EP0072862A1 - Corona charging apparatus. - Google Patents

Abstract

Crown-effect charging device comprising an elongated conductor coated with a dielectric (11) in contact with a control electrode or close to it. In the first version of this device, the control electrode comprises a conductive grid (17) which is mounted against an insulating support (15). In a second version, the control electrode consists of a slot conductor (94), the elongated conductor coated with dielectric (11) being embedded in the slot (96). A variable high voltage potential (25) between the elongate conductor (11) and the control electrode induces a glow discharge in an air region near the two conductors. The control electrode can play the role of an earth member to form a device for discharging the crown relative to a neighboring surface (20). Alternatively, the control electrode can be held at a desired potential (27) to obtain a charging device with an automatically limited voltage. The crown effect charging devices of the invention are characterized by a linear relationship between the output ion currents and a direct current extraction potential. In other versions of the second crown device, the slotted conductor and the conductor coated with a dielectric can be replaced by other structures which give an equivalent enclosure.

Description

CORONA CHARGING APPARATUS

BACKGROUND OF THE INVENTION

The present invention relates to corona charging devices, particularly as used for discharging electrostatic images.

Corona charging devices in The form of thin conducting wires or sharp points are well known in the prior art. Illustrative U.S. Patent Nos. are Vyverberg 2,836,725: L.E. Walkup 2,879,395: P. Lee 3,358,289; Lee P. Prank 3,611,414; A.E. Jvriblis 3,623,123: P.J. McGill 3,715,762; H. Bresnik 3,765,027; and R.A. Potland 3,961,564. Such devices are used almost exclusively in electrostatic copiers to charge photoconductors prior to exposure as well as for discharging. Standard corona discharges provide limited ion currents. Such devices as a rule achieve a maximum discharge current density on the order of 10 microamperes per square centimeter. Additionally, corona wires are small and fragile, and easily broken. Because of their high operating potentials they collect dirt and dust and must be frequently cleaned or replaced, in order to avoid fall-off of the emission current.

Corona discharges which enjoy certain advantages over standard corona apparatus are disclosed in Sarid et al. U.S. Patent No. 4,057,723; Wheeler et al. 4,068,284; and Sarid 4,110,614. These patents disclose various corona charging devices characterized by a conductive wire coated with a thick dielectric material, in contact with or closely spaced from a further conductive member. Various geometries are disclosed in these patents, all fitting within the above general description. These devices utilize an alternating potential in order to generate a source of ions, and a DC extraction potential. The patents disclose a preferred biasing range of 2000-6000 volts, relatively high values which are required in order to obtain significant extraction currents and therefore higher charging rates. These current outputs are exponential in character, in contrast to the fairly linear outputs of the present invention. In addition, these devices are undesirably sensitive to variations in the gap width between the corona and the imaging member.

U.S. Patent No. 4,153,093 discloses ion generating, apparatus which may be used for charge neutralization as well as deposition of net charge. This apparatus is superior to standard corona apparatus, but is difficult to fabricate, and does not provide the high charging rates of the present invention.

Accordingly it is a principal object of the invention to provide charging and neutralizing devices employing corona discharges which have superior performance as compared with prior art corona devices.

Another object of the invention is to provide a corona charging device which achieves high current densities. A related object is the achievement of high charging rates. Another related object is the avoidance of high: biasing potentials in providing such charging rates.

A further object of the invention is to provide a charging device having a rugged and compact structure. A related object is to provide a device having a longer operational life than is customary in corona ion generators. A further related object is the provision of corona apparatus which does not require frequent servicing.

Another object is to provide a corona charging device capable of charging or discharging a remote dielectric or photoreceptor surface to potentials within a few volts of a preselected potential.

Still another object of the invention is the avoidance of emission current fall-off as the ion generator becomes slightly dirty. A related object is the achievement of uniform emission currents. Yet another object of the invention is the provision of a corona charging device with a reliable output potential. SUMMARY OF THE INVENTION

In achieving the above and related objects, the invention provides a corona charging device comprising an elongate conductor with a dielectric sheath, and a control electrode in proximity thereto. In a first type or form of the invention, the control electrode comprises a conductive grid overlying the dielectric sheathed elongate conductor, with both mounted against an insulating substrate. In a second type of the invention, the control electrode comprises a conductive enclosure, which defines a slot in which the sheathed elongate conductor is placed. The apparatus of the invention may be used for corona charging and discharging by means of a time varying potential applied between the elongate conductor and control electrode, which induces a glow discharge in an air region adjacent the control electrode and dielectric sheath.

