Abstract:
A device for generating and guiding an electric discharge current including a first electrode, second electrode and a material arranged between the first and second electrode. The device uses a laser for generating photons having energies equal to the energy between two excited electronic states of the material. The photons fill a region of the material from the first electrode through the material to the second electrode ionizing the region of the material between the first and second electrodes. A voltage generating unit connected to the first and second electrodes applies a voltage between the first and second electrodes thereby generating the electric discharge which follows an ionized path in the region between the first and second electrodes.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to an apparatus and a method for guiding an electric discharge and particularly to a laser guided discharge device capable of guiding a discharge current between two electrodes using only laser light and no guiding wall. 
     2. Description of Related Art 
     Traditional &#34;wall-guided&#34; gas discharge tubes used for example as laser tubes, flashlamps and fluorescent tubes guide an electric discharge between two electrodes by an inner wall of a tube. The inner wall of the tube must be made of electrically insulating materials such as glass or ceramic in order to avoid shorting out the electric discharge. In addition, in order to prevent corrosion and consequently prolong the life of the tube, the inner wall of the tube must be made of materials that do not chemically react with the gas. Satisfying these two requirements severely limits the types of materials which can be used to make discharge tubes. For example, metals such as steel, tungsten and nickel cannot be used to guide an electric discharge between electrodes in &#34;wall-guided&#34; discharge tubes even though they possess particular advantages for housing gases such as high impact resistance, longevity, design flexibility and scalability because they are not electrically insulating. Also, if the gas is a metal vapor, for example, an alkali (group IA of the periodic table), alkaline earth metals (group IIA of the periodic table) or some other transition element (groups IIIA to VIIIA, IB and IIB), no tube known to the inventor can be used to guide the electric discharge. This is mainly because eventually atoms in the gas will accumulate on and chemically or physically attack the inner walls of the tube and will short out or destroy the walls and other components of the device (tube). Further, typical gas discharge tubes can be damaged or destroyed due to high voltages applied across its electrodes. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide a method and an apparatus for guiding a discharge current in a material without using a guiding tube. 
     Another object of the invention is to provide an apparatus and method for guiding an electric discharge in materials that are intrinsically incompatible (chemically or physically) such as metal vapors with present day wall materials. 
     Another object of the invention is to enable the use of metals such as steel, tungsten and nickel as housing material for gases in discharge tubes. 
     Another object of the invention is to enable the use of alkali and alkaline earth metals as gases which support the electric discharge. 
     Another object of the invention is to provide a device capable of creating a discharge between two electrodes maintained at moderate to low voltage levels. 
     One feature of the invention is that it uses a light source such as a laser to output photons tuned to an electronic quasi-resonance of the material in order to create and sustain a channel that guides the electric discharge between electrodes. 
     Another feature of the invention is that it uses independently powered heating wires in order to control an initiation of the channel. 
     Another feature of the invention is that the material containing the electric discharge can be a gas, liquid or solid. 
     The above and other objects, advantages and features of the invention are accomplished by the provision of a device for generating and guiding an electric discharge current, including a first electrode; a second electrode; a material arranged between the first electrode and the second electrode; a photon generating unit for generating photons having energies equal to a difference in energy between two excited electronic states of the material, the photons filling a region of the material from the first electrode through the material to the second electrode ionizing the region of the material between the first and second electrodes; and a voltage generating unit connected to the first and second electrodes for applying a voltage between the first and second electrodes thereby generating the electric discharge which follows an ionized path in the region between the first and second electrodes. 
     In a further aspect of the invention, a housing is provided having a first end and a second end for housing the material and first and second electrodes are arranged at one of the first and second ends of the housing. 
     In an alternative aspect of the invention, a housing is provided having a first end and a second end for housing the material and the first electrode is arranged at the first end of the housing and the second electrode is arranged at the second end of the housing. 
     In another aspect of the invention a power supply and first and second filaments connected to the power supply are arranged along the first and second electrodes. 
     The above and other objects, advantages and features are further accomplished in accordance with the invention by the provision of a method for guiding an electric discharge between a first and second electrode, including the steps of: arranging a material between a first electrode and a second electrode; generating photons having energies equal to a difference in energy between two excited electronic states of the material; filling a region of the material from the first electrode through the material to the second electrode thereby ionizing the region of the material between the first and second electrodes; and applying a voltage between the first and second electrodes thereby generating the electric discharge which follows an ionized path in the region between the first and second electrodes. 
    
