1. Update of the impedance of new LHC experimental beam pipes
M. Galilee, N. Mounet, B. Salvant, G. Schneider, V. Vaccaro

2. Current layout (for 50 mm diameter):
Context
The vacuum group is looking at the opportunity to replace the current LHC experimental beam pipes by composites: titanium (or Aluminum) + carbon-carbon
NEG coating of 3 micron would still be put on the inside
Reason: Beryllium (as now installed in the inner chamber) would be by far the best material (very high Young’s modulus and low atomic number). However cost (150,000 CHF/m!!!) and more importantly toxicity are calling for other options.
Proposed layout 1
Vacuum /NEG(3μm) /Ti (0.3 mm or Al (0.4 mm))
/CC(1% diam 0.5mm)
/vacuum
Current layout (for 50 mm diameter):
Vacuum(25mm) /NEG (3μm)
/Cu(0.2mm)
/SS(1mm)
/vacuum
Proposed layout 1
Vacuum /NEG(3μm) /CC(1% diam0.5mm)
/Ti (0.3 mm or Al (0.4 mm))
/vacuum

3. Preliminary conclusions
The solution with carbon-carbon composite on the inner part of the beam pipe would increase the total longitudinal and transverse impedances of the machine by 5% (if all 160m are at 25mm radius)
The large power losses (10 to 30 W/m) may require cooling.
The 3 micron NEG coating does not affect the power loss but increases significantly the real tune shifts (both longitudinal and transverse).

4. Material parameters used for the resistive wall impedance
• Beryllium: – Resistivity: Ω.m (from specifications) – Permittivity: 1 • Carbon-carbon composite: – Resistivity: Ω.m (from specifications) • NEG: – Resistivity: Ω.m (David Seebacher, F. Caspers, NEG properties in the microwave range, SPSU Meeting, 17th February, CERN) – Permittivity: 10 • Titanium: – Resistivity: Ω.m (from specifications) • Aluminum: – Resistivity: Ω.m (from specifications)

5. Effect of Al coating on the impedance of a carbon pipe (25 mm)
layers
materials
Im(Ztrans) in kohm/m (for 160m)
Im(Zlong) in mOhm (for 160 m)
Power loss in W dissipated in a 1m length 1
Carbon
113 4
12.9 2
Al (1 mic)+ Carbon 14
0.15
2.6
Al (3 mic)+ Carbon
6.3
0.17
0.99
Al (5 mic)+ Carbon
5.1
0.21
0.66
Al (7 mic)+ Carbon
4.9
0.23
0.57
Al (9 mic)+ Carbon
4.83
0.55
Al (11 mic)+ Carbon
4.82
0.54 Al
4.8 3
NEG(3mic)+Al(5 mic)+CC
8.8
0.48
0.67
NEG(3mic)+Al(5 mic)+CC(500 mic)+Ti

6. Impedances as percentage of total LHC for 160 m
Longitudinal impedance
(% of total LHC impedance)
transverse impedance
(% of total LHC impedance)
Already 1 micron Aluminum coating seems sufficient to reach less than 1% of the LHC impedance

7. Power loss
5 micron Aluminum coating seems sufficient to reach less than 1W/m

8. Effect of NEG+Al coating on the impedance of a carbon pipe (25 mm)
layers
materials
Im(Ztrans) in kohm/m (for 160m)
Im(Zlong) in mOhm (for 160 m)
Power loss in W dissipated in a 1m length 2
NEG(3 mic)+Carbon
115
4.1
12.9 3
NEG(3mic)+Al (1 mic)+Carbon 18
0.42
2.62
NEG(3mic)+Al (3 mic)+ Carbon 10
0.44
0.99
NEG(3mic)+Al (5 mic)+ Carbon
8.8
0.48
0.67
NEG(3mic)+Al (7 mic)+ Carbon
8.6
0.5
0.57
NEG(3mic)+Al (9 mic)+ Carbon
8.5
0.55
NEG(3mic)+Al (11 mic)+ Carbon
0.54 1
NEG(3mic)+Al 4
NEG(3mic)+Al(5 mic)+CC(500 mic)+Ti

9. Effect of NEG Longitudinal impedance transverse impedance
(% of total LHC impedance)
transverse impedance
(% of total LHC impedance)
Already 1 micron Aluminum coating seems sufficient to reach less than 1% of the LHC impedance
Power loss doe not change with NEG

10. Recommendations to be discussed
Al coating helps a lot reducing the impedance and power loss of the carbon inside option
For power loss reasons, at least 5 micron Al coating would be desirable

11. Varying the Al coating thickness

12. Varying the NEG coating thickness