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Downloaded by UNIVERSITY OF ADELAIDE on October 25, 2017 | http://arc.aiaa.org | DOI: 10.2514/6.1993-3165
AIAA 93-3165
High Average Pb.ver XeCl and Pulsed
HF Chemical Lasers
H. Brunet, B. Lacour, H. Besaucele and
M. Mabru
LASERDOT (Groupe Aerospatiale)
91460 Marcoussis, FRANCE
AIAA 24th
Plasmadynamics & Lasers Conaence
July 6-9, 1993 / Orlando, FL
For permission to copy or republish, contact the American institute of Aeronautics and Astronautics
370 L'Enfant Promenade, S.W., Washlngton, D.C. 20024
H. BRUNET, B. LACOUR, H. BESAUCELE and M. MABRU
LASERDOT (Groupe AEROSPATIALE)
Route de Nozay, 91460 Marmussis, FRANCE
Downloaded by UNIVERSITY OF ADELAIDE on October 25, 2017 | http://arc.aiaa.org | DOI: 10.2514/6.1993-3165
W
ABSTRACT
11. BACKGROUND
We have developped high-repetition-rate,
high-average power XeCl excimer and HF
chemical lasers based on the use of phototriggered
discharges. Simple and long-life corona
preionization units have been employed. Compact
gas circulation loop were used to achieve highrepetition-rate operation. The XeCl laser has
delivered an average output power of 510 W at a
repetition rate of 650 Hz whereas the HF
chemical laser delivered a multi-line power of
500 W at a frequency of 110 Hz.
The conventional discharge circuits for all
excimer and pulsed chemical lasers are
characterized by very low inductance, very fast
voltage and current risetimes and high peak
currents. A typical circuit is shown in Fig. 1. The
main discharge electrodes are shown but provision
for preionization (UV or X-ray) has been
eliminated for simplicity. Usually a relatively
long preionization pulse is applied to the gas
mixture at the same time the thyratron is
triggered. Once the thyratron is closed, the initial
charge stored on C is transferred to the discharge.
Due to the highly transient nature of the glow
phase, the short lifetime of the upper laser level
and/or the efficient quenching collisions, the
current pulse have to be as short as possible.
Typical voltage and current waveforms are shown
in Fig. 1. The thyratron peak current may reach
several tens of kA with a dI/dt about 5 x 10l1 A/s.
Therefore, severe stresses are put on the thyratron.
I. INTRODUCTION
Since the discovery of chemical and excimer
lasers there has been a considerable effort to
produce high pulse energy and high average laser
power in the mid-infrared or the near UV. Pulsed
lasers are of great interest for material studies and
materials processing.
In the past, conventional electric discharges
using X-ray or W preionization were employed to
produce uniform glow discharges in gas mixtures
containing attaching molecules such as FL HCI or
SFg. In such discharges, the necessity to achieve
short current risetimes and high peak current put
stringent restraints on the electrical circuit and
especially on the switch, typically a thyratron.
For high-repetition-rate, long-life lasers, the
thyratron reliability has a high level of
importance.
W
H
0 1993 American Institute of Aeronautic5 and
Astronautics. Inc All righrs recerved
T
q
v-t PREION
Recently two novel electrical circuits which
do not place undue stress on the thyratron have
been developped : the phototriggered discharge
(1) and the double discharge, also known as
spiker/sustainer excitation technique (2). The two
technologies require that the gas laser itself acts
as a switch in the discharge circuit. Developed in
order to maximise laser performance with high
thought given to reliability, they were recently
used to realize high average power excimer laser
(3) (4). In this paper, phototriggered discharges
have been used to produce high-repetition-rate,
high average laser power operation. We will
present the design and performance of our XeCl
excimer and HF chemical lasers.
Copyright
THYRATRON
-
f f,
50 ns
Fig. 1 - Scheme and electrical waveforms of a
single discharge
In a phototriggered discharge, a fast switch
is not necessary because the gas laser itself acts as
1
Special cares have been taken in the design of
the laser head in order to prevent parasitic corona
discharges or surface flashovers which could
trigger a parasitic discharge before the
phototriggered one. The field along the insulator,
the choice of the material used for this insulator
and the capacitors position have been thorougly
studied. The corona phototriggering device is
placed behind the center flat part of the cathode.
