The lith Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Positron Accumulator Ring for Linear Collider and R&D Plan
Junji URAKAWA, Tachishige HIROSE* and Masakazu WASHIO**Accelerator Laboratory, High Energy Accelerator Research Organization,
Oho1-1, Tsukuba, Ibaraki 305, JAPAN*Department of Physics, Tokyo Metropolitan University,Hachioji, Tokyo 192-03, JAPAN**Laboratory for Quantum Equipment Technology, Sumitomo Heavy Industries, Ltd.
2-1-1, Yato-Cho, Tanashi-City, Tokyo 188, JapanAbstract

The possibility of the complementary positronsource using accumulator ring is discussed for the generation of high intensity multi-bunch positron beam for
the Japan Linear Collider (JLC). The key features ofthis scheme include accelerating positrons at an L-bandfrequency (1428 MHz) to get the high normalized yieldof 0.5e+ / e- · Ge V and using several parallel positrontargets & positron accumulator rings to relax the highradiation activity. For the reliability, maintenability andimproved source efficiency, the design uses several parallel positron generation systems adequately shieldedfrom each other. The invariant transverse acceptanceof the capture system is 0.06m · rad, ensuring an adequate positron beam intensity for the JLC. Since there
are many challenging R&D's to establish the positron
normalized yield of 0.5, the Positron Factory using thepositron target system of the ATF is proposed with
200MeV small accumulator ring, L-band high gradientstructures for acceleration up to 200Me V & deceleration
until 2MeV, and an efficient moderator structure.

mulation for the linear collider using the KEK-ATF isproposed [5]. It is applicable to the high density positron
factory. So, discussion on some applications of proposed
positron factory is given in final section.
Table 1
JLC and NLC-I Positron Source Parameters
Parameters
Drive Electron Energy (GeV)
No. of Bunches per Pulse
Bunch Intensity(10 10 )Repetition Rate (Hz)Beam Power (k W)
Beam(]' on Target (mm)Pulse Energy Density(10 11 GeV/mm 2)
Positron Target Material
Thickness (R.L.)
Tapered Field (T)Uniform Field (T)Flux Concentrator Field (T)Flux Concentrator
Minimum Radius (mm)Accel. RF Frequency (MHz)
Accel. Gradient (MV /m)
Minimum Iris Radius (mm)Edge Emittance ( m · rad)Positron Yield per Electron
Positron Bunch Intensity(10 10 )Positron Normalized Yield

1 Introduction

The proposed JLC positron source based on the existing SLC positron system consists of a dedicated Shand electron accelerator, a conventional positron production and capture system utilizing a high-Z target andan adiabatic matching device, an S-band positron linac,
and a pre-damping ring [1,2]. The JLC is designed tocollide a 85-bunch positron beam with an identical electron beam with a bunch intensity as high as 0.7 x 10 10particles for each machine pulse [3]. The beam pulse
intensity requirement for the JLC is such that 20-times
more positrons need to be produced per pulse, as compared to the operating condition at SLC. There is theprimary challenge in designing the JLC positron source
since a simple scale up of the SLC positron source would
not be feasible due to the excessively high beam energythat the target would have to withstand. Thus, a significant improvement in the positron capture efficiency
is essential in realizing the JLC source. Table 1 lists theimportant parameters for the JLC positron source, alongwith the NLC-1 positron source parameters for comparison [4].
In this paper we present other conventional schemethat is reasonably conservative and uses only existing
technologies. New scheme for complementary positronsource is described in the following section. In section
3 the R&D plan on the positron generation and accu-JLC
10.0
85
0.7
150
147
1.2

NLC-I
3.11
90
1.5
180
121
1.2

6.6
W75Re25
6
1.2
0.8
5.8

4.6
W75Re25
4
1.2
0.5
5.8

3.5
2856
30
130.027
2.1
1.5
0.21

4.5
1428
25
20
0.06
1.4
2.10.45

2 Complementary Positron Source

The JLC positron source is of a conventional typebased on e± pair production from an electromagnetic
shower created in a thick, high-Z target upon bombardment by high energy electrons. The reliability of thepositron production and capture system is specially important since the high radiation levels in these areas
would prevent human access for quick repair in caseof hardware failure. A effective way to increase thereliability is to build several identical positron generation systems, to reduce the electron beam power on thetarget and to increase the capture efficiency. Several
positron generators housing identical positron production and capture systems that are adequately shieldedfrom each other will be built. Fig. 1 shows the schematiclayout of the complementary positron source. This system consists of two drive 0.5-GeV S-band linac, several
positron targets & positron accumulator rings and L--20-

