MSWG Meeting 15-Mar-2013
Present:
N Biancacci, S Persichelli, K Cornelis, S Gilardoni, R Wasef, B Mikulec, V Raginel, L Soby, H Damerau, L
Sermeus, J Tan, A Huschauer, W Bartmann
Agenda:
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Approval of minutes
Main presentations:
o Impedance localisation in the PS – N Biancacci
o Linac2 trajectory measurements – J Tan
AOB
The minutes of the last meeting were approved.
Impedance localisation in the PS – Nicolo Biancacci
Nicolo introduces the concept of localizing impedance sources from phase beating. From the beam
position data of adjacent BPMs after exciting the beam the phase advance can be calculated by a
harmonic analysis. The phase beating as a function of the beam intensity allows then to determine
impedance sources. The global change of the tune as a function of intensity is recorded too.
From these measurements an impedance model is built for the machine at 2 GeV and iterated.
The accuracy of the measurement is estimated in dependence of the noise to signal ratio, the
number of measurements, the monitor quality and the intensity scan limits. This value compared to
the theoretical impedance induced phase beating shows that only big impedance sources can be
localized. After establishing the model, the solution’s stability is checked by a stress test and good
candidates for impedance contribution are identified.
Measurements with a bunch length of 90 ns show a total impedance of 6.5 MOhm/m from the tune
shift and 6.4 MOhm/m from the reconstructed model. For a bunch length of 107 ns the total
impedance amounts to 6.3 MOhm/m (from tune shift) vs. 5.9 MOhm/m (from reconstruction). Clear
candidates are the BFA21 and KFA21 as well as the KFA71.
As a conclusion, the noise of the measurement is well understood, the impedance modelling is
ongoing and two probable impedance locations were found.
Discussion:
K Cornelis asks about the possible impedance location appearing between positions 200 and 300. It
could be the injection region. Its dependency on the bunch length should be investigated.
The red line in the impedance reconstruction plots gives the lower boundary from the accuracy
estimation; different bunch lengths result in a different effective impedance and different amplitude
of the phase beating.
KC asks whether the kickers have an impedance shielding – L Sermeus answers that the KFA21 and 13
have connections pieces between the ground plates and the tank and also between the ground
plates of each module (measurements exist); KFA4 has only one module; KFA71 (9 modules) and
KFA79 (3 modules) have no shielding. The BFAs have a lumped magnet structure.
Concerning the question if impedance poses a problem – S Gilardoni answers that there is instability
at transition with known frequency but unknown source. The aim is to identify the sources, and then
evaluating these elements for longitudinal and transverse impedance contributions.
KC mentions that also in the SPS measurements have been done – it will be presented in one of the
future meetings.
Linac2 trajectory measurements – Jocelyn Tan
Jocelyn presented a comparison of the presently installed magnetic BPMs (UMA) versus modern
strip-lines based on the image current principle. In total, 20 BPMs are installed between Linac2 and
the PSB injection. The new strip-line BPMs do not suffer from magnetic field perturbation; do not
require head amplifiers or analogue “Base Line Restoration” modules as is the case for the old
detectors and electronics. As an added bonus, it is possible to construct a real intensity signal from
each BPM via software calibration factors and the whole system becomes similar to the L4 BPM
acquisition system allowing for consolidation and standardisation.
A prototype BPM was tested in the lab using a wire technique with standing waves of 352 MHz
(operating frequency of L4). The mechanical centring was done via an optical sensor with +/- 0.01
mm precision. The measured error of the Diff/Sum signal vs. the wire displacement was seen to be
3.27% in the horizontal and 0.98% in the vertical plane with respect to the theoretical slope.
Investigations are on-going for understanding these differences.
The strip-line sensitivity depending on beam energy and bunching frequency was simulated with a
Gaussian bunch of 50ps (1 sigma) length. At the Linac2 energy (50MeV/c), the sensitivity is better for
202 MHz than for 352 MHz, while for L4 energies (160MeV/c) and above, the dependence on energy
and frequency becomes negligible. It was mentioned that the energy dependency for the old BPM
system is not known as it has never been measured.
One prototype BPM was installed in the LTB line (LTB.UMA20) for commissioning with 50MeV proton
beams and measurements with a local bump show a good agreement with optics simulations.
The measurements did however reveal unexpected results as the beam induced signal onto the
electrodes is ~50% lower than expected from simulations. Also, the time structure of the beam
intensity at the head of the pulse is different from that given by BCTs which could be explained by
transients in the BPM electronics.
The strip-line resolution was calculated with a SVD analysis and is believed to be better than 10 um
even when taken into account the 50% missing intensity signal. This analysis also detected the good
resolution of M. Gasior’s inductive pick-up. Jocelyn ended his presentation by stating that BE/BI
proposes replacing the old UMA-based detectors with strip-line BPMs and LN4 electronics during
2013 and perform the cold check out in 2014 at the end of LS1.
Discussion:
L Soby asks how the factor 4 reduction in the signal corresponds to 6db attenuation from the cables –
it is the attenuation and the factor 2 missing signal. KC asks whether the matching of the cables is
checked – yes.
LS asks how the amplitude fits in the lab – the amplitude of the wire is not measured, it depends on
vertical positions with respect to the node.
The strange time structure of the beam head as seen by the BPM system is under investigation. It
was asked whether one can reduce the head to see if the difference appears at a fixed time. For L4
beams there won’t be any beam head due to chopping. The discrepancy is believed to be more of
transient BPM electronics, and would very unlikely come from the BCT as all BCTs along the transfer
line give the same pulse intensity shape.
KC asks if one could use inductive pick-ups instead of the presently foreseen strip-lines. The answer
given was that this device is about 5 times more expensive and also the resolution of the strip-lines is
better.
B Mikulec doubts whether the old system provides 100 um resolution, mainly related to the
acquisition system.
KC asks if the system requires beam commissioning or if it can be used from day 1 – LS answers that
it should work from day 1 provided sufficient time is provided during the hardware commissioning
period.
B Mikulec emphasizes the need to understand the missing 50% in order to also use it as an intensity
measurement. It was mentioned that the intensity measurement from BPMs is only relative based on
a cross-calibration with BCTs. A coax test bench will become available to estimate whether the 50%
loss is also observed with L4 beams.
KC asks if the bunch shape can cause a difference. That the bunch length does not cause any
difference in the measurement could come from the fact that the bunch length itself is deduced but
not measured. H Damerau comments that one has to go very far in frequency changes to get a
change in the bunch length.
KC asks why the signal is different towards the end of the line and it was suggested that the
increasing bunch length due to momentum spread is the main cause.
It was mentioned that Linac2 will at least run until end of 2016.
In conclusion to Jocelyn’s presentation and the discussion following, the MSWG meeting endorses
the replacement of the UMA system by the suggested strip-line BPMs.
AOB:
The next meeting will be on a Tuesday, 26th of March. From this meeting on the FOM meeting slot is
being used interleaved with the PS-LIU meeting.