Status of the LHC optics

T. Persson on behalf of the OMC-team

Summary of the measurements• First measurement (April 7-8)

– Issue with the phasing of the BPMs• Solved through re-phasing

• Second measurement (April 9)– Spikes observed in the BPMs

• Solved through disabling the orbit trigger– Calculated local coupling corrections

• Third measurement (April 10)– Beta-beat correction– Dispersion correction

Many thanks to BI for there efforts to solve the problems!

Second measurements (April 9)

We could still use this data by using the first 1700 turns and filter out BPMs with the spikes. This data was used for local coupling corrections

Frequency shift observed for the AC-dipole

4501-5500, blue data set purple are from turns 1-1000 and 3501-4500

Natural Vertical Tune

Ac-dipole waveform (beam 1 vertical)

Nicolas Magnin is investigating. Likely a bad connection.

Ac dipole • These spikes in the AC-dipole waveform could

explain why we observe higher losses when we excite compared to 2012.– To be less sensitive we can excite further away

from the natural tune.

Local coupling corrections• Important for the global correction strategy in the LHC

– In particular when the knobs are used to correct the coupling (only one observation location)

• Example of a correction below. – Found corrections using the data from the two beams.

Local coupling corrections• Before and after correction

– Similar correction to 2012 (slightly stronger) – Correction gives flatter resonance driving terms and a more regular

phase advance of them.

Strength 2012 vs 2015Magnet Name 2012 2015

kqsx3.r1 + 8.0E-04 + 8.0E-04

kqsx3.l1 + 8.0E-04 + 8.0E-04

kqsx3.l2 - 9.0E-04 -1.6E-03

kqsx3.r2 - 9.0E-04 -1.6E-03

kqsx3.l5 + 6.0E-04 + 7.0E-04

kqsx3.r5 + 6.0E-04 + 7.0E-04

kqsx3.l8 - 7.0E-04 - 5.0E-04

kqsx3.r8 - 7.0E-04 - 5.0E-04

Beta-beat and dispersion corrections

• Corrections have been implemented• Still work in progress to understand the different

effects!

Phase-beat comparison with 2012 before any correction

The phase-beat is different and slightly worse compared to 2012.

Things have changed in the machine so not a surprise.

Beta reconstruction• The beta functions are reconstructed from phase and a local model

– A new method was developed for the 2015 run• Uses more BPMs (further away to reconstruct the) beta functions• Uses the b2 errors from the dipole in the local model for the reconstruction.• Possible that in the current implementation it underestimates the uncertainty.• New method gives larger beta-beat.

Beta-beat compared to 2012(old algorithm)

Very small differences between 2012 and 2015

Corrections• We calculate corrections based on the phase-beat so they are

independent of the method used to reconstruct the beta-functions.

• Beam 1 – one global correction of phase-beat and normalized dispersion

• Beam 2 – 3 iterations– 2 iteration trying to correct phase-beat and normalized

dispersion– 1 iteration only trying to correct the phase-beat

Strength of correction 2012 and 2015beam 1

Corrections slightly stronger and more magnets used (MQY) compared to 2012.

Strength of correction 2012 and 2015beam 2

Also slightly stronger than 2012.

Before after correction (new algorithm)

Compare old to the new algorithm

The differences between the two algorithms are under investigationOld method peak ~10%New method peak ~15%

2015 compared to 2012 (old algorithm)

Using the old algorithm the beta-beat is comparable, but slightly worse compared to 2012.

Summary• Many of the technical problems on have been solved!

– A solution to the problems with the spikes in the BPMs is in place– Many thanks to all involved in this!

• Slightly different beta-beat compare to 2012.• After the correction the beta-beat seems to be within the tolerances

– Very close to the situation in 2012. – The differences between the two algorithms is under investigation

• We had to iterate 3 times to correct the beta-beat for beam 2– The reason for this is under investigation

• It would be good when the Ac-dipole is fixed to remeasure to asses the situation