No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12...

44
Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 2006 1 Cold / sticky system Vincent Baglin CERN AT-VAC, Geneva

Transcript of No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12...

Page 1: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 20061

Cold / sticky system

Vincent Baglin

CERN AT-VAC, Geneva

Page 2: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 20062

Cold / sticky system ??

Vacuum ??

Page 3: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 20063

Eureka ??

Page 4: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 20064

Cold / sticky systemOutline

1. Cryopumping

2. Adsorption isotherms

3. Cryosorbers in cold systems

4. He leaks in cold systems

Page 5: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 20065

1. Cryopumping

Page 6: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 20066

Desorption probability

• The desorption probability is a function of the binding energy, E and the temperature, T (first order desorption, Frenkel 1924). The surface coverage, θ, varies like :

• The desorption process is caracterised by the sojourn time, τ :

• For large E and small T, molecules remains onto the surface : CRYOPUMPING

• For some combination of E and T, the molecule is desorbed (bake out)

Tk E-

0 e - νθθ=

dtd

e 0

Tk E

ντ =

(ν0 ~ 1013 Hz, k = 86.17 10-6 eV/K)

Page 7: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 20067

Sojourn timeChemisorbed molecules

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

0 50 100 150 200 250 300 350 400

Degres Celcius

Sojo

urn

time

(h)

H2O_AluminumH2_Stainless steelCO_Stainless steelCO_Stainless steel

~ 1 Year

~ 1 s

0.9 eV1.1 eV

1.2 eV

1.7 eV

• At room temperature :

H2O dominates

> 100 ºC to remove H2O

• A bake out up to 300 ºCremoves molecules with large binding energies which could be desorbed by stimulated desorption

Page 8: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 20068

Cryopumping regimes

Physisorption• Sub-monolayer coverage : attractive force (van der Waals) between a gas molecules and a material• Binding energy for physical adsorption• H2 from 20 to 85 meV for smooth and porous materials resp.• 1 h sojourn time at 5.2 K and 26 K for smooth and porous materials resp.

Condensation

• For thick gas coverage, only forces between gas molecules• Energy of vaporisation 9 to 175 meV for H2 and CO2 resp.• 1 h sojourn time at 2.8 K and 53.4 K for H2 and CO2 resp.

sub-monolayers quantities of gas can be physisorbed at their boiling temperature (ex : H2 boils at 20.3 K and a bake-out above 100 ºC removes water)

Cryotrapping

• Use of a easily condensable carrier (e.g. Ar) to trap molecules withhigh vapor pressure (e.g. He, H2)

Page 9: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 20069

Sticking probability/coefficient

• Probability : 0 < σ < 1ν collision rate (molecules.s-1.cm-2)

• Function of gas, surface, surface coverage, temperature of gas and surface temperature

• Pumping speed

i.e : σ times the conductance of a surface

incident

sticking

incident

departingincident -

νν

ννν

σ ==

vA 41 vA

PP - 1

41 S

sat

σσ ≈⎟⎟⎠

⎞⎜⎜⎝

⎛=

[ ]MT 3.63 cm.l.s S 2-1- σ=

0.6

0.7

0.8

0.9

1

0 100 200 300 400 500 600 700 800 900 1000

Gas temperature (K)

Con

dens

atio

n co

effic

ient

3,5 K3,7 K3,77 K3,9 K

J.N. Chubb et al. Vacuum/vol 15/number 10/491-496

J.N. Chubb et al. J. Phys. D, 1968, vol 1, 361

H2 at 300 K incident onto a surface at 3.1 K

• H2 and CO at 4.2 K :SH2 = 5.3 l.s-1.cm-2

SCO = 1.4 l.s-1.cm-2

Page 10: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 200610

Capture factor, Cf

• Takes into account the geometry of the system :

Baffle in a cryopump Holes in the electron shield of the LHC beam screen

σ 1 2 3 4

0.1 0.48 0.26 0.39 0.43

1 0.68 0.36 0.51 0.57

A.A. Krasnov. Vacuum 73 (2004) 195-199

Cf ~ 0.3

R. Haefer. J. Phys. E : Sci. Instrum., Vol 14, 1981, 273-288

Page 11: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 200611

Thermal transpiration

• Vacuum gauges are located at room temperature to reduce heat load• For small aperture, the collision rate, ν, is conserved at the cold / warm transition

