1. Beam screens in IT phase 1
V. Baglin
CERN AT-VAC, Geneva
1. BS geometry
2. Thermal requirements
3. Gas loads
4. Conclusions
V. Baglin LIUWG, 18/10/07

2. The geometry is defined by
1. BS geometry
The 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 = = 36.9
DID,BS = DCB - 2*0.7-2*1.1 = DCB – 3.6 = = 46.4
V. Baglin LIUWG, 18/10/07

4. The 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 load The 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 :
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 ~ ph/m/s
For 1 W/m dissipated by an electron cloud of <100 eV>, the flux is ~ 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. 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
(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
According to LHC PR 674, the nominal LHC average gas density in the LSS is ~ 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 K will divide by 2 the average gas density
V. Baglin LIUWG, 18/10/07