Chapter 14
Safety System
14.1

Personal Protection System

14.1.1

Overview

The interlock used in the personal protection system (PPS) is structured under thepolicy that The interlock system uses hard wires and dose not uses the network nor
general purpose computer. Programable Logic Controller (PLC) is regarded as having
the same level of reliability as hard wire because its function is specialized in a single
process. It plays an important role in PPS. The PLCs are installed in each local control
room, and the information used for interlock is gathered as ON / OFF logic signal fromthe central PLC system through the optical fiber cable.

14.1.2

Access Control

Overview
The access control of the tunnel is carried out based on the following three levels.
They are 1: Free, 2: Limit, 3: Keep out.
Free is in a state that a person canenter the radiation control area without contacting the central control room. This
state is occurred during a long accelerator shutdown period. Limit indicates a state
temporarily allowing entry into the tunnel during accelerator operation period. In
this case, the entrant needs the shift leader’s permission, and the entry area is strictly
managed by the surveillance camera, the card reader, the personal key. Keep out isa state that no one can enter the tunnel where the beam is injected or accumulated on
the accelerator. At the boundary of each accelerator, "the boundary area" is provided
as much as possible to reduce the radiation eﬀect from the adjacent accelerator whichis under operation. At the boundary area of each accelerator, the key of the door is668

strictly managed as a gate key. Normally, the gate key is stuck in the console of thecentral control room, and if the gate key has been taken out, the operation of theaccelerator is restricted. Figure 14.1 shows the boundary area of the positron damping
ring (PDR), KEKB and their beam transport line.

Figure 14.1: The boundary area around KEKB and PDR

Personal Key control
Access to the accelerator tunnel is controlled through a personal key. A personal keybox, an infrared sensor, a surveillance camera, an operation indicator light, and a card
reader are equipped at the entrance of the tunnel. When an entrant comes in front of
the door, the infrared sensor senses it and it is automatically displayed on the monitor
of the central control room. The entrant makes the entry request to the central control
room through the inter-phone. The operator in the central control room, permits theentry by two steps. The first step is a permission to take the personal key and thesecond step is a permission to open the door with the personal key. As far as theentrant has a personal key in the tunnel, his/her safety is guaranteed.
Monitor
The whole tunnel is constantly monitored by the surveillance cameras and recorded.
The images from the surveillance cameras are displayed on the four monitors in thecentral control room. The monitor is divided into four area and displayed. The surveillance camera is controlled by the PLC s in the Experimental Physics and Industrial

669

Control System (EPICS) framework[1]. The analog cameras are used in the tunnel,
and the high quality digital cameras are used at the entrance to identify the entrant.

14.1.3

Logics of PPS

Whole system
The absence of beams in the tunnel is secured by at least two independent interlocks.
The injector linac can simultaneously supply beams to five rings of PF, PF-AR, KEKB
HER, KEKB LER, PDR. In case of PDR, the beam returns to the injector from PDR.
While one of the accelerator is under operation, a person may access to the otheraccelerator tunnel, so that the logic of the injection beam request becomes complicated.
The accelerator excluding PF requests the beam in the same procedure. The procedurefrom the Free state to the beam operation is shown in Table 14.1.
Table 14.1: The protocol of the beam request
ActionState

Accelerator safety

1
2
3
4
5
6
7

Limit Button ON
Patrol button ON
Keep out button ON
Beam Key ON
The beam injection mode ON
Linac accepts the mode

Free
Limit
Limit
Keep out
Keep out
Keep out
Keep out

8

Beam Request

Keep out

ON the BT unattended confirmation buttons
The open permission of the ring stopper
Wait an acceptance from Injector.
The open permission of beam shutter in BT.
The permission Turn ON Safety Magnet

First, the access level is set to "Limit", and the patrol in the tunnel is required.
Because the beam transport line of KEKB is geometrically complicated, in order not
to overlook person in tunnel, the unattended confirmation buttons are installed along
the tunnel. To move into the "Keep out" level, all buttons have to be pushed at thepatrol. After the patrol the access level moves into "Keep out" level, and the beam key
is turned on. At this level, the open permission of the ring stopper is given. From thedownstream accelerator to the upper stream injector, "the beam ON mode" request issent and the injector Linac supposes to accept the request. At this stage, the safetysystem give a permission to turn on the safety magnet and open the beam shutter.
Finally the beam request signal is sent to the injector linac.
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KEKB Ring and beam transport
To permit the access to KEKB-BT line, two interlocks are required. One is to turnOFF the safety bending magnet installed in the 3rd beam switch yard at the end of
the injector Linac. Another is to insert the beam shutter into the beam chamber toensure safety. For the access to KEKB main ring, in addition above two interlocks, twomore interlocks are required to ensure no stored beam in the ring. Two independent
beam stoppers are inserted in the vacuum chambers in the main ring. Figure 14.2
shows the position of the safety magnets and the beam shutters on the positron andthe electron beam line respectively. For the electron line, three horizontal bending
magnets are connected in series to power supply, and four vertical bending magnets
are connected in series to another power supply so that four vertical bending magnets
and three horizontal bending magnets are used as the safety magnets.

Figure 14.2: KEKB the safety magnets and the beam shutter

Damping ring
To prohibit the injection beam, one beam shutter and safety bending magnets are
installed in the injection beam transport line as shown in Figure 14.3. Since the power
supply drives four safety magnets which are connected in series with each other, thefour magnets are used as safety magnets. No stored beam in PDR must be secured
not only for PDR access, but also for the access to the injector Linac, because the671

accumulated beam in PDR returns to the injector Linac. To ensure no stored beamin PDR, two independent interlocks are prepared. Since only one beam stopper isinstalled in PDR, it is necessary to have another interlock. The PPS requires to turnoﬀ the ring RF cavity once with the beam shutter closed and turned oﬀ the safety
magnets in the injection line. For the injector Linac safety, another safety magnet isinstalled in the extraction line.

