RU2806877C2 - High and ultra high pressure short arc gas discharge lamp - Google Patents

Abstract

FIELD: electrical engineering.

SUBSTANCE: invention is related to gas-discharge lamps of high and ultra-high pressure. In a short-arc gas-discharge lamp of high and ultra-high pressure, the anode surface facing the cathode is made in form of an ellipsoidal surface with the coordinate τ of ellipsoidal coordinates lying in the range - 0.8<τ<0 with cathode coordinate τ₀<0, and for predominant radiation in the ultraviolet and infrared regions of the spectrum, it is made in form of an ellipsoidal surface with the coordinate τ of ellipsoidal coordinates lying in the range 0<τ <0.8 with cathode coordinate τ₀ < 0.

EFFECT: increase in efficiency of generation of optical radiation in a selected region of the spectrum.

1 cl, 9 dwg

Description

Field of technology

The invention relates to electrical engineering, namely to gas-discharge lamps of high and ultra-high pressure. The invention covers high-power short-arc xenon lamps, high-pressure mercury short-arc lamps, as well as all other short-arc lamps with a cathode doped with an easily ionizable additive and an anode with a diameter larger than the diameter of the current channel on the surface of the anode.

State of the art

Currently, there are a large number of designs of electrodes for short-arc gas-discharge lamps, which make it possible to solve a number of technical problems from reducing the thermal load on the electrode to obtaining certain characteristics of the electrodes by varying their composition and design [1-3].

Short-arc high-pressure gas-discharge lamps, as a rule, have cathodes doped with easily ionizable additives. Most often, thorium is used as such an additive. An easily ionizable additive, as a rule, reduces the work function of electrons from the cathode and, evaporating into the discharge volume, can affect the characteristics of the plasma. It is shown in [4, 5] that the presence of thorium atoms reduces the plasma temperature near the cathode without reducing the radiation intensity in the visible region of the spectrum due to the fact that the radiation is recombination bremsstrahlung with the participation of thorium atoms and ions. Fig. 1 shows the longitudinal distribution of plasma characteristics of a short-arc xenon lamp with a power of 250 W [4]. The longitudinal distributions of the concentration of charged particles and plasma temperature obtained from spectral measurements, as well as the intensity of plasma radiation near 550 nm, are presented. The geometry of the discharge is also shown schematically. It can be seen that the presence of thorium near the cathode actually reduces the plasma temperature T and ensures that the concentration of thorium ions exceeds over the concentration of xenon ions in this part of the category. In this case, the radiation intensity in the visible region has a maximum near the cathode and does not correlate with the plasma temperature, which cannot be explained in the case of a discharge that is homogeneous in composition (the presence of only xenon atoms in the plasma).

The results of [4, 5] show a significant influence of the cathode material on the properties of the discharge plasma and, thus, on the emissive characteristics of high-pressure xenon lamps. This conclusion can be generalized to both electrodes: it is obvious that the properties of the anode, in particular its shape, will also greatly influence the characteristics of the arc discharge plasma.

There is a known patent RU 152355 U1, 05/27/2015 [6], which describes a short-arc xenon lamp for a device for optoelectronic counteraction to infrared homing heads of guided missiles with a straight tubular bulb made of colorless leucosapphire, at the opposite ends of which electrode units are hermetically installed coaxially with it - anode and cathode, containing a permanently connected holder of the corresponding electrode, adjacent on part of the side surface to the inner surface of the flask, and a sealing element, the junction of which with the flask is made female, characterized in that in the cathode holder, outside its axis, there is a longitudinal through hole, circular in cross section , in which an ignition electrode is vacuum-tightly installed, separated from the walls of the hole by an insulating element in the form of a straight tube made of a hard dielectric material with high temperature resistance. In this case, the anode of a short-arc xenon lamp has a curvature other than zero. The disadvantage of the proposed lamp design is the inability to control the characteristics of the gas-discharge lamp.

