Surface Properties of LHC Vacuum Chambers

CERN-GSI Electron Cloud Workshop - 7-8 March 2011 1

Surface Properties of LHCVacuum Chambers

V. BaglinCERN TE-VSC, Geneva

1. Vacuum chambers types in LHC2. Electron related surface properties3. Photon related surface properties

2

1. Vacuum Chamber Typesin LHC

CERN-GSI Electron Cloud Workshop - 7-8 March 2011

3CERN-GSI Electron Cloud Workshop - 7-8 March 2011

Cryogenic temperature areas : what’s this ?

• LHC arcs,

• Stand Alone Magnets : triplets, quadrupoles, D1, D2

• At each extremity of EACH cryostat, a vacuum sector valve is installed.

• It defines a so called “cryogenic vacuum” sector

• By definition, a cryogenic vacuum sector is unbaked.

Sector valve


Cryogenic temperature areas : what’s there ?

• Unbaked copper chambers operating at room temperature

• Unbaked copper plated cold warm transitions from RT to 1.9 K

• Unbaked beam screens operating at 5-20 K

1.9 K 300 K

Courtesy N. Kos CERN TE/VSC


Room temperature areas : what’s this ?• LHC experiments : ATLAS, CMS, ALICE, LHC-B

• Room temperature vacuum system between sector valves

Example of CMS beam pipe

Injection kickers


Room temperature areas : what’s there ?

• BI equipment, collimators, kickers, roman pot ….. all baked

• LHC experiments, circular, elliptical vacuum chambers, warm magnets, septa …. all NEG coated and activated• Several geometries :

- ID 52, 63, 80, 100, 130, 212.7, 797- Ellipses 52x30, 59x44, 128x53 can be H or V- Experimental chambers !!- Transition chambers

Warm magnets

CollimatorsRoman pot

7

2. Electrons relatedsurface properties



Secondary electrons curve

• Technical material• Maximum around 200-300 eV• δmax ~ 2 to 3.5

• The electron distribution curve (EDC) shows :

- Component at reflected electron energy - Secondary electrons with low energy

Most of the emitted electrons have low energy

R. Cimino , I.R. Collins, App. Surf. Sci. 235, 231-235, (2004)

electronsincidentelectronsproducedofnumber

312 eV

“as received”

N. Hilleret et al., LHC Project Report 433 2000, EPAC 00


Cu surface : unbaked, baked, pure Cu

N. Hilleret et al., LHC Project Report 433 2000, EPAC 00

• δmax : ~ 2.3 in the unbaked case ~ 1.8 with in-situ bakeout at 300 deg 1.3 with in-situ glow discharge (value of pure Cu)


Activated NEG

• Activated TiZrV film :- δmax ~ 1.1

C. Scheuerlein et al. Appl.Surf.Sci 172(2001)

• Saturated TiZrV film :- δmax < 1.3- H2, H20, CO, CO2

- But δmax >2 when exposed to air !

Since δmax is very low, there are no multipacting in NEG vacuum chambers


LHC : scrubbing under electrons irradiation

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

1.0E-06 1.0E-05 1.0E-04 1.0E-03 1.0E-02 1.0E-01

Dose electrons (C/mm2)

max

0 then 350 V

23 V

44 V

97 V

99 V

340 V

354 V

830 V

350 V

max

V. Baglin et al., Chamonix, 2001

• Reduction of SEY under electron irradiation

• 1 to 10 mC/mm2 is required to have δmax < 1.3

• Growth of a carbon layer (AES, XPS)

maxini

maxfinal00


A very simple curve fit

0.6169 doseln 0.1313- C/mm 10 6 dose if 2.2

max

2-6max

• Fit well 100 eV curve


Scrubbing works also in cryogenic areas !!

• Unbaked by design

• Providing that the beam screen’s surface coverage stays below a monolayer : cool down CB first

• Scrubbing at cryogenic temperature is as much efficient as at room temperature

V. Baglin, R. Cimino


After a stop : slight re-conditioning required

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

-200 -150 -100 -50 0 50 100 150 200 250 300Duree (h)

Max

de

delta Dose = 4 10-3 C/mm2

340 V Bias~ 6 1015 Ph/m/s

Only indirect photons~ 2 1015 Ph/m/s

No beam

max

Time (h)

•Re-conditioning is necessary after a significant stop ~ 0.1 / 10 days for P ~ 10-9 Torr

• Expected to be much faster than the initial conditioning (to be quantified)



What happen to the low energy electrons ?

