Proceedings of IBIC2012, Tsukuba, Japan

MOPA47PLANAR TRANSMISSION LINE BPM FOR HORIZONTAL APERTURE
CHICANE FOR XFEL
A. Angelovski#, A. Penirschke, R. Jakoby, Institut für Mikrowellentechnik und Photonik,
TU Darmstadt, Darmstadt, Germany
U. Mavrič, C.Sydlo, C. Gerth, DESY, Hamburg, Germany

In order to obtain ultra short bunches in the Free
Electron Laser FLASH at DESY, the electron beam iscompressed in magnetic chicanes. Precise knowledge ofthe beam position in the chicane allows for nondestructive energy measurements of each individualbunch in the beam. In the current implementation, theEnergy Beam Position Monitor (EBPM) pick-ups are
coaxial pick-up striplines mounted perpendicularly to thebeam. One can determine the horizontal beam position bymeasuring the phase difference of the beam induced
signal at opposite ends of a pick-up. Due to the different
electrical and mechanical requirements for the European
XFEL a new EBPM has to be developed.
In this paper, we present the design and analysis of aplanar transmission line structure which is planned to beused as an EBPM in the European XFEL. The planardesign of the pick-ups can provide for a proper impedance
match to the subsequent electronics as well as sufficientmechanical stability along the aperture when usingalumina substrate.

INTRODUCTION
The European XFEL will need a zoo of diagnostic tools
for the study of the longitudinal properties of the electron
beam. The Electron Beam Position Monitor (EBPM) will
be used as part of longitudinal diagnostic tools for theEuropean XFEL. It is composed of the pick-up structureand detection electronics.
The EBPM is an instrument used for bunch energy
measurements [1] and it will be used at three different
locations along the European XFEL LINAC. The energy
measurement is done by means of position measurements,
where the transformation from position to bunch energy isdefined by the formalism of the bunch compressor [2]. As
opposed to a standard BPM, the energy BPM measuresphases to define the absolute position of the bunch in thedispersive section of the bunch compressor. Figure 1
shows the current EBPM installed at FLASH and the#principle of phase detection [1]. Two coaxial pick-up
striplines are mounted perpendicular to the beam. At bothends of the pick-up ceramic disks are placed formechanical support and the pick-ups are tapered towards
the connectors. The difference between the measured
phases of the pulses coming from the left and from theright side of the pick-up is proportional to the position (dxin Fig. 1) of the bunch.

Figure 1: CAD model of the EBPM installed in thesecond bunch compressor at FLASH.
The measurement resolution of the bunch position,
hence energy, depends on the frequency of the detection.
Therefore it is advisable to have the phase detection at arather high frequency. The upper limit is defined by themanufacturing limitations of the pick-up, limitations of
the detection electronics and size of the chicane which
imposes the wavelength, below which the phase detection
might become ambiguous [3]. The resolution of theinstrument also depends on the signal spectral density thatthe pick-up structure transfers from the beam to the output
connector.
As the requirements for the future EBPM at theEuropean XFEL (mechanical and electrical) differs
significantly from the ones at FLASH, a new EBPM
needs to be designed. Table 1 summarizes a comparisonbetween the design parameters for FLASH and theEuropean XFEL.

angelovski@imp.tu-darmstadt.deISBN 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 ○Abstract

MOPA47

Proceedings of IBIC2012, Tsukuba, Japan

Table 1: Design Parameters
FLASH and the European XFELofL
[mm]
H[mm]
Detection freq. [GHz]
Bunch charge [pC]

theFLASH
183
81.3
> 200

EBPM

for

XFEL
400
40.5320This paper presents the design of planar pick-ups for
the XFEL EBPM monitor. Two planar structures areproposed and their performances are compared. For thesimulation of the EBPM pick-ups CST PARTICLESTUDIO® is used.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○DESIGN OF PLANAR TRANSMISSION
LINE PICKUPS FOR THE EBPM AT XFELIn order to fulfill the electrical as well as themechanical requirements for the new EBPM, we propose
the planar pick-up electrodes. The pick-ups need to have aresonance-free spectrum up to 6 GHz. To fulfill this, thepick-ups need to be matched to the subsequent
electronics. Although, for now the detection frequency isdetermined to be 3 GHz we design the pick-ups withhigher bandwidth in order to leave margin for future
upgrades to higher frequencies. The connectors areplanned to be perpendicular to the transmission line. With
such a structure the pick-ups will be independent from thechicane and can be mounted or dismounted without
removing the entire chamber. However, this is one of thecritical points which influences the bandwidth of the pickups. Optimization of the transition from the connector tothe transmission line is still in progress. The use of "hard"substrate (Alumina, Glass, etc.) can provide for sufficientmechanical stability. Figure 2 shows the sketch of thisstructure.

