Large Volume Gaseous Electric Discharge System

(1) Field of the Invention

One of the problems encountered when a laser system is designed and intended for use in an industrial environment is the compactness, size and reliability of the system. Lasing action in a system is obtained by subjecting a gas-filled vessel or channel to an electric discharge to form plasma; the electrons provided by the discharge collide with active gas molecules thereby exciting them to higher energy levels, from which they descent to lower energy levels and emit excess energy in the form of photons, or light quanta. The density of particles in the higher energy level must exceed that in the lower energy level to achieve optical gain. The addition of oxygen, water vapor, hydrogen and helium to a mixture of carbon dioxide and nitrogen has been found to yield increased output.

An electrical discharge having a large cross sectional area which will uniformly fill large volume cavities regardless of size or shape is mandatory if powerful and reliable lasers suitable for industrial applications are to be developed. An electrical discharge is normally very restricted in diameter because the temperature in different parts of the discharge is not uniform and this results in lower density and higher current at the inside of the plasma column, thus constricting the column. Ballasting the electrodes offers a partial solution by spreading the emission, but the streams tend to recombine. Judicious use and design of aerodynamic forces to control the ion and electron distribution in a large volume discharge have achieved some success and have resulted in a degree of compactness and reduction in size for the same power output, as measured against the very long discharge systems known in the prior art.

(2) Description of the Prior Art

Commercial application of lasers has been limited in the past because of the limitations of poor reliability, poor beam quality and large size and heavy weight. These problems, coupled with the high cost of lasers per watt of output, have caused the machine tool industry and other industries which can use lasers to proceed very slowly in their use. As an example of the size and weight problem, current high power (1 kw and up), continuous wave lasers measure about 22 feet long by 7 feet wide by 5.5 feet high and weigh several tons.

This invention is directed toward solving this problem. It leads to lower manufacturing costs and lower cost per watt of output. The laser embodied herein has high reliability and exceptional beam quality at high power and reduced size and weight.

Lasers are used to reduce production costs in those applications requiring cutting, welding, drilling, heat treating and other processes. However, according to industry publications, laser sales are only about three percent of the machine tool industry. Use of this invention's improved reliability will enable industry to put lasers on industrial production lines. Improved beam quality will make possible industry applications such as hardening or softening metals or adhering materials to one another, which previously could not be economically done. Miniaturization of the structure opens robotics applications, and eventual lower costs will mean more industries can replace other equipment with lasers.

A significant step forward in controlling the ion and electron distribution in an industrial laser discharge to thereby realize large volume uniform plasma generation and control has been achieved according to this invention by pumping a primary lasing gas stream in a recirculating mode in the path of a plurality of spaced high voltage discharges, and introducing a secondary lasing gas stream through a plurality of ejectors into the primary lasing gas stream, thereby effecting turbulance and diffusion of the gas streams, a pressure rise equal to the recirculating mode pressure drop, and an increase in mass flow. Thus the large pump previously needed for state of the art lasers is sharply reduced in size, weight, and capacity, leading to higher system efficiency, lower cost, and lower input energy requirements. It has been found that an exceptional and unusual characteristic of the ejector contributes significantly to the turbulance and diffusion of the lasing gas streams. As the high velocity gas secondary gas stream emerges from the ejector nozzle it enters a mixing tube of generally cylindrical form and the gas from the primary recirculating stream is thus entrained by massive collisions between gas molecules. Vortices result from the collisions and these vortices blend the several streams homogeneously. The angular velocities of the vortices diminish as the blended gases progress downstream. An aerodynamic contour is formed at the exit area of the mixing tube to develop a radial component of flow to the blended gas streams. This yields an homogeneous longitudinal flow in the laser channel with uniform radial velocity distribution.

In the initial reduction to practice of the 5 kw laser structure to be hereinafter described it has been found that the level of plasma control obtained removes constraints on the laser geometry so that large volume cavities can be uniformly filled with plasma, and one can thus design an optimum optical system. The very large and sturdy optical mirrors which can now be used result in a lower flux density and thus prevent warping and destruction from heat. The technology eliminates the necessity for the very delicate, fragile, and expensive optical systems which are not necessarily adaptable to industrial use.

It is expected that the use of plasma control to uniformly fill a large volume cavity will yield a 5 kw laser having dimensions of approximately two feet by two feet by one foot, thus enabling industry to use the laser on a robot arm.

The invention relates to a method and an apparatus for controlling the lasing gas flow streams in a laser to give a uniform large volume plasma in the laser channels.

It is accordingly a broad object of the invention to provide a laser having a high power output with compact overall dimensions.

It is another object of the invention to provide a uniform large volume plasma in a laser structure.

It is yet another object of the invention to provide a laser system having a high power output and compact size by providing a uniform large volume plasma in the laser channel through diffusing a plurality of lasing gas streams.

It is still another object of the invention to provide a uniform large volume plasma in a laser wherein a plurality of lasing gas streams are diffused by an ejector.