The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

band linac to the rings. In order to increase the positron
capture efficiency, the introduce of the L-band high gradient linac after the positron target, which is the same
one as the NLC design, is essential for the proposed newscheme. Same positron target and capture system as theNLC design except for the thickness of the target will beadopted. The thickness of the positron target is 3.6 radiation length in the case of the drive electron beamenergy of 0.5 GeV. The development of C-band linac
components has been proceeded a.s the back-up option
for the JLC main linac. In this case, 4 parallel positron
generation systems are necessary.

the length of bunch train and kicker rise time. Transverse damping time is proportional to the ring circumference and the bending radius. Since normal conducting
bending field of 2.7[T] is obtained at the routine operation of Aurora-2, the design of the accumulator ring isbased on the magnet design of the Aurora-2 [7]. Thisbending magnet has the gap of 42 mm and good fieldregion of 60 mm. The ring has a racetrack configurationand two 30° & one 120° bending magnets in each arc section to remove dispersion in two straight sections. Table
2 gives preliminary design parameters of the positronaccumulator and the drive electron beam. The electron
beam power to the target is less than 1/12 comparing
the proposed JLC positron source in the Table 1. These
accumulator rings are designed using 6 bending magnets
for simplicity. The value of the single-bunch thresholdindicates large impedance budget. So, the momentum
compaction factor can be reduced by increasing the number of the bending magnets with the constant bending
field. The RF voltage and equilibrium emittance will bereduced by the optimization on the number of the bending magnets. However, the development on short highfield bending magnet will be required.

0.7GeV AC RING

0.25GeV L~band linac

Table 2Preliminary Design Parameters of Positron

JLC X-band Positron Generator System using Acuumulator Ring and l-band Linac(1 ): Bipolar Pulse Magnet(2): Bending Magnet

Fig. 1 Schematic layout
positron source.

(3): Positron T<~rget ( 4): Flu)( Concentrator

oftheAccumulator and Drive Electron Beam
JLC-X
JLC-C
Positron Accumulator:
Beam Energy ( Ge V)
0.7
0.7
Circumference (m)
42.0
66.4
714MHz RF Voltage (MV)
6.4
4.0
Bending Field (T)
2.66
2.66
Bunch Length (mm)3.4
4.3Energy Spread (%)
0.064
0.064
Horizontal Damping Time
8.3 msec
13.2 msec
Single-bunch Threshold (r2) 2.14
1.09
Positron Bunch Intensity
1.4 X 10 10 2.2 X 10 10
Drive Electron Beam:
Electron Energy ( Ge V)
0.5
0.5
No. of Bunches per Pulse
7285
Bunch Intensity (10 10 )2.8
4.44
Repetion Rate (Hz)150
100
Beam Power per Pluse (J)
190.4
255.7
Beam Power (k W)
9.52
12.8

complementary

The drive beam accelerator uses S-band (2856 MHz)RF for acceleration and has an injector consisting ofa thermionic gun, two subharmonic and one S-bandbunchers similar to those in the ATF injector. Beamloading compensation in the accelerator will be accomplished using pairs of structures operated at about 1
MHz above and below the main RF frequency, i.e., theso-called ±C!..f beam loading compensation method. TheL-band design for the JLC positron capture and booster
accelerators is the key to achieving the order of magnitude higher positron beam intensity over that of theSLC positron source. This design is same one as theNLC positron source [6].

2.1 Positron Accumulation
As in the SLC positron source, W75 Re 25 is chosen as
the target material because of its high e± pair production efficiency and excellent thermo-mechanical properties. The input electron beam can be directed to eachsystem via bending magents that are isochronous and ·.
linearly achromatic. The 250 MeV positron beam after the capture accelerator from each system is directed
into a common 0.45 Ge V L-band linac. The first bending
magnet following the capture accelerator also serves toseparate the captured electrons from the positron beam.
Two identical systems allow to be stacked once in RFbuckets already containing positrons.

2.2 Accumulator Ring
Minimum circumference of the ring is determined by

The positron bunch intensity in the Table 2, whichis two times the required intensity of 0.7 x 10 10 at theinteraction point, includes 50% additional beam losses
in downstream accelerator components.

