EP0011203A1 - Device for charging a dielectric layer electrostatically - Google Patents

Abstract

A method and an apparatus for charging a dielectric layer electrostatically to a predetermined potential. An AC electrode is arranged at a distance from the dielectric layer and connected to one output of an AC voltage generator. The AC voltage generator has the other output connected to an output of a DC voltage generator. Between the AC electrode and the dielectric layer there is a DC electrode which is connected to the other output of the AC voltage generator. The dielectric layer rests on a counter-electrode which is connected to the other output of the DC voltage generator and is at ground potential. Each of the electrodes can comprise one or a plurality of mutually insulated single electrodes.

Description

The invention relates to a method for electrostatically charging a dielectric layer to a predetermined potential with the aid of an alternating electrical field and an electrostatic constant field, and a device for carrying out the method.

In the prior art, an electrophotographic development process is known from the reference "Tappi / February 1967, Vol. 50, No. 2, pages 77A-79A", in which an electrophotographic layer is provided with an electrostatic surface charge by means of an electrode to which both a high-frequency high voltage and a DC voltage are applied simultaneously. The electrode, for example a very thin corona wire or fine metallic tips, is arranged close to an insulated metal surface and generates air ions of both polarities due to the alternating voltage, of which those of the corresponding polarity are accelerated towards the electrophotographic layer by the direct voltage. Particularly when the voltage of the electrode is negative, strong inhomogeneities of the air ions close to the wire surface occur and lead to charge fluctuations which have a negative influence on the image formation on the electrophotographic layer in such a way that, for example, a full area of an original is reproduced unevenly. The superimposition of the direct voltage field by the alternating electrical field brings influencing the discha ^g sspannung the electrode, since this is controlled by the impressed alternating voltage in such a manner that the amplitudes of the alternating voltage based on the predetermined DC voltage is superimposed, may whereby voltage peaks occur, leading to a breakdown of the charged layer to lead.

From DE-OS 22 31 530 a method for electrophotographic recording of images on an insulating recording medium is known, which is drawn over a support electrode, while the charge image is recorded by drawing electrodes on the other side of the recording medium via the contact point with the support electrode. For this purpose, an electrode arrangement is used in which a partial current of the corona discharge of a discharge electrode reaches the recording medium through the opening of a slit diaphragm formed of electrodes and causes a charge there over the line of contact of the recording medium with the contact edge. The partial discharge current is controlled via the diaphragm formed by four flat electrodes by means of electrical image signals. For this purpose, at least one of the electrodes is divided into a number of conductor strips, via which the signal voltage is supplied.

DE-OS 24 23 245 describes a method for the electrophotographic recording of images on an insulating recording medium by means of a corona discharge, of which part of the discharge current via a gap aperture removed and used for charging the recording medium. Here, too, the imagewise charging is carried out by means of control voltages on a drawing electrode which is in contact with the recording medium on the side of the recording medium T facing away from the gap, the electrical contact between the drawing electrode and the insulating recording medium by supplying a conductive contact liquid to the contact point will be produced. The recording medium can be charged in flowing nitrogen.

The object of the invention is to provide a method for the gentle and safe electrostatic charging of insulating dielectric layers with avoidance of breakdowns, with which the size of the charging current and the charge distribution can be varied and reproduced with great accuracy with a pronounced linearity between the charging current and a given DC voltage.

At the same time, a device for gentle and safe electrostatic charging of insulating dielectric layers and for avoiding breakdown of these layers is to be created.

This object is achieved according to the invention by a method in such a way that charge carriers are generated at a distance from the surface of the dielectric layer by the alternating electrical field, which charge carriers penetrate under the influence of the electrostatic charge to be charged to the layer to be charged table DC field as a charging current to the surface of the layer.

In an embodiment of the invention, the dielectric layer consists of a photoconductive and / or thermoplastic recording medium, during the charging of which at least one of the two voltage fields is modulated.

The further process steps result from the features of process claims 3 to 18.

In the device for carrying out the method, an electrode is arranged at a distance from the dielectric layer to be charged and is connected to the hot output of an AC generator, and a further electrode is also provided between this electrode and the layer and is connected to the hot output of a DC generator. The layer to be charged lies on a counter electrode to the DC voltage electrode and the counter electrode is at ground potential.

The further configuration of the device results from the features of claims 21 to 45.

The invention overcomes the disadvantages of the prior art corons, which consist in the fact that when a high DC voltage is applied to wire cores or corona needle tips, the control of such corons to achieve a predetermined charging voltage of the insulating layer is possible only to a limited extent, so that the corons are operated in conjunction with additional electrodes in the form of control grids, but the efficiency of the charging voltage, based on the charging current, is low. In addition to the difficulty in controlling the Aufladun sspannung ^g and the quality of charging is often unsatisfactory because breakdowns occur or vary the charging due to contamination of the corona wires or by the combustion of the corona needle tips. Such defects increase with the length of the coronas. Since high voltages of a few thousand volts have to be applied to the corones for ionization, it is necessary to take appropriate safety precautions.

The advantages of the invention are that ions are generated in a high-frequency alternating electrical field, which to a certain extent form a reservoir of charge carriers from which the charging current is transported to the recording layer with the aid of the electrostatic constant field. It is possible to surround the AC voltage electrode with an insulating body, which is a dielectric that increases the field strength in the area of the DC voltage electrode and at the same time protects the electrode against contamination. The possibility of modulating the direct voltage or alternating voltage supply or the potential of the counter electrode of the direct field makes it possible to charge the dielectric layer in a modulated or locally limited manner.

The method and the apparatus can be advantageously used in the production of electrophotographic copies with the aid of an insulating photoconductor layer as a recording medium which is charged, imagewise exposed and developed with toner in order to make the charge image formed on the photoconductor layer into a visible image. The invention can also advantageously be used in the electrophotographic production of relief images, in which the photoconductive and at the same time thermoplastic recording material is first charged, then exposed imagewise and heated until a relief image is formed. It is also possible to generate toner or relief images by imagewise charging a purely thermoplastic recording layer.

