Apparatus and method for applying accelerated electrons to a gaseous medium

Abstract

The invention relates to an apparatus and a method for applying accelerated electrons to a gaseous medium by means of an electron beam generator having at least one cathode (107) for emitting electrons and at least one electron exit window (104; 304), wherein a) the at least one cathode (107) is ring-shaped and the at least one electron exit window (104; 304) is configured as a first hollow cylinder which is ring-shaped, wherein the ring-shaped electron exit window (104; 304) configured as a first hollow cylinder forms an inner wall of a ring-shaped outer housing (101; 301) of the electron beam generator, wherein the electrons emitted by the cathode (107) are accelerated towards a ring axis (103; 303) of the ring-shaped outer housing (101; 301); b) a second hollow cylinder (112; 312) which is ring-shaped is arranged within the electron exit window (104; 304) configured as a first hollow cylinder, which delimits a ring-shaped space (113; 313) between the first hollow cylinder and the second hollow cylinder (112; 312); c) a cooling gas is guided through the ring-shaped space (113; 313) between the first hollow cylinder and the second hollow cylinder (112; 312); and d) a gaseous medium to which the accelerated electrons are to be applied is guided through the second hollow cylinder (112; 312).

Description

TECHNICAL FIELD

[0001] The present invention relates to an apparatus and a method for applying accelerated electrons to a gaseous medium, thereby causing a chemical transformation or a gas synthesis of the gaseous medium. In the meaning of the present invention, a gaseous medium is to be understood as a gas, a gas mixture, a vapor and an aerosol, all of which can also contain solid particles, for example exhaust gases from combustion engines, industrial plants or thermal power plants. BACKGROUND

[0002] For decades, electron beam technology has been used on an industrial scale for the modification of chemical materials and for the disinfection or sterilization of surfaces. Products can be processed economically at atmospheric pressure for which the electrons must first be released in a vacuum, then accelerated and finally decoupled into the treatment zone through an electron beam exit window, usually a thin metal foil. Acceleration voltages of more than 100 kV are usually required to penetrate electron exit windows that can be used on a large scale and are sufficiently robust, and to ensure that a sufficient depth of treatment is achieved in the product.

[0003] Various methods and beam sources have been well established for the surface treatment of flat products such as sheets and strips, while the overall treatment of shaped bodies, bulk materials and fluids remains problematic. Thus, the uniform impingement of electrons on curved surfaces is geometrically problematic due to the shadowing effect and the projection conditions of local differences. The locally varying energy transfer along the path as the electrons propagate in the absorbing medium represents another source of dose inhomogeneity, which also affects the treatment of fluids (gaseous and liquid media).

[0004] For already existing source systems, such as axial irradiators with fast deflection units or strip irradiators with elongated cathodes, both of which are operated using heated thermionic cathodes, the application of a uniform dose of energy to the overall product treatment of shaped bodies or in flowing fluids is only very cumbersome, possibly with additional devices or equipment and / or considerable technical outlay. Electron beam sources based on thermionic emitters are also mechanically complex, difficult to scale and require complex electron aperture voltage sources and electron aperture vacuum systems. If the electron beam exit window is damaged, resulting in a vacuum collapse, the cathode system will be irreversibly damaged, with extremely high repair costs.

[0005] The patent document DE 199 42 142 A1 discloses an apparatus in which a bulk material is guided through an electron beam device in multiple free falls and subjected to accelerated electrons. Due to the multiple passes, in addition to the intermittent mixing of the bulk material, the probability of particles of the bulk material being hit by accelerated electrons from all sides is extremely high in this embodiment. If a fluid is guided through the treatment zone instead of a bulk material, the energy dose of the overall transmission can also be equalized by multiple passes. However, multiple passes require a large amount of time when the treatment process is carried out.

