Laser utilizing a gaseous lasing medium and method for operating the same

The invention relates to an improvement in gas lasers and a method of operating the same. In one aspect, the invention is an improved method for operating a high-power gas laser. The improvement comprises introducing the gas lasing medium tangentially to the laser tube at a pressure establishing a forced vortex in the tube. The vortex defines an axially extending core region characterized by a low pressure and temperature relative to the gas inlet and the exterior of the vortex. An electrical discharge is established in the core region to initiate lasing of the gas. The gas discharge from the tube is passed through a diffuser. As in conventional gas lasers, firing results in a very abrupt increase in gas temperature and in severe disruption of the gas. However, the gas vortex almost immediately restores the gas to its pre-firing condition. That is, almost all of the waste heat is transferred radially to the laser wall, and the original gas-flow pattern is restored. As a result, the power output of the laser is increased significantly, and the laser firing repetition rate is markedly increased.

BACKGROUND OF THE INVENTION 
This invention relates generally to lasers utilizing a gaseous lasing 
medium and more particularly to methods and apparatus for improving the 
operation and cooling thereof. The term "gaseous medium" is used herein to 
include gases, vapors, and mixture thereof. 
In the typical high-power gas laser, an electric discharge or other energy 
source initiates lasing, which is accompanied by an abrupt increase in the 
temperature of the gas and by severe gas disturbances, such as shock waves 
and sound waves. As much as 97% of the power input to the laser may be 
dissipated as heat. Most of this heat from the high-power laser is removed 
by a cooling fluid which is circulated about the outside of the laser 
tube. Before the laser can be re-fired, the gas must be cooled so that the 
electrons again are at their ground-energy state and the above-mentioned 
disturbances minimized. Typically, this is accomplished by gas-purging the 
high-power laser tube several times. The typical purging operation 
comprises (a) circulating the gas discharge from the laser through an 
external loop including a blower, a heat exchanger, and flow-straightening 
means (such as metal guide vanes) and (b) re-directing the conditioned gas 
through the laser. It would be advantageous if the transfer of heat from 
the laser gas to the laser-tube coolant could be accomplished more 
efficiently; this would result in an increase in the laser power output. 
It would also be advantageous if the transfer of heat could be 
accomplished by a technique which acts to suppress laser-gas disturbances 
immediately after firing of the laser occurs. 
The following article describes the use of a forced-vortex heat exchanger 
to cool a gas as it flows through an infrared-detector tube: J. M. Nash, 
"Vortex Heat Exchanger Cooling for IR Detectors," Applied Optics, Vol. 14, 
No. 12 (December 1975). The described vortex heat exchanger comprises an 
annular device which is closed at one end and is provided with a diffuser 
at its other end. Air is introduced tangentially near the closed end and 
forms a vortex which swirls about the exterior of the detector tube. 
However, unlike the standard vortex tube, the vortex heat exchanger is 
connected to a diffuser, allowing the temperature and pressure at the core 
of the vortex to be much lower than at either the tube inlet or at the 
periphery (outer circumference) of the vortex. The article states that the 
provision of a diffuser at the laser-tube outlet markedly increases the 
efficiency of the vortex tube because it permits the pressure at the 
center of the vortex to fall below atmospheric without inflow occurring. 
The following publication describes a gas laser which is provided with 
internal spiral heat-exchange fins for directing the laser gas along 
helical multiple-flow paths while cooling the gas: Laser & Applications 
(September 1982), page 96. The spiral fins periodically direct the gas 
flow through the electrical discharge, which extends along the laser wall; 
that is, the discharge path is laterally offset from the central opening 
defined by the fins. The publication states that the fins provide more 
efficient heat transfer and a longer gas flow path than are obtained in 
axial-flow lasers. As a result, a given volume of gas is used more 
effectively, providing an increase in power output for a given laser 
length. 
