Androula Alekou, Fanouria Antoniou, Hannes Bartosik, Lily Blondel, Leandro Intelisano, Nikos Karastathis, Yannis Papaphilippou, Konstantinos Paraschou, Tirsi Prebibaj, Guido Sterbini, Georges Trad, Natalia Triantafyllou.
Guido opened the meeting by recalling the follow-up of the last meeting on the emittance blow-up due to luminosity burn-off. During the discussion of the past meeting, it was noted the importance to generalize the formalism to the separation leveling. Guido had a discussion with R. Bruce and no major difficulties were spotted for this generalization.
Androula presented the working principle of the crab cavities (CC) and how they are implemented in MAD-X and SixTrack.
A systematic and step-by-step check strategy was deployed and encouraged. This turned out to be crucial for mastering the details of the CC and RF multipoles phasing.
Androula stressed the importance of testing the prototype simulation in SixTrack before launching a complete parametric study in SixDesk.
Yannis praised the systematic approach and commented that similar checks need to be devoted to the HL-LHC masks, starting to clarify how the CC are presently implemented in the masks.
Yannis asked if the CC kicks act on the trace space (y’) or in the phase space (p_y). Androula answered the present implementation is in the trace space. Yannis commented that this is relevant for the symplecticity of the tracking.
Yannis commented that, concerning the LHC case, since the crab bump is closed and local, the adiabaticity of the CC voltage ramp is not as crucial as in the global scheme.
Yannis asked how one could dephase the CC from the accelerating cavities. Androula and Hannes answered that this can be done using the cavity phase, since all phases have a common reference (the reference particle).
Yannis commented that all the valuable efforts to set-up the simulations with all the connected sanity checks should be collected and documented in a Technical Note or Twiki. This effort is very important for the new-comers joining the team.
Natalia presented the simulation results of the CC in the SPS.
This numerical tracking campaign tries to address the general problem of establishing an emittance evolution model for the SPS. In that framework, Natalia presented numerical studies showing the contributions to the emittance blow-up of
the magnet non-linearities,
the power-converter ripples,
The simulations are performed at injection energy and the RMS emittance is computed via a weighted distribution method.
The input for studying the impact of the non-linear elements on the emittance blow-up (BU) comes from the SPS effective model of the non-linear optics. This was performed using an equivalent model constrained with respect to the measured off-momentum detuning for three different SPS optics (Q20, Q22 and Q26). From the equivalent model one can derive only the systematic errors and not the random magnet-by-magnet fluctuations. A study was performed to evaluate the effect of the random error on the DA.
Yannis asked about the effect of the random errors on the chromatic behavior of the optics. Natalia answered that their chromatic effect is minor.
The conclusion of the study was that the impact of the magnet non-linearities in both emittance values and losses is negligible.
A similar analysis was carried out to evaluate the effect of the power supplies ripple. As starting point for the simulations, a 50 Hz ripple was assigned to the main QFs. The amplitude of the ripple was chosen such to originate a 5e-3 tune ripple in the H-plane. The impact of the ripple at 50 Hz for both emittance blow-up and losses is negligible.
Yannis asked about the source of the experimental data shown in SPS tune ripple. Natalia and Hannes explained that these are coming from the BBQ data. It was concluded that the spectrum was indeed driven the beam oscillation and not by the noise of the instrument by comparing the case of excited and not excited beam.
Yannis commented that it would be interesting to perform a similar study on the 600 Hz component or for tones closer to the betatron tunes.
A similar analysis was performed for the CC case. In the presence of CCs the losses are increased by a factor of 1e4. However, they are still considered negligible. No effect on the emittance was observed.
Guido asked about the bench-marking of the simulation with respect to the experimental observations. Natalia and Hannes, commented that due to the entity of the expected results (within the 20 sec time window covered in the simulation), the effect cannot be detected with the present SPS BI hardware.
Yannis asked about the computation cost of the simulation. Natalia answered that in order to simulate 1 min of the SPS one needs approximately 3 days of equivalent CPU times.
Yannis suggested that one should explore how to sub-sampling the core of the distribution, for speed-up the simulation why preserving a proper statistical representation of the bunch.
Implementation of the CC in HL-LHC masks and investigate how to speed-up the weighted-distribution simulations by conveniently sub-sampling the beam core.