SEGMENTED PLASMA EXCITATION RECOMBINATION LIGHT SOURCE

Abstract: In generally known segmented plasma excitation and recombination type of light sources, comprising spaced apart metal strips (101-110) capable of generating a high-density metal-vapor ion plasma (141-149) when an electrical voltage (130) is applied across the strips, the use of strip materials formerly not usable in such devices is made possible by using strips which are significantly thinner (e.g., 10 times thinner) than previously used.

Description: SEGMENTED PLASMA EXCITATION RECOMBINATION LIGHT SOURCE Background of the Invention This invention relates to plasma excitation recombination light sources such as lasers. In Applied Physics Letters, Vol. 36, No. 8, p. 615 (1980) , W. T. Silfvast, L. H. Szeto and 0. R. Wood, II, describe a new electric discharge device developed for producing laser action in the atomic spectra of a number of metal vapors by a segmented plasma excitation and recombination (SPER) mechanism. This laser includes a number of narrow metal strips (of the lasing species) positioned end-to-end on an insulating substrate in such a way as to leave a small gap between each pair of adjacent strips. The strips are positioned in a laser cavity containing either a buffer gas (preferably) or a vacuum and typically are 1 mm thick, 2 mm wide, and 10 mm long (hereinafter "bulk electrodes"). When a high-voltage high-current pulse is applied to the end strips of this arrangement, a high-density metal-vapor ion plasma is formed in each gap. Once formed, these plasmas (consistin primarily of vaporized strip material) expand hemispherically, cool in the presence of the background ga (e.g., helium) at low pressure- and recombine.- -Using this laser configuration, laser action was observed in the near infrared at more than seventy wavelengths between 0.29 and 3.95 μ-m in 11 elements (Ag, Bi, C, Ca, Cd, Cu, In, Mg, Pb, Sn, Zn) , 3 of which (Mg, Zn, In) had not been observed to oscillate in their neutral spectrum before. Some of these results are reported in the aforementioned APL article; others are reported by W. T. Silfvast et al in Applied Physics Letters, Vol. 39 No. 3, p. 212 (1981) and in Optic Letters Vol. 7 No. 1, p. 34 (1982). The SPER laser is simple to construct, can be easily scaled in length and volume, has been shown to be capable of long life, and has the potential for high efficiency. It is the subject matter of U.S. 4,336,506. Lasing action in a SPER laser is not observed with equal facility with all metals, even under conditions which normally facilitate the formation of a plasma, e.g., relatively high pressure of the background gas. A figure of merit, M, can be derived which defines the relative ease of achieving lasing action in a metal vapor. M is defined as follows: . M ~~ (kcpT 2 ) {1) where Jc is the thermal conductivity of the metal, c_ is the specific heat of the metal, p is the density of the metal, and T_ is the absolute temperature of the surface of the metal electrode. This expression is derived from a heat equation for the time necessary to raise the electrode surface temperature to a value T by ion or electron current through the electrode surface. For the value of the temperature at which the vapor pressure of the metal is approximately 0.1 Torr, a list of a number of metals together with calculated values of M normalized to the value for Cd metal, is given in the following table: METAL M Cd 1.00 Na .99 Pb .57 In .41 Zn .39 Li .33 METAL M Ca .27 Sn .11 Ni .03 Al .02 Cu .01 For the alkali metals (Na, Li, Ca) M should be calculated for the oxides that form for these metals. Experimentally metals with M ~ 1 have been found to easily produce segmented metal vapor plasmas necessary for lasing action in SPER lasers even at relatively low background gas pressures (e.g., 1-10 Torr) , whereas metals with M << 1, such as Li, Al, Ca, and Cu, do not produce segmented plasmas. With these metals, as the background gas pressur is reduced, the discharge current is carried by a discharg in the background gas between non-adjacent electrodes, effectively shorting out the intervening metal-vapor arcs, reducing the number of metal vapor plasmas and, hence, lowering the net gain. Summary of the Invention We have found that segmented metal vapor plasma discharges and lasing action in SPER devices can be achieved, even at relatively low background gas pressures, with metal electrodes of materials having M << 1 provided that the metal strips constitute electrodes which are sufficiently thinner (typically about 10 times thinner) than the prior art bulk electrodes so that discharges occu only between adjacent electrodes, thereby eliminating the short circuiting problem associated with bulk electrodes. Using this thin electrode SPER configuration, we have achieved laser action in four metals (Li, Al, Ca, and Cu) I in which laser oscillation was not possible using bulk electrodes and low pressures. In addition, we