BEAM DIAGNOSTICS AND TIMING MONITORING FOR SUPERKEKBINJECTOR LINAC
F. Miyahara#, K. Furukawa, R. Ichimiya, N. Iida, M. Ikeno, H. Kaji, M. Satoh, M. Shoji,
T. Suwada, M. Tanaka, Y. Yano, KEK, Tsukuba, Ibaraki, JapanT. Okazaki, EJIT, Tsuchiura, Ibaraki, Japan
Abstract+-The KEK e /e linac has multiple operation modes forthe electron beam injection into three rings, theSuperKEKB HER (High Energy Ring), PF (PhotonFactory) Ring and PF-AR, and the positron beam
injection into the damping ring (DR) and the SuperKEKBLER (Low Energy Ring). The operation modes can beswitched every 20 millisecond, repetition rate of 50 Hz,
with arbitrary order. The beam parameters such as
charges, energies are different for each of the rings.
Moreover, the bunch charges of the electron beam for
HER, 5nC, is 5 times higher and the transverse emittanceof ~10 mm mrad is 30 times lower than those of theKEKB injector.
Thus, the development for the BPM readout systemwith a wide dynamic range, the installation of opticalfiber detectors with a good S/N ratio for the wire scanners
and bunch-length monitor have been performed. For
stable operation of the linac, many timing signals have tobe monitored as well. To that end we have developed 32bit multi-hit time-to-digital converters (TDCs) with 1-nsresolution. The first beam tests of those systems arereported.

DC solenoids have been installed in sector 1 [4,5]. In caseof beam injection to PF or HER, the electron beam goes
through the hole of the target. To obtain positron beam,
an electron/positron separator chicane has been installedat downstream of the last LAS accelerating structure. Toreducethegeometricemittance
downto41(horizontal)/2(vertical) nm, the positron beam injectedto the damping [6] is stored 40 ms, the store time depends
on the injection rate, then return to the linac. Thus, trigger
timing of upstream (sector A – 2) and downstream (sector3 - 5) for the RF system and the BPMs will be controlled
independently [7].

COMPONENTS OF THE LINAC
The SuperKEKB injector linac is under development toachieve the required performance [1]. The layout of thelinac with energy evolution from downstream of the 180degree arc for each operation modes is shown in Fig. 1.
The linac consists of eight sectors (A – C, 1 – 5). Eachsector has eight accelerating units except for sector A, 1,and 4. The black square represents an accelerating unit
which provides an average energy gain of 160 MeV. In
sector A, a new photocathode RF gun designed to obtainhigh charge (5 nC) and low emittance (10 mm mrad)electron beam [2-3] is set on beginning of the sector. The
linac also has a conventional thermionic gun which isdedicated to PF/AR in sector 3. Thus, electron source for
PR/AR rings can be exchange. A magnetic chicane togive an energy chirp for bunch compression in the arc
section and a diagnostic beamline are located on
downstream of the RF gun.
A new positron source which consists of a tungsten
target with a 2-mm diameter beamline-centered hole forelectron, a beam spoiler, a flux-concentrator and six
large-aperture S-band (LAS) accelerating structures inside___________________________________________#fusashi.miyahara@kek.jp

Figure 1: Schematic layout of the linac and energyevolution patterns for each operation mode.

MONITORING SYSTEM
The beam positions and charges are monitored bystripline BPM. There are more than 90 BPMs and 23
oscilloscopes for fast BPM data acquisition (DAQ)
system [8]. The position resolution of the BPM system isabout 50 µm. To achieve emittance preservation of highbunch charges of 5 nC electron beam, the alignment errorof 0.1 mm for each section and a dedicated orbit
correction are required. These specifications require lessthan 10 µm position resolution for the BPM, but current
DAQ system does not satisfy it. Then, we have beendeveloped a VME-based readout system. The position
resolution of 3 µm was achieved that was measured on
the basis of 3-BPM method in the linac [9]. To measure
the beam size and emittance, Al2O3 ceramic screen(30 µm thick) is set in the vacuum chamber at the chicane
downstream of the gun, 4 sets of four successive wire

scanners are installed in sector B, C, 2, 5 [10]. The
temporal laser profiles for the RF gun and electron bunch
length are monitored by using streak camera. In order toconfirm bunch compression at the arc section and orbit
correction, an X-band RF deflector will be set at the endof the linac.

OPTICAL FIBER LOSS MONITOR
The optical fiber is already used as a beamloss or/and a
profile monitor in TTF1 [11], FLASH [12] and many
facilities. In the KEK linac, the fiber loss monitors havebeen installed around a pulsed bending magnet for PF
ring and the end of sector 5. So far, photomultiplier tube
without any scintillator, the glass in front of thephotocathode works as dielectric medium for Cherenkov
radiation, has been used as the detector for the wire
scanner. But, the way is not suited to the measurement atthe location with heavy beamloss.

Beam Loss Monitor at the End of the Linac

Figure 3: Signals from a PMT without any scintillator(top) and a PMT coupled to the optical fiber (bottom) vswire position. The left, center and right peaks correspondto vertical, oblique and horizontal beam profiles. These
scales of the vertical and horizontal should be divided
by √2.

quadrupoles is much closer to beam axis and recoil
electrons which lose energies are over focus or defocus inthe quadrupole.