The control electrode is maintained at ground potential for charge neutralization, and at a limiting biasing potential for corona charging. The corona charging apparatus is characterized by a linear relationship between output ion currents and a direct current ion extraction potential.

In accordance with one aspect of the first type, the grid electrode comprises a one or two dimensional array of fine conductive members. In a preferred version the grid electrode comprises a fine wire mesh screen. In an alternative version, the grid comprises a parallel array of fine, closely spaced wires, transverse to the axis of the elongate conductor.

In accordance with a preferred version of the second type, the conductive enclosure comprises a unitary structure having a slot with the sheathed elongate conductor embedded in the slot. This unitary structure advantageously consists of a conductive beam having an essentially rectangular slot or channel. In an alternative version of this type, the slotted conductor is replaced by a pair of conductive rods, which are mounted on each side of the dielectric-coated conductor against an insulating support. in accordance with another aspect of the invention, the various dimensions may be altered to modify the output ion characteristics of the corona charging and discharging device. In the first type, the most important parameter is the profile of the grid electrode, and in particular the wrap of the grid electrode over the dielectric sheathed elongate conductor. In the second type, important parameters include: the lateral separation, if any, of the sheathed conductor from the walls of the conductor; the extent of protrusion or indentation of the sheathed conductor with respect to outer surfaces of the enclosure; and the width of the conductive enclosure as compared with the diameter of the sheathed conductor. In the preferred version of the second type the dielectric sheathed conductor contacts the side walls of the conductor and protrudes slightly therefrom; advantageously, the conductive enclosure is only slightly broader than the width of the slot. Another important parameter in either type is the separation of the device from the surface to-be charged or discharged.

Another aspect of the second type relates to the use of a filler at the base of the slot to prevent power loss and dielectric breakdown.

In accordance with a further aspect of the invention the elongate conductor may have a variety of cross sections. In the preferred version of either type, the elongate conductor comprises a cylindrical wire. Alternatively, in the first principal type the electrode may consist of an etched foil. In accordance with a related aspect of the invention, a variety of insulated materials, preferably inorganic, may be utilized in the dielectric sheath for the elongate conductor.

In accordance with yet another aspect of the invention, the dielectric sheathed elongate conductor and control electrode are coextensive structures, preferably forming a linear composite. In the first type, the grid may take a variety of transverse cross sections wherein the grid contacts or is closely spaced from the dielectric sheath at or near its outer surface. In the second type, the conductive enclosure may have a variety of cross sections, subject to the limitation that it must house the dielectric-coated elongate conductor. In an alternative version of the first type, the corona charging apparatus may include a thin dielectric separating the conductive grid from the elongate conductor, but not completely covering the latter member. In a further alternative version of this first type, the insulating substrate may include a slot to house the dielectric sheathed conductor. In this version the dielectric sheathed conductor is embedded in the slot along its length, and the conductive grid is mounted over this member where it protrudes from the slot.

In an alternative version of this second type, a pair of dielectric-sheathed conductors straddle a central conductive rod, all mounted against an insulating board. Preferably, in this version the dielectric-sheathed conductor comprises a glass capillary lined with an inner conductive layer. In this and all versions of the second type, the invention is preferably characterized by a discharge region at or near the upper portion of a slot.

In accordance with yet another aspect of the invention, the time varying potential is advantageously a continuous wave alternating potential in the range 600 to 1500 volts peak, with a frequency in the range 60 Hz to 10 MHz. Alternatively, the varying potential may comprise a pulsed voltage. In the type for corona charging, the extraction potential preferably is on the order of tens or hundreds of volts. Both in charging and neutralizing, the device provides ion output currents which are approximately a linear function of the extraction potential.

In a preferred utilization of the invention, the device is employed for the erasure of electrostatic images on a proximate dielectric member. Alternatively, the device may be employed for charging such a dielectric member to a prescribed voltage. In the latter case the devices of the invention provide automatic control of the charging level. In either utilization, the corona device is preferably disposed at a distance in the range 5-20 mils from the member to be charged or discharged.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects of the invention will become apparent after considering the drawings and detailed description below.

