Cathode screen for gas discharge lamps

Abstract: A mica apertured plate (10, 20) has been developed for the purpose of manufacturing long-life fluorescent tubes in machines with the tubes lying horizontally, where diffusion pumping cannot be performed by the introduction of mercury droplets into the tube, but where it is necessary to purge the tube with alternating tube evacuating steps. In the case of one embodiment, this plate has a circular shape and is provided with slots (12, 13) which extend from the periphery of the circular plate in towards a central hole (11), such as to form intermediate tongues (14). The apertured plate is supported by support devices (8, 9) at a constant distance from the lamp electrode (3), such as to form between the plate (10) and the lamp tube a gap in which electrons and ions of the emission material released from the electrode (3) recombine. In another embodiment, the apertured plate (20) is oval and is provided with a central hole (21) and with a recess (23) which is located in the edge of the plate and in which support devices can engage.

Description: The present invention relates to a device for restricting the spreading of emission material from cathodes of gas discharge lamps, said device having the form of an apertured plate which extends transversely to the lamp discharge path. Cathode screens of various forms and intended for different purposes are known to the art. However, the cathode screen taught by WO-A-81/01344 was the first device to be noticeably effective in restricting the spreading of emission material released from the cathode. Faced with the difficulties of reducing the spread of emission material during lamp operation in the development of gas discharge lamps, so as to extend the useful life of said lamps, lamp manufacturers had, up to that point in time, refrained from investing in means for increasing the burning or running time of such lamps. The device described in the aforesaid patent specification has the form of a can-like casing with that end of the can which faces towards the discharge chamber having the form of an apertured plate, or disc, made of an electrically non-conductive material. Provided in the other end of the casing is an elongated opening of sufficient size to allow the lamp cathode to be inserted into the screen, said screen being made of an electrically conductive material, but spaced from the cathode and its power supply wires. This form of cathode screen restricts the space in which ions and molecules of the emission material with which the cathode is coated are able to move. The emission material is generally comprised of a solid solution of alkali earth metals saturated with barium oxide (BaO). Gas discharge lamps are operated with alternating voltage, and when such a lamp is ignited the electrodes positioned at respective ends of the discharge chamber will function alternately as a cathode and an anode respectively, with a functional change each half period. During the anode phase, the barium ions (Ba²⁺) strive to pass through the emission material and onto the surface of the tungsten filament from which the electrode is spun. This results in the formation of barium tungstate (Ba₃WO₆) on the electrode surface, this substance forming a boundary layer which amplifies electron emission. The free barium formed during the formation of barium tungstate will penetrate up through the emission material during the cathode phase of the electrode and, to some extent, exit into the discharge chamber in the form of ions (and to a very limited extent in the form of molecules). A part of the emission material is knocked out by ion bombardment (in the form of molecules which are ionized in the discharge column), quite soon after the lamp has been ignited, at which time the electrodes have not yet reached operating temperature. The propagation of the barium ions and the emission material in the discharge chamber is restricted by the presence of a cathode screen, and the ions and emission material are prevented essentially from settling on the glass wall surrounding the discharge chamber. Any barium ions and emission material which have settled on the glass wall cannot be recovered. Subsequent establishing a light arc between the anode and the cathode, and therewith establishing the formation of a positive discharge column between anode and cathode, the arc will remain unitary in an axial direction, with the space charge 0 along its longitudinal axis. On the other hand, the ions and electrons generated by the discharge will diffuse towards the lamp wall, the lighter electrons moving more rapidly than the ions. The lamp wall will therefore be negatively charged, and a positive space charge is developed from the centre of the lamp and out towards the lamp wall. The known cathode screen restricts radial movement of the ions in the vicinity of the cathode, to a not insignificant extent. This is assumed to be because the walls of the can-like casing obstruct, or greatly restrict the radial transportation of ionized emission material. It has been found, however, that the mere presence of an apertured plate in front of a cathode will cause the discharge column to be so constricted that ions of the emission material will still be within a range in which they can be influenced by the anode as the current changes direction, so that ions and attractable molecules will again settle on the electrode surface. The constriction of the discharge column through the aperture of the apertured plate will also result in a concentration