Proceedings of IBIC2012, Tsukuba, Japan

TUTA02

BPM ELECTRODE AND HIGH POWER FEEDTHROUGH – SPECIAL
TOPICS IN WIDEBAND FEEDTHROUGH
Makoto Tobiyama#
KEK Accelerator Laboratory, 1-1 Oho, Tsukuba 305-0801, Japan
Since most of the beam in accelerator runs in thevacuum chamber consists of metal, it is needed to have'feedthrough' to get or to put the RF signal from or to
beam. For example, we can get the beam signal by using
button-type electrode which have electrical isolationmaterial to seal the vacuum. Now, many types of vacuum
feedthrough with coaxial structure are available commercially. Nevertheless, it is meaningful to understand thedesign principle of the feedthrough needed for the beam
instrumentation, especially for short bunch length, highbeam current machine. I will show the design method of
the feedthrough such as BPM electrodes or high power
feedthroughs using 3D EM-codes such as HFSS or
GdfidL based on several examples developed forSuperKEKB accelerators.

optimize the S-Parameters, and the other is time domain
method to calculate the beam induced signal such as wake
filed or the output signal of the electrode. Finally, several
examples of the button electrodes and feedthroughs used
KEKB and SuperKEKB accelerators are shown.

REVIEW OF THE TRANSMISSION
THERORY
There are many good textbooks such as reference [1, 2]
to introduce the microwave theory and techniques, wehere only show several definitions used afterwards.

Frequency Domain View
Figure 2 shows a trans-mission line with characteristic
impedance of Z0 with a termination Zr and a resulting
SWR for wavelength.

INTRODUCTION

dmin

/2

In particle accelerators, so many wideband feedthroughs are used to monitor the electromagnetic signal
and to excite or damp the beam. Figure 1 shows examplesof the feedthroughs used in KEKB rings. Though it iseasy to purchase good feedthroughs commercially in themarket, understanding the characteristics of the
feedthrough is still very important.

|V|max|V|minZ0

Zr

Figure 2: Transmission line with a termination.
The Standing Wave Ratio (SWR) S can be written
using Vmax and Vmin or reflection constant  as
(A)

S

(B)

V max
V min



1 
1 

and the load impedance can be estimated as

zr 
(C)

(D)

Figure 1: Feedthroughs used in KEKB rings: (A) Cutmodel of N-type button electrode for LER. (B) Highpower feedthrough for the transverse kicker. (C) Bunch
position detection chamber for bunch-by-bunch feedback.
(D) SMA-type button electrode.
In this tutorial, we at first briefly review thetransmission line theory which is needed to understandthe specification of the feedthrough. Next, we show theexample of the two widely used simulation tools to design
the feedthrough: one is the frequency domain method to___________________________________________#makoto.tobiyama@kek.jp

Z r 1   1  jS tan  d min
Z 0 1   S  j tan d min

where    LC in the case of loss less transmission line.
In a linear two port network shown in Fig. 3, we candefine the S-Parameters as follows: b 1   S11
   
 b 2   S 21

S12  a 1  
S 22  a 2 

I1
Zg

I2

a1

Vg

V1
b1
Port 1.

Z0

A lineartwo-port
network

a2
Z0

ZL

V2
b2
Port 2

Figure 3: A linter two-port network.

ISBN 978-3-95450-119-9
Feedbacks and Beam Stability

1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Abstract

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Proceedings of IBIC2012, Tsukuba, Japan

S11 is the input reflection coefficient (input match) andS22 is the output reflection coefficient (output match). S21
is the forward transmission coefficient (gain or loss) andS12 is the reverse transmission coefficient (isolation). The
S-Parameter has many good points, such as:
 It is relatively easy to measure S-Parameter at highfrequency region.
 It is related to familiar measurements such as gain,
insertion loss SWR, or return loss.
 It can be easily cascaded with multiple devices.
 It has good compatibility with the E-M simulation
software.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Time Domain View
We can estimate the impedance of the load using thevoltage ratio between the forward and reflecting wave. If
we send a short pulse with sharp rise time and flat top, it
is easy to distinguish the forward pulse and the reflecting
pulse using very wideband oscilloscope. This method iscalled as Time Domain Reflectometry (TDR). As thismethod is so intuitive and very easy to understand, it is
widely used to hunt the failure point in transmission line
components such as cables, connectors or feedthroughsaround the accelerator. Note that theoretically it ispossible to estimate the impedance with imaginary
components such as capacitor or inductor, the estimation
easily includes huge error coming from the loss of thecables or small impedance mismatch. Also, theoretically
it is possible to de-convolute the multiple mismatch effectfrom the source to the DUT (device under test),reflections from the far point is easily smeared and noteasy to distinguish the effect.