• Since the average velocity scales like √T

_vn

41 =ν

Bayart-Alpert gauge

Capacitance gauge

IGaz injection

H2 tank

Turbomolecular pump

Injection tube

Thermal screens

Screens

Liquid helium level

Injection holes

Leak valve

Extractor gauge

Screen

Cryostat

P1, T1

P2, T2

1

2

2

1

2

1

2

1

TT

nn

TT

PP

=

=

T (K) 4.2 77

P1/P2 8 2

Page 12: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 200612

Experimental evidence of thermal transpirationStatic conditions

1E-14

1E-13

1E-12

1E-11

1E-10

1E-09

1E-08

1E-07

1E-06

1E-05

0 1E+15 2E+15 3E+15 4E+15 5E+15 6E+15 7E+15

Surface coverage (H2.cm-2)

H2 P

ress

ure

(Tor

r at

4,2

K)

Pressure measured at room temperature with the Bayard-Alpert gauge and corrected from thermal transpiration

Pressure measured with the extractor gauge

located at 4.2 KBayart-Alpert gauge

Capacitance gauge

IGaz injection

H2 tank

Turbomolecular pump

Injection tube

Thermal screens

Screens

Liquid helium level

Injection holes

Leak valve

Extractor gauge

Screen

Cryostat

P1, T1

P2, T2

V. Baglin et al. CERN Vacuum Technical Note 1995

Page 13: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 200613

2. Adsorption isotherms

Page 14: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 200614

Adsorption isotherm

• Measurement, at constant temperature, of the equilibrium pressure for a given gas coverage, θ• Varies with:

molecular speciessurface temperature (under 20 K only H2 and He)surface naturegas composition inside the chamber...

• Models :Henry’s law for low surface coverage

DRK (Dubinin, Radushkevich and Kaganer) for metalic, glass and porous substrate. Valid at low pressure. Good prediction with temperature variation

BET (Brunauer, Emmet and Teller). Multi-monolayer description

θ = c P

( ) ( ) ln = ln - D kT lnPPmSatθ θ ⎛

⎝⎜⎞⎠⎟

⎛⎝⎜

⎞⎠⎟

2

( )( )P

P P = 1

+

- 1

PPSat m m Satθ α θ

αα θ−

Page 15: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 200615

Saturated vapour pressure

• Pressure over liquid or gas phase (many monolayers condensed)• Clausius-Clapeyron equation : Log Psat = A – B/T

Page 16: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 200616

C. Benvenuti et al. J.Vac.Sci. 13(6), Nov/Dec 1976, 1172-1182

H2 adsorption isotherm on stainless steel

A monolayer

• Condensation cryopumps allows to pump large quantities of H2

• CERN ISR condensation cryopump operated with liquid He at 2.3 K (50 Torr on the He bath)

C. Benvenuti et al. Vacuum, 29, 11-12, (1974) 591

Page 17: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

H2 adsorption isotherm at 4.2 K on CO2 condensat

A = 1 1016 CO2.cm-2

B = 2 1016 CO2.cm-2

C = 0 CO2.cm-2

θ

CO2 condensate

E. Wallén, JVSTA 14(5), 2916, Sep./Oct. 1996

• Packing growth for CO2 films

⇒ Porous layer

• DRK adsorption capacity :0.3 H2/CO2

• Electroplated Cu

Page 18: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

• Reduction of the saturated vapour pressure by orders of magnitude

⇒ Cryotrapping

• Electroplated Cu

• In cryopumps CO2 is admitted to enhance the pumping of H2 and He

H2 adsorption isotherm at 4.2 K in co-adsorption with CO2

CO2 concentration

θE. Wallén, JVSTA 14(5), 2916, Sep./Oct. 1996

Page 19: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 200619

He adsorption isotherm from 1.9 to 4.2 K

E. Wallén. J.Vac.Sci.A 15(2), Mar/Apr 1997, 265-274.

• Sub-monolayer range

• Approach of Henry’s law at low coverage

• The isotherms are well described by the DRK model

• θm ~ 1.3 1015 H2/cm2

• Stainless steel

Page 20: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 200620

Cryosorbing materials

• Large capacity• Large pumping speed • Large temperature working range (up to ~ 30 K)

e.g. Activated Charcoal used for cryopumps (see cryopumps talk)Capacity ~ 1022 H2/g i.e. 1021 monolayers (P. Redhead, Physical basis of UHV,1968)

Sticking coefficient ~ 30 % at 30 K (T. Satake, Fus. Tech. Vol 6., Sept. 1984)

20 K cryopanels

Page 21: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 200621

B.E.T surface area – Roughness factor

• Multi-monolayers theory

• Valid for 0.01<P/Psat<0.3

• BET monolayer = θm

• α = exp (ΔE/kT) >>1

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.0E+13 1.0E+14 1.0E+15 1.0E+16 1.0E+17 1.0E+18

