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Field emission in SRF Cavities With the European XFEL + FLASH cavities (beta=1) as 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 § Practical limitations of SRF cavities § Field emission > Cavity Outline > Introduction § Practical limitations of SRF cavities § Field emission > Cavity surface preparation > Handling and assembly > Some results of RF tests Sven Lederer, Detlef Reschke| Field emission in SRF cavities| Dec 18 th, 2012 | Page 2

Niobium Cavities: 1. 3 GHz XFEL- Cavity (β = 1) Detlef Reschke| High Gradients Niobium Cavities: 1. 3 GHz XFEL- Cavity (β = 1) Detlef Reschke| High Gradients in SRF Cavities | April 3 rd, 2012 | Page 3

Practical limitations of SC cavities > text Q-slope Hydrogen Q-disease Courtesy R. Geng Sven Practical limitations of SC cavities > text Q-slope Hydrogen Q-disease Courtesy R. Geng Sven Lederer, Detlef Reschke| Field emission in SRF cavities| Dec 18 th, 2012 | Page 4

Practical limitations > Local thermal (or magnetic) breakdown => “Quench” > Q-drop (without field 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 Sven Lederer, Detlef Reschke| Field emission in SRF cavities| Dec 18 th, 2012 | Page 5

Practical limitations: Field emission > Field induced emission of electrons: - Metallic (conducting) particles 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·( FN E)2/ · exp (- C 3/2 FN E ) - 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. Sven Lederer, Detlef Reschke| Field emission in SRF cavities| Dec 18 th, 2012 | Page 6

Some general statements > Critical for multi-cell cavities, especially in beam operation: > Field Some general statements > Critical for multi-cell cavities, especially in beam operation: > Field Emission!! > P. Kneisel + B. Lewis, SRF Workshop 1995: „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. ” Sven Lederer, Detlef Reschke| Field emission in SRF cavities| Dec 18 th, 2012 | Page 7

Field emission: Cavity limitation > Improved clean preparation techniques allow an increased field emission 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 Sven Lederer, Detlef Reschke| Field emission in SRF cavities| Dec 18 th, 2012 | Page 8

Present picture of field emission: observations > Metallic (conducting) particles or “scratches” of irregular 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·( FN E)2/ · exp (- C 3/2 FN E ) > 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 Sven Lederer, Detlef Reschke| Field emission in SRF cavities| Dec 18 th, 2012 | Page 9

Cavity surface preparation > Standard process for FLASH and European XFEL cavities: - US: 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 Sven Lederer, Detlef Reschke| Field emission in SRF cavities| Dec 18 th, 2012 | Page 10

Cavity surface preparation > European XFEL surface preparation schemes: > EP scheme (similar to Cavity surface preparation > European XFEL surface preparation schemes: > EP scheme (similar to ILC): § 110 µm + 40 µm removal Final EP BCP Flash § 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 Sven Lederer, Detlef Reschke| Field emission in SRF cavities| Dec 18 th, 2012 | Page 11

BCP + EP > Chemical etching (BCP) HF : HNO 3 : H 3 BCP + EP > Chemical etching (BCP) HF : HNO 3 : H 3 PO 4 volume ratio: 1 : 2 removal rate: app. 1 µm/min 0. 5 mm BCP Surface (1µm roughness) or electro polishing (EP) HF : H 2 SO 4 1: 9 app. 0. 4 µm/min 0. 5 mm EP Surface (0. 1µm roughness) Sven Lederer, Detlef Reschke| Field emission in SRF cavities| Dec 18 th, 2012 | Page 12

BCP + EP (ctd. ) > no cleaning, but surface removal > Typically: “main” 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 Sven Lederer, Detlef Reschke| Field emission in SRF cavities| Dec 18 th, 2012 | Page 13

BCP + EP systems Sven Lederer, Detlef Reschke| Field emission in SRF cavities| Dec BCP + EP systems Sven Lederer, Detlef Reschke| Field emission in SRF cavities| Dec 18 th, 2012 | Page 14

Ethanol rinse > Motivation: Field emission can be caused by sulphur > Crystalline sulphur 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 Sven Lederer, Detlef Reschke| Field emission in SRF cavities| Dec 18 th, 2012 | Page 15

High pressure ultra pure water rinsing > High pressure ultra pure water rinsing (cleanroom High pressure ultra pure water rinsing > High pressure ultra pure water rinsing (cleanroom cl. 10 - 100) § repeated inside rinsing i) after final surface treatment (1 x) ii) after final assembly of cavity (> 3 x) § 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 ? Sven Lederer, Detlef Reschke| Field emission in SRF cavities| Dec 18 th, 2012 | Page 16

High pressure ultra pure water rinsing (HPR) > HPR is the final cleaning step 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 Sven Lederer, Detlef Reschke| Field emission in SRF cavities| Dec 18 th, 2012 | Page 17

