Pulsed laser discharge stabilization

Low power discharge preionizers, or species, are provided by a ionizable species generator that preconditions a laser gas to eliminate arcing in lasers, such as pulsed (high repetition rate) lasers. The present invention creates the species in a low power discharge and the species prevents initial arcs from occurring between high voltage electrodes of the laser. The laser comprises a pressure vessel containing the laser gas, a fan for circulating the laser gas, a heat exchanger, a catalyst, and two high voltage discharge electrodes for exciting the laser gas to create lasing. A high voltage power source is coupled to the electrodes for providing a discharge voltage thereto. A primary auxiliary discharge source is disposed adjacent to the electrodes that is used as a preionizer that creates a low density of charged particles in the main discharge volume to act as a uniform seed for the main voltage pulse derived from the electrodes. The ionizable species generator is disposed in the pressure vessel and provides a low power discharge that preconditions the gas to eliminate arcing in the laser. A preionization discharge generated by the ionizable species generator is used to generate species that have low ionization potential. These easily ionized species are then preionized by ultraviolet radiation generated from the primary auxiliary discharge adjacent to the main discharge electrodes to form a uniformly ionized background for the main discharge to work on. The low ionization potential species replace the gas hydrocarbon contaminants that are adsorbed by the catalyst. A method of eliminating arcing in the laser is also disclosed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to pulsed lasers, and more 
particularly, to discharge stabilization of high repetition rate lasers. 
2. Description of Related Art 
The assignee of the present invention has developed TEA CO.sub.2 lasers 
that provide high repetition rate (to 200 Hz) output in a compact, sealed 
package using a catalyst. However, this compact, sealed, high repetition 
rate TEA CO.sub.2 laser, and other such pulsed lasers that utilize a 
catalyst, suffer from discharge arcing in the first several shots just 
after turn-on from a quiescent state. The quiescent state may be as short 
as tens of seconds. Arc elimination is important for laser lifetime and to 
guarantee "first shot" capability in a high data rate situation. 
Heretofore, no means or method of suppressing initial arcing has been 
available. 
High repetition rate TEA CO.sub.2 lasers produce laser output by pumping a 
gas mixture with a high power electrical discharge that takes place 
between two accurately aligned and surfaced electrodes. Lasers of this 
type that operate sealed with a catalyst have the problem that when they 
are first turned on, discharge arcing occurs for the first several shots. 
These arcs are highly localized current paths that prevent laser emission 
and cause inordinate wear of the electrodes, with the danger of permanent 
electrode damage under similar repeated use. However, the initial arcs do 
generate preionizer species that stabilize subsequent discharges. 
The problem of initial shot arcing has been associated with high repetition 
rate (200 Hz) lasers that are compact, with relatively small gas ballast, 
and that operate sealed with a heterogeneous catalyst. In these lasers, 
neutral, low ionization potential hydrocarbons that are necessary for 
discharge stabilization are attached by the catalyst. The initial arcs 
produce low ionization potential species from the main gas constituents 
that replace the hydrocarbons. For very large lasers, operating at reduced 
repetition rates on the order of 20 Hz, initial shot arcing has generally 
not been a problem, but it is not yet known if the quality of laser output 
could be enhanced using the auxiliary discharge concepts of the present 
invention. 
Heretofore, a primary auxiliary discharge located adjacent to the 
electrodes has been used to preionize low density hydrocarbons that then 
act as a uniform background for stabilization of the main voltage pulse. 
This auxiliary discharge is used as a preionizer that creams a low density 
of charged particles in the main discharge volume to act as a uniform seed 
for the main voltage pulse. However, this auxiliary discharge has not 
eliminated the occurrence of the initial arcs, whose overall effect is to 
deteriorate the laser, impair its performance, and limit its operating 
lifetime. 
Accordingly, it is an objective of the present invention to provide for a 
method and apparatus that eliminates the above-described discharge arcing 
problem in pulsed high repetition rate lasers, and the like. It is a 
further objective of the present invention to provide for a method and 
apparatus that provides ionizable species having low ionization potential 
derived from a secondary auxiliary discharge. 
SUMMARY OF THE INVENTION 
The present invention solves the problem of discharge arcing on initial 
shots in pulsed high repetition rate lasers by using a secondary auxiliary 
discharge to produce ionizable species from the laser gas constituents as 
replacements for those adsorbed by a catalyst. The ionizable species 
produced by the present invention prevents initial arcs from occurring 
between the main high voltage electrodes of the laser. This is important 
because it extends laser lifetime and it provides for "first shot" 
capability. No other means of suppressing initial arcing in high 
repetition high repetition rate lasers is known. Therefore, the present 
invention eliminates a basic operational limitation of existing pulsed 
high repetition rate lasers, and the like. 