An outline of the cross-section of the laser head
can be viewed in Fig. 3. The active discharge
volume was 3 (h) x 3 (w) x 50 (1) an3.The discharge
electrodes consisted of a solid anode and a grid
made of nickel or tungsten alloy. The anode had a
modified Chang profile and the grid acted as a
cathode both for the main discharge and the
Corona discharge. This latter was produced by
applying a very short voltage pulse (< 5 ns) to a
Corona anode separated from the grid by 4 mm of
ceramic glass.
Downloaded by UNIVERSITY OF ADELAIDE on October 25, 2017 | http://arc.aiaa.org | DOI: 10.2514/6.1993-3165
a switch, holding off the voltage (above steadystate voltage breakdown) usually for several
microseconds. Fig. 2 shows a schematic of the
circuit together with the voltage and current
waveforms. To initiate the gas breakdown, a very
short preionization pulse is applied to the gas
mixture. Then electrons produced by photoionization rapidly avalanche leading to gas
breakdown and the formation of a uniform
discharge. Due to the high electron multiplication
rate in the gas mixtures considered in this work, it
is very important to apply very short (5 5 ns)
preionization pulse to insure uniform glow
discharge. In our experiments corona preionization
hasbeenused.
v
Hx
ChPACI1OR AND
THYRATRON HOUSING
t PREION
MAT
EXCHANGER
WATER M E T
AND OUTLET
bnlCrOLeCOndeS
e__l
Fig. 3 - Cross sectional structure of the laser
system
Fig. 2 - Scheme and electrical waveforms of a
phototriggered discharge
1.2. Electric circuit
At low repetition rate (< 10 Hz), efficient
operation of both XeCl excimer and HF chemical
laser was achieved using the simple circuit
described above. However, at higher repetition
rates, ineffectual removal of discharge residuals
between two pulses (even using a high gas clearing
ratio) limits the application time of the voltage
to less than 1 ps. Therefore the circuit scheme used
at high repetition rate was of the C-L-C type and
included a thyratron. However, the peak
thyratron current was considerably reduced in
comparison to that required in a conventional
discharge.
The discharge was powered by a charge
transfer circuit. The storage capacitor C1 and the
peaking capacitors C2 are ceramic capacity bank of
capacitance C1 = 200 nF and C2 = 150 nF. The
inductance L1 was chosen to obtain a charging time
of about 500 ns so that the peak current in the
thyratron (EEV 1625) was limited to 14 kA for a
charging voltage of 22 kV. L2 is mainly the
inductance of the laser head estimated to 10 nH.
Figure 4 shows a typical time evolution of the
laser voltage.
To operate the laser at the best efficiency, it
was found necessary to control precisely the photo
triggering time. The Corona HV pulse was
produced by a small thyratron triggered at a
chosen time (usually about 200 ns) by the control
electronin with a jitter less than 10 ns.
111. EXPERIMENTS
1. Excimer laser
1.1. Laser head and corona preionization
2
-
was only 510 W at 650 Hz. This behavior was
supposed to be due to acoustic disturbances.
Recently, we put an acoustic damping material
inside the vessel to suppress the pressure waves.
Good performance as shown on Fig. 5 was obtained,
the average power being presently limited by the
power supply capacity and ultimately by the gas
flow rate.
1.3. Gas handling svstem
Downloaded by UNIVERSITY OF ADELAIDE on October 25, 2017 | http://arc.aiaa.org | DOI: 10.2514/6.1993-3165
w
The laser discharge unit was placed on the
top of a cylindrical stainless steel vessel 1 m in
diameter and 1 m long which contained the gas
circulating and cooling system. A cross-sectional
diagram of the system is given in Fig. 3.