Figure 1 is a sectional view of a corona charging device in accordance with a preferred version of the first type

Figure 2 is a plan view of the charging device of Figure 1:

Figure 3 is a sectional view of a charging device in accordance with the first type, with an alternative grid electrode profile;

Figure 4 is a sectional view of the charging device of

Figure 1 deployed for charging or discharging an adjacent member:

Figure 5 is a sectional view of an alternative charging device design in accordance with the first type;

Figure 6 is a sectional view of a further charging device design in accordance with the first type;

Figure 7 is a sectional view of a charging head with an alternative corona construction in accordance with the first type;

Figure 8 is a plan view of a charging device according to the first type, with an alternative grid electrode;

Figure 9 is a perspective view of a corona charging device in accordance with a preferred version of the second type;

Figure 10 is a sectional schematic view of the corona device of Figure 9 in proximity to an imaging surface;

Figure 11 is a sectional view of the corona device of

Figure 9, including actuating electronics;

Figures 12A, 12B, and 12C are partial sectional views showing various profiles of the device of the type of Figure 9, and the associated air discharge regions:

Figure 13 is a sectional view of a corona device in accordance with an alternative version of the second type: Figure 14 is a sectional view of a corona charging device in accordance with a further alternative version of the second type; and

Figure 15 is a sectional view of a corona charging device in accordance with yet another version of the second type.

DETAILED DESCRIPTION Reference should now be had to Figures 1-15 for a detailed description of the corona charging apparatus of the invention. Two principal types are illustrated in Figures 1 and 9; both of these types are characterized by an elongate conductor with a dielectric sheath, and a control electrode in proximity thereto. In the first type of the invention, shown generally in Figures 1-8, the control electrode takes the form of a conductive grid overlying the dielectric sheathed conductor, all mounted against an insulating support. In the second type, shown in various versions in Figures 9-15, the control electrode consists of a conductive enclosure, which defines a slot in which the dielectric sheathed conductor is placed. A characteristic feature of both corona device 10 (Figure 2) and corona device 90 (Figure 9) is that the corona electrode 11 and control electrode (respectively 17, 99) form a linear structure. The first and second types of the invention are discussed below sequentially.

In both principal types corona electrode 11 consists of a conductive wire 12 (which may consist of any suitable conductor) encased in a thick dielectric material 13. Although a dielectric-coated cylindrical wire is illustrated in the preferred types, the electrode 11 is more generally described as an elongate conductor of indeterminate cross section, "a" with dielectric sheath. Figure 7 illustrates an alternative corona electrode construction in the first principal type. Corona electrode 72 comprises a thin etched electrode with dielectric encapsulation 73. The elongate conductor may rest directly in contact with the insulating support, as long as it is separated from the mesh electrode by the dielectric sheath at the surface 71.

In both principal types, the dielectric 13 should have sufficient dielectric strength to withstand high excitation potentials without dielectric breakdown. It is desirable to minimize the onset voltage, i.e. the excitation voltage at which the dielectric begins to charge. This voltage increases with thicker dielectric layers 13, and decreases with lower dielectric constants of that layer. Organic dielectrics are generally unsuitable for this application, as most such materials tend to degrade with time due to oxidizing products formed in atmospheric electrical discharges. In the preferred type, the dielectric 13 comprises a fused glass layer which is fabricated in order to minimize voids, having a thickness in the range 1-3 mils. Other suitable materials include, for example, sintered ceramics and mica.

In the type of Figures 1-4, corona electrode 11 is placed against an insulating substrate 15. Advantageously, the electrode 11 is constrained by mesh electrode 17, but not bonded to the insulating substrate. This arrangement permits relative movement of these structures due to thermal expansion and contraction. The substrate 15 consists of insulating material of sufficient rigidity to support the coated-wire electrode 11 and mesh electrode 17.

Grid electrode 17 comprises an array of elongate conductors of minute thickness as compared with the diameter of dielectric-coated electrode 11. In the preferred version of this first type, this electrode comprises a fine wire mesh screen, advantageously a screen with a mesh in the range 30-150 apertures/inch, and a wire thickness in the range 0.3-1.2 mils. Preferably, the wire mesh screen is characterized by a high percentage of open area. The screen may consist of any well known metal or metal alloy, such as steels, stainless steels, nickel-chromium alloys, copper alloys, and aluminum alloys. The use of a fine mesh provides a desirably high density of ion generation sites, and avoids overheating at crossover points. In an alternative version, the grid electrode is fabricated by photoetching a screen pattern on a metal foil. In a further alternative version illustrated in Figure 8, grid electrode 87 consists of a parallel array of fine, closely spaced wires running perpendicular to corona electrode 11. The grid electrode is wrapped over electrode 11, and is anchored to insulating substrate 15 at each side of electrode 11. The grid electrode 17 may describe any of a wide variety of profiles as seen from one end. In the preferred type illustrated in Figure 1, the grid electrode 17 is wrapped tightly over the apex of electrode 11, and is bonded to support 15 so as to form a roughly V-shaped profile. An alternative arrangement is shown in Figure 3, wherein the mesh 37 forms an arch over the corona electrode 11. The former profile is preferred, in that the closeness of the mesh 17 to the outer surface of dielectric 13 provides a desirably low cutoff voltage. For this reason, mesh 17 is advantageously bonded or attached to support 15 in such a manner as to tension the mesh to provide firm contact with the electrode 11.