Comparison of the Two Method
As most of the feedthrough has only one RF port (airside) and the other side is open, measuring the SParameters is not easy. Only S11 measurement is possible
and meaningful. In case if there exist dangerous high-Q
trapped structure in the feedthrough, it might show sharp
drop structure in S11 measurement. Nevertheless, if one
need to measure the pure RF characteristics of the
feedthrough, it might be a good idea to prepare speciallydesigned feedthrough with both side has good RF
connector.
On the other hand, TDR measurement does not need
impedance matched port on the vacuum side. You caneasily measure and check the internal structure of the
feedthrough and the connected components after the
installation to the beam chamber.

ITEMS NEEDED TO CONSIDER BEFOREDESIGNING THE FEEDTHROUGH
Size of the Feedthrough—Cutoff Frequency ofTE11 Mode
The signal attenuation of a coaxial cable comes fromimperfect conductors and insulator (loss tangent). Bothcontributions of the attenuation length (dB/m) areproportional to the frequency. If the insulator is fixed, it
is inversely proportional to the diameter of the coaxialline.
The lowest higher-mode propagation in the coaxial lineis the TE11 mode. The cutoff frequency f c of a coaxial
line with inner conductor radius of a and inter radius of
the outer conductor b is expressed as:fc c a  b   R

where c is the speed of light and R is the relative
permittivity of the insulator. For example, the cutoff
frequency of RG-223 cable is around 30 GHz and EIA
7/8” cable is around 6.8 GHz. Part of the signals withfrequency higher than the cutoff frequency change thepropagation mode in the coaxial line from TEM to TE11
mode and cannot be received correctly with thetermination designed to receive TEM mode signal. Also
as in the higher-mode propagation, the phase velocity ofthe signal depends on the frequency, the shape of thesignal in the time domain is wildly deformed. The size ofthe feedthrough should be chosen much smaller than thesize which passes TE11 mode cutoff frequency.

Support of the Inner Conductor
In a connector or a feedthrough, most of the material
between the two conductors is air or vacuum, with  R is
almost 1. It is needed to support the inner conductor atsome point with other dielectric materials. In the air, we
can use lower  R materials of around 2, such aspolyethylene or Teflon. There are mainly four types of
low-reflection bead supports [1] and used as the wideband
connectors in the air. Unfortunately for the feedthroughfor vacuum use, it is not so easy to directly apply suchmethod because of much large R in vacuum sealingmaterial and required mechanical toughness there.
On the feedthrough for vacuum use, alumina ceramic of R ~ 10 or glass  R ~ 4 are widely used for the support ofthe inner conductor and the vacuum sealing. The seal
needed to be stand the mechanical stress coming from thevacuum pressure, thermal stress due to baking process ofthe chamber and heating up due to the passing the beam
power, and huge pressure when connecting the cable orattenuator to the feedthrough. In the view of signal
condition, it is needed to keep good RF contact undermuch severe conditions such as many heat cycles andradiation effect. Note that use of materials including
halogen, such as Teflon or polyvinyl is extremely
dangerous because they easily decompressed by theradiation and damage other accelerator components
including feedthrough itself.
During the fabrication process of the feedthrough, hightemperature (around 800 deg. C) brazing or melting isapplied. For the outer or inner conductor metal, it isdesirable to have very similar thermal expansion
coefficient with the sealing materials. Kovar (Nickelcobalt ferrous alloy) is widely used as it is designed to beISBN 978-3-95450-119-9
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Proceedings of IBIC2012, Tsukuba, Japan

FREQUENCY DOMAIN SIMULATION
For the example of the frequency domain EM
simulation, we show ANSYS HFSS[3]. It is 3D full-wave
electromagnetic field simulator and can calculate Sparameters, field patterns, eigen values or impedances.
Figure 4 shows an example of BPM model for
SuperKEKB BPM feedthrough. Note that both ports (RFconnector port and the button head) are assigned to be thecompletely matched port on the simulation.

