PULSE-TO-PULSE MODE SWITCHING OF KEKB INJECTOR LINAC
T. Kamitani∗ , K. Yokoyama, T. Sugimura, K. Kakihara, M. Ikeda, A. Shirakawa, S. Ohsawa,T. Suwada, K. Furukawa, M. Satoh, M. Kikuchi, T. Mimashi, N. Iida,KEK, Tsukuba, Japan
Abstract
Electron and positron injections from the KEKB injector linac to storage rings have been switched in a typicalcycle of five minutes. The switching involves a mechanical movement of a positron production target from and toa beam line. It prevents from faster switching, like pulseto-pulse switching. A target assembly with a small holebeside a target material and four pulse steering magnets areinstalled. For positron injection pulses, the pulse steering
magnets are off and electron beams irradiate the target to
produce positrons. While, for electron injection pulses, thepulse steering magnets are excited to create a orbit bump inorder for the electron beams to go through the hole and bypass the target. This paper describes details of a system forthe pulse orbit bump and results from recent beam studies.

INTRODUCTION
The KEKB injector linac supplies electron and positron
beams to the KEKB storage rings HER (8.0 GeV e-), LER(3.5 GeV e+) and to the two synchrotron radiation facilityrings PF (2.5 GeV e-) and AR (3.1 GeV e-) as well. Eachring requires different beam properties; a time-structure, aparticle charge, an energy and an intensity. Parameter setting of the linac, called as ”injection mode”, is switched for
an appropriate beam to be generated, accelerated and sent
into a selected ring. In this mode switching, parameters likea choice of the pre-injector, electron gun settings, magnet
currents, acceleration phases, rf pulse timings and so forth
are changed. Mode switching between the electron and thepositron injection for HER and LER rings is frequent in
typically 5 minutes cycle. Each switching takes 30 seconds
in completing to change parameters. Most of the linac operation time is occupied for KEKB continuous injection. It
is interrupted by injections to PF (once a day) and to AR(twice a day), each takes 15 minutes to fill up the storage
ring. Switchings from KEKB to PF or AR and its reverse
have taken a few minutes until recently, since it needed tochange a current slowly for a large bending magnet to keep
its hysteresis cycle. However, recently, a demand of continuous PF injection was brought up for keeping its storage
current constant. It is beneficial for some synchrotron radiation users, but it was not compatible with the KEKB
continuous injection. To solve this issue, much efforts havebeen made to realize quasi-simultaneous electron injection
to KEKB HER and to PF [1]. It can be achieved by injecting both beams from the same pre-injector, installing apulse bending magnet and a new beam transfer line for PF,∗ takuya.kamitani@kek.jp

pulse-to-pulse switching of acceleration rf phases for 8.0
GeV and 2.5 GeV beam energies, setting up focusing andsteering magnets compatible for these different beam energies [2]. This unification of these two injection modes has
almost been completed and it will be used for an autumn
run this year (2008).
Next step of the unification is that of the KEKB electronand positron injections. Positrons are produced by irradiating a metal target with electron beams. A target is placedon a beam line halfway across the KEKB linac. The target
is retracted from the beam line during an electron injection and inserted during a positron injection. Mechanicalmovement of the target takes a few seconds. It is desirable
keep constant storage currents in both of electron ring andpositron ring for a beam collision tuning to achieve superior
luminosity. To realize pulse-to-pulse switching of the electron and positron injection modes, it was proposed to use atarget assembly with a small hole beside a target material
in order to fix it on a beam line and to let electron beams
bypass through the hole with pulse steering magnets only
for electron injection pulses.
In the following chapters, details of the target assembly,
the pulse steering magnets and recent results of beam studies are given.