Figure 14.3: PDR the safety magnets and the beam shutter

14.1.4

System architecture

Rejuvenation from KEKB safety system
The PPS of SuperKEKB and PDR also take care of PF - AR and their beam transportlines. Since it is closely related with each other, the safety system cannot be modified
or updated during operation any of the above accelerators including the injector. Thatis, the period which the system can be updated and tested suﬀers great restrictions by
the operation of the accelerator. Furthermore, due to the construction of a new beamtransportation line of PF-AR and PDR and KEKB main ring, accelerator operation iscarried out while guaranteeing the safety of people working in the accelerator tunnel.
Since the accelerator which completed construction, immediately shifts to the operation
state, it is necessary to update to the corresponding PPS each time. As can be seen fromthe above, it is extremely diﬃcult to remodel a large-scale safety system that manages
672multiple accelerators. The key of the system modification to meet such demands is
modularization of the system.
This system transition started with replacing the PLCs. Since the PLC of eachcontrol rooms and the central PLC system are exchanging logic signals through theoptical fiber cable, the PLC system can mix a plurality of vendor models. Actually
the PLC replacement has been done for several years. In addition, the access control
management system is also configured independently of the main PPS and can communicate with the PPS by the logic signal, so it can be updated separately from themain PPS. The PLC system and the safety console in the central control room had tobe repaired at the same time. By pre-assembling a new system, and connecting theexisting cables there, the downtime is minimized. Another problem is the development
and test of the PLC programing. The debugging of the PLC program with the actual
accelerator is incompatible with the construction and operation of the accelerator. The
programing and its verification has been done with simulation system. All input andoutput for example doors closed state, emergency button status, the input from console
are simulated with the PLC internal relay. AT the final stage, the PLC program isverified with real accelerator.
The safety system hardware• Whole system design:
The interlock used in the PPS is structured under the policy that it uses thehard wires and does not use the network or general purpose computer. The
PLC system is regarded as having the same level of reliability as the hard wire,
because its function is specialized in a single process. It becomes an importantcomponent of the system. The PLC is installed in each local control room, andthe information used for interlock is gathered as ON / OFF logic signal from thecentral PLC system through the optical fiber cable. The information gathered
through the Ethernet are used for monitor purpose, never used for the safety
interlocks.
• PLC system:
To keep the low failure rate, the PLCs have been updated every ten years. For
SuperKEKB PPS, Yokogawa’s FA - M 3 V Leading Edge Controller is chosen.
The PLCs are connected to two networks. Figure 14.4 shows the network conceptual diagram. One of the networks is connected to the sequencer CPU, andused to download the program. This network is an isolated network, it can not
be accessed from outside. The other is a FA - M 3 V dedicated network called FL

673

- Net, which defines a common memory between PLCs and shares information.
FL-Net is used only as a monitoring purpose and does not contribute to interlock.
As shown in Table 2, the FL-net cluster is divided into three clusters and sharesinformation in each cluster.

Figure 14.4: Network Configuration of PPS• Computing system:
The file server and computing server are separated. As a file server, the network
attached storage (NAS) has been chosen. The replica of the main file server isalso installed. As a computing server, the Linux server is chosen. The Linuxserver connects the safety system network and the network of the accelerator
control system.
Software• EPICS based system
EPICS is introduced to KEKB accelerator control system first, then the PPS
adopts EPICS. The information obtained through the FL-net is defined as EPICS
Record on the Input-Output Controller (IOC). Yokogawa’s Embedded Machine
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Controller (e-RT3)[2] with Linux is used as an IOC. The IOC is provided foreach FL-Net cluster shown in Table14.2. Those IOCs are connected to the Linuxserver computer through the third network (see Figure refsafetynet). Informationof the safety system is handed over to KEKB control network through the IOC
for Gateway.
Table 14.2: The FL-net Cluster
1

Accelerator Name

2
3
4
5

KEKB Ring
KEKB/BTPDR
CentralN of Modules

Cluster Name

18
2
2
2

KEKB Cluster
PF-AR Cluster
KEKB Cluster
Central Cluster• User interface:
All inputs that directly aﬀect the interlock are made through the operation console. The console consists of buttons and keys, and it is connected to the input/
output module of the PLCs. Since it is isolated from the network, it cannot
be controlled from outside of the control room. On the other hand, the statusmonitor of PPS connects to the network. The open/close state of the door, thepersonal key return state, the state of the safety magnets and the state of thebeam shutter are gathered in the IOC through FL-NET and sent to the central
control room through the EPICS framework. The information is displayed using
the BOY[3] of CSS (Control System Studio)[4].
• Logging system:
All the collected information is recorded and saved in the CSS archiver. Since
the CSS Archiver has good aﬃnity with EPICS, it is chosen as archiver. The
information is recorded only when the value changes. Compared with pollingrecords, it can save the logging data size as well as network bandwidth. Since
the acquired data is stored using PostgreSQL which is a relational database, it
can be easily displayed by a Web browser.

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14.2

Machine Protection System

14.2.1The beam abort trigger system

Overview
Figure 14.5 shows the abort trigger system configuration[5]. The abort requests fromeach hardware component are converted to optical signals. For the some hardware
components, the 2ch E/O module (1424 module) which accepts TTL, RS422 and relay
inputs, is developed. The optical signals are sent to VME-based 8 channel optical
input module (18K15 module) in 12 local control rooms (LCR). The 18k15 module
outputs an "OR" signal of abort requests. It also latches the signal until receivinga reset signal. These functions are processed on FPGA in the 18k15 module. The
18K15 modules collect more than 130 beam abort request signals. In the central
control room(CCR), all abort requests are collected from these 18k15 module, thenthe abort trigger signal is sent to the abort kicker. In addition to the hardware abort
trigger system, a request activated by the software is used for the abort request that
do not require a quick response. The software abort requests are collected from many
Input/Output Controllers (IOCs), such as IOC of RF, vacuum, monitor, etc. A manual
abort was also prepared to discard the beam in the normal accelerator operation.
Software abort requests and manual abort request are merged into hardwire abort
signals in CCR. The system is controlled in the EPICS frame work.

