OBSERVATION OF WAKEFIELD EFFECTS WITH WIDEBAND
FEEDTHROUGH-BPM AT THE POSITRON CAPTURE SECTION OF THE
SUPERKEKB INJECTOR LINAC
M. A. Rehman∗ , T. Suwada
High Energy Accelerator Research Organization (KEK), Tsukuba, Japan
Abstract
At the SuperKEKB injector linac, positrons are generated
by striking electron beam at tungsten target. The secondary
electrons are also produced during positron creation andaccelerated in the positron capture section. A new wideband feedthrough–beam position monitor (BPM) system
was developed for synchronous detection of secondary produced e− and e+ beams with temporal separation of about
180 ps. When e+ /e− bunches pass through the accelerating
structure or vacuum duct of different radius, they generate
wakefields. These wakefields can be directly observed withthe feedthrough-BPM. A simulation study has also been
carried to validate the observed wakefield effects with thefeedthrough-BPM. The effects of wakefields on beam parameters will be reported in this paper.

INTRODUCTION
The SuperKEKB (SKEKB) [1, 2] is an electron andpositron collider with asymmetric energies to study CP
violation in B mesons and also to search for new physics
beyond the Standard Model, with the target luminosity of
8 × 1035 cm−2 s−1 , which is 40 time higher than its predecessor KEKB [3]. The SKEKB collider consists of e− ande+ rings of energy 7 GeV (HER) and 4 GeV (LER) with thestored beam current of 2.6 A and 3.6 A, respectively.
The SKEKB injector linac generates e− /e+ bunches of
5 nC and 4 nC to directly inject into HER and LER, respectively, at their designed energy. The low emittance e− beam
is produced by a RF–photocathode gun. The e+ beam is
produced by striking the e− beam of energy 3.5 GeV andbunch charge of 10 nC at a tungsten target. The positrons
are generated as secondary particles and have a large transverse emittance. To capture a large amount of positrons, apulsed solenoid called flux concentrator and a large aperture
S–band (LAS) accelerating structures [4] are placed in thedownstream of the e+ target. The secondary electrons arealso produced in a similar amount of charges during thee+ creation process and accelerated in the capture section.
Because of phase slipping process in the capture section,
the time interval between secondary produced e− and e+bunches is about only 135 ps under nominal operation.
Due to low frequency response and high–frequency cable
losses, conventional beam monitors, i.e., stripline beam position monitors, are difficult to detect such closely spaced andopposite polarities bunches in the capture section. For thisreason in the past capture section there were no beam moni∗ rehman@post.kek.jp

toring devices. As a result of a lack of information about thebeam properties such as transverse positions, bunch lengths
and, bunch charges, the e+ beam suffers some amounts ofbeam loss after the capture section.
Therefore, a new wideband feedthrough type BPM wasdeveloped to overcome the above–mentioned challenges.
This new monitor can synchronously detect e− and e+ bunchproperties, i.e, transverse positions, bunch lengths, timeinterval, and bunch charges. It provides an opportunity toenhance positron beam transport through the capture section.
The detailed analysis of bunch properties is reported in [5,6].
The modal analysis of electromagnetic coupling between
SMA-feedthrough and beam is described in [7]. The effect of
wakefields induced by the passage of e− /e+ bunches throughthe end of the accelerating structure on the BPM signal will
be discussed in detail in this report.

