COMMISSIONING OF POSITRON DAMPING RING AND THE BEAM
TRANSPORT FOR SuperKEKB
N. Iida∗ , Y. Funakoshi, H. Ikeda, T. Ishibashi, H. Kaji, T. Kamitani,
M. Kikuchi, T. Kobayashi, H. Koiso, F. Miyahara, T. Mori, Y. Ohnishi,
Y. Seimiya, H. Sugimoto, H. Sugimura, R. Ueki, Y. Yano, D. Zhou,
High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, JapanAbstract
The Positron Damping Ring (DR) for SuperKEKB successfully started its operation in February 2018, and thecommissioning was continued until the end of SuperKEKB
Phase 2 in July without serious troubles. This paper describes achievements of the beam commissioning of injection and extraction lines (LTR and RTL) between the LINACand DR. In the LTR commissioning, the positron beam withhigh emittance, wide energy spread, and high charge weretransported and injected into the DR. In the RTL commissioning, special cares were necessary to preserve the lowemittance. The observed emittance growth in the RTL was
not a problem for Phase 2, but it should be resolved in thecoming Phase 3. In this proceedings, the commissioning ofthe DR will be also reported.

INTRODUCTION
SuperKEKB [1] is a double-ring asymmetric collider with
7 GeV electronic ring (HER) and 4 GeV positron ring (LER)
in which the Belle II detector is installed. The previous
KEKB accelerator [2], which had been in operation from1999 to 2010, recorded the world’s highest luminosity atthat time, 2.1×1034 cm−2 s−1 . In order to increase the peakluminosity by 40 times of KEKB, a beam collision called“Nano beam scheme” are adopted, and the injection beamis required to have low emittance. Since the stored beam
has low emittance and high current, the lifetime is short, sothat the charge of the injection beam to be supplied mustalso be high. We adopted an RF gun [5, 6] for generating
the electron beam, and a flux concentrator [7](FC) and adamping ring for the positron beam. The FC is a pulsedsolenoid installed in right after the positron target to collectpositrons coming from the target with high efficiency. The
resulting longitudinal phase distribution of the positron ishuge, requiring some schemes for transporting the beam toDR efficiently as written in [8].
SuperKEKB is operated in Phases 1, 2, and 3 for eachpurpose. In February 2016, we succeeded Phase 1 [9, 10]
for about 5 months without a Belle II innermost detector,without collision. After remodeling about a year and a half
further, a DR commissioning from January 2018, and a collision experiment (Phase 2 [11]) from March to July werecarried out without incorporating a part of the Belle-II innermost detectors. The required parameters for the positron
injection beam of SuperKEKB-LER are shown in Table 1.
∗ naoko.iida@kek.jp

These values gradually approaches the final values according
to the collision performance such as 𝛽∗ .
As shown in Fig. 1, the DR is a ring with a circumference of about 135.5 m installed about 120 m downstream
the positron target of the LINAC. The positron beam is extracted from the end of sector 2 of the LINAC whose energyis 1.1 GeV, and injected into the DR. Since the beam of enormous energy spread from FC exceeds the energy acceptanceof the DR, an energy compression system (ECS) is installedin the first arc of the LTR. The damped positron beam fromthe DR is returned to the entrance of Sector 3 of the LINAC.
The acceleration frequency of the DR is about 508.9 MHz,
which is same as SuperKEKB, the resulting bunch length is
too long to return to the LINAC with acceleration frequency
2856 MHz (S-band). Thus the RTL has a bunch compression
system (BCS) in the second arc. Fig. 2 shows the particle distribution in the longitudinal phase space simulated with theparameters on Table 2 before and after the DR. The parameters from the DR are modified from the initial design [3].
Since the RF voltage has been changed from the design valueof 1.4 to 1.0 MV, the emittance is changed from 89 𝜇m to64.3 𝜇m. As shown in Figure 2, since positron from the FC
Table 1: Required parameters of injection beam forSuperKEKB-LER Phase 2 and 3
𝛾𝜖𝑥 [𝜇m]𝛾𝜖𝑦 [𝜇m]𝜎𝛿 [%]
Charge [nC]

