Formation of Cavity and Amplification of X-Rays
in a Table-Top X-Ray Laser Pumped by a YAG Laser
N. Yamaguchi, C. Fujikawa, Y. Hisada, A. Ogata, K. Okasaka, T. Ohchi, T. Hara
Toyota Technological Institute,
2-12-1 Hisakata, Tempaku, Nagoya 468, Japan
E-mail: yamagch@toyota-ti.ac.jp
Abstract. A pulse-train YAG laser beam was line focused on a Al slab target
by a focusing lens system consisted of a segmented prism array, a beam
expander and a cylindrical lens. The intensity distribution was uniform along
the focal line including micro dots. Amplification of Li-like Al soft X-ray
transitions was observed in recombining Al plasmas produced by irradiation
of the pulse-train YAG laser with the input energy of 1.5-2 J. The gain
coefficient measured for 15.47 nm line was 3.2 cm-1 and the duration of gain
was longer than 1 ns. Two types of cavities, an unstable cavity and a cavity
with a soft X-ray beam-splitter, were tried. Clear output signals from the
unstable cavity were observed for the first time for Al XI 3p-4d (15.06 nm)
and Al XI 3d-4f (15.47 nm) transition lines. Experiments on the cavity with a
soft X-ray beam-splitter also showed that clear evidences of enhancement in
the two Li-like Al ion lines.
1 Introduction
X-ray lasers which operate at shorter wavelengths less than 30 nm, are important for
many applications, because photons interact strongly with atoms. The majority of
X-ray lasers in the wavelength region are laser driven. Saturated amplification has
been observed in several collisionally pumped Ne-like lasers at high pump-laser
energies, > 1 kJ. Although high-energy X-ray laser systems have demonstrated
considerable gain at short wavelengths, these systems are disadvantaged by a low
operating rate. Development of more compact and more efficient soft X-ray lasers
could make their use widespread in the above applications.
As a route to compact and efficient soft X-ray lasers, we proposed the plasma
production by irradiation of pulse-train lasers [1, 2]. The advantages of the pulse-train
laser are that highly charged ions such as Al10+ are produced efficiently through
successive plasma heating, and that the electron temperature drops rapidly as soon as
laser irradiation ceases, because the decay time of the pulse-train laser is the same as
that of the last pulse. ASE’s of two Li-like Al lines (10.57 nm and 15.47 nm) were
observed using the pulse-train glass laser with only 2.3 J/cm pumping energy [3]. Such
a low energy can be supplied also by a tabletop YAG laser, which has a high operating
rate. The present paper reports the first results of a soft X-ray laser pumped by a YAG
laser.
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N. Yamaguchi et al.
2 Experimental Arrangement
All the components of the YAG
laser system were arranged on
one table (1.5 m x 3.0 m) (see
Fig. 1). The pulse-train with 16
pulses was generated as follows.
A 100 ps laser pulse from the
mode-locked oscillator was
transformed into 8 pulses with
interpulse time of 400 ps by an
optical pulse stacker. The pulsetrain
amplified
by
three
amplifiers was divided into two
beams.
Each
beam
was
Fig. 1. Table-top YAG laser system
amplified up 1.7 J by the final
amplifier, and was combined
again into 16-pulse-train with interpulse time of 200 ps. The maximum output energy
of the combined beam was 3.4 J. The intensity of the latter half of the amplified pulsetrain was reduced less than one fourth that of the first half, because the control of the
envelope of the pulse-train leads to enhance the gain of soft X-ray lines and its
duration through optimizing the recombining process in plasmas [3, 4].
All the output energy of the YAG laser must be used efficiently to produce highly
charged ions. However, if the YAG laser beam is focused by a single cylindrical lens,
the focused laser has an inhomogeneous intensity distribution along the line. To
overcome such difficulty, a focusing technique by using a segmented lens array or
wedge array has been reported [5, 6]. The present focusing system consists of a
segmented prism array, a beam expander and a cylindrical lens. The incident laser
beam is divided into 5 parts by the segmented prism array. Each divided beam is
enlarged by the expander to avoid damage on the window of the vacuum chamber, and
is focused by the cylindrical lens into the same 11 mm-long line on the target. The
focusing system is designed to make a focal line of 10 µm width. Considering the
divergence of the YAG laser, one can expect the line width of about 50 µm. The beam
whose intensity distribution has a uniaxial symmetry can be focused into a smooth line
by this focusing system.
The pulse-train YAG laser was irradiated on an Al slab target. The focal line has
small dots at the center of the line. The width of the focal line was about 150 µm, and
the depth of the dot was about 10 µm at the bottom. Each dot had a diameter of about
50 µm and a spacing of 140 µm. The dotted line pattern was caused by the
interference among individual YAG laser beam segments. As the pattern was almost
same along the line, one can conclude that whole energy of the YAG laser beam was
delivered equally to each dot.
Temporal behaviors of soft X-ray radiation from laser-produced plasmas have
been observed along the axis of a line plasma using a flat-field grazing incidence
spectrograph connected to an X-ray streak camera. Spectroscopic observations have
Formation of Cavitiy and Amplification of X-Rays
V - 49
been made with spatial resolution of less than 0.2 mm in the direction normal to the
target surface.
An unstable resonator consisting of two flat Mo/Si multilayer mirrors, 50 layer
pairs with a layer pair thickness of 7.92 nm, was installed. The calculated reflectivity
of the X-rays at 15.47 nm was 50 % at normal incidence. The two mirrors were set
5 cm apart with a laser-produced plasma placed between them. One of the mirrors has
an output coupler made of a rectangular orifice 0.7 x 0.7 mm2 in size. There was a
shutter between the plasma and the rear mirror.
