JAEA-Review-2010-065.pdf:15.99MB - 日本原子力研究開発機構

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Status Report on Technical Developments of the
AVF Cyclotron
S. Okumura, T. Yuyama, Y. Yuri, T. Ishizaka, I. Ishibori, H. Kashiwagi, S. Kurashima,
N. Miyawaki, K. Yoshida, T. Nara and W. Yokota
Department of Advanced Radiation Technology, TARRI, JAEA
Uniform Irradiation of Ion Beams by Means of a
Nonlinear Focusing Method
The raster scanning method is widely used for large-area
uniform irradiation of ion beams at the TIARA cyclotron
facility. However, some practical problems can arise
because of its irradiation principle that a focused spot beam
is periodically swept; e.g., the local target heating is induced
by irradiation of a high-current beam, a transient
phenomenon of an irradiation sample cannot be measured in
real time, short-time or low-fluence irradiation of high
uniformity is difficult, etc. Therefore, a multipole-magnet
beam profile uniformization system (MuPUS) based on the
nonlinear focusing method, has been developed as an
alternative to the raster scanning method 1-2) . With this
method, the beam profile can be made uniform by the
nonlinear forces of multipole magnets. It is, thus, possible
to irradiate a large-area sample or a number of samples
simultaneously and continuously at a constant particle
fluence rate.
We are developing a measurement system of a large-area
uniform beam using beam-induced fluorescence in order to
evaluate beam characteristics such as the beam size and the
uniformity in real time. It is possible to observe the
uniform irradiation profile of the beam in real time since the
beam spot is stationary on the target, unlike the case of the
scanning method where the beam spot is moving fast on the
target. We have, thus, adopted some of Tb-doped Gd 2O 2S
screens called DRZ (Mitsubishi Chemical) as a fluorescent
screen. They are suitable for real-time tuning of a
low-current-density beam since the optical decay time of
light emission by beam irradiation is much shorter and the
sensitivity to ion beams is higher, as compared with an
ordinary Al 2O 3 screen. The fluorescence signal from the
screen mounted on a target stage was detected by a CCD
camera and converted to the brightness data with a
LabVIEW (National Instruments) system on a Windows
computer. The response of the brightness to the beam
current density, i.e., the fluence rate of the beam was
explored. It is possible to evaluate the uniformity of the
beam intensity distribution in an arbitrary region. To
monitor the uniformity of a large-area beam (typically,
several tens of square centimeters) precisely, the location of
the camera was also optimized.
A 6 cm 6 cm uniform beam of 10-MeV proton was
applied to the test irradiation of space-use solar cells as the
nonlinear focusing method can realize a uniform irradiation
field at a constant fluence rate, closer to the actual space
environment. The irradiation time has been shortened to a
JAEA-Review 2010-065
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fraction of that of raster scanning. We have demonstrated
the high potential of this method. To realize the
uniform-beam formation of heavy ions, R&D studies have
been started.
Quick Change of the Cyclotron Magnetic Field for
Reduction of Beam Changing Time
Reduction of the beam changing time is required to
increase the available beam time for users. We are
developing a technique for reducing the changing time of
the magnetic field of the cyclotron, since a long changing
time is normally required to form the magnetic field for
acceleration. The most time-consuming process is a
cycling of magnetic field strength, which is a start-up
process of about 30 minutes for ensuring stability and
reproducibility of the magnetic field. Since the settling
time of the magnetic field without the cycling process is
longer, as shown in Fig. 1, and the reproducibility is
insufficient, we have started the development of the brief
start-up process, which replaces the cycling process.
References
1) Y. Yuri et al., Phys. Rev. ST Accel. Beams 10 (2007)
104001.
2) Y. Yuri et al., Proc. Euro. Particle Accel. Conf.,
EPAC’08 (2008) 3077.
dB/B
2x10 -4
0
-2x10 -4
-4x10 -4
-6x10 -4
-8x10 -4
-1x10 -3
with cycling
without cycling
0 5 10 15 20
Time (min)
Fig. 1 Changes of the magnetic field strength (B) of the
cyclotron just after the start-up process for 10 MeV
proton beam acceleration.