Proceedings of IBIC2012, Tsukuba, Japan

MOPA01DESIGN STUDY OF THE STRIPLINES FOR THE EXTRACTION KICKER
OF THE CLIC DAMPING RINGS∗
C. Belver-Aguilar, A. Faus-Golfe, IFIC (CSIC-UV), Valencia, Spain
M.J. Barnes, CERN, Geneva, Switzerland
F. Toral, CIEMAT, Madrid, Spain
Abstract

GEOMETRIC DESIGN STUDY
The stripline kicker proposed for the extraction kicker ofthe CLIC DRs consists of two parallel electrodes of 1.7 mlength inside a cylindrical vacuum pipe: each electrode ispowered by an inductive adder [1]. The two electrodes arecharged to opposite polarity; there is a virtual ground midway between the electrodes. The striplines will be powered, via coaxial feedthroughs, from the beam exit end:
the upstream feedthroughs will be connected to resistive
loads, see Fig. 1. The stripline kicker operates as two
coupled transmission lines, each of which should ideallyhave a characteristic impedance matched to 50 Ω. Coupledtransmission line theory shows two operating modes for thestripline kicker, since three conductors are involved in thesignal transmission, i.e. both electrodes and the vacuum
beam pipe. These modes are known as odd and even mode:
odd mode when the electrodes are excited with opposite
polarity voltages, and even mode when the electrodes are
excited by the unkicked circulating beam when passing
through the aperture of the striplines [2].

Flat and Half-moon Electrode Cross-sections
The electrode cross-section was previously selected bystudying several shapes for the striplines and optimizingthem in order to achieve 50 Ω even mode characteristic
impedance, and ±0.01% of field inhomogeneity over a circle of 1 mm radius at the center of the aperture [2]. An ad-Figure 1: 3D model of a stripline kicker [4].
ditional constraint was to achieve an odd mode characteristic impedance as close as possible to 50 Ω. Flat electrodes
allowed for an optimum characteristic impedance and field
homogeneity with a minimum stripline beam pipe radius of
25 mm. A small stripline beam pipe radius diminishes thenon-desirable effects of the wakefields and results in closer
values of even and odd mode characteristic impedances.
Recent studies have shown that a modified half-moon
electrode, with a reduced coverage angle (see Fig. 2), allows for a better optimization of the odd mode characteristic impedance with a stripline beam pipe radius of 20 mm.

Figure 2: Cross-section of the striplines for flat electrodes(left) and modified half-moon electrodes (right). The circle drawn in the center is 3.5 mm radius in both striplines.
For flat electrodes with 50 Ω even mode characteristic impedance, an odd mode characteristic impedance of
36.8 Ω was achieved with a stripline beam pipe radius of
25 mm [2]; for the newly studied modified half-moon electrode the odd mode characteristic impedance is 40.9 Ω. To
increase this value closer to 50 Ω, the capacitance between∗ Work supported by IDC-20101074 and FPA2010-21456-C02-01

ISBN 978-3-95450-119-9
Accelerator Technology

1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Pre-Damping Rings (PDRs) and Damping Rings (DRs)are needed to reduce the beam emittance and, therefore,
to achieve the luminosity requirements for the CLIC mainlinac. Several stripline kicker systems will be used to inject and extract the beam from the PDRs and DRs. Results of initial studies of the stripline cross-section and thebeam coupling impedance, for a non-tapered beam pipe,
have previously been reported. In this paper, we present
the analysis to study the final choice of the cross-section
design, based on impedance matching and field homogeneity requirements, the power reflected in the transition between an electrode and the input coaxial feedthrough, andthe predicted beam coupling impedance. Mechanical tolerances for the stripline manufacturing process are presently
being studied. The striplines are planned to be prototyped
by December 2012.

