ERL related HOM measurements at ELBE

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ERL related HOM measurements at ELBE
Cockcroft Institute

• G. Burt
• Cockcroft Institute, Lancaster University
• E. Wooldridge, B. Spencer, C. Beard, P. McIntosh
• Cockcroft Institute
• G. Staats, J. Teichert, R. Schurig, H. Büttig, P.
  Michel
• FZD

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ELBE
Measurements were made with a 25.9 MeV, 480 mA
beam at 100 kHz CW. Using Cavity C4 of Module 2.
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ELBE Cryomodule
HOM1
Beam
HOM2
The beam was made to traverse the cavity at
several different offsets and the power spectrum
was measured at both HOM couplers as a function
of beam position measured on the upstream and
downstream BPMs.
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Coupler Measurements
We are able to find the loaded and external
quality factors of the cavity and couplers using
the scattering parameters. He we show S22 which
is the ratio of the amplitude reflected from HOM2
to the amplitude sent to HOM2.
We can find the loaded Q from the 3dB bandwidth
of the resonances and the external Q from the
minimum transmission, which is related to the
coupling parameter.
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Loaded Q factors
The measured Q factors were compared to results
published from DESY TESLA-1994-07, Higher Order
Mode Coupler for TESLA, J. Sekutowicz
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Differences between both couplers
HOM1 has significant power at frequencies not
shown in HOM2. By varying the beam current we
showed these powers were not beam driven. My best
guess is klystron harmonics coupling to the HOM
coupler beside it. These are putting a few watts
into the coupler.
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Monopole Spectrum at HOM2
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Integrated Power at 480 mA
We have integrated the power from the HOM coupler
(including cable losses) along the dominant
monopole pass-band from 2.4-2.5 GHz.
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Powers Scaled to 100 mA
We have scaled the results to show what would
happen for a 100 mA beam.
Of course, such weakly coupled HOM couplers would
never be used with a 100 mA beam.
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Dipole Spectrum at HOM2
My analysis will concentrate on the 1st two
dipole pass-bands. Due to the coupling between
the 9 cells in the cavity, each mode of the
single cell splits into 9 separate Eigen-modes.
Each set of 9 Eigen-modes is referred to as a
pass-band.
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Dipole mode Polarisations
In addition each dipole mode has two
polarisations. Due to asymmetries such as
couplers and mechanical errors, the two
polarisations will have slightly different
frequencies.
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Measured Power vs. Simulated loss factor
The power excited in the dipole mode by the beam
should be dependant on the offset, r,
squared. Here we compare the Power/r2 to the
simulated loss-factor/r2. The two are not
strictly comparable but it does give a feeling
for dominant modes.
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Calculated Power vs. Measurements
Using the simulated loss-factors and the
calculated Q factors we can calculate the power
coming out of the HOM couplers for a given
offset. We can then compare this to the
measurements.
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Power vs. Vertical offset
We can use the power measured as a function of
offset to see exactly how the power does vary
with offset. When we do this we find that each
mode is centred on a different position.
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Offset of Mode Centres
An analysis of the data shows that the 3 most
dominant modes in each of the 1st two dipole
pass-bands has their electrical centres offset by
several hundred microns. These are averaged over
7 measurements.
These are calculated from a fit to
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Conclusions

• The measurements have validated the simulations
  within tolerances.
• It has been shown that the modes all have
  different electrical centres.
• That means that the cavities do not have a
  universal on-axis position.
• This will probably effect the cumulative BBU but
  we dont think it will have a big effect on
  regenerative BBU.