There is a known patent US 6614186 B2 [7], which describes the design of the anode of a short-arc gas-discharge lamp, in particular, a high-power xenon lamp, in which the anode design allows increasing the stability of operation due to the appropriate organization of turbulent gas flows caused by non-uniform heating of the gas. This patent describes an anode design that makes it possible to reverse turbulent gas flows in the discharge flask and, thus, eliminate turbulent disturbances in the arc discharge. As shown in FIG. 2, the problem is solved by using a protrusion on the anode surface facing the cathode, which, as claimed by the inventors, prevents the occurrence of gas flows that create discharge turbulent instability. The disadvantage of the proposed anode design is the inability to control the characteristics of the gas-discharge lamp.

There is a known patent DE 102010024240 A1 [8], which describes the design of an anode having a disk-shaped recess coaxial with the anode with a diameter D and depth H. The design of the anode is shown in Fig. 3. By increasing the effective surface of the anode, which dissipates the energy load, the temperature of the central part of the anode surface and the sputtering of the anode material are reduced, which reduces the rate of blackening of the inner surface of the discharge lamp bulb. This also allows for longer lamp life at the required light level and standard discharge lamp brightness. The disadvantage of the proposed anode design is the inability to control the characteristics of the gas-discharge lamp.

There is a known patent JP 2011065756 A [9], which is closest to the claimed invention, which describes the design of an anode having a spherical recess with a diameter smaller than the diameter of the flat part of the anode surface facing the cathode. The anode design is shown in Fig. 4. An increase in the effective area of the working surface of the anode, which takes on the main energy load, due to the recess, reduces the sputtering of the anode material and, thus, reduces the blackening of the inner surface of the discharge bulb, which makes it possible to increase the service life of the lamp at the required level of illumination and standard brightness of the gas-discharge lamps. The disadvantage of the known device, adopted as a prototype, is the inability to effectively control the characteristics of the gas-discharge plasma.

The essence of the invention

The claimed design of the arc lamp anode is free from these disadvantages.

The technical result of the proposed anode design is the ability to influence the properties of the arc discharge plasma and, as a consequence, obtain effective generation of radiation in the visible, ultraviolet and/or infrared regions of the spectrum.

This technical result is achieved by the fact that in a short-arc gas-discharge lamp of high and ultra-high pressure, which includes an anode and a cathode, which are located coaxially opposite each other at a given distance, and the cathode contains an easily ionized additive, and the anode has a larger diameter of the surface facing the cathode or equal to the diameter of the current channel on the surface of the anode, and has the shape of a surface facing the cathode, with an average curvature different from zero, the surface of the anode facing the cathode has an ellipsoidal shape, which for predominant radiation in the visible region of the spectrum is made in the form of an ellipsoidal surface with coordinate τ ellipsoidal coordinates lying in the range - 0.8<τ<0 with cathode coordinate τ ₀ <0, and for predominant radiation in the ultraviolet and infrared regions of the spectrum is made in the form of an ellipsoidal surface with coordinate τ ellipsoidal coordinates lying in the range 0<τ <0.8 at cathode coordinate τ ₀ <0.

Curvature (mathematically) here refers to a quantity that characterizes the deviation of a curve (surface) from a straight line (plane). The deviation of the arc MN of curve L from the tangent MP at point M can be characterized using the so-called. average curvature k _cp of this arc, equal to the ratio of the magnitude of its angle between the tangents at points M and N to the length Δs of the arc MN [10]:

k _cp =α/Δs.

The chosen location of the cathode in the negative half-plane of ellipsoidal coordinates is not important, since this coordinate system is symmetrical with respect to the τ=0 plane.

The claimed anode design and its effect on the plasma characteristics are explained as follows.

