Device for generating a source stream of charge carriers by means of field emission and method for stabilizing a source stream of charge carriers emitted by means of a field emission element

Abstract

Device (100) for generating a source current of charge carriers by means of field emission or field ionization, comprising: - at least one field emission element (1) from which, during operation of the device (100), the charge carriers emerge, leading to an emission stream in the field emission element (1), - at least one extraction electrode (7) for applying an extraction voltage (Vext) to extract the charge carriers from the field emission element (1), wherein a first part of the extracted charge carriers contributes to the source current, and a second part of the extracted charge carriers hits the extraction electrode (7) and leads to an extraction current (Iext) in the extraction electrode (7), - an additional electrode (13) on which the source current of charge carriers impinges and which contributes to an electrode current in the additional electrode (13), wherein the additional electrode (13) is a most positive or most negative electrode of the device (100) and the extraction electrode (7) is located between the additional electrode (13) and the field emission element (1), - a control device (21), and - at least one control element (29) configured to change the emission flow, wherein the control device (21) is configured to actuate the control element (29), wherein the control device (21) is configured to reduce fluctuations of a controlled variable Q characteristic of the source flow during operation of the device (100), and the controlled variable Q is a function of at least the electrode current, such that a setpoint is supplied to or stored in the control device (21), and during operation of the device (100), the controlled variable Q is compared with the setpoint, and if there is a difference, the control element (29) is actuated such that the emission flow changes in a manner that the source flow (3) behaves in such a way that the predetermined setpoint is achieved.

Description

[0001] A device for generating a source current of charge carriers by means of field emission is specified. Furthermore, a method for stabilizing a source current of charge carriers emitted by a field emission element is specified.

[0002] The publication US 2002 / 0130625 A1 relates to a closed-loop control method for controlling charge carrier emission at a cold cathode of an electrode source. Control is based on an emission current.

[0003] From the publication DE 11 2014 002 139 B4 an electron gun for electron microscopes with an electron source, power sources, control electrodes and an acceleration electrode is known.

[0004] The documents US 5 173 634 A and US 5 847 408 A are directed to geometries of emission electrodes.

[0005] One problem to be solved is to provide a device with which a source current of charge carriers can be generated in a time-stable manner.

[0006] This object is achieved by a device and a method having the features of the independent claims. Preferred developments are the subject of the dependent claims.

[0007] According to the invention, the device is configured to generate a source current of charge carriers by means of field emission or field ionization. For the sake of simplicity, the term "field emission" is often used below, but the explanations apply equally to field ionization. In other words, the device described here is suitable as both a field emission element and a field ionization source. The charge carriers are, in particular, electrons or ions.

[0008] According to the invention, the device comprises one or more field emission elements. During operation of the device, the charge carriers escape from the at least one field emission element. The escaped charge carriers lead to an emission current in the field emission element. The field emission element is, for example, one or more emitters. These can specifically be tips, for example, made of a semiconductor such as silicon, a metal such as tungsten, an insulator such as silicon oxide or silicon nitride, a carbon-based material such as diamond-like carbon, carbon nanotubes, graphene, or other materials such as LaB6, to which an extraction voltage is applied. The field emission element is preferably connected as the most positive or most negative electrode, for example as a cathode.

[0009] According to the invention, the device comprises at least one extraction electrode for applying the extraction voltage to extract the charge carriers from the field emission element. The extraction electrode(s) may be, for example, a ring electrode or a grid electrode.

[0010] According to the invention, a first portion of the extracted charge carriers contributes to the source current, and a second portion of the extracted charge carriers impinges on the extraction electrode, resulting in an extraction current in the extraction electrode. In other words, a portion of the charge carriers emitted by the field emission element can impinge on the extraction electrode and be absorbed by the extraction electrode, so that these absorbed charge carriers are no longer part of the source current.

[0011] According to the invention, the device comprises at least one control device. The control device is designed to reduce fluctuations in a controlled variable Q characteristic of the source current during operation of the device. The controlled variable Q is a function of at least the electrode current. The controlled variable Q is, for example, a current at the extraction electrode (extraction current) and / or at the field emission element (emission current) and at an additional electrode (electrode current) and / or any further electrode onto which the charge carriers preferably impinge to generate electromagnetic radiation such as X-rays after passing through the extraction electrode. It is possible to determine the characteristic controlled variable Q by the current in the additional electrode (electrode current).The additional electrode is the most positive electrode or the most negative electrode, depending on whether electrons or positive ions are produced.

[0012] According to at least one embodiment, the device is configured to generate radiation. A photon flux during radiation generation is preferably proportional or approximately proportional to the electrode current in the additional electrode.

[0013] According to the invention, the device serves to generate a source current of charge carriers by means of field emission. The device comprises at least one field emission element from which the charge carriers emerge during operation of the device, leading to an emission current in the field emission element. The device also includes at least one extraction electrode for applying an extraction voltage to extract the charge carriers from the field emission element. A first portion of the extracted charge carriers contributes to the source current, and a second portion of the extracted charge carriers strikes the extraction electrode and leads to an extraction current in the extraction electrode. A control device of the device is also configured to reduce fluctuations in a controlled variable Q characteristic of the source current during operation of the device.

[0014] It has been found that the source current of charge carriers generated by similar devices exhibits relatively strong fluctuations. These fluctuations can be reduced to the controlled variable Q by controlling the device described here.

[0015] According to the invention, the device comprises at least one control element. The control element(s) are configured to change the emission current. Preferably, the control device is configured to control the control element.

[0016] According to at least one embodiment, the device comprises at least one variation electrode configured to change the kinetic energy of the extracted charge carriers of the source current by means of a variation voltage applied to the variation electrode. In particular, for this purpose, the device is configured such that the variation voltage is varied depending on a quantity K characteristic of the kinetic energy of the extracted charge carriers. For example, the characteristic quantity K is a voltage difference between the field emission element and the variation electrode. Such a variation electrode can be present alternatively or in addition to the additional electrode, which preferably serves as the most positive or most negative electrode.

[0017] According to at least one embodiment, the control element controls the emission current. In particular, the control element controls the charge carrier flow to the field emission element or the extraction voltage. For example, the control element changes the emission current by changing a resistance and / or a voltage, in particular toward the field emission element, by controlling a transistor or illuminating it, or by applying a temperature to a diode or transistor. The control element can also be integrated into the field emission element.