Xenon/cm2Xe

non

pres

sure

at 3

00 K

(Tor

r)

304 L & Cuunbaked and baked

Aluminium unbaked and baked

Sealed anodisedaluminium unbaked

Unbaked NEG

Baked NEG

Unsealedanodised

aluminium baked

Unsealed anodised aluminium unbaked

10-2

10-3

10-4

10-5

10-6

10-7

1013 1014 1015 1016 1017 1018

Technical surface Unbaked Baked at 150 ºC

Copper Cu-DHP acid etched 1,4 1,9

Stainless steel 304 L vacuum fired 1,3 1,5 (at 300 ºC)

Aluminium degreased 3,5 3,5

Sealed anodised aluminium 12 V 24,9 not measured

Unsealed anodised aluminium 12 V 537,5 556,0

NEG St 707 70,3 156,3

( )( )P

P P =

1

+ - 1

PP

P

P

Sat m m Sat m Satθ α θαα θ θ−

≈1

R =AA

A A

R

G

m

G

=× θ

0

1

2

3

4

5

6

7

8

9

10

0 0.1 0.2 0.3 0.4 0.5P / PSat

P /

[ ⎝(P

Sat-P

)] [T

orr.

l]-1

V. Baglin. CERN Vacuum Technical Note 1997

Page 22: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 200622

H2 adsorption isotherm on cryosorbersWoven carbon fiber developed by BINP

• Capacity :

1018 H2/cm2 at 6 K1017 H2/cm2 at 30 K

Capacity

V. Baglin et al. EPAC’04, Luzern 2004.

X 25

X 10 000

V. Anashin et al. Vacuum 75 (2004) 293-299

R ~ 103 RCu

Page 23: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 200623

3. Cryosorbers in cold systems.

Case of the LHC superconducting magnets operating at 4.5 K

Page 24: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 200624

The CERN Large Hadron Collider (LHC)

• 26.7 km circumference

• 8 arcs of 2.8 km

• 8 long straight sections of 575 m

• 4 experiments

• 7 TeV

• 1st beam in summer 2007

ATLAS

CMSALICE

LHCb

Page 25: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 200625

LHC dipole vacuum system

• Cold bore (CB) at 1.9 K• Beam screen (BS) at 5-20 K (intercept thermal loads)

Page 26: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 200626

LHC vacuum system principle

• Molecular desorption stimulated by photon, electron and ion bombardment (see beam-vacuum talks)• Desorbed molecules are pumped on the beam vacuum chamber• 100 h beam life time (nuclear scattering) equivalent to ~ 10-8 Torr H2 at 300 K

In room temperature elements

• Molecular chemisorption on NEG coating and in sputter ion pump (see getter talks)

In cryogenic elements

• Molecular physisorption onto cryogenic surfaces (weak binding energy)• Molecules with a low recycling yield are first physisorbed onto the beam screen (CH4, H2O, CO, CO2)and then onto the cold bore• H2 is physisorbed onto the cold bore

Dipole cold bore at 1.9 KDia. 50/53 mm

Beam screen5 - 20 KDia. 46.4/48.5 mm

Cooling tubesDia. 3.7/4.8 mm

Photons

Holepumping

Wallpumping

Desorbed molecules

Electrons stripes

36.8

mm

Page 27: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 200627

LHC dynamic pressure with perforated beam screen

End Window

Ti

Ti

CollimatorValve

`Photon Stop'

ConductanceConductance

DryTMP

DryScrollPump

RotaryPump

TMP

IonPump

Valve`Isolation'

IonPump

TMP

RotaryPump

e-

BA 6BA 4

BA 5

BA 3BA 2 BA 1

RGA 2RGA 3

Extractorgauge

EPA

Capacitancegauge

ControlVolume

Gaz supply

Cold bore

Beam screen(liner) Valve VVS 3

`DownstreamIsolation'

• EC = 194 eV• 3 1016 photons.s-1.m-1

• BS ~ 5 - 100 K• CB ~ 2.5 - 4.2 K

0.E+00

2.E-10

4.E-10

6.E-10

8.E-10

1.E-09

0.0E+00 5.0E+20 1.0E+21 1.5E+21 2.0E+21 2.5E+21

Dose (Photons/m )

RG

A 3

Par

tial P

ress

ure

Incr

ease

(Tor

r)

