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

Design of an Undulator at DSR of RIKEN RI Beam Factory
Masanori W AKASUGI and Takeshi KATAYAMA*RIKEN, The Institute of Physical and Chemical Research, Wako, Saitama 351-01, Japan*Center for Nuclear Study, School of Science, University of Tokyo, Tanashi, Tokyo 188, Japan

Abstract
According to requirements from the X-ray - RI
collision experiment at DSR that is proposed in theRIKEN RI beam factory project, an undulator forelliptically polarized X ray was designed. An Apple-II
type of the undulator was adopted so that both the X-ray
energy and the polarization are continusly changed at thesame time. The photon flux of more than 10 15 photons/(s0.1 %b.w.) is calculated for any polarization in the energy
range of 30- 800 eV.

low emittance electron beam, DBA lattice is adopted in
the arc section of the DSR [4]. As described in Ref. [3],the electron beam having the energy of 0.3 - 2.5 GeV isstored in the DSR. The calculated electron beamspecifications are listed in Table 1. The average current is0.5 A. The beam emittance is obtained to be E/Ey =7.8/13.9 nmrad at the maximum energy of 2.5 GeV.
Since the beta function value at the undulator section isabout 5 m, the electron beam size at the undulator section
is smaller than cr/cry = 197/264Jlm.

3 Design of an Undulator
1 Introduction
3.1 Magnetic Structure
In the RIKEN RI beam factory project, we will
construct not only the heavy ion machines but also theelectron machines. [1,2] The coexistence of bothmachines allow us to make new type of the collisionexperiments. One of them is the X-ray - RI collisionexperiment. Using high brilliant synchrotron radiationfrom one of the DSR ring in which electron beam isstored, we make high resolution X-ray spectroscopy ofhighly charged RI beam stored in another ring of theDSR.
[3] This experiment requires following
specifications on the X ray. The X-ray energy is 30 - 800
eV, it can be continuously scanned with the step of lessthan 0.1 eV. Any polarization of the X ray can be chosen.
The polarization should be kept during the energy
scanning. The X-ray beam intensity is as large as
possible, and the energy resolution is also as high as
possible. To get the X ray beam intensity of more than
10 12 photons/(s 0.01 %b.w.) at the colliding section,
output photon flux from the undulator has to be morethan 10 15 photons/(s 0.1 %b.w.).

2 Electron Beam in the DSR
The intensity and the resolution strongly depend on
the quarity of the electron beam in the DSR. To makeFigure 1 shows the schematical view of the magnetic
structure of an Apple-II type of the undulator recently
proposed by Sasaki et al. [5]. This has two pairs of planerpermanent magnet (Nd-Fe-B) arrays above and below theelectron beam plane. The X-ray energy is scanned bychanging the gap width. The polarization is changed byshifting the relative position of pairs of magnetic arrays.
On-axis magnetic field produced by the phase shifting
makes herical and sinusoidal motions of the electronbeam. The magnetic field in horizontal and verticaldirections are shown in Fig. 2 at three different phases.
The linear polarizations in horizontal and verticaldirections are obtained at the phase shifts D = 0 and D =l/2Au, respectively, where Au is the length of a magnetic
period. The circularly polarized X ray can be get at D =3/8 Au as shown in the figure.
3.2 Gap Width and Length of a Period
The minimum gap width depends on the size of thevacuum tube inserted in the gap. Taking the vertical tube
size of 15 mm, the 1-mm tube thickness and the 1-mm
spacing between the tube nad magnet, the minimum gap
width was decided to be 20 mm.

Table 1 The specification of the electron beam in the DSR.
Electron Beam Energy Ee(GeV)
0.3
Radiation Loss(keY/tum)u
0.1
Emittance(n m rad)0.11
Ex(nm rad)
0.2
Ev4)Energy Resolution
AEjEe
0.82
Bunch Length(em)
(Jz
0.006
Dumping Time
(sec)
6.2
't'x
(sec)
6.1
't'v
(sec)
3.1
'te't
Toushek Lifetime(106sec)
0.008

oo--332-

1
10.6
1.24
2.22
2.7
0.033
0.17
0.17
0.17
0.16

1.5
53.2
2.8
5.01
4.1
0.061
0.05
0.05
0.03
8.1

2
169.1
4.97
8.92
5.5
0.095
0.02
0.02
0.01
180

2.5
412.7
7.76
13.9
6.9
0.134
0.01
0.01
0.005
2.7The lith Symposium on Accelerator Science and Technology, Harima Science Garden City, 19970.30
ByE 0.20:s!
0)

ij;.g0)"~ 0.10~Fig. 1 Magnetic structure of the Apple-II type undulator.
0.0,~ ::::::·:....

050.3~IKAI\Ai1"0·3 0E o.3

20

40

60

80

100120

3.4 Photon Flux

The photon flux is calculated for both case of thelinear polarization (K=0.712) and the circular polarization(Kx=Ky=0.328), and they are shown in Fig. 4. The photonfluxes are larger than 1015 photons/(s 0.1 %b.w.) for bothcases in the whole energy range. The requirement from theexperiment will be completely satisfied. The flux for thecase of the linear polarization is about three times larger
than that for circular polarization because of larger
magnetic field.