• Fig. 1 eine schematische Darstellung der Schaltungsanordnung einer Ausführungsform der Vorrichtung nach der Erfindung,
• Fig. 2 den Verlauf des Aufladestroms in Abhängigkeit von der an einer im elektrischen Wechselfeld befindlichen Elektrode anliegenden Gleichspannung sowie den Verlauf des Aufladestroms ohne elektrisches Wechselfeld,
• Fig. 3-5 schematische Schnitt- und Seitenansichten von Elektrodenanordnungen der Vorrichtung.
• Fig. 6 eine Elektrodenanordnung mit Abschirmung,
• Fig. 7 eine gegenüber Fig. 1 geringfügig abgewandelte Ausführungsform der Vorrichtung,
• Fig. 8 den Aufladestrom in Abhängigkeit von der Gleichspannung an der Gleichspannungselektrode bei verschiedenen Abständen von der Gegenelektrode, mit und ohne elektrisches Wechselfeld,
• Fig. 9 eine spezielle Ausführungsform der Gegenelektrode,
• Fig. 10 eine gegenüber Fig. 6 abgewandelte, weitere Elektrodenanordnung mit Abschirmung,
• Fig. 11 eine Schaltungsanordnung einer anderen Ausführungsform, und
• Fig. 12 eine gegenüber Fig. 11 geringfügig abgewandelte Ausführungsform der Erfindung.

Exemplary embodiments of the invention which are illustrated in the drawings are described in more detail below. Show it:

• 1 is a schematic representation of the circuit arrangement of an embodiment of the device according to the invention,
• 2 shows the course of the charging current as a function of the direct voltage applied to an electrode located in the alternating electrical field and the course of the charging current without an alternating electrical field,
• Fig. 3-5 schematic sectional and side views of electrode arrangements of the device.
• 6 shows an electrode arrangement with shielding,
• 7 shows a slightly modified embodiment of the device compared to FIG. 1,
• 8 shows the charging current as a function of the direct voltage at the direct voltage electrode at different distances from the counter electrode, with and without an alternating electrical field,
• 9 shows a special embodiment of the counter electrode,
• 10 shows a further electrode arrangement with shielding modified from FIG. 6,
• 11 shows a circuit arrangement of another embodiment, and
• Fig. 12 shows a slightly modified embodiment of the invention compared to Fig. 11.

1 comprises a DC voltage generator 1 and an AC voltage generator 2. The DC voltage generator 1 contains a voltage regulator 16 which generates a DC voltage which is variable between zero and a maximum value of a few kV. A smoothing capacitor 33 is connected in parallel with the output of the DC voltage reactor 16 or the DC voltage generator 1. In the conduit to the hot output 1 ₁ of the DC voltage generator 1 is a circuit element 11 having a terminal 14 can be achieved over the voltage modulation of the DC voltage. The cold output 1 ₀ is connected to ground potential via a line 3. The DC voltage U of the DC voltage generator 1 is adjustable between 0 and 20 kV and is from the hot output 1 ₁ via a line 4 to a Electrode 5 applied, which is arranged at a distance from a dielectric layer 8 to be charged. This dielectric layer 8 is, for example, a photoconductive and / or thermoplastic recording material which is charged to a desired voltage, then exposed in terms of image or information and developed with toner. Relief images can also be formed, for the production of which the photoconductive and at the same time thermoplastic recording layer is charged, then exposed imagewise and heated to form the relief images.

The AC voltage generator 2 comprises a voltage regulator 17 and a frequency regulator 18, which are components of an AC voltage source 32. The alternating voltage of the alternating voltage generator 2 is 1 to 10 kV _{eff '} at a frequency between 1 and 100 kHz. The voltage regulator 17 is used to adjust the height of the AC voltage, while the frequency regulator 18 is used to tune the frequency of the AC voltage. The AC voltage generator 2 further comprises an isolating transformer 19, which transforms up the AC voltage supplied by the AC voltage source 32 and ensures a floating cascade connection of the AC voltage to the hot terminal 11 of the DC voltage generator 1. For this purpose, the cold output 2 _{0 of} the AC voltage generator 2 is connected to the hot output 1 _{1 of} the DC voltage generator 1. In the connecting line between the AC voltage source 32 and the isolating transformer 19, a switching element 10 is connected, via the connection 13 a voltage for modulating the AC voltage can be fed. The hot output 2 _{1 of} the AC voltage generator 2 is connected via a line 6 to an electrode 7 which is further away from the dielectric layer 8 to be charged than the DC voltage electrode 5. The AC voltage U ~ of the AC voltage generator 2 is via the line 6 of the AC voltage electrode 7 fed. The layer 8 to be charged to the voltage U ₌ lies on an earthed electrode 9. In the ground line of this electrode 9, which is the counter electrode to the DC voltage electrode 5, a switching element 12 is connected, via the connection 15 of which a voltage can be fed in order to change the potential of the electrode 9.

The electrode 5 also serves as a counter electrode for the AC voltage electrode 7, since the output 2 _{0 of} the AC voltage generator 2 is connected to the output 1 _{1 of} the DC voltage generator 1 and is connected to the DC voltage electrode 5 via line 4.

The special design of the device enables a completely new and special charging technology. The AC voltage U ~ is present between the DC voltage electrode 5, which is a corona electrode for charging, and the AC voltage electrode 7. The electrode 5 consists, for example, of a thin corona wire with a thickness of 50 to 300 μm. However, other charging cores of a suitable design can also be used. An arbitrarily shaped electrode is used as the AC voltage electrode 7, the cross section and surface of which are designed in this way are that no ions are generated in their immediate vicinity. For example, the AC voltage electrode 7 can be a round electrode with a diameter of 2 mm. With the aid of the AC voltage electrode 7, the ambient atmosphere of the electrode 5 is ionized and the size of the AC voltage U ~ is chosen to be sufficiently high that a sufficient number of the required ions are available in the area of the electrode 5 even when the charging current is at a maximum. With strong ionization, a visible glow occurs on the circumference of the electrode 5.

If the electrode 5 is supplied with the DC voltage U, then either a positive or a negative charging current is conducted onto the layer 8, depending on the polarity of the DC voltage U. There is an almost linear relationship between the charging current I (μA) and the direct voltage U ₌ (kV) applied to the electrode 5 over a wide working range, as can be seen from FIG. 2, curve a. This pronounced linearity between the charging current and the predetermined DC voltage enables the layer 8 to be reproducibly charged to the respective predetermined DC voltage. The charging current I begins in the lower current range with direct voltages of a few volts, possibly at a voltage less than 1 volt. For this purpose, the alternating voltage must run extremely symmetrically to the common earth potential, since distortions of the alternating field producing the ions would cause changes in the predetermined charge level. To achieve an extremely precise charge level, a stable, well-adapted mecha is also required African structure of the charging device required. Interfering external fields must be kept away or, if necessary, compensated for by switching on a suitable compensation potential.