    
     The above and other objects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of a laser guided discharge (LGD) device in a longitudinal configuration. 
     FIG. 2 is a schematic representation of the right electrode of the LGD device in FIG. 1 with an insulated filament. 
     FIGS. 3A and 3B, 3C are graphical representations of various electronic states of a general gas and of cesium gas, respectively. 
     FIG. 4 is a schematic representation of the right electrode of the LGD device of FIG. 1 with a nude filament. 
     FIG. 5 is a schematic representation of the LGD device in a transverse configuration. 
     FIG. 6 is a more detailed schematic representation of the left-hand side of the transverse LGD device in FIG. 5 using electrically insulated filaments. 
     FIG. 7 is a more detailed schematic representation of the left-hand side of the transverse LGD device in FIG. 5 using nude filaments. 
     FIG. 8 is a schematic representation of a laser using the LGD device in either a transverse or longitudinal configuration. 
     FIGS. 9A and 9B show the LGD device of FIG. 1 in which the gas is replaced by a liquid and a solid, respectively. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a schematic representation of a laser guided discharge (LGD) device 100 having a longitudinal configuration according to one embodiment of the invention. LGD device 100 has a left electrode 110L and a right electrode 110R on each end of a housing wall 120 which houses a gas 130. Laser 140 outputs a laser beam 144 which enters an electrode window 150 and travels along an electrode wall 160 through an electrode tip 170 and then passes through gas 130 to electrode 110R on the right hand side of LGD device 100. 
     Housing wall 120 has housing ends 180 on its left and right ends in order to provide support for left electrode 110L and right electrode 110R. Housing wall 120 and housing ends 180 comprise housing 182 which can be used to house or contain gas 130. Housing 182 can be cylindrical and can be made of any material capable of containing gas 130 including metals such as steel, nickel and/or tungsten. Gas 130 remains confined in housing 182 because electrode windows 150 seal left and right electrodes 110L and 110R while transmitting laser beam 144. 
     Laser beam 144 output by laser 140 creates a path 186 of ionized gas between left electrode 110L and right electrode 110R. A power source 188 generates a voltage between left electrode 110L and right electrode 110R. Path 186 of ionized gas then serves as a conductive link between electrode tips 170 of left electrode 110L and right electrode 110R. 
     More generally, any region in gas 130 becomes ionized when in contact with the photons from laser 140. Laser beam 144 enters electrode window 150 of left electrode 110L and exits from electrode window 150 of right electrode 110R. Left and right electrodes 110L and 110R are shaped so that laser beam 144 can pass through them without actually coming into contact with electrode tips 170. Otherwise, laser beam 144 would be scattered in multiple directions and consequently would ionize gas 130 in multiple directions. Also, even though laser beam 144 ionizes gas 130 creating ionized path 186, a discharge current induced by the voltage between left electrode 110L and right electrode 110R may not necessarily take path 186 and could instead be between electrode tip 170 of right electrode 110R and housing wall 120. To insure that the discharge current takes path 186, left and right electrodes 110L and 110R are provided with left and right heating power sources 191L and 191R, respectively. Left and right heating power sources 191L and 191R heat left and right regions 192L and 192R in the vicinity of electrode tips 170 of left electrode 110L and right electrode 110R, respectively. Left and right heating power sources 191L and 191R are each connected to a heating element (for example filament 210R in FIG. 2 for right heating power source 191R) via left and right wires 193L and 193R, respectively. 
     Power source 188 can be an AC and/or DC, pulsed, radio frequency (rf) or microwave power source. Wires 189L and 189R connect power source 188 to electrodes 110L and 110R, respectively. Power source 188 then becomes part of a completed circuit comprising wire 189L, electrode 110L, path 186, electrode 110R and wire 189R. A laser guided electric discharge is then generated along path 186. Also, a plasma region 190 is created in the approximate vicinity of path 186 due to electron impact ionization caused by the electric discharge. 
     Gas 130 can be a metal vapor such as alkali and alkaline earth metals (groups IA and IIA of the periodic table) or some other transition element (groups IIIA to VIIIA, IB and IIB). Also the material between left and right electrodes 110L and 110R need not be limited to a gas as will be discussed with reference to FIGS. 9A and 9B. 
     Laser 140 can be any type of laser including a gas laser such as an argon ion laser or a dye-laser. Laser 140 can also be a semi-conductor laser, a solid state laser etc . . . . The source of photons for beam 144 can even be a lamp such as a sodium, mercury or xenon arc lamp. The main requirements on the photon source represented by laser 140 are that its beam 144 be tuned to a frequency such that the energy in each photon corresponds to the difference in energy between two excited electronic states in gas 130 and that its output power be sufficient to completely fill a region between left and right electrodes 110L and 110R, respectively, as will be discussed below. 
