Proceedings of IBIC2012, Tsukuba, Japan

TUPA23PERFORMANCE OF A DOWNCONVERTER TEST-ELECTRONICS WITH
MTCA-BASED DIGITIZERS FOR BEAM POSITION MONITORING IN
3.9GHZ ACCELERATING CAVITIES
T. Wamsat*, N. Baboi, B. Lorbeer, Deutsches Elektronen-Synchrotron DESY, Hamburg, Germany
P. Zhang, The University of Manchester, Manchester, U.K.
Abstract

INTRODUCTION
The ACC39 module in FLASH, DESY [1] contains
four third harmonic cavities operating at 3.9GHz asshown in Fig. 1. An electron bunch passing through thesecavities generates wakefields, which can be decomposed
into higher order modes (HOM). These HOMs, available
from existing coupler, can be used to determine transverse
beam positions in the cavities [2] [3].

Figure 2: Schematics of the downconverter analog box.
A bandpass filter (BPF) with an appropriate frequency
band filters the HOM signal and then it is connected to amixer. A local oscillator (LO) which uses a 81.25MHz
reference from the master oscillator of FLASH generates
the required LO frequency. The IF signal is then filteredwith a 20MHz bandpass filter. After a properamplification, the IF is further filtered by a lowpass filter(LPF) to reduce the noise. To switch the analog box toanother frequency, the bandpass filter was swapped andthe frequency of the LO was changed accordingly toensure an IF of 70MHz.
The analog electronics is shown in Fig. 3.

Figure 1: Schematic of the four cavities within ACC39.
In 3.9GHz cavities HOMs around 5460MHz and
9060MHz are used to determine the beam position [4].

Measurement Setup
The HOM signals are downconverted to an intermediate
frequency (IF) of 70MHz which is then digitized. Figure 2
shows the schematic block diagram of the downconverter,built by FNAL [4].

Figure 3: Downconverter analog box.
The downconverted HOM signal was digitized by theVME and also by the MTCA. The former used a samplingrate? of 216 MHz, and the second 108MHz. The setup
used to make a comparison between the VME and MTCA
is shown in Fig. 4.

___________________________________________*thomas.wamsat@desy.deISBN 978-3-95450-119-9
Beam Position Monitor System1

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Beam-excited higher order modes (HOM) in 3.9GHz
accelerating cavities are planned to be used for beam
position monitoring at the European XFEL. The selected
HOMs are located around 5460MHz, with a bandwidth of
100MHz and 9060MHz, with a bandwidth of 50MHz. A
downconverter electronics, built for tests at FLASH,
converts the HOMs to an intermediate frequency of70MHz.
The MTCA (Micro Telecommunications
Computing Architecture) standard will be used for theXFEL. Thus it is important to have a performance studyof the downconverter test-electronics using the MTCA
digitizer card SIS8300. In the digitizer the IF frequency of70MHz is undersampled with a clock frequency of108MS/s. The paper presents the performance of thedigitizer together with the test-electronics. A comparison
with a 216MS/s VME (Versa Module Eurocard) digitizer
is made.

TUPA23

Proceedings of IBIC2012, Tsukuba, Japan

Figure 4: Set-up with VME and MTCA.
An example of the IF output signal of thedownconverter is shown in Fig. 5.
The MTCA system is described in detail in ref. [6].

Figure 7: Position interpolation into the center of eachcavity.

BEAM POSITION DETERMINATION
To determine the beam position in each cavity aprocedure based on Singular Value Decomposition (SVD)
is used and the details are described in ref. [5].
At the beginning of the measurement a calibration wasmade. The beam was moved in a 2D grid manner bychanging the steerer current. The resultant beam positions
at BPM-A and BPM-B are shown in Fig. 8 and Fig. 9.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 5: Downconverted HOM signal with MTCA.

Measurement S cheme
The schematic of the measurement setup is shown inFig. 6. An electron bunch of approximately 0.5nC ispassing the ACC39 module. Two steering magnets
located in front of ACC1 are used to produce horizontal
and vertical offsets of the electron bunch in ACC39. Twobeam position monitors (BPM-A and BPM-B) are used torecord transverse beam position before and after ACC39.
A straight line trajectory of the bunch is produced
between those two BPMs by switching off theaccelerating field in ACC39 and all quadruples close to it.
Therefore, the transverse offset of the electron bunch ineach cavity can be determined by interpolating thereadouts of the two BMPs, as shown in Fig. 7.