[0018] In a particularly preferred embodiment, the device for generating a source current of charge carriers by means of field emission comprises at least one field emission element from which charge carriers emerge during operation of the device, leading to an emission current in the field emission element and contributing at least partially to the source current of charge carriers. Furthermore, the device comprises at least one control element configured to change the emission current and at least one regulating device, wherein the regulating device is configured to control the control element. Furthermore, at least one variation electrode is provided, which is configured to change the kinetic energy of the extracted charge carriers of the source current by means of a variation voltage applied to the variation electrode.The device is designed such that the variation voltage is varied depending on a quantity K characteristic of the kinetic energy of the extracted charge carriers.

[0019] According to at least one embodiment, the characteristic variable K can be obtained by means of a voltage drop across the control element. The device is preferably designed such that the variation voltage is increased as the voltage drop across the control element increases and / or the variation voltage is reduced as the voltage drop across the control element decreases. In other words, it is possible for the characteristic variable K to be measured by means of the voltage drop across the control element.

[0020] According to at least one embodiment, the device comprises one or more adding elements. The at least one adding element is electrically connected to the variation electrode. The device is preferably configured such that the variation voltage is obtained by adding the extraction voltage and the voltage drop across the control element.

[0021] According to at least one embodiment, the voltage drop across the control element leads to a broadening of the distribution of the kinetic energy of the charge carriers of the source current. This broadening is preferably counteracted by means of the variation electrode.

[0022] According to at least one embodiment, the device comprises a ground connection and / or an electrical supply power source, or power source for short. The control element is arranged in a conduction path between the supply power source and the field emission element. Alternatively, the control element is arranged in a conduction path between the ground connection and the field emission element.

[0023] According to at least one embodiment, the control element is a transistor, specifically a metal-oxide-semiconductor field-effect transistor (MOSFET), and / or a diode, specifically a PIN diode, specifically a silicon drift detector operated as a diode. Furthermore, bipolar transistors and / or IGBTFETs can be used for the control element. It is possible for the control element to control the charge carrier flow in the emitter via an electrically conductive connection to the emitter.

[0024] According to at least one embodiment, the control element is electrically connected to the extraction electrode. The control element is then preferably designed to change the emission current by changing the extraction voltage.

[0025] According to at least one embodiment, the device is designed such that during operation of the device a temporal distribution of the quantity Q characteristic of the source current has a relative standard deviation S Q that is less than or equal to one of the following values: 10%, 5%, 1%, 0.5%, 0.1%, 0.05%, 0.01%, 0.005%, 0.001%. This may cause the emission current through the device to become more or less stable. The quantity characteristic of the source current is approximately a current at the most positive or most negative electrode.

[0026] According to at least one embodiment, the control device has at least one input, to which a current value of the controlled variable is supplied, and at least one output connected to the control element. The device is preferably designed such that the control element is controlled via the precisely one output in particular, such that the controlled variable assumes a predetermined setpoint. The setpoint can be constant or change over time. The controlled variable can also be modulated over time.

[0027] According to at least one embodiment, the device comprises at least one difference-forming element. Q is preferably a function of the difference between the emission current and the extraction current. The difference can also be the voltage drops at the respective detection element, for example, voltage drops across a resistor through which the associated current flows.

[0028] According to at least one embodiment, Q is a function of an electrode current elicited by the source current of charge carriers in the additional electrode onto which the charge carriers impinge.

[0029] According to at least one embodiment, the device comprises one or more additional electrodes onto which the source current of charge carriers impinges and contributes to an electrode current in the additional electrode. The controlled variable Q is preferably a function of the electrode current at this additional electrode, which is in particular a most positive or most negative electrode.

[0030] According to at least one embodiment, the device comprises one or more charge carrier optics. The at least one charge carrier optic can be used to influence and / or control the trajectory and / or kinetic energy of the generated charge carriers. Alternatively or additionally, a variation electrode is provided.

[0031] When determining the controlled variable Q, it is possible to take into account an electric current caused by charge carriers absorbed by the additional electrode. Thus, the controlled variable Q is, in particular, the emission current minus the extraction current, minus a current at the charge carrier optics, minus a current at the additional electrode, and minus a current at the variation electrode. Alternatively, the controlled variable Q is formed by the emission current minus any combination of some of the existing electrodes.

[0032] According to at least one embodiment, the device is designed as a radiation source. The electromagnetic radiation to be emitted by the radiation source is generated by the source current impinging on the additional electrode. This means that, on the one hand, bremsstrahlung is generated by the loss of the kinetic energy of the charge carriers. On the other hand, the charge carriers lift electrons, in particular, in a material of the additional electrode to higher energy levels. When the electrons fall back to their original energy levels, the radiation is generated. The generated radiation is, in particular, electromagnetic radiation in a wavelength range that covers hard X-rays to infrared.For example, the radiation generated is X-ray radiation, for example with an average photon energy of at least 5 keV or 20 keV or 60 keV and / or of at most 250 keV or 120 keV or 30 keV.

[0033] According to at least one embodiment, the device is designed as a charge carrier source. The charge carriers are, in particular, electrons or ions originating from the field emission or field ionization element. They can also be secondary electrons from the additional electrode or ions, for example, from a gas volume into which the source current enters, generating the ions via impact ionization.

[0034] According to at least one embodiment, one or more of the following components of the device are designed to be permeable or partially permeable to the source current, so that during operation of the device, the charge carriers of the source current can pass through this or these elements: the additional electrode, the variation electrode, the charge carrier optics. It is also possible for the additional electrode to form the variation electrode, so that both components can be identical.

[0035] According to at least one embodiment, the device comprises one, an arbitrarily selected plurality of or all of the following current sensing elements: Emission current detection element, Extraction current detection element, Electrode current detection element, Charge carrier optical current detection element, A variation electrode current sensing element. Here, one of the current sensing elements comprises, for example, at least one resistor, wherein the voltage drop across the resistor(s) characterizes the current of the corresponding electrode.

[0036] According to at least one embodiment, the device comprises one, an arbitrarily selected plurality of or all of the following current sensing elements temperature compensation: extraction current sensing element, Emission current detection element, Electrode current detection element, Charge carrier optical current detection element, A variable-electrode current sensing element. Preferably, one, an arbitrarily selected plurality of, or all of the current sensing elements comprise a plurality of series-connected resistors. One of the resistors can be a PTC resistor, and alternatively or additionally, another of the resistors can be an NTC resistor. PTC stands for Positive Temperature Coefficient; a material for this is, for example, a semiconducting, polycrystalline ceramic. NTC stands for Negative Temperature Coefficient; for example, the NTC resistor is made of a wire, such as a platinum alloy or nickel and iron.

