Ⅱ-8. Accelerator

RIKEN Accel. Prog. Rep. 50 (2017)

Beam test using new central region for energy upgrade at RIKEN AVFcyclotron†
J. Ohnishi,䠆1 A. Goto,䠆1 and Y. Kotaka䠆2The structure of the central region of the AVF cyclotron
was renovated in 2009 (S1), and the available energy for
M/Q = 2 ions was increased from 9 MeV/u to 12 MeV/u. 1)
After this modification, in order to further increase thebeam energy of protons to 30 MeV, another structure (S2)which enables an acceleration with a harmonic number (h)of 1 instead of 2 was designed.2) Figure 1 shows thesuperimposed plan views of the structures S1 and S2. Instructure S2, the RF shield is smaller than that of S1, thesecond acceleration gap was moved downstream and theoutside wall at the exit of the second gap was removed.
Figure 2 shows the acceleration performance of the AVF
cyclotron for the three structures of the central region. The
acceleration of the 20±30 MeV proton uses the harmonic
number h = 1.
We replaced the central region S1 with S2 and performed
beam acceleration tests in August 2016. In addition to theacceleration test with h = 1, we investigated the influenceon the transmission efficiencies through the cyclotron in theh = 2 operation. Table 1 presents a summary of theacceleration tests. A total of seven beams were accelerated.
As the scheduled time for study was limited, the mean beamtuning time for the acceleration of one beam wasapproximately 4 h. In Table 1, I36 and C01 indicate thebeam currents at Faraday cups FC-I36 and FC-C01 locatedat the injection and extraction beam lines of the AVF
cyclotron. In the h = 1 operation, while 20-MeV protonswere successfully accelerated with a beam current of 10 PA,
the beam current for 30-MeV protons was as small as 1.1
PA because the dee voltage was insufficient owing to thedeterioration of the RF cavities. In the h = 2 operation, theextracted beam currents at the FC-C01 for 14-MeV protons,
12-MeV/u deuterons and 22Ne7+ were almost the same as
those in the S1 configuration. However, the transmission

Fig. 2. Acceleration performance of the AVF cyclotron.
The yellow, blue, and green areas correspond to theoriginal (before 2009), existing (S1), and currentlytested (S2) geometries, respectively.
Table 1. Summary of acceleration test.
IonEnergyp14 MeV

RF frequency(MHz)23H2+

12 MeV/uHarmonics

I36 (PA)

C01 (PA)

290

14.3

21.25

257

9.6
7.5

22Ne7+

6.0 MeV/u

15.05

242

22Ne5+

4.0 MeV/u

12.3

2122p

20 MeV

13.6

176

10p30 MeV

16.5

1116

1.1

Fe15+

5 MeV/u

13.8

2

1.2

0.12

56

efficiency for 22Ne5+, which simulates 84Kr20+, wasapproximately 60% of the peak performance to date. Thetransmission efficiency for 56Fe15+ beams was only 10%,
while its efficiency has so far greater than 20% or
occasionally 30%. It can be concluded that the transmission
efficiencies in the h = 2 operations with S2 were rather poor
compared to those with S1, although the time for machine
study was not sufficient. Therefore, we decided to revert thetested structure, S2, to the existing one, S1. At present, weare attempting to clarify the reason why the transmission
efficiency was low and search for a new geometry thatrealizes higher transmission efficiencies for both h = 1 andh = 2 operations through a beam simulation study.
References
1) A. Goto et al., RIKEN Accel. Prog. Rep. 43, 127 (2010).
2) S. B. Vorozhtsov et al., RIKEN Accel. Prog. Rep. 44, 105(2011).

Fig. 1. Existing and tested geometries of the central
region. The shaded area indicates the existinggeometry.
†Condensed from the article in Proceedings of Cyclotrons 2016, Zurich,
Switzerland, THP13
1

RIKEN Nishina Center

2

Center for Nuclear Study, the University of Tokyo- 145 -