[0006] Patent document DE 10 2006 012 666 A1 specifies another solution comprising three axial emitters with associated deflection control and three likewise associated electron exit windows. The three electron exit windows are arranged such that they completely surround a free space in the shape of a triangle. If a substrate is guided through this free space, the substrate can be subjected to accelerated electrons over its entire cross section in one treatment process. However, the equipment expenditure in this embodiment is very high, which means that this solution is also extremely expensive. Nevertheless, as soon as the substrate does not have a triangular cross section which coincides with the free space surrounded by the three electron exit windows, the dose distribution of the accelerated electrons impinging on the substrate surface becomes inhomogeneous. In a single pass, the distance to the electron exit windows over the cross section of the treatment zone is very different, so that it is not possible to transfer a homogeneous dose into the fluid volume.

[0007] To solve this problem, patent document WO 01 / 97954 A2 proposes to arrange a plurality of planar electron beam generators along one side of a rectangular treatment chamber and to arrange a plurality of planar electron beam generators symmetrically opposite to each other on the other side. This superposition improves the dose homogeneity in the volume, but again at the expense of high equipment costs. Furthermore, the fluid is usually guided in a pipe having a circular cross section. The transition to the rectangular cross section of the treatment chamber is associated with the formation of eddies which on the one hand increase the flow resistance and on the other hand cause a dose inhomogeneity due to locally varying flow velocities or stationary eddy structures.

[0008] Patent document DE 10 2013 111 650 B3 discloses a ring-shaped device for generating accelerated electrons, wherein all necessary components such as cathodes, anodes and electron exit windows are in a ring-shaped configuration, with which a ring-shaped electron beam can be formed, wherein the accelerated electrons move into the ring. With the aid of this device, for example, a strip-shaped substrate which moves through the ring-shaped opening of the device can be subjected to accelerated electrons over the entire circumference with respect to the cross section of the substrate. The device known from DE 10 2013 111 650 B3 usually has a circular ring shape, but can also be configured in any other ring shape. With the aid of this ring-shaped device, not only rod-shaped substrates can be subjected to accelerated electrons, but also, for example, bulk materials which are guided as free fall through the ring-shaped opening or gases which flow through the ring-shaped opening. However, this device is not suitable for treating hot gases, for example exhaust gases from internal combustion engines or chemical synthesis gases, since the electron exit windows come into contact with the hot gases when the hot gases flow through the ring-shaped opening, but cannot withstand the resulting thermal or corrosive stress.

[0009] Finally, reference is made to the article "Application of radio-frequency electron gun to waste treatment, Radiation Physics and Chemistry" by J. M. Connelly et al. (2019, Book ID 108440), which discloses a star-shaped electron beam source in which a plurality of high-energy axial radiators are uniformly arranged around a liquid or gaseous medium to be treated, such that the beam directions of all axial radiators intersect at a point. With this device it is also possible to treat gases, wherein the gas temperature has an upper limit, since the electron exit windows of the axial radiators are directly exposed to the gas to be treated. Furthermore, this device requires a high level of technical complexity for the control and operation of at least five electron radiators. This is further exacerbated by the need to work with high electron energies in order to penetrate and uniformly impinge on a large cross section, and in particular also by the increased costs for shielding against X-rays as an unavoidable damaging effect. SUMMARY

[0010] In view of this, the technical problem of the present invention is to propose a device and a method for applying and chemically converting gaseous media by means of accelerated electrons, with which the disadvantages of the prior art can be overcome. In particular, with the device according to the invention and the method according to the invention, it is possible to apply accelerated electrons and chemically convert hot gaseous media, such as exhaust gases from internal combustion engines or chemical synthesis gases, with only a small technical outlay.

