The 9th Symposium on Accelerator &ience and Technology, Tsukuba, Japan 1993

Test and Simulation of the Positron Convertor for the SPring-S Linac
Akihiko MIZUNO, Shinsuke SUZUKI, Hiroshi YOSHIKAWA,Toshihiko HORI, Kenichi YANAGIDA, Kenji TAMEZANE,and Hideaki YOKOMIZOJAERI-RIKEN SPring-S Project team
2-28-8 Honkomagome, Bunkyo-ku, TokyoAbstract

For designing an electron/positron convertor (targetand focusing system) for the SPring-8 Linac, we constructeda test apparatus in Tokai Establishment, and obtained energy
spectrums of the generated positrons with various parametersof focusing system. ·Also, we developed a simulation code oftracking particles in the positron focusing section.
Though the test apparatus is simple compared with theplaned one for the SPring-8 Linac, we obtained conversion
efficiency of 0.27% after the focusing section. The dependenceof the experimental results on parameters of focusing system
are coincident with that of simulation one.
I. INTRODUCTION
We are planning to use positrons for SPring-8 Linac
to avoid the ion-trapping in a storage-ring. The positrons will
be generated in the linac at 250MeV section, and accelerated
up to 900MeV at the end of the linac. For getting highintensity light from the storage-ling, we have to achieve highconversion efficiency as possible.
The injection part of the linac (from electron gun tobunching section) has been already constructed, and it'scommissioning is under way. In the test of the 40nsec pulse
widths mode with cathode assembly of "Y-796", we obtained~ the current of 12A at the end of the bunching section, and of
the lnsec mode, 22A[l ].
In order to achieve high conversion efficiency ofSPring-8 Linac, we constructed the test apparatus of positron
generator and focusing system in Tokai Establishment. Aitd
also we developed a code of electron/positron convertor for
simulating the test apparatus in various parameters.
In this paper, we present the results of experiment andsimulation, and a simulation data of a improved focusingsystem.

Eneru Ueasaruent
Section~

DCI Snlenoldol Coli

0Distance

23from Target(m)Tunuten Tarcet

FIG.l The outline of the test apparatus
lOOMeV. Generated positrons are focused and accelerated upto -35MeV in the focusing section. The energy of thepositrons are measured by the analyzer composed of a90°magnet and a Faraday cup.
B. Simulation CodeCalculations for this experiment are using two codes.
One is EGS4" (Electron Gamma Shower version.4)[2] for
positron generation at the tungsten target. Aitd the other is ouroriginal code for tracking through the focusing section.
In the EGS4 simulation, we assumed the profile,radius, and injection angle of the injected electron beam for theinitial condition. Conditions of the target are same as that of inexperiment. The initial conditions of EGS4 are shown inTABLE.l.
TABLE.l Conditions for EGS4 Calculation
Injected electron energy
120MeV
Injected beam radiusl.Omm
Injection angle to the target
0 rad
The number of injected electrons
1,026,000
Proffie of electrons
Gaussian distribution
TargetTungstenTarget figure
Circular plate
Target radius
lO.OmmTarget thickness
6.0mm

II. OUTLINE OF APPARATUS AND SIMULATION
A. The Test Apparatus

A test apparatus is constructed and installed in a beamline of JAERI-linac. The apparatus (see FIG.l) consists of aremovable tungsten target (insert or pull out), focusing section(one pulse-solenoidal coil, one DCl-coil, one DC2-coil, one
S band accelerator structure and one quadrupole magnet), andmeasurement section (energy analysis magnet and Faradaycup). Electrons bombard to the target with an energy of about

Accerelator Structure

EGS4 provides tracking code the initial conditions ofpositrons; such as energy, direction and radial position on thesurface of the target.
The tracking code is based on the forth-ordered
Runge-Kutta method for calculating Maxwell equations in theelectromagnetic field of the focusing section. Since positron
currents are low (-lOrnA in the SPring-8 linac), we neglected-122-TABLE.2 Parameters of Experiment
FIXED

Coulomb interactions between positrons. So each ray trace isindependent, we calculated in parallel by vector processor,
VP2600, at Computing and Information Center JAERI.
The magnetic field is caused by the solenoidal coils.
We adopted calculated longitudinal and radial magnetic fields
along the beam line in the focusing section with assumptionthat cylindrical currents flow at the positions of _!_he coils ..
Longitudinal magnetic field is shown in FIG.2 at the case of
the strongest fields in the experiment, currents of coils are
5.8kA(pulse-'solenoidal coil), 350A(DC1· coil), and 130A(DC2coil) respectively.

see TABLE.l
Target
150nA(average)
Injection electron current
90MeV
Injected electron energy
Spps
Repetition rate
1~-~Sec
Pulse width of injected current
Energy gain in the accelerator structure
33.1MeV
VARIABLE
2.0kA, 4.0kA, 5.8kA
Pulse-solenoidal coil
OA, ZOOA, 350A
DCl coil
OA, SOA, 130A
DC2 coil

E 2.5solenoidal coil sift with current of pulse-solenoidal coil.
Besides, peak due to other coils don't sift with current of pulse
or other coils.[3] In FIG.3, a peak of lower energy corresponds
to the pulse-solenoidal coil because it sifts with pulsesolenoidal coil current. And a peak of higher energy is due toother coils; DCl or DC2, because it does not sift. In thebottom figure, two peaks become same position in abscissa,
and form only one peak. These are coincident between (a) and(b).

~ ,: = :-]:=:1::=:_:{_::::=~tn·····l·····························f················"'··············t············
ii~0 •50012.

