Gas laser generator

Gaseous flux laser generator of the nitrogen and carbon dioxyde type. The dimensions of the enclosure and the conditions of injection of the nitrogen and those of the electrical supply are chosen to set up a luminescent discharge in a swirling flow.

The present addition concerns, like the parent U.S. Pat. No. 3,891,944, a 
gaseous flux laser generator. For convenience's sake, the word (invention) 
will be applied hereinafter to the characteristics taken as a whole 
described in these two texts. 
For example, through a published document by Messrs. Lavarini, Dettini, 
Crancon, Michon, "Laser having electrical excitation and adiabatic 
release" (Reports to the Paris Academy of Sciences, vol. 272, p 335-338, 
Feb. 1, 1971, laser generators in which is produced an electrical 
discharge in a first gas (nitrogen) at a very high speed and that first 
gas is mixed with a second gas (carbon dioxide in an expansion chamber in 
which is arranged a resonant optical cavity are known. The electrical 
discharge has the effect of providing the nitrogen with an excitation 
energy which is transferred to the carbon dioxyde by molecular interaction 
during the mixing. The rapid movement of the mixture in the expansion 
chamber makes it reach the optical cavity before the de-excitation of the 
carbon dioxyde, this enabling the latter to have a stimulated light 
emission within the optical cavity, that is, a laser emission. 
The nitrogen molecules have three possible modes of excitation: thermal, 
rotational and vibrational. The relaxation periods of the first two modes 
are very much shorter than the relaxation time of the last. When the 
nitrogen molecules and the carbon dioxyde molecules mix in the expansion 
chamber, it is the vibrational energy of the nitrogen exclusively which 
subsists and which produces, within the carbon dioxyde, the inversion of 
population, which gives rise to a high-power laser pulse. 
Such devices have, however, a certain number of disadvantages. 
More particularly, the structure as well as the mutual arrangement of the 
elements constituting such generators does not make it possible to obtain 
powerful and homogeneous electrical discharges in the nitrogen, 
whencethere results a limiting of the power as well as of the efficiency 
of the laser emission. 
The present invention makes it possible to overcome such disadvantages and 
it has for its object a gaseous flux laser generator making it possible to 
produce particularly powerful and homogenous electrical discharges and to 
obtain, due to that fact, high power laser emissions, such a generator 
having, moreover, a very great simplicity of structure combined with a 
moderate production cost. 
The invention therefore concerns a gaseous flux laser generator comprising: 
An elongated enclosure provided with transfer orifices at a first end; 
Means for feeding the said enclosure with at least one jet of a first gas 
capable of being excited by an electrical discharge, these feeding means 
comprising an injection nozzle leading into the enclosure in the vicinity 
of its second end; 
Electrodes arranged in the said enclosure and capable of setting up an 
electrical discharge in the said first gas; 
An elongated expansion chamber into which leads the said enclosure through 
the said transfer orifices, the cross-section of the said expansion 
chamber progressively increasing starting from its first and adjacent to 
the said enclosure up to its second end; 
Discharge means for keeping at a low pressure the said second end of the 
expansion chamber and making the said first gas circulate from the said 
injection nozzle up to the said second end of the expansion chamber 
through the said transfer orifices; 
Means for feeding the said expansion chamber with at least a second gas, 
capable of being excited by molecular interaction with the said first gas 
in its excited state, these feed means being situated in the vicinity of 
the said enclosure so as to set up a mixture of the said first andsecond 
gases; 
An optical resonnant cavity capable of setting up a laser emission in the 
presence of the said second gas in its excited state, that cavity being 
arranged in the said expansion chamber so as to have the said gas mixture 
flowing through it; 
That generator being characterized in that the dimensions of the said 
enclosure, the diameter and the position of the said injection nozzle and 
the speed of injection of the said first gas, as well as the dimensions of 
the said transfer orifices are chosen so as to set up a turbulent flow of 
the said first gas in the volume of that enclosure, taken as a whole.

According to FIG. 1, a gaseous flux laser generator comprises a cyindrical 
enclosure 1 having a diameter of .phi.1 into which leads an injection 
nozzle 2 constituting an anode and connected to a voltage generator G 
through a resistor T. That nozzle 2 comprises an axial duct 3 connected to 
a nitrogen source under pressure, SN, shown diagrammatically. Its front 
face comprises an injection orifice 4, diverging towards the inside of the 
enclosure 1. At the other end of the enclosure 1 and at a distance L from 
the end of the nozzle 2 are arranged substantially at an equal distance 
from one another, injectors for carbon dioxyde 5 fed from a carbon dioxyde 
source SC and more particularly described with reference to FIG. 2, such 
injectors being connected electrically to the other pole of the voltage 
generator G by means of conductors 13. 