0
Fig. 4
L
-
V o l t a g e and
current waveforms
(hor.100 nsldiv. - vert. 5 kV/div.)
200
400
800
800
Repetition rale ( Hz )
Fig. 5 - Average laser power vs the repetition rate
The two specially-designed centrifugal
blowers were driven by two external 7.5 kW
motors. The typical gas flow at 5 bars was 700 l/s
for a total pressure drop in the laser head and the
heat exchanger close to 60 mbar. The gas clearing
ratio was about 2 at a repetition frequency of
500 Hz.
The evolution of laser effiaency with respect
to the repetition rate frequency is shown in Fig. 6
for 2 charging voltages. The laser efficiency was
relatively independant of the charging voltage
and hence of the input energy.
An external gas circuit was used to remove
dust from the gas mixture and to inject clean gas
towards the mirrors. Slide valves allowed for the
cleaning of these mirrors without changing the gas
mixture.
2.4
a9
2.2
2.0
1.4. Exoerimental results
V
The gas mixture has been optimised at low
repetition rate (1 Hz) and the corresponding
composition (HC1: 0.7 %o, Xe : 2 %o, Ne : balance)
used all along this work. An energy per pulse of
1.2 J was achieved.The pulse duration was 70 ns.
Satisfactory operation of this laser was confirmed
by the fact that the laser pulse duration (Fig. 4)
was in agreement with the storage line transit
time.
E
Figure 5 shows the repetition rate dependence of the average power. As it can be shown,
the average laser power increases almost linearly
with frequency up to 400 Hz.
Fig. 6 - Efectricaf efficiency us the repetition rate
1.6
E
0
200
400
600
800
Repetition rate (
Hz )
15.Conclusion
Using a phototriggered discharge, we have
realized a compact, phototriggered highrepetition rate laser capable of delivering an
However, above 400 Hz, this was no longer
true and the maximum average power achieved
3
.
average power over 500 W on the XeCl emission.
Our performance compares favourably with those
obtained by other discharge techniques (5) (6) (7).
Furthermore, technical features let us hope a good
reliability for this device which prefigures the
prototype of an industrial excimer laser.
2. Pulsed HF chemical laser
applied to the corona anode was 6 ns and its peak
value was about 15 kV.
Icorona anode
Downloaded by UNIVERSITY OF ADELAIDE on October 25, 2017 | http://arc.aiaa.org | DOI: 10.2514/6.1993-3165
This laser was based on the fast reaction F +
H2 + HF (v) + H where the F atoms were produced
by electrical dissociation of sF6.
4
In gas mixtures containing SF6, the lifetime
of the preionization electrons is very short because
they are rapidly removed from the plasma by
attachment. Consequently the preionization
density produced by the X-ray or VUV pulse could
be considerably reduced at the time of application
of the sustaining voltage. Therefore the discharge
would initiate from too low a preionization
density to be uniform. The phototriggered
discharge presents a decisive advantage over the
conventional one because the preionization pulse is
applied when the voltage across the electrodes is
well above the steady-state breakdown voltage.
There is no longer attachement but instead electron
multiplication. The initial electrons produced by
the VUV corona burst are then immediatly
multiplied by avalanche without any loss.
capacitors
Fig. 7 - Cross sectional structure of the laser head
2.2. Electric circuit
As for the excimer laser, the discharge was
powered by a charge transfer circuit. However,
two elementary circuits were placed in parallel to
feed the discharge, each one including a thyratron
(EEV 1725) This disposition was used because the
charging time was short (- 200 ns) and the peak
current was high. This method also provided for a
lower impedance.
2.1. Laser head and corona meionization
The storage and peaking capadtors consisted
of two banks of TDK ceramic capacitors of total
capacitance 2 C1 = 210 nF and 2 C2 = 115 nF. The
geometrical arrangement of the components is
shown in Fig. 8.