34Distance from the Target (m)FIG.2 Longitudinal magnetic field along the beam fine400

The electric field is caused by the acceleratorstructure. In this experiment, positrons with an energy ofaround lOMeV at the surface of the target are mainly collected.
So their speed is almost equal to light speed, we assume thatstatic field exists in the area of the accelerator structure.
In this code, the positrons are tracked to the finishingpoint of 4 meters from the target, around energy analysis
magnet. During tracking through focusing section, we discard
positrons which radial position is more than 17 rnrn away fromthe center axis.

PuiU•2.0kA

300200,-...
.-::::: 100

c=!

"'

~

~ ::i:::::::~:)·-rrt~~-~: g"'~ .:~~:r--r ;~-~:-_~::;~---tr·

c300

M0..

1000]

j0ao!5200
100

In the following, we show the experimental datacompared with the simulation one. Varied parameters are
currents of the solenoidal coil (2.0kA, 4.0kA, 5.8kA), DCl coil(OA, 200A, 350A), and DC2 coil (OA, 50A, 130A). Otherparameters are fixed. TABLE.2 shows parameters of thisexperiment.

Pul58=4.0kA·;;; 2000.....

093.3MeV••7050400§,_,_.,---,~-,-----~ 6 400·;;;:::>c~.., 300

III. RESULTS OF EXPERIMENT AND SIMULATION~030-e30

riA.
5070

••

I Pulse=5.BkA

1\I~ lA507090

Energy (MeV)
Energy 9.33MeV I div(b) simulation(a) experimentFIG.3 effect of pulse-solenoidal coil
DC1=350A DC2=130A

A. Pulse-Solenoidal Coil

B. DCI CoilAt first we show effects of the pulse-solenoidal coilin FIG.3 (a) (experiment) and (b) (simulation). The data areenergy spectrums of positrons at the Faraday-cup(experiment)
or at the finishing point(simulation). In experimental data,
the abscissa represents an output current of the energy analysis
magnet, and the ordinate is a signal from the Faraday-cup.
Their energy resolution is wider at the right side of abscissa.
Also in the simulation data, energy resolution is proportional
to positron energy, and ordinates are arbitrary unit.
In ;FIG.3, global shape of spectrums and relative
intensity of the positron current are similar in (a) and (b).
We know that peaks in the spectrums due to the pulse-

The results with various currents of DCl coil are
shown in FIG.4 (a) and (b). The global shapes of spectrums
are quite similar in (a) and (b), but relative intensities ofpositrons are different. Perhaps, there are some misalignment
between pulse-solenoidal coil and DCl coil, or detail figuresof this section are different between experiment and simulation.

~123-

variational tendency of global figure of spectrums or positron
intensity from these results.

300400···200-=- 100·a:::>

.e030

~

D. Simulation of a improved geometry
50

7090

We examined the simulation of a improved geometryby this simulation code[3]. This system has 6 DC2 solenoidal
coils mounted over the accelerator structure. The energy gainin the accelerator structure is 40MeV, and the magnetic fieldof this condition along the beam line is shown in FIG.6 .
Except the energy gain and the magnetic field, the conditions
are similar to these of FIG.3 - 5. In this case, we obtained theconversion ratio to be 0.45% at the end of first acceleratorstructure. From these results, we can expect the efficiency tobe more than 0.45% at the end of the first accelerator structurein the SPring-S.

g <OO.C1dDDA~300·;;;~200'0 100
....

U~

D
30507090~ :::~~C1=350A~ 200.

100093.3MeV30507090

Energy (MeV)(b) simulation

Energy 9.33MeV I div(a) experiment

FIG.4 effect of DCl solenoidal coil
Pulse=5.8kA
DC2=130A~I~ 0.5

C. DC2 Coilen

FIG.5 (a) and (b) show the dependencies of thecurrent of DC2 coil. Both global figures of spectrums and therelative intensities of the positrons are similar between (a) and(b). Especially position of small peaks in the abscissa (perhaps
due to DCl coil) are coincident.

200-=- 100·a:::>
030

50

70

90

"'flEe'•"•g..:::= 300·;;;~200t; 100....UFIG.6 Magnetic field of improved geometry
IV. CONCLUSION

Dl~flr'l

V.REFERENCES[1] S.Suzuki et al., "Initial data of Linac Preinjecter for SPringS," 1993 PARTICLE ACCELERATOR CONFERENCE,Washington,D.C., Omni Shoreham Hotel, May, 1993
[2] Walter R.Nelson, Hideo Hirayama and David
W.O.Rogers,"THE EGS4 CODE SYSTEM", SLACReport-265,19S5[3] A Mizuno et al., "Simulation of Electron-positron Convertor
for SPring-S",JAERI-M report 93-030

100

D93.3MeV

5For designing the electron/positron convertor forSPring-S Linac, we constructed the test apparatus anddeveloped the simulation code. Experimental data are
qualitatively explained by the prediction obtained from asimulation code.
In the experiment, we obtained the conversion
efficiency to be 0.27%. From this result and the simulation,
we can expect the conversion efficiency to be over 0.45% at
the end of the first accelerator structure in the SPring-S Linac.

300•oomrnC2:DA!

3

Distance from Target (m)

30

50

7090

Energy 9.33MeV I div
Energy (MeV)(a) experiment(b) simulation
FIG.5 effect of DCZ solenoidal coil
Pulse-5.8kA DC1=350A

In the conditions of bottom one in the above figure,the conversion efficiency (peak current of the obtained
spectrum versus that of the injection one) is 0.27% inexperiment. On the other hand in simulation, it's 0.11% (thenumber of obtained particles versus that of injection one).
Their difference causes from the difference of detail parameters
between experiment and simulation. So we can understand only

-124-