of electrons close to the aperture, resulting in a relatively low anode drop and therewith limited heating of the electrode. The resulting relatively low temperature will keep the departure of emission material from the cathode to within very low values, in relation to a conventional fluorescent tube cathode. This extends the life of the cathode threefold or fourfold over the conventional cathodes. One contributory factor to this long cathode functioning time is the use of krypton as the dominating filling gas in the discharge lamp instead of argon, which has less than half the atomic weight (mass) of krypton. It has been found, somewhat surprisingly, that when the distance travelled by the discharge from the the discharge column through the discharge chamber, this distance being calculated from the axis of the column continuing over the plate periphery and from there to the rearwardly lying electrode, is from two to three times as long as the distance through the aperture to the electrode, the discharge will go through this aperture. Consequently, if the cathode is placed 5 mm behind the apertured plate, the path around the edge of the plate will be from 10 to 15 mm to the nearest part of the cathode, e.g. the ends of the cathode to which the supply wires or filaments are connected. The longer the path, the more certain it is the discharge will pass through the aperture of the disc. An arrangement for restricting the spread of emission material from gas discharge lamps is disclosed in FR-A-0803474. An apertured plate is mounted in front of each of two electrodes functioning altematingly as cathode and anode in the discharge path transversely to the longitudinal axis of the lamp. Each apertured plate is made of an electrically insulating material and terminates close to the lamp wall. The two apertured plates are both carried by the same support devices. In order to further ensure that the discharge will pass through the plate aperture, the plate may be made so large as to terminate close to the lamp wall. A gap between the outer periphery of the plate and the lamp wall is necessary, in order not to damage the fluorescent powder layer applied thereto in the manufacture of the lamp. By optimizing this gap, there is obtained in the present case a recombination zone in which ions are united with electrons that concentrate on the lamp wall, so as to form sluggish molecules. During operation of the lamp, these molecules prevent the discharge from passing around the apertured plate to the cathode. A suitable gap width is 1.5-2.5 mm with a fluorescent tube diameter of 26 mm and when the fluorescent tube is a so-called low energy fluorescent tube, i.e. a tube having a gas pressure of 270-340 Pascal (Pa). The apertured plate is thus larger than the entrance hole preformed at the end of the tube to which a cathode foot or base is fused. A gap of the aforesaid width results in a very high degree of recombination, and ions that are released from the electrode emission material and which do not exit through the plate aperture, but remain in the space downstream thereof, will move towards the lamp wall and meet an excess of electrons in the vicinity of the gap. Molecules are then formed which result in a saturation of barium and other emission material in the proximity of the cathode. This saturation further retards the vaporization of emission material and provides molecules which, together with ionized material, again fall to the cathode surface. This considerably slows down the loss of emission material and the useful life of the electrode is more than sufficient for a so-called long-life fluorescent tube, i.e. a tube having an economic burning time in excess of 30000 hours. (An economic burning time, or running time, is defined as the time when at least 70% of the light yield remains compared to that after a burning time of 100 hours.) The aforesaid recombination also results in inactivation of ions arriving from the discharge chamber to form molecules. This enables the cathode to be made longer than would otherwise be possible, since such ions would be able to carry charges to those parts of the cathode located in the proximity of the outer perimeter of the apertured plate. Thus, in the absence of a recombination zone, the discharge occurring on a long cathode would pass around the edges of the apertured plate to the ends of the cathode. Neither would this longer cathode need to be mounted in an arcuate shape, as in the case with hitherto known can-shaped or cup-shaped cathode screens. The advantage with a long cathode is that the cathode is able to absorb a proportionally greater amount of emission material, and therewith increase the useful life of the lamp. Furthermore, a straight electrode coil or helix has a uniform distance between the wire in each turn of the helix. It is an advantage to obtain a low transverse resistance in the cathode helix when the lamp is operating, which contributes to maintaining the emission material at a low temperature, so as to reduce is operating, which contributes to maintaining the emission material at a low temperature, so as to reduce vaporization and ionization of said material. The cathode screen used in accordance with the aforesaid patent has not been found to cause disturbance in those fluorescent-tube manufacturing methods used hitherto, i.e. with the tubes standing vertically during the course of tube evacuation. Thus, it has been possible to carry out the tube-evacuating pumping process in the form of a diffusion pumping process, by