FIRST SYNCHRONOUS MEASUREMENT OF SINGLE-BUNCHED
ELECTRON AND POSITRON BEAMS WITH A WIDEBAND
FEEDTHROUGH-BPM AT THE POSITRON CAPTURE SECTION OF THE
SUPERKEKB INJECTOR LINAC
M. A. Rehman∗ , T. Suwada, F. MiyaharaHigh Energy Accelerator Research Organization (KEK),
1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan
Abstract
The SuperKEKB is an asymmetric 𝑒 − /𝑒 + collider with40 times higher luminosity than the KEKB project, to explore the new physics beyond the standard model. For theSuperKEKB, the positrons are created by striking the accelerated electrons at a tungsten target. The secondary electrons
are also produced during the positron creation process andaccelerated in the capture section. Because of phase slippingin the capture section, the secondary electron bunch is only
∼180 ps away from the positron. Conventional stripline–
type BPM cannot detect such closely spaced and opposite
polarity signals due to slow frequency response and highcable losses. Therefore, a new wideband feedthrough-type
beam position monitor was developed. It was successfully
employed at the positron capture section of the SuperKEKB
injector linac for the first synchronous measurement of theelectron and positron beams. The cable losses effect also
has been de-embedded to reveal correct signal properties.
This paper describes the initial results of synchronous measurement of 𝑒 − /𝑒 + transverse position.

INTRODUCTION
The SuperKEKB (SKEKB) [1, 2] collider is currently
under operation to study CP violation in B mesons and also
to search for new physics beyond the Standard Model, withthe target luminosity of 8 × 1035 cm−2 s−1 , which is 40 timehigher than its predecessor KEKB [3]. The SKEKB collider
consists of electron (𝑒 − ) and positron (𝑒 + ) rings of energy
7 GeV (HER) and 4 GeV (LER) with the stored beam currentof 2.6 A and 3.6 A, respectively.
The SKEKB injector linac provides 𝑒 − /𝑒 + bunches of
5 nC and 4 nC to HER and LER respectively, at their designed energy. The low emittance 𝑒 − beam is produced bya RF-photocathode gun. The positron beam is produced bystriking the 𝑒 − beam of energy 3.5 GeV and bunch chargeof 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 positron, a pulsed solenoid
called flux concentrator and a large aperture S–band (LAS)
accelerating structure [4] are placed in the downstream direction of the positron target. The secondary electrons arealso produced during the positron creation process and accelerated in the capture section. Because of phase slipping∗ rehman@post.kek.jp

Figure 1: (a) The 3-D model of the new feedthrough-BPM.
(b) Front view of the feedthrough–BPM, where "R", "L", "T"and "B" shows the right, left, top and bottom feedthroughs
respectively.

in the capture section, the secondary electron bunch is only
180 ps away from the positron (depending on the phase ofaccelerating structure). Conventional beam monitors i.e.
stripline–type BPM cannot detect such closely spaced andopposite polarity signals due to slow frequency response
and high cable losses.
Therefore, new wideband feedthrough beam position monitor (BPM) was developed. And successfully employed atthe positron capture section of the SuperKEKB injector linac
for the first synchronous measurement of the electron andpositron beams. This new monitor can detect 𝑒 − and 𝑒 +bunch properties i.e transverse position, bunch length, timeinterval, and bunch charges synchronously. It provides anopportunity to enhance positron transport through the capture section. The detailed analysis of bunch properties isreported in [6]. Here, we report the position response of
feedthrough-BPM.

FEEDTHROUGH BEAM POSITIONMONITORThe feedthrough–BPM consists of a vacuum pipe oflength 431 mm and inner diameter of 38 mm, four SMA-typefeedthroughs having inner conductor made of Kovar with𝜋/2 rotational symmetry are installed at the upstream direction of the vacuum pipe. The diameter of the feedthroughs is
1.8 mm and they extend 1 mm to the center of the beam pipe
from the inner surface of the vacuum pipe. The vacuum pipeof BPM also has bellows installed at the downstream andupstream direction for flexible installation. The 3-D modelof the new feedthrough–BPM and front view are shown inFig. 1 (a) and (b), respectively.