More particularly, laser apparatus of the present invention comprises a 
pressure vessel containing a laser gas, and a fan is disposed in the 
pressure vessel for circulating the laser gas therein. A heat exchanger is 
disposed in the pressure vessel, and a catalyst module containing a 
catalyst is disposed in the pressure vessel. Two high voltage discharge 
electrodes are disposed in the pressure vessel for exciting the laser gas 
to create lasing. A high voltage power source is coupled to the electrodes 
for providing a discharge voltage thereto. The high voltage power source 
comprises a switch, or thyratron, for example, a trigger input for the 
switch, and energy storage means comprising a capacitor and an inductor. A 
primary auxiliary discharge source is disposed adjacent to the main 
discharge electrodes that is used as a preionizer that creates a low 
density of charged particles in the main discharge volume by means of 
ultraviolet radiation to act as a uniform seed for the main voltage pulse 
from the electrodes. An ionizable species generator in accordance with the 
present invention that is adapted to provide a secondary auxiliary 
discharge is disposed through a wall of the pressure vessel, and provides 
a low power discharge that creates ionizable species for the primary 
preionizing auxiliary discharge to act upon. 
The ionizable species generator comprises a low voltage DC discharge source 
coupled to a power supply. The low voltage DC discharge source comprises a 
center electrode, a ceramic insulator disposed around the periphery of the 
center electrode to prevent high voltage flashover outside the pressure 
vessel and ensure that the discharge takes place at the tip of the center 
electrode, and an outer electrode disposed around a portion of the 
periphery of the ceramic insulator that is grounded to the wall of the 
pressure vessel. 
The primary auxiliary preionization discharge is used to provide a uniform 
background of ionization just prior to the main discharge. The present 
invention creates the ionizable species that are a replacement for gas 
hydrocarbon contaminants adsorbed by the catalyst. These ionizable species 
produced by the ionizable species generator include excited neutral 
species, negative ions, and positive ions that have ionization potentials 
well below those of parent molecules that comprise the laser gas mixture. 
In contrast to the attempts of the prior art, in the present invention, the 
second auxiliary discharge is used to generate species that have low 
ionization potential. These easily ionized species are then ionized by 
ultraviolet radiation generated from the primary auxiliary discharge 
adjacent to the main discharge electrodes in the usual way to form a 
uniformly ionized background for the main discharge to work on. The low 
ionization potential species replace the gas hydrocarbon contaminants that 
are adsorbed by the catalyst. 
The present invention also contemplates a method of eliminating arcing in a 
pulsed laser. The method comprises containing a laser gas in a pressure 
vessel and circulating the laser gas within the pressure vessel. A low 
voltage discharge is generated to produce low ionization potential species 
that are circulated within the pressure vessel. The species are ionized to 
form a uniform background for laser discharge. Discharging the laser in 
the presence of the uniformly ionized background eliminates arcing in the 
pulsed laser.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the drawing figures, a cross-sectional view of a pulsed laser 
10, such as a TEA CO.sub.2 laser 10, for example, that incorporates a 
ionizable species generator 20 in accordance with the principles of the 
present invention is shown in FIG. 1. The laser 10 is comprised of a 
pressure vessel 11 that contains a laser gas 18, and in which is disposed 
a fan 12, a heat exchanger 13 and two high voltage discharge electrodes 
14, 14a. A catalyst module 17 that contains a catalyst 17a is disposed 
between the heat exchanger 13 and one of the electrodes 14. A high voltage 
power source 21 is provided that is coupled to the electrodes 14a and to 
the pressure vessel 11 and which comprises a switch 22, such as a 
thyratron 22, for example, and a trigger input 23 for the switch 22. The 
high voltage power source 21 includes an energy storage means 24 
comprising a capacitor 25 and an inductor 26. A primary auxiliary 
discharge source 19 is disposed adjacent to the electrodes 14, 14a that is 
used as a preionizer that creates a low density of charged particles in 
the main discharge volume using ultraviolet radiation to act as a uniform 
seed for the main voltage pulse derived from the electrodes 14, 14a. The 
ionizable species generator 20 is comprised of a DC discharge source 15 
disposed through a wall of the pressure vessel 11 that is coupled to a 
power supply 16. The DC discharge source 15 is adapted to provide a 10 kV 
DC discharge inside the pressure vessel 11. The ionizable species 
generator 20 provides a secondary auxiliary discharge source for the laser 
10. 
