BEAM GATE CONTROL SYSTEM FOR SUPERKEKB
H. Kaji∗ , Y. Ohnishi, S. Sasaki, M. Satoh, H. Sugimura, KEK, Japan
Y. Iitsuka, East Japan Institute of Technology Co., Ltd, JapanT. Kudou, Mitsubishi Electric System & Service Co., Ltd, Japan
Abstract
SuperKEKB is an electron-positron collider at KEK. Its
phase-1 operation is successfully carried out in 2016. We
develop the new Beam Gate control system for the ongoing phase-2 operation. The synchronization of injection
hardware control is improved. In addition, the “DelayedSynchronization” function is installed to operate the newly
constructed damping ring. The new system accepts the control from Bucket Selection. We proceed the phase-2 commissioning of SuperKEKB with the sophisticated Beam Gatecontrol system.

Figure 1: Example of beam operation: the beam currents atLER and HER are plotted. The high beam current at LER
is kept by iterating the open and close of Beam Gate.

INTRODUCTION
SuperKEKB [1,2] is an electron-positron collider with thecenter-of-mass energy of 10.58 GeV. It consists of the injector linac (LINAC) [3], damping ring (DR) for positrons, andtwo main rings (MRs). The MRs for electrons and positronsare called High Energy Ring (HER) and Low Energy Ring(LER), respectively.
The phase-1 operation of SuperKEKB was successfully
carried out in 2016 [4, 5]. The beam doses at LER andHER are 780 Ah and 660 Ah, respectively. The vacuum
scrubbing of beam pipes was smoothly proceeded. The
beam commissioning was implemented.
The phase-2 operation is started from March 2018. The
positron-pulses generated at LINAC are once stored into DR
to suppress the beam emittance and injected into LER inphase-2 while they were directory injected in phase-1. We
successfully realized this injection scheme. Further commissioning of beams and accelerators is ongoing. We observed
the first collision event on April 26th.
The Beam Gate system is one of important items for thecommissioning of SuperKEKB. Figure 1 is an example ofbeam operation. The Beam Gate quickly switches “beamdelivery” and “pause” of beam injection system. During theaccelerator studies, we iterate above process with changing
lots of machine parameters. So the quick switching makes
the studies efficient. Besides, SuperKEKB has an automatic
process to keep beam current at MRs. The software process monitors beam currents of two MRs. It opens (closes)
Beam Gate when the MR beam current becomes low-limit(high-limit). It helps vacuum scrubbing in the machine commissioning.
The upgrade of Beam Gate is required for the phase-2 operation. More precise synchronization of injection hardware
is required. The delayed synchronization to control injectionand extraction components at DR is necessary.
∗ hiroshi.kaji@kek.jp

REQUIREMENTS TO BEAM GATE
We prepare the Beam Gate system for LER and HER
separately. They control (enable or disable) trigger delivery
to the injection hardware, such as electron gun, septum andkicker magnets. The operation of kicker magnets slightly
affect to the stored beams. Therefore we should stop thetrigger deliveries when we do not perform the injection toMRs. It is important for the septum and kicker magnets toavoid unnecessary operation also in terms of their lifetime.
Since the Beam Gate in the phase-1 operation inaccurately
synchronize the controls of injection hardware, it caused alot of accidental beam abort at the early stage of operation.
The trigger delivery to the MR-injection septum is stopped
much earlier than that to electron gun when we closed Beam
Gate. More than ten beam-pulses hit the septum wall in theabsence of bending field. Then the beam-loss monitor atMR requested the beam abort. The precise synchronizationof injection hardware control is required to the new Beam
Gate system.
The “Delayed Synchronization” function described thelater section is necessary to control DR components together
with LER components.

EVENT TIMING SYSTEM
We leave the basic functions of Event Timing System tosomewhere Ref [6]. In this section we describe the module
configuration at SuperKEKB and its important function,Distributed Bus Bit, for the Beam Gate system

Configuration at SuperKEKB
The basic configuration of Event Timing System is as follows: the master module is configured with Event Generator(EVG) and is connected with more than one slave modules,
Event Receivers (EVRs) via dedicated optical network.