Figure 2: A schematic layout of the capture section depicting
the position of new feedthrough-BPM and steering coils.

Positron Capture Section
Two new feedthrough-BPMs namely SP− 15− 25 andSP− 16− 25 were installed in the positron capture section. The
horizontal and vertical steering coils are also installed in thecapture section to optimize the positron transmission throughthe capture section. Three horizontal and one vertical steering magnets have been installed in the 𝑒 + capture section. A
schematic layout of the location of new feedthrough-BPMs
and steering coils is shown in Fig. 2, a detailed descriptionof the capture section can be seen at [6]. The horizontal andvertical steering coils labeled as SX− 15− 15 and SY− 16− 15were used to observe the position response of feedthrough–BPMs. The entire capture section is enclosed in the DC
solenoid coils for efficient transmission of large emittancepositron beam.

Coaxial Cable Losses

Figure 4: The synchronous signal of 𝑒 − /𝑒 + at nominal accelerating phase from new feedthrough–BPM, the cutoff
frequency was set to 𝑓𝑐 = 10 GHz.
The black curve in Fig. 4 shows the raw signal without any
compensation of cable losses and due to high frequency
losses in coaxial cable, 𝑒 − and 𝑒 + waveform are inseparable.
Whereas, the red curve shows the case with high-frequency
cable losses correction and 𝑒 − and 𝑒 + bunches can be distinguish from each other, the cutoff frequency for cable losses
was set to 10 GHz.
When a charged particle travels through any obstacle (i.e.
accelerating structure) it generates electromagnetic (EM)fields that left behind the particle. These left behind EM
fields are called wakefield [10]. The wakefield can act on
subsequent bunches transverse and longitudinal motion. InFig. 4 a subsequent ringing waveform appear. This ringing
waveform may be appeared due to the wakefields of both 𝑒 −and 𝑒 + bunches.

SYNCHRONOUS MEASUREMENT OF
𝒆 − /𝒆 + BUNCHES TRANSVERSEPOSITION
The area of the negative bulge of first (second) bipolar signal is considered for the 𝑒 − (𝑒 + ) position measurement. The𝑒 − /𝑒 + beam position from feedthrough–BPM
was analyzed
Íby the difference-over-sum ratio (4/ ):Figure 3: The coaxial cables losses at high-frequency measured by a vector network analyzer [8].
The SMA connector of the feedthrough is first connectedto the 2 m long semirigid coaxial cable due to its protection
against a high radiation environment. Later the semirigid
cable was connected to the 15 m long 10D and 2 m RG223
coaxial cable [7]. The cable losses at high frequency weremeasured by a vector network analyzer [8]. A typical example of the high-frequency losses of coaxial cables is shownin Fig. 3.
The coaxial cables are then connected to a real–timekeysight oscilloscope of bandwidth 33 GHz and a sampling
rate of 128 GS/s [9]. The typical bipolar waveform at thenominal rf-phase from feedthrough-BPM is shown in Fig. 4.

𝑥(𝑦) = 𝑠 𝑏

𝐴𝑟 ( 𝐴𝑡 ) − 𝐴𝑙 ( 𝐴𝑏 ),𝐴𝑟 ( 𝐴𝑡 ) + 𝐴𝑙 ( 𝐴𝑏 )(1)