[0037] According to at least one embodiment, the device is configured to keep the source current constant. The constant source current enables constant charge carrier generation and / or constant radiation generation. Constant means, for example, that a standard deviation of the source current is at most 10%, 5%, 1%, 0.5%, 0.1%, 0.05%, 0.01%, 0.005%, or 0.001%.

[0038] According to at least one embodiment, all or at least some of the electronic components of the device are integrated into a common microchip or microprocessor. The electronic components are, in particular, semiconductor components such as diodes, operational amplifiers, transistors, adding elements, control units, and memory modules. It is also possible for voltage sources or current sources, or at least their control, to be partially or completely integrated into the microchip or microprocessor.

[0039] According to at least one embodiment of the device, the field emission element and / or the extraction electrode are subjected to electrical voltage in multiple stages. This means, in particular, that several supply elements, such as transistors, for example so-called PowerMOS transistors, are present in series. It is possible for such series circuits to be connected in parallel.

[0040] In summary, the device described here can be operated with the following control types, also in combination: - Anode or cathode current control or control of the current of the most positive or negative electrode - differential current control with at least two, a selection of or all electrodes from each electrode combination, whereby the current can be controlled in any intermediate range, - Emission flow control.

[0041] In all embodiments and for all control types, additional electrodes can be present, one or more, also in any combination: - Variation electrodes, controlled either directly or, for example, via an adder, - one or more electron-optical or ion-optical elements, for example for focusing the charge carriers.

[0042] In all embodiments and for all control types and electrode configurations, the following control parameters can be used: current limiting, for example, to ground or on the so-called "high side," light and temperature, and extraction voltage, for example, via an op-amp. Alternatively or additionally, circuit variations can be implemented: temperature compensation, amplification, decoupling of circuit components, bandwidth, and / or controller type or parameters.

[0043] The device described here can be used in particular for the following applications: X-ray source, electron source, ion source, for example via field ionization or ionization between any electrodes by electrons.

[0044] Furthermore, a method for operating a device as described in connection with one or more of the above-mentioned embodiments is provided.

[0045] According to the invention, the method serves to stabilize a source current of charge carriers emitted by means of a field emission element, wherein the source current is formed by the charge carriers which have already passed an extraction electrode by means of which the charge carriers were extracted from the field emission element, and wherein in the method a variable characteristic of the source current is determined and this variable is transferred to a control device as a controlled variable.

[0046] The generated charge carriers, particularly electrons or positively charged ions, can leave the device directly and be used for various applications. It is also possible for the electrons themselves to be used to generate electromagnetic radiation such as X-rays when they strike a target surface of the device.

[0047] A device and method described herein are explained in more detail below with reference to the drawings using exemplary embodiments. Like reference numerals indicate like elements in the individual figures. However, they are not drawn to scale; rather, individual elements may be exaggerated for clarity.

[0048] They show: Fig. 1 to 9 schematic functional principles based on schematic circuit diagrams for embodiments of devices described here for methods described here for generating a source current of charge carriers by means of field emission or field ionization, Fig. 10 and Fig. 11 schematic representations of structures of embodiments of the device, and Fig. 12 to 14 measurement curves for the electrode current, the emission current and the extraction current as well as the X-ray flux in an embodiment of the device.

[0049] Fig. Figure 1 schematically shows the functional principle of a device 100 proposed herein for generating a source current of charge carriers by means of field emission. Furthermore, Fig. 1 some of the terms used above and below are explained in more detail.

[0050] Fig. 1 and the other embodiments, unless otherwise indicated, are formulated for the generation of electrons as charge carriers. For a different charge carrier polarity, i.e., the generation of positive ions, the opposite applies with regard to the described voltage polarities.

[0051] The device 100 of the Fig. 1 comprises a field emission element 1, consisting of one or more individual emitters such as an emitter array, for example, a metal or a semiconductor such as silicon or an oxide or a carbon-based material such as diamond-like carbon, carbon nanotubes, graphene, or other materials such as LaB6 containing or consisting of it. During operation of the device 100, charge carriers, in particular electrons, can be extracted from this field emission element 1 by means of field emission. The charge carriers extracted from the field emission element 1 are in Fig. 1 is indicated by the arrows with the reference numerals 3 and 5.

[0052] The device 100 further comprises at least one extraction electrode 7. This is assigned to the field emission element 1. By means of the extraction electrode 7, an extraction voltage Vext can be applied to the field emission element 1. For this purpose, the extraction electrode 7 is electrically connected to an electrical power source 9, preferably a voltage source. The extraction electrode 7 can be electrically connected to an extraction electrode terminal of the power source 9.

[0053] In addition to the extraction terminal, the power source 9 may have a ground terminal 91, also referred to as a ground terminal or GND. The extraction electrode terminal is expediently supplied by the voltage source 9 with a higher or lower voltage than the ground terminal 91, depending on the charge carrier polarity. Therefore, the connection option of the power source 9 provided by the extraction electrode terminal is sometimes referred to herein as the high-voltage side or high-side of the power source 9. The ground terminal 91, on the other hand, is sometimes referred to as the low-voltage side or low-side.

[0054] It is also conceivable, and in some cases more practical, for the last electrode to be connected to the ground connection (GND), and for all electrodes closer to the emitter to be connected to a correspondingly more negative or more positive potential. This applies accordingly to all other embodiments. The following only shows the case of an emitter-side ground connection; however, the examples can also be implemented with a corresponding ground potential other than 0 V.

[0055] The field emission element 1 is electrically connected via a supply terminal 11 to a supply power source, via which the field emission element 1 can be supplied with charge carriers that can be extracted from the field emission element 1 by the extraction electrode 7. The supply voltage Vv applied via the supply terminal is expediently lower than the extraction voltage Vext when electrons are generated; the reverse applies when positive ions are generated. For example, the supply voltage is 20 V or less, up to negative voltages. The supply power source can be provided by the power source 9. The supply terminal 11 can be implemented by the ground terminal 91 of the power source 9.

[0056] Typical voltage differences between the field emission element 1 and the extraction electrode 7, which are sufficient to extract charge carriers from the field emission element 1, are in the range of greater than or equal to 50 V or of a few 100 V, for example of greater than or equal to 500 V or greater than or equal to 800 V. The required voltages depend on the material and the geometry of the field emission element.