No holes

~ 2% holes

Parasitic pumping (CB ~ 3.5 K)BS ~ 5 K

BS ~ 19 KCB ~ 4 K

Magnet cold bore at 1.9 KDia. 50/53 mm

Beam screen5 - 20 KDia. 46.4/48.6 mm

Cooling tubesDia. 3.7/4.76 mm

36.8

mm

Photon

Desorbed molecules

Wallpumping

Holepumping

V. Baglin et al. EPAC’00, Vienna 2000.

• Equilibrium pressure

• Equilibrium coverage

n = C

eqη &Γ

CS = m'

0

θηησθ ⎟⎟⎠

⎞⎜⎜⎝

⎛eq

Page 28: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 200628

LHC Long straight section vacuum system

• Dynamic vacuum ⇒ perforated beam screens

• 1.9 K cold bore (~660 m, arc beam screen technology)• ~ 4.5 K cold bore ( ~ 740 m) ⇒ cryosorbers

• Required performances (for installation of 200 cm2/m):• Operates from 5 to 20 K• Capacity larger than 1018 H2/cm2

• Capture coefficient larger than 15 %

Page 29: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 200629

Why cryosorbers ?

Design requires > 100 h life time with 4.5 K cold bore and thick surface coverage⇒ porous surface

Saturated vapour pressure from Honig and Hook (1960)

1E-13

1E-12

1E-11

1E-10

1E-09

1E-08

1E-07

1E-06

1E-05

1E-04

1E-03

1E-02

1E-01

1E+00

1E+01

1E+02

1E+03

1 10 100 1000

Temperature (K)

Pres

sure

(Tor

r, 30

0 K

)

He

H2

CH4

H2O

N2

CO

O2

Ar

CO2

Beam screen

100 h beam life time

4.5 K1.9 K

Page 30: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 200630

Performance with perforated cryosorbing screens

• ~ 2 % holes• Charcoal cryosorber• 206 cm2.m-1

• OFE Cu• 2.2 m• ID 47 mm

Strips with charcoal

4

547 / 50

113

10

6

1.E-11

1.E-10

1.E-09

1.E-08

1.E-07

0 5 10 15 20 25 30Time (h)

CO

LDEX

Par

tial P

ress

ure

(Tor

r)

10

15

20

25

30

35

40

45

50

55

Tem

pera

ture

(K)

Η2CH4COCO2

• 100 monolayers of H2 condensed onto the beam screen prior SR irradiation

• 2 1019 H2/cm2 condensed onto the cryosorber (20 x required capacity)

• CB > 70 K

• BS ~15 and 20 K

• Below design pressure (10-8 Torr)

V. Baglin et al. EPAC’02, Paris 2002.

Page 31: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 200631

LSS cryosorbers regenerationPrinciple of operation

• The cryosorbers installed onto the back of the BS provide the required capacity and pumpingspeed for H2. They are located in cryoelements operating with 4.5 K cold bores

• A regeneration during the annual shutdown for removing the H2 is required

• The active charcoal is regenerated at ~ 80 K

• While regenerating, the beam is OFF and the BS should be warmed up to more than 80 K and the CB held at more than 20 K (emptying cold mass)

• While the H2 is desorbed from the cryosorbers, it is pumped by an external pumped system.

Page 32: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 200632

4. He leaks in cold systems.

LHC beam tube case

Page 33: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 200633

LHC : Superconducting technology

• Air leak or He leaks could appear in the beam tube during operation :the consequences are risk of magnet quench, pressure bump and radiation dose

Page 34: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 200634

Pressure at the level of the leak

Pressure73.5 m

away from the leak

Quench limit :4 10-7 Torr

E. Wallén, JVST A 15(6), Nov/Dec 1997

Example : LHC Test stringLeak rate 6 10-5 Torr.l/sDistance 75.3 m

Description of a He leak

• A He pressure wave is developed with time along the beam vacuum chamber• The He wave can span over several tens of meter without being detected• The local pressure bump gives a local proton loss (risk of quench)

2 Q XF - XF

QQHe front

D

1 mPressure

Distance from leak

xF(t1)x

Po,t2

Po,t1

xF(t2)

PxF

P. Hobson et al. J.Vac.Sci. A. 11(4), Jul/Aug 1993, 1566-1573

Page 35: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 200635

Time to provoke a quench

0

30

60

90

120

150

1.E-07 1.E-06 1.E-05

He leak rate at 300 K (Torr.l/s)

Tim

e in

pre

senc

e of

He

leak

(day

s) Nominal1/3 of nominal1/10 of nominal

He leak rate with risk of quench

• Lower leak rate :Require a pumping of the beam tube on the yearly basis (cold bore >~4K)

• Larger leak rate will provoke a magnet quench within :30 to 100 days beam operation for He leak rate of 10-6 Torr.l/sA day of beam operation for He leak rate of 10-5 Torr.l/s

• 1 year of operation ~ 150 days

Helium leak rate above 5 10-7 Torr.l/s shall be

detected to avoid the risk of a quench

Page 36: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 200636

Vacuum gauge diagnostic

• By design, a vacuum gauge is placed every 3 or 4 cells (320-428 m) i.e. 6 to 8 gauges per arc

• Small probability to detect a He leak in the arcs

• At nominal beam current, leak rates below 2 10-7 Torr.l/s are systematically detected by the arc vacuum gauges before a quench occurs

• So, one needs to find other diagnostic means

The vacuum gauges does not allow a systematic on line detection of the He

leaks in the LHC arc !