Figure 14.5: The abort trigger system at SuperKEKB

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Time stamp
Since abort requests are occurred sequentially, chronological order of the request isimportant to identify a cause of the beam abort. The beam abort system has the timestamp function to keep track of the abort request signal. The time stamp informationis recorded in the IOC located at CCR. Figure 14.6 shows timing diagram of the timestamp system. The time stamps are generated from two counters. One is the trigger
pulse counter which counts as the external clock at the 18k15 module. This trigger
pulse counter is used to synchronize all the 18k15 modules in the system. The external
clock is relatively slow clock and it is generated by the software trigger system[6]. The
software trigger system generates not only synchronization pulse for VME 18k15, butalso the interrupt signal for IOCs. To get precise abort request time, the internal clock
counter of 18K15 module is used. The internal clock counter count up at 10 MHz, andis synchronized to FPGA operation clock. Each counter is 32-bit counter.
Both counter values are cleared when the 18k15 module receives the reset signal.
The internal clock is also cleared by the external clock input. Each counter starts after
detecting the reset signal. When the abort request signal is detected in the module,
both counter is latched. The common external clock determines the time roughly
and the internal clock determines more precisely. The resolution of the timestamp is
0.1µs since the internal clock counter counts up at 10MHz. The time stamp value iscalculated from these two counter value in the IOC. And the eﬀect of transmission
time diﬀerence, device processing delay are also corrected by the software.
Beam abort request
The beam abort system collects four types of abort requests[7].
• Triggers from beam loss monitors• Triggers from synchrotron oscillation phase monitor• Direct triggers from hardware components• Manual abort signal
The circumference of SuperKEKB ring is 3km with 12 LCRs as shown in Figure 14.8
. Loss monitor signals are collected at 4 LCRs, RF signals are collected at 6 LCRs.
Response time of the beam-abort system
The time that the hardware detects the abort alarm, extends over the wide range, i.e.
from a few µs to several second. For example PIN diode detect the beam loss within a677

Figure 14.6: Timing chart of time stamp system.
few µs[8]. On the other hand, loss monitor of ion chamber needs several ms to generate
the signal. Other device such as a magnet power supply, needs several ms to several
second to detect the failure.
After the hardware detects the alarm status, the abort trigger system send theabort request signal to the beam abort system. The beam abort requests that requirea quick response are designed to be transmitted via hardware. The time from a failure
detection to the beam extraction is estimated as follows. Figure 14.7 shows the timenecessary in each stage.
• T1-T0: This is the transmission time from the beam line to the interlock module
at the LCR. At least 2µs is needed if the detector, such as the beam loss monitor,
is located in the tunnel.
• T2-T1: The time necessary for signal processing at LCR plus transmission timefrom LCR to CCR. The abort signal collected at the LCR, is converted to an
optical signal. This process takes 100-200ns that is negligible compared withtransmission time from LCR to CCR. The longest distance from LCR to CCR
is 2km, so that the maximum transmission time is about 10µs . Even for thenearest LCR, 2µs is needed to transmit the signal.
• T3-T2: The revolution time of the SuperKEKB ring is 10µs. The beam is filled678

in every two RF buckets except for consecutive an empty bucket of 200ns thatis used for rise time of the abort kicker magnet. Since abort kicker has to be
fired synchronized with this abort gap, waiting time of 0-10µs is indispensable.
Moreover it takes 3µs to transmit the trigger signal from CCR to abort kicker
power supply as well as to turn on the kicker thyratron switch. In total 13µs is
necessary in this stage.
• T4-T3:The time to extract all bunches from main ring by one set of abort kicker
magnet.
After the device send the abort request, the beam abort system needs 17-35µs to
extract all the beams from the ring. On the other hand the software abort responsetime takes few seconds.

Figure 14.7: Overview of beam loss monitor and abort monitor system at SuperKEKB

Beam abort monitoring system
A beam abort monitor system is prepared to diagnose what caused the beam abort. The
system consists of four data loggers. The data loggers are located in 4 LCRs. In each
LCR, 24-32 signals are monitored. They collect various signals such as beam current
measured by a DCCT; beam loss monitor signals from PIN photo-diodes (PINs) as well
as ion chambers (ICs); various signals related to the RF cavities, i.e. cavity voltages
679

and output power of klystrons; the beam phase signal that measures the deviationfrom the synchronous phase; the injection trigger timing and the abort request signal.
The length of logging time is between 300ms and 600ms for each abort event with asampling time of 1µs to 5µs, which depends on the type of logging apparatus. The
recorded data is sent to the CCR via the KEKB control network.

Figure 14.8: Overview of beam loss monitor and abort monitor system at SuperKEKB

14.2.2

Beam abort system

In order to protect the accelerator components and Belle II detector, each ring isequipped with a beam abort system. It extracts the stored beam and conveys to thebeam dump preventing the beam spraying around the ring. Both of HER and LER
abort system is installed in the Fuji straight section[9].
System requirements
Basic design There are two requirements for the beam abort system. One is totransport the extracted beam to the beam dump safely. In order to extract the beam680

without damaging the extraction window made of titanium (Ti), the energy density
dissipated at the Ti window is less than its allowable maximum value. The otherrequirement comes from the requirement for the stable operation of RF cavities. To
minimize the abrupt change in beam loading, the beam abort gap that is an empty
bucket space reserved for the rise time of the abort kicker magnet, must be less than
200 nsec.
The extraction window The titanium window is chosen as the extraction window,
because of its small mass number and high melting point. When the beam passes
through the extraction window, it deposits the minimum ionizing energy in the window. Since the thickness of the window is suﬃciently thin (1.4mm), compared to theradiation length of 35.6mm, dissipated energy due to the electromagnetic shower isnegligible small. Because the deposited energy due to the collision is in proportional
to the thickness of the window, the temperature rise of the window is independentof its thickness. It depends on the charge density at the window. Since the beamemittance of SuperKEKB is one order of magnitude smaller than that of KEKB, theeﬀective passage area must be enlarged to decrease the charge density at the extraction window. In the present abort system, the beam size is widened deliberately in thehorizontal direction, and the eﬀective beam cross section is expanded by sweeping thebeam position in the vertical direction at the window. The energy density is determined through the product of the horizontal beam size and the sweeping height (H),as shown in Figure 14.9. Note that vertical beam size of the bunches is irrelevant.
In SuperKEKB, 2500 bunches of the beam come in every 4 ns. If the extracted
beam is swept 15mm in the vertical direction, the beam spot moves 6µm per bunch.
In KEKB, the maximum current was 2 A in LER and the eﬀective charge density of
extracted beam at the window is 0.3 A/mm2 . In the new abort system, the charge
density is designed not to exceed this value. In addition, the destruction limit oftitanium window due to the single passage of the beam was investigated using thebeam of KEKB[10], it was no damage at 0.53 A/mm2 , while it was destroyed at 0.77
A/mm2 . Although the eﬀect of multiple passage (eﬀect of fatigue or recrystallization)is still unknown, experience of KEKB tells that the density of 0.3 A/mm2 is safe andhas some margin with respect to the single passage limit of 0.53 A/mm2 . One may
concern diﬀerence of sweeping pattern at the window aﬀects the maximum stress: inthe KEKB it was sinusoidal while simple vertical sweep in the SuperKEKB. In theformer case, the current density maximizes at several stationary points, while in thelatter maximum points lies on the vertical line. Dynamic stress may diﬀer between
these cases. Analysis shows, however that for the Gaussian beam, maximum dynamic