dropping mercury into the tubes, this mercury vaporizing instantaneously at the bottom, hot end of the fluorescent tube. When the mercury droplets vaporize, the mercury ions entrain therewith contaminating substances from the tube in the immediately following evacuation processes. The production method in which the tubes stand vertically, as has hitherto been the case, enables about 1400 fluorescent tubes to be produced per hour. Those attempts to increase this production rate have only resulted in an increase in capacity of some few percent, for a number of reasons. Fluorescent-tubes manufacturing machines are known, however, in which the tubes lie horizontally during the manufacturing process. In this method of manufacture, the tubes are purged with a purging gas, and the contaminants are ventilated from the lamp subsequent to fusing the cathode feet to respective ends of the tubes. The contaminants, or impurities, concerned are mainly carbon dioxide, carbon monoxide, water, gaseous oxygen or gaseous nitrogen, originating essentially from the decomposition of emission material on the electrode surfaces during the process of manufacture. The tubes are evacuated by vacuum pumping between the until the pump tube has been melted-off at one end thereof so as to seal this end of the tube. Horizontal pumping and gas-purging of the tubes during manufacture presents no problem in the case of so-called standard fluorescent tubes which do not include a cathode-enclosing cathode screen. The rate at which such tubes are manufactured is more than twice the manufacturing rate of tubes provided with such a cathode screen, this higher production rate having contributed to maintaining economic viability in the branch. With regard to the manufacture of long-life tubes provided with cathode screens according to the aforesaid patent, problems arise, of which the most serious problem is related to the ventilation of the tube prior to filling the tube with noble gas, and the necessary transportation of gaseous contaminants, originating from the conversion to oxides of the carbonates of the fluorescent powder and emission material. It is not possible to effect diffusion pumping by means of dropping mercury droplets into the tube, since the cycle time is too short in the case of horizontal pumping. Furthermore, it is not possible to introduce mercury in droplet form, or to inject mercury so that the mercury will vaporize against a hot glass surface when the tube lies horizontally. The problem becomes obvious when it is realized that the gaseous contaminants formed by decomposition of the electrode emission material is located in the proximity of the electrodes, i.e. within the can-shaped casing which forms the cathode screen. Gas purging of the aforesaid horizontally lying tubes is effected within a pressure range of from 200-600 Pa, and is performed in two or three stages with intermediate vacuum pumping down to a pressure of 50 Pa. The cycle times are very short, less than 10 seconds. The free cross-sectional area of the pump tubes is smaller than 10 mm², and the area of the aperture in the front wall of the cathode screen is about 50 mm², whereas the annular cross-sectional area between the cathode screen and the wall of the fluorescent is around 275 mm². It will be obvious from this that the major part of the purgative gas will pass through the annular space located outside the cathode screen. Consequently, the contaminants present within the cathode screen will only be ventilated from the tube to a very slight degree. These facts speak against the use of the known cathode screen in the horizontal production of fluorescent tubes. With regard to the aforesaid state of the art, it is found that the inventive apertured plate can be used highly satisfactorily in the manufacture of long-life fluorescent tubes with the aid of horizontally working machines. Thus, the objective of the present invention has been to provide, for the manufacture of long-life fluorescent tubes in horizontally working production machines, a cathode screen which will enable a requisite, clean atmosphere to be created in the lamp, while ensuring, at the same time, that the cathode screen will have an effect which is equally as good as the known cathode screens in limiting the spread of emission material. Another object is to provide a simple support for the cathode screen. These objects are achieved with a lamp having the features set forth in the following claims. By producing the apertured plate from an electrically insulating material it is ensured that the discharge occurring in the fluorescent tube will not be disturbed, and that no ions will be attracted to any particular part of the apertured plate. The plate is supported by support devices in a given position in front of the cathode, so as to leave a gap between the outer perimeter of the plate and the surrounding glass wall of the lamp. In this way, there is formed a recombination zone in which molecules can reform from electrons and ionized emission material deriving from the cathode. The zone need not have a uniform width around the aperture plate, and may even extend around only a part of the plate perimeter. The apertured plate may be in the form of a circular disc, in which case the gap will have a constant width around the circumference of the disc. An advantage is gained when the apertured plate is made of mica. The plate will preferably have a thickness of between 0.06 and 0.12 mm, so that the plate is sufficiently pliable to enable the plate to be deformed when