     A transition between a ground state and an excited state of a gas is generally referred to as a resonance of the gas, whereas a transition between two excited states of a gas is often referred to as a quasi-resonance of the gas. As mentioned above, the energy of photons in laser beam 144 must be equal to the difference in energy between an excited electronic state 2 and an excited electronic state 1, neither of which is the ground state of gas 130. If laser 140 outputs photons tuned to a resonance rather than a quasi-resonance of gas 130, laser beam 144 will be absorbed shortly after entering gas 130 at left electrode 110L. Increasing the power output by laser 140 does increase the distance laser beam 144 can travel in gas 130 before being completely absorbed. However, even if the power in laser beam 144 is as high as several kilowatts, if its photons are tuned to a resonance of gas 130, they will only travel a few centimeters before being absorbed by gas 130. On the other hand, if photons in laser beam 144 are tuned to a quasi-resonance of gas 130, path 186 between left electrode 110L and right electrode 110R can exceed several meters in length even though laser beam 144 is only a few milliwatts. 
     FIG. 2 is a more detailed schematic representation of right electrode 110R of LGD device 100 in FIG. 1. The following discussion is directed to right electrode 110R but is applicable to left electrode 110L as well. Right heating power source 191R is coupled to a wire or filament 210R made of an electrically conductive material such as tungsten via wires 193R. Right heating power source 191R supplies a current through filament 210R which is insulated from gas 130 by an insulator 220R made of an electrically insulating material such as rubber, plastic, magnesium oxide, ceramic or glass and shaped as a hollow doughnut surrounding filament 210R. 
     Without any heating source in a region about electrode tips 170, these regions become regions of higher resistivity through which an electric discharge must travel. However, as filament 210R heats up, it creates a localized area 192R of low resistivity, thereby insuring that path 186 is a low resistance path through which the electric discharge will pass. A further advantage of using left and right filaments 210L and 210R at electrode tips 170 of left and right electrodes 110L and 110R, respectively, is that LGD device 100 can then be operated at lower voltages, i.e., the voltage between left electrode 110L and right electrode 110R can be maintained below 50 volts rather than in kilovolt ranges. Also, once the discharge current takes path 186 of laser beam 144, it produces its own low resistance path and its own magnetic field which in turn keeps the discharge current from deviating from path 186. 
     New photons can be generated by gas 130 and used for some other purpose. That is, atoms or molecules of gas 130 can be excited into excited electronic states via the discharge current. Then, when these excited atoms or molecules undergo electronic transitions from the excited states to lower states they can generate output photons 145 (see FIG. 1) at electrode windows 150 of left and right electrodes 110L and 110R, respectively. Hence, LGD device 100 can be designed in such a way that output photons 145 are its end product. LGD device 100 can then itself function as a laser as will be discussed below. Alternatively, gas 130 together with laser 140 can be selected so that few or no output photons 145 are generated by the discharge current. In this case, the discharge current itself is the end product of LGD device 100. The discharge current can be used, for example, in micro-electronic device fabrication techniques such as deposition and etching processes where the discharge current can be manipulated by manipulating (e.g., focusing) laser beam 144. These adaptions of LGD device 100 will be discussed in more detail after the following discussion regarding selection of laser 140. 
     The required photon energy from laser beam 144 is determined uniquely by the electronic structure of the medium (or media) and its ions. First, it will be shown how the frequency of laser beam 144 is determined for a general atomic element used as gas 130. Then a particular example is given wherein gas 130 includes cesium atoms. Although a gas phase medium is considered here, it is important to realize that many of the principles presented here apply to any medium (or media) in any phase or mixture of phases (this includes liquids, solids, plasmas etc.). The approach centers on determining which processes will occur spontaneously for a given gas (liquid or solid) 130 in the presence of photons from laser beam 144. 
     FIG. 3A shows a generalized energy level diagram (or two-body energy cycle) for a single element medium A as gas 130. Medium A consists of ground state atoms (A) and molecules (A 2 ), excited atoms and molecules (A*, A**, A 2  *, where A* is atom A in an excited state, A** is atom A in another (higher) excited state and A 2  * is molecule A 2  in an excited state) and their ions (A+ and A 2  +) and electrons(e-) (negative ions are neglected). FIG. 3A includes appropriate ionization potentials (IP), optical excitation energies (E), and bond dissociation energies (D) of atoms, molecules and ions in gas 130. Horizontal lines represent the energy of two A atoms in various combinations of electronic structure, bonding, and ionization. 
     On the left side of FIG. 3A it can be seen that 
     
