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

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

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

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

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

Practical limitations: Field emission Field induced emission of electrons: - Metallic (conducting) particles of irregular shape; => typical size: 0,5 - 20 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

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:

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.

Present picture of field emission: observations Metallic (conducting) particles or “scratches” of irregular shape; typical size: 0,5 - 20 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.

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), 800 C firing (=> hydrogen degassing, mechanical stress release (>1200 C postpurification with getter material) Alternative process approaches: - Tumbling (Centrifugal barrel polishing) - Megasonic - Dry-ice cleaning

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

BCP + EP Chemical etching (BCP) or electro polishing (EP) HF : HNO3 : H3PO4 HF : H2SO4 volume ratio: 1 : 1 : 2 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

BCP + EP (ctd.) no cleaning, but surface removal Typically: “main” treatment: 100 – 140 µm final removal 10 - 40 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

BCP + EP systems

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

High pressure ultra pure water rinsing High pressure ultra pure water rinsing (cleanroom cl.10 - 100) repeated inside rinsing i) after final surface treatment (1x) ii) after final assembly of cavity (> 3x) ultra pure water with p = (80 - 150) 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 ?

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

High pressure ultra pure water rinsing (HPR) II Process ultra pure water with p = (80 - 150) 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 !

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

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

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

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

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

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?

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

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

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

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. 28.5 MV/m.

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

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

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.

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