LINAC UPGRADE IN INTENSITY AND EMITTANCE FOR SUPERKEKB*T. Higo#, M. Akemoto, D. Arakawa, Y. Arakida, A. Enomoto, S. Fukuda, K. Furukawa, Y. Higashi,
H. Honma, N. Iida, M. Ikeda, , E. Kadokura, K. Kakihara, T. Kamitani, H. Katagiri, M. Kurashina,
H. Matsushita, S. Matsumoto, T. Matsumoto, S. Michizono, K. Mikawa, T. Miura, F. Miyahara, T.
Mori, H. Nakajima, K. Nakao, T. Natsui, S. Ohsawa, Y. Ogawa, M. Satoh, T. Shidara, A. Shirakawa,T. Suwada, H. Sugimoto, T. Takatomi, T. Takenaka, Y. Yano, K. Yokoyama, M. Yoshida, L. Zangand X. Zhou, KEK, Tsukuba, Japan
D. Satoh, Tokyo Institute of Technology, Tokyo, Japan
Abstract
The SuperKEKB is now in a construction stage. The
injector linac should be upgraded for realizing the 40times higher luminosity than that of KEKB. This requires
both bunch charge increase and reduction of beamemittance. All of the relevant hardware designs for theinjector system are being finalized now to meet thecommissioning of the ring by the end of 2014. In thepresent paper is briefly reviewed the overall progress ofsuch design decisions and hardware developments in thelast two years. A perspective towards the commissioningis also presented.

INTRODUCTION
In order to realize much higher luminosity inSuperKEKB [1] than that of KEKB, the injector linac
should be upgraded both in its intensity and the beamemittance. The relevant parameters are shown in theTable 1. As shown in the table, the electron and positron
beam charge should be increased by about 4-5 times thanKEKB, while the emittance should be reduced by morethan an order of magnitude. The status as of the year 2010
was described in [2] and [3]. In the present paper, wereview the advancement after then.
The intensity and emittance of the electron beam arerealized directly by adopting a photo-cathode RF gun [4].
In contrast, the positron intensity is increased by adopting
a higher capture efficiency system with a flux
concentrator (FC) [5] followed by the large-aperture
accelerators [6], while its emittance is reduced by adamping ring (DR). For preserving such a low emittanceof both beams through the linac [7], the suppression of theemittance growth is crucial. To this end, the alignment of
the accelerator components should be improved to bewithin a sigma of a hundred microns, where we need animprovement by about a factor 10. In addition to thisimprovement, the beam based alignment (BBA) isdefinitely needed with higher-resolution BPM’s [8].
The upgraded linac will serve a 1 nC beam for theinitial commissioning of the rings by the end of 2014. The
low emittance beam will be injected to the rings forcollision run by the end of 2015. Continuously and evenlater than 2015, we will make efforts to increase thebunch charge. We are planning this staged development ofthe beam characteristics depending on the budgetary andcivil engineering limitation.
#toshiyasu.higo@kek.jp

Table 1: Injector Linac Requirements
KEKB Æ SuperKEKB
ItemElectronPositron

Energy (GeV)

8Æ73.5 Æ 4Bunch charge (nC)1Æ51Æ4

Number of bunches

22

Emittance (micron)~100 Æ 20~2100 Æ 10

5050

Rep. Rate

OVERALL DESIGN
Basic Configuration
We are upgrading the injector linac for SuperKEKBwith fully utilizing the existing linac for KEKB so that theschematic of the linac becomes that as shown in Figure 1.
The J-type linac shape should be adopted to assure theelectron energy, even though the energy is reduced from 8
to 7 GeV. In order to install a DR at 1.1 GeV, we shifted
the positron production target from the unit 2-1(beginning of sector 2) to the unit 1-4 (middle of sector 1).
The reduction of the drive electron energy from 4 to 3.5
GeV is compensated by the higher efficiency in the newpositron capture system.

Mission of the Linac
In addition to serving the beams into the two rings ofSuperKEKB, the linac should serve for other two rings at
the same time, PF (Photon Factor) in top-up mode andPF-AR (PF Advance Ring) taking an injection period.
This requires the acceleration and transmission ofdifferent beams, in energy and intensity, simultaneously.
More pulse magnets are installed to improve the opticalcharacteristics for each beam.