681

Figure 14.9: Beam profile at the extraction window
stress does not exceed its initial thermal stress; because speed of sound in the windowmaterial (~5000m/s) is much larger than the sweep speed (2120 m/s), the dynamic
stress does not accumulate; so there is no big diﬀerence in the maximum dynamic
stress between both case[16]. Table 14.3 shows the beam parameters at the extraction
window. Current density is 0.25 and 0.2 A/mm2 for LER and HER, thus having marginof 20 % to that of KEKB. Temperature rise is estimated to be 360◦ C and 290◦ C in
LER and HER. It should be noted that the tensile strength at 400◦ C is as small as 1/5
compared to that at 20◦ C.
Table 14.3: The beam profile at the extraction window

E [GeV]εx [nm]σX at abort window [mm]Vertical Sweep ∆Y [mm]I [A]I/ σX /∆Y [A/mm2 ]KEKB(LER)
3.5
24
0.68
10-15
2
0.3

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SKEKB(LER) SKEKB(HER)
4
7
3.2
4.6
1.22
1.1
12-15
12-15
3.6
2.6
0.2-0.25
0.16-0.2

Beam-abort system of HER
Overview The schematic layout of the HER abort system is shown in Figure 14.10.
The HER beam abort system consists of four horizontal kicker magnets, a verticalkicker magnet, a DC sextupole magnet, a Lambertoson septum magnet, a Ti extraction
window and a beam dump. The horizontal kicker magnets extract the beam fromvacuum chamber through the Ti window. The abort system makes the horizontal
beam size larger by the large horizontal beta function using the sextupole magnet. The
vertical kicker sweeps the beam in the window during one revolution time. In orderto enlarge the horizontal beam size, the DC sextupole magnet is installed between
horizontal kicker magnets and the extraction window. Deflected by the horizontal
kickers, the beam which passes through the sextupole oﬀ-center, is enlarged in thehorizontal size due to the quadrupole component of the oﬀ-centered sextupole. The
extracted beam is transported by the Lambertoson septum magnet to the beam dump.
Since the DC sextupole magnet degrades the dynamic aperture of the stored beam,another sextupole magnet which is connected by −I or I transformation is installed tocancel the geometric aberrations.

Table 14.4: Optics parameters of aborted beams. SI and BI mean synchrotron injection
and betatron injection, respectively.

εx /εy [nm/pm]∆X at abort window [mm]Vertical sweep at abort window [mm]σX at abort window [mm]∆Y at entrance of dump [mm]K0/magnet of H. Kicker (×number) [mrad]
K0 of V. Kicker [mrad]
K0 of Lambertson septum [mrad]
K2 of DC sextupole [/m2 ]K1/magnet of pulsed quadrupole (×number) [/m]HER (SI)
4.6/11.5
38
15
1.2
-217
0.338 (×8)
1.28
79.3
-4.29
—

HER (BI)
4.6/11.5
40
15
1.1
-217
0.332 (×8)
1.31
79.2
-4.36
—

LER (BI)
3.2/8.6
36.8
15
1.22
-129
0.66 (×2)
1.99
121.1
—
-0.036 (×2)

Optics design Two kinds of injection scheme are planed in HER[11]. One is the betatron injection (BI) and the other is the oﬀ-momentum synchrotron injection scheme(SI). The beam abort system is designed to match the both injection scheme[12]. Figure 14.11-(a) and (b) show the optics of the betatron injection scheme for the stored683

Figure 14.10: The schematic layout of the HER abort system. The horizontal axis shows the
position along the electron beam. The transverse orbits at abort system region are shown.
The stored beam passes the center of the abort sextupole. On the other hand, the aborted
beam passes oﬀ-center of the sextupole magnet.

beam and the aborted beam, respectively. The horizontal displacement of the aborted
beam is 40 mm at the extraction window. Using the sextupole magnet, the horizontal
beam size is enlarged from 0.1mm to 1.1 mm at the window. The optics parameters
for the abort system are summarized in Table 14.4.
As shown in Figure 14.11-a, the phase advance between the two sextupole magnets
is 1.5π in the horizontal and π in vertical plane respectively. There is no dispersion
at the sextupole magnet. Figures 14.12-(a) and (b) show the optics for the oﬀ-energy
synchrotron injection scheme of the stored beam and the aborted beam. Since there isa horizontal dispersion in the abort section, optics was adjusted to make a dispersion
function zero at the center of the abort sextupole magnet for the stored beam optics,
while the slop of the dispersion was left free.
Dynamic aperture of HER The influence of the sextupole pair to the dynamic
aperture is estimated. Figure 14.13 shows the dynamic apertures in the HER , where684

Figure 14.11: Optics of the betatron injection scheme for HER. The black and red lines correspond to the horizontal and vertical planes, respectively. (a) The beta functions, dispersion
functions for the stored beam around the abort system region of the HER. The beam comesfrom the right side. (b) Zoom of (a) around the abort system for the abort beam.The beam
comes from the left side.

