The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October2001

DEVELOPMENT OF MUSES ELECTRON COOLER AT RUffiN
M. Nishiura*, T. Tanabe, T. Katayama, RIKEN, 2-1 Hirosawa, Wako-shi, Saitama, 351-0198, Japan
E.Syresin, Joint Institute for Nuclear Research, Dubna, Moscow region 141890, Russia
I. Watanabe, Toshiba Corporation, 20-1 Kansei-Cho, Tsurumi, Yokohama, 230-0034, Japan

2 HARDWARE DESIGNS

Abstract
The radioisotope beam factory (RIBF) is underconstruction at RIKEN [1]. The RIBF has multi-use
experimental storage rings (MUSES) [2], which consistsof an accumulator-cooler ring (ACR) and an electron-RI
beam collider (e-RI Collider). The electron cooler (EC)device is equipped for the ACR. On the basis of theprevious ACR-EC, we refined the design of the electron
gun and the magnetic field strength with the help of thecomputer simulation codes.

1 INTRODUCTION
In order to cool RI beams in the ACR, the stochastic
cooler device (SC) is used in the first phase (it will takeabout 100 ms). After the pre-cooling by the SC, the RI
beams reach less than 0.1% in momentum spread and 0.1
1t mm-mrad in transverse emittance by the stochastic
cooling and the electron cooling.
The design of the electron cooler for the ACR has
recently been proposed and discussed [3]. Although thestrong magnetic expansion was considered to allow us tostudy the atomic and the molecular physics, it was found
not to be necessarily to use the high magnetic field of afew Tesla in a gun section [4]. Syresin et al. [5] reported
the electron beams cooled by the magnetic expansion
factor Bi I Bcool from 1 to 8 is sufficient to cool 92 U238coasting beam withE= 100 MeVIu, oplp = ±1.5 mrad,and Ev = 40 7t·mm·mrad. Here Bi and Bcooi are themagnetic field in a gun and in a cooling section,respectively. As shown in Table 1, the design parametersof the ACR-EC are changed; Bi I Bcool decreases 2.5 times
smaller than that of the previous design. When thefollowing relation k8 T.1. = (Bcool /Bi)l k8 T.1.i is used and theinitial transverse temperature T.1.i is equal to the cathodetemperature Tcath• k8 T.1. in the cooling section is roughly
estimated to increase from 5 meV(previous value) to 12
meV(this time).
In this paper we carried out the computer simulation byusing SAM [6] and EGUN [7], and refined the design ofthe electron gun and the magnetic field strength.

2.1 Electron Gun
The beam current of 4 A at the energy of 250 ke V isrequired for an electron gun. A tungsten cathode of 1.31
em diameter is used. The cathode spec is given as thecurrent density of 3.16 A/cm2 at the working temperatureof 1050 °C. As the results of endurance test for thecathode, the electron gun could be operated for more than2000 hours at the electron current of a few A under Bi =1.35 kG and Bcooi = 500 G, and it was found that thiscathode satisfies our desired ability.
.
T able 1 ACR -rmg_])arameters[4]
Circumference, m
Ion energy, MeVlu
Beam acceptance, Eac"p' 1t·mm·mm
Maximum momentum spread, %Emittance of inj_ected beam,Ein, 7t·mm·mm
Initial momentum spread, %Beta function in cooler (hor./vert.), m
Length ofEC solenoid, m
M~netic field in cooliJ:!g section, kG
Magnetic field in gun section, kG
M~etic expansion factor
Electron beam current, A
Electron energy, keY

134.788
60-400125140±15140±0.15
4114.34
3.6
0.5-2
2-4
1-8430-250

2.2 Comparisons between SAM and EGUN
codes
An electron gun proposed for the ACR-EC consists ofthree electrodes: a flat cathode, a Pierce electrode, and an
anode. The dispenser cathode is 2.75 em diameter. The
cathode-anode gap is biased at 30 kV. The electron beam
will be finally accelerated up to 220 ke V by anacceleration tube (National Electrostatics Co.), andintroduced into the cooling section through a toroid
section. For simplicity, the toroid section is ignored forour simulation.
In order to design the electron gun for the ACR-EC, thecomputer simulations have been carried out by using
SAM and EGUN codes. The input parameters are shownin Table 2.

*mn@postman.riken.go.jp-103-

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

T able 2 Inputparameters t;or EGUN
Electron beam current, A

4Electron energy, keY

220

Cathode-anode voltage, kVCathode diameter, em30
2.57

Magnetic field in gun section, kG

4Magnetic field in cooling section, kG

0.5-2

2.3 Design Issue ofAnodeThe magnetic field on the beam axis is calculated by
MATHEMATICA® [8] in r - z coordinate. The solenoid
field must decrease gradually from the e-gun to thecooling section within the adiabatic expansion. The
adiabatic parameter used here is less than 4x 10-2 under
the condition of Bi I Bcoot = 4kGI0.5kG.
Figure 1 shows the electron beam trajectories.