An alternative construction 50 for a corona device 10 in accordance with the first principal type is shown in Figure 5. The insulating substrate 55 includes a slot 56 in which corona electrode 11 is fitted. The grid electrode 57 is wrapped over substrate 55 and electrode 11 as shown. This arrangement affords ease of positioning and supporting corona electrode 11.

As shown in Figure 6, the conductive core of the corona electrode need not be encased in a dielectric sheath for effective operation. In the alternative structure 60, the dielectric sheath is replaced by a thin, flexible dielectric strip 63. The elongate conductor 62 rests directly against insulating support 65, and is separated from grid electrode 67 by dielectric strip 63. The dielectric 63 may comprise, for example, mica or a thin strip of glass.

In the preferred version of the second principal type, shown at 90 in Figure 9, the corona electrode 11 is embedded in a slot 96 in a conductive beam 94. The dimensions of the various structures are chosen to provide desired operational characteristics of the device 90, as further described below. Significant features of the device in this description include the' side walls 97 and base 98 of slot 96, as well as the outer surfaces 99 adjacent the slot. Figure 10 shows the corona device of Figure 9 as seen in section, in proximity to an imaging surface 20. A number of dimensions are important in describing these devices in structural terms. These include the total radius R of the corona electrode 11 and the thickness T of the dielectric layer 13; the separation G of the corona electrode from the side walls 97, if any: the width W of that portion of the beam 94 at each side of slot 96; the protrusion H of the corona electrode from slot 96 (the corona electrode 11 may be inset from the outer surface in which case H is negative); and the gap width Z between the corona device 90 and the imaging surface 20. In constructing a device 90 in accordance with the parameters, it is generally desirable that G=0, that W be given a minimal value consistent with structural integrity and that H have a small positive value as compared with the magnitude of R. These preferred values provide superior performance characteristics as discussed in detail below.

A nomenclature listing of the reference numerals used in the figures is included at the end of this specification.

With reference to the partial sectional views of 12A-12C, the relationship between the parameter H shown in Figure 10 and the configuration of the discharge region 100 is seen with respect to a variety of profiles of device 90. In all of these profiles G=0 and W is constant. If the electrode 11 protrudes prominently from slot 96 as shown in Figure 12A, the discharge region 100 will largely encompass the outer surface 99 of beam 94. The discharge region 100 is generally determined by the Paschen limits between elongate conductor and conductive beam 94. With the discharge region 100 having the characteristics shown in Figure 12A, there will be considerable inefficiences in the operation of the device 90 due to the loss of ions to the outer portions 99, which acts as a ground plane. This will lead to a diminishing of the ion output current. In the configuration of Figure 12B, the corona electrode 11 protrudes only slightly from the slot 96. In this case, the discharge region 100 comprises a region at the outer portion of the approximately V-shaped area defined by the side walls 97 and the dielectric 13. This area is the optimal location for the ion pool, in that it provides a readily extractable source of ions with minimal ion current loss due to the diversion of ions. If, on the other hand, the corona electrode is embedded considerably below the upper surface 99, as shown in Figure 12C the discharge region 100 will be inset from the surface of slot 96. This will incur the disadvantages that the ions will not be easily extractable and that there will be inevitable ion current loss due to diversion to the outer portion of side walls 97.

In the preferred construction of the corona device of the second principal type, a filler is included in the inner regions of slot 96. In Figures 12A-12C an adhesive filler 95 is contained between dielectric filler 19 and base 98. The use of a filler prevents power losses due to air breakdown in these regions and reduces the risk of dielectric, breakdown due to the heating in these lower regions. Such air breakdown would be similar in form to that depicted in Figures 12A-12C, but would not provide a useful source of ions. It may be seen with reference to Figures 12A-12C that a minimum value for W would be desirable in order to avoid ion current loss, and that a small positive value of H is preferred in order to provide a desirable location for the discharge region 100.

It will be appreciated that while the slot 96 of the conductive beam 94 has been shown with a generally rectangular cross section, the slot 96 alternatively may be in the form of a U-shaped channel that cradles the dielectric coating 13 of the conductive wire 12. This would allow the coated wire 12 to sit on the base of the beam without any need for packaging.