inserting the plate through the opening of a blank tube, or starting tube, during manufacture. In the case of 26 mm tubes, which at present dominate the market, this opening has an inner diameter of 19 mm, whereas the inner diameter of the tube is 24 mm. This is because the tube end is pre-shaped, so that the type of base which surrounds the tube end can be given a diameter smaller than the outer diameter of the lamp. With a specific gap width between the outer perimeter of the apertured plate and the tube wall of 2 mm, the largest cross-dimension or diameter of the apertured plate will thus be 20 mm. A plate which is made of mica will return to its original shape subsequent to having being deformed in order to enable the plate to be inserted through the end-opening of the tube. When it is desired to provide a gas discharge lamp with a very narrow gap between the apertured disc and the smaller, the apertured plate can be made more pliable, by forming slots from the periphery of the disc towards its centre. The width of the slots is from 0.8 to 3.0 mm, depending on the diameter of the lamp tube and the width of the gap. The slot width is chosen in relation to the width of the tongues defined by the slits. With a given continuity, the slits are made broader with increasing width of the tongues. The recombination zone formed in the annular gap around the apertured plate is increased with the slot areas. It would seem that recombination cannot be achieved with certainty at gap widths above 3 mm. Furthermore the slot area and the degree of recombination decrease, when the width of the tongues exceeds 4 mm. A tongue width smaller than 1.5 mm would jeopardize mechanical mounting of the apertured plate, because of the weakness of the plate. The radial extension of the slots from the periphery of the apertured plate is chosen between a value of 3 mm, which affords a noticeable increase in recombination effect, and a value of 6 mm in the case of apertured plates of greater diameter. Slots of greater radial extensions would terminate too close to the centre hole of the plate. Since the requisite recombination effect can be achieved in a fluorescent tube with a gap of varying width, it is possible to use apertured plates of oval or elliptical shape. Such plates are inserted through the end-opening of the tube with the plate bent or curved along the large axis of the ellipse, whereafter the bending force is relieved so that the apertured plate will snap into a position perpendicular to the longitudinal axis of the tube. For example, when such an apertured plate is used in a compact fluorescent tube, the thickness of the plate may be reduced to 0.04 mm. In the case of a 7-foot (2134 mm) fluorescent tube having a diameter of 2 inches (50.8 mm), a suitable thickness is 0.20 mm. A uniform recombination effect is obtained when the disc aperture is located centrally in the path of the discharge to the cathode. The diameter of the aperture is preferably between 25 and 45% of the inner diameter of the lamp tube. An aperture diameter which is smaller than 25% of the inner diameter of the tube causes difficulties in initial ignition of the discharge lamp, particularly at low ambient temperatures. The effect achieved with regard to restriction of ions released from the emission material will be excessively low when the aforesaid aperture diameter is greater than 45% of the inner tube diameter. In the case of a large aperture diameter, these released ions would enter the lamp discharge chamber to an extent such that the running time of the lamp would not be prolonged sufficiently to justify the additional costs associated with a cathode screen and its mounting. The apertured plate is held in position in the lamp in front of the cathode with the aid of metal-wire support devices, for instance pure iron devices. The support devices may be flat-rolled or circular, in which latter case the device is flattened slightly at one end thereof. This end is given the shape of a U, and the apertured disc is inserted into the U, which is thereafter clamped into engagement with the edge of the disc, preferably into a slot when using a circular apertured disc. This enables the apertured plate, or disc, to be inserted straight into the lamp tube. Since the diameter of the end-opening of the tube is smaller than the diameter, or cross-dimension, of the apertured plate, the tongues will spring rearwardly and spring back to positions lying in the plane of the apertured plate when said plate is located within the lamp tube. The support devices are preferably used in pairs, and the wire-ends distal from the apertured plate are fused into the glass of a glass cathode foot, through which power supply wires are drawn to both ends of the cathode. Sufficient insulation between the power supply wires and the support devices can be achieved in this way, even in the case of a compact fluorescent tube, to prevent the support devices from being electrically conductive and influencing the discharge. Oval apertured plates can be fitted to respective tubes in a corresponding manner. A single support device is sufficient in this case, this device engaging around the edge of the plate approximately at the point where the oval plate is dissected by the minor axis. In order to ensure positive engagement of the plate when fitting the plate into a tube, the support devices may be placed in a recess formed in said edge of the plate, so as to enable the plate to be flexed slightly around its small axis when introducing said plate into the tube. The