The characteristics of a short-arc discharge plasma depend, first of all, on the spatial distribution of the electric field strength in the discharge gap. The electric field heats electrons, which ionize and excite gas atoms, and also transfer their energy to gas atoms and ions in collisions, which, under high-pressure arc discharge conditions, leads to the emergence of a locally thermodynamically equilibrium plasma [1, 4, 5]. The electric field intensity distribution obviously depends on the shape of the cathode and anode surfaces. The surfaces of the electrodes are equipotential with potentials determined by the voltage necessary for the existence of a gas discharge. Therefore, if the shape of the surface of the electrodes is such that the effective distance between the electrodes is smaller (the anode has a concave shape), the field strength decreases and its radial distribution becomes flatter, which will lead to an increase in the radius of the current channel. Consequently, the plasma temperature should also decrease. In the case of a convex anode, the effect will be the opposite. Fig. Figure 5 qualitatively illustrates the described effect. The transition from a concave to a convex shape increases the length of the electric field lines, which leads to a decrease in the electric field strength at the periphery of the discharge, which reduces the drift velocity of electrons, therefore, to maintain the magnitude of the flowing current, it is necessary to increase the field strength at the discharge axis. This will lead to an increase in the plasma temperature in the paraxial region.

High-pressure short-arc lamps, as a rule, have electrodes doped with easily ionizable elements. Most often, this additive is thorium (the additive is 2-5% in the composition of the tungsten cathode [1, 2]), the atoms of which have an ionization energy approximately half as low (6.1 eV [11]) as xenon (12.1 eV), while thorium has a lower electron work function (3.35-3.47 eV) compared to tungsten (4.54 eV), and thorium oxide ThO ₂ , with which the tungsten cathode of xenon lamps is doped, is even lower - 2.54-2.67 eV [12]. Thus, doping the cathode reduces the work function of the cathode material (increases the emission of electrons from the cathode) and reduces the cathode drop. In addition to the above, it should be noted that the expected effect of such doping is the possible emission of atoms of an easily ionizable additive (thorium) into the discharge gap: “puddles” of the doped element (thorium) are formed on the cathode surface, the atoms of which evaporate into the discharge volume. This effect was first discovered and described in [4, 5]. The decisive influence of atoms of an easily ionizable additive on the electrokinetic and optical properties of near-cathode plasma was shown. The presence of thorium atoms in the near-cathode region reduced the plasma temperature, which led to the predominance of thorium ions over xenon ions in this region and the generation of recombination bremsstrahlung radiation predominantly with the participation of thorium ions and atoms. In this regard, it can be noted that the atoms of an easily ionizable additive (thorium) are decisive in the generation of radiation from the near-cathode spot of a short-arc xenon discharge. The latter, obviously, also leads to a decrease in the intensity of optical radiation associated with xenon atoms - infrared radiation in the spectral region of 800-1100 nm and ultraviolet recombination bremsstrahlung radiation, which for pure xenon plasma has a higher temperature and, therefore, greater intensity in ultraviolet range.

As noted above, the shape of the anode can affect the spatial distribution of discharge characteristics. In [13], a model of a short-arc xenon discharge at high (ultra-high) pressure was constructed. To describe the spatial distributions of plasma characteristics, ellipsoidal coordinates were used, which, from our point of view, are the most optimal for describing the geometric structure of a short-arc discharge. This model made it possible to obtain both characteristics on the discharge axis and spatial distributions of plasma characteristics. Using this model, the characteristics of the plasma of a short-arc discharge in xenon were calculated for different shapes of the anode surface. As a result, a strong dependence of the plasma characteristics on the shape of the anode surface facing the cathode was obtained.

Fig. Figure 6 illustrates the geometry of a short-arc discharge in ellipsoidal coordinates (prolate ellipsoid of revolution [14]). The cathode is located on the left and is highlighted with a thick line. For the case considered in [13], the cathode coordinate in ellipsoidal coordinates is equal to τ ₀ = -0.924, the anode coordinate τ _L = 0 (flat anode). It is also highlighted with a bold line. The dotted line shows the current channel corresponding to the current spot on the cathode with a diameter of 0.1 cm. The shape of the anode surface is easily determined by choosing the anode coordinate τ _L . In accordance with the claimed invention, it is proposed to select the anode coordinate in the range -0.8<τ _L <0, 0<τ _L <0.8 (coordinate τ _L =0 corresponds to a flat anode).