Figure 14.12: Optics for the synchrotron injection scheme in HER. The black and red lines
correspond to the horizontal and vertical planes, respectively. (a) The beta functions, dispersion functions for the stored beam around the abort system region of the HER. The beam
comes from the right side. (b) Zoom of (a) around the abort system for the abort beam.The
beam comes from the left side.

the beta functions at the interaction point assumed to be the design value of βx,y={25, 0.30} mm. The dynamic aperture is optimized using another sextupole. As the
Touschek beam lifetime is insensitive to the sextupole strength K2, there is no adverse
eﬀect of the abort sextupoles on the dynamic aperture.

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Figure 14.13: The dynamic aperture of HER for the on-energy injection scheme is shown.
Red dots indicates (ϕx0 , ϕy0 )=(0,0) for the initial betatron phases and green dots for(ϕx0 , ϕy0 )=(π/2,π/2). The K2 values of the abort sextupole pair are, (a) zero, (b) 2.78 m−2(half of the design value) and (c) 4.53 m−2 (design value). The Touschek beam lifetimes are
estimated using smaller aperture data as (a) 610 s, (b) 600 s, and (c) 620 s, respectively.

Beam-abort system of LER
The design of abort system for the LER is almost same as that of HER. Instead of a DC
sextupole magnet, two pulsed quadrupole magnets are used to enlarge the horizontal
beam size. Because of geometrical reason, it was impossible to enlarge horizontal beam
size by the sextupole magnet with reasonable parameters. Actually magnification of thebeam size at the window by the pulsed quads is not suﬃcient. Fortunately horizontal
dispersion function at the window helps in large fraction.
Figures 14.15-(a) and (b) show the optics for the stored beam and the aborted
beam respectively. The kickers and the pulsed quadrupole magnets are located acrossthe injection point. One horizontal kicker magnet, a vertical kicker magnet, two pulsed
quadrupole magnets are used. The extraction window placed in front of the Lambertson
septum, is located in the dispersion suppressor area at the end of the Fuji straightsection. Besides the beta function modified by the pulsed quads, the non zero dispersion
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Figure 14.14: Configuration of the LER abort system.. The horizontal axis is the distancealong the positron beam. The transverse orbits at abort region are shown.

at the window, contributes to make the horizontal beam size from 0.52 mm to 1.22 mm
at the window . The parameters of the LER abort system are shown in Table 14.4.
System components Design
Since the most diﬃcult part of the system is the power supply, it was designed withcareful consideration. Other components are developed in harmony with the power
supply design. For easy maintenance, the maximum supply voltage is designed to beless than 40 kV which makes the thyratron switch operable in the air. Horizontal
physical aperture was determined by the optics requirement, and physical apertureof vertical direction was determined to match the kicker power supply. Under thiscondition, the kicker magnet current is set around 1.7 kA. In addition, a local heatingof the surface of the ceramic chamber due to wall current of the high current beam isconsidered to determine the vertical aperture.

687

Figure 14.15: Optics of the LER abort system. The black and red lines correspond to thehorizontal and vertical plane, respectively. (a) The beta functions, dispersion functions, beam
sizes, and orbits for the stored beam in the abort region of the LER. The beam comes fromthe left side. (b) The same parameters as (a) for the aborted beam.

Kicker Magnets and Pulsed Quadrupole Magnets
The kicker magnets are conventional window frame magnets made of ferrite core. Table 14.5 shows the parameter of kicker magnet. The coil of horizontal kicker magnet
excites the magnetic field in the two pieces of 350mm long ferrite core. Water cooled
ceramic chambers are inserted into the gap.
Table 14.5: Parameters of kicker and pulsed quadrupole magnets

B or B’
BL or B’L
No. of Turns (coil)Core Length (mm)Gap (mm)
No. of coils

HER-H
HER-V
23(mT)
87(mT)
63(mTm) 30(mTm)
1
3
350 x 8
350
70
90
4
1

LER-H
LER-V
25(mT) 75(mT)
18(mTm)26
1
3
350 x 2
350
70
90
1
1

LER-Quad
1.4(T/m)
0.96(T)1
350 x270 (Bore radius)2

As the ferrite material, PE 14 (TDK) was used. Its relative permeability is bigger
than 200 up to around 10 MHz, which is suﬃcient for our use. Further, PE14 canbe used up to magnetic field of 3 kGauss . The HER abort kicker located besidethe ARES cavity. To access to the ARES cavity easily for maintenance, the kicker
magnets have to be constructed as compact as possible. As shown Figure 14.16(B),688

Figure 14.16: Horizontal kicker magnetic field simulation
the return yoke is made as thinner as possible under the condition that the ferrite
is not saturated. To minimize the inductance, the single turn coil is chosen for thehorizontal kicker and the pulsed quad. On the other hand, the vertical kicker coil has
three turns. The coil of the quadrupole magnet is molded in one piece. In order toinsert the ceramic chamber into the coil, one of flange is made removable. Flux line ofhorizontal kicker magnet is shown in Figure 14.16(C). The magnetic field uniformity
is shown as a function of horizontal position in Figure 14.16(A). To improve the field
uniformity, the height of the electrode in the core is designed as wide as possible toprevent the horizontal field component penetrating into the beam area. Figure 14.17
shows the configuration of HER kickers in the tunnel. The saturable inductor circuit
is placed beneath the magnets. In order to protect semiconductors used in the circuitfrom radiation damage, the coil electrode is extended vertically and connected to thecircuit at 500 mm below the beam line. Semiconductors are placed at further 200 mm
lower in the tank. Two parallel wide copper-electrode were molded together in a coil.
689