The use of forced-vortex flow to air-cool a solid laser is described in the 
following article: Soviet Journal of Optical Technology, Vol. 35, No. 1, 
January-February 1968. In that arrangement, the air vortex flows about the 
exterior of an elongated lasing body, or crystal. 
As used herein, the terms "forced-vortex" and "forced-vortex flow" refer to 
a vortex having a longitudinal axis and circular motion, the circular 
vortex motion about the axis being that of a rotating solid cylinder. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of this invention to provide a novel method 
for operating laser utilizing a gaseous lasing medium. 
It is another object to provide a gaseous-discharge laser tube of novel 
design. 
It is another object to provide a laser-cooling method characterized by 
nearly immediate heat transfer from the central core region of the laser 
to the inside wall of the laser tube. 
It is another object to provide a cooling method for lasers utilizing a 
gaseous lasing medium, the method effecting rapid suppression of 
lasing-induced disturbances immediately after firing of the laser. 
Other objects and advantages of the invention will be made evident 
hereinafter. 
In one aspect, the invention is an improved method for operating a laser 
utilizing a gaseous lasing medium. The improvement comprises establishing 
forced-vortex flow of the medium in the laser tube, and initiating lasing 
of the medium while in forced-vortex flow. In another aspect, the improved 
method includes the step of introducing the gaseous lasing medium 
tangentially to the laser tube at a pressure establishing therein a forced 
vortex which defines an axially extending core region having a low 
pressure and temperature relative to the periphery of the vortex. An 
electrical discharge is established in the core region to initiate lasing 
of the medium. The gaseous discharge from the tube is passed through a 
diffuser. In another aspect, the invention is a laser system which 
includes a laser tube of circular cross section and which is provided with 
inlet means for tangentially introducing a gaseous lasing medium therein. 
Diffuser means are connected to the tube for conveying the tangentially 
introduced medium to the outlet of the diffuser. Compressor means are 
provided to receive the medium from the diffuser and recycle the same to 
the inlet means for the tube at a pressure establishing forced-vortex flow 
of the medium in the tube. Means are provided for initiating lasing of the 
medium in the region of the tube extending between the tube-inlet means 
and the diffuser means.

DETAILED DESCRIPTION OF THE INVENTION 
The invention is generally applicable to lasers utilizing a gaseous lasing 
medium, but for convenience it will be illustrated in terms of a gas-laser 
system using helium as the lasing medium. 
Referring to the drawing, numeral 5 designates a laser tube which in this 
embodiment is composed of glass and whose ends are close by conventional 
laser mirrors 7 and 9. In accordance with the invention, the laser tube is 
formed with a gas inlet 11 for tangentially introducing helium. The tube 
is connected to discharge the tangentially introduced helium through an 
annular diffuser 13. In the illustrated embodiment, the central section of 
the laser tube is provided with an external jacket 15, through which any 
suitable cooling fluid is passed. In accordance with the invention, the 
outlet of the diffuser 13 is connected to the inlet of a compressor 17, 
whose discharge is fed to the laser gas inlet 11 via a gas cooler 19. The 
laser tube 5 contains an anode 21 and a cathode 23 both of conventional 
design, for energizing the laser. Preferably, the spacing between the 
electrical discharge path and the axis of the laser tube does not exceed 
about one-half of the radius of the tube. Most preferably, the electrical 
discharge path is close to or coincident with the axis of the vortex. An 
electrical circuit including a d.c. power supply 25 and an automatically 
operated power switch 27 is connected across the laser electrodes. The 
diffuser and the various supporting components for the laser may be of 
conventional design. 
In a typical operation of the system shown in the drawing, compressed 
helium is introduced to the nozzle 11 at a pressure which establishes a 
forced vortex characterized by (1) a relatively low vortex-core pressure 
which is compatible with optical/gas requirements for the laser and (2) a 
relatively low vortex-core temperature consistent with effective radial 
heat transfer from the core to the laser wall. Under these conditions, 
firing of the laser results in the usual abrupt heating and gas 
disruptions, but because of the vortex action the helium atoms in the core 
region are almost immediately restored to their pre-lasing ground state. 
