Chapter 5Interaction Region
5.1

Overview

The energy asymmetry of the two beams shifts the interaction point (IP) of SuperKEKB by 471 mm from the geometrical center of Belle II detector. The direction,
towards which the center of mass system of colliding particles is boosted, is called‘Forward direction’ or ‘Right hand side’ of the IP. The other side is called ‘Backward
direction’ or ‘Left hand side’. Figure 5.1 is a plane view of the layout of the detector,
magnets and beam pipes around the IP. The right hand side of the figure is the forward
direction.
The orbits cross with an angle of 83 mrad at the IP. The location of final focus
magnets is very close to the IP as listed in Table 5.1. These are superconducting
magnets. At both sides of the IP, these final focus magnets of each ring are integrated
with compensation solenoids for the detector solenoid field, correctors, cancel coils
against leak fields, magnetic yokes to absorb leak fields, and heavy metal radiation
shields, in a single cryostat. This integrated set is called QCSL in the backward, and
QCSR in the forward, and they are collectively referred as QCS. The fabrication of
QCS with required precision is one of the most challenging issues in the upgrade.
The body of the cryostat of QCS is buried within the Belle II detector. QCS’s arealigned first without the detector. Then the detector rolls in. QCS’s have to temporally
move away from their original position and come back after the roll-in. Therefore, QCS
is supported on a moving stage. The motion of this moving stage must reproduce thealigned position of QCS very precisely. And, the stage must stand for a very strong
force on QCS exerted by the detector solenoid at the final position. After QCS reaches
the final position within the detector, the ‘Single Stretched Wire’ technique is applied
to measure the magnetic centers of the final focus quadrupole magnets with respect tothe design beam lines. In the detector, the transversal shift of the QCS is monitored
104

Figure 5.1: A sketch of an area ±8 m from the IP.

Incoming beam
Positron QC1RP
QC2RP
Electron QC1LE
QC2LE

Distance [m]
0.935
1.925
1.41
2.70

Outgoing beam
QC1LP
QC2LP
QC1RE
QC2RE

Distance [m]
0.935
1.925
1.41
2.925

Table 5.1: Distance of final focus magnets from the IP.
with a gap-sensor set between the cryostat and the surrounding central drift chamber.
The exit of the final bend of LER is about 5.54 m from the IP, the critical energy
of synchrotron radiation (SR) from this bend is 0.95 keV. The end of the final bend of
HER is at about 7.26 m and its critical energy is 1.25 keV. The curvature of the orbit
in these magnets is designed to avoid direct illumination of SR on the central beryllium
pipe of the IP chamber (see below). The detail of the moving stage and compact room
temperature magnets in this region will be described in the magnet section.
The central beam pipe (IP chamber) has a short beryllium tube in the middle.
The length of the IP chamber is 964 mm. Two rings separate within the IP chamber.
The IP chamber is connected to a short tube with a bellows unit and a beam position
monitor (BPM). This bellows unit has a finger-type RF-bridge like the one used for
KEKB. We cannot apply the comb-type RF-bridge (see chapter of the vacuum system),because a flexible design for a small diameter such as 20 mm is not possible with the105

IP IP1
Electron incoming 20 10
Positron outgoing 20 20
IP IP1
Electron outgoing 20 20
Positron incoming 20 10

IP2
QC1LP
QC1LE
20
(21)
34
20 27(V) 21(H)
(44)
IP2
QC1RP
QC1RE
20
(21)
34
20 27(V) 21(H)
(44)

QC2LP
(70)
70
QC2RP
(70)
70

QC2LE Outside
80
80
(80)
80
QC2RE Outside
80
80
(80)
80

Table 5.2: Diameter of beam pipes (mm). The cross section is circular except at
QC1RP and QC1LP. IP1 is the position where two beams separate. For an outgoing
beam, the beam pipe has a horizontal oﬀ-set at IP1. IP2 is the end of the IP chamber.
comb-type ridge. This RF-bridge must withstand a three times higher wall current
than the one experienced at KEKB. The next vacuum chamber is used both as beam
pipe and warm bore of the cryostat to locate the final focus magnets as close to the IP
as possible. By this design we have vacuum flange connections in front of the cryostat,which is not attainable from behind. Physical aperture of beam pipes from IP to theend of QCS is summarized in Table 5.2.
The BPM nearest to IP is located between the IP chamber and the QCS cryostat.
They are on a short beam pipe with bellows as mentioned above. These BPM’s aremechanically fixed to the IP chamber. The next BPM is located in the cryostat. For
HER the position is between QC1 and QC2, however, it is after QC2 for LER. The
third location of HER BPM is at the quadrupole magnet next to the cryostat. However,for LER, the third BPM is beyond two bending magnets from the cryostat.
Mirrors and windows for the large angle beam-strahlung monitor (LABM) system
are provided on the beam pipe for outgoing beam after the cryostat. The distance from
IP is about 4.5 m for LER and 4.8 m for HER. The detail of this monitoring system
will be described in the chapter of the beam monitor system.
The luminosity monitor in the positron beam line will be installed about 7 m
downstream of IP. A signal of positrons whose energy is lost by the radiative Bhabha
process at the interaction point is used as a measure of the luminosity. The locationfor HER is not yet fixed. This beam loss due to the radiative Bhabha process is amajor radiation source in the interaction region.

106

5.2

Final focus superconducting magnet system (QCS)

5.2.1

General configuration of the system

The superconducting magnet system was designed in order to control the beam conditions at the beam interaction point, IP [1]. In the design of the system, the following
constraints are required from the beam optics and the particle detectors: (1) While
we need to keep the same boundaries to the detector components as KEKB, the superconducting magnets must be located closer to IP than KEKB to squeeze a verticalbeam size to around 50 nm at IP. (2) From the requirement of locating the magnets
in the space of the small beam separation between two beam lines of LER and HER,
the magnet components including the conductor sizes of the superconducting coils are
required to be optimized. (3) Since the beta functions for these magnets are so large,
1200 m for horizontal and 6400 m for vertical, a larger space with a good field quality isrequired. (4) These magnets must apply the focusing fields on electrons and positrons,
independently each other. So, the reduction of the non-linear leakage field from theadjacent beam lines is a critical issue to achieve large dynamic aperture. (5) Since two
beams with the crossing angle of 83 mrad are operated under the Belle-II solenoid field
at 1.5 T [2], the solenoid field is required to be cancelled integrally along each beamline. The compensation solenoids [3] are designed to reduce the X-Y coupling and theemittance for the two beams.
Figure 5.2 shows the designed final focus magnet cryostat system in the Belle-IIdetector. The system consists of 8 main quadrupole magnets, 4 compensation solenoids,
35 corrector magnets and 8 special magnets [4][5]to cancel the leakage magnetic fields
on the HER beam lines from QC1LP/RP. The main quadrupole magnets (QC1s andQC2s) are designed to form a quadrupole doublet for each beam, as shown in Fig. 5.3.
Table 5.3 shows the main parameters of the quadrupole magnets at the colliding
energy of 11 GeV of the nominal operation. In the table, the integral field gradients areshown with the field gradients at the magnet center and the magnetic length. The midplanes of the quadrupole fields of the LER magnets roll with the angles as shown in thetable. The QC1LP/RP magnets are located at the closest position, +/- 935 mm, to IP.
They are the quadrupole magnets without magnetic yokes in order to superimpose thefields of the compensation solenoids on the Belle II solenoid field in this area and makethe solenoid field profile for minimizing vertical beam emittance. The QC1RE/LE and
QC2RP/LP magnets have the Permendur yokes. The QC2RE/LE magnets have theiron yokes. The distances of the quadrupole centers from IP and the mid-plane angles
to the horizontal plane are listed in the table. The quadrupole magnets for LER are
assembled with these angles in the cryostats.
107

