24P020-O
Spectroscopic Measurement of Lithium Plasma Generated
by Counter-Facing Plasma Focus System
同軸対向型プラズマフォーカスにより生成されたリチウムプラズマの分光計測
Akiko Yamamoto1, Yuusuke Kuroda1, Hajime Kuwabara2, Mitsuo Nakajima1,Tohru Kawamura1 and Kazuhiko Horioka1
1
山元昭貴子 , 黒田雄介1，桑原 一2，中島充夫1，河村 徹1，堀岡一彦1
Department of Energy Science, Tokyo Institute of Technology1,
Nagatsuta 4259, Midori-ku, Yokohama, 226-8502, Japan
東京工業大学大学院総合理工学研究科 〒226-8502 横浜市緑区長津田町 4259
IHI Corporation2,Shin-nakaharacho 1, Isogo-ku, Yokohama, 235-8501, Japan
株式会社IHI 〒235-8501 横浜市磯子区新中山町１番地
A plasma focus system with counter facing electrodes was proposed for high energy density plasma
creation with long-pulse and/or repetitive high energy density plasma source. We adopted Li as the plasma
source which has a narrow and strong spectral line at 13.5nm. In addition, for improvement of the radial
uniformity and synchronous operation of the current sheets, a system of laser-assisted discharge wasapplied. Prior to the facing operation, one of the pair electrodes was energized at positive and negative
polarity, to examine the basic operating characteristics. We performed spectroscopic measurement of thefocus plasma and emission of the 13.5 nm line was observed at both polarities.

1. Introduction
Recently radiation from high-energy-density
plasma is attracting our attention as an Extreme
Ultra-Violet (EUV) light source for the next
generation semiconductor process. Because they
are cost effective and simple, EUV light sources
based on high-energy-density discharge plasma
have an advantage compared with laser-produced
plasma. High average power and conversion
efficiency are indispensable for a practical use of
extreme-ultraviolet (EUV) light source. However,despite recent efforts in developing adischarge-produced EUV plasma source, anefficient and repetitive light source remains to berealized.
To improve the energy conversion efficiency,
we would like to extend the radiative time ofsource plasma. Then we intend to prolong thelifetime of plasma.[1,2] Previous work achieved
the EUV emission of ~10 µs using counter-facing
plasma guns.[3] In this study, we apply acounter-facing plasma focus system and adopt
Lithium as the plasma source which has a narrowand strong spectral line at 13.5nm. Li plasma is
produced by laser-assisted discharge of which laser
energy is completely lower than the energy ofdischarge.
We conducted the spectroscopic measurement of
the plasma at EUV region using a grazing-incidence

flat field spectrometer in the counter-facing plasma
focus system. The purpose of this study is toexplore the optimum operating parameters throughthe spectroscopic measurement, especially theemission of 13.5 nm-line.
2. Experimental Setup
Figure 1 shows an anticipated behavior of theplasma focus system. In this system, gas or/andplasma of Li is produced by laser ablation, whichinduces trigger of the discharge. Current sheets
generated by the discharges were accelerated to thetop of center electrodes by the magnetic pressure (JBreakdown

Current Sheet

GND

GND+-GND

GND

High Density Li Plasma
GND

GND+GND

B

Reconnection

GND

Fig. 1 Schematic diagram of counter–facing
coaxial plasma focus system

(a)

(a)

(b)

+H.V.

Intensity (arb. units).

40

Nd:YAG Laser

30-H.V.

2010+H.V.

010-H.V.

140(a)Fig. 2 Schematic of (a) electrodes
arrangementand (b)plasma focus and current drive systems

Figure 2 schematically depicts the experimentalarrangement. The plasma focus system consists of apair of plasma guns. The center-electrodes arecharged by plus and minus, respectively, whichhave hollow at the top and groove at the position of
20mm from the top, to implant Li. The
outer-electrode is composed of 6 rods arranged
cylindrically. 6 capacitors were connected between
every
outer-electrodeandthecommon
center-electrode, hence independent electric
discharge circuits are formed. The capacitors
charged with high voltage DC power sources areused to drive the discharges.
A Q-switched Nd:YAG laser operating with asingle pulse mode, 532 nm wavelength and 15 ns
pulse width, was focused on the Li source at middleof the inner-electrode. A grazing-incidence
spectrometer, which has a concave grating(No.001-0437, HITACHI) and a Micro Channel
Plate (F2223-21P, HAMAMATSU), was used forthe observation of plasma.
3. Results
In this study, prior to facing operation, the followof electrodes was energized at positive and negativepolarity. We performed spectroscopic measurementof the plasma to examine the operating
characteristics, in particular, polarity effect on thespectrum output. Typical charged voltage of120

Intensity (arb. units).

capacitors was 3kV, or 7kV and the laser energyB) and collide between the electrodes. Because
was 8.0 108 W/cm2 , or 12.8 108 W/cm2 .
voltage pulses with opposite polarities can drive adischarge between the center electrodes, the current
Fig. 3 shows the emission spectra measured withsheets should be reconstructed and shrunk into anthis system. As shown, emission of 13.5 nm wasellipsoidal shape after the collision. Self-magnetic
observed at each of polarity. This result shows thatpressure should compress the plasma, while the (b) the system has capability of Li supply to themagnetic pressure gradient should suppress particle
pinching region.
Even
Nd:YAG
Laserthe one-sided operation of+H.V.
loss in the axial direction. Hence the current pathguns, it was able to produce high-H.V.temperature
should create a stable condition, the plasma
plasma enough to radiate the EUV at 13.5 nm. This
Nd:YAG Laser
becomes dense and high temperature.
In order tomeans that radiation duration of 13.5 nm from Li-H.V.
Nd:YAG Laser
achievethe stable condition, it is necessary that the +H.V.plasma can be extended to microseconds-H.V. by using-H.V.
uniform current sheets in the radial direction are
the counter-facing systems.
80
exhausted at the same timing from both side guns.
70 (a)The laser assisted discharge system is expected to60-H.V.
have a potential to produce
suchcondition of the-H.V.
50
plasma sheets.
13.5nm121416182022241618202224

Wavelength (nm)(b)

13.5nm100806040200101214

Wavelength (nm)Fig. 3 Emission spectra of 13.5 nm at(a) positive and (b) negative polarity

4. Summary
We proposed a counter-facing plasma focus
system, in which Li was adoped as the plasma
source. On this system, Li plasma was produced bylaser-assisted discharge of which laser energy iscompletely lower than the energy of discharge.
Prior to facing operation, we investigated theemission characteristic of EUV with one-sided
operation. With both of polarity, emission of 13.5
nm was confirmed. This indicates that thecounter-facing plasma focus system has potential toprolong the radiation of 13.5 nm from Li plasma for
more than several microseconds.
References[1] M. Masnavi, M. Nakajima, E. Hotta and K. Horioka:
J. Appl. Phys. 101 (2007) 033306.
[2] M. Masnavi, M. Nakajima, E. Hotta and K. Horioka:
J. Appl. Phys. 103 (2008) 013303.
[3] Y. Aoyama, M. Nakajima and K.Horioka: Phys.
Plasmas 16 (2009) 110701.