Curve b in FIG. 2 shows the charging current as a function of the DC voltage of a known charging corona of the same size, which works without an alternating electrical field. It can be seen from curve b that charging only begins at a DC voltage greater than 8 kV and very quickly approaches the breakdown voltage for the layer to be charged, which is around 9 kV, for example, asymptotically. The charging current I according to curve b, which results for a voltage just below the breakdown voltage, can be achieved according to curve a with a much lower DC voltage, which is approximately lower by the corona voltage. This reduced DC voltage, as can be seen from FIG. 2, curve a, is approximately 2.2 kV. This DC voltage, which is reduced by the corona threshold voltage, in conjunction with the generation of ions separated from the corona discharge by means of the high-frequency electrical alternating field, considerably reduces the number of breakdowns of the layer 8. If, according to the invention, the DC voltage electrode 5 is operated as a corona electrode with the same voltage as conventional corona devices, a much larger charging current can be achieved than with the known Korcna devices.

The electrodes 5 and 7 are expediently in electrical the arrangements summarized, some of which are shown schematically in Figures 3-5.

The electrode arrangement in FIG. 3 comprises a thick wire as an AC voltage electrode 7 and a thin wire as a DC voltage electrode 5, which are tensioned by two insulating pieces 20 and fixed in their position relative to one another and to the layer 8 to be charged or to the counter electrode 9. For example, a wire made of copper or another metal with a diameter of 1 to several millimeters is used for the electrode 7. Instead of a wire, another profile made of metal can also be used. A tungsten or steel wire of approximately 10 to a few 100 μm thick is preferably selected for the electrode 5.

A distance 21 between the electrode 5 and the layer 8 to be charged and a distance 22 between the two electrodes 5 and 7 are 1 to about 20 mm. In the electrode arrangement shown in FIG. 4, the AC voltage electrode 7 is enclosed by an insulating body 23. For this purpose, for example, the electrode 7 can be melted or pushed into a glass tube. This firstly results in better insulation between the electrode 7 and the direct voltage electrode 5 and secondly gives a higher field strength in the air space between the electrode 5 and the insulating body 23 when the alternating voltage is the same as in the electrode arrangement according to FIG. 3. This results from the high dielectric constant of about "5" for glass compared to Air, since it is known that the individual field strengths are inversely proportional to the dielectric constants of different materials.

The freely clamped electrode 5 made of thin wire has a slight tendency to mechanical vibrations, especially when there are large span lengths. The tendency to oscillate can be suppressed in part by correspondingly large clamping forces on the electrode 5. The problem of the tendency of the electrode 5 to oscillate can be solved more favorably with the electrode arrangement according to FIG. 5, in which the electrode 5 is guided in direct contact with the surface of the insulating body 23. For this purpose, the electrode 5 can easily be clamped on the surface of the insulating body 23 or melted into the surface of the same. Furthermore, the electrode 5 can be applied to the insulating body 23 galvanically or by baking. Such an electrode arrangement with an electrode 5 fixed on the insulating body 23 is particularly suitable for elongated coronas up to a length of 1 m and beyond, which are used, for example, in electrophotographic copiers for producing copies of large-area originals such as technical drawings.

The electrode arrangements described above also enable, as mentioned, very low charges of the layer 8 with a voltage of 1 volt and below, so that an extensive neutralization of undesired surface or residual charges on electrophotographic recording materials is possible. For example X-ray intensity patterns irradiated in ionization chambers are converted into corresponding charge patterns on insulating layers which, after development with toner, give visible images of the X-ray intensity distribution. Before the X-ray intensity pattern is irradiated, it may be necessary to neutralize surface charges on the insulating layers, which arise, for example, from triboelectric contact with other layers, so that they cannot undesirably overlap the charge patterns. The neutralization takes place, for example, in such a way that the DC voltage electrode 5 is switched to earth potential and the AC voltage electrode 7 receives such a high AC voltage that the residual charge when the layer 8 passes under the electrode 5 becomes negligibly small. For this purpose, a special electrode setting and balancing of the AC voltage may have to be carried out and electrostatic external fields must be kept away or compensated for.

As already mentioned, in the device according to FIG. 1 there is the possibility of modulated charges, for which there is a versatile need. Thus, it is generally known that when developing charge images with large solid areas, a preferred toner deposition takes place at the edges of the image, which results in so-called edge images if no special measures such as the provision of additional development electrodes are taken. Another way to achieve full toner deposition and improve half sound reproduction consists in the rasterization of the charge image. In this case, charging is generally carried out homogeneously first and then exposed in a grid pattern. It is also possible to apply the screening with good quality as a continuous screen, for example with a sinusoidal charge distribution, or as a modulated screen, for example with a rectangular charge distribution, together with the charging in one process step. A grid of up to 20 lines / mm, preferably 5 to 10 lines / mm, is completely sufficient for the requirements of qualitatively perfect office copies.

The charge images on which the relief images are based must also be rasterized for the halftone representation of relief images by Schlieren projection. Screenings of up to 10 lines / mm are also required when using electrostatic relief imaging technology, in which the rapid development by deformation without the addition of additional developer is used for X-ray image recordings on insulating deformable layers in ionization chambers or on suitable photoconductor layers, for example selenium alloys.

The electrode arrangement of the device is due to the pronounced linearity between load current and DC voltage also very ^g ut electrostatic copiers, such as computer Ausdrucker and fax machine. In these Anwendunqsfällen the line-decomposed information is supplied as an electrical signal corresponding to the sequentially opierer _K, the dielectric on a Aufzeich Apply a corresponding charge pattern to the insulating layer line by line, generally with the aid of an electrode matrix composed of individually controllable individual electrodes. The charge pattern is made visible with toner or creates a relief image on a thermally deformed layer. The pronounced linearity between the charging current and the signal voltage, which replaces the DC voltage, enables a local surface charge on the recording medium which is proportional to the respective signal voltage. Depending on the surface charge, toner is deposited or the depth of the relief image is modulated, so that good halftone reproduction is ensured. Because of the large linear modulation range of the electrode arrangement, the halftones can be reproduced in small increments.