TIME DOMAIN SIMULATION
Another kind of 3D electromagnetic simulation is thetime domain simulation which simulate the beam
response such as wake function or port outputs in time
domain. For an example, GdfidL[4] simulator is shown.
GdfidL is an electromagnetic filed simulator which cancalculate time dependent fields in loss-free or lossy
structure. The field may be excited by port mode or
relativistic line charge. Also resonant fields in loss-free or
lossy structure can be calculated. From the calculated
results we can easily extract the S-parameters, includingtime evolution of the extracted power to the port, wakepotentials including loss factor, coupling impedance, or Q
values and shunt impedance of a resonant structure. As itis time domain simulator, it needs large-scale parallel
computing resources to finish the simulation within arealistic time. Also, as the input is the simple text, unlike
the beautiful 3D CAD like HFSS, it is not so user friendly.
The text input has, however, merit we can change someparameters by changing only one line in the input.
Translation script from other CAD interface or 3D EM
simulation code[5] might help for complicated structure.
Figure 6 shows the cut plot of BPM electrode for
KEKB feedback system. In Fig. 7, the simulated BPM
output for the =6 mm bunch (A) and the longitudinal
wake potential (B).

Figure 4: HFSS Modeler.
We set the target frequency, accepted delta-S andfrequency range to sweep. HFSS automatically generate
suitable mesh and iterate the calculation until delta-S
condition satisfies. An example of calculated Sparameters of this model is shown in Fig. 5.
Figure 6: GdfidL model of KEKB-FB BPM .

(A)

Figure 5: Simulated S-parameters.
In the case of most feedthroughs, HFSS simulation timeneeded is fairly short.

(B)

Figure 7: Examples of simulated results of GdifdL : (A)Output of BPM with 1 pC charge with =6 mm, (B)Longitudinal wake potential and loss factor.

ISBN 978-3-95450-119-9
Feedbacks and Beam Stability

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○compatible with the thermal expansion characteristics ofborosilicate glass. Note that Kovar is magnetic substanceso need care to use near the strong magnetic field such as
quadrupole magnets. Thermal conductivity of the Kovar
is also not so good so need to care about the heating up of
the inner conductor or stripline plate or button connectedto the inner conductor. For the use of strong magnetic
field such as near the superconducting magnets, use oftitanium might be a good candidate. Other materials, suchas cupper or aluminium might be used.

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Proceedings of IBIC2012, Tsukuba, Japan

In GdfidL, the user needs to decide the mesh size andwake length. Typical simulation time strongly depends onthe mesh size, total size of the object and the length of
wake. The example shown here (wake calculated up to5m) took about 4 hours using 64 cpu cores.

EXAMPLE OF FEEDTHROUGH DESIGN

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Button electrodeThe design of the button electrodes for SuperKEKB
rings has already been reported in Ref. [6] so here weshow some tips to design the electrode.
 Normal SMA-J connector, which has split-pin on the
feedthrough side, has difficulty in RF contact
because we cannot expect any spring action of thecenter-pin material after the blazing process. We
have modified the split-pin to solid-pin like P-sideconnector (“Reverse J-type connector”) for bettercontact.
 We prefer gold-plating at the RF connector also forbetter RF contact. During the blazing process, gold
easily diffused into the bulk metal. It is not so easy tocontrol the thickness. Also, we need to care about the
reduction of the resistance between the center
conductor and outer conductor due to the plating
residuals.
 We need to pay attention to the possible trappedmodes in the sealing structure, which might damage
the seal on high beam, multi-bunch condition. The
trapped mode can be estimated using eigen-mode
solver or fine frequency sweep in S-parameter
calculation.
 The narrow gap between the button electrode and thevacuum chamber might cause multipactoring after
passing the bunch, especially for high single-bunch
current machine such as light source or traditional
high energy colliders.
 The longitudinal coupling impedance of the button
electrode might not be negligible on large
circumference, high beam current machine. It mightcause longitudinal coupled-bunch instability.
Though the current simulation codes are fairly reliable,
it is still meaningful to test the devices using real beam,such as short pulse from linac. Figure 8 shows an example
of measured beam signal from KEKB-LER FB electrode.
We can make the bunch length of the linac beam much
shorter than the beam in the ring using pulse compression
at the transport line. The beam response of the device
form such short beam includes very high frequency
characteristics of the target device. In this case, the use ofvery wideband realtime oscilloscope might be needed
because the beam from linac might change pulse by pulse,and preparing good timing signal for the trigger might not
be easy. Figure 9 shows examples of beam response of(A) KEKB-FB button electrode and (B) SuperKEKB-FB
button electrode using test beam line of KEK-PF beamtransport line. As expected, better time response is shownin SuperKEKB-FB button electrode which uses low Rglass as sealing material.