In operation, the fan 12 causes, laser gas 18 to flow across the two high 
voltage discharge electrodes 14, 14a and to pass through the catalyst 
module 17 and heat exchanger 13. The high voltage power source 21, charged 
to 25 kV, supplies pulsed power to the electrodes 14, 14a and this power 
is pumped into the laser gas 18 through an avalanche discharge of about 
200 nsec duration. Uniformity of this discharge is critical in achieving 
reliable laser output. If the discharge arcs, then all the discharge 
current is carded in a hot current channel of about 1 mm diameter and the 
laser gas 18 is not pumped, with the result that laser action is not 
achieved. Discharge uniformity is therefore an important requisite for 
laser output. 
In order to achieve discharge uniformity, it is essential to have low 
ionization potential species contained in the laser gas 18 that are 
preionized by a precursor discharge that takes place about 25 nsec before 
application of the main discharge voltage pulse by the high voltage power 
source 21. These low ionization potential species are usually hydrocarbon 
contaminants. The primary auxiliary discharge source 19 ionizes these 
species by ultraviolet radiation creating a low density of charged 
particles in the main discharge volume prior to pulsing of the electrodes 
14, 14a. The ionized species act as a uniform seed for the main voltage 
pulse from the electrodes 14, 14a. However, in the presence of the 
catalyst 17a, the low ionization potential hydrocarbons are adsorbed. 
The role of the ionizable species generator 20, or secondary auxiliary 
discharge source, is to generate excited species 34 (shown in FIG. 2) that 
take the place of the low ionization potential hydrocarbons that are 
depleted by the catalyst 17a. These species 34 produced by the ionizable 
species generator 20 are carded into the main discharge by the gas flow. 
The use of the ionizable species generator 20 (DC discharge source 15), 
therefore guarantees that the first several laser shots of the laser 10 
are arc-free. On subsequent shots, the laser discharge itself generates 
excited species 34 and discharge uniformity is self-perpetuating. If the 
ionizable species generator 20 is turned off for tens of seconds in the 
absence of any auxiliary DC discharge preionizer generation, the excited 
state preionizers are lost through recombination and the main laser 
discharge arcs for the first few shots when the laser 10 is turned on 
again. 
Under normal operating conditions in a sensor, or other electrooptical 
device, for example, the laser 10 is turned on and off many times in the 
course of a normal test session, for example. Therefore, without the 
generation of preionizers, the main discharge electrodes 14, 14a 
experience a great number of arcs. These arcs cause erosion and pitting of 
the electrodes 14, 14a, which may limit the lifetime of the laser 10 and 
may occasionally cause catastrophic failure if pitting of the electrodes 
14, 14a is highly localized. The arcing process is statistical in nature 
and lifetime degradation or catastrophic failure cannot be predicted. The 
auxiliary preionization discharge provided by the present invention solves 
these problems. 
A detailed illustration of the ionizable species generator 20 is shown in 
FIG. 2. Voltage is applied to a center electrode 31 of the discharge 
source 15 and an outer electrode 32 is grounded to the wall of the 
pressure vessel 11. A ceramic insulator 33 prevents high voltage flashover 
outside the pressure vessel 11 and ensures that the auxiliary discharge 
takes place at the tip of the center electrode 31. The ionizable species 
generator 20 is hermetically sealed with welded joints between the ceramic 
33 and center electrode 31. In addition, the ceramic 33 is welded into a 
fitting that makes the connection to the wall of the pressure vessel 11. 
The electrodes 31, 32 are made of stainless steel and have not shown any 
wear to date. The power supply 16 operates at low voltage, typically less 
than 5 kV, and is current limited. The secondary auxiliary discharge 
caused by the ionizable species generator 20 generates the species 34 as a 
result of the gas discharge at the tip of the center electrode 31. These 
species 34 are entrained in the gas flow and carded into the area adjacent 
the main discharge electrodes 14, 14 a where they assist in discharge 
stabilization. 
During operation of the laser 10, the high voltage power supply 16 is 
turned on and the auxiliary discharge starts immediately. A glow discharge 
has a negative resistance characteristic; therefore, a power supply 
current limiter (not shown) is needed to prevent the power supply 16 from 
entering current saturation. The discharge enters steady state within 1 
msec and continues to generate low ionization potential species 34 until 
the laser 10 is turned off. This is true even if the laser 10 operates in 
burst mode or intermittently with long periods of nonfiring. 