Figure 6: Schematic of measurement setup (not to scale,cavities in ACC1 are approximately three times larger
than those in ACC39).

Figure 8: The readouts of BPM-A during the 2D grid scan(calibration: blue, validation: red).

Figure 9: The readouts of BPM-B during the 2D grid scan(calibration: blue, validation: red).
The set of beam positions marked in blue were used forcalibration of the HOM signals into beam offsets. The
ones in red were applied this calibration and were used toestimate the system performance.
A result of the beam position prediction for calibrationand validation using the HOM signal with SVD comparedto the interpolated beam position (x,y) using BPM-A andBPM-B are shown in Fig. 10 and Fig. 11.

ISBN 978-3-95450-119-92Beam Position Monitor System

Proceedings of IBIC2012, Tsukuba, Japan

TUPA23RESOLUTION ESTIMATION
For this measurement couplers C1H1, C2H1 and C3H1
were used (see Fig. 1). To estimate the resolution ahundred beam pulses were taken for each of several
positions shown in Fig. 13 as measured with BPM-B. In
this measurement only the MTCA digitizer was used.

Figure 10: Measurement (blue) and prediction (red) of
beam position from calibration.

Figure 11: Measurement (blue) and prediction (red) of
beam positions from validation.

Figure14 shows the analysis of one measurement. Figure 14(a) shows the calibration with the position determinationfrom the BPM in blue and the prediction from the SVD in
red. In Fig. 14(b) all 100 beam pulses are painted within it
and the deviation between these hundred beam pulses canbe obtained. Figure 14(c) and (d) show the resultant
resolution for x and y direction at a specific position.

The difference between measured and predictedtransverse beam position from calibration and validationis shown in Fig. 12.

Figure 14: Analysis of the 100 beam pulses taken fromcoupler C3H1 at 9060MHz with MTCA at BPM-B
position (2.37, -1.58), see Fig. 13.
Figure 12: Difference of measured and predicted beampositions.

In Fig. 15 the measured and predicted beam positions
for the same position, three cavities are shown.

Note that the denoted resolution in the plots is in a global
sense, since the RMS is calculated on the prediction
errors for all different beam offsets. It is not the resolution
which is usually used in beam diagnostics for certainpositions. This will be clarified in the next section.

ISBN 978-3-95450-119-9
Beam Position Monitor System

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 13: Selected beam positions for resolution studies.

TUPA23Proceedings of IBIC2012, Tsukuba, Japanc 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 17: Position resolution of x and y in C2.

Figure 18: Position resolution of x and y in C3.
Figure 15: Measured (blue) and predicted (red) beam
positions in the three cavities C1 to C3.
Figure 16 to Figure 18 show the resolution results takenwith a hundred beam pulses with the MTCA for the selectedbeam positions shown in Fig. 13. The dashed lines
indicate the MTCA resolution in a global sense using across-validation technique [5]. The solid lines are resultsfrom the VME digitizers.

The reached resolution with both digitizers is similar.
But it has to be considered that the VME digitizer works
with 216MS/s and the MTCA with 108MS/s which means
an undersampling of the used 70MHz. The resolution
obtained for several beam positions is in general better
than the global resolution.

CONCLUSIONSThe MTCA SIS8300 digitizer card is a reasonable
alternative to the VME digitizer. The resolution in thisapplication is nearby the same although the sampling rateof the MTCA causes an undersampling.

OUTLOOK
A downconverter with an IF of 30MHz is currently
built by FNAL in order to avoid undersampling. This will
be installed at FLASH. A MTCA-based digitizer cardwith a higher sampling rate (500Ms/s), underdevelopment at DESY, is considered for the XFEL.

REFERENCES
Figure 16: Position resolution of x and y in C1.

[1]
[2]
[3]
[4]
[5]
[6]

S. Schreiber, et al., IPAC12, TUPPP052
P. Zhang, et al., Rev. Sci. Instrum. 83, 085117(2012)
N. Baboi, et al., SRF2011, MOPO060
N. Baboi, this conference, TUPA16
P. Zhang, et al., DESY12-143, 2012B. Lorbeer, et al., this conference, TUPA19

ISBN 978-3-95450-119-9
4Beam Position Monitor System