Recovery from Earthquake to SuperKEKB
The linac suffered from a mess due to the earthquake in
11 March, 2011 [9]. The supports for the accelerator
girders were bent, many of Q magnets were fallen andbellows with BPM’s were ruptured. These should bereplaced, reinforced and realigned. The alignment
requirement for SuperKEKB is much stringent so that thealignment method is also being refined.

Figure 1: Schematic for injector linac for SuperKEKB.

ELECTRON PRODUCTION
The electron bunch of 5nC bunch charge with low
emittance is generated by a photo-cathode RF gun [7].
The candidate cathode materials are such as LaB6 or Ir5Ce.
These will be illuminated by a laser of 266 nm with a few
mJ per pulse [10] to get 5 nC without heating. If Qe is nothigh enough by some reason, we consider the introductionof the heating. The RF cavity for extraction andacceleration of the electrons is performed by a DAW
cavity with curved cathode shape [4]. It allows the highcharge and the low emittance with relatively low cavity
gradient.
Due to the earthquake, the test system was located at
the sector 3 and the preliminary test has been performed
as shown in Figure 2. Now a bunch charge of 1 nC wasextracted and the emittance will be estimated in near
future.

Figure 2: RF gun system installed at 3-2 section.

POSITRON PRODUCTION
The positron bunch is produced by a W target with theelectron bunch of 10 nC at 3.5 GeV energy. The capture
efficiency should be improved by a factor 5 than KEKB.
To realize this high capture efficiency, the FC [5] is usedinstead of the pulse coil. It is followed by a large aperture
RF system with the negative RF phase [6].
A flux concentrator has been developed under thecollaboration with BINP Novosibirsk [3]. Even if thisdesign opens the possibility to a very high field as 10Tesla, we decided to start with the type developed by
SLAC [11] to meet the requirement to operate from late2013. The detailed information of the type was given

from IHEP China [12]. The shaping and the fieldmeasurement of the prototypes are ongoing. We may use
a special copper material which is supposed to keep thehardness even after silver brazing cycle.
The capture section is composed of two L-bandtravelling-wave accelerator structures with the beam holediameter of larger than 35 mm. The RF power isterminated by the collinear load section to get rid of theouter wave guide and minimize the solenoid bore to be
260 mm in diameter. Load material is atmosphericplasma-sprayed Kanthal. The first L-band accelerator
structure is shown in Figure 3. This is the one used forbunch compression after DR as shown in Figure 1, but themain part of it will be reproduced in a collinear design for
positron capture. This structure in Figure 3 is soon highpower tested. It produced 10 MV/m with 15 MW input.
The choice of 5/11 times S-band linac frequency offers
two features, larger longitudinal acceptance and thereduction of the injection loss at the damping ring bysuppressing the S-band satellite [13].

Figure 3: Prototype L-band accelerator structure for BCS.

EMITTANCE PRESERVATIONThe emittance preservation is one of the big upgrade
items for the injector linac, because the requirement for
the KEKB was much looser than the present one. Most
basic strategy for suppressing emittance growth is torealize the better alignment. From the beam simulation,such a tolerance as 100 microns over a hundred meters or
so is required. We will firstly realign the whole linac
components which were heavily moved by the earthquake[9]. The first step of this realignment, equivalent to theinitial installation, will be done with respect to the laser
beam monitored by quadrant photo detectors. The

transmission of the small-aperture laser beam over 500
meters was confirmed and it will be used for the initialsetting. The actual position of each component withrespect to the reference line is measured by a laser tracker
system. The alignment will be gradually refined byintroducing the beam-based alignment. The resolution ofthe BPM’s will be improved from 50 microns to 10-20
microns by applying the better signal processing [8].
The emittance growth due to the short-range wake field
will be reduced by shortening of the bunch length andalso by introducing the beam offset [14]. We will preparesuch tuning knob by adding pulse steering magnets.
In order to evaluate the low emittance along the linac,
the wire scammers are used for the beam emittancemeasurement. We found that the beam loss monitor by
detecting Cerenkov light emitted in the optical fiber canbe used for the detection of the beam intersection with thewires with better S/N ratio than the case with PMT [15].
The vertical emittance as a function of position along abunch will be measured by introducing a deflector cavity
at the end of linac. This activity is ongoing undercollaboration with SLAC.