where 𝐴𝑟 ( 𝐴𝑡 ) and 𝐴𝑙 ( 𝐴𝑏 ) are calculated areas from theright (top) and the left (bottom) feedthroughs signals (seeFig. 1), respectively, and 𝑠 𝑏 is the BPM sensitivity given as:𝑅,(2)2
where 𝑅 is the radius of the vacuum pipe. The transverse position change of 𝑒 − /𝑒 + bunches at SP− 15− 25 and SP− 16− 25
are measured as a function of the field strength of horizontal and vertical steering magnet. The bunch charges werecalculated by a summation of four pickup signals as given
below:
Õ𝑄=𝐺𝐴𝑖 ,(3)𝑠𝑏 =𝑖Figure 5: The synchronous transverse position of 𝑒 − /𝑒 + measured by new feedthrough–BPM namely SP− 15− 25 andSP− 16− 25. The transverse position of 𝑒 − /𝑒 + bunches were varied by the air-core horizontal and vertical steering magnets.
The color band shows the positron yield variation due to the transverse position change of 𝑒 − /𝑒 + bunches. (a) The transverse
position change of 𝑒 − /𝑒 + bunches at SP− 15− 25 with the horizontal steering magnet (SX− 15− 15). The transverse positionchange of 𝑒 − /𝑒 + bunches at SP− 16− 25 with (b) the vertical (SY− 16− 15) and (c) horizontal (SX− 15− 15) steering magnets,respectively.
where 𝑄 is the beam charge and 𝐺 is a conversion factor tocalculate the beam charge. The detailed description is givenin [6]. And the yield ratio is defined as the ratio of intensityof secondary produced bunch to the primary electron bunch.
Figure 5 shows the 𝑒 − /𝑒 + bunches transverse position response as a function of the field strength of horizontal andvertical steering magnets and the color band in Fig. 5 corresponds to the 𝑒 − and 𝑒 + yield ratio. Figure 5 (a) shows transverse position change of 𝑒 − /𝑒 + bunches at SP− 15− 25 withthe horizontal steering magnet (SX− 15− 15). Figure 5 (b)and (c) presents the the transverse position change of 𝑒 − /𝑒 +bunches at SP− 16− 25 with the vertical (SY− 16− 15) and horizontal (SX− 15− 15) steering magnets, respectively. Whenthe strength of the horizontal steering magnet (SX− 15− 15)
was changed the major position change in the vertical direction was observed at the SP− 15− 25 as shown in Fig. 5(a). As explained in earlier section 𝑒 + capture section is enclosed in the solenoid to confine large transverse emittance𝑒 + beam. This interesting beam motion arises due to thehelical motion of charged particles under the solenoid field.
When charged particles enter the solenoid field, they startrotating with the cyclotron period.
If the charged particle transverse position is observed
at longitudinal position where the cyclotron period is notcompleted, then the position change will be observed in theopposite transverse position compared to the kick applied by
the steering magnet. Whereas, at the full cyclotron period
longitudinal position the transverse position change will beobserved in the same direction as applied kick as observedin Fig. 5 (b) and (c). This interesting phenomenon was confirmed by the charged particle tracking (RK–4 method) underthe solenoid field with the consideration of different kickfields. The color band in Fig. 5 shows the 𝑒 − and 𝑒 + yieldratio as a function of the horizontal and vertical kick. This
simultaneous information of bunch transverse positions andcharges was used to optimize the 𝑒 + transmission through thepositron capture section. The error bars on different transverse positions in Fig. 5 shows the systematic errors dueto the frequency characteristics and wakefield effects. The
frequency characteristics error is calculated by setting thedifferent cutoff frequencies for high frequency cable losses.
The error due to the wakefield in detail discussed in [6]. The
total transverse position error is the root-mean-square sumof these two systematic errors, which is about 11%.

SUMMARY
The first synchronous measurements of 𝑒 − and 𝑒 + bunches
have been successfully performed with the wideband
feedthrough-BPM system at the 𝑒 + capture section of the SuperKEKB factory. The synchronous transverse position andintensity of the 𝑒 − and 𝑒 + bunches were independently measured. The transverse motion of the 𝑒 − and 𝑒 + bunches wasobserved by the new feedthrough-BPM under the combined
field of solenoid and steering magnets. The new wideband
feedthrough-BPM proved as an excellent monitor to optimize
𝑒 + transmission through the 𝑒 + capture section.

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