[0057] The field emission element 1 has a tip on its side facing the extraction electrode 7, which promotes the extraction of charge carriers. The extraction electrode is preferably recessed to allow the passage of extracted charge carriers through the extraction electrode. The charge carriers extracted from the field emission element 1 collectively contribute to an emission current Iemission flowing away from the field emission element toward the supply terminal 11. The emission current Iemission is caused by the totality of all charge carriers extracted from the field emission element 1. This means that the emission current is equal to the sum of the extraction current and the source current.

[0058] The charge carriers extracted from the field emission element 1 have two components, components 3 and 5. The charge carrier component 3 passes through the extraction electrode 7. For this purpose, the extraction electrode 7 is recessed, preferably in a region in which the field emission predominantly takes place, expediently in the region lying above the at least one tip of the field emission element 1. A second charge carrier component 5 of the extracted charge carriers reaches the extraction electrode 7, in particular immediately after extraction from the field emission element 1, and generates an extraction current Iext in the extraction electrode. If the charge carriers are electrons, the technical current flow direction is from the power source 9 to the extraction electrode 7, as in Fig. 1 illustrates.

[0059] The charge carrier fraction 3 that has passed through the extraction electrode 7 contributes to or forms the source current of charge carriers generated by the field emission element 1 by field emission. Consequently, fraction 3 is less than the total extracted charge carriers by the amount of the charge carrier fraction 5 absorbed in the extraction electrode 7.

[0060] In addition, the device 100 comprises the Fig. 1 shows a control device 21, which is illustrated only in a highly simplified manner. Preferably, the control device 21 has a Fig. 1 not shown GND terminal and / or is connected to another voltage supply. Furthermore, the control device 21 preferably influences the electrical potential of the extraction electrode 7, the field emission element 1 and / or the additional electrode 13, wherein the electrical connection required for this purpose is in Fig. 1 are not shown or are shown only very schematically. The control device 21 is explained in more detail in connection with the following exemplary embodiments.

[0061] The device 100 with the extraction electrode 7 and the field emission element 1 can serve as a charge carrier source, for example, for applications requiring free charge carriers. Such applications include, for example, an X-ray tube, electron beam lithography or e-beam lithography, gas ion mobility spectroscopy, a smoke detector, mass spectrometry, or similar applications. The device 100 can be designed, in particular, as a device for the aforementioned purposes and thus, apart from the aforementioned components, can also comprise further components, such as those required for an electron beam lithography device or an X-ray tube.

[0062] As in Fig. As shown in Figure 1, the device 100 has an additional electrode 13, which is a most positive or most negative electrode. The source current of charge carriers, formed by the charge carrier portion 3 that has passed through the extraction electrode 7, can impinge on the additional electrode 13.

[0063] The additional electrode 13 can be electrically connected to an electrode terminal of a power source 15. The electrode can be supplied with the electrode voltage Velectrode via the power source 15, so that this voltage is applied to the additional electrode 13. The electrode voltage Velectrode is preferably the most positive or the most negative. The power source 15 can be provided by the power source 9 or another power source, wherein the electrode terminal is expediently different from the extraction electrode terminal. The horizontal line in Fig. 1 symbolizes the ground connection 151, 91 of the respective power source.

[0064] The charge carriers impinging on the additional electrode 13 result in an electrode current Ielectrode flowing in the additional electrode 13. If the charge carriers are electrons, the technical current flow direction is from the power source 15 to the additional electrode 13. Above and below, the electrode current and electrode voltage may also be referred to as anode current and anode voltage.

[0065] The charge carriers striking the additional electrode 13 can generate electromagnetic radiation, in particular X-rays. The radiation can be emitted by the additional electrode 13. This is Fig. 1 by the arrow with reference number 17, which symbolizes the generated electromagnetic radiation. In the case of an electron or ion source, the emission of the electrons preferably occurs either through an opening of the additional electrode 13 or by transmission through a closed additional electrode 13.

[0066] Alternatively or additionally, a variation electrode 75 can be provided, as in all other embodiments. The variation electrode 75 is preferably arranged between the additional electrode 13 and the extraction electrode 7. The charge carrier flow thus first passes through the variation electrode 75 and can only later reach the additional electrode 13. Furthermore, the device 100 can have an optical element 76, in particular an electron-optical element, of the charge carrier optics. Such optical elements 76 include, for example, pinholes, Wehnelt cylinders, capacitor plates, and / or magnets such as ring magnets.

[0067] This optical element 76 can be arranged downstream of the extraction electrode 7 and / or the variation electrode 75, as seen along the charge carrier flow. For example, the optical element 76 of the charge carrier optics can be used to appropriately focus the source stream of charge carriers so that it impinges on the additional electrode 13 in a defined area.

[0068] Deviating from the representation in Fig. 1, the variation electrode 75 and the optical element 76 can also be arranged in the reverse order. If the device 100 is used as an electron or ion source, so that the charge carriers are transmitted through the additional electrode 13, then in particular the optical element 76 can also be arranged after the additional electrode 13 and / or the variation electrode 75 can be used as an electron or ion transmission window. The same applies to all other embodiments.

[0069] In Fig. 1, it is optionally possible to control and influence the behavior of, in particular, the field emission element 1 and / or the control device 21 by means of a temperature unit 48 and alternatively or additionally by means of a lighting unit 49. With the temperature unit 48, a temperature of, for example, the field emission element 1 and / or the control device 21 can be set. With the lighting unit 49, the field emission element 1 and / or the control device 21 can preferably be illuminated, for example with near-ultraviolet radiation, visible light or infrared radiation. Such a temperature unit 48 and / or lighting unit 49, which in Fig. 1 is only shown very schematically, can also be present in all other embodiments.

[0070] The following describes measures by which the source current of charge carriers can be stabilized. One or more of these measures are present in each of the exemplary embodiments. This particularly means reducing fluctuations in the electrode current, in the generated electromagnetic radiation, and / or the width of the distribution of the kinetic energy of the electrons.

[0071] Using one or a combination of these measures, a particularly stable charge carrier source or radiation source can be provided. The proposed charge carrier source is particularly suitable for use in a device designed as a radiation source, since the stable source current also ensures that the generated radiation exhibits only very small intensity fluctuations. Charge carrier optics, which serve, for example, for electrostatically focusing the emitted charge carriers, can also be designed in a simplified manner due to the advantageously narrow spectrum of the charge carrier kinetic energy distribution.

