J. Plasma Fusion Res. SERIES, Vol.7 (2006) 142-144

Energetic Ion Generation by an Ultra-Intense Laser Pulse
on Various Plasma Foil Targets
OKADA Toshio, OKUBO Kota, MIKADO Yuichi and SUGIE Michio
Tokyo University of Agriculture and Technology, Tokyo 184-8588, Japan（Received: 4 October 2004 / Accepted : 24 May 2005）

Abstract
High energetic ion generation from thin foil targets irradiated by 30 fs laser pulses at intensity 10 20 W/cm2
has been studied as a function of various targets with various concaves. We have demonstrated an increased
acceleration and higher ion energies for the targets with triangle concave by using particle-in-cell (PIC) simulations. The optimum target plasma conditions for the maximum ion acceleration are found. Mechanism offast (multi-MeV) ion acceleration in the rear of the targets with triangle concaves is analyzed.
Keywords:
intense ultra-short laser pulse, fast electron generation, fast ion generation
1．Introduction
Fast particles generated by laser-plasma interactions
can be used in many applications, from manufacturing
to medicine and even for the initiation of tabletop nuclear reactions. Fast ion generation by the interactionof an ultra-short high intensity laser pulse with a plasma
has been demonstrated in recent theoretical [1] and experimental [2] papers, with maximum ion energies of up
to 55 MeV having been observed. Diﬀerent methods offast ion generation have been proposed for both gas [3]and solid [4] targets. It has been shown that the energyof a laser pulse can be eﬃciently converted into fast ion
energy using foil targets. Simulation [5] has shown thatthe mechanisms for generating ion acceleration are theambipolar field and the Coulomb explosion. It has also
been shown that fast electrons ejected from the foil by
the laser field create a strong ambipolar field, which is
the main source of acceleration of ions ejected from theback of the foil. Thus a collimated ion beam can be produced by focusing an intense laser onto the surface of asolid film. Fast ions accelerate normally to the foil surface because this is the direction of the ambipolar field[6]. We have demonstrated fast ion generation to get amost energetic proton generation for the slab target, thetarget with triangle concave and the target with circle
concave [7]. Most experimental high power lasers produce a pre-pulse, which generates a plasma layer witha smooth density gradient on the surface of the foil. In
this paper, we attempt to study fast ion bunch generation by ultra-intense laser pulse in front plasma layerswith smooth density gradients and rear plasma layerswith triangle concaves.

2．PIC simulation
We apply a PIC method to simulate the interactionof a plasma layer with an intense ultra-short laser pulse.
The method is based on the electromagnetic PIC and isappropriate for analysis of the dynamics of over dense
plasmas created by arbitrarily polarized, obliquely incident laser pulses. The 2D (using a Cartesian coordinatesystem) relativistic, electromagnetic code is used to calculate the interaction of an intense laser pulse with anover-dense plasma. Simulations are performed for laser
wave length of 1.06 µm and laser intensity 1020 W/cm2with a 30 fs pulse width. The transverse focal spot profile is Gaussian with a diameter of 4 µm. The background electron density is 20 nc . Simulations are madefor target thickness of 3 µm with 1 µm density gradient. Simulation box is 12 µm wide 70 µm long. Thetime step is chosen to be 0.1/ω L where ωL is the laser
frequency, and the grid size is chosen to be 0.2 c/ω L
where c is the speed of light. The time step normalized
by the electron plasma frequency becomes about 0.447
which is large for the simulation, but the simulation results are valid because that the total energy is conservedin the simulation time. The number of spatial grids andparticles are 2000 × 360 and 106 , respectively.
3．Fast ion production from the rear surface ofthe foilIons begin to accelerate as soon as the ambipolar
field is generated. We note that ions are ejected fromboth sides of the target but fewer fast electrons, andC 2006 by The Japan Society of Plasma

Science and Nuclear Fusion Research
Corresponding author’s e-mail: okada@cc.tuat.ac.jp142Energetic Ion Generation by an Ultra-Intense Laser Pulse on Various Plasma Foil Targetsyω L /cRω L /c=6ω Lt = 400

6055
5045
40353025201530
2520151050100 105 110 115 120 125 130 135 140xωL /cFig. 4 Spatial distribution of ion density.
Fig. 1 Proﬁle of a target containing a concave.

Average ion energy [MeV]ω Lt = 600yω L /cRω L /c=16055
5045
403530252015ω Lt = 400302520151050.45
0.44
0.43
0.42
0.41
0.4
0.39
0.38
0.37
0.36
0.35
0.34
00

12Fig. 2 Spatial distribution of ion density.

Peak ion energy [MeV]yω L/c5656w Lt = 600

RωL / c = 3
ω Lt=4004Fig. 5 Average ion energy vs RωL /c.

100 105 110 115 120 125 130 135 140xω L /c60
55
50
45
40
35
30
2520
15

3RωL/c30
25
20
15
10526
25.5
25
24.5
24
23.5
23
22.5
22
21.5
21
20.5
20
19.5
1900

1

2

3RwL/c4Fig. 6 Peak ion energy vs RωL /c.

100 105 110 115 120 125 130 135 140xω L /cFig. 3 Spatial distribution of ion density.

peak ion energy is changed from 19.8 to 25.2 MeV as
the concave depth is increased from 0 to 1 µm.

hence fewer ions, are ejected from the front side. Thus,
we only consider the rear side of the target. In Fig. 1,
profile of a target containing a concave is shown. Thewidth of the concave L is fixed at 4 µm and the depth R
is changed. In Figs. 2-4, spatial distributions of ion density are shown at ωL t = 400 from the targets containing
various concave depth. In Fig. 5, average ion energy isshown for the concave depth R. Figure 6 shows how the4．Conclusion
We have shown numerically that a laser pulse of intensity 1020 W/cm2 and duration of 30 fs will generate
an intense ion bunch that propagates directly from therear surface of the foil. It is clear that the peak ion energy is maximum at RωL /c = 2.3 in Fig. 6. The physical mechanism of the existence of maximum ion energy
143

Energetic Ion Generation by an Ultra-Intense Laser Pulse on Various Plasma Foil Targets

will be the diﬀerence of the electron density distribution. The more detailed explanation will be appeared inforthcoming paper.

[4]
[5]

[1]
[2]
[3]

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