TARGET BYPASS ORBIT FOR
PULSE-TO-PULSE MODE SWITCHING
Target bypass orbit
In designing a bypass orbit to avoid the target retraction,
first considered has been a large orbit bump to bypass not
only the target but positron focusing solenoids and a capture system. It needed an independent bypass beam line
apart more than 300 mm from the original beam line andalso huge pulse magnets to bend 4-GeV electron beams
in large angle. Thus, it was too costly. More practicaland economical solution was to make a small hole in atarget assembly beside the target material and to make abump orbit of only some millimeters height. This small
orbit bump can be contained in a present beam ducts andin apertures of accelerating structures. Fig. 1 shows a bypass orbit bump and a beam line layout around the positron
production target. A component denoted as PT21T in thefigure is a positron production target and the thick green
lines show an aperture of the target hole. Center of thehole is 5.2 mm aside horizontally from a beam line center. Focusing quadrupoles setting is based on a candidate
beam optical calculation which is compatible for HER andPF injections. A purple line shows a bump orbit and red

Figure 1: Target bypass orbit and beam-line component layout around the positron production target.
lines show a beam envelope in 3-sigma extent assuming
a design emittance of electron beam for HER injection:γǫ = 1.0 × 10−4 m. A envelope of PF injection beams is
not described here since its size is typically much less thanthat of HER beam. About 3 meters upstream of the target,
there is a quadrupole quartet (illustrated in green rectangles) to focus electron beam onto the target. At just downstream of the target, there is a positron-focusing solenoid
region in 8 meters long and capture accelerating structures (blue rectangles) in it. Three horizontal pulse steering
magnets (PX17C, PX17C5, PX2145) and a vertical steering (PY2145) are installed (orange rectangles). Horizontal
steering magnets make a closed orbit bump to let electron
beams go through the hole. Since the focusing quadrupole
quartet is inside the bump, it contributes to kick the beam
toward the beam line center and help the second steering
magnet. The vertical steering magnet is used to compensate a vertical kick from edges of the solenoidal field.

Target assembly with a small hole
For positron production, 4.0-GeV electron beam (beamintensity 10 nC/bunch, 1 or 2 bunches/pulse, maximum
pulse repetition 50 Hz) irradiates a tungsten target. A 14mm thick amorphous tungsten had been used as a target
material. Recently, a 10-mm thick crystal tungsten has
been developed for practical use, which resulted in 25%
increase of the positron intensity [3]. Transverse cross section of the crystal tungsten is a square in shape, 4.5 mm ona side. The target is combined in a copper support by the
hot isostatic pressing method. Cooling water pipe is woundaround the support and connected by brazing. In a present
injection mode switching, the target assembly is mechanically moved. The target is retracted from a beam line forelectron beams to pass through during electron injections,and it is inserted in the beam line for positron production
during positron injections. The target is driven by a pressurized air and it takes a few seconds to complete the movement. A cycle period of the retraction and the insertion islimited to be longer than 5 minutes for avoiding a breakage
of a vacuum bellows in a long-term operation. To realize abeam mode switching without moving the target, a hole 5.0mm in diameter is penetrated beside the target as shown in

Fig. 2.

Figure 2: Positron production target assembly with a hole.

Pulse steering magnets
To make a bump height of the target bypass orbit sufficient for the target hole, pulse steering magnets should generate 1.0-mrad transverse kick angle for the 4-GeV electron beam. To achieve this specification, modification ofused magnets in the TRISTAN accelerator was considered.
The candidate magnet has dimensions of 330 mm in yokewidth, 280 mm in yoke height and 170 mm in pole length.
This size is suitable for installing into available spaces in
the linac beam line. It could generate 0.0593 Tesla dipole
field with a maximum DC current of 10 A, which corresponds to 0.56-mrad kick angle for 4.0 GeV beam. To meetthe specification requirement on the field strength, a polegap of the magnet was modified from 70 mm to 35 mm, byfabricating additional pole head and welding to an existing pole. It doubled the field strength at the same current tomeet the specification. This modification resulted in significant increase of a coil inductance from 32 mH to 57 mH. In
this situation, the maximum current which can be supplied
to the magnet is limited by a induction voltage a power
supply can manage. We use a sine-like pulse shape witha offset to make an initial and a final time derivates of thesupplied current to be zero, because an edged shape of thepulse current give larger induction voltage. Bipolar pulse
power supplies, BP4610 and BP4620 of the NF corporation
are used as amplifiers of input pulses from function gener-

