Update on BGV impedance studies

1. Update on BGV impedance studies Alexej Grudiev, Berengere Luthi, Benoit Salvant for the impedance team Many thanks to Bernd Dehning, Massimiliano Ferro-Luzzi, Plamen Hopchev, Nicolas Mounet, Elena Shaposhnikova.

2. Agenda • BGV design • Impedance studies for the LHC • First studies with 147 mm diameter • Studies with smaller diameters and various geometries • Conclusions and next steps

3. Design of new LHC BGV (Beam Gas Vertex detector)to be installed in LS1 • request by Plamen, Bernd (BE-BI) and Massimiliano (LHCb)

4. Agenda • BGV design • Impedance studies for the LHC • First studies with 147 mm diameter • Studies with smaller diameters and various geometries • Conclusions and next steps

5. Impedance studies in LHC • We study the electromagnetic fields generated by the LHC beam when passing through the BGV. • These fields perturb the guiding fields, and can lead to • Beam instabilities (longitudinal and transverse)  beam losses and/or emittance growth (many occurrence of transverse instabilities in 2012) • Beam induced heating of the surrounding  loss of performance, outgassing, deformation, or destruction of the equipment (many examples in 2012: TDI, BSRT, ALFA, MKI, TOTEM, vacuum bellows) • In view of higher brightness after LS1, we need to carefully study all planned installation and modifications of LHC hardware.

6. Agenda • BGV design • Impedance studies for the LHC • First studies with 147 mm diameter • Studies with smaller diameters and various geometries • Conclusions and next steps

7. First studies with CST (with initial radius of 147 mm) Scan over Angle 2 Time domain wakefield simulations Angle 1=15 degrees Longitudinal impedance in Ohm (underestimated) Frequency in GHz  Many longitudinal resonances whatever the angle from 800 MHz onwards.

8. Angle IN: 10 degrees Angle Out: 10 degrees Angle IN Angle OUT With eigenmode solver: Largest longitudinal mode at ~1 GHz: R~1 MOhm, Q= 40,000 Angle IN: 30 degrees Angle Out: 10 degrees Angle IN Angle OUT With eigenmode solver: Largest longitudinal mode at ~1 GHz: R~0.8 MOhm, Q= 65,000  Very large resonances, despite the longer taper

9. New geometry (smaller radius requested by Plamen : 130 mm) : taper IN : 6 degrees and taper OUT: 30 degrees Re(Zlong) Frequency (GHz) Shunt impedance With eigenmode solver: Many longitudinal modes after 900 MHz: R~0.07 MOhm, Q between 40,000 and 65,000 Mode number Still quite large, but factor 10 reduction.  What is the acceptable limit?

10. What is the acceptable limit (1/2) • Limit for longitudinal instabilities • Limit from design report in 400 MHz RF system: 200 kOhmfor ultimate intensity, 2.5 eVs longitudinal emittance at 7TeV (E. Shaposhnikova BE/RF-BR). • Hard limit below 500 MHz. In principle, less critical above 500 MHz. • However, much safer to stay below 200 kOhm for all frequency range

11. What is the acceptable limit (2/2) • Limit for beam induced heating: • The cooling system should be dimensioned to cope with the power lost in the device • Ex: 70 kOhm at 900 MHz with 50 ns beam at 1.6e11 p/b Ploss~ 700 W • Ex: 70 kOhm at 1100 MHz with 50 ns beam at 1.6e11 p/b Ploss~ 100 W • Ex: 70 kOhm at 1200 MHz with 50 ns beam at 1.6e11 p/b Ploss~ 5 W  It is critical for both limits to:  push the mode frequencies as high as possible  reduce the shunt impedance below 200 kOhm

12. Agenda • BGV design • Impedance studies for the LHC • First studies with 147 mm diameter • Studies with smaller diameters and various geometries • Impact of cavity length • Impact of taper length • Conclusions and next steps

13. Impact of cavity length on shunt impedance of the highest mode • 106 mm radius (smaller radius push frequencies higher) • Copper coating (increases shunt impedance by a factor ~ 6 for 316LN) Cavity length • Not monotonic • The length of the cavity should not be too small • Frequency of the modes is not plotted, but is also important to assess their effects

14. Agenda • BGV design • Impedance studies for the LHC • First studies with 147 mm diameter • Studies with smaller diameters and various geometries • Impact of cavity length • Impact of taper length • Conclusions and next steps

15. L = 0.5 m Importance of taper (L=0.5m) l L l • 106 mm radius (smaller radius push frequencies higher) • Copper coating (increases shunt impedance by a factor ~ 6 for 316LN)  The longer taper, the better

16. L = 1 m Importance of taper (L=1m) l L • 106 mm radius (smaller radius push frequencies higher) • Copper coating (increases shunt impedance by a factor ~ 6 for 316LN) l  The longer taper, the better

17. L = 1.5 m Importance of taper (L=1.5m) l L • 106 mm radius (smaller radius push frequencies higher) • Copper coating (increases shunt impedance by a factor ~ 6 for 316LN) l • The longer taper, the better! • The longer cavity length, the better (at least above , complete study ongoing)

18. Agenda • BGV design • Impedance studies for the LHC • First studies with 147 mm diameter • Studies with smaller diameters and various geometries • Impact of cavity length • Impact of taper length • What is the best if total length= 2m? • Conclusions and next steps

19. A more realistic geometry • 106 mm radius (smaller radius push frequencies higher) • Copper coating (increases shunt impedance by a factor ~ 6 for 316LN) • Full length of about 2 m (taper included) l L l L+2l = 2 m Cavity length increases  Taper length decreases

20. Zoom below the limit • The longer the taper, the better (for the symmetric case) • Even with copper coating, well below the limit below 1.5 m of flat length (with Ploss of 40 W  is it acceptable from mechanical point of view?).

21. Conclusions and next steps • There is hope with 106 mm radius! • Can the system take ~ 50 W of power loss? • Actual mechanical constraints to be added to the next round of simulations  What is feasible? • Checks of the transverse impedance