V. Baglin LIUWG, 18/10/07 1

Beam screens in IT phase 1

V. BaglinCERN AT-VAC, Geneva

1. BS geometry2. Thermal requirements

3. Gas loads4. Conclusions

V. Baglin LIUWG, 18/10/07 2

1. BS geometryThe geometry is defined by

• Beam aperture requirements

• Cooling capillaries requirements

• Thermal conductivity requirements :• sliding rings• thermal anchoring of capillaries

• Thickness of colaminated copper defined by impedance requirements

• Quench force requirements for mechanical stability

• Shape and thickness of pumping slots to reduce electromagnetic leakage towards the BS/CB coaxial space

• Transparency defined by gas load

• Integration of a mask defined by the amount of heat to be extracted at the level of the cold mass

V. Baglin LIUWG, 18/10/07 3

BS geometry (2)First results

• Beam screen thickness of 2.5 to 3 mm if 75 μm copper coating (see C. Rathjen estimations)

• hmax = DCB – 2*0.7-2*4.76-2*3 = DCB – 17• DID,BS = DCB - 2*0.7-2*3 = DCB – 7.4

• Arc beam screen : hmax = DCB – 2*0.7-2*4.76-2*1.1 = DCB – 13.1 = 50-13 = 36.9 DID,BS = DCB - 2*0.7-2*1.1 = DCB – 3.6 = 50-3.6 = 46.4

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 LIUWG, 18/10/07 4

2. Thermal requirementsThe thermal requirements are defined by

• Beam screen operating temperature (5-20 K)

• Longitudinal temperature profile is defined by the heat load and the cooling capacity

• Transverse temperature profile defined by the heat load and the transverse conductibility (copper + welding of the cooling capillary)

• Mask operating temperature : 5-20 K or higher ?

V. Baglin LIUWG, 18/10/07 5

3. Gas loadThe gas loads are due to

• Synchrotron radiation

• Electron cloud

• Ion stimulated desorption (beam-gas ionisation, photoionisation, electron-ionisation)

• Debris coming from the IP

• The conductance must fulfill : 32

13

ion

DebrisECSReq m/H10

IC

-1 C

QQQP

e

e

I

P

QQQC ion

eq

DebrisECSR

V. Baglin LIUWG, 18/10/07 6

Gas load (2)• The reduction of the pumping speed due to the ion contribution is negligible

• The contribution of debris is ignored :

• For 15 mm off-axis beam, the SR flux ~ 5 1016 ph/m/s• For 1 W/m dissipated by an electron cloud of <100 eV>, the flux is ~ 6 1016 e/m/s

• After a beam conditioning time of ~ 30 days, scrubbing could be achieved and the desorption yields are also reduced. The dose are 1023 ph/m and 16 mC/mm2.

eq

DebrisECSR

P

QQQC

eq

ECECphSR

PC

H2 CH4 CO CO2

SR 5 10-5 2 10-6 2 10-5 3 10-6

EC 1 10-3 1 10-5 1 10-4 1 10-4

(inputs to be consolidated)

V. Baglin LIUWG, 18/10/07 7

Gas load (3)• The gas load is dominated by the electron stimulated desorption• Equivalent gas density for 2 % and 5% transparency

• Average gas density in the LSS

Equivalent pressure in inner triplet phase 1

0.0E+00

5.0E+14

1.0E+15

1.5E+15

2.0E+15

2.5E+15

3.0E+15

3.5E+15

5 6 7 8 9 10 11 12 13 14 15

Diameter (cm)

Gas

den

sity

(H

2_eq

/m3)

0.05

0.02

Average gas density in the LSS as a function of IT density

0.0E+00

2.0E+13

4.0E+13

6.0E+13

8.0E+13

1.0E+14

1.2E+14

1.4E+14

1.6E+14

1.8E+14

2.0E+14

0.00E+00 2.00E+14 4.00E+14 6.00E+14 8.00E+14 1.00E+15 1.20E+15

Inner triplet gas density (H2_eq/m3)

Ave

rag

e L

SS

den

sity

(H

2_eq

/m3)

0

2

4

6

8

10

12

14

16

18

20

incr

ease

fac

tor

According to LHC PR 674, the nominal LHC average gas density in the LSS is ~ 1.5 1013 H2_eq/m3

V. Baglin LIUWG, 18/10/07 8

4. Conclusions

• The quench force defines the beam screen thickness to ~ 3 mm

• The capillaries dimensions are defined by the heat load

• Present estimations of the gas load in the inner triplet phase 1, show that the average gas density in the LSS will be increased by a factor ~ 10 compared to the LHC nominal.

• However, in the nominal design, the multipacting effect was not taken into account since the surface was assumed to be fully conditionned (LHC PR 674). Here, under the same assumption, the dissipated power by the electron cloud in the IT phase 1 would be 20 mW/m (instead of 1 W/m) such that the average gas density in the LSS will be increased by a factor less than 2 compared to the LHC nominal.

• Changing the beam screen operating temperature to 40-60 K will divide by 2 the average gas density