[0072] In order to keep the source current of charge carriers as uniform and constant as possible, or to modulate it in a specific way, various control concepts are proposed here. One group of concepts is based on the idea of ​​using a variable for controlling the source current that is characteristic of the source current itself. In particular, this is a controlled variable that takes into account that a portion of the extracted charge carriers does not contribute to the source current but leads to currents in one or more of the electrodes, as explained above. Another group of concepts relates to the fact that, in addition to the additional electrode 13, the variation electrode 75 is provided, by means of which the kinetic energy of the extracted charge carriers of the source current can be changed.

[0073] The source current can be stabilized by controlling the controlled variable Q, which is characteristic of the source current. The variation electrode 75 can be used in addition to control via the controlled variable Q, or also with other control systems, particularly those that result in the kinetic energy of the extracted charge carriers fluctuating due to the control, so that the fluctuations can be reduced via the variation electrode 75. The variation electrode 75 can therefore lead to an optical element 76 of the charge carrier optics arranged downstream of the variation electrode 75 being easier to shape due to the narrower distribution of the kinetic energy of the charge carriers after the kinetic energy has been influenced by the variation electrode 75.

[0074] The following applies in particular to the controlled variable Q: Q = Iemission - Iext - Ielectrode - Ivar - Iopt, where Ivar is the current through the optional variation electrode 75 and Iopt is the current through the optional optical element 76. Thus, Q is, in particular, the emission current minus all electrode currents up to and including the most positive electrode. To control the current in a selected intermediate range, only a portion of the available electrodes can be used to form the difference.

[0075] The source current can be measured by measuring the electrode current.

[0076] An embodiment of such a control using a controlled variable that does not depend on the extraction current is shown in Fig. 2 is shown schematically.

[0077] The basic elements of the device 100 shown are already in Fig. 1. Reference is made to this point. In the Fig. In the concept proposed in 2, a variable characteristic of the electrode current Ielectrode is used as the controlled variable. The electrode current is caused by the charge carriers of the source current 3 in the additional electrode 13, which is why the electrode current is characteristic of the source current.

[0078] To detect the electrode current, the device 100 has an electrode current detection element 19, formed, for example, by a measuring resistor Rmess. The voltage drop Vactual across the electrode current detection element 19 is characteristic of the electrode current Ielectrode.

[0079] Furthermore, the device 100 includes the control device 21. In the present example, the control device 21 is formed by an operational amplifier, or OPA for short. Alternatively or in addition to an OPA, RF amplifiers or ASICs can also be used for the control device 21 or for other optionally present OPA. RF stands for radio frequency, and ASIC stands for application-specific integrated circuit.

[0080] The part of the device 100 that generates the extracted charge carriers can be influenced via the control device 21. The control device 21 has an input 23 to which the controlled variable Q, i.e., Vact, is supplied. The input 23 is thus coupled to the branch of the additional electrode 13. A setpoint Vsetpoint is also supplied to the control device 21 or is stored therein. The control device 21 can have a further input 25 for supplying the setpoint. The setpoint can be constant over time or, if modulation of the source current is desired, can change over time in a predetermined manner. The control device 21 also has an output 27.

[0081] The device 100 also has a control element 29, in particular a current control element. The charge carrier flow from the supply power source 9 to the field emission element 1 can be controlled via the control element 29. The output 27 of the control device 21 is expediently electrically connected to the control element 29. During operation of the device, the controlled variable Q is compared with the setpoint value, and if a difference exists, the control element 29 is controlled such that the emission current changes in such a way that the source current 3 behaves such that the predetermined setpoint value is reached. In particular, if the control device 21 is designed as an operational amplifier, the controlled variable is compared with the setpoint value, and if a difference exists, the difference is output with the predetermined and set gain and / or the set integration constant and / or differentiation constant (PID controller).Any other type of control is also conceivable, such as fuzzy logic.

[0082] The control element 29 can be implemented, for example, as a transistor, in particular as a field-effect transistor, such as a MOSFET. Preferably, the output 27 of the operational amplifier 21 is connected to the gate of the transistor of the control element 29, so that charge is integrated onto the gate of the transistor until the source current 3 is so large that the actual value assumes the specified target value.

[0083] The extraction voltage Vext is particularly preferably selected such that the control device 21 has sufficient reserve to regulate the emission current. This means, for example, that the operating point of the field emission element 1 is significantly above the desired current. Thus, the control reduces the current at the operating point, and a change in the field emission properties can be counteracted.

[0084] In the embodiment according to Fig. 2, the additional electrode 13 is impermeable to the source current 3. Deviating from the illustration in Fig. 2, it is likewise possible for the additional electrode 13 to be permeable or partially permeable to the source current 3. If the additional electrode 13 is permeable, it can either be penetrated by a portion of the source current, for example if it is very thin, or, like the electrode 7, which is designed, for example, as a ring electrode or a grid electrode, it can have one or more recesses through which at least a portion of the charge carriers of the source current pass. A further portion of these charge carriers can be absorbed in the additional electrode 13 and contribute to the electrode current, which is then characteristic of the source current. Targeted, intended ionization, for example of gas in the space between the additional electrode 13 and the field emission element 1, is also conceivable.

[0085] The control variant described above, in which the controlled variable Q is obtained directly from the electrode current at the additional electrode 13, is also referred to as electrode current control or anode current control or most positive electrode current control.

[0086] A variant of the electrode current control is in Fig. 3. In contrast to Fig. 2 is in Fig. 3, the charge carrier inflow to the field emission element 1 is not regulated, but the extraction electrode 7 is controlled by means of the control device 21, so that the extraction electrode 7 serves as a control element 29. Since the emission current, caused by the totality of the extracted charge carriers, is controlled via the extraction electrode 7, the source current is also indirectly controlled, since this is caused by a part of the extracted charge carriers, namely the portion 3 according to Fig. 2. In this case, a comparison between an actual value Vist and a setpoint value Vsoll is preferably carried out in the control device 21, as in connection with Fig. 2 explained.

[0087] However, the electrode current at the additional electrode 13 cannot always be directly measured. This applies, for example, when the device 100 is operated as a pure charge carrier source or is a device for ionizing gas. Therefore, a control concept is described below in which the proportion of the source current 3 to the extracted charge carriers is calculated from the available variables. Such a concept is based on Fig. 4 and is also referred to as differential current control.