[0011] First of all, the device according to the application can have all the features of the ring-shaped electron beam source as disclosed in DE 10 2013 11 650 B3 and DE 10 2013 113668 B3. Thus, the device according to the application for applying accelerated electrons to a gas, a gas mixture, a vapor and an aerosol, summarized under the term "gaseous medium", possibly carrying solid particles, comprises at least one electron beam generator having at least one cathode for emitting electrons and at least one electron exit window, wherein the at least one cathode is ring-shaped and the at least one electron exit window is configured as a first hollow cylinder which is ring-shaped. The ring-shaped electron exit window configured as a first hollow cylinder forms an inner wall of a ring-shaped housing of the electron beam generator, wherein the electrons emitted by the cathode can be accelerated towards a ring axis of the ring-shaped housing. According to the application, a second hollow cylinder which is ring-shaped is arranged within the electron exit window configured as a first hollow cylinder, which delimits a ring-shaped space between the first hollow cylinder and the second hollow cylinder. In the device according to the application, a cooling gas flows through the ring-shaped space between the first hollow cylinder and the second hollow cylinder, and the gaseous medium to which the accelerated electrons are to be applied flows through the second hollow cylinder. Due to the cooling gas flowing through the ring-shaped space between the first hollow cylinder and the second hollow cylinder, overheating of the electron exit window and contamination by condensate and solid particles and the resulting damage can be avoided, so that also a hot gaseous medium carrying particles can be treated with the device according to the application.

[0012] It should be expressly pointed out in this connection that the term "ring-shaped" in the context of the present application is not limited to a circular ring in all the ring-shaped devices and elements described hereinafter. The cross section of the ring-shaped piece of the device according to the application is circular in the preferred embodiment of the application, but can also have any other geometric shape in the broadest sense of the application.

[0013] In the method according to the application for applying accelerated electrons to a gaseous medium and for chemically converting the resulting gaseous medium, an electron beam generator is used which has at least one cathode for emitting electrons and at least one electron exit window. In this case, the at least one cathode (107) is ring-shaped and the at least one electron exit window is configured as a first hollow cylinder which is ring-shaped, wherein the ring-shaped electron exit window configured as a first hollow cylinder forms an inner wall of a ring-shaped housing of the electron beam generator, wherein the electrons emitted by the cathode are accelerated towards a ring axis of the ring-shaped housing. Furthermore, a second hollow cylinder which is ring-shaped is arranged within the electron exit window configured as a first hollow cylinder, which delimits a ring-shaped space between the first hollow cylinder and the second hollow cylinder. Furthermore, a cooling gas is guided to flow through the ring-shaped space between the first hollow cylinder and the second hollow cylinder, and a gas or a gas mixture to which the accelerated electrons are to be applied is guided to flow through the second hollow cylinder. BRIEF DESCRIPTION OF DRAWINGS

[0014] The application will be described in detail below with reference to the embodiments. In the drawings:

[0015] Figure 1 A schematic perspective sectional view of a device according to the application is shown;

[0016] Figure 2 A schematic top view of the ring of a device according to the application is shown; Figure 1

[0017] Figure 3 A schematic sectional view of an alternative device according to the application is shown;

[0018] Figure 4 A schematic sectional view of another alternative device according to the application is shown;

[0019] Figure 5 A schematic sectional view of a device according to the application with an electronic reflector is shown;

[0020] Figure 6 A schematic sectional view of a device according to the application with an alternative shape- conforming aerodynamic electronic reflector and with means for injecting a chemical agent into a fluid flow and with means for effecting an additional gas discharge is shown; and

[0021] Figure 7 A schematic sectional view of a device according to the application with alternative means for injecting a chemical agent into a gas flow of a gaseous medium to be subjected to accelerated electrons and with means for effecting an additional gas discharge is shown. DETAILED DESCRIPTION

[0022] In this connection, for a more clear understanding of the application, the terms "ring column" and "ring disc" are defined with respect to ring-shaped objects. A quantity is obtained by subtracting the inner radius of a circular ring from the outer radius of the circular ring. If this quantity is smaller than the extension of the ring in the direction of its ring axis, the ring is configured as a ring column. Conversely, if this quantity is larger than the extension of the ring in the direction of its ring axis, the ring is configured as a ring disc.

