Follow up from last meeting

PS intensity limitations

• There was a question on the transverse feedback system in the PS: it seems to be required for a variety of functions (injection oscillations against kicker rise time/ripple, headtail instabilities at injection, electron cloud instability at flat top, kicking out a bunch for the 80b production scheme). Guido remarked that the transverse damper is probably not the only way to address the points above, but it significantly facilitates operation --> What is its status and is it solidly in the LIU baseline? Guido will present this subject in detail at the next meeting
• Although simulations had previously indicated that up to 3e11 p/b can be extracted from the PS with 25 ns beams, first tests made last year with the longitudinal feedback (~60% functional) showed that 1.7e11 p/b could be extracted with the required beam quality, with a quadrupolar instability appearing in the last stages of the beam production. More MDs will have to be conducted in 2016 to learn how to master higher bunch intensities for 25 ns beams. The PS is the first machine in which we are able to test the performance reach in terms of beam intensity with the LIU upgrades in place already at this stage (e.g. SPS estimations are all based on calculations and simulations). While the experimental verifications will be taking place, the beam parameters that we assume should remain those extrapolated from simulations. We will periodically review the progress of the PS MD activity within this WG.

Longitudinal emittance blow up in the PSB (Simon Albright)

• The goal for future LHC beams is to transfer from the PSB bunches with ~3.0 eVs longitudinal emittance. This will significantly alleviate space charge in the PS due to the longer bunch (compatibly with the the other constraints on the kicker rise times) and the large momentum spread (through dispersion), which should allow to reach the beam brightness goal. 
• Two possible options can be considered to apply longitudinal emittance blow up in the PSB, i.e. resonant excitation and injection of band limited noise (like in the SPS and LHC). Only the former option is discussed in this meeting, while the latter one is being investigated by Danilo in simulations.
• Presently the bunches are transferred with ~1.2 eVs and a moderate longitudinal emittance blow up (~20-30%) is applied via phase modulation of the C16 RF system. A maximum value of 1.8 eVs was achieved by Steve in MDs in 2015, but no specific effort to implement a resonant scheme was made (issues with C16 reproducibility and controllability). To be noted that the longitudinal emittance blow up in the PSB can only be applied after ~c600 (i.e. 200 ms before extraction), when the bucket area becomes sufficiently large. Once the PSB magnetic cycle will go to 2 GeV within the same cycle time after LS2, the situation may become even more critical because the longitudinal motion will be slowed down by getting closer to transition, and so will be any process based on longitudinal excitation. In addition, the bucket area will be smaller due to the faster ramp rate and so less time within the cycle with sufficiently large bucket area for the longitudinal blow-up will be available.
• A scheme for resonant excitation for single harmonic bunches is presented here, which has been conceived using the theory of the controlled longitudinal emittance blow up via resonance excitation in macropaticle simulations. The process consists of five steps, in which some excitation parameters are changed. In particular five parameters could be in principle changed in this case with single RF: voltage and harmonic number of C16 + offset (theta₁₆), amplitude and frequency (as a multiple of the small amplitude synchrotron frequency) of the excitation. Out of this 5D parameter space, it is chosen to keep the first two parameters + theta₁₆ constant and vary the last two along the five steps. This process results in a beam with longitudinal emittance >3 eVs. Longitudinal losses during the process seem to be very low, i.e. less than 1% (maybe losses due to dispersion should also be included). However intensity effects (e.g. longitudinal space charge, impedance) are not included yet. It would be interesting to design a (simplified) scheme based on this principle also for the PS and test it experimentally. Action: Simon, Heiko and Steve.
• While this scheme proves that on paper longitudinal emittance blow up can be obtained in the PSB through resonant excitation, it also shows that it requires a rather complex process relying on precise parameter control (sensitivity to parameters to be assessed), which could be well beyond the LLRF of the PSB. In fact, even for simpler schemes that do not require all the outlined steps, it is necessary to first ensure the reproducibility of C16 (depending on firmware to be deployed in these days → to be followed up) and also improve the controllability of the parameters (theta₁₆ is not controllable to the necessary level of accuracy, improving this requires more LLRF work → to be followed up)
• For the future activity on emittance blow up in the PSB, progress can be made on many fronts (Action: Simon, Danilo):
  • Use double harmonic bucket (although this increases the dimension of the parameter space!), maybe the blow up can be started earlier due to a possibly larger bucket area.
  • Include intensity effects in the simulations (e.g. space charge, impedance)
  • Simulate the accelerating cycle to 2 GeV
  • Injection of phase noise (also considering the three points above)
  • Carry out machine studies as soon as the firmware for reproducibility of C16 will be deployed.