II-9. Instrumentation

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Development of Resonant-Extraction Charge Breeder (RECB)
R. Ogawara,∗1,∗2 S. Takagi,∗2 K. Tsukada,∗2 H. Tongu,∗2 Y. Kuriyama,∗2 and M. Wakasugi∗1,∗2
Potential oscillation ON (Non)Potential oscillation OFF(Noff)

Event rate [cps]

4030
20
10050

100

150

200

250300Frequency of potential oscillation ω [kHz]Fig. 2. Spetrum of extracted 12 C4+ ions.
12 4+C

2.5

Enhancement factor

In the RUNBA project,1) RI ions from ERIS2) are
accelerated in the storage ring (RUNBA) for nuclear reaction experiments. For eﬃcient acceleration, RI ionsshould be a highly charged ion beam. Although an EBIT
type charge breeder (CB) is widely used to increase
the charge state, an eﬃciency of only 20% hase beenachieved do far because of ﬁnite spread in charge statedistribution.3) We developed a prototype of a resonantextraction charge breeder (RECB) to improve the eﬃciency, the RECB can selectable extract only the desired
charge state ions.
In a RECB, a longitudinal electrostatic potential for
ion trapping is designed as a quadratic shape on an electron beam axis (Fig. 1). The longitudinal motion ofions in the potential is a simple harmonic oscillation forwhich the frequency depends on the mass-to-charge ratio. Thus, the ion motion is excited by adding an oscillation to the electrostatic potential at the resonant
frequency. Then, ions of a selected charge state are extracted from the trapping regions, and the others areleft in the RECB. When we apply a time-dependent potential Vtrap (z, t) = (a + b sin(ωt))z 2 (z, ω, and a andb represent the position, frequency of the potential oscillation, and constants, respectively), the ion motion is
described by Mathieu’s diﬀerential equation.
As shown in Fig. 1, RECB consists of an electron gun,a solenoid coil, an electron beam collector, and 20 electrodes that form the trapping potential Vtrap (z, t) usinga DC power supply and a function generator. In thetrapping region, the energy, current, and beam radius,of the electron beam were −32 keV, 10 mA, and 0.06 mm,respectively. Collection eﬃciency at the electron beamcollector was more than 99.5% when the solenoid magnetic ﬁeld was 0.13 T. Extracted ions with an energyof 10 keV/q from the RECB were separated by the analyzing magnet, and then, they were detected with achanneltron.

(Non-Noff) / Noff

2
1.5
10.5+

H

H2+12 2+ 16 2+

C

12 +CO14 +N2+ 16 +O

H2O++OH+ 19F+

CO0CH30510Mass-to-charge ratio (A/q)

1520Fig. 3. Enhancement factors of extracted of 12 C4+ ions.

We evaluate the performance of the potential oscillation by measuring the extracted residual gas ions (vacuum pressure = 5 × 10−6 Pa). Figure 2 shows an example of a 12 C4+ ion extraction where the depth of thequadratic shape potential is 150 V and the function generator supplies a 1.0 Vpp sine wave for a short durationof 0.5 ms with a repetition rate of 500 Hz. The red andblue plots indicate the spectrum of the extracted 12 C4+ions from the RECB with and without the potential
oscillation (Non and Noﬀ ), respectively. We conﬁrmedthat 12 C4+ ions were extracted at their fundamental frequency (90 kHz) and second order harmonic frequency(180 kHz). The spread of the spectrum was attributed
to a distortion of the electrostatic potential produced by
the space charge of trapped ions. Figure 3 shows the enhancement factor estimated by the event rate for (Non −Noﬀ ) divided by that for Noﬀ at a frequency of 90 kHz.
The enhancements for the 12 C4+ ions were 30, 42, and
9.0 times greater than those for the 12 C+ , 12 C2+ , and12 6+C ) ions, respectively. In future
H+2 (same A/q ofwork, we will optimize the electrostatic potential form
to improve charge state selectivity for extraction.
ReferencesFig. 1. The schematic diagram of the prototype RECB.
∗1∗2

RIKEN Nishina Center
Institute for Chemical Research, Kyoto University

1) M. Wakasugi et al., in this report.
2) T. Ohnishi et al., Nucl. Instrum. Methods Phys. Res. B,
317, 357 (2013).
3) F. Wenander, J. Instrum. 5, C10004 (2010).
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