II-8. Accelerator

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

R&D in AVF cyclotron
J. Ohnishi,∗1 A. Goto,∗2 and Y. Kotaka∗3
We aim to increase the beam intensity and energy
in the AVF cyclotron. In 2020, 7.25 MeV/nucleon Hebeams with intensity greater than 40 µA were stably supplied for RI-production experiments, and themaximum beam energy was successfully increased from14 MeV to 30 MeV for protons. This paper describes thesupply of a 30 MeV proton beam and the temperaturemonitoring system in the deﬂector septum for extracting
beams with an intensity greater than 1 kW.
Previously, the maximum energy of the AVF cyclotron
for protons was 14 MeV; however, in the summer of
2017, the new central region was installed to acceleratebeams with higher energies, for example, a 30 MeV proton (Harmonic H = 1 acceleration) and 14 MeV/nucleon
deuteron.1) The shape of the old and new central regions,orbits of 30 MeV protons, and the central orbit of normal H = 2 operation are shown in Fig. 1. In February
2020, an acceleration test was conducted because the RIproduction experiment using the 30 MeV proton beam
was scheduled. The results of this test showed that apart of the beam in the second turn hit the phase slit,
which was modiﬁed in 2019, and the shape of the slit wascorrected in a hurry. As a result, a 30 MeV proton beamwith a current of 10 µA was successfully accelerated andsupplied for the user’s experiment in June. The passing
eﬃciency in the AVF cyclotron was approximately 7%.
To prevent the deﬂector septum from melting due tobeam loss, a system was installed to monitor the temperature of the septum with thermocouples (TCs) andto stop the beam when an abnormal temperature riseis detected. Figure 2 shows a part of the deﬂector towhich the TCs are attached. The septum is made ofFig. 1. New and old central region of the AVF Cyclotron.
Hatched area indicates the old region. Beam orbits for
30 MeV protons and the central orbit of the normal H = 2
acceleration are also shown.
∗1
∗2
∗3

RIKEN Nishina Center
Institute of Materials Structure Science, KEK
Center for Nuclear Study, University of TokyoFig. 2. Tip of the deflector septum to which thermocouples
are attached.

Fig. 3. Time response of temperatures on the septum calculated by ansys.

copper, and it has a thickness of 0.3 mm and a heightof 12 mm. As shown in the ﬁgure, the septum has anotch with a length of 20 mm in the beam direction for
dispersing beam loss. The two TCs were attached 6 mmabove and below the tip of the septum notch. Figure 3
shows the time response of the temperatures at the tipof the septum notch (beam loss point) and the TC position calculated with ansys2) for a beam loss of 300 W(assuming a beam diameter of 2 mm). These calculation
results show that the maximum temperature of the septum exceeds the melting point of copper (1085◦ C) within
1 s with a beam loss of approximately 370 W. Further,
it is found that the temperature rise at the TC position
was as small as one tenth of the maximum temperatureand its response speed was slow. Therefore, we will stop
the beam before the septum melts when the temperaturerise for each 0.1 s measurement interval exceeds a predetermined value owing to any abnormal beam loss. The
system will be used in 2021 when the permission limiton the radiation protection for the 7.25 MeV/nucleon He
beams will be increased to 70 µA.
References

1) J. Ohnishi et al., RIKEN Accel. Prog. Rep. 51, 137 (2018).
2) https://www.ansys.com/ .

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