~3~020406080100120~l [·(c).•. .•.•. ··•·•··•· •.•:: •.•. •. .•.· :•.• 1
020406080

10015Fig. 3 Maximum magnetic field as a function of thephase shift.

~ ~IkQ):~YBx0<:A: •.~:::;::

10Phase Shift (mm)120Z-Axis Length (mm)1016 ~~••~•••~•••~.~••• -. ..-...-...~.. -...- .-.. -. ..-. ..~.•~...-...-.. -. .. ~--~

Fig. 2 Magnetic field on the electron beam axis. (a), (b)and (c) corrspond to the phase shift of 0 mm,11.25 mm and 15 mm, respectively.
~

1015..ci

The length of period is decided from the following
considerations. The space of 6.7-m length is available for
insertion devices in the DSR. The photon flux of morethan 10 15 photons/(s 0.1 %b.w.) has to be obtained using
the electron beam having the energy of more than 0.3
GeV and the gap width of more than 20 mm over the Xray energy range of 30 - 800 eV. Thus the length of theperiod was decided to be 3 em. In this case, the maximum
K value is 0.712 by assuming the magnetic field strengthof 1.3 Tat the pole tip of the Nd-Fe-B magnet. The X ray
having the energy of 30 - 800 eV can be produced bychanging the gap width from 20 to 26.7 mm and theelectron beam energy of0.3- 1.7 GeV.

~0~!

10"

co====...=..=··=1.. =..=·.=...=.. ====-=.. =·-·=..=·-=.. =·~~i~~-~ml~~;,];v1~~m2~W1-1.tm.~!~.1·_1·11~..,.J~>.

400

600

800

1000

X-Ray Energy (eV)Fig. 4 Photon flux for the linearly polarized X ray (a) andthe circularly polarized X ray (b).

3.3 Magnetic Field on the Beam Axis

The calculated magnetic fields are shown in Fig. 2,
and the relations between the gap width, the phase shift
and the field are shown in Fig. 3. The maximum
magnetic field is 0.245 T. For any gap width, thecircularly polarized X ray is obtained at the phase shift isaround 11 mm. To keep the polarization during the energy
scanning, both the gap width and the phase have to be
simultaneously changed as shown by an arrow in thefigure.

3.5 Mechanical Structure

Whole system of the undulator having the total lengthof 6:24 m is divided into two undulator units because it is
too ling to construct a 6-m undulator with high accuracy.
Both are exactry the same and they are coupled in tandemwith the distance of 60 mm. Figure 5 shows the front
view and the side view of the unit of the undulator. It-333-

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

consists of the magnet array, array holders, saport stands,
a vacuum tube, correction magnets, valves, and gap andphase driving system. In one unit, the length of themagnet array is 2400 mm and the number of periods is
80. Total weight is about 7.5 t per unit.
The vacuum tube inserted in between the magnets has
special form because not only the low emittance electron

beam but also the heavy ion beam are stored in the DSR
depending on the operation mode [4]. The tube has threerooms; those are for the low emittance electron beam, forthe heavy ion beam and for the NEG pump as shown inFig. 6. When the DSR is used as the heavy ion storage
ring, the tube is moved without breaking vacuum after
expanding the gap width to 300 mm.

3120
2400

Electron
Beam Line-Gear Box

II~Electron
Beam LinellFig. 5 The Apple- II undulator unit.

r------n'nm-~~===-==~""""
Room.1....:::::-,~:wlliffkiillttt{·

Electr~nBeam L1ne"""""
Fig. 6

Cross section of the vacuum tube inserted mbetween the magnets.

3.6 Gap and Phase DriverThe gap driving system consists of a servomotor, twodecelerator, a gear box, four linear guide, position sensors
and interlock system. The magnetic force acting on amagnet array is calculated to be about 1 t in the horizontal
direction, 0.2 t in the vertical direction and 0.8 t in thebeam direction. The accuracy of the gap driving can beachieved to be less than 5 J.lm that comes from therequirement of the minimum X-ray energy scanning stepof 0.1 e V. The driving system has a feed back systemfrom the position sensors, and it can keep the gap width
within 3 J..Lm. The phase driver has the same kind of thesystem.
References[1] Y. Yano et al., Proc. 5th European Particle
Accelerator Conf., p. 536 (1996).
[2] T. Katayama et al., Proc. 5th European Particle
Accelerator Conf., p. 563 (1996).
[3] M. Wakasugi et al., Proc. 5th European Particle
Accelerator Conf., p. 611 (1996).
[4] N. Inabe et al., Proc. 5th European Particle
Accelerator Conf., p. 926 (1996).
[5] S. Sasaki et al., Nucl. Instrum. Meth. A331, 763(1993).

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