length of the support device is such that the apertured plate will be located at a distance of from 4 to 9 mm in front of the cathode, as seen in the discharge direction. When this distance is shorter than 4 mm, the ends of the cathode will not be adequately screened and there is a danger than ionized emission material will escape into the discharge chamber. When the aforesaid distance is greater than 9 mm, the space behind the apertured plate is relatively large, resulting in a decrease in the extent to which ionized emission material is restored. This results in a reduction in the total lamp running time. An advantage is gained when the lamp tube is filled with a relatively heavy noble gas. Although argon is a relatively in expensive gas, when used alone it generates a high operating voltage, or burning voltage, in the tube and the collision between the ions deriving from emission material are much too elastic to achieve the desired length of useful life of the lamp. Consequently, argon corresponds to a quarter of the gas filling, while the remainder of the noble gas filling is comprised of krypton in a concentration corresponding to 75-90% of the total. A support ring may be placed adjacent the support devices and behind the apertured disc, as seen from the discharge chamber. This ring is preferably made of pure iron and is co-rolled with amalgam, or alternatively is coated with amalgam. Subsequent to filling the lamp tube with noble gas and melting-off the pump tubes in the lamp manufacturing process, the support ring is subjected to a high-frequency treatment process which causes the amalgam to cleave or split so as to give-off mercury. The composition of the amalgam used is such that the amount of mercury required to operate the lamp over a given life time will be available in the lamp tube. Two alternative embodiments of the invention will now be described with reference to the accompanying drawing, in which Figure 1 illustrates one end of a fluorescent tube and shows the insertion of an apertured disc onto a support device attached to a cathode foot; and Figure 2 is a cross-sectional view of one end of a fluorescent tube and shows a generally elliptical apertured plate and a cathode located behind said plate. An electrode 3 is inserted onto a cathode foot 2 in the lamp tube 1 of a fluorescent lamp. The electrode 3 is carried by current conducting wires 4, 5 connected to the ends 6, 7 of the electrode helix 3. Fused into the cathode foot 2 are two wire-like support devices 8, 9. These devices support a thin mica disc 10 having a central aperture 11 located centrally opposite the electrode 3. The support devices 8, 9 have U-shaped ends 8', 9' which engage around the peripheral edge of the mica disc 10. In the case of the illustrated embodiment, the disc 10 is provided with two mutually opposed slots 12, 13 which extend from the periphery of the disc 10 in towards its centre, wherein the U-shaped ends 8', 9' of the support devices engage in the inner ends of said two slots. The slots define tongues 14 of sufficient resiliency to enable the mica disc 10 to be bent and inserted at right angles to the end opening 15 of the tube 1, despite the diameter of said end-opening being smaller than the diameter of the disc 10. The cathode foot 2 has a plate-like bottom part 2' and when the cathode foot is moved to a position in which said bottom part is in abutment with and covers the end-opening 15, the lamp tube and the cathode foot 2 are fused together. The tube is then purged with gas, alternated with evacuation of the tube 1 with the aid of a pump pipe 16 which extends through the cathode foot 2. Finally, a mixture of argon and krypton is introduced into the lamp tube at a pressure of about 300 Pa and the pump pipe 16 is melted-off. Although not shown, getter rings may be mounted behind the plate 10 and between the support devices 8, 9. Subsequent to sealing the lamp tube, the ring is subjected to high frequency treatment so as to release mercury in an amount at which a partial pressure of mercury of 0.7 Pa will be maintained during operation of the lamp. In the Figure 2 illustration, an apertured mica plate 20 has a partially elliptical shape and is provided with a circular, centrally located hole or aperture 21. The plate is intended to be used in a manner corresponding to the aforedescribed. In the case of this embodiment, however, recombination zones 22 are formed only at those parts which are located close to the ends of the major axis of the ellipse. In order to enable only one support device to be used, a recess 23 has been formed in the edge of the apertured plate 20, preferably at the point where said edge is intersected by the minor axis of the ellipse. The support device has an U-shaped end which engages around the apertured plate 20 at the recess 23, such that said support device will not protrude beyond the periphery of the plate. This is advantageous when fitting the apertured plate, since the plate 20 can be bent or curved in the U-shaped engaging part of the support device and the support device inserted into the lamp tube with the apertured plate 20 arcuately curved in its longitudinal direction and transversely to the longitudinal axis of the lamp tube. This method of fitting the support device and apertured plate enables the transverse measurement of the plate to be generally equal to the diameter of the end-opening of the lamp tube. The same conditions as those described with reference to the embodiment first described also apply with the second embodiment.

