1. LHC collimation review follow-up Impedance with IR3MBC option & comparison with phase 1 tight settings
N. Mounet, B. Salvant and E. Métral
Acknowledgements: Collimation team, LHC operation, G. Rumolo.
!! Still preliminary results !!
N. Mounet, B. Salvant and E. Métral - BE/ABP/ICE - 15/06/2011

2. N. Mounet, B. Salvant and E. Métral - BE/ABP/ICE - 15/06/2011
Introduction
IR3MBC is the collimation layout with a combined momentum – betatron cleaning in IR3, and no cleaning in IR7.
Impedance model:
Collimators open in IR7, closer to the beam in IR3 and some additional collimators, including in cold sections (settings provided by A. Rossi),
Rest of the machine as for phase 1:
Beam screens,
Warm pipe,
MBW+MQW warm magnets,
Broadband impedance from design report.
We compare this to the worst of the three cases studied for the review (14/06/2011):
3.5 TeV, tight collimator settings achieved in MD (07/05/2011), in mm,
7 TeV, tight coll. settings from MD, in mm,
7 TeV, tight coll. settings from MD in nominal sigmas, converted into mm for this energy (i.e. divided by sqrt(2) ).
The worst case (for the impedance) is case 3, and the horizontal plane is the most critical.
N. Mounet, B. Salvant and E. Métral - BE/ABP/ICE - 15/06/2011

3. Horizontal and vertical dipolar (driving) impedance
Higher vertical impedance with IR3MBC (magenta) w.r.t. tight settings (7TeV in sigmas – case 3 in previous slide) (blue)
N. Mounet, B. Salvant and E. Métral - BE/ABP/ICE - 15/06/2011

4. Largest impedance contributors in IR3MBC configuration
For those horizontal collimators:
- very small halfgap due to small s (from small bx),
quite large by,
 Large vertical impedance (but not horizontal since bx is 10 times smaller).
N. Mounet, B. Salvant and E. Métral - BE/ABP/ICE - 15/06/2011

5. IR3MBC vs. tight settings case 3 – single-bunch TMC instability threshold (from Headtail)
IR3MBC vertical
Tight settings case 3 horizontal
Octupoles and chromaticity put to zero
For IR3MBC, TMC around p+/bunch in the vertical plane, vs in the horizontal plane for tight settings case 3.
N. Mounet, B. Salvant and E. Métral - BE/ABP/ICE - 15/06/2011

6. N. Mounet, B. Salvant and E. Métral - BE/ABP/ICE - 15/06/2011
IR3MBC vertical coupled-bunch modes (from Sacherer formula) with octupoles stability diagram
25ns, p+/bunch, e=2
50ns, p+/bunch, e=2
Rigid bunch modes (should be damped by feedback, if no issues with it – e.g. emittance blow-up)
Coupled-bunch headtail modes (with intrabunch motion): no feedback, they are unstable if not inside stability diagram curves.
N. Mounet, B. Salvant and E. Métral - BE/ABP/ICE - 15/06/2011

7. N. Mounet, B. Salvant and E. Métral - BE/ABP/ICE - 15/06/2011
Tight settings case 3 – horizontal coupled-bunch modes (from Sacherer formula) with octupoles stability diagram
25ns, p+/bunch, e=2
50ns, p+/bunch, e=2
→ For IR3MBC, coupled-bunch headtail modes are even more outside the stability diagram than with the tight settings case 3 (with high intensity – low emittance beams).
N. Mounet, B. Salvant and E. Métral - BE/ABP/ICE - 15/06/2011

8. N. Mounet, B. Salvant and E. Métral - BE/ABP/ICE - 15/06/2011
Conclusions
At 7TeV, with the IR3MBC option, it was already known that the horizontal impedance is lower than for nominal phase 1 settings, but the vertical one is much larger (for the imaginary part, factor between 1.5 at 10kHz and 3 at 10 GHz)
→ see e.g.
IR3MBC option is also worse than all the tight settings options studied for the collimation review. In particular the TMC threshold is lower (about p/b) and coupled-bunch headtail modes even further away from the stability diagram of the octupoles.
N. Mounet, B. Salvant and E. Métral - BE/ABP/ICE - 15/06/2011