3 R&D Plan
It is necessary to investigate the feasibility on thenormalized yield of 0.5e+ / e- · Ge V. The essential
R&D is to establish the technology on the high gradient L-band structure and the positron capture system.
The device including the flux concentrator, the tapered
field solenoids and the target was made as the ATF
positron source. The ATF linac is designed including
the positron-source test facility before the linac beam-21-

The lith Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

dump [5].
The L-band linac with the accelerating gradient of
25 MV /m should be constructed after the positron target. There is space to install about 10 m-long linac anda small ring near the end of the ATF linac. Fig. 2 shows
the 200 MeV accumulator ring to stack the positron
beam with the 0.06 edge emittance. Inner size of thevacuum chamber for the positron accumulator is horizontally 60 mm and vertically 35 mm in full width. The
vertical width of 50 mm is necessary to accept the beamwith the edge emittance of 0.06m · rad. It is difficult
to enlarge the gap of the bending magnet because thebending field is inversely proportional to the gap. TheR&D on normal conducting bending magnet which has
60 mm gap and 2.1 T is important.
The essential R&D regarding the accumulator ringis to study the beam acceptance. Table 3 shows thedesign parameters of the 200 MeV positron accumulator and the ATF drive electron beam. Further detailed
analyses should be carried out concerning the L-band
accelerator structures, beam instability, reliability of thecomponents, and rotating target.

Bend

Quad

Quad

Qu•d

Qu•d

Quad

Bend

Bend

Table 3
Design Parameters of 200Me V Positron Accumulator
and ATF Drive Electron Beam
Positron Accumulator:
Circumference (m)21.0714MHz RF Voltage (MV)0.4
Bending Field (T)2.08Bunch Length (mm)4.5Energy Spread (%)
0.064
Horizontal Damping Time
70 msec
Touschek Lifetime
78 secSingle-bunch Threshold (0)0.275
Positron Bunch Intensity
3.1 X 10 10
Normalized Emittance of Beam 5 x 10- 5 m · rad
ATF Drive Electron Beam:
Electron Energy ( Ge V)
1.54
No. of Bunches per Pulse
20
Bunch Intensity (10 10 )2.0
Repetion Rate (Hz)
6.25
Beam Power per Pluse (J)98.6
Beam Power (k W)
0.62
by S. Okada et. al. [8]. The 2 MeV slow positron can befocused on the moderator until the beam size of about
1 mm(rms). This high density positron factory will givethe promotion of precise experiments for material scienceand the possibility of the experiment on Bose-Einsteincondensation of positronium using laser cooling [9].
Acknowledgement

9.390mFig. 2 Accumulation Ring for Positron Factory.

One of Authors ( J. Urakawa) would like to thankProf. K. Oide for helpful advice on SAD code which heused to make optics calculations. He also wish to thankDr. N. Terunuma for useful discussion on the vaccumchamber.
References

Fig. 3 Lattice Parameters of Half Ring and Half Edge
Size of Injected Beam.

4 High Density Positron Factory
The L-band linac can decelerate the bunched beamwith the normalized emittance of 5 x 10- 5 m · rad until 2 MeV. About 108 slow positrons per pulse are obtained using the moderator assembly with a honeycomblike structure enclosed by reflectors which was proposed[1] "SLC Design Handbook", SLAC (1984).
[2] J. Urakawa et al., "Preliminary Designs of 1.54Ge V
Damping Ring and Bunch Compressor for the JLC",
Proc. 1990 Linear Accel. Conf., (1990) 689.
[3] Ed. by N. Toge, "JLC Design Study", KEK Report97-1, April, 1997.
[4] The NLC Design Group, "Zeroth-order Design Report for the Next Linear Collider", SLAC Report
474, 1996.
[5] Ed. by F. Hinode et al., "ATF Design and Study
Report", KEK Internal 95-4, June, 1995.
[6] H. Tang et al., "Design of the NLC Positron Source",
Proc. 1996 Linear Accel. Conf., (1996) 647.
[7] T. Hori and T. Takayama, "" AURORA-2" Compact Advanced SR Ring as an X-ray Source", Proc.
ICSRS-AFSR'95, Pohang, Korea, (1995).
[8] S. Okada et al., "Design Studies for the Positron Factory", Proc. 1996 Linear Accel. Conf., (1996) 249.
[9] T. Hirose, "Design of a laser cooling system for highdensity ortho- positronium", Submitted to Workshop
on Low Energy Positron and Positronium Physics(Nottingham England July 16-18, 1997)-22-