The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

Preliminary Design of a New Central Region for the RIKEN AVF Cyclotron
Jong-Won KIM and Akira GOTO
The Institute of Physical and Chemical Research (RIKEN)
2-1, Hirosawa, Wako-shi, Saitama 351-01, Japan

Abstract
The central region of the AVF cyclotron at RIKEN
has been newly designed for operation with higher injection voltages in the future. This upgrade plan is aimed
to attain higher beam intensities in a stand-alone modeof the cyclotron as requested by nuclear science users.
The design is preliminary, but main design requirements
seem to be met by the current work.

1 Introduction
The AVF cyclotron at RIKEN has been used as aninjector to the RRC and also in a stand-alone mode foratomic physics experiments [1]. Beams extracted from10 GHz ECR source are axially injected, and then inflected onto the median plane by means of spiral inflector. Detailed design parameters and operation status of
the cyclotron can be found such as in refs. 2,3.
Increase of the injection voltage for instance from
10 kV to 20 kV for a beam of N5+ has been conceived
to achieve higher beam intensities. The higher voltage
will enable extraction of higher beam currents from thesource according to the well-known Child law, and also
help reduce space charge effects during the beam transport along the injection line. A first step toward thisupgrade is to redesign the inflector and central region.
15.2 +--___.J__ _c____L______.J__ _c____ L __For acceleration of other nuclei, a constant orbit
mode can be applied in operation with the injection voltages varied according to the ratios of ~B5.

2 Electromagnetic fieldsThe electric fields of the central region were calculated using RELAX3D for 3-d electrode geometries. The
horizontal meshes were sized to be 1 mm, and the gridwas 201 x 201 x 21. The magnetic field employed, on
the other hand, is a measured data for the N5+ beam.
Figure 1 shows azimuthally averaged distributions ofmeasured and isochronized fields. The central corn field
extends to 20 em to enhance the axial focusing.

___L_-+

15.0
14.8

8

14.6

6

14.4

Measuredm14.2

Isochronous

14.0
13.8
13.6 + - - - - - , - - , - - - , - - - - - - - , - - - , - - - - , - - - - - r - - - 1 03010402050706080
R (em)

Fig. 1 Radial distributions of measured and isochronized magnetic fields which are azimuthally averaged for a beam of N5+ accelerating to 7MeV/u.
In the original design of the central region, the electric fields were calculated assuming analytic distributions of the fields [2]. Thus practical effects such as
field penetrations into the dees were not fully considered. In the new design the fields were calculated witha relaxation code RELAX3D [4]. First, to make orbit
computations for comparison with previous results, theelectric fields were computed for the existent geometry.
The design ion is N5+ accelerating to 7 MeV /u for whichmeasured magnetic fields are available.

Fig. 2 Orbit trajectory of a N5+ ray starting at To=350 with an injection voltage of 10 kV, which isdrawn on the equipotential map of the original
central region.

3 Central region design
To calculate the orbits the code Z3CYCLONE [5]was used. The code utilizes the magnetic fields on themedian plane, and a linear expansion in z for the horizontal fields to calculate verical motion.
Orbit trajectories have been first computed for thepresent central region to compare with results already-418-The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

available. In Fig. 2 is shown a trajectory on equipotentials for the N5+ beam which is injected at r= -35° atthe exit of the inflector (rf voltage is in sine wave). This
result agrees with that of the original design.
When the injection voltage is doubled, the magnetic
bending radius of the injected beam becomes larger by
the square root of two. The new geometry was foundwith trial and error on the basis of the original geometry.
One restriction was that the radial location of the beam
exiting from the inflector is limited by 3 em radius of theexisting center hole. It turned out that major modifications are needed in the location of injection beam andin the azimuths of extraction electrodes. The centeringerror which is a main design consideration is tried tobe maintained less than ±1 mm. The orbit trajectories
traced with a beam emittance of 250 1r mm·mrad areplotted in Fig. 3.

2+---------l_________l_________--+

L __ _ _ _ _ _Z0 =3 mm, P, 0 =0 mrod
Z0 =0 mm, P, 0 =30 mrod

E' o+-L_--~~~~~--~~--~r-~~~~~~~
N-1-2+---------,---------,---------,--------+
0510
Turn Number

1520Fig. 4 Vertical motions for two different initial conditions.
4 Current status

A preliminary design of the central region was carried out, which can accept the beams injected withhigher voltages. This is a first step to improve the beam
currents extracted from the RIKEN AVF cyclotron.
Works will continue as the project is initiated in thefuture.
References[1] Y. Yano, "Recent achievements at the RIKEN ringcyclotron", Proc. of 13th Int'l Conf. on Cyclotrons
and their applications (1992) 102.
[2] A. Goto et al., "Study of beam optics for the injector
AVF cyclotron at RIKEN", Proc. of 12th Int'l Con£.
on Cyclotrons and their applications (1989) 439.
[3] A. Goto et al., "Injector AVF cyclotron for RIKEN
ring cyclotron", RIKEN Accel. Prog. Rep. 21 (1987)
210.
[4] C.J. Kost and F.W. Jones, "RELAX3D User's guideand reference manual", TRI-CD-88-01 (1988).
[5] H. Blosser, "Optimization of the cyclotron central
region for the nuclear physics user". Proc. of 5th Int'l
Conf. on Cyclotrons and their applications (1971)
257.
[6] D. Rifuggiato et al., "Axial injection in the LNS superconducting cyclotron", Proc. of 15th Int'l Conf.
on Cyclotrons and their applications (1998) 646.
Fig. 3 Orbit trajectories on the newly designed central region for a beam with emittance of 250 1rmm·mrad.
The initial conditions of injected beams were attained using analytic equations of motion in a spiral inflector. But studies have indicated that orbit trajectories
are slightly different from the results which involve more
realistic inflector electric fields (6]. The present design
did not consider this effect, but it appears that the strayfield effect can be readily handled.
The vertical motion has been computed also withZ3CYCLONE, and trajectories are plotted in Fig. 4 for
two linearly independent initial conditions. The verticalacceptance is expected to be about the same as in thepresent design.

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