MOPA01

Proceedings of IBIC2012, Tsukuba, Japanc 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○an electrode and the virtual ground, midway between bothelectrodes, must be decreased relative to the magnitude ofcapacitance to the beam pipe ground. This can be achieved
either by increasing the distance between the electrode andthe virtual ground, or reducing the cross-sectional area of
each electrode. The distance between an electrode and thevirtual ground, 10 mm, is a fixed value, which corresponds
to the half-aperture for the CLIC DR. The modified halfmoon electrode, which is thicker than the flat electrode,allows for a smaller coverage angle of the electrodes andhence a reduced cross-sectional area.
In addition, field inhomogeneity requirements for the
CLIC DR, i.e. ±0.01% over 1 mm radius, can also beachieved by the half-moon electrode shape. As shown inFig. 3, the field homogeneity is better in the case of themodified half-moon electrode. Furthermore, a better efficiency can be expected since the electric field between thestriplines is higher for the half-moon electrode shape.

Figure 3: Field homogeneity over 1 mm radius for flat electrodes (blue) and half-moon electrodes (red).
The modified half-moon electrodes have been studied
further and, in the following, the features of the half-moon
and flat electrodes (Fig. 2) are compared, which allows thefinal geometric design of the striplines to be chosen.

COAXIAL FEEDTHROUGH TOSTRIPLINE TRANSITION
A total of four coaxial feedthroughs are required to transfer power from the inductive adders to the two electrodes
and from the electrodes to the two, matched impedance,loads. The feedthroughs are coaxial outside of the stripline
beam pipe but the connection from a feedthrough to anelectrode cannot be coaxial (see Fig. 4): hence the characteristic impedance of the connection to the electrode is
not 50 Ω. Furthermore, during the kicker operation, thestripline characteristic impedance is lower than 50 Ω (seeabove). These impedance mismatches result in reflections:the impedance of the connection to the electrode can becalculated using S-parameters.
HFSS has been used to calculate the reflection parameter S11 , looking into an input port when the corresponding
output port is resistively terminated. Initially the value ofthe termination was modelled as 50Ω, however future simulations will consider values not exactly equal to 50Ω. The
results of the initial simulations are shown in Fig. 5.
The simulation used a frequency sweep between
100 kHz to 1 GHz and the results were extrapolated to DC.

Figure 4: Feedthrough simulation with HFSS.
For a good signal transmission, the S11 parameter should beless than 0.1 for frequencies below 0.5/T, where T is the risetime of the pulse from the inductive adder [4]. In the caseof the CLIC DR, this frequency corresponds to 0.89 MHz
and 0.5 MHz for the 1 GHz and 2 GHz baselines, respectively [2].

Figure 5: S11 looking into an input port, when the corresponding output port is terminated with 50 Ω, for flat electrode (blue) and modified half-moon electrode (red), for afrequency range from DC to 1 GHz.

Figure 5 shows that the reflection parameter changes
with a maximum-minimum pattern: the shape of thecurves depend on the coaxial feedthrough to striplinetransition. For CLIC the S11 parameter must be below
0.1 for a good signal transmission. The modified half-moon
electrode shows an excellent transmission, i.e. low
reflection up to 300 MHz, whereas for the flat electrodeshape the reflection parameter is above 0.1 over thewhole frequency range. Therefore, from the powertransmission point of view the modified half-moon
electrode presents a better transmission.

BEAM COUPLING IMPEDANCE
Beam coupling impedance is a very important parameterfor the striplines. Since the DRs are periodic, it is importantto reduce as much as possible the effect of the field produced by the beam itself, in the striplines, when the kicker
is not operating, in order to diminish the wakefield effects
in the unkicked circulating beam. The permissible beamcoupling impedance, of the striplines, is 0.05 Ω/turn and
200 kΩ/m for the longitudinal and transverse beam coupling impedance, respectively [5].
The beam coupling impedance, for an untapered beam
pipe with flat electrodes, has previously been studied andISBN 978-3-95450-119-9
2Accelerator Technology

Proceedings of IBIC2012, Tsukuba, Japan

Figure 6: Longitudinal (top) and transverse (bottom) beamcoupling impedance for flat and modified half-moon electrodes, in a frequency range from DC to 1 GHz.

