Download LIU-PS and IPAC16

Title: New Working Point for CERN Proton Synchrotron
Authors: F. Sperati, D. Schoerling, M. Serluca, S. Gilardoni, A. Beaumont
Abstract: The LHC High-luminosity project requests high brightness and intensity beams from the CERN
Proton Synchrotron (PS). The generation of such beams is limited due to resonance effects at injection.
The impact of resonances can be minimized by performing appropriate correction with dedicated magnets
and by optimizing the tune working point. Currently the tune working point at injection is naturally set by
the quadrupolar component generated by the one hundred combined function normal conducting
magnets installed in the PS, and slightly corrected by low energy quadrupole magnets. In this paper, a
study is presented exploiting the use of the available five auxiliary individually powered circuits to adjust
the quadrupolar and higher-order multipole components for changing the tune integer at injection. Due
to the non-linear contribution of each circuit to the magnetic field distribution a finite-element magnetic
model was prepared to predict the required currents in the auxiliary coils. The magnetic model was
benchmarked with magnetic measurements and then tested in the PS machine during dedicated machine
development times.
Title: Simulation of instability at transition energy with a new impedance model for CERN PS
Authors: N. Wang, G. Sterbini, S. Aumon, S. Persichelli, M. Migliorati, N. Biancacci
Abstract: Instabilities driven by the transverse impedance are proven to be one of the limitation for the
high intensity reach of the CERN PS. Since several years, fast single bunch vertical instability at transition
energy has been observed with the high intensity bunch serving the neutron Time-of-Flight facility (nToF).
In order to better understand the instability mechanism, a dedicated measurement campaign took place.
The results were compared with macro-particle simulations with PyHEADTAIL based on the new
impedance model developed for the PS. Instability threshold and growth rate for different longitudinal
emittance and beam intensities were studied. A good agreement was found between simulations and
measurements.
Title: Emittance characterisation of high brightness beams in the CERN PS
Authors: G. Sterbini, J.-F. Comblin, V. Forte, A. Guerrero, E. Piselli
Abstract: In the framework of the LHC Injector Upgrade (LIU), measurements in the CERN PS showed that
the characterisation of the horizontal emittance for the LIU beam is challenging. It is known that one of
the limit of the present approach is to assume a Gaussian energy distribution of the bunch to reconstruct
the beam betatronic profile. In this paper the performance of a numerical deconvolution algorithm
between measured longitudinal and transverse profile is evaluated. In addition to the overall beam
emittance, the bunch-by-bunch emittance measurement will be necessary to evaluate the performance
of LIU beams presently under study. In 2015 the first bunch-by-bunch measurement chain was setup. The
results are reported and discussed.
Title: Excitation of longitudinal coupled-bunch oscillations with the wide-band cavity in the CERN PS
Authors: L. Ventura, H. Damerau, M. Migliorati, G. Sterbini
Abstract: Longitudinal coupled-bunch oscillations in the CERN Proton Synchrotron have been studied in
the past years and they have been recognized as one of the major challenges to reach the high brightness
beam required by the High Luminosity LHC project. In the frame of the LHC Injectors Upgrade project in
2014 a new wide-band Finemet cavity has been installed in the PS as a part of the coupled-bunch feedback
system. To explore the Finemet potential during 2015 a dedicated measurement campaign has been
performed, coupled bunch oscillations have been excited with the cavity around each harmonic of the
revolution frequency with both a uniform and uneven filling pattern, coupled bunch modes have been
extracted and growth rates have been measured. In the following the measurements procedure and
results are presented.
Title: Measurements of the CERN PS longitudinal resistive coupling impedance
Authors: M. Migliorati, N. Biancacci, H. Damerau, G. Sterbini, S. Persichelli, L. Ventura
Abstract: The longitudinal coupling impedance of the CERN PS has been studied in the past years in order
to better understand collective effects which could produce beam intensity limitations for the LHC
Injectors Upgrade project. By measuring the incoherent quadrupole synchrotron frequency as a function
of beam intensity, the inductive impedance was evaluated and compared with the impedance model
obtained by considering the contribution of the most important machine devices. In this paper, we
present the results of the measurements, performed during a dedicated campaign, of the synchronous
phase shift as a function of beam intensity. The phase shift has been measured by using two different
techniques: in one case we injected in the machine two bunches, one used as a reference and constant,
and the second one changing its intensity; in the second case, more conventional, we measured the bunch
position with respect to the RF signal of the 40 MHz cavities. The obtained dependence of the synchrotron
phase with intensity is then related to the loss factor and the resistive coupling impedance and compared
with the real part of the PS impedance model.
Title: The PS 10 MHz High Level RF System Upgrade
Authors: G. Favia, H. Damerau, V. Desquiens, S. Energico, M. Morvillo, D. Perrelet, C. Rossi
Abstract: In view of the upgrade of the injectors for the High Luminosity LHC, significantly higher bunch
intensity is required for LHC-type beams. In this context an upgrade of the main accelerating RF system of
the Proton Synchrotron (PS) is necessary, aiming at reducing the cavity impedance which is the source of
longitudinal coupled-bunch oscillations. These instabilities pose as a major limitation for the increase of
the beam intensity as planned after LS2. The 10 MHz RF system consists in 11 ferrite loaded cavities, driven
by tube-based power amplifiers for reasons of radiation hardness. The cavity-amplifier system is equipped
with a wide-band feedback that reduces the beam induced voltage. A further reduction of the beam
loading is foreseen by upgrading the feedback system, which can be reasonably achieved by increasing
the loop gain of the existing amplification chain. This paper describes the progress of the design of the
upgraded feedback system and shows the results of the tests on the new amplifier prototype, installed in
the PS during the 2015-16 technical stop. It also reports the first results of its performance with beam,
observed in the beginning of the 2016 run.
Title: Study of the beam-cavity interaction in the PS 10 MHz RF system
Authors: G. Favia, H. Damerau, M. Morvillo, C. Rossi
Abstract: The eleven main accelerating cavities of the Proton Synchrotron (PS) at CERN consist of two
ferrite-loaded coaxial lambda/4 resonators each. Both resonators oscillate in phase, as their gaps are
electrically connected by short bars. They are in addition magnetically coupled via the bias loop used for
cavity tuning. The cavities are equipped with a wide-band feedback system, limiting the beam loading,
and a further reduction of the beam induced voltage is achieved by relays which short-circuit each halfresonator gap when the cavity is not in use. Asymmetries of the beam induced voltage observed in the
two half-cavities indicate that the coupling between the two resonators is not as tight as expected. The
total cavity impedance coupling to the beam may be affected differently by the contributions of both
resonators. A dedicated measurement campaign with high-intensity proton beam and numerical
simulation have been performed to investigate the beam-cavity interaction. This paper reports the result
of the study and the work aiming at the development of a model of the system, including the wide-band
feedback, which reproduces this interaction.
Title: Flat Bunches with a Hollow Distribution for Space Charge Mitigation
Authors: A. Oeftiger, H. Bartosik, A. Findlay, S. Hancock, G. Rumolo
Abstract: Longitudinally hollow bunches provide one means to mitigate the impact of transverse space
charge. The hollow distributions are created via dipolar parametric excitation during acceleration in
CERN's Proton Synchrotron Booster. We present simulation work and beam measurements. Particular
emphasis is given to the alleviation of space charge effects on the long injection plateau of the
downstream Proton Synchrotron machine, which is the main goal of this study.