We mainly utilize the EVG and EVR modules developedby MRF [6]. Those developed at SINAP are partially install at MR. They configure sub-station to provide injection
triggers to MR hardware.
The master EPICS IOC [7, 8] for Event Timing System
consists of three EVGs and one EVR. They are connected
to form three-layers. The two EVGs installed in the lowest
layer work for controlling the 1st half and 2nd half of LINAC,respectively. The detailed specification of master IOC is
described elsewhere [9–11].

Distributed Bus Bit
We utilize the Distributed Bus Bit (Dbus) function ofEvent Timing System for delivering Beam Gate information
towards individual injection hardware. The Dbus systemmonitors the input TTL level signals at EVG and delivers
them toward all downstream EVRs. The information isdelivered via Event network without disturbing machine
operation with the Event delivery.
In more detail, the Dsub accepts 8 TTL inputs. They areconverted into one byte data packet and transfered toward
EVRs. Therefore we can deliver 8 different level signals
simultaneously via the single optical cable. The input sampling rate is one or half of Event clock, it depends on operation parameters of EVG. It is 57 MHz in case of SuperKEKB.
By upgrading the device/driver of MRF modules [8], thetransfered information can be used in two ways at individualEVRs. The status of individual Dbus bits can be outputtedas the NIM or TTL level signals. They can be monitored
also via the EPICS PV.

INJECTION CONTROL
General Specification
The Beam Gate system receives inputs from the individual
MRs. The electron gun at LINAC, injection and extraction
hardware at DR, and injection hardware at two MRs arecontrolled with these inputs.
Totally 8 kinds of control signals are provided with Beam
Gate main software. They are summarized in Table 1. The
LER Beam Gate input controls the LER and DR components
while the HER input controls the only HER components.
Figure 2 is the schematic view of Beam Gate control system at main Event IOC. The control signals of electron gun
are outputted from EVG module and delivered to the nearby
circuit which generates triggers to electron gun. All othercontrol signals are newly developed for the SuperKEKB
phase-2 operation. They are delivered with Dbus toward
remote IOCs. As written in the previous section, the Dbus
utilizes the Event network which has already been configured and Therefore we increase the control signal withoutcabling new optical lines.
The electron gun at LINAC is controlled with both LERand HER gun control signals. The gun circuit receives twocontrol signals. However it accepts only LER (HER) control

Table 1: Summary of Beam Gate Signals: note, the LER
Beam Gate controls LER and DR components while theHER Beam Gate controls the only HER components. Theinjection kicker and septum for HER (LER) are controlledwith only one signal labeled ”Injection Hardware”.
Components

Latency (ms)Dbus

LER GunDR Injection Kicker
DR Injection Septum
DR Extraction Kicker
DR Extraction Septum
LER Injection Hardware

20020
Tstr
Tstr + 20
Tstr + 20

None
EVG1 bit4
EVG1 bit5
EVG2 bit4
EVG2 bit5
EVG2 bit1HER Gun
HER Injection Hardware

2020None
EVG2 bit3Figure 2: Schematic view of Beam Gate circuit: the output
for electron gun is provided as the level signal while others
are provided as the Dbus.
signal when LINAC performs LER (HER) injection.1 The
by-pass lines connect between input and output for electron
gun works as the “immediate close”. For example, when theMR is aborted, the Beam Gate is closed the abort trigger
system. The “immediate close” function is able such kindof cases.
The other injection hardware is controlled by the EVR
modules. The EVRs provide injection triggers to the injection hardware, such as the septum and kicker magnets. They
switch the enabling and disabling of trigger output with theDbus information when they receive the Event interruption.
The core part of Beam Gate system (labeled as “EVGs”
in Figure 2) is software process. We newly develop main
software of Beam Gate. It receives the Beam Gate control
signal and provide the 8 kinds of output control signals which
are listed in Table 1.
Even though the individual injection components are controlled by different EVRs, we can precisely synchronize
their switching processes since the main software controls
the Dbus information.
The main software intendedly makes latency between input and output. We can set different latencies listed in Table 1
to the individual outputs so that the delayed synchronizationof Beam Gate is realized. The details of delayed synchro1 The gun circuit is developed in the former KEKB project. The Beam

Gate signal is directory inputted into this circuit in that period. The same
control is implemented also for the light source ring on this circuit.

nization and main software itself are described in following
subsections.