DR Extraction
64.3
3.2
0.055
1.5

Phase2< 200< 400.16
1.5

Phase3–< 100< 150.10
4.0

Table 2: Design Parameters of the Injection and Extraction
Beam for DR (∗ shows a full width.)ParametersECSin

𝛾𝜀𝑥 [𝜇m]𝛾𝜀𝑦 [𝜇m]
𝜎𝑧 [mm]𝜎𝛿 [%]
𝑅56 [m]𝑉𝑐 [MV]

±8∗
±5∗
–0.61
41

ECSout
=DRin
2800
2600
±30∗
±1.5∗

BCSin
=DRout
64.3
3.2
6.6
0.055

BCSout

1.3
0.8
–1.05
21.5

has a long energy tail in the LTR, the beam passing throughthe ECS causes beam loss in the LTR and the DR. We setup 4 collimators in the arc section of the ECS cutting theFigure 1: The LINAC consists of eight sectors from Sector A, B, C, and 1 to 5 starting from the electron sources. Theelectron and positron beams are accelerated up to 7 GeV and 4 GeV, respectively, and are injected into the HER and LER ofSuperKEKB via each beam transport line (BT). Both of the injection/extraction lines for the DR (LTR/RTL) have two arc
sections with straight sections between them.
the DR, LTR and RTL was strictly controlled because ofthe radiation limit. Figure 3 shows the beam optics of theLTR. Four collimators are installed in the first arc where thehorizontal dispersion is large.

Tuning of LTR

Figure 2: Simulated distributions of longitudinal phase space
before and after ECS, and those of BCS.
energy tail beforehand (red part in (a) Before ECS) to makethe energy spread within ±5%. In the ECS downstream it
is compressed to ±1.5%, which safely enters into the DRbucket height ±1.5%. The bunch length of the beam extracted from the DR is compressed from 6.6 mm to 1.3 mm
with the BCS. Although for 𝑉𝑐 =18.4 MV, the bunch length
is compressed to the minimum at the BCS exit, 21.5 MV wasadopted, since the simulation shows that the energy spreadat the injection point to LER is small. Because the bunch
length of the beam in the BCS and ECS is longer than thefrequency of S-band, the sine wave of S-band is visible asin “After ECS/BCS” in Fig. 2. By choosing the parameters(𝑅56 , 𝑉𝑐 ) as in Table 2, the beam can be transported almost
without loss.

COMMISSIONING OF LTR
LTR commissioning was started on Jan. 23rd, 2018. Initially the beam was guided by inserting a beam shutter inthe end of the LTR not to go to the DR, and on the nextday, reached the beam shutter. At that time, the FC was set
to standby and the charge was 0.75 nC. The beam loss inActually in order to adjust the beam with asymmetric
distribution from the FC as shown in Fig. 2-(a), the information from beam position monitors (BPMs) should not be
used for the steering. Therefore, we first made core “Ginjo”[12] beam by using the four collimators. As shown in theupper row in Figure 4, the energy peak of the beam wasadjusted as to come to the center of the vacuum chamberusing the energy knob in the sector 2, and collimated so that
only the peak part is left, which is called “Ginjo” beam. The
zero-crossing of the ECS acceleration phase is measured
with the “Ginjo” beam. Specifically, the phase where theFigure 3: Beam optics of the LTR. The upper graph shows
the horizontal (blue) and vertical (red) beta functions. Thelower graph shows the horizontal dispersion. The horizontal
axis is the distance in meter from the end of Sector 2 inthe LINAC. 𝑅56 of the first arc of the LTR and 𝑉𝑐 of thedownstream accelerating structure constitute the ECS.

Table 3: Measured emittances by wire scanners at the straightsection of LTR when the FC is stand-by and on.
𝛾𝜖𝑥 [𝜇m]𝛾𝜖𝑦 [𝜇m]
Charge [nC]
Figure 4: Distributions in the longitudinal phase space ateach location. The upper row shows the distributions of the“Ginjo (core) beam” for the tuning of the LTR. The small
circles are the center of the longitudinal phase space, wherethe “Ginjo” beam is to be positioned. The lower row shows
those of the operation beam with wider spread.
beam energy does not change between on/off of the ECS
acceleration is measured. It was confirmed by the displacement of the downstream LTR 2nd arc with a large horizontal
dispersion.

