RIKEN Accel. Prog. Rep. 48 (2015)

Ⅱ-9. Instrumentation

Current status of the gas cell ion beam cooler-buncher at SLOWRI
Y. Ito,∗1 F. Arai,∗1,∗2 I. Katayama,∗1 M. Reponen,∗1 P. Schury,∗1 T. Sonoda,∗1 M. Wada,∗1 and H. Wollnik∗1,∗3

Efficiency

1.0
0.8
0.6

K+
fRF = 9.3 MHz
VRF = 112 Vpp
fAF = 70 kHz(vion ~ 90 m/s)
1.0

1.5

2.0

2.5

3.0

3.5

PHe (mbar)

Epush (V/cm)
51015

4.0

4.5102

Acetylcellulose
d = 1.22-1.34 g/cm3101

152025

Incident Energy (keV)

3010152025Incident Energy (keV)

30Fig. 2. Ranges as a function of the incident energy of an
incoming ion beam for several elements with both SiNand Acetylcellulose membranes. The bands describe
the range straggling in ±1σ and the dashed lines indicate the minimum thicknesses for each material.

5.0Fig. 1. Transport and extraction efficiency as a function ofthe He gas pressure for several Epush values.

As shown in Fig. 1, at He gas pressures higher than
2 mbar, efficiencies of ≈100% were obtained for a wide∗3NaKRbCs

Si3N4

d = 3.44 g/cm3

100.2∗2Si3N4

101

0.4

∗1102

Range (nm)

1.2

0.0

range of push electric field strengths. For an even lower
gas pressure, such as 1 mbar, the efficiency slightly decreased; efficiencies of more than 60% were achieved
even with the highest push electric field strengths.
Compared to the efficiency of 22% seen with a finepitch RFC at 5 mbar, a rougher pitch can be expected
to make the GCCB highly efficient.
Another concern was the injection of a 30 keV beam
into the gas cell. The gas cell has no window to transport the low-energy ion beam, and thus all elements
can be injected into the gas cell. However the incoming beam might be scattered due to the leak gas aroundthe entrance. As a complementary idea, we are considering using nm-thick windows for the entrance. One
option is a SiN membrane and the other is a Acetylcellulose membrane. The minimum thicknesses of SiNand Acetylcellulose membranes are 10 nm with 5×5mm2 area and 10 nm with ∼10 mm diameter, respectively. The ranges for both windows were calculated by
the SRIM code as shown in Fig. 2. In both cases, theranges at 30 keV for the elements as heavy as Cs arelarger than the minimum thicknesses, indicating thations can penetrate the windows. Because the residual
ion energy after the window will be much lower than
the windowless case, the size of the gas cell can bemuch smaller, i.e., total length of <100 mm.

Range (nm)Conversion of a continuous ion beam to a pulsed
ion beam will be an essential process for various precision experiments at the SLOWRI facility1) . Continuous ion beams from SLOWRI gas cells can be stopped
and provided as low-emittance pulsed ion beams witha gas cell ion beam cooler-buncher (GCCB)2) . The
GCCB consists of a gas cell with an RF carpet (RFC),
an RF quadrupole ion guide, and a flat trap3) . Ion
beams of energy as high as 30 keV will be injected intoa low-pressure He gas cell, and then decelerated andthermalized solely by gas collisions. The ions will thenbe efficiently transported by the RFC and the RFQ ion
guide to the flat trap, and then bunched and cooled.
In this process, there was a concern regarding theperformance of the RF carpet at such unusually low
pressure – as low as 2 mbar. Simulations indicatedthat a rough-pitch RFC could transport ions more efficiently than a fine-pitch RFC4) . The transport andextraction efficiency was investigated experimentallyusing an RFC with a pitch of 0.32 mm and an exit holewith a diameter of 0.64 mm with K+ ions. Here thepitch was twice as large as the fine-pitch RFC used inhigh pressure gas cells. The experimental parameters
were optimized to achieve a high efficiency (describedin Fig. 1), which is defined as the ratio of the ion current leaving the exit hole measured by a Faraday cup
placed after the RFC to the ion current measured on
the RFC electrodes when the RF and AF voltages areturned off.

RIKEN Nishina Center
Department of Physics University of Tsukuba
Department of Chemistry and Biochemistry, New Mexico
State University

A performance test of these windows, i.e., a stresstest for the pressure difference and the heat cycle between the room temperature and cryogenic temperature, will be performed soon. Using a 30 keV ion beamof stable nuclei, the overall efficiency of the GCCB withand without a window will be evaluated in FY2015.
References
1) M. Wada et al., Nucl. Instr. Meth. B 204, 570 (2003)
2) Y. Ito et al., JPS Conf. Proc. 6 (2015) (in press)
3) Y. Ito et al., Nucl. Instr. Meth. B 317, 544 (2013)
4) Y. Ito et al., RIKEN Accl. Prog. Rep. 47, 205 (2013)- 226 -完全版2014_本文.indd 226

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