In the experiment on a cavity with a soft X-ray beam-splitter, a Mo/Si multilayer
on Si membrane, 4 x 4 mm2 size, has been used as a front mirror to the spectrograph
instead of the mirror with an output coupler. The beam splitter used was a 20 layer
pair multilayer on 1 µm-thick Si or a 10 layer pair one on 1.5 µm-thick Si. The
thickness of a layer pair was 7.95 nm. The calculated reflectivity was 59.2 % or
37.8 % and the transmittance was 1.96 % or 2.35 %, respectively. The actual
reflectivity might be lower than the above value because of roughness on the
multilayer interface originated in the manufacturing process of the beam splitters.
3 Experimental Results
3.1 Measurement of Gain
To measure the gain coefficient for the soft X-ray transition of Li-like Al, plasma
length was varied by changing the width of a mask in front of the target, where X-ray
mirrors were not used. We have succeeded in observing the two ASE signals at
15.47 nm and 15.06 nm with the pumping energy of 1.5-2 J. Intensities of Al XI 3d-4f
transition were measured for the plasma lengths of 6, 8.5 and 11 mm. The best fit of
Linford formula [7] gave the gain coefficient of 3.2 cm-1 for the 15.47 nm line. The
temporal change of gain coefficient can be determined by calculating the ratio of line
intensity of 11 mm plasma to that of 6 mm plasma and by using Linford formula. It
was confirmed that the duration of gain higher than 2.5 cm-1 was longer than 1 ns. The
spatial distribution of gain along the target normal was also measured by moving the
target and the lens system together and fixing the observation axis. As a result, it was
found that the gain coefficient for 15.47 nm line had a beak at 0.2 mm from the target
surface.
3.2 Double-Pass Amplification
We have performed experiments of double-pass amplification by using a flat Mo/Si
multilayer mirror which was located at a distance of 25 mm from the center of a line
plasma. The reflectivity of X-ray at 15.47 nm was 34 % at normal incidence. When
the shutter between the plasma and the mirror was opened, the Li-like Al transition
line (3d-4f, 15.47 nm) was enhanced up to 3 times that of the closed case. The
15.06 nm line was amplified by up to 50 %, because this mirror had the reflectivity of
13 % for 15.06 nm.
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N. Yamaguchi et al.
3.3 Unstable Cavity Experiment
Fig. 2. X-ray streak camera images in the unstable
cavity experiment. Input energy is 1.8 J/cm of pumping
energy per unit length of the focusing line.
a: Streak image when the shutter is closed (cavity: off).
b: When the shutter is open (cavity: on).
The schematics of unstable cavity experiment is shown
in the inset
Typical examples of soft X-ray spectra observed by using the streak camera are shown
in Fig. 2. These images were taken at a distance of 0.4 mm from the target surface.
Figure 2a shows the spectrum when the cavity is off. Here, the temporal change of the
plasma emission is viewed through the orifice. When the shutter was opened, two
sharp lines preceding the plasma emission were observed as shown in Fig. 2b, which
were identified as Al XI 3p-4d (15.06 nm) and Al XI 3d-4f (15.47 nm) transition
lines. Comparisons of Al XI 3d-4f line intensity have been made among the plasma
emission, the double-pass signal and the unstable cavity output. We have found that
the cavity signal has more than three times the intensity of the double-pass amplified
signal.
The time interval of the onset of the preceding emission and that of the plasma
emission decreased when the position of the edge of orifice was moved toward the
target, becoming zero at 0.15 mm from the target. It has been confirmed
experimentally that the plasma has a high gain for the Li-like Al X-ray transitions
beyond this position. The time interval of about 1.5 ns shown in Fig. 2b is equal to the
required time for the plasma front to pass through a length of about 0.2 mm at the
velocity of 1.4 x 107 cm/s which has been determined experimentally. It is considered
that the soft X-ray radiation originated from the region of the target side to the edge of
orifice has a round-trip between the cavity mirrors and has been amplified
successively. And finally it appears ahead of the plasma emission through the orifice.
3.4 Experiments on Cavity with Soft X-Ray Beam-Splitter
Clear enhancement of the two Li-like Al lines has been observed in the cavity
experiment with the beam splitter of 10 Mo/Si multilayers on 1 µm thick silicon. In
this case, the Li-like Al line intensities was enhanced by 50 to 100 % of that of the
double pass amplification. The output beam had not an enough intensity in the case of
Formation of Cavitiy and Amplification of X-Rays
V - 51
the beam-splitter cavity, because X-rays through the beam-splitter suffered from
absorption and besides the reflectivity of the used beam-splitter was not so high.
Beam-splitters with high-reflectivity are anticipated to progress cavity experiments.
4 Summary
A pulse-train YAG laser system has been developed as a pumping source for a
tabletop X-ray laser. The focusing system consisted of a segmented prism array, a
beam expander and a cylindrical lens has also been developed to obtain a uniform
11 mm-long focal line. X-ray lasing of Li-like Al ion in the recombination scheme
have been investigated. ASE signals of the two Li-like Al transitions (3d-4f, 15.47 nm
and 3p-4d, 15.06 nm) have been observed with only 1.5-2 J of pumping energy. The
gain coefficient measured for 15.47 nm line was 3.2 cm-1 and the duration of that was
longer than 1 ns. This means that the two requirements for soft X-ray cavity
experiment have been satisfied. Two types of cavity experiments have been carried
out, which are the unstable cavity with a coupling hole and the cavity with a beam
splitter. Clear enhancement has been observed in the Li-like Al transition lines at
15.47 and 15.06 nm for both types of cavities. Significant X-ray laser output could be
extracted by using more efficient cavity geometry or beam splitters with high
reflectivity. Beam characteristics of output X-ray will be investigated. Applications of
X-ray lasers are now planned for microbeam forming or X-ray spectromicroscopy.
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