Proceedings of IBIC2012, Tsukuba, Japan

MOPA46REALIZATION AND MEASUREMENTS OF CONE-SHAPED PICKUPSFOR BUNCH ARRIVAL-TIME MONITORS FOR FLASH AND XFEL*Abstract
At the Free Electron Laser FLASH at DESY, the stateof the art Bunch Arrival-time Monitors (BAMs) have atime resolution better than 10 fs for bunch charges ofmore than 500 pC [1]. With the extension of FLASH II
and the European X-ray Free Electron Laser Project(XFEL) a low charge operation mode with 20 pC bunchcharge is planned. The time resolution of the BAMs
significantly drops as the bunch charge reduces [2]. Highbandwidth BAMs are essential for a femto-second time
resolution of the measured arrival-time in case of low
charge and high charge operation mode of the freeelectron lasers (FELs). The proposed cone-shaped pickup
electrodes with bandwidth up to 40 GHz are
manufactured and measured. Due to the different beam
pipe apertures for FLASH and XFEL, two hermetic
bodies are manufactured. The RF properties of thepickups are measured and compared to the simulation
results obtained by CST MICROWAVE STUDIO®.

The slope steepness reduces with lower bunch charges
leading to significant performance degradations [2]. Thiscan be compensated by increasing the bandwidth of theBAM components. The BAMs comprise RF-pickups, anelectro-optical front-end and read-out electronics [3].
In this paper we present the realization and themeasurements of cone-shaped pickups for the BAMs withbandwidth up to 40 GHz proposed in [4]. The pickups arecharacterized by scattering parameters and the results arecompared to the simulations performed with CSTMICROWAVE STUDIO®.

HIGH BANDWIDTH CONE-SHAPEDPICKUPS FOR BAM
In order to achieve sub-10 fs time resolution for bunchcharges of 20 pC or lower, the bandwidth of the BAM
pickups should be increased up to 40 GHz. With thisbandwidth the voltage slope at the first zero crossing of
the pickup signal is more than 300 mV/ps at 20 pC [6].

INTRODUCTION
For providing an optimal operation of the free-electron
lasers (FELs), which generate ultra short X-ray pulses, thearrival-time of the electron bunches has to be measured
with femtosecond precision. For that purpose Bunch
Arrival-time Monitors (BAMs) are developed andinstalled at the Free Electron Laser FLASH at DESY.
These arrival-time monitors combine the beam induced
signal with an electro-optical detection scheme as
described in [1]. The time resolution of such a detection
scheme depends on the steepness of the pickup voltage
slope at the first zero-crossing. The state of the art BAMs
have an intrinsic time resolution better than 10 fs forbunch charges of more than 500 pC [1]. With theextension of FLASH II and the XFEL, a low charge
operation mode with bunch charge of 20 pC is planned.
___________________________________________*Work supported by the Federal Ministry of Education and Research(BMBF) within FSP 301 under the contract numbers 05H10GU2
and 05K10RDA#angelovski@imp.tu-darmstadt.deFigure 1: Cross-section of the cone-shaped pickups withthe denoted ports.
Figure 1 shows the cross-section of the high bandwidthcone-shaped pickups. The design details can be found in[4 - 6]. Eight pickups have been produced by Orient
Microwave Corp [7]. The pickups are mounted in thestainless steel hermetic body. Due to the different beam
pipe apertures for FLASH and XFEL, two hermetic

ISBN 978-3-95450-119-9
Longitudinal Diagnostics and Synchronization

1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○A. Angelovski#, M. Hansli, A. Penirschke, R. Jakoby, Institut für Mikrowellentechnik und Photonik,
TU Darmstadt, Darmstadt, Germany
C.Sydlo, M.K. Czwalinna, H. Schlarb, M. Bousonville, DESY, Hamburg, Germany
A. Kuhl, Universität Hamburg, Institut für Experimentalphysik
Gruppe Beschleunigerphysik, Hamburg, Germany
S. M. Schnepp, Laboratory for Electromagnetic Fields and Microwave Electronics,
ETH Zürich, Switzerland
T. Weiland, Institut für Theorie Elektromagnetischer Felder,
TU Darmstadt, Darmstadt, Germany

MOPA46

Proceedings of IBIC2012, Tsukuba, Japanbodies are produced with flanges CF63 and CF50
respectively. Figure 2 shows a single pickup (top and sideview) and the pickups mounted in the hermetic body.

The resonance at 32 GHz contributes to the ringing of thevoltage signal.

MEASUREMENT OF THE PICKUPS

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○For characterizing the pickups and validating thesimulation results several measurements are performed
with a Vector Network Analyzer up to 40 GHz. At first,
the pickups shown in Fig. 2 (top left/right) are measured
in free space. The reflection coefficients (S11) obtained
from the eight pickups was compared to a reference one
and the relative error was calculated. Figure 5 shows theobtained results.

Figure 2: Top view of the pickup (top left) and side view(top right) . Pickups mounted in hermetic body (bottom).
During the production process of the hermetic body, thethickness of the copper gasket was not taken into
consideration. This prevents the pickup to be in thevicinity of the beam pipe aperture and creates a step
(0.5 mm) from the beam pipe to the pickup (Fig. 3).

