Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 51 (2018)

Present status of ERIS at the SCRIT electron scattering facility
T. Ohnishi,∗1 S. Ichikawa,∗1 and M. Wakasugi∗1
The Electron-beam-driven RI separator for SCRIT(ERIS)1) at the SCRIT electron scattering facility2) isan online isotope separator system used to produce lowenergy RI beams for electron scattering experiments ofunstable nuclei. Recently, ion stacking and pulse extraction were performed at ERIS in order to inject RI
beams into a dc-to-pulse converter named fringing-RFﬁeld activated dc-to-pulse converter (FRAC).3) The results were reported in Ref. 4). In the present year, wemodiﬁed the ionization chamber to increase the intensity of ion beams. In this paper, we report the results
and present status of ERIS.
Figure 1 shows a schematic of the new ionizationchamber of ERIS. The new ionization chamber consistsof a curved cathode, an ionization chamber (anode),and entrance and exit grids. The entrance grid is acurved tungsten-wire mesh. The wire diameter of themesh is 30 µm, and the mesh pitch is 0.85 mm. The
entrance and exit grids are connected to the ionizationchamber through an insulator. Ion stacking and extraction are controlled by switching the voltage of theexit grid. Neutral atoms enter the ionization chamber,passing through the cathode. They are ionized by collision with thermionic electrons emitted from the cathode, which is maintained at approximately 2000◦ C.
Though ionization occurs everywhere inside the ionization chamber, ions located near the exit hole aremainly extracted, because the potential is low enoughfor ion extraction only around the exit hole owing to
its small diameter of 2.5 mm. To increase the number of ions near the exit hole, the focusing of electron
beams on the exit hole is proposed using a new curved
cathode and a curved entrance grid, as shown in Fig. 1.
The bending of the cathode and entrance grid was determined based on the electron track simulation using
the SIMION code.5)
The properties of the new ionization chamber werestudied using 10-keV 132 Xe ion beams. The voltagesof the cathode and anode were set to 0 and 180 V, respectively. The exit-grid voltages at the stacking andextraction points were 185 and 0 V, respectively. The
entrance-grid voltage needed to prevent the escape ofions from the entrance grid was 200 V. This voltage ishigher than the voltage in the case of the non-curved entrance grid, 182 V,4) which indicates the large diﬀerenceof potential distribution between the curved and noncurved electrodes. Under the same condition for theanode current, ∼20 mA, the ion beam currents with thecurved and non-curved electrodes were approximately
2 and 6 nA, respectively. Pulse shapes measured at theexit of ERIS with the curved and non-curved electrodes∗1RIKEN Nishina Centerare shown in Fig. 2. The stacking time was 10 ms, andthe extraction period was 300 µs. These pulse shapes
are plotted as the ratio of the pulse beam current to thecontinuous beam current. With the curved electrodes,
it takes almost 1.5 ms to extract all of the stacked ions.
In the case of the non-curved electrodes, the stacked
ions are extracted within 0.5 ms. These results showthat the ionization with the curved electrodes occurs
mainly far from the exit hole contrary to the calculatedresults. More detailed calculation including the eﬀectof the space charge and other eﬀects is needed in orderto understand the obtained results.
In summary, we tested the new ionization chamberwith the curved electrodes at ERIS. The expected improvement of the ion beam intensity was not observed.
More detailed study and developments are in process.

Fig. 1. Schematic of the new ionization chamber of ERIS.

Fig. 2. Pulse shapes extracted from ERIS after the stacking. The pulse shape is plotted as the ratio of the pulse
beam current to the continuous beam current. The redand blue shapes are measured with the curved and noncurved electrodes, respectively.

References

1) T. Ohnishi et al., Nucl. Instrum. Methods Phys. Res. B
317, 357 (2013).
2) M. Wakasugi et al., Nucl. Instrum. Methods Phys. Res.
B 317, 668 (2013).
3) M. Wakasugi et al., in this report.
4) T. Ohnishi et al., Accel. Prog. Rep. 50, 192 (2017).
5) SIMION, http://simion.com/- 174 -