23P161-P
Ionization dynamics in laser-matter interaction
レーザー物質相互作用における電離ダイナミックス
Yasuaki Kishimoto, Satoshi Nakano, Tomoya Matsuda and Natsumi Iwata
岸本泰明，中野諭，松田知也，岩田夏弥
The Graduate School of Energy Science, Kyoto University, Uji, Kyoto 611-0011, Japan
京都大学大学院エネルギー科学研究科 〒611-0011 京都府宇治市五ヶ庄
By using a particle code including ionization and atomic processes, we investigated the ionization
dynamics of carbon film irradiated by an intense laser pulse. We found two types of ionization dynamics,
namely, a fast time scale convective propagation of the ionization front with C4+ triggered by the field
ionization due to the induced plasma wake, and a slow time scale global ionization beyond whole spatial
region with C5+ and C6+ triggered by electrons impact ionization due to heated hot electrons. The plasma
wave is found to be self-sustained by the pressure gradient of the propagating ionization front.

1. Introduction
The development of high-intensity short-pulse
lasers opens up various applications based onlaser-matter interaction, such as laser fusion,compact particle accelerators, high intensity shortpulse X-ray and neutron sources, etc.[1]. Manycomputational works using the particle-in-cellmodel have been done; however, most of themmade a priori assumption of ideal plasma as aninitial condition. For applications utilizing
relatively high-Z materials, atomic and relaxation
processes play an important role in determining theinteraction. In order to study such applications, wehave developed a particle based integrated code
(EPIC3D) which includes various atomic andrelaxation processes [2]. By using the code, weinvestigated the ionization dynamics in carbon film
irradiated by a short-pulse high-power laser, which
had not been clarified previously.
2. Simulation parameter
Here, we carry out one-dimensional simulations
in a box of size Ly = 2.46μm, where carbon filmwith 1/20 of solid atom density, i.e. ni = 0.88×1022
cm-3, is set in 0.41μm < y < 1.23μm. A p-polarizedlaser is emitted in the x-direction from the antenna
placed at y=0.04 μm. Here, we employ a transparent
boundary condition in the x-direction. The laserparameters consist of a wavelength of λ = 0.82μm,
a pulse width of 100 fs, and a peak intensity of
5.1×1019 W/cm2. As for the atomic process, weconsider a tunneling field ionization and an
ionization by electron impact, where the crosssections are given by ADK [3] and the BEB
formula [4]. The relaxation process is modeled by
the Monte-Carlo method via successive binary
collisions of particle pairs.

3.

Numerical results
The time histories of ion abundance for eachcharge state Cq+ are shown in Fig. 1. After the laser
pulse hits the film, ionizations are successively
triggered from C1+ to C2+. The charge state q = 4evolves slowly at first, but found to be suddenly
accelerated around t = 23fs. The abundance of C1+,
C2+, and C4+ shows clear linear dependences withtime. On the other hand, the number of C5+ and C6+
increases with time, but the rate is small comparedwith that for C4+. Furthermore, the number of C5+
and C6+ ions gradually increases even after thedirect interaction with the laser pulse has ceased.
Figure 2 shows the electron charge density
profiles (Σjqjnj/en0) at different time scales after
t=10 fs, i.e., panel (a) shows profiles from 10.7fs to
23.5fs, (b) from 23.5fs to 33.8fs, (c) from 34.6fs to
68.8fs, respectively. Here nj and qj(= je) represent
the ion density and charge of the charge state j, andn0 is the solid carbon density. It is found that thecharge state up to C2+ is produced directly by thelaser field which initially propagates inside thecarbon film since the film is initially neutral. Fromthe localized high density region near the surface,
the ionization for C4+ starts as seen in Fig. 2 (a), but
the evolution is slow as also seen in Fig. 1.
However, around t= 23fs, the front of C4+ starts toconvectively propagates deep inside with higher
speed and reaches the rear side (t=33.8fs), leading
to a flat density structure over the whole carbon
film. The density in the flat region corresponds tothe density at which the carbon is thoroughly
ionized to C4+. Hence, it is concluded that thepropagation observed in Fig.1 corresponds to an
ionization front to the charge state q = 4.
Figure 3 shows the coordinate of ionization front
C4+ as a function of time. It is clearly seen that thefront speed is changed at t ~ 23fs from 2×107 m/s

to 1.17×108 m/s (roughly 1/3 of the speed of light).
The speed is also found to be accelerated to 1.68×108m/s around t=29 fs. Followed by the propagationof C4+, ionizations to higher charge states, i.e. q = 5
and q = 6, are proceeded, but the dynamics isdifferent from that of C4+, i.e. the charge states
gradually increase in whole region from the front torear side.
We investigated the ionization process for eachelectronic state of carbon ions. We found that theionizations up to q=4 were due to field ionization,
whereas those to q=5 and q=6 were due to electronimpacts. Therefore, it is concluded that the fast
convective propagation results from field ionization,
whereas the subsequent slow one results fromionization due to electron impact.
Figure 3 illustrate the spatial distributions of theion charge density (ρi) and longitudinal electricfield Ey at t = 25 fs (a) and t = 32 fs. As can be seen,
the oscillating electric fields which propagates with
the ionization front with a similar speed are foundto be excited, suggesting that the ionization for C4+
results from this field. It is possible to conclude thatthe observed longitudinal electric field is thepropagating plasma wave, i.e. wake field. As is wellknown, the wake field is generally excited by thespatially localized laser field. However, the laser
field does not exist inside the film due to the higher
density than the cut-off. From the detailed analysesof the simulation, we found that the wake field isexcited by the density gradient of the ionization
front due to its steep pressure gradient, which plays
a similar role as that of photon pressure of the laser
field. Namely, the ionization front of C4+ isself-sustained such that the front ionization isproceeded by the plasma wake, but it is excited by
the pressure gradient at the ionization front.
Summary
In conclusion, the ionization dynamics in highdensity carbon medium is found to evolve throughmultiples stages with different time scales. The field
ionization sustained by the plasma wake found toplay an important role leading to fast convective
ionization.

Fig.1 Time history of ion abundance for each

(a)

(b)

(c)

Fig. 2 Electron density profiles(a)(b)

4.

References[1] D. Umstadter: Phys.Plasmas 8 (2001) 5.
[2] Y. Kishimoto:
Annual Report of the Earth
Simulator Center, Apr.’03-Mar.04’ (2004) p.199.
[3] M. V. Ammosov et al.: JETP 64 (1986) 1191.
[4] K. Kim et al.: Phys. Rev, A 50 (1994) 3954.

Fig. 3 Electron density and longitudinal electricfield profiles at (a) t=24.79fs and (b) 32.05fs