(A)KEKB-FB(B)SuperKEKB-FB

Figure 9: Button signal using short linac pulse: (A)KEKB-FB type electrode (B) SuperKEKB-FB typeelectrode with glass seal.

High Power FeedthroughFigure 8: Measured beam signal from FB electrode of
KEKB-LER. Sampling scope of bandwidth of 20 GHz
was used.

For the bunch feedback use, we need (semi-)wideband
feedthrough to pass power at the range of kW. As thetarget center frequency and the bandwidth is muchsmaller and narrower than the case of button electrode,
we can use simple but solid structure for the vacuum
sealing. In KEKB, we have used 20D-type structure, thesize were just same as EIA-7/8”, for the feedthroughs ofISBN 978-3-95450-119-9
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Proceedings of IBIC2012, Tsukuba, Japan(A)Simulated S21 and S11

We have also tested the increase of the temperature of
the ceramic using 1.3 GHz high power amplifier. The
measured temperature rise without water cooling outside
the ceramic was 4 deg per 100 W. Using water coolingchannel outside the ceramic structure, we expect the
feedthrough can be usable up to around 5 kW CW.

SUMMARY
In this tutorial, we at first reviewed the transmission
line theory briefly. The S-parameters, SWR, reflection
constant and the Time Domain Reflectometry has beenreviewed.
Next, two well-known 3D E-M simulation software,
HFSS for frequency domain simulation and GdfidL fortime domain simulation has been shown.
Finally, we showed several examples of the
feedthroughs used KEKB rings and prepared forSuperKEKB rings with some tips useful to design the
button electrodes and feedthroughs.
Though it is fairly easy to buy good feedthroughscommercially available in the market, designing andfabricating your own special feedthrough might help toincrease the total performance of your diagnostic systembecause the feedthroughs are placed nearest position to
the beam.

ACKNOWLEGEMENT
The author would like to express his sincere
applications to the colleagues of beam instrumentationgroup of KEKB, KEK-PF for their help to design,
fabricate and evaluate the button electrodes andfeedthroughs.

REFERENCES(B) Simulated SWR[1] Peter A. Rizzi, “Microwave Engineering”, Prentice
Hall, ISBN 0135867029.
[2] Harry A. Atwater, “Introduction to Microwave
Theory”, Krieger Pub Co; ISBN 0898741920.
[3] ANSYS HFSS, http://www.ansys.com/[4] GdfidL : http://www.gdfidl.de/[5] T. Abe, Transportation script from Microwave Studioto GdfidL, private communication[6] M. Tobiyama et. al., in proceedings of BIW10.
[7] P. McIntosh, et. al., in proceedings of PAC 2003,
p3141 (2003).

Figure 10: Simulated frequency response of the highpower feedthrough for SuperKEKB longitudinal kicker.

ISBN 978-3-95450-119-9
Feedbacks and Beam Stability

5c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○the transverse stripline kickers and cavity-typelongitudinal kickers. They worked very well withoutalmost no trouble until the shutdown of the rings. Only
one exception was the vacuum leak of one feedthrough of
the longitudinal kicker due to intrinsic inferior of the 20D
anchor contact at the air side: the RF contact had failed
due to passing beam power and became open-contact sogigantic SWR had attacked the ceramic structure to breakit.
During the design work of the new longitudinal kicker
for PEP-II, we found that the small change of SWR of the
feedthrough within the pass band affects the quality factorof the kicker too much[7]. For the longitudinal kicker for
SuperKEKB rings, we have designed the EIA-7/8” feedthrough with the VSWR in the pass band (500 MHz to 2GHz) less than 1.4. Also, to reduce the insertion loss atthe vacuum seal, Si3N4 was used as the ceramic. Figure 10
shows simulated S-parameters and SWR. We haveconfirmed this type of feedthrough does not affect the Qof the longitudinal kicker for SuperKEKB by the HFSS
simulation.

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