Figure 14.17: HER kicker magnet configuration at the beam line.
It minimizes the coil inductance. In order to prevent from traveling discharge on thecoil surface, unevenness was introduced on the mold surface to make discharge pathlonger.
Power supplies
Power supply design There are two requirements for the power supply. One is avery fast rise time that is less than 200 nsec. The other is to keep current constant intime longer than one revolution time. In order to obtain a fast rise time, a saturable
inductor switch was adopted and in order to feed the constant current, a power crowbar circuit was combined. Figure 14.18 shows a conceptual circuit diagram. During
the first 200 nsec, the current flows through the saturable inductor circuit. At the
beginning, the inductance is high and the current virtually does not flow until the
integrated current exceeds a threshold where the core of transformer saturate, thereby
the inductance decreases abruptly and the current starts to flow rapidly. Because thechange of inductance is based on the characteristic of the core material, the switching
speed is so high that rise time is determined only by the external circuit. On the other
hand, the current of power crowbar circuit rises slowly. It reaches its maximum value
when the current of the saturable inductor becomes zero, and keeps the current morethan 10µsec. Figure 14.19 shows output current of the power supply, illustrating the690

transition of the two circuits. The current waveform of the vertical kicker is half sinewith its maximum at about 15µ sec, which makes the vertical beam position at thewindow move uniformly during one revolution time. The charger and thyratron housing are located at the klystron gallery on the ground. The main capacitor, saturable
inductor and power crowbar diodes are placed underneath the magnet. Since same
voltage of the common high voltage power supply is applied to each horizontal andvertical kicker’s main capacitor, relative strength between them can be adjusted byadding the extra resister series to the vertical kicker magnet. The applied voltage to

Figure 14.18: Power supply configuration. (HER)
the switching thyratron is at most 40 kV, so that it can be operated in the air. Howeverat the main capacitance and the saturable inductor circuit, the voltage becomes about
twice of the voltage at the thyratron, and they are housed in insulating oil tank placed
beneath the magnet. Since it is undesirable to stock inflammable material inside thetunnel, silicone oil was used as insulating oil. Furthermore, the conservator is designed
to be closed system. The thermal expansion of silicone oil is absorbed by attaching abellows to the conservator. In order to facilitate access to the surrounding equipment,691

Figure 14.19: Output current in the horizontal kicker power supply. (A) Current component
diagramRed part is the current from saturable inductance circuit and blue is the current frompower crober circuit. (B) Output current f the horizontal kicker power supply. (C) First 1µsecof (B).

the conservator is placed just above the magnet. In designing the power supply, oneof the important concept is modularization. If the power supply breaks down, it must
be recovered promptly. Especially high voltages handling parts must be modularizedfor quick replacement with spare module in case of failure. Since the main capacitanceand the saturable inductor are implemented in an oil tank, it is modularized and canbe replaced easily. In addition, the parameters of the kicker magnet system are chosen
the spare parts to be shared in HER and LER. In order to reduce the noise from kicker
system, the ground is separated from other DC magnets. The thyratron unit and themain capacitor are connected with a double-shield coaxial cable, and the outer shieldis connected to the ground on one end. Trigger signal and control network are sent
through optical fibers.
Damage of accelerator components caused by the power supply failure
Damage of the accelerator components caused by the failure of the abort kicker power
supply is investigated and the results is summarized in Table 14.6. All magnets areoperated by a common switching thyratron to prevent either kicker magnet from improper firing. When the thyratron has a trouble, none of them are fired, and the abort
does not occur. In this case the beam is sprayed in the whole ring by switching oﬀ
the RF cavities or by turning oﬀ one of the weak bend magnets. It is ignited by theinterlock system to prevent the damage of the vacuum chamber.
If the vertical kicker does not sweep because of the capacitor breakdown, the beamwould hit a single point at the extraction window inducing a horrible damage. To avoid
the damage, the main capacitance and the transformer of the vertical kicker circuit are
divided into two identical parts, and connected to the kicker coil in parallel. The dual
circuit system makes it possible to perform the vertical sweep even if either capacitance692

Table 14.6: Abort system failure possibility
Failure events
Thyratron break down
Gate circuit mulfunction
Thyratron misfire
Gate circuit misfire
H-kicker cap break down (charging)V-kicker cap break down (charging)
Charger fault
H-kicker bias circuit fault
H-kicker power crober fault
V-kicker cap break down (discharge)
H-kicker cap break down (discharge)

What happen in the system
2% improper beam loss
2% improper beam loss
Oﬀ one bending magnet
Oﬀ one bending magnet
Oﬀ one bending magnet
Oﬀ one bending magnet
Oﬀ one bending magnet
Oﬀ one bending magnet
Oﬀ one bending magnet
Small vertical sweep
Peak current low

Damage
No
No
No
No
No
No
No
No
No
No
Damage ?

is short-circuited. The damage caused by the horizontal kicker trouble is investigated.
In the case of LER, because there is only one horizontal kicker, the beam is not kicked
out, and the beam is lost while sweeping in the vertical direction. The beam does not
hit stationaly any single point of the vacuum chamber. In the case of HER, even if
one of the kicker magnet circuit fails, except for the one at the most upstream, still
the beam passes through the Ti extraction window. If the kicker magnet on the most
upstream side of HER fails, there is a possibility giving the damage to the junction
point of Ti window chamber and nearby vacuum chamber. To cover this case, strengthof the most upstream kicker is adjusted to be weaker than other kickers.
Radiation protection The nuclear reaction that generates neutrons in SuperKEKB
is mainly photo-nuclear reaction. It is close to fission neutron at Eth: 10-19 MeV (light),
4-6 MeV (Heavy). When considering shielding from the neutron in above energy range,
the reference can be made to the neutron data of the spontaneous fission source of 252 Cf.
The average neutron energy of 252 Cf is about 2 MeV. As shown in Figure 14.20, theneutron flux can be reduced to 1/20 with 20 cm of polyethylene. About the γ rayshield, the critical energy in SuperKEKB is about 5-10 KeV. The γ ray from 99m TC is142.7 keV which is shown in Figure 14.20. So it is better shielded than 99m TC . The
sandwich of lead plate and polyethylene (lead 7 mm + polyethylene 20 cm + lead 1
mm ) makes that γ ray to be on the order of 10−6 and neutron rays to be 1/20. γ ray
generated by neutron in polyethylene shield is stopped with 1mm lead plate.
693

Figure 14.20: Radiation shielding eﬀect [13].
Monitor for the beam abort system
Three kinds of beam abort monitoring system is prepared.
• A screen monitor is installed in front of the beam dump. It is used usuallyat start-up of the accelerator operation after long shut-down. The abort kicker
trigger timing is adjusted with the screen monitor.
• The output current of the kicker power supply is observed with an oscilloscope
together with abort trigger signal. The monitoring plays a role as watch-dog of
the abort kicker power supply.
• A beam position monitor is installed in front of the beam dump[14]. The signal
from the BPM is sent to the oscilloscope, and the position of each bunch ismeasured. Even the abort kicker works properly, the beam loss may occur inthe ring before the beam abort. The abort BPM logs the situation of extracted
beam all the time, and see if the storage beam is extracted properly to the beam
dump. The Figure 14.21 shows the typical aborted position of the bunches at
the KEKB abort BPM. If the plots are almost same as the reference, the beam
has been extracted safely.

