Ⅱ-8. Accelerator

RIKEN Accel. Prog. Rep. 47 (2014)

Interaction of plasmas in laser ion source with double laser system
Y. Fuwa, 1, 2 S. Ikeda,

1, 3

M. Kumaki,

1, 4

M. Sekine,

1, 5

D. Cinquegrani, 6 M. Romanelli, 7T. Kanesue, 8 M. Okamura, 8 and Y. Iwashita 9A laser ion source can provide intense and low-emittancepulsed ion beams. For these advantages, various
applications have been studied, such as DPIS (DirectPlasma Injection Scheme) to RFQ (Radio FrequencyQuadrupole) Linac1) and a seed-ion beam provider for theEBIS (Electron Beam Ion Source) at BNL (BrookhavenNational Laboratory)2), 3). The laser ion source functions ona simple principle. Figure 1 shows the schematic layout of alaser ion source. Laser irradiation with an energy density
above tplasma. This
plasma drifts to an extracting electrode and an ion beam isformed. A change in the laser power density on the target
expanding velocity of plasma.
In conventional laser ion sources, a nano-second laser has
been used. With single nano-second laser irradiation,
thermal mechanisms are the dominant processes in plasma
production and it results in a Maxwellian ion energy
distribution. Since the plasma expands in three dimensions
in the drift region from the target to the extracting electrode,an ion beam pulse width is proportional to the drift length L,
and the peak ion beam current is inversely proportional toL-3. Therefore, if we need a longer beam pulse width, thetotal peak current is steeply decreased.
A multi-pulse laser system may be used to elongate theion beam pulse length or to intensify the beam current. To
test the feasibility of these ideas, a double-pulse laser
system was used in BNL.
Previous research showed that a multiple laser shot
scheme is useful in extending the ion beam pulse length fora low-charge state mode. However, if the interval between
the two laser is less than 10 µs, the observed current profile
is not just sum of two laser plasmas4). In this research, we
carried out a more detailed study for the case in which theinterval between laser pulses was less than 10 µs, in orderto understand this phenomenon.
In our experiment, each laser energy on the target was560 mJ at maximum and the laser spot was an ellipse ofheight 3 mm and width 4 mm. The estimated laser power
density of each laser shot was between 108 and 109 W/cm2,
Condensed from the article in Rev. Sci. Instrum. 85, 02B916 (2014).
1
RIKEN Nishina Center2
Graduate School of Science, Kyoto University
3
Interdisciplinary Graduate School of Science and Engineering, Tokyo
Institute of Technology
4
Research Institute for Science and Engineering, Waseda University
5
Department of Nuclear Engineering, Tokyo Institute of Technology
6
Nuclear Engineering and Radiological Science, University of Michigan
7
School of Applied and Engineering Physics, Cornell University
8Collider-Accelerator Department, Brookhaven National Laboratory
9Institute for Chemical Research, Kyoto University- 162 -Fig. 1. Schematic layout of a laser ion source
and the induced charge states of ions were mostly single.
Ion current was measured using a Faraday cup. Twolasers were operated with various intervals of trigger timing
range from 0.1 µs to 10 µs. In the measured current profile,a prominent peak that does not correspond to any single
ure 2 shows atypical result. This peak had a maximum peak current for
the laser interval from 1 µs to 1.5 µs and the peak height
was multiplied five times. This peak appears to have formed
owing to the interaction between the second laser and theneutral vapor or particles produced by the first laser.

Fig. 2. Comparison of the measured ion current profile
between single-laser plasma (blue and green plots) anddouble-laser (red plots).
References

1) M. Okamura et al.: Rev. Sci. Instrum. 79, 02B314 (2008).
2) J. G. Alessi et al.: Rev. Sci. Instrum. 81, 02A509 (2010).
3) K. Kondo et al.: Rev. Sci. Instrum. 81, 02B716 (2010).
4) M. Okamura et al.: Rev. Sci. Instrum. 83, 02B308 (2012).