The previously mentioned transfer orifices are constituted by the open gaps 
such as 50 between two injectors or between an injector and the wall of 
the enclosure 1. 
The enclosure 1 leads by transfer orifices into an expansion chamber 6 
provided with two mirrors 7 and 8 constituting an optical resonant cavity, 
the mirror 8 being semi-transparent and hence ensuring the laser emission 
in the direction of the arrow F. 
The largest end of the expansion chamber is kept at very low pressure by 
discharge means constituted by ducts connecting it to a vacuum reserve SV, 
shown diagrammatically and having sufficient dimensions for the pressure 
to remain practically zero therein during the whole operation of the laser 
generator. 
FIG. 2 enables the structure of the injectors 5, each comprising a profiled 
metallic body 9 in which has been formed, on the one hand, a duct 10 
connected by its two ends, to a source of carbon dioxyde (CO2) and of 
helium and, on the other hand, a duct 11 in which a cooling fluid, in this 
case, water (H.sub.2 O) flows. The said duct 10 feeds several tubes 12 
having a constant cross-section for the injecting of the carbon dioxyde. 
Such a generator operates as follows: 
The nitrogen inserted under pressure in the axial passage 3 of the nozzle 2 
is injected at supersonic speed in the enclosure 1 by means of the 
injection orifice 4. Subsequent to a suitable choice of the parameters of 
the generators such as L and .phi.1 previously defined, a swirling "main" 
flow of the nitrogen in the enclosure 1 is formed. That flow is shown by 
arrows in continuous lines 14. It ensures a homogenous distribution of the 
electrical discharge set off by feeding the anode 2 and the conductors 13 
by means of the generator G. 
A part of the nitrogen drawn away by the main flow then flows between the 
injectors 5 and the walls of the enclosure 1, forming a secondary flow 
shown by arrows in discontinuous lines and draws away the carbon dioxyde 
and the helium injected by the tubes 12. The carbon dioxyde is then 
excited in the way described previously and produces a laser emission in 
the direction of the arrow F. 
According to the embodiment shown in FIG. 3, similar to FIG. 1, the passage 
3 is subdivided into two passages 16 and 17 ending respectively in two 
injecting orifices 18 and 19, divergent towards the inside of the 
enclosure 1. 
The structure and the operation of the generator are equivalent in the 
assembly to what has been set forth hereinabove inasmuch as concerns FIG. 
1. Nevertheless, by means of a suitable choice of parameters, previously 
defined, a double swirling flow of the nitrogen takes place in the 
enclosure 1, this being shown by the arrows 20 and 21. This ensures a 
particularly homogenous distribution of the electrical discharge set off 
by feeding the anode 2 and the conductors 13 by means of the electrical 
generator (not shown in FIG. 3). 
Of course, it will be understood easily that it is possible to implement a 
nozzle 2 comprising more than two injector orifices, as well as several 
nozzles 2 comprising one or several injector orifices. 
The enclosure may, on the other hand, have a form other than cylindrical, 
parallelepipedical, for example. The nozzle 2 may then be formed in such a 
way as to inject in the enclosure 1, instead of a jet of nitrogen having a 
circular cross-section, a large and not very thick layer, parallel to one 
of the wells of the enclosure 1. 
Such an embodiment is shown in FIGS. 4 and 5. 
The nozzle 2 has an injection orifice 40 in the shape of a slot which is 
narrow in the vertical direction and having a width approximately equal to 
that of the enclosure 1 in the horizontal direction. The main flow of the 
nitrogen is then effected in horizontal layers: a middle layer shown by 
arrows 41 is directed from the nozzle 2 towards the injectors 5 and two 
layers, the one an upper layer and the other a lower layer, shown by 
arrows 42 and 43 respectively, are directed from the injectors 5 towards 
the nozzle 2. 
In the embodiments which have just been described, the electrical discharge 
is longitudinal, that is, it is parallel to the direction of the average 
flow of the nitrogen in the enclosure 1. The present invention may 
nevertheless quite well be implemented if the discharge is transversal, 
that is, perpendicular to that same direction. It is necessary only for 
the greater part of the volume of the enclosure 1 to be crossed by the 
electrical discharge. 
The way in which the various parameters of the generator according to the 
invention must be chosen to obtain proper operation will now be set forth 
more precisely. 