[0023] Figure 1 and Figure 2 The same device 100 according to the application is shown schematically in Figure 1 a perspective sectional view thereof is shown in Figure 2 a top view of the ring thereof. The device according to the application first comprises a ring-shaped housing 101 which delimits a space 102 which can be evacuated at least in one region, the space 102 being divided into spaces 102a and 102b which can be evacuated. Due to the shape of the housing, the space 102 which can be evacuated is also configured in a ring shape. In Figure 1 and​Figure 2 In the embodiment of Fig. 1, the housing 101 is configured as a radially symmetrical ring having a ring axis 103. All elements described below in connection with the device 100 and referred to as ring-shaped are also configured as radially symmetrical rings and have the same ring axis 103. Inside the ring of the housing 101, the housing 101 is configured as an electron exit window 104 in the form of a first hollow cylinder, i.e. viewed in the direction of electron exit, the surface of the electron exit window 104 is perpendicular to the interior of the ring, as in the case of a hollow cylinder, it is parallel to the ring axis 103. The working gas enters the evacuatable space 102 through at least one inlet (not shown in the figures) in the housing 101 and the vacuum in the evacuatable space 102 is maintained by at least one pump device (also not shown in the figures) in the range of 0.1 Pa to 20 Pa, preferably in the range of 1 Pa to 3 Pa. Figure 1 Figure 1

[0024] The device according to the application also has at least one first cathode and at least one first anode between which a glow discharge plasma can be generated in the evacuatable space 102a by an applicable first voltage provided by a first power supply device. In the exemplary embodiment, two wall regions shaped as ring segments in the housing 101 are configured as first cathodes 105a and 105b, which delimit the space 102a on opposite sides. In the device 100, the housing 101 and the first cathodes 105a, 105b have the same potential, which is also the ground potential of the device 100.

[0025] The first anode of the device according to the application comprises a plurality of wire electrodes, which extend through the space 102a and, in a housing in the shape of a circular ring, such as the housing 101, are preferably arranged on the same radius and equidistant from one another around the axis 103. In comparison with the housing 101, the wire electrodes 111 can have a slightly positive voltage potential in the range of +0.25 kV to +5.0 kV, which penetrates the housing 101 and the first cathodes 105a, 105b in an electrically insulating manner. Due to the applied voltage between the wire electrodes 111 and the first cathodes 105a, 105b, a plasma is formed in the space 102a. Therefore, in the following, the space 102a is also referred to as plasma space 102a.

[0026] The device according to the application also has at least one second cathode and at least one second anode between which a second voltage is applied by a second power supply device. In the device 100, the cathode 107 is configured as the second cathode and the grid-shaped anode 108 is configured as the second anode. Both the cathode 107 and the anode 108 are configured in the shape of a ring.

[0027] ​​In the device according to the application, the second cathode represents a cathode for emitting secondary electrons, which are subsequently accelerated, for which a high voltage potential is applied, preferably in the range of -80 kV to -300 kV. The second cathode 107 is electrically insulated from the housing 101 by means of the insulator 109.

[0028] In Figure 1 In the described embodiment of the application, the second anode 108 and the first cathodes 105a, 105b have the same potential, which is formed as a ground potential. Alternatively, the second anode and the first cathodes can also have different potentials.

[0029] By applying a high voltage potential in the range of -80 kV to -300 kV, the ions from the plasma 106 in the space 102a are accelerated by the grid-shaped second anode 108 in the direction of the second cathode 107. Here, the ions hit the surface area 110 of the second cathode 107, the surface of which is perpendicular to the ring interior of the housing, i.e. parallel to the ring axis 103. When the ions hit the surface area 110, the ions have already dropped a potential difference which essentially corresponds to the acceleration voltage of the device 100. When the ions hit, the energy of the ions is released in an extremely thin edge layer in the surface area 110 of the cathode 107, which leads to the release of secondary electrons. At the above-mentioned voltage of the second cathode 107, the ratio of released electrons to hitting ions is in the order of 10, so that the production of accelerated electrons is very efficient in this way. The applied electric field accelerates the generated secondary electrons considerably and causes them to fly through the grid-shaped anode 108, which is configured in the form of a ring column, and the plasma 106 in the space 102a. After passing through the electron exit window 104, which can be configured, for example, as a thin metal foil, the electrons enter the free space surrounded by the ring-shaped housing 101, in which a higher pressure can prevail than in the space 102. All materials known from the prior art for electron exit windows, such as titanium, can be used as the material for the electron exit window 104. In addition, in order to provide the electron exit window 104 with higher mechanical stability, it is advantageously provided with a support grid, as is also known from the prior art.

