lhc impedance: comparison between phase 1 and ir3mbc – follow-up

N. Mounet, B. Salvant and E. Métral - BE/ABP/LIS - 24/11/2010

LHC impedance: Comparison between phase 1 and IR3MBC – follow-up

N. Mounet, B. Salvant and E. Métral

Acknowledgements: G. Rumolo, A. Rossi.

!! Still preliminary results !!


Introduction• IR3MBC is the collimation layout foreseen as an intermediate step before

the phase 2 collimation → combined momentum – betatron cleaning in IR3.

• 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.

• Headtail simulations:– Non linear bucket,– Chromaticity = 2 in both planes,– Kick=0.1 sigma in both planes,– Nominal parameters for top energy

(7TeV/c, Qx=64.31, Qy=59.32, norm. transverse emittances 3.75 mm.mrad (1 sig), long. emittance 2.5 eVs, RF voltage 16 MV, bunch length 7.55 cm, E/E (2 sig)=0.22e-3 ).


Multibunch modes: horizontal

Better in horizontal, but…

Stability diagrams from the octupoles


Multibunch modes: vertical

… worse in vertical.

In particular, huge tune shift (~1.3 10-3) with IR3MBC option.

BUT this is supposed to be damped by transverse feedback.

Now, is the real part of the tune shift an issue (for transverse mode coupling instability - TMCI - threshold) ?

Stability diagrams from the octupoles


Horizontal and vertical dipolar (driving) impedance

Confirm a much stronger vertical impedance with IR3MBC w.r.t. phase 1 (red vs.magenta) above ~105 Hz)


Largest impedance contributors in IR3MBC configuration

For those horizontal collimators:

- very small halfgap due to small (from small x),

- quite large y,

Large vertical impedance (but not horizontal since x is 10 times smaller).


Vertical dipolar impedance: largest contributors

Rest of the machine = everything that is not a collimator in the current impedance model

Above a few MHz, one single IR3MBC collimator gives the same imag. part as the totality of the phase 1 collimators.


Single-bunch TMCI instability threshold (Headtail)

Vertical modes(coupling due to tilted collimator)

In horizontal In vertical

Large positive tune shift of vertical mode -1 TMCI in the vertical plane at 1.4 1011 p+/bunch (vs 3.4 1011 in phase 1)

Without octupoles


Single-bunch TMCI instability thresholdIn horizontal In vertical

The TMCI threshold seems higher (~ 1.9 1011 p+/b), but we need to simulate more turns to be sure.

With octupoles

at 500A


Conclusions

At 7TeV, with the IR3MBC option, the horizontal impedance is lower than in phase 1, but the vertical one is much larger (for the imaginary part, factor between 1.5 at 10kHz and 3 at 10 GHz).

In consequence, the multibunch transverse instability (thought to be the most critical effect at 7TeV) is a more critical issue than in phase 1 (well beyond the stability diagram). But this should be damped by the transverse feedback system.

Without octupoles, the effect on single-bunch TMC (transverse mode coupling) instability threshold of such an impedance is worse by more than a factor two between IR3MBC and phase 1, according to Headtail simulations. With octupoles currents one needs to simulate more turns to be sure, but threshold is at least of 1.9 1011 protons per bunch..

The multibunch effect on the TMC instability threshold has still to be checked (involve parallel Headtail multibunch simulation, code is still under development). According to earlier results by Scott Berg it could be worse (http://www.slac.stanford.edu/cgi-wrap/getdoc/slac-pub-6829.pdf).

http://www.slac.stanford.edu/cgi-wrap/getdoc/slac-pub-6829.pdf

lhc impedance: comparison between phase 1 and ir3mbc – follow-up

Documents

mtral beabplis

horizontal impedance

vertical dipolar impedance

large vertical impedance

stronger vertical impedance

broadband impedance

lhc impedance

vertical plane