One important aim is to obtain high energy efficiency, that is as high a 
ratio as possible between the light energy produced and the electrical 
energy consumed by the discharge in the enclosure 1. For that purpose, it 
is necessary for the discharge to be of the luminescent type. It is known 
that such a discharge is kept up by electrons which come essentially from 
the secondary electrnic emission of the cold cathode bombarded by ions or 
other particles, or, even, from an emission of the cold cathode by field 
effect. It is also known that it is easily distinguished from an electric 
arc in which a high rise in temperature appears with a high ionization of 
the gas and a great thermo-electronic emission at the cathode. 
In a luminescent discharge, the speed of the electrons, within the positive 
column, is such that their kinetic energy is transferred with high 
efficiency, greater than 80%, to the nitrogen molecules encountered, with 
excitation of their vibrational mode. 
In an electric arc, the high temperature reached makes the transfer 
efficiency small within the enclosure 1. Moreover, the energy efficiency 
of the processes which take place in the expansion chamber 6 is also 
reduced. 
When it is sought to increase the power of known laser generators by 
increasing the volume of the enclosure 1, the pressure of the nitrogen and 
the density of the electric current in that enclosure, the homogeneous 
luminescent discharge becomes unstable and is transformed into a 
multiplicity of electric arcs in the form of filaments. The efficiency of 
these generators then decreases greatly. Due to the swirling flow 
according to the invention, the discharge remains of the luminescent and 
homogenous type, while enabling high power to be obtained. 
For that purpose, in the case of FIGS. 1 and 2, that is, of a cylindrical 
enclosure and of a longitudinal electric discharge, it is necessary, 
firstly, to choose the ratio of the length L of the enclosure in relation 
to its diameter .phi.1 between approximately five and seven: 
EQU 5&lt;L/.phi.1&lt;7 
It is ideal for the quantity of movement Qm injected per second by the 
nozzle 2, that is, the product of the output-to-weight ratio and the 
injection speed, be comprised between one and thirty, if the units adopted 
are the meter, the second and the kilogramme: 
EQU 1 Kg.m/s2&lt;Qm&lt;30 Kg.m/s.sup.2 
The output permeability coefficient K, that is, the ratio between the 
output surface left free between the injectors 5 and the surface of the 
cross-section of the enclosure, is, to great advantage, comprised between 
5% and 50%. 
EQU 5%&lt;K&lt;50% 
It should be observed, in this respect, that the pressure in the enclosure 
1 is close to or greater than twice the pressure in the expansion chamber 
6, so that the flow of the nitrogen between the injectors 5 reaches the 
speed of sound. 
Inasmuch as concerns the pressure P of the nitrogen in the enclosure 1, the 
present invention is more especially and advantage if that pressure is 
greater than 0.1 bar. It may reach several bars without the appearance of 
an electric arc. 
The injection speed V for the nitrogen through the nozzle 2 is greater than 
100: 
EQU V&gt;100 m/s 
The current density j in the enclosure, that is, the ratio between the 
intensity and the surface of the cross-section of the enclosure, must be 
increased if it is required to increase the power of the generator. High 
current densities have been obtained before the present invention in 
enclosures having a small cross-section. A homogenous electrical discharge 
was then set up. Out of it was required to increase the power of the 
generator by increasing the cross-section of the enclosure, electric arcs 
in the form of filaments appeared. The present invention enables, when the 
current density is high, the cross-section of the enclosure 1 to be 
increased. The result obtained is, to great advantage: 
EQU 10 mA/squ.cm&lt;j&lt;200 mA/squ.cm 
The electric energy Wm injected per unit of mass of the nitrogen may be 
expressed in joules per gramme: 
EQU 500 J/g&lt;Wm&lt;5000 J/g 
It may be an advantage to calculate the ratio E/N between the electronic 
field E in the enclosure 1 and the number N of molecules of nitrogen per 
cubic centimeter, for that ratio is connected with the average speed of an 
electron striking a nitrogen molecule. The result then obtained is 
preferably: 
EQU 10.sup.-17 V/squ.cm E/N 10.sup.-14 V/squ.cm 
The values of the previously defined parameters will be given hereinbelow 
in two examples of the implementing of the invention, corresponding to the 
embodiment in FIG. 1. 
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AMETER UNIT 1st EXAMPLE 2nd EXAMPLE 
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L mm 300 300 
0 1 mm 50 50 
Qm Kgm/s2 3.5 17.5 
K % 10 15 
P Millibar 200 500 
V m/s 320 580 
j mA/squ. cm 
20 60 
Wm J/g 690 1600 
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