Ⅱ-8. Accelerator

RIKEN Accel. Prog. Rep. 49 (2016)

Lithiumion
ion production
production for
Lithiumfor laser
laser ion
ion sourcesource††
M. Okamura,*1,*2 K. Palm,*3 C. Stifler,*4 D. Steski,*1 S. Ikeda,*5,*2 M. Kumaki,*6,*2 and T. Kanesue*1
The use of laser ion source (LIS) is an effective andsimple method for creating a wide variety of ions. However,this method has not yet been extensively tested on alkali
metal targets owing to their high reactivity. In this report,
we investigated the Li plasma properties in laser ablation
for the future application of alkali metal beam acceleration
because Li ion beams can be used for neutron beam
creation1).
Li oxidizes rapidly when exposed to the atmosphere. To
limit the exposure of the targets, special precautions weretaken when producing the targets. The metals were polished,rolled to an appropriate thickness using a rolling mill, andimmediately stored in a portable vacuum chamber. Whenthe targets were installed, argon gas flow was employed toreduce the exposure to oxygen. Then, the target chamberwas immediately pumped down to vacuum. The attempted
target thickness was 1.0 mm, although it was not easy toachieve good accuracy. The targets were irradiated with aneodymium-doped yttrium aluminum garnet (Nd:YAG)
laser operating at 1.4 J, with a wavelength of 1064 nm and apulse length of 6 ns. The laser was focused on the target
with a plano-convex focusing lens with a focal length of
100 mm. The distance of the lens from the target was
adjustable from outside of the target vacuum chamber, thus
allowing the beam to be focused and defocused. Theincident angle between the laser path and the beam line was20°. The target chamber and subsequent beam line were
held below 1.8×10-4 Pa for the duration of the experiment.
A Faraday cup (FC) with a ĳ = 10 mm aperture was placed
2.4 m away from the targets to measure the beam current.
The suppressor voltage of the FC was set to í3.5 kV. The
positively charged particles in the plasma were analyzed
using an electrostatic ion analyzer (EIA). The selected ionswere detected by a secondary electron multiplier (SEM).
The total distance between the detector and target was 3.7m. The ion species and charge states were determined by
the time of flight information in the SEM signal and theapplied voltage to the EIA.
In the experiment, we detected H, Li, and O ions.
Unfortunately, the apparatus cannot distinguish ions ofdifferent species but which have the same mass to charge
ratio. Therefore, some detected signals may be contributedby different species. The entire view of the experimental
setup was explained in our previous publication2).
†Condensed from the article in Rev. Sci. Instrum. 87, 02A901 (2016)
Collider-Accelerator Department, BNL*2
RIKEN Nishina Center*3
Department of Physics, Cornell University*4Engineering Physics Systems Department, Providence College*5Interdisciplinary Graduate School of Science and Engineering, Tokyo
Institute of Technology*6
Research Institute for Science and Engineering, Waseda University*1- 152 -A focused laser beam with power density of the order of1012 W/cm2 was used to create high-charge-state Li beams.
A fresh laser spot was always supplied in the motorizedstage for every laser shot. The analysis showed oxygencharge states ranging from O1+ to O8+ as well as 7Li3+, 7Li2+,7 1+
Li , and 6Li3+. The beam composition data can be seen inFig. 1.

Fig. 1. Species and charge state distribution in the plasma
from the Li target. The charges were counted at 1.0 m fromthe target with a 1.0 cm2 sensing area.
The LIS could create high-charge-state ions from the Litarget. There was an apparent oxygen contamination in thebeams. We also found that the Li target was clearly oxidizedin the vacuum chamber held below 1.8 x 10-4 Pa. To adopt alaser ion source with a Li target for industrial applications,oxidization and the time-consuming target preparation need
to be overcome.
References

1) E. Norbeck, Jr. and C. S. Littlejohn, Phys. Rev. 108, 3 (1957).
2) S. Ikeda et al., Rev. Sci. Instrum. 85, 02B913 (2014).
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