Figure 5: Relative error of the reflection parameters of thepickup compared to pickup No. 0.

Figure 3: Pickup mounted in the body with a step to thebeam pipe.
The step creates an impedance jump and influence the RF
properties of the device. The simulation results in Figure4 show the effect of the gap on the beam induced signal.
The output signal strength is reduced by 5 dB.

The pickups are enumerated from 0 to 7. The pickup
marked as pickup No.0 was taken as a reference. From theplots one can see that the pickup Nr. 7 has the largest
difference in the reflection (up to 16% at 40 GHz)
compared to the other pickups. It is anticipated that thedeviations result from the different manufacturing
tolerances of this pickup. However, up to 35 GHz thepickup has deviation less than 5%. The other pickups(except pickup Nr. 1) have deviation less than 4% in theentire frequency range up to 40 GHz. This is sufficient fordelivering a symmetric pickup signal at the output. Afterthe mounting of the pickups in the hermetic bodies, thecross-talk between the pickups is measured with open
body in free space. The results obtained from themeasurements are compared to the simulation performed
with CST MICROWAVE STUDIO®. Figure 6 shows themeasurement and the simulation of cross-talk for theXFEL body.

Figure 4: Comparison of the spectrum of the pickup
voltage signal with step and no step transition to the beampipe.

ISBN 978-3-95450-119-92Longitudinal Diagnostics and Synchronization

Proceedings of IBIC2012, Tsukuba, Japan

MOPA46

CONCLUSION

There is a relatively good agreement between themeasurements and the simulation in the position of theresonant peaks. The level of the peaks and the Q factor isdifferent due to the different losses in the simulated andthe measured body as well as the relatively low resolutionof the simulation. The big dimensions of the structure aswell as the wide frequency range require very big
computational power for high resolution simulation.
The cross-talk between the ports (see Fig. 1) is below-18 dB. As presented in [5], this minimizes thecontribution of the cross-talk signal to the ringing. Figure
5 shows the measurement and simulation of the cross-talkfor the FLASH body. From the S31 plot ( Fig. 7 (bottom) )one can see that not all the peaks of the measurement fit
with the simulation. The calibration uncertainties andrelatively low simulation resolution can explain thisbehavior. Further investigations are in progress.

Figure 7: Simulation and measurement of the cross-talkfor the FLASH hermetic body.

ACKNOWLEDGMENT
The authors would like to thank the company CST for
providing the CST MICROWAVE STUDIO software
package.
The authors would like to thank Jürgen Kruse,
S. Vilcins-Czvitkovits, P. Gessler, M. Pelzer of DESY,
Hamburg and J. Rönsch-Schulenburg, J. Roßbach of theInstitute for Experimental Physics at Hamburg University,Hamburg for the support.

REFERENCE[1] F. Löhl et al., "Electron Bunch Timing with
Femtosecond Precision in a Superconducting FreeElectron Laser", Physical Review Letters, Vol. 104,
Issue 14, id. 144801, April 2010.
[2] M. K. Bock et al., "Benchmarking the performanceof the present bunch arrival time monitors at FLASH", DIPAC 2011, Hamburg, Germany, 2011, p. 365.
[3] K. Hacker, Ph.D. thesis, Universität Hamburg, 2010.
[4] A. Angelovski et al., "Pickup Design for High
Resolution Bunch Arrival time Monitor for FLAHand XFEL", DIPAC 2011, Hamburg, Germany,2011, p. 122.
[5] A. Kuhl et al., "Sensitivity and Tolerance Analysis ofa New Bunch Arrival-time Monitor Pickup Designfor FLASH and XFEL ",Proceedings of IPAC2011,
San Sebastián, Spain, 2011, p. 1186.
[6] A. Angelovski et al., "High Bandwidth Pickup Designfor Bunch Arrival-time Monitors for Free-ElectronLasers", submitted to PRST-AB in Feb.2012.
[7] http://www.orient-microwave.com/

ISBN 978-3-95450-119-9
Longitudinal Diagnostics and Synchronization

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 6: Simulation and measurement of the cross-talkfor the XFEL hermetic body.

In this paper we have presented the realization and themeasurements of cone-shaped pickup electrodes for theBAMs for FLASH and XFEL. The manufactured pickups
are characterized with S-parameters and the results arecompared to the simulations. From the relative error
analysis we can conclude that two of the eight pickups
show deviations in the reflection coefficient. The otherpickups have deviations in the reflection less than 4%.
The measured cross-talk for both bodies is below -18 dB.
There is a good agreement between measurement andsimulation for the pickups mounted in the XFEL body.
For the FLASH body pickups one can see that some of
the measured resonant peaks (S31 in Fig. 7) do not fit to
the resonant peaks obtained from the simulation.
However, the trend of the measured curve corresponds tothe simulation. The step between the pickups and thebeam line introduced during the mounting influences theoutput signal strength (reduced for -5 dB compared to thecase with no step). The resonance effect at 32 GHz
influence the ringing of the output signal. In the future
implementation the step should be avoided in order tomaximize the output signal with reduced ringing.