First (preliminary) results from BTF measurements in the LHC

First (preliminary) results from BTF measurements in the LHC C. Tambasco X. Buffat, T. Pieloni, J. Barranco Acknowledgements: G. Trad, A. Boccardi, M. Gasior, T. Levens, T. Lefevre, M.Khun W. Hofle, E. Metral, D. Valuch, G. Kotzian. OP: M. Pojer, R. Giachino, B. Salvachia,M. Solfaroli, J. Wenninger and all the crews on shift (Kajtan, Markus, Giulia, Lasse, Reyes) And all the other MD users for spare time

BTF measurements have been performed for the first time in LHC The BTF system has been developed by the BI team BTF system has been set-up (no MD time was allocated) to be transparent to the beams (no emittance neither losses observed) Several configurations have been tested and will be presented Experimental data - simulations comparisons will be discussed (preliminary) Next step: SD computation (PySSD) from SixTrack particle distribution Summary

What are BTF measurements? Beam Transfer Function: beam amplitude response per driving excitation frequency ADT kicker beam BBQ pickup Ain, φin, Ω Network Analyzer Beam Response Aout, φout, Ω BTF: Amplitude (Ω), phase (Ω )

BTF measurements btf_gui.py

BTF measurements Plane

BTF measurements Plane Excitation amplitude

BTF measurements Plane Excitation amplitude Frequency range

BTF measurements Plane Excitation amplitude Frequency range Frequency step

Why BTF measurements? Particle distribution BTF= Frequency Distribution ( Landau Damping with two dimensional betatron tune spread, J. S. Berg and F. Ruggero) Tunes and coherent mode observations Spread measurements etc Direct measurements of the Landau Stability Diagrams Sensitive to particle distribution changes BTF measurement is the only way to test the model of the Stability diagrams

Summary of the BTF measurements BTF measurements tested on pilot and nominal bunches at injection and at flat top Excitation amplitude scan: good settings have been found to be transparent Octupole scan (at injection and flat top) Beams in collision (at injection and after the squeeze) ADT gain scan (at flat top) Chromaticity scan (at flat top) LR contribution (end of the squeeze, LR separation ~14σ )

Amplitude scan: safe BTF configuration BSRT data, injection, pilot bunches The excitation amplitude is not calibrated (restricted time for BTF setup) 0.018 0.058 0.036 0.01 BTF excitations use about 0.01 % of the damper power

Synchrotron sidebands in BTF response Synchrotron sidebands visible in the BTF signal expected at ±0.005 at injection from the peak of the tune Q ~7 units Phase jump clearly visible in correspondence of the synchrotron sidebands and coherent tune

ADT effect on BTF amplitude response ADT effect on the BTF Sidebands still visible, very high, Q ~7 We corrected chromaticity and switched off the ADT (here already collision tunes) Q ~2 Q=±0.005

MD 22-07-2015: Octupole scan injection

MD 22-07-2015: Octupole scan injection Losses for 26 A octupole current

Octupole scan at Injection: BTF amplitude response Octupole scan at injection with pilot bunches at collision tunes Q ~2 εnorm=2 μm [a.u.] [a.u.] Each BTF signal has been normalized at the maximum value

Octupole scan at Injection: BTF phase Phase Phase The slope of the phase increase while increasing the spread Bad phase in vertical, could not be used to reproduce SD in vertical

Octupole scan at Injection: Footprint Q ~2 εnorm=2 μm

Octupole scan at Injection: Footprint Q ~2 εnorm=2 μm Vertical losses observed (D. Mirarchi)

BTF simulation with COMBI A sinusoidal excitation is applied to the beam: Positive LOF 6.5 A Horizontal/Vertical plane excitation amplitude Initial excitation frequency Final excitation frequency Resolution of the frequency step Number of turns for each step of the BTF excitation The tune spread is computed with MAD-X and given as an input in COMBI (linear spread)

Some first observations and preliminary results Preliminary results B1 H At 0 octupole current, the spread is not the one expected from simulations

Some first observations and preliminary results Preliminary results B1 H The spread at 0 A is reproduced with ~5 A octupole current This is consistent with measured spread at injection equivalent to 3 A (R. Tomas, further checks needed)

Some first observations and preliminary results Preliminary results B1 H

Some first observations and preliminary results Preliminary results B1 H 10 A octupole current is needed to reproduce 6.5 A BTF measurements

Some first observations and preliminary results Preliminary results B1 H

Some first observations and preliminary results Preliminary results B1 H Very noise signal for COMBI simulation, still needed to be understood, but some of the visible peaks are also present in measurements

Preliminary results B1 V

Some first observations and preliminary results Preliminary results B1 V

Some first observations and preliminary results Preliminary results B1 V Measured spread smaller than the one expected

Beam in collision at injection BTF signal when beams in collisions in IP1&5: pi and sigma-mode visible Optimization of the IPs was performed looking at the shift from the BTF Blow up observed during BTF possibly due to the excitation of the pi mode (ADT off) π mode V Measured tune shift by BTF ΔQ~0.0129 (expected ΔQ=0.013) π mode H

Beam in collision at flat top Looking at the π-mode position we can optimize collisions Incoherent spread visible between σ-mode and π-mode No emittance blow up observed (ADT Off) Measured tune shift by BTF ΔQ~0.0075 (expected ΔQ=0.008)