Table 5.3: Main Quadrupole Parameters
MagnetGL, T (T/m × m)

Type

Z, mm

θ, mrad

QC2RE

13.58 (32.41 × 0.419)

Yoked

2925

0

QC2RP

11.56 (26.28 × 0.410)

Yoked

1925

-2.114QC1RE

26.45 (70.89 × 0.373)

Yoked

1410

0

QC1RP

22.98 (68.89 × 0.334)

no Yoke

935

7.204

QC1LP

22.97 (68.94 × 0.334)

no Yoke-935-13.65QC1LE

26.94 (72.21 × 0.373)

Yoked

-1410

0

QC2LP

11.50 (28.44 × 0.410)

Yoked

-1925

-3.725QC2LE

15.27 (28.44 × 0.537)

Yoked

-2700

0

Since the solenoid field and the leakage field of QC1P magnets change the e- beam
orbits, the beam orbits will be controlled by dipole corrector magnets. In order tominimize the required magnetic fields of the corrector magnets, the magnet axes of
QC1s and QC2s are shifted as shown in Table 5.4. In the table, ∆Y is the shift amountin vertical direction from the horizontal plane including IP and the negative sign means
downward. The plus ∆X means the horizontal shift toward the outer direction of theaccelerator ring.
Since the QC1LP/RP magnets are non-yoked magnets, the leakage field from themagnets to the e- beam line contains all field components as shown in Fig. 5.4. In the
figure, the horizontal axis corresponds to the distance from IP on the HER beam line.
In order to cancel these field components, the B 3 , B 4 , B 5 and B 6 cancel magnets arewound on the coil bobbins as the helium inner vessel. The cancel magnets are designed
to have a small twist in the windings for cancelling the skew field component which iscontained in the leakage field. The B 1 and B 2 field components are included in the
optics design, and then the B 1 and B 2 cancel magnets are not incorporated in thesystem.
The compensation solenoids, ESL, ESR1, ESR2 and ESR3, are designed to cancel
the Belle-II solenoid field integrally along the beam lines. In the ESL and ESR1
solenoids, two or three quadrupoles are assembled, respectively, as shown in Figs. 5.2
and 5.3. In the right cryostat, the additional solenoids, ESR2 and ESR3, are also
installed.

108Figure 5.2: The final focus magnet-cryostat system in the Belle-II detector.

5.2.2

Superconducting quadrupole magnet design

The four cross sections of the quadrupole magnets are shown in Fig. 5.5. The magnet
designs are diﬀerent in order to accommodate the change of the beam pipe diameters in
the cryostats. Table 5.5 shows the design parameters of the magnets. The parameters
are listed as the maximum operation points when the colliding energy of 12 GeV. The
magnets consist of two layer coils by the double pancake technique, and the cables aresame specification except for the keystone angle.
QC1P magnet design
The QC1P magnets are designed without iron yokes because in the location of themagnets the solenoid fields by ESL and ESR1 integrally cancel the Belle-II solenoid
field. The four superconducting coils are clamped with the YUS-130S stainless steel
109

Figure 5.3: Superconducting magnet configuration in the interaction region of SuperKEKB.

Figure 5.4: Leakage field profiles from QC1P magnet along the e- beam line.
collars. The design field gradient at the magnet center is 76.4 T/m at the currentof 1800 A, and the eﬀective magnetic length is 0.3336 m. The operating point withrespect to the critical point of the superconductor is 72 % at 4.7 K. The inner radiusof the QC1P magnet is 25 mm, and the physical magnet length is 409.3 mm. The
QC1P magnets are very small magnets. Three corrector magnets of a 1 , b 1 and a 2
are assembled inside of the magnet bore. The three corrector magnets are directly
wound on the support bobbin, which has the function of the helium vessel. The b 4
corrector magnet for the QC1LP and the b 4 and a 3 corrector magnets for the QC1RP
are assembled on the outer periphery of the stacked collars.
The field quality is designed on the reference radius, R r , of 10 mm in the magnet
bore. The calculated multipole field components up to the 20-th pole (n=10) are listed
in Table 5.6. The multipole field components are described by the following equations,−4

By + iBx = 10 B2∞∑((bn + ian )

n=1

110

x + iy
Rref

)n−1
,

(5.1)

Table 5.4: Shift amount of magnet axis.
Magnet
QC1RP
QC2RP
QC1LP
QC2LP

∆Y
-1.0 mm-1.0 mm-1.5 mm-1.5 mm

Magnet
QC1RE
QC2RE
QC1LE
QC2LE

∆Y
-0.7 mm-0.7 mm+0.7 mm+0.7 mmwhere bn and an are the normalized multipole field components as B2 (quadrupole
field amplitude value), and they are the coeﬃcients of the normal and the skew fieldcomponents (units).
The higher order multipole field components are optimized with the magnet crosssections (in 2 D model) and the whole magnet models (in 3D model), in which themagnet both ends are included. From the symmetry configuration of the four coils,
the error field of the non-listed multipole numbers are 0.
Because the higher order multipole field profile along the magnet length is important
for the beam optics, the profiles of the field components are calculated and they areincluded in the optics model. The multipole field profiles are shown in the Fig. 5.6,
with the coil geometries of two layers. The field components are normalized by thequadrupole field at the magnet center as 10000. In order to exclude the skew quadrupole
components in the coil lead end, the quadrant coils have mirror symmetry to theneighbor coils, while b 4n components are induced. In the lead ends, the b 4 components
show the peaks of +25.0/-26.0 units.
The magnetic field by the QC1P magnet goes through the e- beam line, whichis the counter beam line, because this magnet does not have a magnetic yoke. The
magnetic field is expanded into the multipole field components, and the profiles of thecomponents are shown in Fig. 5.4. In order to remove the influence of the QC1P
magnetic field on the operation of the electron beam, the cancel system of the leakfield, which consists of twisted b 3 , b 4 , b 5 and b 6 magnets, is designed. These magnets
ae described in the section of the superconducting corrector magnet.
QC1E magnet design
The QC1E magnets locate on the e- beam line behind the QC1P magnets from IP. The
design field gradient at the magnet center is 91.6 T/m at the current of 2000 A, and themagnetic length is 0.3731 m. The design operating point to the superconductor limit
is 73 % at 4.7 K. The inner radius of the magnet is 33 mm, and the physical magnet
111

length is 455.4 mm. The QC1E magnets have the magnetic yokes, and the material
is Permendur, which has the saturated magnetization, Ms, of 2.3 T in order to avoid
magnetic saturation in the yokes due to local imperfection in cancelling Belle-II fieldby ESL and ESR1. Especially, in the front part of the magnet, the inner helium vessel
for the positron beam line has an interference with the magnetic yokes of the QC1E
magnets, and therefore some yokes are gouged.
The higher order multipole field components have been calculated with the 2D crosssection, and they are listed in Table 5.6. The largest component at R r of 15 mm is
b 10 of -0.34 units. The whole magnet field quality was calculated as same as QC1P.
The designed coil ends are shown in Fig. 5.6 with the profiles of the multipole fieldcomponents. The multipole field components of the whole magnet are listed in Table
5.6, and the amplitudes of them are to be smaller than 0.5 units.
Figure 5.7 shows the leak field into the positron beam line from QC1LE magnet.
The magnet current is 1577 A at the 4S physics operation. In the yoke and the magnetic
shield, the magnetic field is assumed to be 1 T. The upper plot shows the calculated
magnetic profile in the positron beam line in case of the iron yoke of QC1LE and theiron magnetic shield, and the magnetic field strength from 0 to 0.007 T is shown withthe contour lines. The lower plot is the calculated result in case of Permendur for theyoke and the magnetic shield. The magnetic field strength in the positron beam line
is from 0 to 0.00035 T. By using Permendur as material of the yokes and the magnetic
shields, the leak field to the e+ beam line is reduced to the twentieth part of the fieldwith iron.
The QC1E magnets have the 4 correctors inside of each magnet bore. The QC1LE
magnet has the a 1 , b 1 , a 2 and b 4 corrector magnets, and the QC1RE magnet has thea 1 , b 1 , a 2 and a 3 corrector magnets.
QC2P, QC2LE/RE magnet designs
The inner and outer coil radii of the QC2P are 53.8 mm and 59.2 mm, respectively. The
four coils are cramped with the YUS130S collars, and Permendur yokes are assembled
on the collars. With the yokes, the leak field from QC2P to the e- beam line can be
reduced from 50 Gauss to 2 Gauss for the magnetic field of 1 T in the yokes in caseof 1 % imperfection of cancelling the detector solenoid field by ESL or ESR1. The
design field gradient of QC2P at 1000 A is 32.0 T/m, and the magnetic length is 409.9
mm. The nominal operation currents of QC2LP and QC2RP are 877.4 A and 882.1 A,
respectively.
The cross section designs of the QC2LE and QC2RE magnets are diﬀerent while
the collared coils of QC2LE and QC2RE are the same design. QC2RE has the com112

