Follow up from last meeting

• The beam parameter tables for protons and ions were reviewed in the last two meetings
• The new beam parameters were used for the C&S Review, in which the HL-LHC requirements were always quoted as LIU target, but levels of risk was associated to the achievement of these parameters due to some clear bottlenecks that are still under study
• Now the official LIU beam parameters will be updated and they will be released officially by next meeting

News on the PSB-PS transfer and PS space charge studies (Hannes Bartosik, Vincenzo Forte, Wolfgang Bartmann, Guido Sterbini)

• Space charge and emittance blow up in the PS (Hannes)
  • Beam losses and emittance growth were monitored along the PS cycle (from PSB extraction to PS extraction) for the operational BCMS beams (studies have also started for standard LHC beams).
  • It was first noted that measurements with WS at PSB extraction (cross-checked and validated with SEM grids) and PS injection show systematically ~10% emittance growth in the vertical plane and ~25% emittance growth in the horizontal plane. The large value measured in H is presently being studied by improving the emittance measurement (deconvolution from measured dp/p profile from tomoscope), studying the optics (dispersion) mismatch at PS injection as well as the dependence on the energy of the injected beam.
  • After the growth right at injection, the emittance monitored all along the PS flat bottom seems quite constant in H but grows by an additional 10-15% in V.
  • Scanning the vertical tune, one sees that this emittance growth is indeed measured for values above the quarter integer, where the losses are significant (4-8%), while for tune values below the quarter integer the losses tend to disappear (<1%) but the measured emittance growth quickly reaches 50%
  • The evolution of the vertical emittance along the cycle shows another significant growth (~15%) after the first short ramp to 2.5 GeV (for the longitudinal manipulation before acceleration on h=21) and a final 10% until extraction.
  • Plotting the calculated space charge tune spread along the first short ramp, one finds that it increases by about 20% at the beginning of the ramp (due to the bunch shortening) before decreasing, which could be the origin of the blow up observed. Indeed, by adjusting the voltage along the ramp keeping a constant bucket area, the additional blow up along this ramp disappears. If in addition, the working point is chosen such as to minimise the blow up along the flat bottom, the vertical emittance remains about flat all over the cycle, except the final 10% growth from beginning of the ramp to extraction.
  • The constant bucket area voltage program along the first ramp has also a beneficial effect on the horizontal emittance blow up in this part of the cycle.
• Dispersion matching at PS injection (Vincenzo)
  • The dispersions in the BTi-BT-BTP lines have been measured for present LHC25-like beams. While  R1 and R4 exhibit a very good agreement with the MadX model for the horizontal dispersion, the agreement for R2 and R3 is less good.
  • The PS 1st turn dispersion has been measured from the PSB 4 rings.  R1 and R4 exhibit smaller beating compared to R1 and R2 (~10% less)  The horizontal mismatch is large (~x2) on the maximum values.
  • A mismatch with respect to the PS closed solution is visible in measurements. 
  • The PSB-BTi-BT-BTP-PS MadX model shows a dispersion mismatch with amplitude similar to the simulations. A re-matching of the BT line quads has improved the dispersion matching in simulation and measurements.
  • The sem-grids in 48, 52, 54 after PS injection have been re-used in dedicated MD with LHCINDIV (since it is not clear up to which intensity they can be actually used) after a long time. This is potentially a very interesting tool for matching and cross-check emittance measurement of the injected beam.
• Emittance growth at PS injection according to model (Wolfgang)
  • With the betatron and dispersion mismatch according to our models, the expected emittance growth after injection into the PS can be estimated using an analytical model
  • In the present situation, one can expect a horizontal emittance growth of 20% with BCMS beams for the standard longitudinal parameters in the transfer, but if we assumed the large longitudinal emittance from the PSB, this value can become as high as 65%. For the standard 25 ns beam, the values would be 10% and 30%, respectively. The expected vertical emittance growth would be more 5 to 15%.
  • In the future, with the BT-BTP upgrade and increased flexibility for rematching, one can expect negligible emittance blow up in H and up to 15% in the vertical plane with the large longitudinal emittance from the PSB.
• Status of controlled longitudinal emittance blow-up to 3.0 eVs (Simon)
  • The technique of parametric resonance excitation has been put in place in the PSB to blow up the longitudinal emittance of the LHC beams to the desired value (~2.8 eVs) and provide the desired bunch length at extraction. This requires specific trims of the C02, C04 and C16 voltage functions.
  • Results are in general good and reproducible (actually even very good for Ring1, less good for the other rings).
  • These bunches are now good enough to be taken in the PS and first tests already took place (to be continued)
• Status of the transverse feedback system in operation (Guido)
  • Transverse damper is now in operation for a series of users (including LHC beams) to damp injection oscillations
  • The required damping time of ~100 turns is achieved in both planes and is visible from the measurements of the injection oscillations with and without damper.
  • A clone of the LHC beam was also used to run with reduced chromaticity and coupling, avoiding the losses due to the H headtail instability thanks to the damper. But this is not implemented in the operational beam.
  • One can then gate the transverse damper off on one bunch alone after the triple splitting. The result is that this bunch is lost and a train of 80 bunches can be produced in the PS. This is an alternative way to produce trains of 80 bunches (which is however not 100% reproducible because it relies on the loss of one bunch due to instability) to the one with the kicker used in the 'standard' production of the 80b scheme.