Ⅱ-8. Accelerator

RIKEN Accel. Prog. Rep. 51 (2018)

Installation of new central region for energy upgrade
at RIKEN AVF cyclotron
J. Ohnishi,∗1 A. Goto,∗1 and Y. Kotaka∗2x

Acceptance (pmmmrad)

300

S1
S2200

100

0-160 -120 -80-40

Acceptance (pmmmrad)

040 80Phase (deg)120 160 200

y300

S1S2200

1000-160 -120 -80-40

040 80Phase (deg)120 160 200

Fig. 1. Injection acceptance sizes for 12-MeV/nucleon
deuterons in two orthogonal directions.
40%

Transmission efficiency

We changed the structure of the central region of theAVF cyclotron to a new one in September 2017. The
new structure is the same as the one that was beamtested in August 2016, except that a part of the innerwall of its RF shield was scraped by hand so that theinﬂector can be rotated. In the beam test that was conducted in 2016, protons were successfully accelerated toan energy of 30 MeV with harmonic H = 1 acceleration mode. However, the structure was reverted to theprevious one because the transmission eﬃciency of 5MeV/nucleon 56 Fe15+ beam was 10% and worse thanhalf of the conventional eﬃciencies.1) Although the injection acceptance for the original (S1) and new (S2)structures had been calculated in 2016, more detailed
simulations including a buncher located at I36 (2.03 m
above the median plane of the cyclotron) were executed
again in 2017.
Figure 1 shows the injection acceptance sizes of 12MeV/nucleon deuterons for S1 and S2. The injection
acceptance is deﬁned as the phase-space size of the particles accelerated beyond a radius of 180 mm. The abscissa indicates the relative phase angle to the RF at thestarting point of the tracking calculation located justabove the buncher. Both the dee voltages were 40 kV.
The injection acceptances for S1 and S2 were almost thesame. The same applies to the other beams. Considering the results of the beam test that was conducted
in 2016, the extraction eﬃciencies for S2 may be worse
than those for S1. The reasons for this are not clear
because simulations for the extraction eﬃciencies were
not done suﬃciently due to the involvement of many adjustable parameters. On the other hand, in the machinetime of 9.1-MeV/nucleon 12 B4+ performed in July 2017,only half of the beam current of 300 particle nA that wasrequested by the experimenters could be supplied. Thiswas due to large beam losses in the ﬁrst turn for S1, because this beam needed high acceleration voltages. The
new structure S2 can reduce the injection beam losses
for beams that need high voltages. Therefore, from thesetwo points, we decided to install S2 again and to use it
in the machine time for the experiments.
Figure 2 shows a comparison of the transmission efﬁciencies before and after the installation of S2. The
transmission eﬃciencies are deﬁned as the ratio of
the beam currents of Faraday cups FC-I36 and FCC01 located at the injection and extraction beam linesof the cyclotron. The transmission eﬃciency of 9.1MeV/nucleon 12 B4+ beams increased four times and theextracted beam current reached 800 particle nA. Although the transmission eﬃciencies depend on the kind

30%
20%
10%
0%

d+
B4+
Kr20+ Ar11+
C4+
12
9.1
3.8
3.8
7.0
MeV/u MeV/u MeV/u MeV/u MeV/up12
MeV

Fe15+ Ne7+
55MeV/u MeV/uFig. 2. Transmission eﬃciencies of the AVF cyclotron afterand before the installation of the new structure S2. Filledbars indicate the transmission eﬃciencies after the installation and empty bars indicate those before the installation.

of beams and the time for operational adjustment of
the cyclotron, it seems that the transmission eﬃciencies for S2 are as good as those for S1 or much betterin some cases. Moreover, in the machine study after
the installation of S2, we succeeded in accelerating 14MeV/nucleon deuterons which could not be accelerated
before. However, while their injection eﬃciency was 40–
50% as high as other beams, the extraction eﬃciencies
were poor, approximately 10%, which is too low and cansupply only weak beams of 1–2 particle µA. Since the increase in the energy of deuterons are strongly requested
by experimenters, we plan to do beam simulations andmachine study to increase the extraction eﬃciencies for
14 MeV/nucleon deuterons.
Reference∗1∗2

RIKEN Nishina Center
Center for Nuclear Study, the University of Tokyo

1) J. Ohnishi et al., RIKEN Accel. Prog. Rep. 50, 145(2017).
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