It is advantageous to place the corona electrode in contact with the side walls 97 (i.e. G=0) in order to avoid erratic behavior in the operation of the device. This characteristic poses difficulties in the type of Figures 9-12 in keeping the dielectric-coated electrode in contact with the side walls throughout the length of the device. Figure 13 gives a sectional view of a corona device 110 in accordance with an alternative type, wherein this difficulty is overcome. In the charging device 110, the slotted conductive beam 94 of Figures 12A-12C is replaced with a pair of conductive rods 116 and 117, illustratively with a rectangular cross section. The conductive rods and dielectric-coated electrode are mounted on an insulating support block 115. Rods 116 and 117 are flexible metallic structures which may be conformed to the dielectriccoated electrode 111 throughout its length, thereby ensuring that G will be negligible for the entire length of the device.

The mounting arrangement of Figure 13 may be further modified by altering the spacial arrangement of the various electrodes. In the sectional view of Figure 14, a pair of dielectric-coated elongate conductors straddle a central conductive rod. Illustratively, the conductive rod comprises a thick cylindrical wire 121, and each of the dielectric-coated electrodes 122 and 126 comprise a glass capillary of rectangular cross section filled with a metallic core material. Desirably, the metallic core material is characterized by a low melting point, and has a coefficient of expansion which is compatible with that of the capillary material. As in the case of the device 110 of Figure 13, the charging device 120 is fabricated by mounting the electrodes 121, 122, and 126 on an insulating base 125 so that these electrodes closely conform to each other throughout the length of the device. The corona device 120 is actuated by applying time-varying potentials between each of the respective metallic cores 123 and 127 and the central electrode 121.

Figure 15 illustrates a modified version 130 of the device of 120 of Figure 14. In corona device 130, the glass capillaries are not completely filled with a metallic core material, but are lined with an inner metallic layer of sufficient thickness to conduct the energizing current. Suitable metals for the core structures of Figures 14 and 15 include for example low melting alloys of bismuth, and indium alloys.

The corona devices of both principal types may be employed for the generation of ions both for charge neutralization and for charging a proximate dielectric surface to a predetermined potential. This is illustrated for the respective principal types in Figures 4 and 11, respectively. The former figure will be discussed for illustrative purposes, but both devices are essentially identical in operation and the discussion that follows applies to the device 90 of Figure 11 as well.

In the sectional view of Figure 4, the device 10 is employed for the generation of ions by application of a timevarying potential 23 between the elongate conductor 12 and grid electrode 17. This causes a pool of positive and negative ions to be formed in an air space in the vicinity of that portion of grid 17 which is in .contact with or close proximity to dielectric 13. This phenomenon is herein termed "glow discharge". With a periodically varying potential 23, air gap breakdown occurs during each half cycle if the excitation potential exceeds approximately 1400 volts peak-to-peak, if the dielectric sheath thickness is in the range of two to three mils. The dielectric 13 will receive a net charge, thereby extinguishing the discharge, and preventing the direct flow of an in-phase current between grid electrode 17 and elongate conductor 12.

With the switch in position x, the ion generator 10 acts as a charge neutralizing device with respect to an electrostatic image carried on a proximate member. As seen in Figure 4, the device 10 is disposed adjacent a dielectric surface 20 having a conductive backing 25, and the mesh electrode 17 is grounded to counterelectrode 25. The electrical behavior of this device may be measured as a plot of output current, i, as a function of the voltage V between surface 20 and electrode 17. Typically, the devices of the invention are characterized by roughly linear i-V curves. It is preferable to have a low offset voltage V₀, i.e. voltage at which i=0.

If dielectric surface 20 carries any net positive or negative charge, this charge will establish an electrical field to grid electrode 17, causing the extraction of ions of the opposite polarity from the ion pool 18. If the ion generator iθ is thus disposed for a sufficient period of time, the surface 20 will be completely neutralized. The surface 20 bears little or no residual charge under these circumstances. Another desirable feature is that of the typically high charging/discharge rates of this device.

Advantageously, the corona device 10 is disposed at a distance in the range 5-20 mils from surface 20, most preferably around 15 mils, as measured from the outer surface of grid electrode 17. A further advantageous feature of the invention is that the offset voltage of this device is relatively insensitive to changes in gap width within this range.

With further reference to Figure 4, the device 10 may be utilized to deposit a net positive or negative charge on surface 20 when switch 21 is at position y. This places a DC bias potential 22 on grid electrode 17. With a positive bias to electrode 17, for example, a positive charge of equal magnitude will be deposited on surface 20. When operated in this mode, the corona device 10 provides automatic limiting of the charging potential.

In a preferred utilization of the corona device 10, a relative motion is provided between the device 10 and surface 20, so that the device will be adjacent to various surface areas over time. Layer 20 may comprise, for example, the surface of a rotatable drum with a dielectric or photoconductive surface. It is generally desirable to minimize variations of the gap width Z between corona device 10 and surface 20 during such relative motion. When operating in the corona charging mode during such motion, the device will generally provide a surface potential which is a fraction of the bias potential; this fraction will increase with lower surface speeds.