observed segmented vapor plasma discharges in a SPER with Ni electrodes. Brief Description of the Drawing FIG. 1 is a schematic diagram of a SPER laser in accordance with an illustrative embodiment of our invention; FIG. 2 is a graph of relative output power as a function of helium pressure for a Cd SPER laser operating on the 1.433 μm transition in Cd I and the 537.8 nm transition in CD II; FIG. 3 is a graph of the relative intensity of the spontaneous emission as a function of gap width on the 394.4 nm Al I resonance line, on the 281.6 nm Al II line, and on the 569.6 nm Al III line from a single-gap Al SPER device operating at a helium pressure of 3 Torr; FIG. 4 is a graph of the minimum helium pressure for reliable operation of an 11-gap Al SPER laser as a function of gap width for three values of electrode thickness; namely, 1.2 mm, 0.5 mm, 0.125 mm; and FIG- 5 is a schematic of an alternative embodiment of our invention utilizing bi-meta lic strips. Detailed Description An embodiment of the present invention used for the production of a segmented plasma-excitation and recombination (SPER) laser in a metal vapor is shown in FIG. 1. A plurality of metal strips 101-110, each constituting, in accordance with this invention, a particularly thin electrode, are positioned end-to-end on an electrically insulating plate 120 in such a manner as to leave a small gap between each pair of adjacent strips. This electrode arrangement is then installed in a gas cell 125. A capacitor 130 and a switch 132 are connected in series across the first (101) and last (110) electrodes. The capacitor 130 is charged to a few tens of kilovolts and is then discharged across the series of metal strips with a C$ spark gap. Operation at much lower voltages (e.g., a few tens of volts) is also possible, however. The resulting current pulse produces a bright metal vapor plasma in eac gap. Areas 141-149 in FIG. 1 depict the shape of the plasmas after they have expanded hemispherically outward from the gaps into a background gas. The entirety of each strip need not constitute a material which is vaporizable into a plasma. As shown in FIG. 5, it is sufficient if the cathode ends (202b, 203b, etc.) constitute such a material and that the anode ends (202a, 203a, etc.) constitute a non-vaporizable material under the operating condition of the device. In this embodiment, for example, each strip would comprise tandem segments (e.g., 202a and 202b) of non-vaporizable and vaporizable material electrically connected to one another Within the scope of applications intended here, an example of a non-vaporizable material is tungsten. Moreover, strips of different vaporizable materials can be mixed within a single device so as to yield a multi-color source. Two dielectric spherical mirrors 150 and 151 are coated for maximum reflectivity at the desired lasing wavelength to form a resonator for the laser radiation. Illustratively, these mirrors are mounted. on the ends of a tube which forms gas cell 125. The optical axis 160 of this resonator is positioned parallel to and slightly abov the row of foil electrodes. The output from this resonator, shown as line 170, is focused through suitable filters onto a suitable photodetector (not shown) . In prior work with Cd SPER lasers, capacitor 130 had a capacitance of 0.01 yF and was charged to 21 kV to produce an 840 A current pulse having a ringing frequency of 1.2 MHz. The background gas was helium at a low pressure of 5 Torr. The mirrors had a 3 radius of curvature and formed a 12 cm long resonator for the laser radiation with the optical axis positioned 7 mm above the foil electrodes. The photodetector was a room temperature Ge diode . EXAMPLE Using an arrangement of the type shown in FIG. 1 we have made SPER lasers in the visible and near infrared at wavelengths between 569.6 nm and 5460 nm in four metal elements having M « 1: Li, Al, Ca and Cu. Table I lists the observed wavelengths and the transition assignments fo these elements. In our experimental arrangement on these SPER lasers we used a background gas of helium at low pressures of 1-10 Torr. Forty-one thin electrodes were arranged in tandem on a glass substrate. The electrodes were typicall 25 mm long by 2.5 mm wide by 0.1-0.2 mm thick. The gap width between adjacent electrodes was preferably about 0.2 mm for Li and Ca and about 0.5 mm for Al and Cu. Importantly, this thin electrode geometry differs from tha used in previous experiments where 1.0 mm thick bulk electrodes spaced 1.0 to 2.0 mm apart were used. When thi 41-electrode assembly was excited with 15—30 kV pulses, producing a 5 μsec current pulse of 50-200 A, a metal vapo plasma was formed in each gap. The 40 metal vapor plasmas expanded 20-25 cm away from the gap regions into the low pressure helium gas (1-10 Torr) , and in the afterglow cooled to the point that collisional recombination dominated all other electron loss processes. Collisional de-excitation of the high