The discharge model makes it possible to calculate the electrokinetic characteristics of the plasma, in particular, the electric field strength, plasma temperature, concentration and composition of ions, the concentration of thorium atoms emitted by the cathode into the discharge gap, and the radial distributions of these characteristics. In the next Fig. Figure 7 shows the characteristics of the plasma along the discharge axis: plasma temperature T, electric field strength E, concentrations of thorium atoms and ions N _Th and , xenon ions and electron density _ne [13]. The calculation was carried out for conditions close to the operating conditions of an ultra-high pressure xenon lamp with a power of 250 W: xenon pressure in a cold lamp is 20 atm, discharge current is 14 A, the distance between the cathode and anode is 0.3 cm, the outer diameter of the lamp is ~2.5 cm.

As can be seen from FIG. 7, the presence of thorium atoms near the cathode noticeably reduces the plasma temperature. This leads to a radical change in the composition of ions near the cathode: the concentration of thorium ions exceeds the concentration of xenon ions, which is close to zero in this region. The plasma radiation of a short-arc xenon discharge is recombination-bremsstrahlung radiation and radiation of xenon spectral lines, the main contribution to which is made by infrared lines in the region of 800-1100 nm. A decrease in plasma temperature and the almost zero concentration of xenon ions near the cathode (this means that the excitation of xenon atoms is small in this region) leads to the fact that the emission spectrum is dominated by recombination bremsstrahlung radiation created by thorium atoms and ions, which has a continuous spectrum with an effective temperature (6500-7200) K (see Fig. 1 and Fig. 7). The region near the cathode creates the main radiation flux of the discharge, so it is obvious that the radiation of the xenon lamp will be mainly a continuous spectrum with the temperature indicated above. This radiation is close to the spectrum of solar radiation and is therefore used in lighting when it is necessary to ensure the best color rendition. The intensity of the IR spectral lines of xenon atoms and UV radiation will be relatively low.

However, there are technical tasks that require high intensity UV radiation (etching processes, lithography, surface disinfection, etc.) or strong IR radiation (high-temperature exposure to matter, IR spotlights, etc.). The constructed model [13] allows, by selecting the appropriate shape of the anode surface, to change the plasma radiation spectrum and, thus, expand the scope of application and efficiency of using a high-pressure short-arc discharge. There is also a need to obtain optical radiation with a temperature below 6000-6500 K. Such radiation can be used for special purposes: the use of “close” to Planck emitters with temperatures up to 4000 K and below, the creation of comfortable “warm” radiation, the design of art objects, etc. .

The stated result is confirmed by theoretical analysis conducted at St. Petersburg State University.

In FIG. Figure 8 shows the plasma temperature for different shapes of the anode surface. The shape of the anode is selected corresponding to the coordinate τ _L in ellipsoidal coordinates: -0.4, -0.2, 0, +0.2, +0.4 (see Fig. 6 for a depiction of coordinates in ellipsoidal coordinates). The distance between the cathode and anode in all cases is chosen equal to 0.3 cm. It can be seen from the figure that the shape of the anode surface radically affects the plasma temperature in the discharge gap. In the case of a concave anode surface (τ _L = -0.4, -0.2), the plasma temperature in the main volume of the discharge is (6000-7500) K, while for a convex anode (τ _L = +0.2, +0.4) the temperature reaches (8500 -9000) K. The case τ _L =0 describes a flat anode.

The following Figs. 9a (τ _L =0), Fig. 96 (τ _L =+0.4) and Fig. 9c (τ _L = -0.4) show the distribution of concentrations of thorium atoms N _Th , thorium ions and xenon ions with different anode shapes - from concave (τ _L =-0.4) to convex (τ _L =+0.4). In the case of a flat anode (Fig. 9a), the concentration of xenon ions at the cathode is practically zero and further increases as it approaches the anode. The concentration of thorium ions near the cathode is significantly higher than that for xenon ions, which leads to a noticeable drop in the plasma temperature in this region (see Fig. 8).

