- Motivation - Cleaning mechanism, technique & apparatus - Nb cavity results - Copper rf gun...

- Motivation

- Cleaning mechanism, technique & apparatus

- Nb cavity results

- Copper rf gun cleaning

- Summary, open topics + next steps

Detlef Reschke, Arne Brinkmann

IWLC-2010, Oct. 21st, 2010

Dry-ice (CO2-snow) Cleaning

Detlef Reschke, Arne Brinkmann | IWLC-2010 | Oct. 21, 2010 |

Motivation for dry-ice cleaning

> High cleaning potential for removal of particles + field emission by dry-ice cleaning proven on samples=> publications by Univ. Wuppertal (e.g. SRF Workshops, …)

> Additional cleaning option: no replacement of high pressure water rinse !

> advantages of dry-ice cleaning:

Effective removal of particulate and film contamination

Dry cleaning process => horizontal cleaning option of Nb cavities=> final cleaning just before string

assembly=> all applications unsuitable for water e.g. application to Cu gun cavity => no drying procedure necessary


Effect of DIC on a flake-like emitter (courtesy Univ. Wuppertal)

emitter of ~ 20 µm size destroyed by DICemitter of ~ 20 µm size destroyed by DICremnants emitting at higher Eon!

EDX: no foreign element detected EDX: no foreign element detected (probably oxide of Nb)(probably oxide of Nb)

2.4 × 10-13 1.2 × 10-15S (m2)

8.3 × 10-132 × 10-17S (m2)

38.051.2

35.467.4

62.854.3 Eon (MV/m)

HPR+DICHPRemitter


Motivation for dry-ice cleaning

> High cleaning potential for removal of particles + field emission by dry-ice cleaning proven on samples=> publications by Univ. Wuppertal (e.g. SRF Workshops, …)

> Additional cleaning option: no replacement of high pressure water rinse !

> advantages of dry-ice cleaning:

Effective removal of particulate and film contamination

Dry cleaning process => horizontal cleaning option of Nb cavities=> final cleaning just before string

assembly=> all applications unsuitable for water e.g. application to Cu gun cavity => no drying procedure necessary


Cleaning mechanism

> dry-ice “snow”: mechanical, thermal + chemical cleaning forces - thermo-mechanical: i) embrittling by shock-freezing

ii) shearing forces by high momentumiii) drastic volume increase by sublimation

- chemical: liquid CO2 acts as solvent for hydrocarbons + silicone

=> embrittling, blasting, shearing, dissolving, washing

> removal of particles down to < 100nm

> local, dry, without residues

> simple checks with air and surface particle counters possible


Cleaning technique

> patent-registered nozzle design for CO2 surrounded by nitrogen designed by Fraunhofer IPA, Stuttgart, Germany

> spontaneous formation of snow/gas mixture by relaxation of liquid CO2

- app. 40-45% snow at -78,9°C; ~50-55 bar

> surrounding supersonic nitrogen gas (20°C; (12-18)bar)=> accelerating + focussing of jet=> (partially) avoidance of condensation of humidity


Apparatus: General

> schematic of dry-ice cleaning set-up

nitr

ogen

carb

on d

ioxi

de

1

1

2

3

4

5

51 valve2 pressure reducer3 gas purifier4 chiller5 filter


Apparatus: key components + operation


Apparatus: key components + operation

> system for horizontal cleaning of (1-3)-cell cavities in stable operation


Single cell Nb cavity results

> Results with actual cleaning parameters:

=> 3 of 5 tests show no fieldemission up to 35 MV/m; 2 tests with moderate FE

> Potential to remove HPR resistant field emitters ??

1.00E+09

1.00E+10

1.00E+11

0 5 10 15 20 25 30 35 40MV/m

Qo

1DE10, test 4 at 1.8K (leak at 2K)1DE11, Test 3: Q(E) at 2K1DE11, Test 4: Q(E) at 2K1DE11, Test 5: Q(E) at 2K1DE11, Test 7 at 1.8K (leak at 2K)

1.00E+09

1.00E+10

1.00E+11

0 5 10 15 20 25 30 35 40MV/m

Qo

first test after bake + HPR

test 2 after add. DIC with old parameters

test 3 after add. DIC with improved parameters


Copper rf gun cavity cleaning

> Task: cleaning of the copper rf gun cavity of the photo injector for FLASH and European XFEL

> Goal: effective removal of particles => low dark current with no oxidation of Cu

Photo injector area with rfgun cavity at FLASH


Copper rf gun cavity cleaning (ctd.)

> new vertical cleaning stand with modified movable nozzle


Copper rf gun cavity cleaning: first result

> three guns cleanedexample: gun cavity 4.2 conditioning at PITZ => dark current during gun processing app. factor 20 reduced

1 2 3 4 5 6 70

1000

2000

3000

4000

5000

ma

xim

um

da

rk c

urr

ent

(A

)

rf power (MW)

Gun 3.1 (standard cleaned) Cs2Te #58.1 (2006-05-08)

Gun 3.2 (standard cleaned) Cs2Te #42.4 (2007-08-05)

Gun 4.2 (CO2 cleaned)

Mo 2008-01-14: 60 s Mo 2008-02-06: 400 s Mo 2008-02-11: 700 s


Open topics + ideas

> Further optimisation of cleaning parameters:reduced CO2-capillary size, cleaning speed=> reduce consumption of CO2 => reduce/avoid moisture condensation=> avoid heating ??

>What is the “better” nozzle head: One movable nozzle vs. two fixed nozzles? (angle of nozzles?)

>Heating of cavity or inert gas atmosphere ??

> Improved drive system of cavity


Summary + Outlook

> In operation:- horizontal cleaning of (1-3) cell cavities => successful

- cleaning of 1.3 GHz Cu gun cavity with movable nozzle => successful

> Future:- Cleaning of REGAE (Relativistic Electron Gun for Atomic Explorations)

- Regular cleaning of 1.3 GHz gun cavities for FLASH and European XFEL

- Cleaning of water sensitive special parts

>Options:

- Extension to Nb nine-cell accelerator cavities ?

- Cleaning of full accelerator modules ???

- Motivation - Cleaning mechanism, technique & apparatus - Nb cavity results - Copper rf gun...

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