         E(A.sub.2 *)+IP(A.sub.2 *)=IP(A.sub.2)+D(A.sub.2 +)        (1) 
    
     and 
     
         IP(A.sub.2)+D(A.sub.2 +)=D(A.sub.2)+E(A*)+IP(A*).          (2) 
    
     Similarly, other equations may be written 
     
         E(A**)+IP(A**)=IP(A)                                       (3) 
    
     and, 
     
         D(A.sub.2)+IP(A)=IP(A.sub.2)+D(A.sub.2 +) etc.             (4) 
    
     When the photon energy (hv) in laser beam 144 is equal to one of the energy differences, an electronic transition corresponding to the double arrow is possible. For example, as shown in FIG. 3A in direct photo-ionization of A*, hv is equal to or greater than IP(A*). For a resonant excitation, hv can be equal to E(A*) (where A in its ground state undergoes a transition to A* representing A in a first excited state) or E(A**) (where A in its ground state undergoes a transition to A** representing A in a second excited state). Hence, for a quasi-resonant excitation, hv is equal to E(A**)-E(A*). 
     Collisions are represented in FIG. 3A by horizontal lines. For example, the horizontal line beneath A*+A represents the energy of a collision between A* and A. Three examples will now be given of how gas 130 (i.e., atoms A and molecules A 2 ) can be excited. The three examples are direct photo-ionization, associative ionization and Penning ionization. 
     In direct photo-ionization, photons from laser beam 144 have sufficient energy (hv) to ionize constituents of gas 130 directly by absorption. 
     
         A+hv→A.sup.+ +e.sup.-                               (5) 
    
     
         A.sub.2 +hv→A.sub.2.sup.+ +e.sup.-                  (6) 
    
     
         A+hv→A*                                             (7a) 
    
     
         A*+hv→A.sup.+ +e.sup.-                              (7b) 
    
     
         A.sub.2 +hv→A.sub.2 *                               (8a) 
    
     
         A.sub.2 *+hv→A.sub.2.sup.+ +e.sup.-                 (8b) 
    