[0088] This figure also corresponds in its essential components to the Fig. 2. The block with the additional electrode 13 is shown in dashed lines.

[0089] Since the electrode current does not need to be detected, no electrode current detection element is provided. In the concept according to Fig. 4, however, two other currents are detected, namely the extraction current Iext and the emission current Iemission. For this purpose, the device 100 has an extraction current detection element 31, for example, a resistor. The extraction current can be detected via the voltage drop across the resistor. Furthermore, the device 100 has an emission current detection element 33, for example, a resistor. The emission current can be detected via the voltage drop across the emission current detection element 33.

[0090] A controlled variable determination element 35 is connected upstream of the control device 21 and in particular to the input 23. The controlled variable determination element 35 can have one or more inputs 37, 39 and can be designed, for example, as an OPV. Furthermore, the controlled variable determination element 35 has an output 41. This is preferably conductively connected, in particular directly connected, to the input 23 of the control device 21. The controlled variable determination element 35 can, for example, be a difference-forming element in which the difference between a variable characteristic of the emission current, for example the voltage drop across the emission current detection element 33, and a variable characteristic of the extraction current, for example the voltage drop across the extraction current detection element 31, is formed.In addition, some or more of the currents in other electrodes can be used to determine the controlled variable, for example by Q = Iemission -Iext - Ielectrode - Ivar - Iopt, compare the explanations for . Fig. 1.

[0091] One of the inputs, in the exemplary embodiment, input 39, is conductively connected to the extraction current detection element 31, so that the value characteristic of the extraction current is supplied to this input. The same applies to input 37 and the emission current detection element 33. The controlled variable Q, which is supplied to input 23 of the control device 21, is accordingly a function of the difference between values ​​characterizing the extraction current and the emission current. For example, the value characteristic of the extraction current, which is usually smaller, is subtracted from the value characteristic of the emission current.The difference between these values ​​can be used to estimate the electrode current, which may not be easily determined. This is because, as shown, the source current 3 of charge carriers, which can generate the electrode current, ideally corresponds approximately to the emission current, except for the charge carrier component 5, which is responsible for the extraction current. If additional electrodes are present, differential control can also be achieved using other control variables, as explained above.

[0092] In Fig. Figure 5 illustrates another embodiment of device 100. The voltage drop across the control element 29, particularly formed by the MOSFET control, results in a broadened spectrum of the kinetic energy of the emitted charge carriers due to the variation of the drop. This artificially broadens the spectrum of pure field emission, which is narrow compared to thermal emission. This can significantly complicate the electrostatic focusing of the emitted charge carriers, especially electrons.

[0093] This voltage drop can be compensated for by an additional electrode 77 for a so-called tetrode configuration and by an adding element 78. Although the kinetic energy of the charge carriers then varies at the additional electrode 13, an optional electron optic (not shown) could be located exactly between the additional electrode 13 and the additional electrode 77, or ionization could occur, for example, exactly between these electrodes, meaning that a constant current and constant kinetic energy are required in this area. The additional electrode 77 can be the variation electrode 75, or the additional electrode 77 can be different from the variation electrode 75. The adding element 78 is preferably formed by an adder, the two inputs of which are connected in particular to the field emission element 1 and the extraction electrode 7, and the output of which is connected to the additional electrode 77 and / or the variation electrode 75.Deviating from the representation in . Fig. 5, the additional electrode 13 may be identical to the further electrode 77.

[0094] In Fig. 5 shows the tetrode configuration together with the adding element 78 for the electrode current control of the Fig. 2 and Fig. 3. Such an arrangement can also be used for differential current control of the Fig. 4 are used. In a variation of the embodiments of the Fig. 3 and Fig. 4, an OPV with differential control can also be used directly. The different aspects in the embodiments of the Fig. 3 to 5 can also be advantageously combined with each other.

[0095] In the example of Fig. 6 shows optional, additional aspects which may also be present individually or in combination in all other embodiments.

[0096] In a first aspect, temperature compensation is implemented. For this purpose, it may be expedient to compensate for the temperature dependence, particularly of the current measurements, using a suitable circuit or to minimize it using suitable components. For this purpose, the electrode current detection element 19 can, for example, comprise or consist of a series circuit with two resistors with opposite temperature coefficients.

[0097] In a second aspect, the gain can be reduced. That is, the OPV of the control device 21 is operated with a reduced gain through feedback with a resistor 45 and an additional series resistor 43.

[0098] In a third aspect, transistor 29 and regulator 21 can be decoupled. For this purpose, a resistor 44 is located between the gate of the MOSFET of control element 29 and the output of control device 21. This allows for more stable operation, in particular, reduced susceptibility to oscillation.

[0099] In a fourth aspect, the controller can be adapted, for example, the integration component by an additional capacitor 46 in the feedback of the control device 21, in particular in addition to a gate capacitance of the control element 29. Alternatively, other controller concepts, such as fuzzy logic, are also conceivable.

[0100] In a fifth aspect, all in Fig. The bandwidth can be adjusted using the additional components shown in Figure 6 or other components.

[0101] The additional aspects in Fig. 6 for an electrode current control can be used in the same way for the differential current control, compare Fig. 4, are used.

[0102] In all embodiments for electrode current control, a further OPV circuit (such as the OPV 22 with the resistors 41 and 42 in Fig. 6) should be used with the lowest possible input current to decouple the current measurements and / or to adjust the voltages to favorable values. Decoupling can also be achieved with resistors between the circuit components.

[0103] In the examples of the Fig. Figures 7 to 9 each show a so-called high-side measurement, i.e., a measurement on the "high" side, using the example of detecting the extractor current. This type of current measurement is also possible for all other currents, such as the current at electrode 13. In this case, another, in particular a second, current measurement is performed for differential current regulation on the "high" side of the power supply. This is achieved by a series of PowerMOS transistors 29', M. A single stage allows a voltage dependent on the permissible drain-source voltage of the respective transistor. With currently available transistors, for example, approximately 400 V per stage can be achieved. For higher voltages, several stages M1-M4 are provided accordingly. In the embodiments of the Fig. 7 to 9 each have a current detection element 31 on the high side.