That is: (1) The initial low gas temperature in the core is re-established 
almost immediately by radial transfer of almost all of the waste heat 
through the vortex, to the wall of the laser. Thus, much more heat can be 
removed by the above-mentioned external cooling fluid than is the case 
with prior cooling arrangements, and the laser power output (watts per 
unit length) is increased. (2) After the laser is fired, the disrupted 
vortex flow pattern of the helium atoms is restored to the pre-firing 
condition rapidly--probably within the period of a few revolutions of the 
vortex--permitting the laser to be fired at significantly higher 
repetition rates. 
The following is a more specific illustration of the invention as utilized 
in the system shown in the drawing. The laser tube 5 and diffuser 13 may, 
for example, have the following dimensions: (1) laser tube: length, 25"; 
inside diameter, 5"; (2) diffuser: diameter, 6". In this illustration it 
is assumed that the forced-vortex cooling system is functioning but that 
the power supply 25 has not been energized. Gaseous helium is introduced 
to the laser inlet nozzle 11 at a pressure of 6 atmospheres and a 
temperature of 40.degree. F. The helium leaves the diffuser outlet 16 at 
1.5 atmospheres and 39.degree. F. The pressure drop across the vortex 
diffuser 13 is 4:1 (P.sub.2 /P.sub.1 =6.0/1.5), at which value the 
performance of the laser tube is at its maximum. (Exceeding this pressure 
ratio would cause the gas velocity in the axial direction of the tube to 
exceed the sonic velocity of the gas. At this point the special 
forced-vortex effect would stop). The forced vortex which forms within the 
tube cools the helium gas at the central core at constant entropy (s). 
Assuming 90% efficiency, with the diffuser doubling the resultant change 
in enthalpy (.DELTA.H), the temperature of the helium gas at the core 24 
of the vortex is -340.degree. F. The gas pressure in the core would be 0.1 
atmosphere. Calculations based on the conservation of energy and the 
change in enthalpy (.DELTA.H) of helium would indicate a gas temperature 
at the wall 22 of +420.degree. F. Thus, the total change in gas 
temperature within the forced vortex is 760.degree. F. 
Under the conditions just cited, the tangential and axial velocity 
components of the forced vortex are very high. At the tube wall 22, the 
gas tangential velocity is almost 22,000 feet per second, (V.sub.t =Mach 
6.2). The calculated axial gas velocity at the wall 22 is 2,800 feet per 
second, (V.sub.AX =Mach 0.99). At the vortex core 24, the gas tangential 
velocity is very small. Theoretically, it is zero, (V.sub.t =0). The axial 
gas velocity in the core does not change, (V.sub.AX =Mach 0.99). 
A possible laser firing repetition rate for the laser just described would 
be once for each revolution of the forced vortex. At a tangential velocity 
of 20,000 feet per second, this laser conceivably could fire 15,000 times 
a second. 
The following calculations are based on the above-described laser system 
operating in the fully functional mode--i.e., with the laser power supply 
operating and the resulting waste heat being removed by the forced-vortex 
cooling system. To have a meaningful basis of comparison, it is assumed 
that the waste heat being generated in the vortex core 24 is sufficient to 
increase the core gas temperature to 40.degree. F., the same temperature 
as at the tangential inlet 11. Under these conditions the gas in the whole 
forced vortex increases in temperature proportionally. The gas at wall 22 
now is at 800.degree. F. Waste heat is being removed from the system by 
two modes, described immediately below. 
First, the forced vortex is removing heat from the core 24 radially 
outwardly, to the wall 22. Because of the extremely high velocity of the 
gas, the experimentally determined heat-transfer rates were found to be as 
much as ten times higher than if the laser gas were cooled by conventional 
gas thermal-conduction means. The heat-transfer rates were found to be so 
high that it was virtually as if the system were submerged in a liquid. In 
the foregoing example, about 28,000 joules per second of power (28,000 
watts of heat) are being removed from the core via the forced vortex. 