Figure 3: Microstrip transmission line (top). GCPW
transmission line (bottom).
The connectors for both structures are perpendicular to
the transmission line. In the case of GCPW, vias areplaced in order to short-cut the field. Alumina withɛr = 9.8 is used as a substrate and copper as metallization
layer for both structures. The design parameters areshown in Table 2.
Table 2: Design Parameters for the Planar Transmission
Lines
Substrate height [mm]Line thickness [mm]Line width[mm]Line gap[mm]Loss[dB/m]

Microstrip
1.3
0.006
1.55/2.4

GCPW
1.50.006
1.3
20
3.6

Using the planar transmission lines (microstrip andGCPW) two rectangular chambers are built (as shown inFig. 2) and simulated. The simulation is performed with
CST PARTICLE STUDIO® with a beam charge of
100 pC and a beam width of 1.8 mm. Figure 4 shows theresults of the simulation.

Figure 2: Cross-section of planar transmission line pickups for the EBPM.
Two planar structures are investigated in this paper:
The microstrip transmission line and Grounded Co-planar
Waveguide (GCPW) transmission line. Details of thedesign and analysis are presented in [4]. Planar structures
used as pick-up/kicker electrodes for stochastic coolingare described in [5,6]. Figure 2 shows the CAD design ofthe planar structures.

ISBN 978-3-95450-119-9
2Longitudinal Diagnostics and Synchronization

Proceedings of IBIC2012, Tsukuba, Japan

MOPA47

From the results we can see that the coaxial pick-ups
become resonant at 2.7 GHz. The resonant peaks in thespectrum are due to the miss-match (the ceramic discs inFig. 1) and correspond to twice the length of the pick-up.
The FoM for the coaxial pick-ups is -11.17 dB/Hz and islower (-0.7 dB/Hz) compared to that of the microstrip
line.

CONCLUSION AND FUTURE WORKThe simulation shows that both planar transmission
lines have similar performance. However, the output
signal for the GCPW transmission line rings longer andwith higher amplitude. As a Figure of Merit (FoM) weuse the voltage level at 3 GHz. The microstrip
transmission line has -0.7 dB/Hz and the GCPW line has-2.175 dB/Hz. The lower FoM for the GCPW line
corresponds to the higher losses and the higher ringing
compared to the microstrip line. Based on the simulation
results as well as on the fabrication complexity, for
producing the pick-ups we have selected the microstrip
transmission line pick-ups to be the candidate for theEBPM at the European XFEL.
In order to assess the output signal of the microstrip
transmission line pick-ups we have compared it to thesignal obtained from the coaxial pick-ups (see Fig. 1).
The dimensions of the chamber are the same for bothscenarios. Figure 5 shows the spectra for the planar andcoaxial pick-ups.

ACKNOWLEDGMENT
The authors would like to thank the company CST for
providing the CST PARTICLE STUDIO® software
package. The authors are grateful to Silke VilcinsCzvitkovits for helpful discussions and comments.

REFERENCE[1] K. Hacker, "Measuring the Electron Beam Energy ina Magnetic Bunch Compressor", PhD Thesis,
Hamburg, 2012.
[2] P. Castro, "Beam trajectory calculations in bunch
compressors of TTF2", DESY Technical Note 03-01,April 2003.
[3] U. Mavric et al., "Preliminary measurement results ofthe upgraded Energy BPM at FLASH", Proceedingsof IPAC2012, May 2012, New Orleans, Louisiana,USA, p.813.
[4] J. Coonrod et al., "Comparing Microstrip and CPW
Performance", Microwave Journal, Vol. 55, No. 7,
p. 74, July 2012.
[5] F. Caspers, "Planar Slotline Pick-Ups and Kickers
for Stochastic Cooling", Proceedings of the 12th
IEEE Particle Accelerator Conference, 16-19 Mar
1987, Washington, DC., p.1866.
[6] C. Peschke et al., "Planar pick-up electrodes forstochastic cooling", Nuclear Instruments andMethods in Physics Research Section A:Accelerators,Spectrometers,Detectors
andAssociated Equipment, Vol. 532, Issues 1–2,Oct. 2004, p. 459.

Figure 5: Comparison of the spectrum for microstrip
transmission line and coaxial pick-ups.

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 4: CST PARTICLE STUDIO simulation of theplanar pick-ups. Voltage in time domain (top), voltage infrequency domain (bottom).

In this paper we have presented the design of planartransmission line pick-ups for the EBPM to be installed at
the European XFEL. Two types of planar structures areproposed and analyzed. Based on the obtained simulation
results the microstrip transmission line pick-ups wereselected as a candidate for the new EBPM. The microstrip
transmission line pick-ups show the best FoM comparedto the GCPW and the current coaxial pick-up design.
A non-hermetic prototype of the EBPM with planar
pick-ups is planned to be built in near future formeasurement purposes and prove of concept.
Optimization of the transition from the connectors to themictrostrip line is in progress as well as analysis for
maximizing the signal for different substrate thickness.