation process
What does that mean?
If have not removed the critical contamination up to now, it will stay
Afterwards it will not become better, only worse
Set-up
HPR stand in ISO 4 / 5
no moving parts inside the cavity
adequate materials (HPR pump!) for high pressure + ultra pure water => not all stainless steels are qualified
final filter (< 0.1 µm) between HPR pump + spraying head
outside rinsing maybe helpful to avoid transport of contamination into the assembly area

18. High pressure ultra pure water rinsing (HPR) II
Process
ultra pure water with p = ( ) bar
repeated inside rinsing i) after final surface treatment (1x) ii) after final assembly of cavity (> 3x)
well adjusted amount of water for your purpose => too much water => circulating wave, no direct draining => a “plastic” cavity / dummy is very useful !

19. HPR systems at DESY Sketch of “new” HPR system
Rinsing cabinet of “new“ HPR system with “plastic” cavity
Rinsing cabinet of “old“ HPR system
Jets of single-cell HPR system

20. Pumping systems, leak check and (automated) venting
Leak check + venting
oil-free pump stations with He leak check and residual gas analyzer
laminar (automated) venting with pure, particle filtered N2 or Ar
Scheme of oil free pump station
Scheme of manual venting system

21. Automated venting + pumping
Motivation of automated venting + pumping
Avoid particle transport from outside into the vacuum system and particles already in the vacuum system should not be moved
For abs. pressure p > 1 mbar and differential pressure Δp > 1 mbar (e.g. opening of valves, start pumping) => movement of particles observed
For abs. pressure p < 1 mbar => no movement of particles observed
Manual needle valves cannot safely avoid particle transport

22. Vacuum: Cleaning
Cleaning of all vacuum components in the cavity environment to the same level as the cavity itself
No hydrocarbons, no particles

23. Vacuum: Cleaning
DESY: Separate cleanroom for cleaning of vacuum components

24. Handling and assembly Key points of handling and assembly
well cleaned components (flanges, power coupler, bolts, nuts)
Precisely defined procedures
well-trained and motivated personal
keep duration of actions at open cavity short
simple flange & gasket design e.g. NbTi-flange with Al-gasket
check of cleanliness?

25. Handling and assembly (ctd.)
Power coupler and string assembly at DESY cleanroom

26. Alternative approaches
Tumbling (Centrifugal Barrel Polishing)
Replaces the main surface removal by BCP or EP
Low removal rate => long processing time
Electro chemical treatment afterwards recommended
Megasonic cleaning
Effective removal of sub-micron particles
No (?) activities in the last years
Dry Ice Cleaning (CO2 Snow)
Additional final cleaning technique for particles + film contaminations
Mechanical, thermal + chemical cleaning forces
Local, dry, without residues

27. Cleanroom Compatible Tools for Cavity Tuning
Avoid re-contamination of the cavity after surface preparation during He-tank welding!
Final check and correction of field profile possible just before He-tank welding (welding of “ring” and “bellow” already done)
Assembly of “FMS” (field profile measurement system) for bead pull in ISO 4 cleanroom and venting with Ar

28. Some results of rf vertical acceptance tests
What are typical cavity results ?
Cavities for a XFEL prototype module (PXFEL3_1) (2K, cw/long pulse operation)
Average gradient for all cavities
above Q0=1010 and
Xray below 10-2 mGy/min
approx MV/m.

29. From vertical acceptance tests to horizontal module test
Same cavities after string + module assembly:

30. Some results of rf vertical acceptance tests (ctd.)
One of the best nine-cell cavities
Large grain cavity AC155 after EP-treatment (2K, cw)
Eacc = 45 MV/m corresponds to 192 mT magnetic surface field

31. Conclusion Field emission may limit SRF cavity performance
Essential countermeasures
Cleaning
Handling
Proper pumping and venting
By means of the current established procedures the required quality of SRF cavities for XFEL is achieved.

32. Thank you !
Thanks to all colleagues for their support and transparencies.
The end !