Tests have been performed on the laser 10 incorporating the ionizable 
species generator 20 of the present invention and test results are 
described below. The primary auxiliary discharge operates in three modes 
as shown in FIG. 3 in which the discharge current is plotted against the 
voltage produced by the power source 21. For low supply voltages below 
about 7 kV, no current flows because the voltage is insufficient to break 
down the gas. Between 7 kV and 16 kV, however, the gas breakdown voltage 
is exceeded and the discharge takes the character of a relaxation 
oscillator. Typical discharge voltage traces as a function of time are 
shown in FIG. 4. In this case, the power source 21 charges up stray 
capacitance through a large value resistor (not shown) until the breakdown 
voltage is reached, at which point the discharge is ignited dumping energy 
out of the stray capacitance. At breakdown, the voltage quickly falls to 
effectively zero, because the arc impedance is very low. After this energy 
dumping, the discharge turns off again for lack of input power and the 
power source 21 recharges the stray capacitance. This cycle is repeated at 
frequencies from 300 Hz to 600 Hz, depending upon applied voltage. The 
higher the charge voltage, the shorter the charge time. The discharge 
ignites at different locations from shot-to-shot, giving the effect of a 
series of arcs that fill an annulus between the inner and outer electrodes 
14, 14a. Beyond a power source 21 voltage of about 16 kV, the relaxation 
oscillator frequency is so high that it transitions to a steady state 
discharge, but at somewhat lower average input power. This is a third 
phase in which a steady, single arc forms. The single arc has 
characteristics typical of a glow discharge. That is, it has a negative 
resistance in which an increase in current leads to a decrease of 
discharge impedance. 
When using the ionizable species generator 20, it is desirable to operate 
the power source 21 as a relaxation oscillator for the following reasons. 
In the relaxation oscillator mode, the arc covers a much larger effective 
area than the steady arc, thereby reducing wear on the electrodes 14, 14a. 
A relaxation oscillator discharge is characterized by a high voltage 
phase, followed by relatively high current at low voltage. This has been 
shown to be desirable for generation of gas dissociation products that 
benefit from preionization. Finally, the relaxation oscillator operates at 
lower applied voltage than a steady arc. The optimum operating point is an 
applied voltage of about 10 kV with discharge input power of about 1.0 
Watt. 
The various parameters relevant to the results outlined above are as 
follows: 
______________________________________ 
Stray capacitance 40 pF 
Input resistor 50 Mohm 
Gas/pressure CO.sub.2 /N.sub.2 /He = 1/1/3 at 1 atm 
Voltage/polarity 10 kV/Negative 
Typical input energy/pulse in 
2 mJ 
relaxation mode 
Typical relaxation oscillator 
400 Hz 
frequency 
Typical average discharge power 
0.8 W 
Typical average total power 
1.6 W 
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In initial tests, it was found that the ionizable species generator 20 was 
effective in eliminating first pulse arcing at a laser discharge 
repetition rate of 1 Hz in which the recirculating fan 12 was not used and 
the catalyst 17a was at room temperature. Thus, the auxiliary discharge 
provided by the ionizable species generator 20 has been demonstrated to be 
effective in eliminating arcing at laser repetition rates of 1 Hz. At 
higher laser repetition rates, it is necessary to place the primary 
auxiliary discharge in close proximity to the main laser discharge and to 
pump it at high average power. Consequently, it is expected that the 
ionizable species generator 20 will effectively eliminate arcing in the 
laser 10 with appropriate placement of the discharge electrode 31. 
In view of the above, the present invention also provides for a method of 
eliminating arcing in a pulsed laser 10. The method comprises containing 
the laser gas 18 in the pressure vessel 11 and circulating the laser gas 
18 within the pressure vessel 11. A low voltage discharge is generated to 
produce species 34 having low ionization potential that are circulated 
within the pressure vessel 11. The species 34 are preionized to form a 
uniformly ionized background for laser discharge. Discharging the laser 10 
in the presence of the uniformly ionized background of the preionized 
species 34 substantially eliminates arcing in the pulsed laser 10. 
Thus there has been described a new and improved discharge stabilizer for 
use in pulsed high repetition rate lasers, such as TEA CO.sub.2 lasers. It 
is to be understood that the above-described embodiment is merely 
illustrative of some of the many specific embodiments which represent 
applications of the principles of the present invention. Clearly, numerous 
and other arrangements can be readily devised by those skilled in the art 
without departing from the scope of the invention.