Loss Monitor in Downstream of the Positron
SourceFigure 2: Schematic Layout of the linac end (sector 5).
The schematic layout of the linac end is shown in
Fig. 2. The optical fiber is set along the beamline. The
fiber passes through the pulsed bending magnet, steering
magnets (BX, BY) and quadrupoles (QF, QD). It wasrevealed that the electron beam tail hits the vacuum
chamber installed in the pulsed bending magnet fromanalysis of the signal of the fiber. The beam sizes
measured by a wire scanner are shown in Fig. 3. The
signals measured by the PMT without fiber is deteriorated
due to the background of heavy beamloss. On the otherhand, the data measured by the fiber shows clear beamprofile. That good S/N ratio has been achieved by to set
appropriate ADC gate timing that corresponds toquadrupoles. It appears that the fiber passed through in

The target system for positron production was changed
that electron beams go through a small hole with 2.0 mm
diameter on axis, while the beam had been bypassed thetarget in the KEKB injector. Although the radius of thehole is designed so that the electron beam passes throughwithout any beamloss, the target could cause beamloss
due to any position jitter, beam halo or other reasons.
The construction of positron source was started in 2013
and wire scanner system in sector 2 had been installed.
Then, initial positron beam commissioning has started.
To find the good S/N position for wire scanner, theoptical wire was set along the beamline (Fig. 4). The
background signal originating from beamloss at the target
is shown in Fig. 5. Although, the primary electron beam
passed through the target hole without any clear charge
losses monitored by BPMs, six strong signals thatcorresponds quadrupoles were observed. The bottom inFig. 5 presents signals with insertion of the wire scanner.
The growth of 5th and 6th signal indicates electron beam

Figure 4: Layout of the beamloss monitor at sector-2 end which located ~100 m-downstream of the positron source.

scattering in the wire. In case of two-bunch operation,
those peaks are overlap other. Thus, the optical fiber
dedicated to beamsize measurement using wire scannerhas been set in last quadrupoles (QF_28_4, QD_28_4 inFig. 4). The beam profile was measured, neverthelesshigh background as shown in Fig. 6.

TIMING MONITORING SYSTEM
The trigger timing system of the linac is based on Event
Timing System on VME [13]. The timing system
distributes trigger timings and parameters for the linac
components, such as RF phases, currents of magnets, etc.,
through event code and data buffer. The injection beam
for the HER and LER requires timing jitter of less than 30
ps. It is confirmed that the system satisfies that therequirement under a variation of temperature in the linac[14]. To accomplish multiple injections to HER, DR,LER, PF ring and PF-AR, sequence of injection will beput every ~2 s. The trigger timings and parameters are
decided by each beam mode. Those trigger timings range
from ns to several hundred µs and the interval between
the beam pulses is 20±1 ms and should be scheduled
precisely. Thus, timing monitoring system with a wide
dynamic range of more than 20 ms and a resolution of 1
ns is required. Then, a FPGA-based TDC has beendeveloped [15].

FPGA-BASED TDC

Figure 5: Background signals due to a beamloss at thepositron source, in case of the primary electron beam
passes through the target hole. The top/bottom
corresponds to without/with insertion of the wire scanner.

Figure 6: Vertical beam size at the wire scanner.

The basic specification of the FPGA-based TDC listed
in Table 1. The TDC has a common strat input and 16
multi-stop input channels. The inputs to the modules are
standard NIM logic levels. Since handling FPGA with
1 GHz is not easy, the 4X oversampling technique isemployed to the TDC with a 250MHz clock. The timing
charts are shown in Fig. 7. Four clocks which have π/2
relative phase reference, then the time duration T is givena resolution of Tclk/4 isby T=NTclk+∆T1+∆T2 andobtained.
Table 1: Margin Specifications
Parameters

Value

Number of common start
Number of stop channels
Number of multi-stops
Dynamic range
Resolution
Clock1164
4.3 s
1 ns250 MHz

The performance test has been performed by using theTDC and the Digital Delay generator (SRS, Model
DG645, rubidium time base) from the delay time of 1 ns
to 800 ms. The accuracy which is given by the absolute
time difference divided by the setting delay time andvariations in the time difference of the measured to thesetting delay time are shown in Fig. 8. The accuracy is
0.13 ppm up to ~3 ms which corresponds to the roll-off
point. Those results are acceptable in our trigger timing
monitoring system.

from ns to several hundred µs and the modulators of highpower klystron should be triggered by 20±1 ms. Thus,
wide dynamic range TDC with 1 ns resolution has beendeveloped. The TDC shows good performance enough tomonitor the linac timing system.

REFERENCES

Figure 7: Schematic timing diagram for the FPGA-based
TDC.

Figure 8: Accuracy and timing jitter. The bar shows
standard deviation.

CONCLUSION
In the SuperKEKB injector linac, a new RF gun for low
emittance, high bunch charge electron beam and a newpositron source had been installed. Although the beamcommissioning was just started, in case of the electron
beam passes through the hole on the positron target, abeamloss was found by using the optical fiber set along
the beamline. The optical fiber dedicated to beamsizemeasurement by using wire scanner has been also
installed. A clear beam profile was obtained by using thesystem under the background which stems from thetarget. Trigger timings for the linac components ranges[1] T. Miura et al., “Upgrade Status of Injector Linac forSuperKEKB”, MOPRO001, IPAC’14, Dresden,
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