     In Equations 5 and 6 ground state atoms A and molecules A 2  are directly photo-ionized (→is read: &#34;. . . is transformed into . . . &#34;). In Equations 5 and 6 the necessary conditions are that hv≧IP(A) and hv≧IP(A 2 ) (see FIG. 3A). In Equations 7a and 7b atoms A are ionized in two steps. Namely, atoms A are first electronically excited which requires that (hv=E(A*)) and then photo-ionized which requires that hv≧IP(A*). Equations 8a and 8b represent an analogous mechanism for molecules A 2  (hv=E(A 2  *) and hv≧IP(A 2  * )). 
     In associative ionization, ions are produced as a result of collisions involving electronically excited atoms and molecules (A*, A**, A 2 .sup. *). Consider the mechanism represented by Equation 9 (A* are formed via Equation 7a), wherein an excited atom (A*) collides with a ground state atom (A) and associatively ionizes to produce a dimer ion (A 2   + ) and an electron. 
     
         A*+A→A.sub.2.sup.+ +e.sup.-                         (9) 
    
     
         A*+A*→A.sub.2.sup.+ +e.sup.-                        (10) 
    
     Using exoergicity as the criterion for spontaneity, Equation 9 will proceed spontaneously (from left to right) provided that (see FIG. 3A), 
     
         E(A*)+D(A.sub.2)≧IP(A.sub.2).                       (11) 
    
     However, FIG. 3A shows instead that 
     
         E(A*)+D(A.sub.2)&lt;IP(A.sub.2)                               (12) 
    
     and therefore, the mechanism represented by Equation 9 will not proceed spontaneously and consequently will not produce ions. However, equation 10 can proceed spontaneously, if 
     
         2E(A*)+D(A.sub.2)≧IP(A.sub.2).                      (13) 
    
     FIG. 3A shows that indeed Equation 13 holds and therefore the mechanism represented by Equation 10 will proceed spontaneously. Furthermore, the mechanism 
     
         A**+A→A.sub.2.sup.+ +e.sup.-                        (14) 
    
     also proceeds spontaneously (since E(A**)+D(A 2 )&gt;IP(A 2 )). Hence, another way to produce a quasi-resonant excitation of A, is to select a laser 140 which outputs photons (144) having frequencies such that hv=E(A**)-E(A*). Ionization then occurs spontaneously due to the associative ionization mechanisms represented by Equations 10 and 14. There are other associative ionization mechanisms that involve electronically excited molecules (A 2  *). 
     In Penning ionization, ions are also produced by collisions involving excited atoms and molecules. For example, 
     
         A**+A**→A.sup.+ +A+e.sup.-                          (15) 
    
     and 
     
         A**+A.sub.2 →A.sub.2.sup.+ +A+e.sup.31 .            (16) 
    
     By inspection of FIG. 3A, it can be seen that the mechanisms represented by Equations 15 and 16 will proceed spontaneously (since 2E(A**)&gt;IP(A) and D(A 2 )+E(A**)&gt;IP(A 2 )) when hv=E(A**) or hv=E(A**)-E(A*). Furthermore, the mechanism 
     
         A**+A→A.sup.+ +A+e.sup.-                            (17) 
    
     will not proceed spontaneously because E(A**)&lt;IP(A). 
     Finally, if a second element B had been chosen to be present, it would have been possible to use the mechanism 
     
         A*+B→B.sup.+ +A+e.sup.-                             (18) 
    
     provided that E(A*)≧IP(B). 
     The general principles discussed above with reference to FIG. 3A will be applied to the case where gas 130 includes ground state cesium atoms (Cs), electronically excited cesium atoms (CS(6P), Cs(5D), etc.), cesium ions (Cs + , Cs 2   + , Cs 3   + , and Cs - ) and electrons. Cesium dimers (Cs 2 ), cesium trimers (Cs 3 ) etc. and direct photo-ionization also left out. Second and third order collisional mechanisms are included. 
     FIG. 3B shows a relevant energy level diagram (here a modified Grotrian diagram) for cesium. The vertical scale (energy) is in thousands of wavenumbers (cm-1). Atomic states are represented by short solid horizontal lines and are labeled by their principal quantum numbers (those in a given vertical column have the same angular momentum quantum number). Four long solid horizontal lines (labeled a-d) represent threshold energies for four collisional mechanisms to be discussed below. Two long dashed horizontal lines give the energy of the collision: Cs(6P) +Cs(6P) (both spin-orbit component combinations are shown). 
     Mechanism (a) in FIG. 3B produces an ion pair and may be written: 
     