ators. The power supplies can tolerate an induction voltage
by a pulse longer than 40-ms cycle. Thus, it can excite abump orbit at maximum repetition of 25 Hz. In a case of50 Hz injection, we will switch the power supplies to a DC
mode. A timing of the orbit bump excitation with respect
to beam arrival has 0.5 ms jitter from a constraint in bucket
selection.

Figure 3: Pulse steering magnet for target bypass orbit

Beam transmission studies
Preliminary beam studies have clarified some issues on
this scheme of mode switching using a bump orbit throughthe target hole:• When the bump height is large, certain beam loss occurs because some fraction of electrons hit vacuum
ducts around the region of the quadrupole quartet.
Vacuum ducts whose inner shape was a circle of 10.0
mm in radius was replaced by that of elliptical shapewhose major axis is 14.5 mm.
• Preliminary target hole diameter 3.0 mm was not sufficiently tolerant for beam transmission. It was enlarged
to 5.0 mm in the most recent target.
• Setting of the pulse steering magnets for the bump orbit and beam transmission performance are dependent
upon a focusing strength of the quadrupole quartet. In
the most promising optical setting in views of beamtransmission and matching to a downstream optics,
the quadrupole strength was set to a medium value between that for an previous HER injection optics andthat for optimum focusing onto the target [2].
Beam studies are performed quite recently to measure abeam transmission performance after these improvements.
Fig. 4 shows beam transmission ratios for 1st and 2nd
bunches of the HER injection beam and for the PF beam.
We define a standard charge by a beam charge measured
at a downstream of the bump orbit when bump height iszero and the target extracted. The transmission ratio in vertical axis of the figure is calculated as a measured beam
charge divided by the standard charge. The bump heightin horizontal axis is defined as a difference of an orbit position at the target from that when pulse steering magnets
are off. It should be noticed that these values are not absolute orbit position. Dashed the blue and aqua lines show
measured beam transmission ratio of two bunches of a HER
beam with the target extracted. Degradation in a right-handshoulder is caused by beam loss at vacuum ducts. While,the solid lines in same colors are those for the case thetarget is inserted. At each bump height of the maximum
transmission for the first and second bunches, differencesof the values for the cases with and without the target isless than 5%. Thus, it shows that 95% of the beams canbe pass through even with an aperture limited by the hole.
Optimum bump heights for the first and second bunches ofthe HER coincides in a range of 0.1 mm. It shows that theorbits for them are very close in 0.1 mm and a common
bump height can achieve a transmission close to the optimum. Dashed and solid red lines show the transmission forPF beams. The optimum transmission for PF beam is better than 95%, but the optimum bump height differs about
0.6 mm from those of HER beams. This significant orbit
difference forces a compromised bump height in order tomake it common for the HER and PF beams. It results inslightly degraded transmission. It need more careful tuning to make these orbit to be closer for better tranmission.
If the tuning cannot improve the situation, pulse-to-pulse
variation of the bump height will be necessary and requires
some upgrades in the pulse power supply control system.
After all, even in a compromised bump height, 90% transmission is achieved both for HER beams and for PF beam.

Figure 4: Measured beam transmission performances.

REFERENCES[1] M. Satoh, et al., “Present Status of the KEKB injector upgrade for the Fast Beam-Mode Switch”, LINAC’08, Victoria,Canada, September 2008, TUP013.
[2] Y. Ohnishi, et al., “Design and Performance of Optics forMulti-energy Injector Linac”, LINAC’08, Victoria, Canada,
September 2008, TUP012.
[3] T. Suwada, et al., “First application of a tungsten singlecrystal positron source at the KEK B-factory”, Phys. Rev. STAccel. Beams 10, 073501 (2007).