[0104] The high-side measurement is first performed using Fig. 7, without the actual PowerMOS stages. The extraction current leads to a voltage drop V_sense across a measuring resistor Rmess, 31. Similar to the control concept presented in the previous figures, the op-amp U1 regulates the current through Rin via transistor 29' such that no voltage drop occurs across the op-amp inputs. This means that in the regulated state, the voltage drop across Rin is equal to the voltage drop V_sense across the measuring element Rmess, 31. The resulting current through transistor 29' is thus: I=V_sense / R_in.

[0105] To minimize measurement error, an op-amp with the highest possible impedance is preferred, so that the current flowing into the input of the op-amp U1 is as small as possible. The current can be transformed into a voltage using Rout. The resulting voltage is thus: V_out=V_sense∗R_out / R_in .

[0106] The area of ​​application of the Fig. 7 is limited by the voltage capability of the amplifier U1 and the transistor 29. In Fig. Figure 8 shows a concept that allows the maximum permissible voltage to be increased.

[0107] To decouple the high voltage from the low voltage side, additional transistors M, especially MOSFETs, can be installed between transistor 29' and resistor Rout and the negative supply input of the op-amp U1 and ground. The MOSFETs used are preferably high-voltage-resistant p-channel MOSFETs, for example, of the type FQD3P50. To achieve higher voltages than the maximum drain-source voltage of a MOSFET, this voltage decoupling can be constructed several times in succession, see transistors M1-M4. In the case of Fig. Figure 8 shows a two-stage configuration. In this case, the maximum isolation voltage for an n-stage configuration with n=2 is V_max=n*V_DSS. Diode D1 can, by choosing a suitable breakdown voltage, provide the necessary voltage difference to supply the op-amp U1.

[0108] The voltage Vout of the high-side measurement can now be used for one of the presented types of control. Fig. 9 shows a single-stage version of the high-side measurement in conjunction with the differential current control from Fig. 4. The high-side measurement is also compatible with other control types, such as the emission current control from Fig. 2 and Fig. 3, possible.

[0109] For this purpose, it is advisable to adapt the voltage Vout to the requirements of the further circuit and to decouple it from it by means of a non-inverting amplifier 36. After this optional intermediate stage, for example, exactly as in Fig. 4 the difference between the emission current and the extractor current measured on the high side is regulated.

[0110] In Fig. 10 illustrates a structure of the device 100 in the area of ​​the field emission element 1. The field emission element 1 is formed by an emitter array 101 with a plurality of tips, see the schematic cross-sectional drawing in Fig. 10A and the scanning electron microscope images in the Fig. 10B and Fig. 10C. The tips, for example, are etched from silicon and optionally coated.

[0111] A metallic contact 105 of the emitter structure is located beneath the emitter array 101. The emitter array 101 is electrically insulated from the extraction electrode 7 in the form of an extraction grid by a layer 103 of an insulating material, for example, mica, in particular with a thickness between 20 µm and 150 µm. A metallic contact 107 of the extraction grid is located on the extraction electrode 7. Typical values ​​for the electrical voltage between the extraction electrode 7 and the tips in such a configuration are, in particular, between 200 V and 1000 V.

[0112] In the embodiment of the Fig. 11, the device 100 comprises a field emission element 1 with integrated extraction electrode 7, see the schematic representation in Fig. 11A and the scanning electron microscope images in the Fig. 11B to 11D.

[0113] An array 101 of tips for the field emission element 1 is provided. This array 101 is etched, for example, from a semiconductor material such as silicon. The insulating layer 103, which is made, for example, of silicon dioxide and in particular has a thickness of approximately 1 µm, is applied to the emitter array 101. The metallization 107 for the extraction electrode 7 is formed as a grid and is made, for example, of gold, in particular with a thickness of approximately 200 nm. The tips of the array 101 can project beyond the extraction electrode 7. Overall, the extraction electrode 7 and the tips can be located in a recess of a carrier formed by the material for the tips. The emitter array 101 and thus the field emission element 1 are electrically contacted via the planar metallization 105. Typical voltage values ​​for this structure are in the range between 20 V and 200 V.

[0114] In Fig. 11B shows the electrical connections 108 for the metallizations 105, 107, which are designed, for example, as bond pads. The emitter array 101 with the grid-shaped extraction electrode 7 is shown in Fig. 11C. A single tip of the field emission element 1 with the associated extraction electrode 7 is shown in Fig. 11D shown.

[0115] The other parts or all electronic components of the device 100 such as the transistors, operational amplifiers, diodes, adders and / or resistors can be used in both cases, an integrated (see Fig. 11) or mechanically applied extractor electrode (see Fig. 10), in the carrier made of a semiconducting material, for example Si, Ge or SiGe, from which the tips are made, are monolithically integrated. Furthermore, several of the other optionally present electrodes 13, 75, 76, 77 can also be formed by a thin-film structure, similar to the application of the gate metal in Fig. 11, monolithically integrated. It is also conceivable to implement the presented circuit concepts partially or completely integrated for each individual emitter or group of individual emitters.

[0116] In Fig. Figure 12 shows measurement curves for the electrode current Ielectrode, the emission current Iemission, and the extraction current Iext of an exemplary device 100. The electrode current Ielectrode is approximately 40% of the emission current Iemission, but may also be at least 0.05% or 0.2% and / or at most 100% or 2% or 1% of the extraction current Iext.

[0117] In the Fig. 13 and Fig. Figure 14 illustrates the emission current Iemission and the flux of X-ray photons θX-Ray as a function of time, with device 100 used as an X-ray radiation source. The emission current Iemission is plotted in A or µA, and the flux θX-Ray is plotted in counting units or thousands of counting units per second (count rate). Field emission element 1 is an array of 100 individual emitters made of p-doped silicon columns with a multitude of tips realized by an etching process, and is operated in saturation.

[0118] In Fig. Figure 13 shows that in uncontrolled operation, the X-ray flux stability is lower than the emission current stability. In controlled operation, the X-ray flux stability increases, but the emission current stability decreases. The present invention is based in particular on this result.

