協同的トムソン散乱法を用いたレーザー照射時間帯におけるレーザー生成プラズマ挙動の研究

概要

九州大学学術情報リポジトリ
Kyushu University Institutional Repository

Investigation of dynamics of laser-produced
plasmas during the laser-irradiation using
collective Thomson scattering
潘, 奕明

https://hdl.handle.net/2324/6787641
出版情報：Kyushu University, 2022, 博士（理学）, 課程博士
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名 ：潘 奕明

論 文 名 ：Investigation of dynamics of laser-produced plasmas during the laser-irradiation using collective
Thomson scattering （協同的トムソン散乱法を用いたレーザー照射時間帯におけるレーザー生
成プラズマ挙動の研究）

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分 ：甲

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論 文 内 容 の 要 旨
Thesis Summary
Plasma temperature, density, and flow velocity are the critical physical properties of laser produced plasma
(LPP) to reveal the ablation dynamics, energy transport, and hydrodynamic evolution. This knowledge, in turn, leads
to promising improvements in LPP-based applications, such as pulsed laser deposition (PLD) and extreme
ultraviolet (EUV) light source. In the time window during and just after laser irradiation, experimental investigations
are very scarce so that many theoretical models remain untested. A clear evolution history of LPP expansion
dynamics during and just after the laser pulse time, and in a region very close to the target (typically 0.13–0.6 mm) is
investigated using collective Thomson scattering (CTS) method. The electron density (ne), temperature (Te), and drift
velocity (Vd) in the LPPs were measured, providing a space- and time-resolved 2D profile of the LPP.
As the first step, carbon LPPs were studied to reveal the expansion dynamics of LPPs in this time window. A
table-top Nd: YAG laser (at 1064 nm, intensity 6 ×109 W/cm2, pulse 7 ns in FWHM) was used to generate the LPP
from a planar graphite target, whose width was arranged to be smaller than the laser spot diameter to produce a
one-dimensional planar expansion plasma near the target. A second Nd: YAG laser (at 532 nm, pulse 4 ns) was
used as the probe laser.
The experimental observations made it possible for the expansion dynamics to be compared directly with the
LPP expansion models. The results suggest that during the laser pulse, the LPP is approximately isothermal and
expands predominantly one-dimensionally in the target normal direction, in which the LPP drift velocity is found to
increase linearly with distance. The linear extrapolation of the velocity indicates that the LPP has a considerable
velocity at the initial target surface; this velocity is approximately the speed of sound derived from the observed Te.
The experimental results were found to be in moderate agreement with the 1-D self-similar isothermal expansion
model. The ratio of the internal to kinetic energy in the observed area was ~0.6, as predicted by the isothermal
expansion model. The experimental findings were compared with the results of the 2-D hybrid code STAR, and good
agreement was obtained.
As a next step, we have expanded our diagnostic technique to laser-produced high-Z Tin (Sn) and gadolinium
(Gd) LPPs. These are widely used as the light source for nanolithography in modern semiconductor industry. The
density (ne) and temperature (Te) profiles in LPPs during the laser pulse time, and the dependence of Te on laser
intensities IL are crucial for LPP light source applications but remain not understood, because the lack of direct
measurement results.
Therefore, direct measurements on spatial and temporal profiles of electron density ne and temperature Te in
laser produced gadolinium (Gd) plasmas was carried out, for the first time. The required relation between Te and
laser intensity are experimentally studied, over a range of laser intensities approximately 1010 − 1011 W / cm2. It is
revealed that, the measured maximum Te in Gd LPPs increase with increasing laser intensity with a dependence
Te ∝ (IL)0.37. Laser intensity of 1.9 × 1012 W cm-2 is needed to achieve the optimum condition (Te ~100 eV and ion
charge states ~ Gd+18, at ne ~ (2−3) × 1019 cm-3) for efficient beyond EUV (BEUV) light source. In addition, two spot
size (150um and 250 um diameter) are used to study the effect of spot size on Te profile. Our experimental findings,
supported by the simulations, exhibit that bigger spot can create uniform Te profiles, higher maximum Te and larger
high Te regions in LPPs. Radiation-hydrodynamic simulation using 2-D hybrid code STAR was performed under
identical conditions as used in the experiment, and extended further to higher laser intensities, up to 6 × 1011 W / cm2,

and the results are found in good agreement with the experiment observations.
In conclusion, in this thesis, first direct measurements of electron density (ne), temperature (Te), and drift velocity
(Vd) inside LPP during laser-irradiation timing have been performed using CTS. In addition, the experimental results
were compared with hydro-dynamic simulations. The quantitative comparison between the experiment and
simulation study firstly reveals that LPP dynamics during laser-irradiation timing were well explained by 1D
self-similar solution models. In addition, this novel technique for LPP have been expanded to high-Z LPP, which
were important for light-source application.

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