The laser head shown in Fig. 7, not very
different from that used for the excimer laser, was
made of aluminium and Teflon (PTFE). The active
discharge length was 45 cm while the discharge
gap was fixed at 3 cm. The discharge electrodes
consisted of profiled nickel-coated brass bar and a
flat grid made of nickel. The transverse cross
section of the solid anode was designed to obtain a
uniform electric field in the discharge region over
a width of 4 cm. An analytic compact profile with
k = 0.08 due to Chang was adopted. The grid acted
as cathode both for the main discharge and the
corona discharge. The preionization was produced
by the application of a very short voltage pulse to
the corona anode separated from the grid by 3 nun
of dielectric (Al2O3). The rapid change of voltage
between the electrodes produces a conducting
channel on the surface of the dielectric followed
by the production of large amounts of VUV
radiation in the main discharge gap. High purity
alumina was chosen as the dielectric material
because it has a high dielectric constant (E = 9) and
at the same time, a high dielectric strength
(17 kV/mm). This permits a high-capacitance
value to be achieved using a not too thin wall.
Furthermore alumina is inert to F, F2 or HF. It also
possesses a high thermal conductivity which
limits temperature gradients on the surface.
Typically, the risetime of the voltage waveform
LJ
Fig. 8 - Geometrical arrangement of the laser
head components
The discharge was triggered by the very fast
voltage applied to the corona electrode about 50 ns
before the maximum of the C2 voltage.
Primary power was provided to the charge
transfer circuit by 4 high voltage power supplies
(ALE 402) placed in parallel. Each unit provides
an average charging rate of 4 kJ/s over an output
4
u'
..
voltage of 50 kV. The trigger pulses to the
thyratrons and the preionization unit were
delivered by a home-made pulser driven by a
variable pulse repetition-rate clock circuit.
Laser performance was firstly studied
in single shot operation. Using a SFg-C2Hg-hk =
0.5-0.05-0.45 gas mixture at a pressure of 200 mbar,
a HF laser pulse energy of 5.4 J was obtained at a
charging voltage of 4 2 k V . This result was
achieved by using an optical cavity consisting of
a 10-m radius of curvature full reflector and a
flat CaF2 output coupler. The laser burn pattern
was rectangular with dimensions 3(h) x 4(w) cm2.
2.3. Gas handling svstem
Downloaded by UNIVERSITY OF ADELAIDE on October 25, 2017 | http://arc.aiaa.org | DOI: 10.2514/6.1993-3165
W
The clearing of the discharge region of
discharge products between pulses is especially
important in HF/DF chemical lasers because
residual HF/DF molecules would degrade laser
output. A flow velocity of at least 4 m/s in the
discharge region was necessary to achieve an
equivalent of even one full gas exchange between
pulses at a repetition frequency of 100 Hz.
Actually 2 to 3 times more velocity was required
due to the backstreaming caused by the discharge.
Therefore the flow velocity was fixed at 12 m/s
which corresponds to a flow rate of about 300 l/s.
Two centrifugal fans driven by a common 400 Hzmotor were placed inside the vacuum vessel. Each
fan is capable of moving 150 l/s of gas against a
30 mbar pressure differential at a total pressure of
150 mbar. The gas mixture was cooled by passing
through a stainless steel heat exchanger which
removed about 2OkW from the gas flow at the
repetition rate of 110 Hz.
L/
In repetitive operation, the average output
power increases almost linearly with repetition
rate up to about 100 Hz. Fig. 10 shows an
oscillogram of a short sequence of output laser
pulses at a repetition frequency of 75 Hz with the
46th pulse ex+nded.
I
The probable deleterious contaminants in the
gas mixture were HF and S,F,-type
species.
These impurities were efficiently removed by
passing the gas through a chemical trap located in
the region of slowest flow. The trap consisted of a
rectangular stainless steel box filled with a
mixture of 5 A and 13 X molecular sieves closed by
coarse stainless steel gauze mounted on the
sidewalls. Figure 9 presents a schematic of the
laser system.
I
I
I
Fig. 10 - Sequence of output laser pulses at 75 Hz
w i t h the 46ih p u l s e
expanded
(hor. 200 nsldiv.)