High pressure ultra pure water rinsing (HPR) II > Process § ultra pure water 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 (1 x) ii) after final assembly of cavity (> 3 x) § well adjusted amount of water for your purpose => too much water => circulating wave, no direct draining => a “plastic” cavity / dummy is very useful ! Sven Lederer, Detlef Reschke| Field emission in SRF cavities| Dec 18 th, 2012 | Page 18

HPR systems at DESY Sketch of “new” HPR system Rinsing cabinet of “old“ HPR HPR systems at DESY Sketch of “new” HPR system Rinsing cabinet of “old“ HPR system Rinsing cabinet of “new“ HPR system Sven Lederer, Detlef Reschke| Field emission in SRF cavities| Dec 18 th, 2012 | Page 19 Jets of single-cell HPR system with “plastic” cavity

Pumping systems, leak check and (automated) venting > Leak check + venting § oil-free 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 N 2 or Ar Scheme of oil free pump station Scheme of manual venting system Sven Lederer, Detlef Reschke| Field emission in SRF cavities| Dec 18 th, 2012 | Page 20

Automated venting + pumping > Motivation of automated venting + pumping § Avoid particle 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 Sven Lederer, Detlef Reschke| Field emission in SRF cavities| Dec 18 th, 2012 | Page 21

Vacuum: Cleaning > Cleaning of all vacuum components in the cavity environment to the Vacuum: Cleaning > Cleaning of all vacuum components in the cavity environment to the same level as the cavity itself > No hydrocarbons, no particles Sven Lederer, Detlef Reschke| Field emission in SRF cavities| Dec 18 th, 2012 | Page 22

Vacuum: Cleaning > DESY: Separate cleanroom for cleaning of vacuum components Sven Lederer, Detlef Vacuum: Cleaning > DESY: Separate cleanroom for cleaning of vacuum components Sven Lederer, Detlef Reschke| Field emission in SRF cavities| Dec 18 th, 2012 | Page 23

Handling and assembly > Key points of handling and assembly § well cleaned 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. Nb. Ti-flange with Al-gasket § check of cleanliness? Sven Lederer, Detlef Reschke| Field emission in SRF cavities| Dec 18 th, 2012 | Page 24

Handling and assembly (ctd. ) Power coupler and string assembly at DESY cleanroom Sven Handling and assembly (ctd. ) Power coupler and string assembly at DESY cleanroom Sven Lederer, Detlef Reschke| Field emission in SRF cavities| Dec 18 th, 2012 | Page 25

Alternative approaches > Tumbling (Centrifugal Barrel Polishing) § Replaces the main surface removal by 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 (CO 2 Snow) § Additional final cleaning technique for particles + film contaminations § Mechanical, thermal + chemical cleaning forces § Local, dry, without residues Sven Lederer, Detlef Reschke| Field emission in SRF cavities| Dec 18 th, 2012 | Page 26

Cleanroom Compatible Tools for Cavity Tuning > Avoid re-contamination of the cavity after surface 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 Sven Lederer, Detlef Reschke| Field emission in SRF cavities| Dec 18 th, 2012 | Page 27

Some results of rf vertical acceptance tests > What are typical cavity results ? Some results of rf vertical acceptance tests > What are typical cavity results ? > Cavities for a XFEL prototype module (PXFEL 3_1) (2 K, cw/long pulse operation) Average gradient for all cavities • above Q 0=1010 and • Xray below 10 -2 m. Gy/min approx. 28. 5 MV/m. Sven Lederer, Detlef Reschke| Field emission in SRF cavities| Dec 18 th, 2012 | Page 28

From vertical acceptance tests to horizontal module test Same cavities after string + module From vertical acceptance tests to horizontal module test Same cavities after string + module assembly: Sven Lederer, Detlef Reschke| Field emission in SRF cavities| Dec 18 th, 2012 | Page 29

Some results of rf vertical acceptance tests (ctd. ) > One of the best Some results of rf vertical acceptance tests (ctd. ) > One of the best nine-cell cavities § Large grain cavity AC 155 after EP-treatment (2 K, cw) Eacc = 45 MV/m corresponds to 192 m. T magnetic surface field Sven Lederer, Detlef Reschke| Field emission in SRF cavities| Dec 18 th, 2012 | Page 30

Conclusion > Field emission may limit SRF cavity performance > Essential countermeasures § Cleaning 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. Sven Lederer, Detlef Reschke| Field emission in SRF cavities| Dec 18 th, 2012 | Page 31

Thank you ! Thanks to all colleagues for their support and transparencies. The end Thank you ! Thanks to all colleagues for their support and transparencies. The end ! Sven Lederer, Detlef Reschke| Field emission in SRF cavities| Dec 18 th, 2012 | Page 32