WIDEBAND FEEDTHROUGH BEAM
POSITION MONITOR
The feedthrough–BPM consists of a vacuum pipe oflength 431 mm which has an inner diameter of 38 mm, four
SMA-type feedthroughs having inner conductor made ofKovar with π/2 rotational symmetry are installed at the upstream direction of the vacuum pipe. The diameter of thecentral connector pin of feedthroughs is 1.8 mm, and theyextend 1 mm to the center of the beam pipe from the innersurface of the vacuum pipe. The vacuum pipe of BPM also
has bellows mounted at the downstream and upstream direction for flexible installation. The upstream bellow is shieldedin order to any suppress unwanted wakefields. In the upstream direction, BPM is attached with the LAS accelerating
structure. The 3-D model of the new wideband feedthrough–BPM with LAS structure is shown in Fig. 1 (a). Figure 1 (b)shows the front view of the new monitor with dimensions.
Two new feedthrough–BPMs were installed in the positron
capture section. The horizontal and vertical steering coils
are also installed at the same locations in the capture section
to optimize the e+ transmission through the capture section.
The entire capture section is enclosed in the DC solenoid
coils for efficient transmission of large emittance e+ beam.
For the sake of simplicity, they have been omitted from Fig. 1.
The details can be found elsewhere [2, 5].
The SMA connectors of the feedthroughs are first connected to 2 m–long semirigid coaxial cable due to its protection against a high radiation environment. Later the semirigid cable was connected to 15 m–long 10D and 2 m–long
RG223 coaxial cable [8]. The RG223 coaxial cables are thenconnected to a real–time keysight oscilloscope of bandwidthFigure 1: (a) The mechanical model of the feedthrough–BPM with LAS accelerating structure (only last three structuresand output coupler). The cabling and connections of feedthrough-BPM to wideband oscilloscope are also shown. (b) Front
view of the feedthrough-BPM.
33 GHz and a sampling rate of 128 GS/s [9] with a 10-dB
attenuator. The cable losses have been measured in advanceby a vector network analyzer [10] and are de-embedded fromthe feedthrough–BPM signals by the oscilloscope.

WAKEFIELD SIMULATION OF
WIDEBAND FEEDTHROUGH–BPM
When a charged particle travels through any obstacle (i.e.,
accelerating structure, beam pipe of different radii, etc.), it
generates electromagnetic (EM) fields that are left behind theparticle. This EM field is called wakefield [11]. These wakefields can act on subsequent bunches (long–range wakefield)
and can also act on trailing particles of the same driving
bunch (short–range wakefield). In order to understand thewakefield field effect on the feedthrough-BPM signal, a timedomain simulation in CST-wakefield solver [12] has been
done. In this section, simulation of single and double bunch
beams and their effects for the feedthrough–BPM signals
will be described.
The 3-D model used in the simulation is shown in Fig. 1(a). Figures 2 (a) and 2 (b) present the simulation results ofa single bunch e− beam through the new monitor with andwithout accelerating structure, respectively. For the simulation the e− bunch length (1σ) was assumed 10 ps with thebunch charge of 5 nC. Since the cutoff frequency in the experiment was chosen to 10 GHz by applying a 4th–order Bessel
filter. Therefore, a Bessel filter with the cutoff frequency of
10 GHz is also applied to simulation results. In Fig. 2 (a)the typical bipolar signal from the feedthrough–BPM [13](without accelerating structure) appears and after the passage of high–energy e− bunch, a small noise appears due tothe wakefield produced by the protrusion of feedthrough.

Figure 2: (a) The CST-wakefield simulation of the high–energy e− single bunch of the feedthrough–BPM without
LAS structure. No large ringing signal appears after the bipolar signal from the feedthrough–BPM. A very small noise
appears only due to the 1 mm protrusion of the feedthroughs
toward the center of the vacuum duct. (b) The CST-wakefield
simulation of the feedthrough–BPM with the last three cavities and output coupler of LAS structure. The large ringing
signal immediately appears after the bipolar signal of the e−
beam.
Whereas in the case of a simulation with the accelerating
structure, a large ringing signal appears after the main bipo-

bipolar signal differ with some errors from the bipolar signal
without accelerating structure.

EXPERIMENTAL RESULTS
In order to estimate the effect of the wakefield on theexperimental signal of the double bunch, it is assumed thatthe wakefield–induced voltage oscillates and decays [14] asfollows:Õ−ω n(1)V(t) =
an sin[ωn t + ϕn ]e 2Q n t ,nwhere V(t) is the induced voltage by the wakefields, n represents the mode number, ωn resonant angular frequency of
the accelerating structure, Q n is the quality factor, and ϕnshows the phase delay between different excited modes.