In the preferred type, time varying potential 23 comprises a high frequency, high voltage sinusoid. Preferably, excitation potential 23 has a magnitude in the range 1700-2500 volts peak-to-peak, most advantageously around 2000 volts peakto-peak. Excitation potential 23 may comprise a continuous wave alternating potential, preferably of a frequency in the range 10KHz to 1 MHz. Driving voltages at higher frequencies have been observed to cause overheating of the corona device, while lower frequency waveforms may provide inadequate output currents. A continuous wave frequency. of 100 KHz provides desirably high emission currents without a serious risk of overheating device 10. Alternatively, excitation potential 19 may comprise a pulsed voltage which may be specified by the parameters of peak-to-peak voltage, repetition period, pulse width, and base frequency. The device 10 has been operated at frequencies as high as 1 MHz applied in short bursts having a duty cycle near 10 percent.

Both principal types of the invention are further illustrated in the following nonlimiting examples:

Example 1 A corona charging device of the type shown in Figure 1 was constructed as follows. The insulating support was fabricated of glass epoxy G-10 laminate. The corona electrode consisted of a 7 mil diameter stainless steel wire having a 2 mil thick glass coating. After laying the coated wire on the support block, a fine woven wire screen was stretched over the glass coated wire and bonded with a thermoset adhesive to the sides of the support. The screen was composed of a plain woven 1 mil stainless steel wire, having a mesh count of 100 and an open area of approximately 90 percent. The coated wire electrode was not bonded to the support block, and was constrained only by the overlying screen.

A 100 KHz, 2000 volt continuous wave alternating potential was placed between the coated wire and the mesh electrode. The outer surface of the mesh electrode was located 15 mils from the surface of an imaging drum having a thin photoconductive surface layer, with a capacitance of 100 picofarads per cm². The photoconductive surface was charged to 500 volts with a charging rate of 10³ cm²/sec., by imposing a 500 volt direct current potential between the mesh electrode and the drum's conductive core. This represented an average corona output current of 10 microamperes per cm. length of corona.

Example 2 The apparatus of Example 1 was employed as a corona discharge device by grounding the mesh electrode to the photoreceptor drum's conductive core. In this mode, the device neutralized electrostatic images at rates comparable to the charging rates of Example 1, leaving virtually no residual electrostatic image.

Example 3

The apparatus of Example 1 was modified as follows to provide a corona charging device of the type shown in Figure 7. The corona electrode was fabricated by laminating a 1 mil stainless steel foil to the support block using a pressure sensitive adhesive, and photoetching an electrode with a line width of 8 mils. The electrode was encapsulated with a 1.5 mil thick layer of glass by silk-screening a glass frit over the etched electrode, and sintering the glass at a high temperature to form a continuous glass coating.

This apparatus exhibited equivalent performance to the structure of Example 1, in both the charging and neutralizing modes. Example 4

A corona charging device 90 of the type shown in Figure 9 was constructed as follows. The corona electrode consisted of a 7 mil diameter stainless steel wire having a 2 mil thick glass coating. The coated wire was embedded in an 11 mil wide, 10 rail deep rectangular slot in a stainless steel beam of total dimensions 50 mil wide and 50 mil deep, after inserting adhesive filler at the bottom of the slot. This provided a beam width of 14.5 mil on each side of the slot.

A 100 KHz, 2000 volt peak-to-peak continuous wave alternating potential was placed between the coated wire and the steel beam. The outer surface of the corona electrode was located 15 mils from the surface of an imaging drum having a thin photoconductive surface layer, with a capacitance of 100 picofarads per cm.². The imaging drum was rotated at a surface speed of 25 cm/second relative to the corona device, and was charged to 500 volts by imposing a 1000 volt direct current potential between the steel beam and the drum's conductive core. This represented an average corona output current of 1.25 micro-amperes per centimeter length of corona.

Example 5 The apparatus of Example 4 was employed as a corona discharge device by grounding the mesh electrode to the photoreceptor drum's conductive core. In this mode, the device neutralized electrostatic images at rates comparable to the charging rates of Example 4, leaving virtually no residual electrostatic image.

Example 6 The apparatus of Example 4 was modified as follows to provide a corona charging device of the type shown in Figure 13. A glass-coated tungsten wire as in Example 4 was bonded to an insulating support consisting of glass epoxy G-10 laminate. Two tantalum wires of 10 mil X 10 mil square cross-sections were bonded to the base on either side of the glass-coated wire, contacting the dielectric sheath along its length. This apparatus exhibited equivalent performance to the structure of Example 4, in both the charging and neutralizing modes.