lying. excited states of the recombined ions or atoms populated levels where radiative decay dominated over collisional decay and established population inversions. Laser action was observed in these plasmas when the axis of a 115 cm-long laser resonator was positioned parallel to and 7 mm above the foil electrodes. A resonator formed by two, 2 meter-radius of curvature, highly reflecting dielectric coated mirrors 150 and 151 wa used for those lasers with wavelengths less than 1900 nm, and a resonator formed by two, 2 meter-radius of curvature gold coated mirrors (one mirror was provided with a 2 mm diameter central coupling hole) was used for wavelengths - 1 - greater than 1900 nm. The laser output was observed through a 1/4 meter monochromator and (depending on wavelength) was detected with either a photomultiplier, a intrinsic germanium detector, or a liquid helium cooled Ge:Cu detector. The 30 new recombination lasers produced using our electrode SPER configuration are listed in Table I. Since recombination of the stages of ionization always occurs in descending order, the temporal behavior of the laser output aided in the identification of the particula neutral or ion stage involved in laser action. Assignmen within a particular stage also were not ' difficult since recombination lasers generally occur across energy gaps i the spectrum of each ionization stage. In order to obtain recombination laser action, particularly in higher ion stages, the plasmas should be segmented as in FIG. 1 to maximize cooling, and the plasm should be produced in a background gas (e.g., helium) at low pressure where, as FIG. 2 indicates, the gain will be highest. The technique of enhancing gain through plasma segmentation has been discussed in the aforecited Silfvast et al articles and patent where it was shown tha in a properly operating SPER laser, gas breakdown is followed by the formation of a metal vapor arc in each ga As mentioned earlier, however, with metals having a low figure of merit M, as the helium pressure is reduced, the discharge current is carried by a discharge in the helium gas between non-adjacent electrodes, effectively shorting out the intervening metal vapor arcs, reducing the number of metal vapor plasmas and, hence, lowering the net laser gain. The path which the arc discharge ultimately takes depends on formative time lags and the rate of growth of ionization along each path since the applied voltage exceeds that necessary for breakdown not only for the gaps but also along paths in the buffer gas between non-adjacen bulk electrodes as well. Ionization growth between non- adjacent bulk electrodes is dominated by ionization phenomena in the background gas and is expected to be roughly constant for different metals and electrode geometries. On the other hand, techniques that enhance the metal vapor production in the gap regions should lead to increased ionization growth and further vapor production and, hence, are expected to be determined to some extent by the properties of the particular metal used as the electrodes. The background gas is believed to serve two purposes in the plasma expansion process: (1) to cool the electrons thereby inducing electron-ion recombination, and (2) to confine the region of inversion to the mode volume of the optical cavity. A typical curve of laser output versus pressure for a SPER-type device in the neutral atom (Cd I) resulting from recombination of the Cd + ion and in the single ion (Cd II) resulting from recombination of Cd ÷+ is shown in FIG. 2. It can be seen that, especially for higher ion stages, low background gas pressures are important. For recombination laser action the gap width should be chosen to optimize the spontaneous emission intensity from the metal vapor arcs. The dependence of the spontaneous emission intensity of the 394.4 nm Al I . resonance line in a foil electrode Al SPER laser on increasing gap width is shown in FIG. 3. The emission intensities of lines in the Al II and Al III spectra were also found to vary with gap width in roughly the same way. When the gap width was increased beyond 2.0 mm the emission from the metal vapor dropped, and a helium discharge appeared between the metal vapor-dominated spherically-shaped plasmas. The results shown in FIG. 3 demonstrate that the intensity of the spontaneous emission (and, if the electron density is not too high, the gain) increases with gap width and has a maximum in an electrode Al SPER device at approximately 1.5 mm. The effects of gap width and electrode thicknes on the ability of plasmas to form at relatively low pressure were tested in the following way. A number of eleven-gap thin electrode Al-SPER devices (each with a different gap width) were constructed by careful machining to achieve gaps with macroscopic uniformity. As the pressure of the helium background gas was gradually reduced, there was a well-defined pressure at which one or more gaps were shorted out by the occurrence