For periodic, grid-shaped modulations for generating rectangular charge distributions on the insulating layer, for example the AC voltage of the AC voltage generator 2 of the device according to FIG. 1 is modulated via the switching element 10. For this purpose, pulses are given to the switching element 10 via the connection 13, for example an electromechanical relay which is opened and closed. Ions are only generated between the electrodes 5 and 7 when the switching element 10 is closed. The relays used can be operated at 200 Hz, for example. Instead of electromechanical relays, electronic switches can also be used as switching element 10, which allow switching frequencies of 100 kH2 and beyond.

At a switching frequency of 500 Hz, for example, screened charge patterns can be applied to a record carrier moving at a speed of 10 cm / s, which have a screen of 5 lines / mm.

Shielded electrode arrangements, as shown in FIGS. 6 and 10, are preferably used for the modulated charging.

In the arrangement according to FIG. 6, the DC voltage electrode 5 and the AC voltage electrode 7 with the insulation body 23 are located in an open shielding housing 24 made of electrically insulating material. The shielding housing 24 has a gap 25, on the edge of which the electrode 5 lies and under which the layer 8 is moved past. The gap width is approximately 1 mm and the distance between the electrode 5 and the layer 8 is between 5 and 15 mm. The ions generated in the interior of the shielding housing 24 emerge through the gap and strike the layer 8. The shielding housing 24 shields the light-sensitive layer 8 to a large extent against a coronal glow of the electrode 5 and enables the generation of ions and charging within a protective gas atmosphere, for example of nitrogen, which is introduced into the shielding housing 24 and exits through the gap 25. When filling with pure nitrogen with a degree of purity of 99 or better, the charging current is increased with otherwise unchanged settings of the electrodes. In addition, a slight overpressure in the corona hub rich the DC voltage electrode 5, which works as a corona electrode, protected from contamination.

There are further modulation options via the switching element 11 in the DC voltage generator 1 and the switching element 12 in the ground line of the counterelectrode 9. These switching elements 11, 12 can be electromechanical relays or electronic switches and are controlled via the connections 14 and 15, respectively. When switching elements 11 and 12 are opened, the existing contacts are interrupted and signals that are variable in terms of time and amplitude can be entered. The modulation can also be carried out in such a way that the switching elements 10, 11, 12 are controlled so that the existing contacts are not interrupted but only the alternating or constant field is weakened during the modulation phase.

Circuit relationships that are easy to miss are obtained when the potential of the counterelectrode 9 is modulated by driving the switching element 12 via the connection 15. The switching element 12 is particularly suitable for actuation by means of strongly changing signals. In the case of composite signals, such as those printed out on computers or facsimiles, the electrode 9 is then broken down into a number of individually controllable electrode sections across the recording width, over which the insulating recording layer, for example a homogeneous dielectric paper or a film, is guided. The information fed in via the switching element 12 can additionally be scanned via the periodically excited switching element 10 if necessary.

The arrangement shown in FIG. 7 largely corresponds to the device according to FIG. 1 in the circuit structure, with the differences that there are no switching elements and that the aluminum counter electrode 9 is connected to ground potential via a direct current meter 26. This arrangement served to accommodate the curves a-d and a'-d 'shown in FIG. 8.

Fig. 8 shows the linear relationship between the charging current I ( _{/ U} A) and the DC voltage U = (kV) of curves a, b, c, d for different distances between the DC voltage electrode 5 and the counter electrode 9. For comparison purposes, the corresponding Curves a ', b', c ', d' are drawn for the same different distances between the DC voltage electrode and the counter electrode, but the electrode arrangement is only operated with DC voltage, ie there is no alternating electrical field. The end points of the individual curves a - d and a '- d' represent the charging currents shortly before voltage breakdowns occur in the layer to be charged. From the curves of Fig. 8 it can be seen that at approximately the same breakdown voltages for charging the layer with direct voltage, supported by an alternating electrical field, and with direct voltage alone, without an alternating electrical field, the achievable charging currents in the former case are substantially greater than those of the second If lie.

9 shows a metallic counterelectrode 9, for example a copper layer, into which raster lines are etched photomechanically on one side. This counter electrode 9 is coated with an insulating recording layer 8. The grid lines of the counter electrode 9 are connected to a center tap 27 of a potentiometer 28, the center tap being moved under the DC voltage electrode over the one-side grounded potentiometer 28, to which a voltage U is applied, while the counter electrode 9 is moving past. In this way, a voltage drop of, for example, U = -300 V to 0 V can be generated at the counter electrode 9 during the recording, as a result of which a modulation of the recording is obtained as a result of this change in the potential at the counter electrode 9.

10 shows a further electrode arrangement which surrounds a shielding housing 24. The DC voltage electrode 5 consists of a number of individual metal wires which are cemented at intervals from one another and isolated from one another between two facet-like hand-ground glass plates 30. The wire tips as well as the wire ends protrude at the front and rear ends of the glass plates 30. The wire ends have connections 31 for applying the DC voltage. The surface of the counter electrode 9 is slightly curved, so that a dielectric paper composed of an insulating cover layer 8 and a conductive paper carrier 29 changes its direction of movement in the region of the counter electrode 9 in accordance with the curvature of the counter electrode 9. Corresponding to the number of electrode wires 5a, 5b, 5c, ... there are just as many connections 31a, 31b, 31c, ... at the wire ends of the DC electrodes.

FIG. 11 schematically shows the circuit structure of the device with which the electrode arrangement according to FIG. 10 can be operated, for example. The DC voltage electrode 5, as already mentioned, consists of individually controllable electrodes 5a, 5b, 5c, ..., which have a corresponding number of switching elements 11a, 11b, 11c, ... with connections 14a, 14b, 14c. .. be controlled in terms of voltage. The switching elements 11a, 11b, 11c, ... are connected to the connections 31a, 31b, 31c, ... of the individual electrodes 5a, 5b, 5c, .... Otherwise, the circuit structure corresponds to that according to FIG.

FIG. 12 shows a circuit arrangement of the device in which each of the electrodes 5, 7 and 9 consists of a plurality of individual electrodes 5a, 5b,... 7a, 7b, ...; 9a, 9b, ... exists. The individual electrodes 5a, 5b, ... of the direct voltage electrode 5 and the individual electrodes 7a, 7b, ... of the alternating voltage electrode 7 are connected to switching elements 11a, 11b, ... or 10a, 10b, ... which are connected via corresponding Connections 14a, 14b, ... or 13a, 13b, ... with voltages for modulating the voltage of each individual electrode in sections. The voltages applied to the individual electrodes for modulation can be of different sizes. The other parts of FIG. 12 correspond to those of FIGS. 11 and 1. This is the AC voltage generator 2 with the AC voltage source 32, which comprises the voltage regulator 17 and the frequency regulator 18, and the isolating transformer 19.