While various aspects of the invention have been set forth by the drawings and the specification, it is to be understood that the foregoing detailed description is for illustration only and that various changes in parts, as well as the substitution of equivalent constituents for those shown and described, may be made without departing from the spirit and scope of" the invention as set forth in the appended claims.

NOMENCLATURE

10. Corona device, first aspect, (Fig. 2)

11. Corona electrode

12. Conductive wire

13. Dielectric coating (e.g. glass)

15. Insulating substrate (e.g. plastic) 17. Control electrode

19. Dielectric filler (Figs. 12A-12C)

20. Imaging surface (Figs. 4 and 10) (e.g. plastic) R Radius of corona electrode 11

T Thickness of dielectric layer 13 G Separation of corona electrode from side walls W Width of beam 94

H Protrusion of corona electrode from slot Z Gap width between corona device and imaging surface

21. Switch x "switch off" position y "switch on" position

22. DC biasing source 23. Excitation potential

25. Conductive backing (Fig. 4)

26. Excitation potential

27. DC biasing source

30. Corona device, alternate arrangement (Fig. 3) 37. Mesh

50. Corona device alternative structure (Fig. 5) 55. Insulating substrate (e.g. plastic)

56. Slot

57. Grid electrode

6θ. Corona device, alternative structure (Fig. 6)

62. Elongate conductor

63. Dielectric strip

65. Insulating support (e.g. plastic)

67. Grid electrode

71. Corona electrode surface (Fig. 7) 72. Conductive wire

73. Dielectric encapsulation (e.g. plastic)

75. Insulating Support (e.g. plastic)

80. Corona device, alternative structure (Fig. 8)

85. Insulating support

87. Grid electrode (Fig. 8)

90. Corona device, alternative structure (Fig. 9)

94. Conductive beam

95. Adhesive Filler (Fig. 12A) 96. Slot

97. Side wall

98. Base

99. Control electrode (Figs. 9 and 12C)

100. Discharge region (Fig. 12A)

110. Corona device (Fig. 13)

111. Dielectric coated electrode 113. Dielectric coating (e.g. glass) 115. Support block (e.g. plastic)

116. Conductive rod

117. Conductive rod

120. Charging device (Fig. 14)

121. Thick cylindrical wire (electrode)

122. Dielectric coated electrode

123. Metallic core

124. Dielectric coating (e.g. glass)

125. Support block (.e.g. plastic)

126. Dielectric coated electrode

127. Metallic core

128. Dielectric coating (e.g. glass)

130. Modification of device 120 (Fig. 15)

131. Cylindrical electrode

132. Dielectric coated electrode

133. Hollow metallic core

134. Dielectric coating (e.g. glass)

135. Support block (e.g. plastic) 136. Dielectric coated electrode

137. Hollow metallic core

138. Dielectric coating (e.g. glass)

Claims

WE CLAIM:

1. Apparatus for generating ions, comprising: an elongate conductor: a dielectric sheath for said elongate conductor: a control electrode in proximity to said dielectric sheath; a time-varying potential applied between said elongate conductor and said control electrode in order to create a glow discharge in an air region adjacent said control electrode and dielectric sheath: and an extraction potential for extracting ions from said glow discharge to produce an output ion current approximately proportional to said extraction potential.

2. Apparatus as defined in claim 1, wherein said control electrode comprises a conductive grid contacting said dielectric sheath, further comprising an insulating support for the elongate conductor and dielectric sheath.

3. Apparatus as defined in claim 2: wherein the conductive grid comprises a conductive mesh electrode: wherein the conductive grid comprises a conductive mesh electrode comprising a wire mesh screen; a wire mesh screen with a mesh in the range 30150 apertures per inch; a wire mesh electrode with a high open area ratio; a wire mesh screen comprising a lattice of wires having a thickness in the range 0.3-1.2 mils; a metal foil etched in a mesh pattern; wherein the conductive grid comprises an array of essentially parallel conductors; wherein the conductive grid contacts the dielectric sheath along a line coextensive with the elongate conductor; wherein the elongate conductor and dielectric sheath comprise a dielectric-coated wire; wherein the dielectric sheath has a thickness in the range 1-3 mils. wherein the elongate conductor and dielectric sheath comprise a conductive strip contacting the insulating support, with an encapsulating dielectric layer; wherein the conductive grid is anchored against the insulating support on each side of the elongate conductor and dielectric sheath: wherein the conductive grid has an approximately inverseV-shaped lateral cross section; wherein the conductive grid has an arcuate lateral cross section: or wherein the elongate conductor and dielectric sheath are housed in a slot in said insulating support, with said conductive grid contacting the dielectric sheath above the slot.