pensation solenoid between the collars and the yokes, as shown in Fig. 5.5. The innerand outer coil radii of QC2E are 59.3 mm and 64.7 mm, respectively. The materialof the yokes for QC2LE and QC2RE is iron because the amount of iron in the beam
separation is suﬃcient to reduce the leakage field to the e+ beam line to almost zero.
The design field gradients of QC2LE and QC2RE are 35.3 T/m and 38.1 T/m withthe currents of 1250 A and 1350 A, respectively, and the magnetic lengths are 537 mm
and 419 mm, respectively. The nominal operation currents of QC2LE and QC2RE are1007 A and 1312.8 A, respectively. From the low background noise requirement by
the Belle-II detector, the QC2LE magnet is designed to be operated with the lower
quadrupole field than the QC2RE magnet, and then QC2LE is physically longer than
QC2RE.
The calculated multipole field components with the 2D cross sections and with the
3D models are listed in Table 5.6. The field qualities of QC2P and QC2E are designed
at R r of 30 mm and 35 mm, respectively. Under the space constraint in the cryostat,
the coil ends are designed to be as short as possible, as shown in Fig. 5.8. The coil endsof QC2P and QC2E are designed within the space of 51.6 mm and 57.5 mm including
the G-10 spacers, respectively. In Fig. 5.8, the field profiles along the magnet lengths
of QC2P and QC2LE are shown, too. In the lead ends, the b 4 components show
the peaks of +7.0/-5.0 units for QC2P and +6.0/-6.6 units for QC2LE. The integral
b 4 components of QC2P and QC2LE were controlled at -0.06 units and 0.07 units,
respectively, as shown in Table 5.6. The maximum error field components are b 10 , andthey are -1.43 units and -1.92 units for QC2P and QC2LE, respectively.
The QC2P magnets and the QC2LE/RE magnets have the 4 correctors inside of
the magnet bore. The QC2LP and QC2LE magnets have the a 1 , b 1 , a 2 and b 4
corrector magnets, and the QC2RP and the QC2RE magnets have the a 1 , b 1 , a 2 anda 3 corrector magnets.

5.2.3

Superconducting corrector magnet design

The 43 superconducting corrector magnets including the leak field cancel magnets for
the QC1P magnets are installed into the two cryostats. A few millimetres of radial
space is available for corrector coils atop support bobbins that serve as the inner coldmass containment wall and the main quadrupole coils’ inner surface. The correctors are
directly wound on the support bobbins [6], and the productions of the correctors are
performed by the superconducting magnet division of the Brookhaven National Laboratory under the US-Japan research collaboration. The corrector requirements evolved
in response to results from optics optimization and tracking studies are summarized inTable 5.7, and An and B n are the skew and normal correctors, respectively. The A1
113

Table 5.5: Design Parameters of QC1 and QC2 Magnets.

Field gradient at I d , G, T/m
Design current, I d, A
Peak field in coil winding, T
Magnetic yoke
Coil I.R., mm
Coil O.R., mm
Collar or Yoke O.R., mm
Turns in a pole
Magnetic length, mm
Inductance, mH
Physical magnet length, mm
Id /I c, %S.C. Cable
Cable size in bare, mm
Cable size with insulation, mm
Cable keystone angle, degree
Critical current at 5T and 4.22 K

QC1P
QC1E
QC2P
QC2LE/RE
76.4
91.6
32.0
35.3/38.1
1800
2000
1000
1250/1350
4.56
3.50
2.43
2.63/2.84
No
Permendur Permendur
Iron
25.0
33.0
53.8
59.3
30.49
38.49
59.29
64.79
35.5
70
93
115
25
34
54
58
333.6
373.1
409.9
537/419
0.88
2.19
7.32
13.28/10.36
409.3
455.4
495.5
618.9/560.9
72
73
47
50/49
NbTi Rutherford cable, 10 strands, ϕ0.5mm of strand
2.5 in height and 0.93 in mid-thickness
2.65 in height and 1.08 in mid-thickness
2.14
1.66
1.05
0.94
3160
3080
3070
3130

Table 5.6: Error field components of QC1 and QC2 Magnets.

Ref. radius, mm
2D cal.

3D cal.

n=2
6
10
n=2
4
6
8
10

QC1P
10
an
bn
0 10000
00.10
0 -0.210 10000
0
0.24
0
0.54
0
0.01
0 -0.21

QC1E
15
an
bn
0 10000
0 -0.06
0 -0.34
0 10000
0 -0.02
0 -0.04
0 -0.42
0
0.05

114

QC2P
30
an
bn
0 10000
00.00
0 -0.15
0 10000
0 -0.06
0
0.28
0
0.11
0 -1.43

QC2 LE/RE
35
an
bn
0
10000
0
-0.24
0
-0.14
0
10000
0
0.05/0.07
0 -0.15/-0.13
00.05/0.06
0 -1.53/-1.92

Figure 5.5: Quadrupole magnet cross sections.
and B 1 correctors are designed to be able to shift the magnet axis in the range of 0.5
mm to 1.0 mm, in the horizontal and vertical direction, respectively. The A2 corrector
can rotate the mid-plane of the quadrupole field phase from 12 mrad to 15 mrad in theamount of angle. The B 4 corrector magnets increase the dynamic transverse aperture.
The corrector magnet production for QCS-L commenced first using preliminary specifications provided before the right-side requirements were set. Later optics studies
found that sextupole A3 and B 3 correctors are needed in order to cancel the error
fields of the quadrupole magnets induced by magnet assembly errors. The required
integral sextupole field strengths are equivalent to 1 × 10-3 of the quadrupole fields.
Installation of sextupole correctors was decided from the magnetic field measurement
results of QC1P/1E proto-type magnets [7].
The majority of the corrector coils are located inside a main quadrupole coil; however, with insuﬃcient space inside the QC1LP/RP quadrupole magnet bores, their B 4
correctors are wound on bobbins places just outside the main coil (along with the A3
and B 4 correctors for QC1RP). Because these QC1LP/RP quadrupoles have insuﬃcient space for magnetic flux return yokes between their coils and the nearby HER
beam, there is significant external field leakage that must be dealt with so as not toadversely impact the HER optics. Requirements for these additional external field115

Figure 5.6: Two layer coil geometries and multipole field profiles along the magnet
axes of QC1P and QC1E. Z-position=0 is the magnet center, and the reference radii
are 10 mm and 15 mm for QC1P and QC1E, respectively.