         Cs*+Cs→Cs.sup.+ +Cs.sup.-                           (19) 
    
     The threshold for this transformation is at 27,603 cm-1. Hence the threshold excited atomic state for this mechanism is 9P. All cesium in atomic states that lie at the same or a higher energy than 9P can ionize by this mechanism (all cesium atomic states lower in energy than 9P cannot ionize by this mechanism). 
     Mechanism (b) in FIG. 3B is a second order associative ionization to form Cs 2  + and may be written 
     
         Cs*+Cs(6S)→Cs.sub.2.sup.+ +e.sup.-.                 (20) 
    
     Mechanism (b) has its threshold at 26,648 cm-1. Cesium in excited states 9S, 5F and 5G and higher will transform spontaneously by associative ionization and all cesium in states with energies lower than 26,648 cm-1 will not. 
     Mechanism (c) in FIG. 3B is a second order associative ionization to form Cs 3   +  (trimer ion formation), 
     
         Cs*+Cs.sub.2 →Cs.sub.3.sup.+ +e.sup.-.              (21) 
    
     Mechanism (c) has its threshold at 24,600 cm-1. Cesium atoms in excited states 8P, 7D and higher will transform spontaneously by this mechanism into Cs 3   +  and e - . Cesium in states 8S and 4F are very near threshold and may ionize at sufficiently high temperatures. Cesium in states below 8S and 4F will not ionize. 
     Mechanism (d) in FIG. 3B is a third order associative ionization used to form Cs 3   +  and proceeds by the simultaneous collision of three atoms. 
     
         Cs*+Cs(6S)+Cs(6S)→Cs.sub.3.sup.+ e.sup.-.           (22) 
    
     Mechanism (d) has its threshold at 20,922 cm-1 (third order mechanisms play an important role at high densities). In this case, cesium in states 7P, 6D and higher will spontaneously transform to give Cs 3   +  and e -  and cesium atoms and molecules in states below 20,922 cm-1 will not spontaneously transform to give Cs 3   +  and e-. 
     The two dashed lines in FIG. 3B represent the energy of two colliding Cs(6P) atoms. The figure shows that the (second order) mechanism. 
     
         Cs(6P)+Cs(6P)→Cs.sub.2.sup.+ +e.sup.-               (23) 
    
     will not proceed spontaneously since the dashed lines lie below the threshold for Cs 2  + formation (which is at 26,648 cm-1). However, the third order mechanism, 
     
         Cs(6P)+Cs(6P)+Cs(6S)→Cs.sub.3.sup.+ +e.sup.-        (24) 
    
     does proceed spontaneously since the dashed lines lie above the threshold for Cs 3   +  formation (which is at 20,922 cm-1). 
     To specify an electronic transition is to also specify possible mechanisms. For example, if laser 140 is used to excite one of the quasi-resonant transitions, 
     
         Cs(6P)+hv→Cs(5D)                                    (25) 
    
     laser 140 would be an infrared laser tuned to either 3.0111 or 3.4904 microns and ionization would occur via the following third order mechanisms: 
     
         Cs(5D)+Cs(5D)+Cs(6S)→Cs.sup.+ +Cs.sup.- +Cs(6S)     (26) 
    
     
         Cs(5D)+Cs(5D)+Cs(6S)→Cs.sub.2.sup.+ +e.sup.-        (27) 
    
     and 
     
         Cs(5D)+Cs(5D)+Cs(6S)→Cs.sub.3.sup.+ +e.sup.-        (28) 
    
     As another example, a near infrared laser at 0.8764 or 0.9175 micron would excite the quasi-resonant transition 
     
         Cs(6P)+hv→Cs(6D) and                                (29) 
    
     the primary mechanism of ionization would be 
     
         Cs(6D)+Cs(6S)+Cs(6S)→Cs.sub.3.sup.+ +e.sup.-.       (30) 
    
