Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 53

The 3rd report on oﬄine test for RF carpet transportation in RF ion
guide gas cell at the SLOWRI facility
A. Takamine,∗1 S. Iimura,∗1 M. Rosenbusch,∗2 M. Wada,∗2 S. Chen,∗3 J. Liu,∗3 P. Schury,∗2 T. Sonoda,∗1T. M. Kojima,∗1 I. Katayama,∗1 Y. X. Watanabe,∗2 H. Ueno,∗1 and H. Ishiyama∗1∗1∗2∗3

RIKEN Nishina Center
Wako Nuclear Science Center (WNSC), IPNS, KEK
Department of Physics, The University of Hong Kong

Cs 8.34 MHZ
Cs 5.75 MHz
Cs 4.80 MHzCs 2.79 MHz
Cs 1.75 MHz(a)Transport efficiency on 1st RFCP

1.210.8
0.6
0.4
0.20(b)Transport efficiency on 1st RFCP

A gas catcher cell using a radio-frequency (RF) ionguide method1) is being developed to provide ultra-slow
RI beams at the SLOWRI facility. We built a gutterstructure ion guide gas cell, which consists of two-stage
RF carpets (RFCPs).2) We previously used strip electrodes with a pitch of 0.32 mm (electrode width of0.16 mm and spacing of 0.16 mm) for the 1st RFCP. Thetransport eﬃciency was limited to ≈60% in 133 mbar ofHe gas and even lower in a higher pressure gas. We
replaced the 1st RFCP with a ﬁner pitch of 0.25 mm(electrode width of 0.1 mm and spacing of 0.15 mm)
to improve the performance. However, ﬁner-pitch RF
electrodes have a larger capacitance, which would practically limit the RF resonance frequency for the RFCP.
Therefore, we conducted an oﬄine test to investigate theRF transport performance on the new 1st RFCP at different RFs by changing the resonance coil of the resonant
circuit.
The oﬄine test was performed using surface-ionization
ion sources of Cs and K which were placed at the innerwall of the gas cell. The setup is shown in a ﬁgure inRef. 2). The ions produced from the ion sources were collected onto the 1st RFCP, transported on the 1st RFCP,
and collected onto the 2nd RFCP. We measured the ion
currents, I1 , on the 1st RFCP and those on the 2nd
RFCP, I2 , by a pA meter using each of the RFCPs as a
Faraday cup. The transported eﬃciency was deﬁned as
I2 /I1 . It should be noted that for the measurements ofI1 , the DC potential distribution was slightly diﬀerent
from that in the case of I2 because we could apply a DC
gradient on the on the 1st RFCP only for the measurements of I2 .
Figure 1 shows the test results in 133 mbar of Hegas for (a) Cs+ ions and (b) K+ ions at diﬀerent RFfrequencies. The drag DC ﬁeld on the 1st RFCP was
4 V/cm, and the extraction DC ﬁeld on 2nd RFCP was
20 V/cm. We achieved a higher transport eﬃciency compared with the previous RFCP,2) although we found thatit requires a correction to the eﬃciencies by a factor ofnot more than ∼0.9, which results from the DC-ﬁeld
distribution diﬀerence mentioned above. The results indicate that an RF frequency of ∼6 MHz was suﬃcientfor Cs+ (m/q = 133), although it did not saturate theK+ (m/q = 39) transport eﬃciency. According to theresults, we will operate the RF ion guide gas cell at 7–
8 MHz in the commissioning at the end of the ZeroDegree beam line in spring FY2020.
The plots clearly show that higher RF voltages are required to transport the lighter ions at a similar RF fre-050100150
RF voltages on 1st RFCP [Vpp]

1.2

200

K 8.05 MHz
K 5.75 MHz
K 3.49 MHz

10.80.6
0.4
0.20050100150
RF voltages on 1st RFCP [Vpp]200Fig. 1. RF voltage dependence of the RF transportation efficiencies for (a) Cs+ and (b) K+ .

quency, which can be straightforwardly interpreted fromthe expression of the RF repelling force.1) The plot (a)also shows that there exist RF voltages maximizing thetransport eﬃciency for each RF frequency. We observedsome discharge-like ion currents at a higher voltage thanthe eﬃciency peak points; consequently, the discharge
seemingly disturbed the ion transport. However thetransport eﬃciency would be diﬀerent depending on theposition in an RFCP stability diagram,3) deﬁned by the
RF and DC voltage values, which has a possibility toexplain this result. An attempt at quantitative interpretation is underway.
References

1) M. Wada et al., Nucl. Instrum. Methods Phys. Res. B
204, 570 (2003).
2) A. Takamine et al., RIKEN Accel. Prog. Rep. 52, 139 (2019).
3) S. Schwarz, Int. J. Mass Spectrom. 299, 71 (2011).

- 108 -