[0119] In Fig.Figure 14 shows that the control can also be used to switch the emission current on and off. For this purpose, the setpoint V_actual was modulated over time (on / off). In the switched-on state, control is immediately applied to the setpoint. The control time for switching on is less than 10 ms. The switch-off time is not determined by a physical effect, but by the circuit. Through circuit optimization, both the switch-on and switch-off times can be further reduced. In the optimized case, a switch-off time equal to the switch-on time can be achieved. List of reference symbols 100 device 1 field emission element (cathode) 3 Charge carriers of the source current 5 Charge carriers of the extraction current 7 Extraction electrode 11 Supply connection 13 additional electrode (anode / most positive electrode) 15 Power source 17 electromagnetic radiation 19 Electrode current detection element 21 Control device 22 additional operational amplifiers 23 Input of the control device 25 additional input of the control device 27 Output of the control device 29 Control 31 Extraction current detection element 33 Emission current detection element 35 Control variable determination element 36 non-inverting amplifier 41, 42, 43 electrical resistors 44, 45 electrical resistors 46 Capacitor 48 temperature units 49 Lighting unit 75 Variation electrode 76 electron-optical element 77 additional electrodes 78 Adding element 91, 151 Ground connection 101 Emitter array 103 Layer of an insulating material 105 metallic contact 107 metallic contact 108 electrical connections Ielectrode Electrode current in the additional electrode Iemission emission current Iext extraction current Iopt current through the optical element Ivar current through the variation electrode M power transistor R resistance U1 additional amplifierVelectrode electrode voltage at the electrode Vext extraction voltage Vv supply voltage

Claims

[1] Device (100) for generating a source current of charge carriers by means of field emission or field ionization, comprising: - at least one field emission element (1) from which the charge carriers emerge during operation of the device (100), which lead to an emission current in the field emission element (1), - at least one extraction electrode (7) for applying an extraction voltage (Vext) to extract the charge carriers from the field emission element (1), wherein a first part of the extracted charge carriers contributes to the source current, and a second part of the extracted charge carriers impinges on the extraction electrode (7) and leads to an extraction current (Iext) in the extraction electrode (7), - an additional electrode (13) onto which the source current of charge carriers impinges and which contributes to an electrode current in the additional electrode (13), wherein the additional electrode (13) is a most positive or most negative electrode of the device (100) and the extraction electrode (7) is located between the additional electrode (13) and the field emission element (1), - a control device (21), and - at least one control element (29) which is designed to change the emission current, wherein the control device (21) is designed to control the control element (29), wherein the control device (21) is designed to reduce fluctuations in a controlled variable Q characteristic of the source current during operation of the device (100), and the controlled variable Q is a function of at least the electrode current, so that a setpoint is supplied to the control device (21) or is stored therein, and during operation of the device (100), the controlled variable Q is compared with the setpoint and, if there is a difference, the control element (29) is controlled such that the emission current changes in such a way that the source current (3) behaves such that the predetermined setpoint is reached. [2] Device (100) according to the preceding claim, with at least one variation electrode (75) which is designed to change a kinetic energy of the extracted charge carriers of the source current by means of a variation voltage applied to the variation electrode (75), wherein the device (100) is designed such that the variation voltage is varied as a function of a variable K which is characteristic of the kinetic energy of the extracted charge carriers. [3] Device (100) according to the preceding claim, wherein K is obtained by means of a voltage drop across the control element (29) and the device (100) is designed such that with increasing voltage drop across the control element (29) the variation voltage is increased and with decreasing voltage drop across the control element (29) the variation voltage is reduced. [4] Device (100) according to one of the two preceding claims, comprising an adding element (78) conductively connected to the variation electrode (75), the device being arranged such that the variation voltage is obtained by addition of the extraction voltage and the voltage drop across the control element. [5] Device (100) according to one of the preceding claims, further comprising a ground terminal (91) and an electrical supply power source (9), wherein the control element (29) is arranged either in a conduction path between the supply power source (9) and the field emission element (1) or in a conduction path between the ground terminal (91) and the field emission element (1). [6] Device (100) according to one of the preceding claims, wherein the control element (29) is electrically conductively connected to the extraction electrode (7) and is designed to change the emission current by changing the extraction voltage. [7] Device (100) according to one of the preceding claims, which is designed such that during operation of the device (100) a temporal distribution of the quantity Q characteristic of the source current has a standard deviation S Q which is less than or equal to 0.5%. [8] Device (100) according to one of the preceding claims, comprising a difference forming element, where Q is a function of the difference between the emission current and the extraction current. [9] Device (100) according to claim 1, further comprising at least one electron-optical element (76) and at least one variation electrode (75) and at least one additional electrode (13), where Q is equal to the emission current reduced by the currents through the variation electrode (75), the electron-optical element (76) and the additional electrode (13) or reduced by a part of these currents. [10] Device (100) according to one of the preceding claims, further comprising a plurality of power transistors (M, PMOS) which are at least partially electrically connected in series, wherein the power transistors (M, PMOS) are mounted between the field emission element (1) and the extraction electrode (7) or the additional electrode (13) to realize the current measurement and isolation of voltages above the dielectric strength of the components used. [11] Device (100) according to one of the preceding claims, which is designed as a radiation source, wherein the electromagnetic radiation to be emitted by the radiation source is generated by means of the source current impinging on the additional electrode (13), and wherein the radiation is X-ray radiation. [12] Device (100) according to one of claims 1 to 11, wherein the device (100) is designed as a charge carrier source. [13] Device (100) according to one of the preceding claims, the device comprising one, an arbitrarily selected plurality or all of the following current sensing elements: Emission current detection element (33), Extraction current detection element (31), Electrode current detection element (19), Charge carrier optical current detection element, Variation electrode current detection element, wherein the emission current detection element (33) has at least one resistor and the voltage drop across the emission current detection element (33) characterizes the emission current, and / or wherein the extraction current detection element (31) has at least one resistor, wherein the voltage drop across the extraction current detection element (31) characterizes the extraction current, and / or wherein the electrode current detection element (19) has at least one resistor, wherein the voltage drop across the electrode current detection element (31) characterizes the electrode current. [14] Device (100) according to the preceding claim, wherein one, an arbitrarily selected plurality or all of the following current sensing elements have temperature compensation: extraction current sensing element (31), emission current sensing element (33), electrode current sensing element (19), charge carrier optical current sensing element, variation electrode current sensing element, wherein one, an arbitrarily selected plurality or all of these current sensing elements have a plurality of resistors connected in series, wherein one of the resistors is a PTC resistor and another of the resistors is an NTC resistor. [15] Method for stabilising a source current of charge carriers emitted by a field emission element (1), which is carried out with a device (100) according to one of the preceding claims, wherein the source current is formed by the charge carriers, which have already passed an extraction electrode (7) by means of which the charge carriers were extracted from the field emission element (1), and wherein in the method a variable Q characteristic of the source current is determined and this variable is transferred to a control device (21) as a controlled variable.

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