of a helium plasma between non-adjacent electrodes. This pressure, th lowest pressure the device reliably provided segmented plasmas, is denoted P m i n « The results of measurements of p min on a ser "i es °f devices with different gap widths and electrode thicknesses, when excited with a peak current of 200 A and a duration of 5 μsec, are shown in FIG. 4. In contrast with the strong dependence on electrode thickness little effect was found in varying the width of the electrodes. For gap widths greater than 0.1 mm only the thinnest electrodes (0.125 mm) resulted in low values for P m in an ~- n segmented! Al vapor plasmas. Thus, to produce the highest net gain and, hence, to maximize the chances o obtaining recombination laser action in Al, gap widths of approximately 1.0 mm and electrode thicknesses of approximately 0.1 mm should be used, although the actual gap width used will be determined by the lowest pressure necessary for laser action. It-is to be understood that the above-described arrangements are merely illustrative of the many possible specific embodiments which can be devised using the principles of the invention. It is theorized that thin electrodes work well because they enhance the thermal processes that act at the surface of the electrodes by reducing the heat lost due to conduction into the electrode. In the case of bulk electrodes the heat flow into the electrode is three dimensional, but in a thin geometry the heat flow is more nearly two dimensional. Hence, heat loss due to conductio is less for thin geometries, and the surface of the thin electrode reaches a given temperature in a shorter period of time. This effect is thought to lead to higher vapor pressures and enhanced ionization growth rates. Regardless of the mechanism, however, thin electrodes not only permit the use of metals with a low figure of merit in segmented plasma devices at low pressures, but also permit the generation of plasmas in reactive metals (e.g., alkali metals) which have oxide layers on their surfaces. In addition, our thin electrode SPER device has enabled us to observe spontaneous emission in Ni (M = 0.03 at wavelengths characteristic of its visible emission. TABLE I Metal Wavelength (μm) Transition Assignment Li I 1 . 279 5f - 3d 1.870 5d 2 D 5/2 - 4f 5/2 2.689 3d - 3s 3/2 1 /2 4.057 4f 2p °7/2 - - 3d 2 °5/2 A1 I .1.126 4f ~F' 1/2 - 3d -D 5/2 A1 II 0.7042 4p 3 P β 2 - 4s 0.7471 4f - 3d 'D- \ 0.7515 (unknown) 1.008 4p - 3d . 1.012 4p 3 P° 0 - 3d 3 D1 1.246 5d 1 D 2 - 5p 1 P * 1 1.406 7ρ V-, - 6s 1 S 0 A1 III 0.5696 4 ? ,p °3/2 " 4S S 1/2 0.5722 4P 2 P° 1/2 " 4s 2 S 1/2 TABLE I (Cont'd) Metal Wavelength (μ ) Transition Assignment Ca I 1.304 3d - 4p' ό F° Ca II 1.184 5p 2 P β 1/2 - 5s 1/2 1.195 5 ∑> 2p 3/2 " 5S 2S 1/2 Cu I 1.257 5f 2 F « 7/2 - 4d D 5/2 1.818 4f 2 F β 7-,/,2„ - 4d 2 D5/2 3.089 (unknown) 3.726 4ρ' 2 P β 1/2 " 5s 2s 1/2 5.460 7d 2 D '3,/,2., - 7p 2 P β 1/2 Cu II 0.7807 6s 3 D 3 - 5ρ 3 F β 1.787 (unknown) OM

Claims: Claims 1. A recombination light source comprising: at least two strips (101-110) having a gap therebetween proving a discharge path; and means (130,132) 5 for applying an electrical pulse to said at least two strips; at least a portion (202b) of said at least tw strips being fabricated from a material which is converted into a plasma as a result of the application of said 10 electrical pulse, which plasma cools and recombines to generate radiation; characterized in that said portion comprises a material having a figure of merit M much less than unity and is sufficiently thin so that said discharge path occurs only between 15 adjacent ones of said at least two strips. 2. The source of claim 1 wherein said material has a figure of • merit M < 0.33. 3. The source of claim 2 wherein said material is a metal selected from the group consisting of Li, Al, Ca, 20 Ni, and Cu. 4. The source of claim 1 wherein said at least two strips are each approximately 0.1-0.2 mm thick. 5. The source of claim 1 further including a gas cell containing helium at a pressure of about 1-10 Torr, ^ 25 said at least two strips being positioned within said gas cell. 6. The source of claim 1 wherein said at least one gap has a width of approximately 0.2-0.5 mm. 7. The source of claim 1 wherein the entirety of 30 each of said at least two strips comprise said material. 8. The source of claims 1, 2, 4, 5, 6 or 7 for use as a laser and further including a resonant laser cavity adapted to permit egress of said radiation therefrom, 35 said excitation means being adapted to produce said radiation in said cavity. O 9. The source of claim 8 wherein said material is selected from the group consisting of Li, Al, Ca, and Cu.

Inventors: MACKLIN JOHN JOSEPH, SILFVAST WILLIAM THOMAS, WOOD III OBERT REEVES, MACKLIN, JOHN, JOSEPH, SILFVAST, WILLIAM, THOMAS, WOOD III, OBERT, REEVES
Applicants: WESTERN ELECTRIC CO, WESTERN ELECTRIC COMPANY, INC.
Application year: 1983
Main classification: H01S3
IPC codes: H01S3