116

Table 5.7: Corrector integral field requirements. The right-side b 3 magnets are places
between main magnets. The leakage field cancel magnets for the HER are not includedin this table.
Magnet
QC1RP
QC2RP
QC1RE
QC2RE
QC1LP
QC2LP
QC1LE
QC2LE

Rr
mm
10
30
15
35
10
30
15
35

A1
Tm
0.016
0.03
0.027
0.015
0.016
0.03
0.027
0.015

B1
Tm
0.016
0.03
0.046
0.015
0.016
0.03
0.046
0.015

A2
T
0.64
0.31
0.75
0.37
0.64
0.31
0.75
0.37

A3
T/m
7.6
1.36
7
1.5
-

B3
T/m
17.2
@R r =30
27@R r =15
-

B4
T/m2
60
60
60
60
60

cancel coils for the HER are discussed in Chapter 1.
Both the corrector and cancel coils are attached to support bobbins via the Direct
Wind technique using 0.35 mm diameter, single-strand superconducting round wirefrom Furukawa with a 1:1 Cu:NbTi ratio and critical current at 4.2K greater than 130
R overwrap insulated andamps in a 5 T background field [5]. The wire is Kapton⃝adhesive coated to be compatible for use with BNL Direct Wind ultrasonic bonding
technology.

5.2.4

Compensation solenoid design

Figure 5.9 shows the solenoid field profile (red line with dots) only by the Belle-IIsolenoid and the combined field profile (black line) with ESL, ESR1, ESR2 and ESR3
with the locations of the quadrupole magnets. As shown in Fig. 5.9, the Belle-IIsolenoid generates the solenoid field of 1.5 T at the center of the detector, and thefield is mainly cancelled by the reverse solenoid fields by ESL and ESR1 in the areas of
QC1LP and QC1RP. In order to produce the combined solenoid field profiles requiredby beam optics, ESL and ESR1 are divided into 12 and 15 small solenoids as shown in
Fig. 5.10, and they produce the integral fields of 2.31 T m and 3.69 T m, respectively.
These integral fields correspond to the integrated Belle-II solenoid fields from the IP.
The maximum fields in the ESL and ESR1 coils with the Belle-II fields are 3.4 T and
3.2 T, respectively.
Since the QC1E magnets and the QC2P magnets have the Permendur yokes and117

Table 5.8: Design Parameters of Compensation Solenoids.
ESL
MagnetIntegral field, T m
Design current, A
Inductance, H
Maximum field, T
Magnet length, mm
No. of component coils
Total turns
Id /I c, %Superconducting cable
Ic at 4T and 4.22 K, A
Cu/NbTi ratio
Cable size, mm

ESR

ESR2/3

2.31
390
2.5
3.4
905
12
4610.1
52

3.69
0.17
450
151
8.0
0.14
3.2
0.48
1575
720
15
1
6237.1
1356
51
11
NbTi
1814
1.7
0.932 × 1.384

the counter beam lines are covered by the magnetic shields, the combined fields on
the beam lines are designed to be less than 0.01 T. The residual solenoid fields on the
beam lines are negligible on the beam lines. Between QC2RP and QC2RE, the Belle-IIfringe field is in the range of 0.6 T because of no magnetic shields. This fringe field iscancelled by the ESR2 on HER or the ESR3 solenoid on LER in the area of QC2RE.
The solenoid parameters are summarized in Table 5.8.

5.2.5

Cryostat

Cryostat system design
Two cryostats are designed to be installed on each side of IP as shown by Fig. 5.2, the
QCS-L cryostat for the left side and the QCS-R cryostat for the right side. The crosssections of the QCS-R cryostat are shown in Fig. 5.11 [8]. The QCS-L cryostat has
the same configuration as QCS-R. The cryostat consists of a magnet cryostat, a service
cryostat, a cryogenic tube and a support table. The magnet cryostats are supported
in a cantilever way inside the Belle-II detector by the support table, as shown in Figs.
5.2 and 5.11. The cryostat parameters are listed in Table 5.9.
As shown in Fig. 5.11, the superconducting magnets are assembled in the twohelium vessel (front vessel in IP side, and rear vessel in non-IP side). At the service
118

cryostat, the cryogenic transfer tubes from the cold box are connected to the cryostat.
The current leads are mounted in the service cryostat. Since each magnet is connectedto the individual power supply, the QCS-L and QCS-R service cryostats have 25 pairs
plus 1 leads and 29 pairs plus 1 current leads, respectively. The ESR2 and ESR3 havethe same magnet parameters, and then these solenoid magnets are connected in series.
The current leads for the QC1/QC2 magnets and the ESL and ESR1 solenoids areconventional types which are cooled with vaporized helium gas. The current leads forthe corrector magnets and the ESR2/3 solenoids are compact assembled unit where
8 leads are embedded [9]. The leads have the HTS superconducting tapes in the lowtemperature region below 80 K to reduce the heat load by 0.24 W/unit due to joule
heating, and the leads are cooled with the evaporated He gas. The additional 1 lead
is for tuning the solenoid field profiles along the axes of ESL and ESR1. In the front
vessel, the radiation shields of tungsten alloy of which density is 18 g/cm3 are installed
in order to protect the Belle-II components. The total weights of the radiation shieldare 231 kg and 1271 kg for QCS-L and QCS-R, respectively.
Concerning for the 80K thermal shield in the magnet cryostats, SS304 as the material was used for reducing an eddy current by magnet quench. The eddy currents
generate electro-magnetic force (EMF) and deformation of the shield plate. Becauseof a low thermal conductivity of SS304, the temperature profile of the plate was calculated, and the temperature diﬀerence in the plate was confirmed to be within 5 degree.
In the service cryostat, aluminum alloy was used because of high thermal conductanceand the very weak background magnetic field.
The material of the support table was SS400 because the tables had the function
as the magnetic shield against the fringe field from the Belle-II solenoid for the conventional magnets which were assembled inside of the table.
Mechanical design of the cryostat
The helium vessels are supported by a suspension system of eight titanium alloy (Ti6Al-4V) rods. In Fig. 5.12, the system for the front vessel of the QCS-R cryostat isshown. The lengths of the rods are 75.5 mm and 108 mm in the front and back sides,
respectively. The rods are designed to have a tilt angle so that the center between the
supporting points of the rods on the helium vessel does not change when the vessel
is cooled down to 4K. The angles of the front and back rods at room temperatureare 20.16 degree and 20.14 degree, respectively, and after cool-down to 4 K, they arechanged to 18.83 degree and 19.72 degree, respectively. With thermal contraction
during cool-down, the quadrupole magnet centers change as shown in Fig. 5.4. The
QC1RP center changes 2.35 mm and 0.12 mm in the axial and horizontal directions,119

