RIKEN Accel. Prog. Rep. 50 (2017)

Ⅱ-9. Instrumentation

Ion stacking and pulse extraction at ERIS
T. Ohnishi,∗1 S. Ichikawa,∗1 M. Togasaki,∗1,∗2 K. Yamada,∗1,∗2 K. Kurita,∗1,∗2 M. Watanabe,∗1and M. Wakasugi∗1
The ERIS (electron-beam-driven RI separator forSCRIT)1) at the SCRIT electron scattering facility2)
is an online isotope separator system for the electron
scattering of unstable nuclei. Ion beams from theERIS are transported to the FRAC (fringing-RF-ﬁeldactivated ion beam compressor).3) The FRAC realizes
the continuous injection followed by trapping, and after the appropriate accumulation time, pulsed beams
from the FRAC are injected to the SCRIT system.
During the accumulation inside the FRAC, some of
the trapped ions escape through the entrance because
the entrance potential is lower than the beam energy.
In order to reduce the ratio of escaped ions to injected
ions, it is necessary to shorten the opening period of
the entrance and inject same number of ions as in thecontinuous injection. Ion stacking and pulse extraction4) are proposed, and we report the results obtainedat the ERIS in this paper.
Figure 1 shows a schematic drawing of the newionization chamber of the ERIS. The new ionizationchamber consists of a cathode, an ionization chamber(anode), and entrance and exit grids. The entrance
and exit grids are connected to the ionization chamber
through an insulator. The schematic potential structure of these electrodes is also shown in Fig. 1. The
ion stacking and extraction are controlled by switching
the voltage of the exit grid. Neutral atoms continuously enter the ionization chamber, passing throughthe cathode. They are ionized by electrons emitted from the surface of the cathode, which is kept
at approximately 2000 ◦ C. In the longitudinal direction, ions are trapped between the entrance and exitgrids. The number of stacked ions inside the ionization chamber is determined by the ionization rate andion-trapping lifetime.
The properties of the ion stacking and pulse extraction at the ERIS were studied using 6-keV 132 Xe ionbeams. The voltages of the cathode, anode, and entrance grid were set to 0, 180, and 182 V, respectively.
The exit-grid voltages at the stacking and extraction
were 182 and 40 V, respectively. The typical pulse
shape, measured at the entrance of the FRAC witha 1-ms stacking time and a 300-μs extraction period,is shown in Fig. 2(a). The pulse height is about ﬁvetimes larger than that of the continuous beam. Figure 2(b) shows the stacking ratio, which is the ratio of
the total charge within a 300-μs pulse width to the total charge obtained by integrating the continuous beamover the stacking time and extraction period. This re∗1
∗2

RIKEN Nishina Center
Department of Physics, Rikkyo University- 192 -sults shows that a pulsed beam with the same numberof ions as those of the continuous injection is provided
with a 1-ms stacking time and 300-μs pulse width. Using this scheme, the number of ions accumulated inside
the FRAC is 2–3 times larger than when using continuous injection.5)
The decay time of the stacking ratio corresponds
to the ion-trapping lifetime, which is estimated to be
4 ms. In order to extend the ion-trapping lifetime, atransverse potential like an RFQ potential is needed.
In addition, in Fig. 2(b), the stacking ratio exceeds
one when the stacking time is shorter than 1 ms. Thisis believed to be because the extraction eﬃciency is
diﬀerent from that in the continuous extraction. One
possible reason for this is that the potential distribution inside the ionization chamber is modiﬁed by thecharge of the stacked ions.
In summary, we performed ion stacking and pulse
extraction at the ERIS. The obtained accumulationeﬃciency at FRAC is still half of the target value5) .
More detailed study and developments are in process.

Fig. 1. Schematic drawing of the new ionization chamberof the ERIS. The schematic potential structure is alsoshown.

Fig. 2. (a) Pulse shape extracted from the ERIS after thestacking. (b) Stacking time dependence of the stacking
ratio. Details are given in the main text.

References
1) T. Ohnishi et al., Nucl. Instr. Meth. B317, 357 (2013).
2) M. Wakasugi et al., Nucl. Instr. Meth. B317, 668(2013).
3) T. Togasaki et al., Proceedings of HIAT2015, WEPB25(2015).
4) Y. Shirakabe et al., Nucl. Instr. Meth. A337, 11 (1993).
5) K. Yamada et al.: in this report.