The electron gun mentioned above is designed to keep ahigh vacuum and to prevent the beams from hitting theanode. It is desirable that electron beams for an EC havea low transverse temperature. Since the initial velocity ofelectrons emitted from the cathode dominates the final
transverse temperature, the electric field in front of thecathode should be parallel to the cathode surface.
The conductance of the anode A2 (133 sec- 1) becomes
the same level as that of the At (I12 sec- 1) to reach ultrahigh vacuum of less than 1o-s Pa. The beam edge keeps
the clearance of 8 mm from the A2. The electron beam
current reaches 5A at the anode voltage of 30 kV.

Fig. 3 Cross sectional view of previous anode shape (At)and modified anode shape (A2).

z[mm]Fig. 1 The electron beam trajectories from the electron
gun to the cooling section, which are calculated by EGUN
code in the case of Bi I Bcool = 4kGilkG.
Figure 2 shows the dependence of the transverse
temperature Etra on Bcool calculated by SAM and EGUN.
The transverse temperature by SAM tends to be lower
than by EGUN at the beam edge. In both cases Etra
becomes less than I e V at the beam edge, and less than
100 meV at r = 5.436 mm.

Two configurations for anode shapes are comparedusing the EGUN code. Figure 4 shows the dependenceof the transverse temperature in the cases of At and A2.
The value of Etra at the beam edge dropped from I eV to100 meV; At Bcool = 2 kG, the transverse temperature at
the beam edge is lower than that at 5.436 mm. In our case,the cathode temperature is equal to or more than thetransverse temperature. When Tcath 0.1 e V and Bi I Bcool
I, the transverse temperature would be less than 200meV. From the beam trajectory, the electron beamincreases the radius of 35 mm under the condition of Bi I
Bcool _8 by the expansion and the drift. This spec of theelectron gun is acceptable for our purpose.

===1000
100.s~

101000;-..........

100centerJ~0.1

r=0.055mm

I!!

0.01
0.5

1.0

1.5

2.05.436mm.s

uf
0.0

10

r=0.055mm

0.1

2.5
0.01

Bcool (kG]

0.001

Fig. 2 Transverse electron energy in the cooling section
calculated by SAM (dotted lines) and EGUN (solid lines).
The value of r indicates the radial beam position at thedispenser cathode.

0.0

0.51.01.5

2.0

2.5

Bcool (kG]

Fig. 4 Dependence of the transverse temperature on themagnetic field in the cooling section. Open and closed
circles indicate the cases of AI andA2, respectively.

-104-The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001•

2.4 Collector Section
Tanabe et al. have already proposed the collector designfor the ACR-EC in Ref. [9]. The same design is used forthe beam trajectory calculation, as shown in Fig. 8. The
initial beam parameters are taken from the final beam
parameters calculated above in the case of Bi I Bcool =4kG/(2 - 0.5kG). When the expansion factor is higher,
the beam radius is very close to the deceleration tube
radius. In order to avoid beams hitting the wall and theelectrodes, the focus lens should be placed before thebeams passing through the deceleration tube.

3D simulation for electron beams and magnetic
fields by MAFIA code [ 10]. (The preliminary
result of magnetic field is shown in Fig. 6.)

453

-699

CST

Fig. 6 Profile of the magnetic field near the electron gun
calculated by the MAFIA code. The red curve shows themagnetic field on the z-axis in the z component. Themagnetic shield is drawn in green.

5 REFERENCES
Fig. 5 Electron beam trajectories for Bgun
kG, Bcol = 2 kG.

=4 kG, Bcool =2

3 FUTURE PLAN
The present status of the ACR-EC has been presented inthis paper. In the near future, we will accomplish thefollowing plans:•Development of a precise magnetic fieldmeasurement system (resolution of 10"5) ofsolenoids from the e-gun to the collector.

[1] Y. Yano et al., the proc. of Cyclotrons 2001.
[2] T. Katayama et al., p.529, the proc. ofEPAC'98[3] T. Tanabe et al., NIM A441, 104(2000).
[4] T. Tanabe et al., the proc. of Cooling workshop 2001,Germany.
[5] E. Syresin et al., the proc. of Cooling workshop 2001,Germany.
[6] M. Tiunov, B. Fomel, and V. Yakovlev, Preprint INP,
Novosibirsk, 85-159. ·[7] W. H. Herrmannsfeldt, SLAC-226, 1979.
[8] Radia(ESRF), http://www.esrf.fr/[9] T. Tanabe et al., the proc. ofPAC'99.
[10] MAFIA(CST), http://www.cst.de/-105-