respectively. The magnets and components at 300 K are designed so that the magnet
centers at 4 K are positioned on the beam lines.
As the result, the axial repulsive EMFs act on the solenoids. The calculated values
are summarized in Table 5.10. The EMFs on the ESL and ESR1 are 52.6 kN and35.7 kN, respectively. By these forces, the front helium vessels where the solenoids
are placed are pushed out from IP. In case that the ESL and ESR1 solenoids are notexcited like a magnet quench, the magnetic components in the vessels are drawn into
IP, and the forces are calculated to be 57.3 kN and 23.5 kN, respectively. The rods aredesigned to withstand the forces and the weight of the helium vessel. The cross section
areas of the rods for QCS-L/R are 182.3 mm2 and 81 mm2 , and the maximum stressin the rods is 174 MPa and 205 MPa, respectively. The stress is suﬃciently smaller
than the yield strength of the titanium alloy at room temperature (825 MPa).
Thermal design of the cryostat
The thermal design of the magnet cryostat is based on the condition that the superconducting magnets are cooled with single phase liquid helium (LHe) for the stable
magnet excitations during the SuperKEKB beam operation. Each cryostat is cooledwith the individual helium refrigerator of which the cooling power is 250 W at 4.4
K. The cryogenic system can supply subcooled LHe of 20 g/s to the cryostat. LHeat the inlet of the cryostat is in a subcooled condition at 0.161 MPa and 4.4 K. The
allowable thermal load to the helium vessels is 40 W for the helium to be subcooled.
The thermal loads for two cryostats were calculated and the results are summarized inTable 5.11. In the magnet cryostats of QCS-L and QCS-R, the loads are 16.3 W and
15.9 W, respectively. Both loads are below 40 W and all magnets can be cooled bysubcooled LHe.
In this thermal design, while the outer surfaces of the helium vessels have thethermal radiation shields cooled by liquid nitrogen, the surfaces of the bores for the
beam pipes do not have the thermal shield because of the limited space between thebores and the pipes. In Fig. 5.13, the horizontal cross section of the front part of the
QCS-R cryostat is shown. The helium vessel has two bores at 4 K for the beam pipes
at 300 K, and the corrector magnets are directly attached to the outer surfaces of thebores. The QC1RP magnet is assembled outside of the corrector magnets. The rightfigure shows the QC1RP cross section in detail. In the design, the minimum distance
between the bore and the beam pipe is only 3.5 mm. In order to reduce the heat loadby thermal radiation, the beam pipes are wrapped with 5 layers of a superinsulation
sheet, and the bore surface of SS316L is mirror-processed and coated with Al by a vapor
deposition. The emissivity coeﬃcient of this bore surface is evaluated to be smaller120

than 0.02, and the heat load by the thermal radiation is reduced by 50 %, comparedto the surface of SS316L as the bore material.
The QCS-L and QCS-R service cryostats have 16 and 17 ports for installing thecurrent leads, respectively. The pipes of the ports are connected between the vacuum
vessel and the cooling pipe, and the thermal loads through the pipes of QCS-L andQCS-R are 11.5 W and 12.5 W, respectively. After cooling the magnets, LHe of about
1 g/s is used for cooling the current leads, and then the total thermal loads of the
QCS-L/R cryostats are 40.5 W + 1.00 g/s and 40.7 W + 1.03 g/s, respectively.

5.2.6

Cryogenics

The magnet-cryostat at each side of IP is cooled with the individual cooling system.
The cooling system consists of a helium refrigerator, a helium compressor, a liquid
helium subcooler, two helium gas tanks and a liquid nitrogen storage tank. The helium
refrigerator and the liquid helium subcooler are installed in the 4th basement of the
Tsukuba experimental building as shown in Fig. 5.14. The subcooler and the magnetcryostat are connected with the multi-channel cryogenic transfer tube. The pressurized
helium gas at 5K supplied by the refrigerator is changed in the subcooled liquid heliumat 4.44 K and 0.161 MPa with the heat exchanger. The system can supply the subcooled
liquid helium of 20 g/s for one cryostat. The subcooled liquid helium can cool the heatload of 40 W under the condition of the liquid phase. The evaluated heat load tothe helium vessels in the cryostat is 23.9 W. In the thermal design of the cryostat,
the margin of the heat load is 15.5 W. The measured cooling power of the helium
refrigerator is 160W + 28.4 L/h. The total heat load of the system is calculated to be
85.2 W + 28.4 L/h. The refrigerator has the margin of 74.8 W. The heat loads of one
system are summarized in Table 5.12.

5.3

Beam pipe and vacuum system

5.3.1

IP chamber

The inner profile of the IP chamber is determined by taking account of following
conditions.
• The middle part is a straight pipe parallel to the magnetic field of Belle II witha diameter of 20 mm.
• Make no cavity inside the IP chamber so that HOM created by a crotch may
propagate away.
121

Table 5.9: Cryostat parameters.

Magnet cryostat
Vac. vessel length (mm)Vac. vessel max. diameter (mm)Vac. vessel mass (kg)
4 K Cold Mass
Front magnets and others (kg)Tungsten radiation shield (kg)Rear magnets and others (kg)
80 K thermal radiation shield (kg)Service cryostat
Vac. vessel length (mm)Vac. vessel height (mm)Vac. vessel wide (mm)Vac. vessel mass (kg)
80 K thermal rad. shield, kg
He gas-cooled current leads
Conventional leads
Compact 8 terminal leads
Control valves
Support table
Length, mm
Mass, kg
Total cryo. system length (mm)Total cryo. system mass (kg)

122

QCS-L

QCS-R

2724
ϕ1100
1570

3287
ϕ638
1472

949
231
342
45

805
1271
1063
36

2757
917
900
2523
79

2757
917
863
2501
77

10 pairs+1
5 units2

10 pairs
7 units
2

3810
6279
6533
12550

3810
6061
7087
15000

Table 5.10: Electro-Magnetic forces on ESL, ESRs and iron components.
Excitation
QCS-R

ON
OFF
ON
OFF

QCS-L

EMF onsolenoids, N
3.58 × 104
0-4.69 × 104
0

EMF on iron, N

Total, N-0.60 × 104-2.37 × 104
0.42 × 104
8.04 × 104

2.98 × 104-2.37 × 104-4.27 × 104
8.04 × 104

Table 5.11: Design heat load of cryostats at 4 K

Magnet cryostats
Support rod, W
Thermal radiation, W
Sub-total, W
Service cryostat
Thermal radiation, W
Current lead pipe, W
Instrument wire, W
He gas for C. L., g/s
Sub-total
Total

QCS-L

QCS-R

9.7
6.6
16.3

5.8
10.1
15.9

1.9
11.5
3.8
1.00
23.2 W + 1.00 g/s
40.5 W + 1.00 g/s

2.1
12.5
3.4
1.03
24.0 W + 1.03 g/s
40.7 W + 1.03 g/s

Table 5.12: Estimated heat load and the required amount of liquid helium for the
QCSL cryogenic system.
CryostatCurrent Leads
Transfer Tube
Total

(including the service cryostat)
QC1P, QC1E, QC2P, QC2E, ESL
20 corrector magnets
˜13 m length + 1 connection box