FC: Stand-by
2350
2310
0.75

FC: 5kV2760
2450
1.5

increased to about 1.5 nC which roughly doubled that without the FC. The measured emittance by the four wire scanners [13] installed in the straight section of the LTR is shownin Table 3. The results are almost same as the design emittances as shown in “DRin” of Table 2, and there was no
significant difference in the emittance measured on and offof FC. The beam loss measured by the beam loss monitors [14] around the DR were small enough as shown inFig. 5-(b).

COMMISSIONING OF DR

Figure 5: (a) The energy acceptance of the LTR. The horizontal axis represents the energy of the “Ginjo” beam in
an arbitrary unit. (b) The green bars show the beam loss
measured the beam loss monitors around the DR, which arewithin the allowable level.
The result of measured energy acceptance of the LTR 1starc by using the “Ginjo” beam is shown in Fig. 5-(a). It iscut by the collimator during operation where the low energy
beam of the first arc passes indicated by the dashed line on
the left side of the figure. Tuning of injection to the DR
is also performed only using the “Ginjo” beam, and once
adjusted, the ECS and the DR injection parameters should
not be tuned with the beam for operation. At the end of thesetuning processes, we returned to the operation beam, opened
the collimators, and increased the beam energy by +5%
higher than the “Ginjo” beam. By this tuning, the operating
beam rotates around the center of the phase space where the“Ginjo” beam has passed, that corresponds to the center ofthe vacuum chamber and the longitudinal phase space, thenthe beam loss is minimized. Other basic measurements, suchas 3-BPM measurements, local bump study, beam basedalignment (BBA), and single kick response measurement,were made.

Effect of FC
On February 22nd, the FC was turn on. By turning on
the voltage of the FC at 5 kV, the charge passing the LTR

Figure 6: (a1, a2)The measured optics before and after correction, respectively. The upper two rows represent the horizontal and vertical beta beats, and the third and fourth show
the deviation of betatron phases from the of the design. The
green dots in (a2) show the corrected value. (b) The rows are
the design horizontal dispersion, the measured difference ofthe horizontal and vertical dispersions. The correction wasapplied twice, from left to right.
The first commissioning of the DR and the beam transportation to the end of LINAC were done in three days. That
was, the tuning of timing for the injection septum, kickers,and BPMs in the DR, orbit tuning, RF capturing, the tuningof timing for the extraction septum, kickers, and BPMs inthe RTL, orbit tuning, BCS of the RTL tuning, and LINACtuning. The optics corrections of the DR were done as shownin Fig. 6. As shown in Fig. 6-(a2), the 𝛽-functions are somewhat improved by the correction, but the phase advances
have systematic slopes. Further after the 𝛽 correction, thedispersion corrections were done two times, but a patternreflecting the horizontal dispersion of the model graduallyappears. Further investigation is necessary.

The model lattice of the DR was wrongly specified in
SAD in the definition of the fringe field of dipoles at the first
operation time. After corrected that, the imbalance between
the correction coefficients of the quads were improved from
11% to 5%. The reason why the coefficient is still too large
is under investigation. The calibration of magnetic measurements are in question. More basic measurements including
the beam size, tunes, and so on should be done.

COMMISSIONING OF RTL

Figure 7: Beam optics of the RTL. The BCS consists of𝑉𝑐 of the S-band acceleration tube installed in the straightsection of RTL and 𝑅56 of the second arc. The notations aresame as Fig. 3.
On February 8th, as the beam revolved around the DR, it
was extracted to the RTL at the same time, and on February
10th the beam after tuning of the BCS was transported tothe beam dump at the end of the LINAC. The low emittancebeam from the DR has fewer worries of beam loss unlike
at the time of the LTR, but there is a difficulty in keeping
the low emittance during the transportation. Since the beam
distribution from the DR is clearly Gaussian, it is already“Ginjo” beam. Figure 7 shows the optics of the RTL.

