Ⅱ-8. Accelerator

RIKEN Accel. Prog. Rep. 47 (2014)

Analyses of the plasma generated by laser irradiation on sputtered target for
determination of the target thickness used for plasma generation†
M. Kumaki,*1,*2 Y. Fuwa,*1,*3 S. Ikeda,*1,*4 M. Sekine,*1,*4 N.Munemoto,*1,*4D. Cinquegrani,*5 T. Kanesue,*6 M. Okamura,*1, *6 and M. Washio*2
A Laser Ion Source (LIS) has been developed at
Brookhaven National Laboratory (BNL) 1). A focusedhigh-power laser is used to generate plasmas containing
highly charged ions from solid targets. For every laser shot,
we provide a new surface because the irradiation creates acrater, and the second irradiation on the same spot causes
beam instability. However, the depth of the target required
to generate the plasma is not yet clear. We assumed that
only the surface layers of the material were converted to theplasma, and knowledge of the surface layers to be converted
to the plasma is necessary to understand the initialprocesses of laser-ablation plasma creation. We prepared acarbon-coated aluminum plate as a target. By analyzing thecontents of the ablation plasma, the effective depth required
to generate the laser plasma was investigated.
The target surface was divided into four segments, andeach segment has different carbon coating thickness; thethicknesses were about 25 nm, 125 nm, 250 nm, and 500nm. To generate ablation plasma, the segmented target withmulti thickness coating was irradiated by a Nd:YAG 1064
nm focused laser (Brilliant Quantel, Energy: 728 ± 5 mJ(rms); Pulse Width: 6 ns). We analyzed the generatedplasma using an electrostatic ion analyzer (EIA) and asecondary electron multiplier (SEM) to measure the current
distribution of ions of each charge state 2).
Figures 1 and 2 show the charge-state distribution of
measured ions of the un-coated aluminum target and 500
nm carbon-sputtered target, respectively. These current
distributions were reconstructed from the signals obtainedusing the SEM with the scanning EIA voltage. The C6+
could be clearly separated, but C5+ and C4+ were containedby the Al11+ and Al9+ signals.

Fig. 1. Current distribution of each charge state (pure Al)†Condensed from the article in proceedings of ICIS 2013
RIKEN Nishina Center*2
Cooperative Major in Nuclear Energy, Waseda University*3
Department of Physics and Astronomy, Kyoto University*4
Department of Energy Sciences, Tokyo Institute of Technology*5
American Nuclear Society, University of Michigan*6
Brookhaven National Laboratory*1

Fig. 2. Current distribution of each charge state (500nm)
Figure 2 indicates that the plasma was generated from thelayers deeper than 500 nm because the Al ions still occupy
the biggest fraction of the plasma contents. It is also notedthat the C6+ ions appeared in the earliest part of theobserved plasma, and the combination of Al+9 and C4+
produced the highest yield of ions at the peak position.

Fig. 3. Total C6+ particle number
Figure 3 shows the total yield of C6+ ions for eachcarbon-thickness case. The amount of C6+ ions was not
linearly increased by the sputtered carbon thickness. The
result implies that the surface layer has less contribution toform the ablation plasma. The layers from 250 nm to 500
nm were used more efficiently than the surface layer up to250 nm depth to generate the plasma.
Using the carbon-coated aluminum target, thecharge-state distribution was measured. We confirmed thatthe required thickness of the target for the plasma
generation is more than 500 nm. We also found that thesurface layer up to a few hundred nanometers in depth hasless contribution than the deeper carbon layers. To
investigate further, we need to prepare thicker
carbon-sputtered targets thicker than 500 nm.
References

1) M. Okamura, et al., High current carbon beam production withDPIS, Rev. Sci. Inst. 77, 03B303, 2006.
2) S. Kondrashev, T. Kaneue, M. Okamura, K. Sakakibara
Journal of applied physics 100,1 (2006)- 164 -