Ⅱ-8. Accelerator

RIKEN Accel. Prog. Rep. 51 (2018)

An innovative method for 12 C4+ suppression in 18 O6+ beam production
in an electron cyclotron resonance ion source†
H. Muto,∗1 Y. Ohshiro,∗2 Y. Kotaka,∗2 H. Yamaguchi,∗2 Y. Sakemi,∗2 M. Kase,∗1 S. Kubono,∗1,∗2and S. Shimoura∗1
It is a major and complex task to accelerate an ionthat has the same charge to mass ratio with strong contaminant ions, such as 12 C4+ in the 18 O6+ beam. An
innovative method has been developed to suppress thecontaminant ions in the Electron Cyclotron Resonance(ECR) ion source by introducing Li vapor. The ion distribution inside the ECR zone was obtained by the optical analysis of ions inside the ECR ion source. The
12 4+
C ions were suppressed as much as by a factor of10, whereas the 18 O6+ beam changed little with the useof this technique.
A 14 GHz Hyper-Electron Cyclotron Resonance(Hyper-ECR) ion source has been successfully used as aninjector for the RIKEN Azimuthal Varying Field (AVF)
cyclotron.1) A grating monochromator was installed at
the Hyper-ECR ion source, and the emitted light intensities of gaseous and metal ions were observed during
ion beam tuning.2,3) Recently, the 18 O6+ beam was produced at the Hyper-ECR ion source and injected into
the RIKEN AVF cyclotron. After the beam time wasstarted, the users noticed a signiﬁcant contamination of
the 18 O6+ beam with the 12 C4+ ions. The plasma chamber is not hydrocarbon free, and a counter-ﬂow of rotary
oil is also suspected for the presence of the carbon atoms.
During the beam tuning process, the charge distributionof ions extracted from ECR plasma was measured by amagnetic beam analyzer and a Faraday cup. However,the charge to mass ratio (q/m) of 18 O6+ is 0.33335, andthat of 12 C4+ is 0.33333. Even our high-resolution AVF
cyclotron (Δm/m =1/12000) is unable to separate these
two ions.
During the experiment a total beam current of theq/m =1/3 ranged from 30 to 50 μA. Although the 12 C4+beam current contaminating the main beam had to bemeasured, a direct measurement was impossible. Therefore, we measured the light intensity of the C IV line
spectrum during the beam experiment.4,5) The strengthof the light intensity measured by a photomultiplier anda digital voltmeter constantly showed 2.5 mV throughout the experiment. Figure 1 shows the light intensity of
the C IV line spectrum from the residual gas plasma asa function of the analyzed 12 C4+ beam intensity. From
this result, the 2.5 mV light intensity corresponded to
the 12 μA of 12 C4+ beam intensity. Therefore, the contamination ratio of the 12 C4+ ions in the total beam was24∼40%.
When an 7 Li2+ beam was produced and extracted
from the Hyper-ECR ion source, there was no C IV light†∗1∗2

Condensed from the article in Rev. Sci. Instrum. 89, 016103(2018).
RIKEN Nishina Center
Center for Nuclear Study, University of Tokyo

Fig. 1. Light intensity of C IV line spectrum as a function ofan analyzed 12 C4+ beam intensity.

Fig. 2. Time chart of 16 O6+ beam intensity and C IV lightintensity with 7 Li crucible in operation.

intensity present.
Then, we attempted to tune the 16 O6+ ion beam whilevaporizing a small quantity of Li atoms in the plasmachamber. Figure 2 shows a time chart of 16 O6+ ion beam
current and C IV light intensity under 16 O6+ ion beamtuning with Li evaporation.
By adjusting the distance between the Li crucible andplasma, the brightness of the Li II light intensity waskept at 1.1 mV. To prevent excessive Li crucible heating,
the C IV light intensity was kept at less than 0.9 mV,
and the beam current was controlled at 48 μA of 16 O6+ .
The 12 C4+ beam current became ∼1.0 μA when this C
IV brightness was converted into the beam current asshown in Fig. 1. The amount of impurities was drastically improved to around 2% from 40% by the Li ion
pumping eﬀect.
References
1) Y. Ohshiro et al., Rev. Sci. Instrum. 85, 02A912 (2014).
2) H. Muto et al., Rev. Sci. Instrum. 84, 073304 (2013).
3) H. Muto et al., Rev. Sci. Instrum. 85, 02A905 (2014).
4) H. Muto et al., Rev. Sci. Instrum. 85, 126107 (2014).
5) NIST Atomic Spectra Database, www.nist.gov/pml/
data/asd.cfm.

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