123

40.5 W
16 W
56.4 W

24.4 L/h
4.0 L/h28.4 L/h• Allow no direct hit of SR on the middle pipe from the final bend, and stop other
SR as possible as one can.
Figure 5.15 shows a plane view of the inner profile.
The design of the middle straight pipe of the IP chamber is shown in Fig. 5.16. The‘window’ through which elementary particles from IP appears of is double beryllium
tubes. The ‘window’ is open to an azimuthal angle from 17 degree (forward) to 150
degree (backward). The thickness of the inner beryllium tube is 0.6 mm and theinner diameter is 20 mm. The outer tube has a 1 mm gap with respect to the innertube and has a thickness of 0.4 mm. They are brazed to titanium manifolds. The
gap is used to cool the inner pipe with paraﬃn. Paraﬃn is provided through thetitanium manifolds. Mechanical structure of the titanium manifolds is able to absorb
the diﬀerence of expansion between two beryllium tubes. This straight pipe is sputtercoated with 10˜20 µm gold to stop scattered photons below 10 keV. The uniformity of
the coating is tested with a model tube. It was within 5% fluctuation for a beryllium
part.
Titanium manifolds are welded to tantalum branch chambers via Ti-Ta transition.
The tantalum chambers are sputter-coated with 10 µm copper to reduce the ohmic lossof wall current. The beam pipe in the branch chamber for incoming beam has a slope
towards the interaction point to reduce direct irradiation of SR through the pipe fromthe final bend. Since the SR from the last bend illuminates only outside wall (lower sidein Fig. 5.15) of the beam duct, the direct SR passes through the central tube without
hitting its wall. The minimum diameter of the beam pipe for an in-coming beam is 10mm. At this narrowest aperture, a 30σ x radius of the beam is 2˜3 mm. Therefore itis suﬃciently large for the beam. The whole view of the IP chamber is shown in Fig.
5.17 with some fabrication information. The IP chamber has a mechanically delicate
central pipe and heavy wings. Though it is confirmed by mechanical analysis by S.
Koike (MEC) that the IP chamber is self-supporting, handling must be done with care.
To avoid the hitting of scattered light on the beryllium surface, the inside of the pipe
for an in-coming beam has ridges. The beryllium surface is hidden in a space definedby the illuminated slope of ridges. The eﬀectiveness of the ridge structure depends on
the amount of photons which may reach the beryllium surface due to tip-scattering on
top of the ridge. T. Ishibashi and K. Sonnard (Cornell University) performed very clear
experiment at Chess G2 beam line (Cornell University) on this issue. A gold coatedridge of tantalum is illuminated by the 9 keV photon beam. One side of the ridge isvertical. The other side has a slope of 18.4 (Fig. 5.18 (a)). The top of the ridge has afillet of 0.5 R. The vertical beam size is about 0.14 mm. The boundary of the beam is
not clear-cut due to scattering on air. The sample is set so that half of the beam hit
124the ridge. As seen by the vertical beam size, the beam hit only the fillet of the ridge.
The ratio of photons scattered oﬀ from the primary beam was measured. The set up
geometry is shown in Fig. 5.18 (b). The result is shown in Fig. 5.18 (c). From thisresult, one can see that the number of photons scattered into the hidden space by theslope of the ridge is negligible. This experiment was performed using a monochromatic
light. However it gives clear suggestion on an amount of tip-scattered photon.
The pipe for outgoing beam has the same diameter as the middle straight pipe
to allow for created HOM to escape. The structural loss factor of the IP chamber iscalculated by K. Shibata with GdfidL for each version of the design. At present wedon’t have estimation for the last design. We refer here the estimation just before thelast as 1.67 x 1010 V/C (for a bunch length of 5 mm), 1.10 x 1010 V/C (for a bunch
length of 6 mm). The power loss of the beam is 458 W for HER, and 574 W for LER.
The contribution of the ridge in the impedance is about 20˜60 %.

5.3.2

Ta beam pipe in the cryostat

To place final focus magnets as close as to the interaction point, the inner bore of
QC1 is minimized by using a warm inner bore of the cryostat as a beam pipe. The
thickness of this beam pipe is 4 mm. The inner diameter changes in four steps asshown in Table 5.2. Its vertical dimension is elongated in QC1RP and QC1LP. The
central axis of the electron beam pipe has a horizontal oﬀset of 0.7 mm between QC1
and a remaining front part. It is made of Ta according to the requirement by thedetector group. Cooling water is necessary to absorb heats due to the direct SR fromthe last bending magnet and the ohmic loss on a wall. Inside of the beam pipe iscoated with 10 micron Cu to reduce the wall loss. Beam position monitor is installedon this pipe. A typical image of this pipe is shown in Fig. 5.19. Since this beam pipe
is surrounded with He-temperature surfaces, a care is necessary not to freeze coolingwater. As the outside of these beam pipes is limited, water channel must be designed
closely along the body. At some locations water passes a welded seam whose otherside is a vacuum. This kind of structure is avoided in a standard design. However,
because Ta is chemically stable, we judged that no corrosion will develop. As an R&D
we made a short Ta pipe, which has a cooling channel in the wall and a welded seam
between water and vacuum. After a test with tap water over months, no water leakage
was observed. At the same time it is found Ta has a very low thermal outgassing rate
(H. Hisamatsu, see 3-4).

1255.3.3

Other beam pipes and Pressure estimation

Other beam pipes within the first gate valves from IP are made of copper. Theirstandard inner diameter is 80 mm. The duct for positron bending magnet has a groove
on top and bottom to reduce the electron cloud density. To fabricate this groove,straight beam pipe is adopted for a bending magnet, which reduces the horizontal
aperture. All positron beam pipes outside cryostat are TiN coated also to reduce theelectron cloud density. The main pump in this region is non-evaporable getter (NEG).
NEG strip is installed as a distributed pump along a bending chamber. Between
magnets high-capacity NEG modules are installed at pump ports. The arrangementof vacuum pumps is shown in Fig. 5.20. From IP to the end of the cryostat, no spacefor vacuum pump is available. The average pressure around IP will be of the order of
10-6 Pa even after suﬃcient scrubbing with SR (when photo-desorption coeﬃcient of
the QCS beam pipe is ˜ 10-6 molecules/photon). Detailed calculation of the pressure
profile around IP is planned.

5.3.4

Some experimental vacuum data

Special material and coating are used in vacuum components of this region. To compare with standard material with respect to thermal desorption and photon-desorption
property, some measurements are performed. Though they not always directly applicable to the final design, those data are useful from a general viewpoint and are reported
here.
Photon desorption coeﬃcient of Ta, Cu, Au-coat on Ta (Y. Tanimoto and
T. Nogami)
Photo-desorption coeﬃcient was measured at the beam line BL21 of Photon Factory(PF). The source of synchrotron radiation is a bending magnet and the critical energyof SR is 4 keV. A sample was exposed to 5 mm x 5 mm photon beam at normal
incidence. The value of each desorption coeﬃcient is similar up to a photon dose of1021 . Then the decrease of the desorption coeﬃcients of Ta and Au become slower
than Cu. And Au shows much slower decrease. At present we have no explanation on
the reason why the decrease of the desorption coeﬃcient of inactive material becomesslow at high photon dose. These data are shown in Fig. 5.21 (a)-(c).

126Thermal outgassing rate of Ta pipe (H. Hisamatsu)
A test pipe is 478 mm long. A 346 mm tube with ID 21 mm is connected to a 34 mm
long ID 26 mm tube with a taper. It was cleaned with HNO3 + HF + Water. Thenit was heated in a vacuum furnace with a cryo-pump at 1100-1150 ◦ C. The outgassing
rate is shown in Fig. 5.21 (d), The outgassing rate is lower than standard titanium
and copper.

5.4

Installation scenario

We have vacuum connections between IP chamber and cryostat beam pipe. The accessby human hands to these connections at their final location is impossible in the forward
part, and is diﬃcult in the backward part. How to connect these flanges is combinedwith how to install the vertex detector (VXD). At present two options are considered.
Baseline scenarioFirst VXD and QCSR are connected outside the detector. Then QCSR moves forward
pushing VXD. After VXD is mechanically detached from QCSR and fixed on CDC,QCSL moves forward. Then remaining connection is performed by hand. The last
step is now becoming more diﬃcult with an advance of detailed detector design andcryostat design. This scenario assumes the Belle II detector is in the roll-in position.
When the replacement of the BPM-bellows unit is necessary, VXD must be pulled out.
Fig. 5.22 illustrates the whole process of this scenario.
Alternative installation method (AIM)This method makes use of a remote vacuum connection system (RVC) in front of
the cryostat. The RVC system was proposed by K. Gadow and C. Niebuhr (both
DESY). The prototype of the RVC was tested and succeeded in performing vacuum
connection. A 3D-picture of the prototype and a mock-up are shown in Fig. 5.23. In
this case VXD is installed by hand using some sliding supports. A concrete designof this support system is being developed and tested at MPI by C. Kiesling and B.
Müller. The Major concern is that the use of RVC in the forward inaccessible region.
A disassembling method in case the RVC stacks must be established.