With a convex anode (Fig. 9b) and a plasma temperature of up to 9000 K, the concentration of xenon ions at the maximum increases noticeably (about two times compared to a flat anode), and the maximum itself approaches the cathode. The distributions of thorium atoms and ions are pressed toward the cathode, which leads to a significant decrease in the intensity of optical radiation associated with thorium atoms and ions emitted from the cathode spot. For a concave anode (Fig. 9c), the picture is the opposite: the plasma temperature in almost the entire discharge gap does not exceed 7400 K, and the concentration of thorium ions in almost the entire gap exceeds the concentration of xenon ions.

The results shown in Fig. 8 and 9 confirm the strong influence of the anode shape on the characteristics of the plasma and on the optical radiation emitted by the plasma.

High and ultra-high pressure xenon light sources have thoriated cathodes. Thorium is a radioactive element. Therefore, recently, work has been carried out to replace thorium with other elements, in particular rare earth elements, which can provide an effect similar to the effect of using thorium. In particular, the compounds La ₂ 0 ₃ -W, ZrO ₂ -W, CeO ₂ -W, Y ₂ 0 ₃ -W [15], Ce-W, La-W, YW [16] are being studied. These additives have a melting point lower than the melting point for tungsten and the excitation and ionization energies of atoms emitted into the plasma are noticeably lower than those for xenon atoms, therefore the claimed invention will also be applicable to short-arc discharges with electrodes doped with other elements.

The technical and economic efficiency of the claimed invention consists in increasing the efficiency of generation of optical radiation in the selected region of the spectrum. Achieving this result allows the claimed invention to be used to create new and effective environmentally friendly sources of optical radiation (light sources), expands the possibilities of using the claimed invention in various technological processes, and will also find wide application in industrial and household lighting.

List of information sources used

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3. Gas discharge lamps. Catalog 09.5.01-80. M.: Informmelektro. 1980; Catalog 09.50.07-75. M: Informelektro. 1975

4. N.A. Timofeev, V.S. Sukhomlinov, G. Zissis, I. Yu. Mukharaeva, & Dupuis P. IEEE Trans. on Plasma Sci. 47, No 7, (2019) PP. 3266-3270

5. N. Timofeev, V. Sukhomlinov, G. Zissis, I. Mukharaeva, D. Mikhailov, P. Dupuis. Technical Physics, 64, No 10, (2019) PP. 1473-1479; DOI: 10.1134/S1063784219100207

6. Patent RU 152355 U1, IPC H01J 61/02 (2006.01)

7. Patent US 6614186 B2, IPC H01J-061/073*, |H01J-061/12,| H01J-061/86

8. Patent DE 102010024240 A1, IPC HOU-061/073/2*, |H01J-061/82

9. Patent JP 2011065756 A, IPC H01J-061/073* (prototype)

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13. N.A. Timofeev et al. "Modeling of high pressure short-arc xenon discharge with a thoriated cathode," IEEE Trans, on Plasma Sci., 2021, August, Doi: 10.1109/TPS.2021.3093816

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Claims (1)

A short-arc gas-discharge lamp of high and ultra-high pressure, including an anode and a cathode, which are located coaxially opposite each other at a given distance, wherein the cathode contains an easily ionized additive, and the anode has a diameter of the surface facing the cathode greater than or equal to the diameter of the current channel on the surface of the anode , and has the shape of a surface facing the cathode with an average curvature different from zero, characterized in that the surface of the anode facing the cathode has an ellipsoidal shape, which for predominant radiation in the visible region of the spectrum is made in the form of an ellipsoidal surface with coordinate τ ellipsoidal coordinates lying in the range - 0.8<τ<0 at the cathode coordinate τ ₀ <0, and for predominant radiation in the ultraviolet and infrared regions of the spectrum is made in the form of an ellipsoidal surface with coordinate τ ellipsoidal coordinates lying in the range 0<τ<0.8 at the coordinate cathode τ ₀ <0.