It is yet still another object of the invention to provide a method of pumping a CO.sub.2 laser to yield a uniform large volume plasma whereby a primary lasing gas stream flows in a recirculating path and a secondary lasing gas stream is introduced into the primary stream through an ejector, use is made of magnetic fields which cross couple any stray thermal and attachment instabilities to thereby make possible the introduction of greater energy into a given gas volume without an arc resulting.

As more fully disclosed hereinafter, this invention provides a structure and a method for mixing and pumping lasing gas streams in a laser having a primary, recirculating, gas stream, and an ejector to introduce a secondary gas stream to effect diffusion, turbulance, a pressure rise, and an increase in mass flow. Ballast for the discharge streams may be provided by means of the device shown in my U.S. Pat. No. 3,581,146 identified above.

FIG. 1 is a schematic sectional view of a gas laser, looking down from above, according to the invention.

FIG. 2 is a sectional view of a detail of the laser of FIG. 1 showing an ejector diffusing a plurality of lasing gas streams.

FIG. 3 is a sectional view of a detail of the ejector of FIG. 2 showing the blending of lasing gas streams.

FIG. 4 is a sectional view of a detail of the ejector of FIG. 2 showing the radial component of gas stream and showing the uniform velocity distribution of the lasing gas.

FIG. 5 is a sectional view of a detail of the laser showing one manner of applying magnetic fields to the lasing gas stream.

FIG. 6 is a sectional view of a detail of the laser showing another manner of applying magnetic fields to the lasing gas stream.

FIG. 7 is a sectional view of a detail of the laser showing yet another manner of applying magnetic fields to the lasing gas stream.

FIG. 8 is a sectional view of a detail of the laser showing still yet another manner of applying magnetic fields to the lasing gas stream.

Referring now to FIG. 1 of the drawings which shows a gas laser having the structure shown in more detail in FIGS. 2, 3, and 4. The laser assembly has a cylindrical housing 10 engaging and sealed to end walls 12 and 14. A laser channel 16 is disposed concentric with housing 10, and mirror 18 and output coupler 20 are fixed at each respective end of the channel. Channel 16 is joined to a frustro conical cylindrical member 22 or ejector having a plurality of equally spaced, cylindrical passages or tubes 24, 26 formed at an acute angle with the axis of symmetry of the channel and serving as mixing tubes for the gas streams. Main nozzles 28 and 30 are fixed adjacent to each respective mixing tube. Hollow electrodes 32 and 34 are positioned within and concentric with each respective main nozzle 28, 30 and are connected to a source of high voltage 50 through circuit conductors 56 to provide the electric discharge for lasing action.

FIGS. 2, 3, and 4 show the profile of the mixing tubes and, more specifically, where an aerodynamic contour is formed at the exit area. As viewed in section the tubes terminate on one side in a reverse ogee 29 comprising a first curve 31 blending into a second curve 33, and a radius section 35 opposed to the reverse ogee, and registering with the second curve. This aerodynamic contour is material in the development of a radial component in the gas stream as it exits the mixing tube, and the reaction of the gas stream with the exit contour results in an homogeneous gas flow stream in the contour area and the laser channel with microturbulant, uniform velocity distribution as shown by the flow arrows and wave front profile. Unless the velocity distribution is uniform in cross section, those regions which flow more slowly will become preferentially heated, and these hotter regions become less dense. This leads to an optical disturbance of the laser beam, and can ultimately produce an electric arc.

Laser channel 16 is interrupted as shown by reference characters 40 and 42 to allow a recirculation of gas through the housing and into the mixing tubes as shown by the flow arrows. Hollow electrodes 44 and 46 are connected to voltage source 50 to close the discharge circuit. The arrows in FIGS. 1, 2, 3, and 4 illustrate the gas streams flow. The primary stream is the recirculating stream which flows through channel 16, out interruptions 40 and 42, through the volume between channel 16 and housing 10 and is entrained by the ejectors which introduce a high velocity secondary gas stream and vortices into the mixing tubes as seen in FIG. 3. A series of cylindrical finned heat exchangers shown generally by reference character 48 are positioned adjacent interruptions 40 and 42 to control the lasing gas temperature. Helium under pressure in tanks 52 and 54 is connected to electrodes 32, 34, 44 and 46 by piping shown generally as 56 and 58.

A lasing gas bleed system for optimum continuous and predictable results and to remove impurities including, for example, oil, carbon monoxide, and hydrocarbons from the lasing gas comprises an exhaust pump 60 connected to housing 10 by pipe 62 and having another exhaust pipe 63 leading to an appropriate storage vessel (not shown). A positive-displacement circulating pump 64 is connected to housing 10 by pipe 66 and heat exchanger 68 substantially as shown. Nitrogen gas and carbon dioxide gas under pressure are held in tanks 70 and 72 and are connected to piping 74 and thence to primary nozzles 32 and 34. A catalytic converter 67 by chemical reaction converts any CO formed in the system to CO.sub.2. An electric heating system (not shown) is provided to maintain the gas temperature at the proper level for the converter to function. A molecular sieve 69 is provided to prevent the gas from being poisoned by impurities.

A plurality of annular magnets 80 which are shown as permanent magnets embrace laser channel 16 and are longitudinally spaced thereon for a purpose to be later explained.