694

Figure 14.21: (a) The horizontal axis shows the time from the abort trigger. The verticalaxes show the horizontal/vertical bunch position, and the bunch current. (b) Two-dimensional
position plots are shown. The origin is the center of the BPM chamber. The gray points show
the reference plots aborted manually.

Ceramic chamber
Ceramic chambers are used for the pulsed magnets in the abort system.
They enable time-varying external magnetic-field to penetrate the vacuum chamber.
A thin metallic coating is necessary to carry the beam image current and also toprotect outside components from the beam field. The power loss in the ceramic tube
is produced by two types of induced currents. One is the image currents of the beamand the other is the eddy currents induced by the kicker’s magnetic field. In case ofceramic chambers used in SuperKEKB, the power dissipation due to the beam image
current is dominant. Since the design bunch length is almost same as that of KEKB,
the power dissipation at the ceramic chambers can be estimated using the data in
KEKB operation. The power dissipation is proportional to the number of bunches andthe bunch current squared. For the SuperKEKB design current (HER 2.6A and LER
3.6A), the power loss of one ceramic tube is estimated to be 2.7kW in LER and 1.8kW
in HER. Thus the water-cooled ceramic chambers are inevitable. Assuming water flowrate of 3 ℓ/min, the temperature rise of the ceramic tube is estimated to be 8◦ C in
HER, and 14◦ C in LER. An independent cooling water system for the ceramic chamber

695

Figure 14.22: (A) Simulation result of the field distribution of the horizontal kicker along
the beam. (B) Ratio of the integrated field to the total integrated field in whole region asfunction of z.

to the facility cooling water system directory is adopted, because of possible damages
to the ceramic due to the water hammer that occurs when the valve is forced suddenly
to close. The requirements for HER and LER ceramic chambers are slightly diﬀerent.
Since the beam energy is higher in HER, more kicker magnets are needed, so it isnecessary to shorten the overall length. On the other hand, the design beam current isbigger in LER, the power loss in LER ceramic chamber is 50% bigger than that of HER.
The better cooling ability is required in LER ceramic chamber. Parameters of ceramicchamber used in SuperKEKB are summarized in Table 14.7. The ceramic chamberis racetrack shape in its inner side with diameter of 60mm and 40mm in horizontal
and vertical respectively. Alumina ceramic was chosen as chamber material because ofits mechanical strength and good metalizing characteristic for braze. The dimensional
accuracy was required to the inner wall of ceramic. It was ground so that the flatnessof inner wall surface is less than 0.1 mm in 10 mm square area. In order to match thelevel at the junction of ceramic and sleeve metal less than ±0.2 mm diﬀerence, taper
structure was chosen at the end. Figure 14.22(a) shows a simulation result of verticalmagnetic field By as a function of longitudinal position Z where length of the ferrite
core is assumed to be 350 mm. Figure 14.22(b) shows a ratio of eﬀective magnetic field
as a function of Z. Ceramic tube with a length of 500 mm covers 95% of the total field.
The ceramic chamber was developed to be more compact than KEKB. The gap ofhorizontal kicker magnet was reduced from 90mm (KEKB) to 70mm (SuperKEKB). It
696

reduces the power of the power supply by 40%. Mo-Mn is a typical braze material to theceramic. However it is weaker against water, because plated NI which is indispensablefor Mn-Mo metallization, dissolves into water. In stead of Mn-Mo metallization, Ti
with activated metallize method has been adopted. This technique has been chosen in
KEKB, and no troubles has been seen during KEKB operation.

Figure 14.23: Field penetration of the kicker magnet.
Penetration of the kicker magnetic field was investigated with the test bench. The
rise time of horizontal the kicker magnetic field is measured by integrating a signal
from a pickup coil inserted in the magnet gap. Figure 14.23 shows power supply
output current as well as magnetic field with/without Ti-coated ceramic chamber.
The thickness of Ti coating is set to 6 µm. Observed rise time is less than 200 nsec
that satisfy the requirement.
Ceramic chamber for HER Two types of ceramic chambers were developed for
the HER beam abort system[15]. One is used for the vertical kicker magnet and theother is used for horizontal kicker magnets. The length of the vertical kicker chamber
697

Table 14.7: Parameters of ceramic chambers

Power Loss/ceramic [kW]Ti coating [µm]Cu coating [µm]
inner dimensions [mm]Height/Width [mm]Braze
Braze Metallization
Ceramic material
No. of ceramic tubes
Ceramic length [mm]Chamber Length [mm]Structure
Temperature rise [◦ C]

HER-H
HER-V
1.8
1.8
5
5
100
100
60x40
60x40
67.5x87.5 67.5x87.5
Ag-Cu
Ag-Cu
Ti
Ti
Alumina Alumina
2x4
1
500x8
500
1128x4
580
Straw
Straw
8
8

