Minutes from the Machine Studies Working group
13/02/2015
Present:
J. L. Abelleira, Karel Cornelis, Haiko Damerau, Simone Gilardoni, Klaus Hanke, Lars Jensen , Gerd
Kotzian, Bettina Mikulec, Giovanni Rumolo, Benoit Salvant, Andrea Santamaria, Maurizio Serluca,
Kyrre Sjobaek, Rende Steerenberg.
Talks:

New optics for the PSB measurement line, Jose Abelleira
Jose presented the new optics for the PSB measurement line (BTM). This is a new set of
optics that should replace the present set of four optics: dump, vertical emittance
measurement, horizontal emittance measurement (low dispersion) and horizontal emittance
measurement (large dispersion). The envelope of the beam size for the worst case in all the
four optics shows a beam size reduction at the location of the elements BTM.BHZ10 and
BT.QNO20. This would reduce the present levels of radiation for the operation at 1.4 GeV
(run2) and at the same time reduces the specifications for the new magnets that need to be
built for the 2-GeV operation (after LS2). In particular it shows a considerably reduction of
the vertical beam size at BTM.BHZ10. Regarding the conditions for the optical functions at
the SEM grids for the emittance measurements, the new optics has improved them. The
second part of the talk describes the limitations of the present grids for the future beams.
These beams would be in principle smaller, due to the adiabatic damping at the larger
energy. The (1-sigma) beam sizes would be in all cases above the wire separation (condition
for an appropriate adjustment to a Gaussian function). This in principle should ensure a good
emittance calculation. To prove this, an MD was carried out for the smallest beam size
available (LHC probe), comparing the emittance measurement for the same shots taken by
the PSB wire scanner and by the SEM grids. The analysis of these results is ongoing, but the
rough conclusions are that the SEM grids tend to overestimate the emittance for the vertical
plane around 15-30 % (if the adjustment to the Gaussian function takes into account all the
points) or by 15-20 % (if the adjustment takes only the points within 2 sigma). Regarding
the horizontal plane, there is larger overestimation (30-50 %), but this may be caused by a
wrong value of the dispersion at the location of the wire scanner (to be checked out)
Discussions:
Simone suggested taking the beam from ring 1 (and not ring 3) as worst case due to the
largest vertical dispersion. Karel commented if one could consider performing the
measurements with screens However, Bettina and Klaus said that the energy of the line is
too low. In fact in HiRadMat the screens are working very well (However is a different case
as the energy is much higher). It was also commented that the fitting routine could not be
the optimal for the Wire Scanner; but Bettina pointed out that a decision was taken in the
past to have a single routine for all the machines. Rende commented that when the shape of
the very small beams is not Gaussian, adding more fitting points (less wire separation) is
useless as we are not adjusting to the proper function. Karel also suggested for future
studies to compare the optics given by the beam size measurement with those of the model.
Jose added that this will be done and compare in the new MD. Bettina said that the correct
position of the SEM grids should be updated by Olav in the MADX model.

SPS damper, Gerd Kotzian
Gerd explained, on behalf of the damper team, the present SPS damper. The function of that
element is to damp transverse beam oscillations by a feedback system that detects bunch
displacements by BPMs and acts on them by means of electrostatic kickers. The SPS damper
is being upgraded under the LIU project. Several improvements have already been done
during LS1. Among them, power system consolidated, 6 new BPMs installed, new digital
signal processing for the beam types pLHC, pFT, Ions, Scrub implemented, and new clock
distribution with additional optical fiber links between BA3 and BA2. Results of the 2014
performance of the pFT and pLHC modules were shown. The outlook of the works related to
scrubbing for 2015 was discussed when the corresponding scrubbing module will be
evaluated, with full I/Q signal processing and both scrub and pLHC damper running in
parallel. In 2015 a new SPS Damper user interface will be implemented. This requires
refactoring of the damper loops FESA class to V2.0, make LSA consistent with this new FESA
class, and implement a GUI in JAVA. Finally, Gerd summarized the MD requests that will aim
on evaluating concepts for improvements of the damping performance, develop methods to
facilitate loop diagnostics, and the exploitation of new features.
Discussions:
Karel asked if for the fixed target beam we have the full bandwidth of the system (20 MHz)
which requires the fine delay precisely adjusted. Gerd replay that we are limited to 16 MHz
by a digital low pass filter with the loop delay fine adjusted up to this frequency. Karel asked
the status for the ions. Gerd pointed out that the system will be prepared to be ready for the
LHC ion run at the end of the year.
Karel mentioned that with ions at low intensity the former system needed to have
attenuators removed, and Gerd replied that this will be re-evaluated with beam, however,
with the new system we have a large input dynamic range readily available.
Karel also asked if we could use other pick-ups for LHC beams. In fact, from the 6 pick-ups, 4
are used for p+ fixed target beams (electrostatic pick-ups, at baseband and suffering from
electron cloud) and 2 for LHC-type beams (at 200 MHZ). If the electron cloud would be
mitigated we could use also the other 4, but this is not the case for the moment.
Rende raised a question about the tune. He asked if it would be possible to readjust the
feedback phase if the tune is changed. The answer is that we are able to do this by means of
pre-programmed functions. However this is not an issue as the tune variation during
acceleration is very small for the SPS. Gerd suggested an MD dedicated to feedback
performance evaluations with different phase shift filters. The goal would be to make the
feedback phase insensitive to tune variations by means of different phase shift filter
implementations.
Finally, Simone asked if it would a problem for the BBQ that the damper is active. Gerd
commented that in case the damper is active then the observable signal in BBQ is actively
damped. For the purpose of a clean BBQ measurement and as the SPS is a PPM machine one
could lower the damper gain or even switch off the damper for a cycle - if beam stability
permits.
Jose L. Abelleira