The smoothing capacitor 33 is located parallel to the outputs of the DC voltage regulator 16 and the DC voltage generator 1.

The switching elements 10, 11, 12 known from the device according to FIG. 1 are characterized by the aforementioned switching elements 10a, 10b, ...; 11a, 11b, ... and 1 ₂ a, 12b, .. replaced, which are connected to the corresponding individual electrodes of the AC voltage, DC voltage and counterelectrode. The switching elements 10a, 10b, ...; 12a, 12b, ... are constructed analogously to the switching elements 11a, 11b, ..., ie they can switch back and forth between two positions, depending on whether a modulation voltage or a modulation signal is fed in or not.

In the following examples, operating data and parameters of the device are given.

example 1

• Wechselspannungselektrode 7: 1,8 mm dicker Kupferdraht Gleichspannungselektrode 5: 50 µm dicker Wolframdraht Isolationskörper 23: 5 mm dickes Polytetrafluoräthylen Länge der Gleichspannungselektrode 5: 40 cm Wechselspannung: 5 kV_eff /30 kHz.

An electrode arrangement according to FIG. 4 was used in a device according to FIG. 7. At a distance of 4 mm below the DC voltage electrode 5, the plate-shaped counter-electrode 9 made of aluminum was arranged, which was connected to earth potential via the DC current meter 26. Other dates were:

• AC voltage electrode 7: 1.8 mm thick copper wire DC voltage electrode 5: 50 µm thick tungsten wire insulation body 23: 5 mm thick polytetrafluoroethylene Length of the DC voltage electrode 5: 40 cm AC voltage: 5 kV _eff / 30 kHz.

The length of the DC voltage electrode 5, which forms the corona electrode, corresponds to the usual lengths of corons in office copiers. A current of 2 µA is set at +300 V DC, 11 _/ uA at +700 V and 22 _/ uA at +1200 V. Similar current values were obtained when negative DC voltage was applied to a DC voltage electrode 5. These charging currents were measured at DC voltages below the required threshold voltage of the DC voltage electrode. If no alternating field was applied to the alternating voltage electrode 7, the charging current failed.

With exact adjustment of the AC voltage with regard to its symmetry and freedom from distortion, a charging current with an AC field applied to the AC voltage electrode 7 could already be measured with a DC voltage close to 0 volts. When making the exact setting, it should be noted that if the isolating transformer 19 of the AC voltage generator 2 is operated in the area of natural resonance, albeit with great attenuation, frequency readjustment leads to phase shifts and influences on the sine curve of the AC voltage.

Example 2

• Wechselspannungselektrode 7: 1,8 mm dicker Kupferdraht Gleichspannungselektrode 5: 100 µm dicker Stahldraht Isolationskörper 23: Glasrohr mit 14 mm Durchmesser Länge der Gleichspannungselektrode 5: 780 mm Wechselspannung: 5,5 kV_eff/30 kHz.

The electrode arrangement according to FIG. 4 was used in the device according to FIG. 7. The data were:

• AC electrode 7: 1.8 mm thick copper wire DC voltage electrode 5: 100 µm thick steel wire insulation body 23: glass tube with 14 mm diameter. Length of the DC voltage electrode 5: 780 mm AC voltage: 5.5 kV _eff / 30 kHz.

The results of these measurements, which were carried out for distances between the DC voltage electrode 5 and the counter electrode 9 of 5, 8, 10 and 13 mm, are shown in FIG. 8 in curves a, b, c and d.

Example ₃

The measurements of Example 2 were carried out with a similar but longer DC voltage electrode. The length of the DC voltage electrode 5 was 1290 mm and a 4 mm thick VA steel wire was used as the AC voltage electrode 7. The distances between the DC voltage electrode 5 and the counter electrode 9 were the same as in Example 2. The charging currents were approximately 1.5 times the values according to Example 2.

Example 4

• Wechselspannungselektrode 7: 1,8 mm dicker Kupferdraht Länge der Gleichspannungselektrode 5: 620 mm Wechselspannung: 3 kV_eff/20 kHz

An electrode arrangement corresponding to FIG. 5 was used in the device according to FIG. 7. As a DC voltage electrode 5, an approximately 1 mm wide strip of gold-palladium was applied to an insulating body 23 made of ceramic and 8 mm in diameter and baked. The rest of the data was:

• AC voltage electrode 7: 1.8 mm thick copper wire Length of the DC voltage electrode 5: 620 mm AC voltage: 3 kV _eff / 20 kHz

The measured linear current rise as a function of the DC voltage, which was varied between 1 and 7 kV, is shown in curve a in FIG. 2. As can be seen from curve b in FIG. 2, without AC voltage support, a charging current only occurs above a DC voltage of 8 kV, with voltage breakdowns occurring on the layer 8 to be charged above 9 kV.

Example 5

A photoconductor layer with a thickness of 10 μm and equal molar proportions of poly-N-vinylcarbazole and trinitrofluorenone, which are applied to a conductive support made of aluminized polyester film, was charged to a direct voltage of -800 V.

• Wechselspannungselektrode 7:.1,8 mm dicker Kupferdraht Gleichspannungselektrode 5: 100.um dicker Stahldraht Länge der Gleichspannungselektrode 5: 300 mm Isolationskörper 23: Glasrohr von 14 mm Durchmesser Abschirmgehäuse 24: 3 mm dicker Kunststoff Spaltbreite: 3 mm An die Gleichspannungselektrode 5 wurde eine Gleichspannung U₌ = -80C V angelegt.

For this purpose, the photoconductor layer 8 was passed at a distance of 5 mm below the DC voltage electrode 5 of the electrode arrangement according to FIG. 6 at a speed of 30 cm / s. Other data were:

• AC voltage electrode 7: .1.8 mm thick copper wire DC voltage electrode 5: 100 µm thick steel wire Length of the DC voltage electrode 5: 300 mm Insulation body 23: Glass tube of 14 mm diameter shielding housing 24: 3 mm thick plastic gap width: 3 mm A DC voltage U ₌ = -80C V was applied to the DC voltage electrode 5.