LER-H
LER-V
LER-Q
2.7
2.7
2.7
6
6
6
All CuAll CuAll Cu
60x40
60x40
60x40
88x68
88x68
88x68
Ag-Cu
Ag-Cu
Ag-Cu
Ti
Ti
Ti
Alumina
Alumina
Alumina
2x1
1
1x2
500x2
500
500x2
1230
634
634x2
Double tube Double tube Double tube
14
14
14

is 580 mm. The chamber has 500 mm long ceramic tube with a thin Ti conduction
layer deposited on the inner wall. The flange made of Cu has been chosen to minimize
heating due to image beam current. The ceramic is connected to the Cu flange througha sleeve made of kovar. Kovar was chosen as metal brazes ring. It provides low thermal
stress hermetic seal, because kovar has almost same coeﬃcient of thermal expansion as
alumina ceramic. And it also provides flexible transition between ceramic and massive
flange. Kovar for vacuum seal and that of cooling water were brazed separately, becauseeven one of them had damage, it won t give any damage to the other braze. The
electron beam welding is used to connect Cu flange and kovar sleeve to avoid annealingof Cu flange. In order to suppress heating due to image current on the kovar, which
has high permeability, copper electroforming is applied to deposit 100 µm thick Culayer on the inner wall of Kovar sleeve.
The ceramic chamber for the horizontal kicker magnet comprises two pieces ofceramic tubes. Two ceramic tubes are combined with kovar sleeve, making a single
chamber with 1128.3 mm. Each ceramic tube has cooling water path. The waterflows into the ceramic tube at upper and lower inlet in one end and gets out from theother end. The ceramic chamber made from one piece of ceramic so that it has simple
structure and relatively easy to assemble. Figure 14.24 shows copper electroforming
inner wall of kovar sleeve and cut model of ceramic tube. The simple structure makes
the ceramic chamber shorter.
698

Figure 14.24: The ceramic chamber used in HER abort system. (A) shows kovar sleeve with
100 µm thick Cu conducting layer at the inner wall. (B) is cut model of the ceramic tube
which include cooling water path in the ceramic tube.

The Ti metallization of ceramic inner wall was produced by DC magnetron sputtering. Argon gas was chosen to make the plasma between the cathode of Ti rod andthe anode which is a stainless-steel screen. Coating uniformity has been checked withthickness meter utilizing eddy-current. Target value of Ti coating thickness was set
to 5 µm. Coating is relatively thick in the middle of ceramic tube and thin near theend. (Figure 14.25 ) When the target value is set to 5µm, minimum 1µm Ti coating
is guaranteed.
Ceramic chamber for LER In a similar way of HER, two types of ceramic chambers
are developed. The pulsed quadrupole magnets and the vertical kicker use same typeof ceramic chamber. The diﬀerence is only that the chamber of pulsed quadrupole
magnet has a removal flange on one end. The length of ceramic chamber is 634 mm
with 500 mm long ceramic tube having a thin Ti conduction layer deposited on the
inner wall. Because the LER design current is larger than HER, the Cu sleeve, insteadof kovar, has been adopted to connect to the Cu flange. Figure 14.26 shows Cu sleeveconnected directory to the ceramic tube. Cu sleeve is connected to the Cu flange by
the electron beam welding.
As same as HER, the ceramic chamber for horizontal kicker magnet is made of two
ceramic tubes. They are combined via Cu sleeve, and total length of the chamber is
1230mm which is longer than HER. LER ceramic chamber has a double tube structure.
Each ceramic tube is 5mm thick and secured adequate mechanical strength. Cooling699

Figure 14.25: Ti coating thickness distribution in the inner wall of the ceramic along thebeam direction.

water flows between inner and outer ceramic tubes. Since the total surface of the innertube is cooled by water, it has a good cooling ability. As same as HER ceramic tube,
the cooling water enters from the upper and lower tubes of one side and exits from theother side. Compared with HER ceramic chamber, the cooling ability is better, but
the structure is complicated and total length became longer.
The extraction window and other vacuum chambers
The size of the abort window was designed so that the beam can pass through it safely
full filling requirements of a 3.5σ beam size, an energy deviation of ±1%, and a coherent
oscillation simultaneously. The maximum vertical coherent oscillation is assumed to beabout 50σy which is maximum acceptance limited by the vertical collimators to protect
the Belle II detector. As for the horizontal coherent oscillation, we assume 5.6σx at
maximum in the case of HER-BI and the beam abort is planed to be triggered when theoscillation amplitude exceeds this value. The designed cross section of the chambers for
the HER and the LER are shown in Figure 14.27 and Figure 14.28, respectively. With
the above considerations, the closest positions of the aborted beam to the inner surfaceof the chamber between the kickers and the Lambertson, are shown in Figure 14.27(a)and 14.28(a). The all chambers are designed to pass the aborted beam safely.
At the window, the aborted beam is confined within the region shown with the redrectangle in Figure 14.27(b) and 14.28(b). As shown in Figure 14.11(b) and 14.12(b),the horizontal orbit deviation of the aborted beam for the HER becomes maximum at

700

Figure 14.26: LER ceramic chamber with Cu sleeve connected directly to the ceramic tube.
the “Abort sextupole (1)” in the beam pipe. The horizontal and vertical sizes of thisregion are summarized in Table 14.8. Note that the vertical beam size of the aborted
beam is larger than that of the stored beam due to the vertical dispersion created by
the vertical kicker. We consider the tail of the aborted bunch train. The beam abort
window was designed so as to include the red rectangle region. These beam pipes musthave a larger aperture than the abort window.

701

Figure 14.27: HER: (a) The cross section of the inner chamber of quadrupoles QX6RE
and QX7RE which locate between the abort kickers and the entrance of Lambertson. The
dots show the positions where the beam are closest to the chamber walls with the coherent
oscillation. (b) The cross section of the abort window. The red rectangle shows the region
where the aborted beam passes.

Table 14.8: The inner size of the Abort chamber which are installed in the Lambertson
septum. The sizes full fills requirements of a 3.5σ beam size, an energy deviation of±1%, and a coherent oscillation simultaneously.

Horizontal size
Vertical size

HER (SI)
5.7σx
47σy

702

HER (BI)
5.6σx
51σy

LER (BI)
3.2σx
121σy

Figure 14.28: LER: (a) The cross section of the chambers for the inner chamber ofquadrupoles QS1FLP (dotted line), and QSBFLP and the entrance of Lambertson (solidlines), which locate between the abort kickers and the Lambertson. The points show thepositions where the beam are closest to the chamber walls with the coherent oscillation. (b)The cross section of the abort window. The red rectangle shows the region where the aborted
beam occupies.

703

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704

[14] N. Iida et al., "A Position Monitor for the Aborted Beam in KEKB", EPAC2008,
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