Second, waste heat is being removed by gas-purging the system. The laser 
tube is being sweep-purged axially at a rate of 2,800 feet per second. The 
helium within the tube is flushed completely 1,300 times a second. This 
action removes heat at the rate of 144,000 joules per second. By 
comparison, in a conventionally sweep-purged laser the purging would take 
place at a rate of about 100 feet per second. 
The two heat-removal mechanisms just described are capable collectively of 
removing over 170,000 joules of waste heat from the laser system. This is 
a maximum rate. By decreasing the pressure ratio of the inlet gas versus 
the outlet gas, it is possible to reduce the heat removal by a factor of 
four. 
Still referring to the foregoing example using helium as the lasing medium, 
essentially any laser tube modified in accordance with the invention will 
achieve the temperatures, pressures, and velocities cited. Changes in the 
size of the laser tube have little effect on these parameters. 
Forced-vortex cooling can be established in tubes at least as small as 
5"-6" in length and 3/8" in inside diameter, and in tubes at least as 
large as 20' in length and 30" in inside diameter. However, heat removal 
is dependent upon the gas throughput and heat-transfer surface area 
available. The heat-removal rate will range over several orders of 
magnitude for the range of sizes cited. Given the teachings herein, one 
skilled in the art will be able to design a suitable forced-vortex cooling 
system of the kind described, without resorting to more than routine 
experimentation. The operating temperature at the core of the vortex may 
be selected from a wide range of values and depends on such factors as the 
inlet temperature and pressure of the gas and its outlet pressure. The 
pressure ratio across the vortex tube may, for example, be in the range 
from about 1.5:1 to 4:1. The length-to-diameter ratio for the tube may be 
in the range from about 1.5:1 to 5:1. 
A laser system designed in accordance with the invention can produce very 
low core pressures--e.g., 200.mu.--and may be operated with high inlet 
pressures--e.g., 5000 psia. The core temperature may approach absolute 
zero, when helium is the lasing medium. Preferably, the lasing medium is a 
monatomic gas (Ne, He, Kr) but suitable polyatomic gases may be used--as, 
for example, H.sub.2 S, O.sub.2, and N.sub.2. Metallic vapors (e.g., Cu or 
Hg) also are suitable. Both single gases and mixtures of gases (such as 
He-CO.sub.2-N2) or metallic vapors may be utilized in accordance with the 
invention. Preferably, the laser electrodes are positioned to establish 
electrical discharge along the axis of the vortex, in order to take full 
advantage of the refrigeration in the core region. 
The diffuser 13 may be of any suitable design which converts the velocity 
of the gaseous vortex leaving the laser tube into an increase in pressure 
(potential energy, .DELTA.P). Because the exit pressure from the diffuser 
is fixed by the suction pressure of the compressor, the pressure ratio 
translates into a very low pressure in the vortex core. The diffuser 
significantly decreases the core temperature of the forced vortex, 
increasing efficiency significantly. Any suitable means may optionally be 
used to remove heat from the outside wall of the laser tube. For instance, 
an external vortex heat exchanger of the kind described above (see 
"Background") may be so used. 
Although the invention has been illustrated in terms of lasers of the 
closed (circulated-gas) type, it is also applicable to the open type. 
Given the teachings presented herein one versed in the art can determine 
the most suitable operating conditions for the invention as applied to a 
particular laser. 
The foregoing description of a specific embodiment of the invention has 
been presented to explain the principles of the invention and to enable 
others skilled in the art to utilize the invention in various embodiments 
and various modifications as are suited to a particular contemplated use. 
It is not intended to be exhaustive; obviously, many modifications and 
variations are possible in light of the above teaching. It is intended 
that the scope of the invention be determined from the appended claims.