1
EXPERIMENTAL STUDY OF THE LASER-ACTIVE
ELEMENT CHARACTERISTICS FOR HIGH-POWER
NUCLEAR REACTOR-PUMPED LASER SYSTEM
E.D.Poletaev, A.F.Gamaly, P.P. Dyachenko, V.N.Smolsky, M.Yu.Zaitsev,
V.R.Agafonov, Yu.A. Dyuzhov, A.V.Gulevich, B.V.Kachanov, O.F.Kukharchuk
Institute for Physics and Power Engineering,
1,Bondarenko Sq., Obninsk 249020,Russia
Abstract. The results of experimental studying characteristics of the laser-active element with
the Ar-Xe mixture on the 1.73 m transition of Xe atoms for the high-power pulsed reactorpumped laser system are presented . The experiments have been carried out on the BARS-6
two-core pulsed reactor using a special-purpose setup that is also described.
INTRODUCTION
One of the promising directions in a nuclear energy utilization is the
development of the nuclear pumped lasers. At present time , a pilot model of highpower laser system based on optical quantum nuclear-power amplifier (OKUYAN) is
under construction at the Institute of Physics and Power Engineering [1]. The main
components of the laser module for this system are the laser-active elements (LAELs)
with Ar-Xe mixture. The Ar-Xe mixture is used as a working medium for LAEL
because of its detail studying under conditions of nuclear pumping [2-5]. Applying this
mixture, nuclear-pumped lasers have been realized on the various transitions of Xe
atoms in the microwave range with rather high output parameters - specific energy is up
to 1 J/l, and efficiency of nuclear-to-laser energy conversion is up to 3.5 %.
This paper is devoted to an experimental study of laser characteristics for the
accepted variant of the Ar-Xe LAEL which will be applied in laser module of
OKUYAN.
EXPERIMENTAL SETUP
Proc. Intern. Conf. ICENES’98, 1998
 - 1998 Institute for Physics and Power Engineering, Technical Physics Laboratory
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The experiments have been carried out on the BARS-6 two-core pulsed reactor
using a special-purpose setup for studies of nuclear-pumped lasers. Schematic layout of
experimental setup is shown in Fig. 1. The laser-active element placed into the
cylindrical
polyethylene neutron moderator, was arranged near reactor cores at a
distance of 68 cm. The LAEL is a thin walled stainless steel tube with outer diameter 49
mm and length 2500 mm with an internal 5 m -thick metal uranium-235 coating. The
windows with a dielectric antireflection coating for the 1.73 m
quartz optical
wavelength were mounted on both ends of the LAEL.
1
2
5
6
4
3
7
9
8
6
6
6
6
16
12
13
14
15
He-Ne
6
11
10
10
Fig.1. Schematic layout of the experimental setup.
1 - nuclear reactor cores, 2 - neutron moderator, 3 - LAEL, 4, 5 - cavity mirrors,
6 - deflecting mirrors, 7, 10,11 - beamsplitters, 8 - lens, 9 - calorimeter, 12, 13,
14 - photoreceivers, 15 - alignment laser, 16 - shielding wall
A vacuum pump, a filling system, a circulation line including the silicagel,
titanium and sodium traps, and a bellows pump for continuos purification of Ar-Xe
medium, were attached to the LAEL. The purification control was carried out by
measuring intensity of radiation for bands of both the impurity molecules (N
2 ,
OH,
Proc. Intern. Conf. ICENES’98, 1998
 - 1998 Institute for Physics and Power Engineering, Technical Physics Laboratory
http://www-tpl.ippe.obninsk.ru
E-mail: kuh@ippe.obninsk.ru
3
etc.) and the heteronuclear ion of Ar+Xe under excitation by -particles. For this
purpose a cell with -source (Pu-238) was included to a circulation line . The excited
spectrum was measured by the MDR-6 monochromator .
The optical scheme of experiment has two channels formed by beamsplitter (7)
which is a mirror with a dielectric coating on 1.73 m having a reflection coefficient
equal to 96
%. Therefore , the most part of laser beam entered a thermocouple
calorimeter to detect the laser pulse energy. Another part of laser beam was directed by
the plate aluminum mirrors to a measuring system including a set of various
photoreceivers for registration of the laser pulse time shape. The alignment of optical
system was carried out using a He-Ne laser.
The copper and tungsten activation indicators placed into neutron moderator
near surface of the LAEL were used to measure the thermal neutron fluence distribution
along the LAEL. The time dependence of thermal neutron pumping pulse in moderator
was measured by vacuum fission chamber with
235
U. In this experiment, the pressure
jump in the LAEL was measured during the pumping pulse using the DMI-2 transducer
in order to determinate the energy deposition of fission fragments into the gas.
RESULTS AND DISCUSSION
Measurement of the LAEL laser characteristics have been carried out in a free
oscillation mode. Results of such experiments allow to obtain both optimal ratio and
total pressure of laser mixture, and also so important parameters of active medium as an
unsaturated gain, a saturation intensity and an efficiency of the nuclear-to-inversion
energy conversion.