The tuning of the alternating voltage generator 2 as a function of the geometric structure of the electrode arrangement is carried out via the voltage regulator 17 and frequency regulator 18. In the control range U _~ for alternating voltage = 1 to 5.7 kV _eff , the voltage on the photoconductive layer 8 was initially less than the predetermined direct voltage, rose however, with increasing AC voltage up to the specified setpoint. At an alternating voltage U ~ = 5.7 kV _eff , the voltage level was relatively independent of the frequency and corresponded to the specified direct voltage value. The greatest charging current at this AC voltage was measured at 34 kHz with a half width of approximately ± 4 kHz.

In the control range U ~ = 5.7 to 10 kV _eff , depending on the frequency setting, the photoconductor layer 8 was partially charged slightly higher, sometimes lower than to -800 V.

The photoconductor layer was charged to -800 V overall under the specified conditions with good reproducibility and without strikethrough of the photoconductor layer. After charging, the image was exposed, developed with toner and the toner image was transferred to paper.

Example 6

A photoconductive thermoplastic recording layer 8 on a polyester support of 50 _/ um thickness which lies on a glass plate with a transparent conductive layer was charged to +5200 V.

The recording layer 8 consisted of an approximately 1 μm thick partial layer of bromopyrene resin, to which 1/5 part by weight of dicyanomethylenetrinitrofluorenone and 1/2 part by weight of a copolymer of vinyl chloride and vinyl acetate had been added. Above it was a second, about 0.5 µm thick sub-layer made of the glycerol ester of the hydrogenated rosin.

The electrode arrangement was set as in Example 5, with the only difference that the DC voltage at the DC voltage electrode 5 was +5200 V. The charge was reproducible without any strikethrough on the recording layer.

After charging, an intensity pattern of 820 lines / mm was irradiated with interfering light from a He / Ne laser, after which the recording layer was heated to 70 ° C. over a period of 1/10 s, a relief grating being formed, which diffracted the irradiated laser light .

Example 7

• Wechselspannungselektrode 7: 1,8 mm dicker Kupferdraht Gleichspannungselektrode 5: Wolframdraht mit 50 µm Durchmesser Isolationskörper 23: Glasrohr mit 9 mm Außendurchmesser

A charge image corresponding to the irradiated X-ray intensity pattern, which was made visible with toner, was generated on a polyester film 50 μm thick in an ionization chamber. Here, fluctuations in density of the separated amount of toner also occurred in points of equal intensity. These fluctuations were avoided by subsequent neutralizing charging of the polyester film with the following electrode arrangement:

• AC voltage electrode 7: 1.8 mm thick copper wire DC voltage electrode 5: tungsten wire with 50 µm diameter Insulation body 23: glass tube with 9 mm outside diameter

The DC voltage electrode 5 was arranged at a distance of 5 mm from the polyester layer and the AC voltage electrode 7 was arranged at a distance of 10 mm from the outer diameter of the insulating body 23. The DC voltage electrode 5 was set to earth potential and the AC voltage electrode 7 was initially operated at 3 kV _eff . The polyester layer was moved past several times under the DC voltage electrode 5. The AC voltage was gradually increased to 4.5 kV _cff . To further neutralize the residual charge on the polyester film, its frequency was gradually changed in the range between 30 and 40 kHz, starting from 35 kHz, in order to symmetrize the alternating voltage, until an optimal charge neutralization was obtained. The frequency was 32 kHz.

With these settings, the polyester film could be neutralized to such an extent that only a residual voltage of 1.5 V was measured on the surface using a non-contact electrostatic voltage measuring device.

Example 8

A thermoplastic recording layer 8 on a polyester support of 50 _/ um thickness was charged grid pattern to +5 kV. The 20 _/ um thick recording layer consisted of a Glykolester of hydrogenated rosin.

• Wechselspannungselektrode 7: 1,8 mm dicker Kupferdraht Gleichspannungselektrode 5: 50 µm dicker Wolframdraht Isolationskörper 23: Glasrohr mit 9 mm Durchmesser, Spaltbreite 1 mm Wechselspannung: 5 kV_eff/30 kHz Abstand zwischen den Elektroden 5 und 7: 6,5 mm

The recording material was moved on a grounded base at a distance of 5 mm from the DC voltage electrode 5 under an electrode arrangement according to FIG. 6 at a speed of 10 cm / s. The rest of the data was:

• AC voltage electrode 7: 1.8 mm thick copper wire DC voltage electrode 5: 50 µm thick tungsten wire Insulation body 23: glass tube with 9 mm diameter, gap width 1 mm AC voltage: 5 kV _{eff /} 30 kHz distance between electrodes 5 and 7: 6.5 mm

In order to modulate the charging voltage of the recording layer, the alternating voltage was periodically interrupted with the switching element 10 according to FIG. 1, for which purpose a frequency generator was connected to the connection 13. The switching element 10, which consists of an integrated semiconductor switch, enabled practically instantaneous control of the AC voltage.

The pulse duration and the dead time in which the switch was open or closed was 10 ms in each case. At the DC voltage electrode 5 a is a DC voltage of +5 ^g of kV.

After the grid-like charging, the recording material was heated with warm air at about 50 ° C., a relief grid of 5 lines / mm being produced.

Example 9

Example 8 was repeated, with a further charge pattern being superimposed on the grid-shaped charge. For this purpose, the recording material charged in a grid shape was introduced into an ionization chamber together with the conductive base. The conductive pad consisted of a 5x5 cm glass plate with a conductive transparent layer with reinforced electrodes on opposite sides. The plate electrodes were connected to lines leading to the outside. A second transparent electrode was located above this plate at a distance of 1 cm. The housing of the ionization chamber consisted of 15 mm thick plexiglass. The chamber was evacuated and filled with xenon under a slight positive pressure. The electrode with the overlying recording layer was connected to earth potential and a voltage of -8 kV was applied to the upper electrode. Before the X-ray exposure a flat lead wedge was brought into the beam path with 80 kV X-rays. When the lower electrode was heated by a voltage surge of 70 V for 0.1 s, a relief image was formed on the thermoplastic recording layer, which was read out using Schlieren optics. In the areas of attenuated X-ray radiation through the lead wedge, its outlines, superimposed by a linear grid structure, could be seen. The raster intensity increased with the thickness of the lead wedge during the X-ray exposure, which made a corresponding halftone reproduction of the Eleikeil possible.