The pulse energy was nearly constant from
pulse to pulse. The laser pulse duration was about
160 ns. Fig. 11 shows an oscillogram of the
corresponding voltage pulses. The risetime of the
voltage as seen on the expanded 46th pulse was
about 200 ns.
Fig. 11
t
rimental results
Sequence of discharge voltage pulses at
50 H z with the 46th pulse expanded
-
The operation regularity at high repetition
rate is illustrated in Fig. 12 which shows an
oscillogram of a sequence of output laser pulses at a
repetition frequency of 100Hz at a charging
voltage of 35 kV. It can be seen that the pulse
energy is almost constant from pulse to pulse,
Fig. 9 - Cross sectional structure of the lase?
system
5
The work on the HF chemical laser was
supported by Direction des Recherches Etudes et
Techniques, Paris.
REFERENCES
0. de Witte, B. Lacour and C. Vannier,
Conference on Lasers and Electro-Optics,
Phoeniw, AZ,April 1982.
W.H. Long, M.J. Plummer and E.A. Stappaerts,
Appl. Phys. Lett. 43, 735 (1983).
Downloaded by UNIVERSITY OF ADELAIDE on October 25, 2017 | http://arc.aiaa.org | DOI: 10.2514/6.1993-3165
Fig. 12 - Sequence of output laser pulses at 100 H z
H. Nagai, g t h Symposium on Gas Flow and
Chemical Lasers, paper WdEL2, Heraklion,
Greece, Sept. 92
At high repetition rates, the charging voltage
had to be limited to 38 kV. Therefore a maximum
average power of 500 W was achieved at 110 Hz.
The electric efficiency calculated as the ratio of
the power delivered to the storage capacitors to
the laser power was 3 %.
B. Lacour, H. Brunet, H. Besaucele and
C. Gagnol, g t h Symposium on Gas FIow and
Chemical Lasers, paper WdEP2, Heraklion,
Greece, Sept. 92
The use of a chemical trap together with a
large storage vessel allowed for a relatively long
time operation without a significant decrease of
the output laser power. Typically, more than
lo4 pulses have been produced per gas fill. By
continuously adding fresh gas mixture, it should be
possible to run the laser over a long period because
sulfur deposit was very small and did not alter
laser behavior.
H. Frowein and B. Basting, gtk Symposium on
Gas Flow and Chemical h e r s , paper WdEPl,
Heraklion, Greece, Sept. 92
B. Godard, P. Laborde, and P. Murer, g t h
Symposium on Gas Flow and Chemical Lasers,
paper WdEP3, Heraklion, Greece, Sept. 92
M. Stehl6, J. Bonnet and D. Pigache
To our knowledge, any high average power,
recirculating HF chemical laser has never been
reported. Howeve!,
a previously described laser
has achieved very high repetition rate with low
power (9).
7/ H.M. von Bergmann and P.H. Swart, 8 t h
Symposium on Gas Flow and Chemical lasers,
SPIE, vol. 1397, pp. 63-66, Madrid (1990)
8/ V.M.Borisov, A. Yu. Vinokhodov and Yu. B.
Kiryuklin, Sov. J. Quantum Electron. 17, 595
2.5. Conclusion
(1986)
A phototriggered discharge has been used to
realize a repetitively pulsed HF chemical laser.
It has produced about 5 J per pulse with an average
power of 500 W at 110 Hz.The electrical efficiency
was 3 %.
9/ R.I. Rudko, Z. Drozdowicz, S. Linhares and
D. Bua, "High-repetition-rate, recirculating
hydrogenldeuterium fluoride laser", Rev. Sci.
Inshum., 53,452, (1982).
111. CONCLUSION
We have demonstrated that phototriggered
discharges can be successfully used to realize
efficient gas lasers. Initially this type of
discharge technique was developed in order to
increase the component lifetimes and therefore the
reliability. Our results have shown that laser
performance have not been sacrified.
ACKNOWLEDGMENTS
The work on the XeCl excimer laser was
supported by the french ministers MRE and MICE
as a part of the EUREKA project EU 205.
6
W'
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