MECHANICAL TOLERANCES
Two possible errors during the manufacturing process
are being taken into account when studying both designsof electrodes: (1) the position and inclination angle of anelectrode (see Fig. 7), and (2) the dimensions of both electrodes and the beam pipe. The tolerances being studied arein a range of ± 0.5 mm and ± 1 degree.
In order to define mechanical tolerances both evenand odd modes must be taken into account, by studying
the variation of the common mode and differential mode
impedances as a function of various mechanical parameters. Common and differential mode impedances are related to even and odd mode impedances, respectively, byZcomm = Zeven /2 and Zdiff = 2Zodd .
Studies of the tolerances are presently being carried out:
initial predictions show that an error in the position of anelectrode will affect the characteristic impedance of thestripline, whereas if an electrode is placed with a certain inclination angle, both the characteristic impedance and thefield homogeneity will be affected.

Figure 7: Offset of ± 5 mm in the position of an electrode(left) and for an inclination angle of ± 15deg (right): thesevalues are only used for a better visualization.

CONCLUSIONS
Studies of the cross-section of the striplines for the extraction kicker of the CLIC DRs have shown that for a 50 Ωeven mode characteristic impedance of the striplines, themodified half-moon electrode shape results in an odd modecharacteristic impedance closer to 50 Ω than the flat electrodes. Furthermore, a good field homogeneity and higher
efficiency is expected with this modified half-moon electrodes. Simulations show that the longitudinal beam coupling impedance is lower for the modified half-moon electrodes than for the flat electrodes, whereas the flat electrode shape seems to be better from the transverse beamcoupling impedance point of view. The reflection coefficient predicted looking into the input port, with a 50 Ω oneach output port, shows that the transmission is better inthe case of modified half-moon electrodes. Therefore, themodified half-moon electrode is now considered as the best
choice for the cross-section of the striplines for the extraction kicker of the CLIC DRs. However, further studies ofbeam coupling impedance are required.

ACKNOWLEDGMENTS
The authors thank Trinos Vacuum-Projects S.L., responsible for building the striplines, and especially
D. Gutiérrez.

REFERENCES[1] J. Holma, M.J. Barnes, Evaluation of components for the highprecision inductive adder for the CLIC Damping Rings, Proc.
IPAC 2012[2] C. Belver-Aguilar et al., Final Cross-section Design of theStripline Kicker for the CLIC Damping Rings, Proc. of IPAC
2012.
[3] D.M. Pozar, Microwave Engineering, Third Edition.
[4] I. Rodrı́guez, Calculation Methodology and Fabrication Procedures for Particle Accelerator Strip-line Kickers: Application to the CTF3 Combiner Ring Extraction Kicker and TL2
Tail Clippers, PhD thesis (2009).
[5] G. Rumolo, CERN, private communication.
[6] C. Belver-Aguilar et al., Beam Impedance Study of theStripline Kicker for the CLIC Damping Ring, Proc. IPAC
2012.

ISBN 978-3-95450-119-9
Accelerator Technology

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○the results, reported in [6], show good agreement between
analytical calculations and predictions from simulations.
These previous results for a flat electrode, together with
the new CST Particle Studio predictions for the modified
half-moon electrode, are shown in Fig. 6. The modified
half-moon electrodes have lower longitudinal beam coupling impedance than the flat electrodes. This is easily understandable by using the analytical formulas [6], because
in the modified halfmoon the coverage angle is reduced
from 2 radians to 0.9 radians, decreasing the longitudinalbeam coupling impedance. The simulations give a higher
value for the transverse beam coupling impedance in thecase of half-moon electrode than in the case of flat
electrode, that is in contradiction to the analytical formulas
because the stripline beam pipe radius is reduced from25 mm to 20 mm. Therefore, further simulations of beamcoupling impedance are required.

MOPA01