Example 10

A 300 µm thick selenium layer on a 2 mm thick aluminum plate was coated with a 20 µm thick thermoplastic recording layer consisting of the glycol ester of the hydrogenated rosin. With the settings of Example 8, a rastered charge to +1800 V was carried out, X-rays of 80 kV being irradiated through a flat lead wedge and heating with warm air. This resulted in a relief image of the lead wedge, which was read out with a reflective streak optic. The relief image was rasterized in the form of lines at the points protected from the X-ray exposure by the lead wedge. The raster intensity increased with the thickness of the lead wedge during the X-ray exposure, whereby a halftone image of the lead wedge was obtained.

Example 11

A charge pattern with correspondingly variable input data was applied to an insulating recording layer 8.

• Wechselspannungselektrode 7: 1,8 mm dicker Kupferdraht Gleichspannungselektrode 5: 50_/um dicker Wolframdraht Isolationskörper 23: Glasrohr von 9 mm Durchmesser, Wechselspannung: 5 kV_eff/30 kHz Gleichspannung: - 300 V

The recording layer 8 was passed at a distance of 5 mm under an electrode arrangement according to FIG. 6 at a speed of 45 cm / s. The rest of the data was:

• AC voltage electrode 7: 1.8 mm thick copper wire DC voltage electrode 5: 50 _/ um thick tungsten wire Insulation body 23: glass tube of 9 mm diameter, AC voltage: 5 kV _eff / 30 kHz DC voltage: - 300 V

The electrode arrangement with a gap width of 1 mm was used in a device according to FIG. 1, the modulation taking place via the connection 15 of the switching element 12 in the ground line of the counter electrode 9.

The counter electrode 9 consisted of a plastic plate with a copper coating, as used for the production of printed circuit boards. Raster lines were not photo-mechanically etched into the copper as the counter electrode 9, as can be seen in FIG. During the recording, a voltage drop of -300 V to 0 V was generated at the counter electrode 9.

When developing with liquid toner, increasingly more toner was deposited on the recording layer 8 along the running path. The toner image transferred to paper clearly showed the increase in blackening along the running distance and the raster lines of the counter electrode 9 were clearly reproduced. No breakdown of the recording layer 8 occurred.

Example 12

A charge pattern corresponding to variable input data was applied to an insulating thermoplastic recording layer 8 to form relief structures.

The recording layer 8 consisted of a 20 .mu.m thick layer of the glycol ester of the hydrogenated rosin on a 50 .mu.m thick polyester film and was charged according to the settings in Example 11, the predetermined direct voltage being U_ = -5 kV. The alternating voltage of 5 kV _eff and 30 kHz was periodically modulated via the switching element 10 and the connection 13 by a frequency generator with 3 kHz.

When developing the recording layer 8 with warm air at about 50 ° C., a relief image of the counter electrode 9 was obtained. The relief depth increased along the route. When projected with Schlieren optics, the counter electrode 9 became increasingly darker in the direction of travel.

Example 13

• Wechselspannungselektrode 7: 1,8 mm dicker Kupferdraht Gleichspannungselektrode 5: einzelne Wolframdrähte mit 150 µm Dicke, die in Abständen von etwa 300 _/um gegeneinander isoliert angeordnet waren Wechselspannung: 5 kV_eff/30 kHz

A charge pattern with correspondingly variable input data was applied to the insulating recording layer 8 of a dielectric paper with a conductive paper carrier 29. The dielectric paper was moved about 0.5 mm apart under an electrode arrangement according to FIG. 10 at a speed of 25 cm / s. The rest of the data was:

• AC voltage electrode 7: 1.8 mm thick copper wire DC voltage electrode 5: single tungsten wires of 150 micron thickness which were arranged to isolated against each other at intervals of about 300 _/ AC voltage: 5 kV _eff / 30 kHz

The gap width of the shield housing 24 was 1 mm. The electrode arrangement was used in a device according to FIG. 11. The DC voltage U was applied to the associated individual electrodes 5a, 5b, 5c, ... via the switching elements 11a, 11b, 11c,. The individual control functions for controlling the individual electrodes 5a, 5b, 5c, ... were given to the connections 14a, 14b, 14c, ... of the switching elements 11a, 11b, 11c, ...

First, all the wire ends were conductively connected to one another via the connections 31a, 31b, 31c, ... and it was similar to Example 11 via a potentiometer tap 27 _(FIG. 9) has a constantly changing from -500 V to 0 V DC voltage U applied to the wire ends of the individual electrodes. When developing with liquid toner, increasingly less toner was deposited on the dielectric paper in the direction of travel. No line-shaped traces of writing of the individual electrodes 5a, 5b, 5c, ... were produced. There were no strikethroughs and the halftone reproduction was consistently satisfactory. The lines of adjacent individual electrodes joined together without dividing lines, so that the transition from one individual electrode to the adjacent individual electrode could not be determined optically.

In this example, a direct voltage U ₌ = -500 V was also applied to only a single electrode, for example electrode 5a, or a group of individual electrodes, while the remaining individual electrodes were at ground potential. The applied DC voltage was then often interrupted for different lengths of time during the recording. The image developed with liquid toner showed traces of writing down to a width of about 0.3 mm in the direction of travel and across. No breakthroughs occurred.

Claims (45)