A typical time-dependent laser oscillation on the 1.73 m transition is shown in
Fig.2 together with a thermal neutron pumping pulse. The total laser output energy
measured by calorimeter was about 1 J at optimal conditions. These data were used to
normalize a laser output signal from Ge-photodiode recording a laser pulse shape. The
thermal neutron signal from the
235
U vacuum chamber was normalized to the neutron
fluence measured by activation method. Study of laser output energy as a function of
Proc. Intern. Conf. ICENES’98, 1998
 - 1998 Institute for Physics and Power Engineering, Technical Physics Laboratory
http://www-tpl.ippe.obninsk.ru
E-mail: kuh@ippe.obninsk.ru
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ratio and total pressure of Ar-Xe mixture has shown that optimal parameters of medium
is close to those of other studies [2-4].
300
Laser power , kW
250
8
200
150
2
4
100
1
0
0
200
Pumping power , W / cm
3
12
50
400
600
0
800
Time , s
Fig.2. Laser (1) and pumping (2) pulses.
In Fig. 3, the results of measuring a jump of pressure in the laser active medium
at P0=0.5 atm are compared with computed data obtained as a result of gasdynamic
simulation. It is shown that side by side with common increase of pressure in laser
volume, a sound wave is brought about by non-uniformity in the energy deposition into
the gas along the LAEL. In time dependence of pressure, a slight increasing of the
average pressure is observed right up to 50 ms. One of the reasons of such effect can be
an action of the delayed reactor neutrons, and also neutrons reflected from the reactor
box walls and the experimental equipment. Experiment and calculation have shown
that contribution of these neutrons can make up around 20 % of the total energy
deposited into the gas. The result of modeling used above assumption is shown in Fig. 3
by the smooth solid line with diamond symbols.
Proc. Intern. Conf. ICENES’98, 1998
 - 1998 Institute for Physics and Power Engineering, Technical Physics Laboratory
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E-mail: kuh@ippe.obninsk.ru
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P/P0
0.8
0.6
0.4
- Experiment
0.2
- Calculation
- Model
0.0
0.00
0.02
0.04
0.06
0.08
0.10
t, s
Fig. 3. Comparison of experimental relative pressure jump with results of
gasdynamic calculation and modeling.
Energy deposition , mJ /cm
3
40
30
20
10
0
0.0
0.2
0.4
0.6
0.8
1.0
Pressure , atm
Fig. 4. Specific energy deposition into the gas vs. pressure.
Proc. Intern. Conf. ICENES’98, 1998
 - 1998 Institute for Physics and Power Engineering, Technical Physics Laboratory
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E-mail: kuh@ippe.obninsk.ru
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The specific energy deposition into the medium of the LAEL volume as a
function of total medium pressure
obtained by treatment of the pressure
jump
experimental data, is shown in Fig. 4. These results turn out to be less on 12 % than
those obtained by the direct neutron-physical simulation [6,7]. Such a difference lies
within the accuracy of both measurement and simulation.
Three-dimensional distribution of fission fragment energy deposition in the
LAEL at time of the neutron pulse maximum is presented in Fig. 5. It is shown that for
our experimental geometry the rate of energy deposition into the gas is changed very
strongly both along length and radius of the LAEL.
Fig.5. Three-dimensional distribution of energy deposition rate at time of the
neutron pulse maximum.
Measurement of the laser energy dependence as a function of the output mirror
transmittance have been carried out for the ratio Ar:Xe = 200:1 at the total mixture
pressure of 0.5 atm, and the distance between reactor cores of 70 cm. Analysis of this
dependence in the frame of the laser radiation transfer equation has allowed to evaluate
Proc. Intern. Conf. ICENES’98, 1998
 - 1998 Institute for Physics and Power Engineering, Technical Physics Laboratory
http://www-tpl.ippe.obninsk.ru
E-mail: kuh@ippe.obninsk.ru
7
the stored inversion energy that appeared equal to 1.4 J . Therefore , the efficiency of
the nuclear -to-inversion energy is estimated equal to about 1 %.
12
Output power , kW
10
8
6
4
2
0
0
20
40
60
80
100
Transmittance , %
Fig.6. The output laser power as a function of the output mirror transmittance.
Measurement of the medium unsaturated gain have been performed using a
method of calibrated losses within resonator. The gain value was found to be equal to
2.6 .10-2 cm-1 at the pumping power of 220 W/cm3. The gain coefficient obtained has
been applied to calculate the saturation intensity parameter using the Rigrod’s equation
[8] to approximate experimental data on the output laser power as a function of output
mirror transmittance (Fig.6). As a result, the value of the saturation intensity has been
determined equal to ~ 215 W/cm2 at the pumping power of 220 W/cm3. Using these
data, we can estimate a value of power efficiency that appears equal to 2.5 % which is
in agreement with result of [3].
CONCLUSION
Thus, the results of studying the laser characteristics for the Ar-Xe LAEL carried
out on the BARS-6 reactor facilities, have shown some good scope for applying this cell
Proc. Intern. Conf. ICENES’98, 1998
 - 1998 Institute for Physics and Power Engineering, Technical Physics Laboratory
http://www-tpl.ippe.obninsk.ru
E-mail: kuh@ippe.obninsk.ru
8
in the laser module of a demonstrative model of the high-power laser system. Also, it
should be noted that most of experimental and calculated results are in a good
agreement.
REFERENCES
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Proc. Intern. Conf. ICENES’98, 1998
 - 1998 Institute for Physics and Power Engineering, Technical Physics Laboratory
http://www-tpl.ippe.obninsk.ru
E-mail: kuh@ippe.obninsk.ru