Delayed Synchronization
In principle, the trigger deliveries to all injection hardware
are controlled synchronously. The new Beam Gate system
precisely synchronize the EVR processes with main softwareand Dbus.
There is another important feature on the Beam Gate mainsoftware. It works when we control the LER Beam Gate.
The positron pulse is once stored at DR to damp the beamemittance. The DR storage time and entire period of LER
injection becomes longer than one LINAC operation periodof 20 ms. In this case the trigger delivery is controlled fromupstream to downstream. Therefore the delayed synchronization is implemented with the arbitrary latency of individualoutputs.
The delay of the DR-extraction and LER-injection fromthe DR-injection depends on the positron storage time atDR, Tstr . The latency of output signals for DR-extraction
and LER-injection are Tstr ms longer than the DR-injection
components. The Tstr is defined as follows:
Tstr = min(TM AX , 1000 · Npulse / fin j ),(1)

where the fin j and Npulse are injection frequency of LER
and the operation pulse of DR. The TM AX defines the maximum storage time. It is decided to be 200 ms owing to thespecification of trigger circuit. Therefore, for example, theTstr becomes 40 ms when we perform LER injections withfin j = 25 Hz and Npulse = 1. Note, the Tstr value is definedin 20 ms step.
The delayed synchronization is important. For example,
when we close LER Beam Gate, if we stop trigger delivery
to the DR-extraction components earlier than schedule, thepositron pulse remains at DR. On the other hand, if we stop
the delivery later than schedule, they are fired unnecessary.
It makes their lifetime shorter.

Main Software
The main software is configured on the EPICS process.
Figure 3 is the flow chart of Beam Gate core process. We develop the new software record named “bgate”. Two software
PVs are provided for LER and HER, separately.
The bgate has one input field and eight output fields. TheBeam Gate status is once converted to the EPICS PV which
is linked with the input field of bgate. It is transfered intooutput field after waiting the latency defined as follows.
The bgate record has an internal counter. The counter isincremented when the PV is processed and initialized when
the input status is changed. The PV is processed in 50 Hzby the interruption of Event Timing System.
The latency listed in Table 1 is set to the delay field inunit of this counter. For example, we set “1” to the delay
field when the latency is 20 ms. Then the bgate transfers theinput status to the output when the internal counter equals
to the value of delay field.

Figure 3: Flow chart of Beam Gate core process on EPICS
IOC: it consists of bgate PVs, reflective memory PVs, andcalcout PVs configured as “AND” logic.

The Event Timing System launches interruptions precisely
between injections of LINAC2 . Therefore the delayed synchronization of Beam Gate control is realized precisely eventhough it is the software process.

Input from Bucket Selection
As seen in Figure 3, We put the “AND” logic PV which
receive inputs from the bgate PV and the reflective memory
PV (BS PV). The Dbus PV for DR and LER components
are controlled with this “AND” PV.
By using this “AND” logic we can control injection hardware also from Bucket Selection. For example, during DR
commissioning, we can store beam pulse at DR when theBS1 and BS2 PVs are “HIGH” and all others are “LOW”.
We can throw beam from DR with the opposite condition.
Bucket Selection manage such kind of control by using
the reflective memory PVs.

CONCLUSION
We develop the new Beam Gate system for the phase-2
operation of SuperKEKB. The controls of trigger delivery
to individual injection hardware are precisely synchronizedwith the new system. Besides, the “Delayed Synchronization”
function is installed for the LER injection which includes theoperation of newly constructed DR. The injection hardware
can be controlled also with Bucket Selection. It makes thebeam commissioning efficient.
The DR is commissioned successfully in February 2018,
before the phase-2 operation. The phase-2 operation is ongoing with the sophisticated Beam Gate system.

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2 The feasibility and robustness of Event interruption system has alreadydemonstrated in the long term operation. LINAC switches operation
parameters, pulse by pulse, in its usual operation. It realizes the simultaneous top-up filling into more than one acclelerator rings.

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[6] MRF website:
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http://www.mrf.fi/index.php/[7] EPICS website: http://www.aps.anl.gov/epics/