ADT gain scan at Flat top High chromaticity > very high sidebands Q ~15 units The coherent tune is visible for an ADT gain of 100 turns (knob of 0.02)

Chromaticity scan at Flat top Chroma 15 units gives large response, higher than the coherent tune amplitude Sidebands go down decreasing the chromaticity, and the amplitude of the coherent tune increases

Chromaticity scan at Flat top Data normalized to the amplitude of the coherent tune to compare measurements

Chromaticity scan: sidebands ratio Sideband ratio: A= S1/S0 > Independent from excitation amplitude Quadratic behavior of the amplitude ratio of the sidebands To be verified and further investigated vs analytical expectations (S. Fartoukh LHC LHC Project Report 986 )

Chromaticity scan: tune shift from sextupoles High sensitivity in tune measurements with BTF amplitude response > Feed-down effect from sextupoles on the tune while changing chromaticity

Octupole scan at Flat top: BTF amplitude response εnorm=2.4 μm εnorm=2.2 μm Data normalized to the BTF amplitude of the coherent tune Less impact on the BTF amplitude width while changing octupole current

Octupole scan at Flat top: BTF phase

Octupole scan at Flat top: BTF phase The resolution in the phase is not good to reproduce SD

Octupole scan at Flat top VS PySSD simulations PySSD and BTF data comparison B1 H B1 V

Octupole scan at Flat top VS PySSD simulations B1 H B1 V

BTF measurements end of the squeeze εnorm=2.24 μm εnorm=2.19 μm Beam-beam LR at ~14σ : really small contribution as expected with a such separation However there is an asymmetry w.r.t. the vertical plane

PySSD and BTF measurements comparison B1 H B1 V

PySSD and BTF measurements comparison B1 H B1 V Simulations predict a small increase in the spread, not observed in the vertical plane

Crossing angle asymmetry Emittance difference in the planes of 2.3%, can this explain the different behavior? Can this be related with uncertainty on crossing angle? Update on crossing angles in IR1 and IR5 J. Wenninger (LBOC meeting 27/10/2015)

Measured Stability Diagram at Injection 11.5 A octupole current ε=2.0 μm

Next step: integrate the distribution from SixTrack to compute SD with PySSD We needed to smooth the SixTrack distribution in order to integrate the dispersion integral A bilinear interpolation (2D) has been used to smooth the histogram of the distribution Known case, analytical linear detuning from octupoles only (-100 A) Initial uniform distribution at the 1st turn in SixTrack Jy Jx

SixTrack distribution Known case, analytical linear detuning from octupoles only (-100 A) Weighted distribution for gaussian shape exp(jx,jy) 100 bins Interpolated Jy Jx

SixTrack distribution Known case, analytical linear detuning from octupoles only (-100 A) Distribution after 1e6 turns in SixTrack 100 bins Jy Jy Jx Jx

Stability Diagram from SixTrack distribution Known case, analytical linear detuning from octupoles only -100 A, 4 TeV, ε=2.2 μm The integration is working, we can reproduce the SD from PySSD integrating the SixTrack distribution

Stability Diagram from SixTrack distribution Known case, analytical linear detuning from octupoles only -100 A, 4 TeV, ε=2.2 μm

Technical summary for LHC 2016 start up BTF measurement have been set-up to be completely transparent on the beams (while separated) BTF system has been tested on several machine configuration (on pilot and nominal bunches, at injection and flat top, with different ADT gains and chroma, on colliding and non colliding beams) Outlook for the 2016: LHC 2016 > Setup of the system to be transparent to the beam (start up stage) Need calibration of the excitation amplitude with the BI team collaboration More flexible BTF GUI B2 Vertical plane signal to be investigated

BTF Summary Extra spread at injection with 0 A octupole current, probably due to non linearities (reproducible by simulations with 5 A octupole current) Consistent with optics measurements at injection (further checks with R. Tomas ref: Collecting amplitude detuning measurements from 2012 R. Tomas et all) From BTF measurements we can measure the tune and coherent modes with a resolution of 10^-4 ADT gain scan: coherent modes visible for an ADT gain of 100 turns Chromaticity scan: the ratio of the heights of the tune peak and sidebands have a parabolic behavior, we can measure the feed-down effects due to sextupoles with a sensitivity of 10^-4 Octupole scan at flat top: not large impact on spread with different octupole, good agreement with simulations (better in vertical plane) End of the squeeze: observed asymmetry between H and V possible explanations: different emittance, crossing angle, multipolar errors at triplets?

Ongoing & Outlook Confirm with simulations spread of non linearities at injection and understand how it adds-up to the spread for higher octupole currents Further simulations required to explain the measured spread after the squeeze: combine uncertainty on the x-angle and smaller emittance in V Further investigation on analytical expectations for chromaticity scan (S. Fartoukh LHC Project Report 986 ) Integrate SixTrack distribution in PySSD, problem on the histogram binning to be solved (convergence problem)

Stability Diagram from SixTrack distribution Known case, analytical linear detuning from octupoles only -100 A, 4 TeV, ε=2.2 μm There is a dependency on the chosen binning to create the distribution histogram from SixTrack Convergence needs to be found

Measured Stability Diagram at Injection 6.5 A octupole current ε=3.75 μm