4. Apparatus as defined in claim 1, wherein said control electrode comprises a conductive enclosure having inner walls surrounding the sheathed elongate conductor and further including an opening to expose said sheathed elongate conductor.

5. Apparatus as defined in claim 4: wherein said conductive enclosure comprises a conductive beam having a slot, and wherein the sheathed, elongate conductor is embedded in said slot; further including an insulating base, wherein the sheathed elongate conductor is mounted against said insulating base, and wherein the conductive enclosure comprises a pair of conductive side members mounted against said insulating base; wherein the conductive enclosure comprises conductive rods mounted against said insulating base and fitted against the sheathed elongate conductor along its length; wherein the inner walls of the conductive enclosure contact opposite sides of the sheathed elongate conductor: wherein the inner walls of the conductive enclosure straddle said dielectric sheath along lines coextensive with said elongate conductor; wherein the conductive enclosure has outer surfaces which form corners with its inner walls: wherein the sheathed elongate conductor protrudes beyond said outer surfaces for a small fraction of its thickness: or wherein the outer surfaces are narrow as compared with the thickness of the sheathed elongate conductor.

6. Apparatus as defined in claim 1, wherein the control electrode comprises a conductive rod, and the dielectric sheathed elongate conductor comprises a pair of elongate side members straddling said conductive rod, with the time-varying alternating potential applied between each of said elongate side members and said conductive rod; further comprising an insulating base for the conductive rod; wherein the elongate side members are mounted against said insulating base and fitted, to either side of said conductive rod; wherein each of the elongate side members comprises a glass capillary tube with a conductive inner lining: wherein each of the elongate side members comprises a glass capillary tube with a solid conductive core: or wherein each of the elongate side members comprises a glass capillary tube with a core comprised of a material chosen from the class consisting of low-melting alloys of bismuth, and indium alloys.

7. Apparatus as defined in claim 1: wherein the dielectric sheath is comprised of an inorganic dielectric material; wherein the dielectric sheath is comprised of a material selected from the class consisting of glass, mica, and sintered ceramic materials; wherein the apparatus is proximate to an imaging surface to be discharged, said imaging surface having a backing electrode, and wherein said control electrode is grounded to said backing electrode; wherein the apparatus is proximate to an imaging surface to be charged, said imaging surface having a backing electrode, and wherein said extraction potential comprises a bias potential between the control electrode and backing electrode; or wherein the control electrode is disposed at a distance in the range 5-20 mils from a member to be charged or discharged.

8. Apparatus as defined in claim 1: wherein the extraction potential comprises a direct current potential between the control electrode and a counterelectrode; wherein the extraction potential comprises a direct current potential of a magnitude from tens to hundreds of volts; wherein the time-varying potential comprises a high voltage alternating potential; wherein the time-varying potential comprises a high voltage alternating potential of a frequency in the range 60 Hz to 4 MHz; or wherein the time-varying potential comprises a pulsed voltage.

9. A method for electrostatic discharging, comprising the steps of: disposing a corona device near a member to be discharged, said corona device comprising an elongate conductor, a dielectric sheath for the elongate conductor, and a control electrode in proximity to the dielectric sheath: applying a time-varying potential between said elongate conductor and said control electrode to induce a glow discharge in an air region adjacent the control electrode and dielectric sheath: and grounding said control electrode to a counterelectrode for the member to be discharged; wherein the control electrode comprises a conductive grid in contact with said dielectric sheath, further comprising an insulating support for the dielectric sheathed elongate conductor: or wherein the control electrode comprises a conductive enclosure having inner walls surrounding the dielectric sheathed elongate conductor and including an opening to expose the dielectric sheath.

10. A method for electrostatic charging comprising the steps of: disposing a corona device near a member to be discharged, said corona device comprising an elongate conductor, a dielectric sheath for the elongate conductor, and a control electrode in proximity to the dielectric sheath; applying a time-varying potential between said elongate conductor and said control electrode to induce a glow discharge in an air region adjacent the control electrode and dielectric sheath; and applying a bias potential between said control electrode and a counterelectrode for the member to be charged; wherein the control electrode comprises a conductive grid in contact with said dielectric sheath, further comprising an insulating support for the dielectric sheathed elongate conductor or wherein the control electrode comprises a conductive enclosure having inner walls surrounding the dielectric sheathed elongate conductor and including an opening to expose the dielectric sheath.