1. A method for electrostatically charging a dielectric layer to a predetermined potential with the aid of an alternating electrical field and an electrostatic constant field, characterized in that charge carriers are generated at a distance from the surface of the dielectric layer by the alternating electrical field, which under the influence of the to be charged Layer penetrating electrostatic constant field as a charging current to the surface of the layer. 2. The method according to claim 1, characterized in that the dielectric layer consists of a photoconductive and / or thermoplastic recording medium, when charged at least one of the two voltage fields is modulated. 3. The method according to claims 1 and 2, characterized in that the alternating electric field is variable in strength and is set so that the ionization of the ambient atmosphere takes place up to the saturation value. 4. The method according to claim 1, characterized in that the potential of the counter electrode of the DC field is modulated during the charging of the dielectric layer. 5. The method according to claim 1, characterized in that a charge current of 0 to a corresponding maximum value is obtained in almost linear dependence on the DC voltage by regulating the DC voltage of the electrostatic DC field between zero and a predetermined maximum value. 6. The method according to claim 1, characterized in that the frequency of the alternating electric field is adjustable in the range from 5 to 100 kHz. 7. The method according to claim 2, characterized in that the alternating field is modulated in voltage. 8. The method according to claim 7, characterized in that the alternating field is modulated by externally applied voltages. 9. The method according to claims 7 and 8, characterized in that the alternating field is modulated with periodic square-wave voltage pulses. 10. The method according to claim 7, characterized in that the alternating field is modulated with an informational span- now ^g . 11. The method according to claim 2, characterized in that the DC field is modulated in voltage. 12. The method according to claim 11, characterized in that the DC field is periodically modulated. 13. The method according to claims 11 and 12, characterized in that the DC field is modulated with an informational voltage. 14. The method according to claims 11 to 13, characterized in that the modulation of the constant field is carried out in discrete spatial sections transverse to the dielectric layer. 15. The method according to claims 7 and 11, characterized in that the alternating field and the constant field are modulated. 16. The method according to claims 1 to 15, characterized in that the charged dielectric layer is acted upon by an ion current or by electromagnetic radiation in order to modify the electrostatic charge. 17. The method according to claims 1 to 16, characterized in that the dielectric layer is charged to zero potential for deleting residual charges from previous charges. 18. The method according to claims 1 to 17, characterized in that the charging in a Fremdgasatmosphä- _'re takes place. 19. Device for performing the method according to claims 1 to 18, characterized in that a Electrode (7) is arranged at a distance from the dielectric layer (8) to be charged and is connected to the hot output (2.) of an AC voltage generator (2) and that between this electrode (7) and the layer (8) there is a further electrode (5 ) is provided, which is connected to the hot output (1 ₁ ) of a DC voltage generator (1). 20. The apparatus according to claim 19, characterized in that the layer to be charged (8) rests on a counter electrode (9) to the DC voltage electrode (5) and that the counter electrode (9) is located on ^p tential Massep. 21. Device according to claims 19 and 20, characterized in that the cold output (2 ₀ ) of the AC voltage generator (2) with the hot output (1 ₁ ) of the DC voltage generator (1) is connected. 22. Device according to claims 19 to 21, characterized in that the DC voltage U = of the DC voltage generator (1) is adjustable between zero and 20 kV. 23. Device according to claims 19 to 21, characterized in that the alternating voltage of the alternating voltage generator (2) is 1 to 10 kV _eff at a frequency between 1 and 100 kHz. 24. The device according to claim 19, characterized in that the known AC voltage generator (2) an AC voltage source (32) ^q from a voltage regulator (17) for height adjustment of the AC voltage and a Fre uenzregler (18) for tuning the frequency of the AC voltage. 25. The device according to claim 24, characterized in that the AC voltage generator (2) comprises an isolating transformer (19) which transforms up the AC voltage supplied by the AC voltage source (32) and an ungrounded cascade connection of the AC voltage to the hot terminal (1 ₁ ) of the DC voltage generator (1) forms. 26. Device according to claims 24 and 25, characterized in that between the AC voltage source (32) and the isolating transformer (19), a switching element (10) is connected, to which a voltage for modulating the AC voltage can be supplied via a connection (13). 27. The device according to claims 19 to 22, characterized in that in the line to the hot output (1 ₁ ) of the DC voltage generator (1), a switching element (11) is connected, which via a connection (14) with a voltage for modulating the DC voltage can be applied. 28. Device according to claims 19 and 20, characterized in that in the ground line of the counter electrode (9), a switching element (12) is connected, which can be acted upon via a connection (15) with a voltage for changing the voltage of the counter electrode (9) is. 29. The device according to claim 19, characterized in that each of the electrodes (5,7) consists of a plurality of individual electrodes (5a, 5b, 5c, ...; 7a, 7b, 7c, ...) which are insulated from one another. 30. The device according to claim 20, characterized in that the counter electrode (9) consists of a plurality of mutually insulated individual electrodes (9a, 9b, 9c ...). 31. Device according to claims 29 and 30, characterized in that each of the individual electrodes (5a, 5b, 5c, ...; 9a, 9b, 9c, ...) of the DC voltage electrode (5) and the counter electrode ( 9) is connected to a switching element (lla, llb, llc, ...; 12a, 12b, 12c, ...) which is connected via a corresponding connection (14a, 14b, 14c, ...; 15a, 15b, 15c, ...) can be acted upon with a voltage for section-wise modulation of the direct voltage or for changing the voltage of the individual counter electrodes and that the voltages applied to the individual electrodes are of different sizes. 32. Device according to claims 26 to 28 and 30, characterized in that the switching element (10; 11; 12; 10a; lOb; ...; lla; llb, ...; 12a; 12b; ...) a electromechanical switch like a relay or an electronic switch. 33. Apparatus according to claim 19, characterized in that the DC voltage electrode (5) consists of a corona wire with a diameter of 5 µm to 2 mm. 34. Apparatus according to claim 19, characterized in that the DC voltage electrode (5) is a metal strip with a rectangular or a strip cross-section, which has a thickness between 5 microns and 2 mm. 35. Device according to claims 19 and 29, characterized in that the DC voltage electrode (5) consists of a needle arrangement with mutually insulated, individually controllable needles. 36. Apparatus according to claim 19, characterized in that an insulating body (23) encloses the AC voltage electrode (7). 37. Device according to claims 19 and 36, characterized in that the direct voltage electrode (5) of the insulating body (23) enveloping the alternating voltage electrode (7) is arranged at a distance of 1 to 10 mm. 38. Device according to claims 19 and 36, characterized in that the DC voltage electrode (5) bears directly on the insulating body (23) enclosing the AC voltage electrode (7). 39. Device according to claims 19 and 36 to 38, characterized in that the distance between the DC voltage electrode (5) and the AC voltage electrode (7) is 1 to 20 mm. 40. Device according to claims 19 and 29, characterized in that the alternating voltage electrode (7) consists of a metal wire with a diameter of 1 to 20 mm or of a metal profile with a cross section of corresponding thickness to the wire diameter. 41. Device according to claims 19, 20 and 29, 30, characterized in that the distance between the DC voltage electrode (5) and the counter electrode (9) is 1 to 20 mm. 42. Device according to claims 19 to 35, characterized in that an opaque shield (24) is arranged between the DC voltage electrode (5) and the dielectric layer (8). 43. Apparatus according to claim 42, characterized in that the shield (24) of the DC voltage electrode (5) has an outlet gap (25) for the charging current. 44. Device according to claims 42 and 43, characterized in that the shield (24) forms a shell filled with extraneous gas. 45. Device according to claims 42 to 44, characterized in that the region of the DC voltage electrode (5) is under positive pressure.

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