Page 37: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 200637

Beam loss monitor based diagnostic

• BLM provide a measurement every 53 m at each arc quadrupole

• All quench due a He wave half-length larger than 26 m are detected by the BLM

At nominal beam current, leak rates below 1.3 10-6 Torr.l/s are systematically detected by the BLM system before a quench

Maximum He leak rate which can be detected by the BLM system before a quench occurs

1.0E-06

1.0E-05

1.0E-04

0.0 0.1 0.2 0.3 0.4 0.5 0.6

Beam current (A)

He

leak

rat

e at

300

K (T

orr.

l/s)

Nominal1st year

Page 38: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 200638

Cryogenic based diagnostic

• Per cell (102 m), a power of ~ 1 W/m can be measured in the cold masses

• Requires stable operation to integrate over a long time (~ 6h)

At nominal beam current, leak rates below 1 10-5 Torr.l/s are systematically detected by the

cryogenic system before a quench

Remaining time, after the dissipation of 1 W/m in the cold mass, before a quench occurs

0

50

100

150

1.E-07 1.E-06 1.E-05 1.E-04

He leak rate at 300 K (Torr.l/s)

Tim

e be

fore

que

nch

(day

s)

Nominal1/3 nominal1/10 nominal

0

6

12

18

24

1.E-06 1.E-05 1.E-04

He leak rate at 300 K (Torr.l/s)

Tim

e be

fore

que

nch

(h)

Nominal1/3 nominal1/10 nominal

Remaining time, after the dissipation of 1 W/m in the cold mass, before a quench occurs

0

50

100

150

1.E-07 1.E-06 1.E-05 1.E-04

He leak rate at 300 K (Torr.l/s)

Tim

e be

fore

que

nch

(day

s)

Nominal1/3 nominal1/10 nominal

0

6

12

18

24

1.E-06 1.E-05 1.E-04

He leak rate at 300 K (Torr.l/s)

Tim

e be

fore

que

nch

(h)

Nominal1/3 nominal1/10 nominal

Page 39: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 200639

Suspected leak in a given area

(1)• If possible, the cold mass temperature shall be temporarily increased to 4 K to move the He fronttowards the nearest short straight section

• Vacuum gauge or residual gas analyser can be locally installed, at this short straight section, in the tunnel for a leak detection

• Mobile radiation monitors can be installed to monitor a “radiation front” while the beam is circulating

(2)

Page 40: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 200640

In case of a quench

• After a quench, the faulty magnet is identified by the triggered diode.

• The cold bore is warmed up to more than 30-40 K during the quench

• The He is flushed to the nearest unquenched magnet and condensed over ~ 10 m

• The He shall be flush to the nearest SSS to perform a leak detection with a RGA

Page 41: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 200641

Repair

• Warm up of the cold bore to > 4 K and pump out of the He every month allows to operate with leak rates up to:

- 2 10-6 Torr.l/s and 1/3 of nominal beam current- 4 10-6 Torr.l/s and 1/10 of nominal beam current

• Time estimate ~ 1 day

• Leak rates larger than 4 10-6 Torr.l/s require an exchange of the magnet

Page 42: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 200642

Exchange of a magnet

Time estimate ~ few weeks !BE SURE that the OBSERVED QUENCH is due to a He LEAK at THE GIVEN MAGNET

… otherwise …

Page 43: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 200643

Exchange of a magnetBE SURE that the OBSERVED QUENCH is due to a He LEAK at THE GIVEN MAGNET

… otherwise …

Lyn EvansLHC Project Leader

Page 44: No Slide Titlecas.web.cern.ch/sites/cas.web.cern.ch/files/lectures/platjadaro-2006/baglin.pdf12 Experimental evidence of thermal transpiration Static conditions 1E-14 1E-13 1E-12 1E-11

Vincent Baglin CERN Accelerator School - Platja d'Aro - 16/24 May 200644

Thank you for your attention !!!

The guy responsible of the magnet exchange will be …