5.5

Radiation shield

New concrete shields are added to an existing gate shield.
1271) On top of the gate shield 45 cm concrete is added.
2) Front of the gate shield is filled with concrete leaving a necessary aperture foraccelerator components..
3) The gap between the detector and Accelerator floor is filled with concrete.
4) The gap between the ceiling of accelerator tunnel and the gate shield is filled with
80 cm concrete.
5) The shield wall of the experimental hall is extended towards IP.
These additional concrete shields are depicted in Fig. 5.24 except 3). By these concrete
shields, the radiation level of the experimental hall will be 1˜10 µSv/h for design
parameters. A training as a radiation worker is necessary to access the hall. To reduce
further, local shield around radiation sources will be added. Especially beam pipes
where the beam loss due to the radiative Bhabha process occurs must be shielded toreduce air activation. It is suggested that shields to cover an opening for cable trays
connecting the experimental hall and the accelerator tunnel are eﬀective to reduce theradiation level of the hall. (T. Sanami)

5.6

The design for Phase 1 commissioning

128

Figure 5.7: Contour plots of the magnetic field in the positron beam line laying sideby-side with QC1LE. QC1LE magnet current is 1577 A as 4s operation. In the upper
plot, the materials of the QC1LE yoke and the magnetic shield of the position beamline are iron, and in the lower plot, they are Permendur.

129

Figure 5.8: Two layer coil geometries and multipole field profiles along the magnet
axes of QC2P and QC2LE. Z-position=0 is the magnet center, and the reference radii
are 30 mm and 35 mm for QC2P and QC2LE, respectively.

130

Figure 5.9: Solenoid field profiles along the beam lines.

(a)

(b)

Figure 5.10: Compensation solenoids. The drawings of (a) and (b) show the ESL andthe ESR1 solenoid magnets, respectively.

131

Figure 5.11: Overall view of QCS-R cryostat and the cross section of the magnet
cryostat. The top figure is the whole cryostat viewed from the top. The middle figureis the horizontal cross section of the magnet cryostat, and the bottom figures are thevertical cross sections of the magnet cryostat at the individual quadrupole magnet
center.

132

Figure 5.12: Support rods of the front helium vessel of the QCS-R magnet cryostat(top left), the rod design (top right) and the magnet center changes of four quadrupole
magnets by cool-down (bottom). The quadrupole magnet centers at 300 K and 4 K
are shown with symbols of triangle and circle, respectively. The position changes areshown in mm.

Figure 5.13: Cross section of the front part of the QCS-R magnet cryostat. The left
figure shows the horizontal cross section of the QCS-R cryostat at the QC1RP magnet.
The right figure is the QC1RP cross section.

133

Figure 5.14: Cryogenic systems for the final focus superconducting magnet system.

Figure 5.15: Inner profile of the IP chamber. Vertical scale is expanded ten times.
BSR (beam stay clear) corresponds to about 30σ x radius.

134

Figure 5.16: The middle straight pipe of the IP chamber designed by T. Kohriki (IPNS).

135

Figure 5.17: Whole view of the IP chamber.

Figure 5.18: Measurement of the tip-scattering of photon. (a) Sample dimension. (b)Experimental set-up. (c) The ratio of scattered photon with respect to the primarybeam. The horizontal axis of this graph is ‘δ’ in the picture of (b).

136

Figure 5.19: Ta beam pipe for QCSL positron beam line.

Figure 5.20: Arrangement of vacuum pumps in the interaction region.

137

Figure 5.21: Outgassing data. (a) Photo-desorption coeﬃcient of Cu. (b) Photodesorption coeﬃcient of Ta. (c) Photo-desorption coeﬃcient of Au coat on Ta. (d)Thermal outgassing rate of Ta.

138

Figure 5.22: Baseline installation scenario. Start is left top, and end is right bottom.

Figure 5.23: RVC mechanism. (a) A cut view of a test mock-up. (b) The mock-up for
vacuum test.

139

Figure 5.24: RVC mechanism. (a) A cut view of a test mock-up. (b) The mock-up for
vacuum test.

140

Figure 5.25: Phase 1 layout of the interaction region with concrete shield

The commissioning of the SuperKEKB starts from a vacuum scrubbing phase (Phase1). The main purpose of the Phase 1 is the scrubbing of new LER beam pipes withsynchrotron radiation to reduce the photo-desorption coeﬃcient. In this phase, BelleII detector is out of the interaction region (in the roll-out position) and there are no
final focus QCS magnets. Temporal Al beam pipes are set at the interaction point and
the QCS location. A compact concrete shield covers these beam pipes. This shieldkeeps the experimental hall as a non-controlled area under the Phase 1 operational
condition. Fig. 5.25 shows a layout of the interaction region during Phase 1.

5.7

Beam Background

With increased luminosity at SuperKEKB, we need to carefully estimate the beam
background impact on Belle-II detector. If beam background exceeds the acceptablelevel, radiation might kill Si devices, fake hits might deteriorate detector resolution,and high occupancy might use up bandwidth of data acquisition path. Major sourcesof beam background at SuperKEKB are: beam-induced backgrounds sources(Touschek
141scattering, beam-gas scattering), luminosity-dependent background sources (radiative
Bhabha process, two-photon process), synchrotron radiation, and so on.

5.7.1

Beam background sources• Touschek eﬀect
Touschek eﬀect is an intra-bunch scattering between two beam particles in thesame bunch [10]. The scattered particles with too little or too much energy start
to deviate from the design orbit while they travel along the ring, and will finallybe hi ting the beam pipe and generate electromagnetic showers. If the beam loss
position is in the neighborhood of the Belle-II detector, the shower particles canreach the detector.
At SuperKEKB, simple extrapolation using beam size parameter predicts thatthe beam background by Touschek eﬀect will increase by factor of ∼20 comparedto that of KEKB. However, it can be dramatically mitigated by movable collimators which can stop the deviated particles before they reach the interaction
region.
Horizontal collimators are installed at both inner and outer side of the beam
orbit. Collimators are located at the positions where horizontal beta function or
the dispersion becomes local-maximum. Collimator opening widths are selectedto achieve good background mitigation and beam lifetime at the same time.
• Beam-gas eﬀect
Beam particles scatted by the residual gas atoms in the beam pipe are alsodeviated from the design orbit, like Touschek-scattered particles. Here we assume
both Coulomb scattering and Bremsstrahlung scattering.
The beam-gas scattering rate is proportional to the vacuum level and the beam
current. At SuperKEKB, the beam currents will be ∼ 2 times higher than thatof KEKB, and the vacuum level except for the interaction region will be the samelevel as KEKB. Therefore we have been expected the same order of magnitude (afew times higher) beam-gas background in the past publications [12]. However,
our latest simulation study reveals that Coulomb scattering rate is higher byfactor of ∼ 100 than that of KEKB, since IR beam pipe aperture is smaller andthe maximum vertical beta function is larger.
Beam-gas Coulomb background can be mitigated by vertical collimators, but thecollimator half width should be very small (few mm), due to the Transverse Mode142