• Detailed analysis of the SEY curves, revealed the presence of reflected electrons at low energy

•Low energy electrons are present in as received and scrubbed state

• The reflected part might be described by an exponential behaviour

2

2

max

maxmax 2

E-exp a

EE 1-s

EE s

(E)

s



Electron reflectivity of Cu• Measure of the EDC for several primary energies (Ep)• Electrons with energy below 20 eV have large reflectivity (> 50 %)

0 4 8 12 16Kin. En. (eV)

Ep=11 eVSecondaries

Reflected electrons

0 4 8 12 16

Ep=3.7eV

Kin. En. (eV)

Secondaries

Reflected electrons


0.00

20.0

40.0

60.0

80.0

100

0 50 100 150 200 250 300 350

Perc

enta

ge

Primary Energy (eV)

Secondaries

Reflected electrons

0 20 40 60 80 100 120Kinetic Energy (eV)

Ep=112 eV

Secondaries

Reflected electrons


A consequence : impact on conditioning efficiency

• The conditioning rate is less efficient for electrons below 50 eV

R. Cimino et. al. EPAC 2008, Genoa, Italy


SEY at cryogenic temperature• Cu can be scrubbed BUT

• avoid gas condensation (H2O, CO2) MAX VERSUS COVERAGE

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

2.1

2.2

1.E+12 1.E+13 1.E+14 1.E+15 1.E+16 1.E+17COVERAGE (MOL.CM-2)

SEY

CH4COCO2 6CO2 7CH4 RANGE 350 eVCO RANGE 300 eVCO2 RANGE 350 eV

Variation of maximum yield with amount of adsorbed water

1.7

1.8

1.9

2

2.1

2.2

2.3

2.4

0 20 40 60 80 100 120 140 160 180 200 220Number of monolayers

Y. Bozhko, N. HilleretAT-VAC CERN 1995

N. Hilleret et. al. Chamonix 2000

N. Hilleret. LHC MAC December 2004


0.0

0.20

0.40

0.60

0.80

1.0

1.2

0 50 100 150 200 250 300 350

Primary Energy (eV)

total

Contribution of secondaries

to

Contribution of reflected electrons to

Fully scrubbed Cu

19

3. Photons relatedsurface properties



LHC SR spectrum : UV• With nominal parameters : 7 TeV and 585 mA• With reduced beam current, 90 mA, and reduced beam energy


Energy of emitted photoelectrons• Most of the photoelectrons have energies below 10 eV

R. Cimino et al. Phys. Rev. AB-ST 2 063201 (1999)

• A conditioning is observed under SR or glow discharge

Cu colaminated


EDC under SR irradiation

• The total yield is decreased by 40 % after 1 day of nominal LHC operation

• SR irradiation reduce the amount of low energy photoelectrons

-2 0 2 4 6 8 10 12 14

As received surface; PY=0.103(dose<1 min. LHC operation)

After ~ 1 day LHC operation; PY=0.063

OFE Colaminated Copper

Electron energy above the vacuum level (eV)

Inte

nsity

(a.u

.)



LHC design• Sawteeth are provided in the LHC beam screen to reduce the photoelectron yield and the forward reflectivity



~ 40 mm

~ 500 mm


Photon reflectivities of Cu materials• Measured at ELLETRA with SR of 26 mrad grazing incidence (4.5 mrad in LHC)

• LHC sawtooth provides low : - forward reflection - back scattering - diffuse light

Flat Cu Saw tooth

N. Mahne et al. App. Surf. Sci. 235, 221-226, (2004).

LHC Beam Screens


1.5E-02

2.0E-02

2.5E-02

3.0E-02

3.5E-02

4.0E-02

1.E+19 1.E+20 1.E+21 1.E+22 1.E+23 1.E+24Photon Dose (Photons/m)

Phot

oele

ctro

n yi

eld

Y* (e

/ph)

Vented to atm with N2

Rforward = 6 %

V. Baglin et al., CERN Chamonix XI, 2001

PEY, Reflectivity

Sawteeth, Ec=194 eV

11.5 TeV LHC !!

Forward reflectivity = 6 %• SR irradiation at EPA

• The photoyield decrease with beam conditioning

• It varies from 4 to 1 % under perpendicular incidence

1 day

Behaviour with critical energy ?


• SR irradiation at EPA

• Grazing incidence, 11 mrad

• The photoyield increases when increasing critical energy.

• Photon reflectivity slightly decreases when increasing critical energy

I.R. Collins et al. EPAC 1998,Stockholm, Sweeden

NB : molecular desorption yields are linear in the range, 10 – 300 eV. So the photoelectron yield should be also proportional to critical energy

c E~ *PY

Behaviour of technical materials under different treatments ?



• WL irradiation at BESSY

• Value ranges from 4 to 10 %

• Al exhibit the highest yield

• Colaminated Cu is 6 %


SEY vs dose photon, EPA #12, Ech A -45V, direct , 194 eVchambre dents de scie

1.00

1.50

2.00

2.50

1.E+19 1.E+20 1.E+21 1.E+22 1.E+23 1.E+24Dose (ph/m)

Del

ta a

240

V

Direct photons (-45 V)

1 year of nominal LHC operation yields to ~ 1.5

Photon scrubbing vs photon dose

max

• A minor reduction of the SEY due to SR can be observed

• Cannot rely on SR to scrub the LHC


Thank you for your attention !!!

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Surface Properties of LHC Vacuum Chambers

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