STAGED UPGRADE SCENARIO
The upgrade scenario of the injector linac has been
redefined reflecting various boundary conditions, not only
the technical perspective on the readiness of the hardware
but also the budgetary profile and the civil engineeringschedule. Our rough idea on the staged goal for the beam
is listed in the Table 2. By the end of 2013, both electron
and positron with a bunch charge of 1 nC will becommissioned even in a low rep. rate mode and in a
reduced intensity as required from the radiation safety.
The electron will be produced by the RF gun from thebeginning. From the end of the year 2014, 1 nC beam will
be injected to the rings to serve for initial commissioning
and vacuum scrabbling of the rings. Finally with theintroduction of the damping ring by the end of 2015, 1 nC
bunch with the emittance for the SuperKEKB will beinjected for collision run. Then later on, the intensity ofthe positron will be increased by gradually increasing theaxial magnetic field of the flux concentrator and thefollowing DC solenoid field. Note that the latter is thecase that we cannot prepare enough cooling and electric
power for magnets before this time.

PERSPECTIVE
The quadrant photo diode suffers from the radiation.
Therefore, the laser alignment cannot be a method ofmonitoring the reference line in a longer period,
especially after establishing a high current operation for
SuperKEKB. During the long-term operation of collision
run, the monitoring of the reference line will be done witha different laser system for Fresnel lens [16], which does
not suffer from the radiation. However, it can be replacedby beam based alignment, once the alignment isestablished and the misalignment does not develop so big
in a short period.

The synchrotron injection scheme to the high energy
ring has being studied as a possible better injection
scheme [17]. This requires lower bunch energy spreadfrom the linac and sophisticated septum design.
Table 2: Staged Upgrade Scenario
Q (bunch charge), PRF (Rep. rate), H (emittance)nCHzmicronStage

ElectronPositronyear

Q

PRF

H

Q

PRF

H

KEKB

150 ~100

150 ~2000

2013/E

1~5 Study~1~5n

2014/E150n

1n

2015/E

1n20

1n

10

SuperKEKB

55020

45010

REFERENCES[1] H. Koiso et al, “Design Progress and Construction Status ofSuperKEKB,” TUPPR006, IPAC12, proceedings of thisconference.
[2] T. Sugimura et al, “The LINAC Upgrade Plan for
SuperKEKB,” THPD007, IPAC10, Kyoto, Japan, 2010.
[3] T. Kamitani et al, “Injector Linac Upgrade forSuperKEKB,” MOP011, LINAC2010, Tsukuba, Japan,2010.
[4] T. Natsui et al, “High Charge Low Emittance RF Gun for
SuperKEKB,” TUPPD057, IPAC12, proceedings of thisconference.
[5] L. Zang, “Design Optimization of Flux Concentrator for
SuperKEKB”, TUPPD032, ibid.
[6] F. Miyahara et al, “Development of L-Band Positron
Capture Accelerating Structure with Kanthal-coatedCollinear Load for Super KEKB”, TUPPR051, ibid.
[7] M. Yoshida et al, “SuperKEKB Injector Upgrade for High
Charge and Low Emittance Electron Beam”, TUPPD035,ibid.
[8] M. Satoh et al, “The BPM DAQ System Upgrade forSuperKEKB Injector Linac”, MOPPR025, ibid.
[9] A. Enomoto et al, “Quick Recovery of the KEK e-/e+
Injector Linac from the Great East Japan Earthquake”,
WEPPD064, ibid.
[10] X. Zhou, “Ytterbium Fiber Laser System of DAW RF Gunfor SuperKEKB”, WEPPD056, ibid.
[11] Kurikov et al, “SLC Positron Source Pulsed Flux
Concentrator”[12] P. Guoxi, IHEP, China, private communication.
[13] N. Iida et al, “”, THPD004, IPAC10, Kyoto, Japan, 2010.
[14] H. Sugimoto et al, “Design Study on KEK Injector Linac
Upgrade for High-current and Low-emittance Beams”,
TUPPC021, IPAC12, proceedings of this conference.
[15] Y. Yano et al, “Development of Optical Fiber Beam Loss
Monitor for KEKB Injector Linac”, MOPPR027, ibid.
[16] T. Suwada, “Development of a Laser-based Alignment
System Utilizing Fresnel Zone Plates at the KEKB InjectorLinac”, WEPPD065, ibid.
[17] T. Mori et al, “Design Study of Beam Injection for
SuperKEKB Main Ring”, TUPPR089, ibid.