Ⅱ-8. Accelerator

RIKEN Accel. Prog. Rep. 50 (2017)

Mg ion pumping effect during 24Mg8+ ion beam production
at the Hyper-ECR ion source
H. Muto,䠆1 M. Kase,䠆1 K. Kobayashi,䠆2 M. Nishimura,䠆2 S. Kubono,䠆1Y. Ohshiro,䠆3 Y. Kotaka,䠆3 H. Yamaguchi,䠆3 Y. Sakemi,䠆3 and S. Shimoura䠆3A grating monochromator with a photomultiplier has
been used for beam tuning at the Center for Nuclear
Study Hyper-Electron Cyclotron Resonance (ECR) ion
source.1,2) The Hyper-ECR ion source has beensuccessfully used as an injector of the multi-charged ion
beams of high intensity for the RIKEN AVF cyclotron.3)
24
Mg8+ ions have been produced in the 14.2-GHz
Hyper-ECR ion source. At the beginning of the chamberbaking an RF power of 100 W was fed to the residual gasof the plasma chamber. The extraction voltage was set to10 kV. Then the vacuum gauge reading rapidly dropped
to less than 10-4 Pa from the order of 10-5 Pa and abrakedown of the high-voltage power supply occured
because of the huge drain current. A few hours later theextraction voltage was recovered and the vacuum gauge
reading was restored to the order of 10-5 Pa. The RFpower was gradually increased to 600 W until a goodvacuum condition (1~5 x 10-5 Pa) and low drain currentof less than 2 mA were obtained. After baking theplasma chamber an MgO rod was gradually inserted into
the chamber without excessive heat. The RF power
ranges from 500 to 600 W for 24Mg8+ ion production. A
grating monochromator (JASCO CT-25C) and aphotomultiplier (Photosensor module H11462-031,Hamamatsu Photonics) were used for light intensity
observation during chamber baking and beam
operation.3) The wavelengths of the observed lines were
determined in accordance with the NIST Atomic Spectra
Database.4) Figure 1 shows the optical line spectrum of
the Hyper-ECR ion source under plasma chamber baking.
This spectrum was taken three hours from baking start.
The vacuum gauge reading was 5.7 x 10-5 Pa. The draincurrent was 12 mA. The RF power was 100 W. In thisfigure most of the peaks were from Fe I and Fe II. The
relative intensities of those Fe I and Fe II peaks are quite
high. Figure 2 shows an optical line spectrum of the ECR
plasma during 24Mg8+ ion beam tuning. The vacuum
gauge reading was 1.7 x 10-5 Pa. The drain current was
1.8 mA. The RF power was 611 W. The line intensities
of Fe I and Fe II almost decreased to zero and Mg line
intensities appeared. Especially, the Mg VIII line
spectrum (O=279.64 nm) was clearly obtained to identify
the existence of 24Mg8+ ions in the ECR plasma. The ion
source was tuned for the production of the 24Mg8+ ions.
The H+ ions were also reduced during 24Mg8+ ion beam
operation. From these results the ion pumping effect ofMg ions was visually demonstrated by the optical grating
monochromator with a photomultiplier.
䠆1
䠆2
䠆3RIKEN Nishina Center
Sumitomo Accelerator Service (SAS)
CNS, University of Tokyo- 147 -Figure 1. Light intensity spectrum of the residual gas
ions during baking for three hours.

Figure 2. Optical line spectrum during 24Mg8+ ion beamtuning.
References
1) H. Muto et al., Rev. Sci. Instrum. 84, 073304 (2013).
2) H. Muto et al., Rev. Sci. Instrum. 85, 02A905 (2014).
3) H. Muto et al., Physics Procedia 66, 140 (2015).
4) www.nist.gov/pml/data/asd.cfm for NIST atomic
spectrum data base.