Figure 3: (a) The independent simulated signals of secondary e− and e+ bunches. (b) The interference of secondarye− and e+ signals.

lar signal as shown in Fig. 2 (b). The long–range wakefield
generated by the passage of e− bunch through the accelerating structure will reach to the feedthrough–BPM muchlater. Moreover, the output coupler of the accelerating structure also acts as a dumper for the higher-order modes of
wakefield.
When e− bunch passes through the end duct of accelerating structure, which has an inner diameter of 30 mm, tothe feedthrough-BPM with the inner diameter of 38 mm, a
wakefield produced due to this abrupt change of vacuum
duct diameter. This short–range wakefield travels with the
speed of light and emerges as an immediate ringing signal
after the beam signal, as shown in Fig. 2 (b). Therefore, thearea and bunch width for the second lobe of the bipolar signal differ with some errors from the bipolar signal without
accelerating structure.
Since it is not possible to simulate the double bunch
structure with the opposite polarities in the CST wakefieldsolver,e− and e+ bunches were considered independently
and superimposed on each other to find the total wakefield
effect. Figure 3 (a) shows the e− and e+ bunches independently in blue and red colors, respectively. In the nominal
operation, the intensity of e+ charges is about twice of secondary e− , therefore e+ intensity was assumed higher thanthat of the e− bunch. The time interval between the electron
and positron bunch was taken 135 ps. Figure 3 (b) shows
the superposition of e− and e+ bunches. The e− bunch (firstbunch) exposed to only short-range wakefield, whereas thee+ bunch experiences the wakefield generated by the e−bunch and also self-generated wakefields. The area, bunch
width, and time interval for the e+ bunch’s first lobe of the

Figure 4: The synchronous signal of e− (first)/e+ (second)
bunch at nominal accelerating phase from the new
feedthrough–BPM, the cutoff frequency was set to
fc = 10 GHz. The fast oscillation and decay signal arises
due to the wakefields of the e− /e+ bunches.
The model described in eq. (1) is used to fit the experimental data and then extrapolated it on the bipolar signal to estimate the effect of the wakefields on the beam
parameters. Figure 4 shows the synchronous signal ofe− (first)/e+ (second) at nominal accelerating phase fromthe new feedthrough–BPM, the cutoff frequency was set
to fc = 10 GHz. The red curve in Fig. 4 is a double bipolarsignal from the e− and e+ bunches, whereas the curve shownin black presents induced voltage on the feedthrough–BPM
due to the wakefield, which exhibits fast oscillation and decay. These wakefields induced voltages were fitted by using
eq. (1) as shown in blue color in Fig. 4.
The induced voltages by the passage of the e− /e+ bunches
through the different radius of vacuum duct are a superposition of the several higher–order modes excited by the wakefields. Therefore, fitting to experimental data was done withthe different values of n. Figure 5 shows the variation in
reduced χ2 to experimental data as a function of the higher–
order modes "n". It can clearly be seen at the value of n = 6
the reduced χ2 value becomes ∼ 1. Therefore, n = 6 waschosen for the fitting function.
The fitting function is also a superposition of the wakefield of e− and e+ bunches, as the e− bunch is ahead of thee+ bunch therefore, the e− bunch does not witness any wakefields by the e+ bunch. For this reason, the fitting function
was extrapolated only to the bipolar signal of the e+ bunch.
In Fig. 4 the magenta curve is the extrapolation of the fitting
function to the e+ bipolar signal. The green curve in Fig. 4
shows the constructive interference of the extrapolated fitting
function to the e+ bunch. The interference of the wakefields
induced voltages on the e+ bipolar signal contributes to errors in area, bunch width, and time interval with taking into
account the phase, respectively for the e+ bunch’s first lobeof the bipolar signal. The comparison of the wakefield errors
for the beam parameters between experiment and simulation
is summarized in Table 1. The estimated wakefield errors
for the beam parameters are consistent in experiment andsimulation.
Table 1: Comparison of the Wakefield Errors for Beam
Parameters Between Experiment and Simulation
Wakefield Errors
Beam Parameters
Area
Bunch width
Time Interval

Experiment

Simulation

41%
1.2 ps
2.5 ps38%0.72 ps
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Figure 5: Variation in the reduced χ2 of the fitting function
to the ringing signal of e− /e+ bunch. For n = 6 reduced χ2
becomes ∼ 1.

SUMMARY
The short–range wakefield has been observed as a ringing
signal after a double bipolar signal in synchronous measurements of the e− and e+ bunches with the wideband
feedthrough-BPM system at the e+ capture section of theSuperKEKB factory. A simulation study and analytical function fitting on experimental data have been successfully carried out to estimate the effect of the wakefield on the beam
parameters measured by the new beam monitor.

ACKNOWLEDGMENTS
We thank Profs. Toshiyasu Higo and Tetsuo Abe for theirgenerous support and guidance for this project.

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