Tuning of BCS from RTL to the end of LINAC
Like the ECS of the LTR, the BCS acceleration was set
to standby first, the orbit was corrected, and even if the acceleration was turned on, the RF phase where the orbit doesnot move was set at the zero cross. Since a streak camera
in Sector 3 was not ready yet at that time, we distinguished
whether the RF phase is zero or 𝜋 by looking at screen
monitors where the horizontal dispersion exists. However,
without the judgment, it can also be confirmed that in thecase of the opposite phase, the beams do not pass well from
Sector 3 to Sector 5. After setting the phase of the BCS, weadjusted the RF phase of the acceleration of Sector 3 to 5.
The energy spread at the end of LINAC was ±0.3% in full
width in the screen monitor, which satisfies the requirementfrom SuperKEKB-LER of Table 1.

Table 4: Measured emittances by wire scanners at Sector 3
𝛾𝜀𝑥 [𝜇m]𝛾𝜀𝑦 [𝜇m]

Measured Emittance
126±8.2
1.5±0.1

DR Design64.3

EMITTANCE GROWTH
The emittances of the positron beam from the DR andRTL are measured at Sector 3 in LINAC with four wirescanners as shown in Table 4. Assuming that the horizontal
emittance is same as the design value of the DR and thevertical emittance of the DR is less than the measured valuein Sector 3, the emittance ratio of the DR is presumed to beless than 2.3%. The question is that the measured horizontal
emittance at Sector 3 is larger than the design value by afactor of 2. The measured emittances have two dependencies. One is the BCS-voltage dependence of the horizontal

Figure 8: BCS-voltage dependence of the horizontal emittance. The blue and red dots show the measured normalized
horizontal and vertical emittances. The blue dashed line
shows the design emittance of the DR. The operation voltage of the BCS is 23 MV.
Table 5: Bunch charge dependence of the horizontal emittance
Charge [nC]
0.7
1.5

V𝐵𝐶𝑆 [MV]00𝛾𝜀𝑥 [𝜇m]
88±7.6
104±7.4

𝛾𝜀𝑦 [𝜇m]
1.4±0.4
3.7±0.5

emittance as shown in Figure 8, another is bunch charge
dependence as shown in Table 5. The measured horizontal
emittance depends on the bunch length and/or energy spread.
The 𝛾𝜀𝑥 measured at Sector 3, even minimum of them, islarger than the design value of the DR. Furthermore, theemittance growth of both the horizontal and vertical emittances are observed by increasing the bunch charge. We
suspected the blowup of the emittance is caused by longitudinal coherent synchrotron radiation (CSR) in the bending
magnets at the arc sections of the RTL. The wake potentialfrom the longitudinal CSR is shown in Figure 9 [15] by a numerical simulation. The values are in good agreement in thetheoretical ones. The resulting emittance growth by tracking
simulation is shown in Table 6. Clearly seen in the table, theeffect of the longitudinal CSR effect on the emittance growthis negligibly small. A possibility of another cause is thatthe residual dispersion from the arcs of the RTL makes thebeam size blow-up, and the transverse wake field in Sector
3 also causes an emittance growth with the charge increase.
Thus these emittance growth is still mystery. Anyway, themeasurement of the emittance in the DR is also necessary.

Figure 9: The wake potential of the longitudinal CSR of
single-bunch effects in RTL. The blue and red lines show
the wake potential from the theoretical calculation and simulation, respectively.
Table 6: The effects of CSR on emittance
V𝐵𝐶𝑆 [MV]21.5
21.50Q [nC]
0.7
4.0
4.0

Δ𝜀𝑥 /𝜀𝑥3.2×10−6
3.1×10−5
1.1×10−8

CONCLUSION
The first commissioning of the LTR, DR, and the RTL
for SuperKEKB were successfully and quickly done. Noserious trouble was occurred. The injection into the LER of
SuperKEKB in Phase 2 has been done safely and continuously. The basic parameters of DR should be measured inthe next run. An emittance growth from the DR to Sector
3 was observed. The BCS-voltage and bunch charge dependences of the horizontal emittance have been mystery.
We have more serious emittance growths in the LINAC andBeam Transport line (BT) to LER [17]. More investigations
should be done before SuperKEKB Phase 3 operation.

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