In the operation of the device the circulator pump 64 establishes a pressure to create a high velocity lasing gas flow through heat exchanger 68 to remove excess heat, piping 74, catalytic converter through the annular array of primary nozzles 28 and 30, through the annular array of mixing tubes 24, 26 into and through laser channel 16, out interruptions 40 and 42, around vanes 76 and 78, past heat exchanges 48 and back into passages 24 and 26 in a recirculating mode flow substantially as shown by the arrows.

The operation of the ejector in mixing and diffusing the lasing gas streams results in a uniform, large volume gaseous discharge in the laser channel. Referring now to the detail drawing of FIGS. 2 and 3 it will be seen that very high velocity gas emerges from each primary nozzle and enters its respective mixing tube. As represented in the figures a relatively conical boundary layer of gas enters the mixing tube and flares outwardly in the tube as it progresses down its length. The gas molecules from the recirculating primary gas stream are entrained by the boundary layer's mass velocity. Gas vortices result from the collisions between gas stream molecules traveling at different velocities and the vortices intimately mix and blend the streams. The annular velocity of the vortices diminish as the blended gas stream move downstream and result in complete diffusion of the gas streams and an homogeneous and uniform gas mixture. In the system actually reduced to practice the cross-sectional area ratio of each respective mixing tube to the throat of each respective primary nozzle is about 27.5 to 1. This ratio is given as an example only and not in a limiting sense because many modifications can be made without departing from the spirit and the intent of the invention.

The laser beam output of the invention emerges to the left of FIG. 1. By a rearrangement of the optical system used the beam output will be made to emerge at a right angle to the axis of channel 16. This system is well and truly shown and illustrated in FIG. 12 of U.S. Pat. No. 3,795,838 issued Mar. 5, 1974. In the embodiment, a horizontally distributed ejector pump array is substituted for the nozzle block shown in FIG. 12 of the above identified patent.

In the closed cycle or recirculating lasing gas mode, contamination of the lasing gas by oil, dust, impurities, ozone, carbon monoxide or chemical reactions within the system gives rise to serious attachment instability problems and will affect system power output and reliability by poisoning the gas. Catalytic converter 67 and molecular sieve 69 prevent the gas from being poisoned. Heat exchanger 68 dissipates heat and thereby cools the lasing gas to optimum temperature. An exhaust pump 60 and its associated plumbing 10, 62, and 63 is provided to allow the composition of the lasing gas to be monitored, as by taking periodic samples, so that the optimum gas mixture can be maintained if necessary by introducing fresh gas from tanks 52, 54, 70, and 72.

Referring now to FIGS. 5 and 8 inclusive, it has been found that in the design of an industrial laser wherein continuity of operation and reliability are material factors, the addition of a magnetic field which interacts with the plasma flowstream insures an homogeneous discharge plasma by inhibiting the growth of thermal instabilities within the discharge. Thermal instabilities are incipient arcs which align with the flowstream and unless controlled prevent a laser from delivering its full power output. If an arc develops it could burn and destroy the laser optical system and will short circuit the laser cavity.

FIG. 5 shows the static magnetic field produced by the permanent magnets 80 with poles opposed. The interaction of the field, in this example, with the flowstream does no work and neither adds nor substracts energy from the system. The lines of force produce a static resultant undulating magnetic field which rises and falls in strength or intensity in the direction of plasma flow. As the lasing gas flows it reacts with the undulating magnetic field thus produced and changes the electron flow pattern by giving the electrons a velocity component at right angles to the flow, causing migration of charged particles and mixing of hot and colder regions to thereby prevent the growth of thermal instabilities. A more uniform plasma results.

FIG. 6 shows another species wherein a static magnetic field is produced by D.C. excited electromagnets 78. The field reacts with the lasing gas as in the preceding example and thus changes the electron flow pattern to yield an homogeneous plasma. It follows, however, that the magnetic field produced by voltage source 82 is variable in intensity and can be controlled externally by controlling the applied D.C. voltage.

The species of FIG. 7 shows an elongated electromagnet 84 embracing a substantial segment of the channel 16. When excited by a D.C. voltage source 83 an axially uniform distributed magnetic field results which prevents development of thermal instabilities by distributing the magnetic lines of force in the direction of the lasing gas flowstream. The action of this field will prevent thermal instabilities by guiding the charged particles in substantially parallel paths in the laser channel.

The species of FIG. 8 shows an oscillator 86 which superimposes an A.C. field upon the D.C. field from power supply 85. The interaction of the two energy sources with the distributed coil 84 creates an undulating magnetic field at right angles to the direction of lasing gas flow. The work to compress and expand the field is supplied by the oscillator and causes a radial direction of motion of charged particles to homogenize the gas. To avoid standing wave-patterns, the oscillator frequency must exceed the gas flow-through time several fold.

There is thus provided a high power laser gaseous discharge system which has high reliability and exceptional beam quality at high power and reduced size and weight. It will be apparent to those skilled in the art that many changes may be made in the construction and arrangements of parts without necessarily departing from the scope of the invention as defined in the appended claims.