Coupling (TMC) instability caused by the collimator. For more details, pleaserefer [11].
In addition, slight misalignment of the vertical collimator can easily lead to significant increase of the background level. Therefore sum 50µm precision for the
movable collimator system is required.
• Radiative Bhabha process
Gammas emitted from radiative Bhabha scattering by beam collisions travel
straight to the downstream and interact with beam pipes or magnets. In theseinteractions, neutrons are copiously produced via the giant photo-nuclear resonance mechanism. Those neutrons scattered back to the detector is dangerous
for the outermost KL and muon detectors (KLMs).
Electrons or positrons lose energy after radiative Bhabha scattering by beamcollision. In case of KEKB, since we employed the shared final quadrupole magnet
scheme, downstream beam orbits go through oﬀ-center of the quadrupole magnets
and therefore the spent beam particles were over-bent strongly. At SuperKEKB,
we introduce individual final quadrupole magnet scheme, therefore spent beam
particles are more likely to go through the interaction region without hitting thebeam pipe. However, if they lose so much energy, they can still hit the beam pipe
inside or near the Belle-II detector (typically particles below 2 GeV can be lost
inside the detector), and generated shower particles can reach to the detector.
Radiative Bhabha background is proportional to the luminosity. With 40 times
higher luminosity at SuperKEKB, radiative Bhabha background is the most dominant background source, after the mitigation of T??o??u??s??c??h??e??k?? andbeam-gas background by movable collimators.
• Two-photon process
Low momentum electron-positron pairs produced via the two-photon process (ee → eeee ) by beam collision. This background source is important for the innerdetectors. At SuperKEKB, the radius of the innermost detector is less than thatof KEKB since we introduce the pixel detector close to the IP. The two-photon
background rate increases roughly as 1/r2 . Curling tracks might be dangerous,
since they might hit the inner detectors many times while curling.
• Synchrotron radiation
Synchrotron radiation photons are generated from the beam particles when theorbit is bent. It might penetrate beam pipe and reach the inner detectors. To
143absorb the synchrotron radiations before they reach the inner detectors, the innersurface of the Beryllium beam pipe are coated with gold plate. The shape of IR
beam pipe is carefully designed to avoid direct SR hits at the detector. Theenergy of SR which we are worring is from few keV to tens of keV.
• Other background sources
Beam background during the injection is also important background source. We
need to apply timing veto on the innermost detector, to achieve comfortable dataacquisition rate.
Beam-beam kick eﬀect, the interaction of a beam bunch with the electric fieldof the other beam bunch at collision point, changes the beam shape to distorted
distribution, not a Gaussian. This eﬀect should be carefully taken into account,especially when estimating the halo eﬀect on synchrotron radiation.
Simulation tools
For Touschek and beam-gas background, we use theoretical formulae [16] to generate
distribution of scattered beam particle energy and direction. For radiative Bhabha
background, we use BBBREM [13] generator. Generated particles are then tracked
using SAD [15] code, until they are lost by hitting beam pipes. Then loss particles
information is provided to Geant4 [17], and Geant4 takes care of simulating shower
propagation and detector response.
2-photon background is generated by Diag36(BDK) [14] generator, and provided directly to Geant4. Synchrotron radiation is generated inside Geant4 framework, andsimulated by Geant4.
Beam loss distribution
Figure 5.26 shows the beam loss distribution inside Belle-II detector, weighted byenergy of lost beam particle. Particles lost by radiative Bhabha process are mainly lessthan 2 GeV, while Touschek and beam-gas particles are almost 4 GeV (positrons) or
7 GeV (electrons).
Table 5.13 summarizes the loss rates within |z| < 4 m. Radiative Bhabha components
give dominant impact on the detector.
Impact on Belle-II detector
We simulate the impact of the beam background on the sub-detectors of Belle-II detector: PXD, SVD, CDC, TOP, ARICH, ECL, and KLM. By adding few more shielding
144

Figure 5.26: Beam loss distribution inside the Belle-II detector, weighted by beam
particle energy.

Radiative Bhabha
Touschek
Coulomb

LER
0.63 W
0.07 W
0.07 W

HER
0.88 W
0.02 W
0.001 W

Table 5.13: Beam loss rate inside the Belle-II detector, weighted by beam particle
energy.

structure inside the detector, we have confirmed that the estimated background levels
are acceptable for most of our sub-detectors, assuming 10 years of machine operation
at full design luminosity:• PXD/SVD occupancy are OK• CDC wire hit rates are OK• radiation dose on PXD/SVD chips, ECL crystals/diodes are OK• neutron rate on CDC FPGA chips, ARICH HAPD sensors, and end-cap KLMs
are OK
However, we found TOP PMT photocathode might not survive for 10 years at full
design luminosity. We are planning to replace them with new PMTs, after few yearsof operation.
Validity of those simulation are confirmed by BEAST experiments prior to the start ofphysics runs. We install BEAST sensors to measure radiation levels inside the detector
145during the machine comissioning period, to make sure we can safely install our Belle-IIdetectors. Details of BEAST experiments can be found elsewhere[18].

146Bibliography[1] N. Ohuchi et al., “Design of The Superconducting Magnets System for The SuperKEKB
Interaction Region,” WEODA1, Proceedings of NA-PAC2013, Pasadena, September
2013, pp. 759-761 (2013).
[2] [Y. Makida, et al., “Development of a superconducting solenoid magnet system for the
B-Factory detector (BELLE),” Advances in Cryogenic Engineering, Vol. 43A, 1998, pp.
221-228.
[3] H. Yamaoka et al., “Solenoid Field Calculation of the SuperKEKB Interaction Region,”
THPPD023, Proceedings of IPAC2012, New Orleans, May 2012, p. 3548 (2012).
[4] B. Parker et al., “Superconducting Corrector IR Magnet Production for SuperKEKB,”
THPBA07, Proceedings of NA-PAC2013, Pasadena, September 2013, pp. 1241-1243(2013).
[5] B. Parker et al., “Direct Wind Superconducting Corrector Magnets for the SuperKEKB
IR,” IPAC12, New Orleans, LA, WEEPPB013, p. 1291 (2012).
[6] B. Parker, et al., “BNL direct wind superconducting magnets”, IEEE Trans. on Appl.
Supercond., Vol. 22, No. 3, 2012, 4101604.
[7] N. Ohuchi et al., “Design and Construction of the Proto-type Quadrupole Magnets
for the SuperKEKB Interaction Region,” THPBA04, Proceedings of NA-PAC2013,
Pasadena, September 2013, pp. 1241-1243 (2013).
[8] N. Ohuchi et al., “Design and Construction of the Magnet Cryostats for the SuperKEKB
Interaction Region,” IEEE Trans. on Appl. Supercond., Vol. 28, No. 3, 2018, 4003204.
[9] Z. Zong, N. Ohuchi, K. Tsuchiya, Y. Arimoto “Development of a compact HTS lead
unit for the SC correction coils of the SuperKEKB final focusing magnet system”, NIM
PR-A 830, 2016, pp. 279-286.
[10] A. Piwinski, “The Touschek eﬀect in strong focusing storage rings,” physics/9903034.

147

[11] Y. Funakoshi, K. Ohmi, Y. Ohnishi, K. Kanazawa, Y. Suetsugu, H. Nakayama andH. Nakano, “Small-Beta Collimation at SuperKEKB to Stop Beam-Gas Scattered Particles and to Avoid Transverse Mode Coupling Instability,” Conf. Proc. C 1205201,
1104 (2012).
[12] T. Abe et al. [Belle-II Collaboration],
arXiv:1011.0352 [physics.ins-det].

“Belle II Technical Design Report,”[13] R. Kleiss and H. Burkhardt, “BBBREM: Monte Carlo simulation of radiative Bhabha
scattering in the very forward direction,” Comput. Phys. Commun. 81, 372 (1994) [hepph/9401333].
[14] F. A. Berends, P. H. Daverveldt and R. Kleiss, “Monte Carlo Simulation of Two Photon
Processes. 2. Complete Lowest Order Calculations for Four Lepton Production Processes
in electron Positron Collisions,” Comput. Phys. Commun. 40, 285 (1986).
[15] http://acc-physics.kek.jp/SAD/sad.html[16] Y. Ohnishi, T. Abe, T. Adachi, K. Akai, Y. Arimoto, K. Ebihara, K. Egawa and J. Flanagan et al., “Accelerator design at SuperKEKB,” PTEP 2013, 03A011 (2013).
[17] S. Agostinelli et al. [GEANT4 Collaboration], “GEANT4: A Simulation toolkit,” Nucl.
Instrum. Meth. A 506, 250 (2003).
[18] http://belleweb.pnnl.gov/forTDRreview/TDR-SLAC-13Dec8.pdf

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