Claims: An arrangement for restricting the spread of emission material from cathodes in gas discharge lamps of the kind which comprise an apertured plate (10) mounted in front of the cathode (3) in the discharge path transversely to the longitudinal axis of the lamp, the apertured plate (10) being carried by at least one support device (8,9) and being made of an electrically insulating material and terminating close to the lamp wall such as to leave a narrow gap (22) between the apertured plate (10) and said wall, whereby said narrow gap (22) forms a recombination zone (22), wherein said gap extends around at least a part of the plate perimeter, and wherein said at least one support device or devices (8,9) has the form of round-section or flat wire, and the one end of said support device or devices (8,9) engages the apertured plate (10), and the other end of said support device or devices (8,9) is fused in the cathode foot material (2) in which foot electrically conductive wires (4,5) carrying the cathode (3) are totally isolated from the support device or devices (8,9). An arrangement according to claim 1 used in fluorescent lamps, characterized in that the apertured plate (10) is circular and is comprised of mica having a thickness of between 0.06 and 0.12 mm; in that the circular plate is provided around its periphery with slots (12,13) which are separated by tongues (14) having a width of 1.5-4 mm and a length of 3-6 mm; and in that the gap formed between the lamp wall and the free extrimities of the tongues (14) in the apertured plate (10) corresponds to 2-10% of the inner diameter of the tube (1), said support devices preferably functioning as a pair with the ends (8',9') of said devices engaging the apertured plate (10) in the innermost part of diametrically opposed slots (12,13). An arrangement according to claim 2, characterized in that the slots (12,13) separating the tongues (14) of the plate (10) have a width of 0.8 to 3.0 mm. An arrangement according to claim 1 used in fluorescent lamps, characterized in that the apertured plate (20) has an approximate elliptical configuration with the length of the minor axis equalling 80-90% of the length of the major axis and the length of the major axis equalling up to 90% of the inner diameter of the fluorescent tube (1); in that the thickness of the apertured plate (20) is between 0.04 and 0.20 mm; and in that the apertured plate has a centrally located circular aperture (21) whose diameter is between 25 and 45% of the inner diameter of the flurescent tube, one end of the support device engaging around the outer perimeter edge of the apertured plate, preferably at a location at which the minor axis of the plate intersects its perimeter. An arrangement according to any one of the preceding claims, characterized in that the apertured plate (10,20) is positioned at a distance of from 4 to 9 mm in front of the cathode (3), calculated in the direction of the discharge column. An arrangement according to any one of the preceding claims, characterized in that the noble gas filling of the lamp is comprised of 75-90% krypton and 25-10% argon. An arrangement according to claim 2 or 3, characterized in that the diameter of the apertured plate (10) is greater than the diameter of the inlet opening of the fluorescent tube in which a preformed end of a cathode foot is fused. An arrangement according to any one of the preceding claims, characterized in that the arrangement includes a metal support ring which is located behind the apertured plate (10), as seen from the discharge chamber, and which is connected to the support device (9) and which is preferably partially made of amalgam.

Inventors: BJOERKMAN AKE, BJOERKMAN, AKE, BJÖRKMAN, Ake
Applicants: LUMINOVA AB, LUMINOVA AKTIEBOLAG
Application year: 1992
Main classification: H01J61
IPC codes: H01J61