     This process can become especially important because nearly every Cs (6D) , once formed, results in ionization. Cs (6D) atoms that radiatively decay must do so primarily to the 6P state, the laser then re-excites these atoms back to the 6D state (they are radiatively trapped). If a Cs(6D) atom collides with a ground state cesium atom (6S), the following (&#34;energy pooling&#34;) mechanism occurs, 
     
         Cs(6D)+Cs(6S)→Cs(6P)+Cs(6P).                        (31) 
    
     The Cs (6P) atoms formed in Equation 31 are then returned to the 6D level by the laser in accordance with Equation 29. Together these mechanisms form an important &#34;feedback&#34; process. In addition, if two 6D atoms collide, Penning ionization will occur, i.e., 
     
         Cs(6D)+Cs(6D)→Cs.sup.+ +Cs(6S)+e.sup.-.             (32) 
    
     In this example, collisional mechanisms will tend to form Cs +  and Cs 3   +  but not Cs 2   + . The fact that these quasi-resonant plasmas exist at very low laser powers (&lt;1 milliwatt) strongly suggest highly efficient &#34;feedback&#34; processes. 
     FIG. 3C shows two quasi-resonances 6P-8D and 6P-9D of cesium to which a 10 milliwatt dye-laser (Coherent 599) was tuned. The 6P-8D transition corresponds to wavelengths of 601 or 621 nanometers and the 6P-9D transition corresponds to wavelengths of 567 or 585 nanometers. Note that the 6S electronic state is a ground state for cesium, and consequently, tuning laser 140 to the 6S-6P resonances (not quasi-resonances) at 852.4 and 894.6 nanometers would result in strong absorption of laser beam 144. This limits the size of LGD device 100 due to large attenuation of laser beam 144 by gas 130. 
     Since laser 140 is tuned to a quasi-resonance, it is only slightly attenuated as it travels from one electrode to the other, because the population of electronically excited 6P states is much smaller than that of the ground 6S states. Consequently, laser beam 144 can penetrate long distances in gas 130. This also makes it possible to scale LGD device 100 to high powers. 
     FIG. 4 shows right electrode 110R with a nude filament 410R and insulation feed through 420R which insulates wires-193R from electrode wall 160. In this case, no insulator is used to insulate filament 410R at localized area 192R of low resistivity. Wire 189R is connected to wires 193R via a capacitor 430R. Capacitor 430R provides a dc block so that only higher frequency ac power from power source 188 reaches filament 410R. Filament 410R serves as both a heating element and electrode tip 170 and consequently the discharge current remains between left electrode 110L and right electrode 110R, while wall 160 remains electrically isolated from filament 410R. 
     FIG. 5 is a schematic representation of a transverse LGD device 500 according to another embodiment of the invention. Identical reference numerals are used for those elements of transverse LGD device 500 which are common to longitudinal LGD device 100 of FIG. 1. Here however, the direction of the electric discharge flow (i.e., the electric discharge current vector) is transverse to the path of beam 144. Also, instead of having left and right electrodes such as 110L and 110R in FIG. 1, transverse LGD device 500 has a top electrode 510T and a bottom electrode 510B between which the discharge current is directed. A top conductive element 530T interconnects top electrodes 510T on the left and right hand sides of transverse LGD device 500 and a bottom conductive element 530B interconnects bottom electrodes 510B on the left and right hand sides of transverse LGD device 500. As with longitudinal LGD device 100, transverse LGD device 500 can output photons 145 at either its left end or its right end. Plasma 190 is produced between top and bottom conductive elements 530T and 530B. 
     A top heating power source 591T is used to ionize gas 130 in the vicinity of top electrode 510T and top conductive element 530T. Again, this provides a conductive path to guide the discharge current between top electrode 510T with top conductive element 530T and bottom electrode 510B with bottom conductive element 530B connected to power source 188 via top and bottom wires 589T and 589B, respectively. Similarly, bottom heating power source 591B ionizes gas 130 in the vicinity of bottom electrode 510B and bottom conductive element 530B, thereby further aiding in producing the discharge current in the transverse direction as indicated in FIG. 5. 
     As with LGD device 100, laser beam 144 is directed from the left side of transverse LGD device 500 through gas 130 between top conductive element 530T and bottom conductive element 530B to the right hand side of transverse LGD device 500. Gas 130 between top and bottom conductive elements 530T and 530B is ionized by laser beam 144 so long as laser beam 144 fills the space between the top and bottom conductive elements. Localized areas 592T and 592B (analogous to localized areas 192L and 192R for longitudinal LGD device 100) which guide the discharge current, are formed using top and bottom heating power sources 591T and 591B, respectively, as will be discussed below. Top and bottom heating power sources 591T and 591B are connected to filaments (see FIG. 6) via wires 593T and 593B, respectively. 
     FIG. 6 is a more detailed schematic representation of the left hand side of transverse LGD device 500. In particular, FIG. 6 shows how top and bottom heating power sources 591T and 591B are connected to top and bottom filaments or wires 610T and 610B via wires 593T and 593B, respectively. Top filament 610T and bottom filament 610B are electrically isolated from plasma 190 via top and bottom insulators 620T and 620B, respectively. Here, however, top and bottom localized areas 592T and 592B are located in the vicinity of top and bottom filaments 610T and 610B, respectively. Top and bottom filaments 610T and 610B together with respective top and bottom insulators 620T and 620B can extend along the entire length of respective top and bottom conductive elements 530T and 530B (FIG. 5). 
     FIG. 7 shows the left side of transverse LGD device 500 using top and bottom electrodes 510T and 510B together with top and bottom nude filaments 710T and 710B, respectively. Here, a capacitor 730 is used as a dc block for power source 188 which in this case produces pulsed or ac voltages between top electrode 510T and bottom electrode 510B. Top and bottom electrical insulators 720T and 720B electrically insulate top wire 589T and bottom wire 589B from housing ends 180, hence, electrically insulating power source 188 from the housing ends 180 and walls 120 (FIG. 5). 
     FIG. 8 shows how LGD device 100 or 500 can be used in a laser 800 which outputs coherent photons 845 from output photons 145. A left and right reflector 810L and 810R are used to create a laser cavity. Left reflector 810L should be partially transmissive for photons from laser beam 144 while being reflective for output photons 845 and 145 generated in plasma region 190 (see FIGS. 1 and 5). On the other hand, right reflector 810R should be reflective for generated photons 145 in plasma region 190 (FIGS. 1 and 5) and laser beam 144 and partially transmissive for output photons 845. The output of laser 800 is coherent photons 845 which are those photons in output photons 145 which have been amplified as a result of being in a stable mode of a resonator formed from reflectors 810L and 810R. 
     FIG. 9A shows a longitudinal laser guided discharge device 900a in which gas 130 is replaced with a liquid 930a. Again, the frequency of laser beam 144 output by laser 140 must be tuned to a quasi-resonance of liquid 930a, i.e., the energy of photons in laser beam 144 must be equal to the difference in energy between two excited electronic states in liquid 930a. Details of left and right electrodes 110L and 110R as well as heating power sources 191L and 191R have not been shown but are identical to those shown in FIG. 1. Similarly, transverse LGD device 500 in FIG. 5 can be modified to contain a liquid in place of gas 130. Examples of liquids 930a include water, alcohols, liquid hydrocarbons, liquid nitrogen and liquid argon. 
     FIG. 9B shows a modification of LGD device 100 in which gas 130 is replaced by a solid 930b with left and right sides 910L and 910R, respectively. As in FIG. 9A, details of left and right electrodes 110L and 110R as well as left and right heating power sources 191L and 191R have been omitted for clarity. Again, laser 140 must output photons tuned to a quasi-resonance of solid 930b. Note that solid 930b need not have housing wall 120 or housing ends 180. Examples of solids 930b include doped semi-conductors, doped glass, doped crystals and photoconductive materials. 
     Numerous and additional modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically claimed.