[0030] Due to the ring shape of the above-mentioned components of the device according to the application, the curtain of accelerated electrons is also generated in the form of a ring, the direction of movement of the accelerated electrons being oriented into the free space surrounded by the housing. In the radially symmetrical device according to the application, such as the device 100, the direction of movement of the accelerated electrons is preferably parallel to the ring axis 103.

[0031] For the sake of completeness, it should be noted here that the device according to the application also has means for cooling the device, which are known from the prior art for devices for generating accelerated electrons. For example, the means for cooling the device according to the application can comprise cooling channels which extend inside the insulator 109 and through which a cooling medium flows.

[0032] In the device according to the application, the second anode 108 is preferably configured as a ring-shaped column in the form of a grid and represents a spatial boundary between the evacuatable spaces 102a and 102b, which second anode 108 serves three basic purposes. On the one hand, the second anode 108 causes ions extracted from the plasma to be accelerated in the direction of the second cathode due to the voltage difference between the second anode 108 and the second cathode 107. On the other hand, it also causes secondary electrons generated by ion bombardment to be accelerated in the direction of the electron exit window 104. The grid structure of the second anode 108 is configured parallel to the secondary electron emission surface 110 of the second cathode 107, thus forming an electric field, so that the trajectories of the accelerated secondary electrons are also essentially parallel. In addition, the second anode 108 screens the plasma from the voltage potential of the second cathode 107, preventing too many ions from drifting in the direction of the second cathode 107, thus helping to maintain the plasma 106 in the space 102a.

[0033] According to the application, a second hollow cylinder 112 is arranged inside the electron exit window 104 configured as a first hollow cylinder, wherein the diameter of the second hollow cylinder 112 is smaller than the diameter of the ring-shaped electron exit window 104, so that the second hollow cylinder 112 delimits a ring-shaped space 113 between the second hollow cylinder 112 and the electron exit window 104 configured as a first hollow cylinder.

[0034] The second hollow cylinder 112 delimits a free space 114 inside the hollow cylinder 112. A gaseous medium to be supplied with accelerated electrons flows through the free space 114 enclosed by the second hollow cylinder 112, which gaseous medium can also contain solid particles, for example in the case of exhaust gases from an internal combustion engine.

[0035] The electron exit window of the electron beam generator only accepts heat input from the bombardment of charged particles inside the electron beam generator. If the above-mentioned electron beam generator 100 is then additionally used to treat hot gases flowing through the free space 114 delimited by the second hollow cylinder 112 as well, the electron exit window 104 is subjected to additional thermal stress, which can lead to damage to the electron exit window 104. Therefore, according to the application, a cooling gas flows through the ring-shaped space 113, which dissipates thermal energy and thus protects the electron exit window 104 from thermal damage.

[0036] In one embodiment, the cylinder wall of the second hollow cylinder 112 has a plurality of openings. Accelerated electrons can pass unhindered through these openings into the process space in front of the electron exit window 104. For example, the cylinder wall of the second hollow cylinder 112 can have a grid structure or be made of a wire mesh. The grid structure or wire mesh of the second hollow cylinder 112 can be made, for example, of a heat-resistant material having a melting point above 1250°C. With a device according to the application thus configured, hot gaseous media can also be processed. Such a heat-resistant material can be made, for example, of at least one metal or metal compound, for example yttrium-stabilized zirconium oxide or stainless steel or a refractory metal. The second hollow cylinder can be made entirely of one material or, alternatively, comprise a base body made, for example, of a metal or metal compound, at least one layer of a heat-resistant material having a melting point above 1250°C being deposited on the inner wall thereof.

[0037] With such a second hollow cylinder, gaseous media having a high temperature, for example exhaust gases from internal combustion engines or chemical synthesis gases, can also be subjected to accelerated electrons and chemically converted.

[0038] If the cylinder wall of the second hollow cylinder 112 has a plurality of openings, it is advantageous if the gaseous medium to be processed flowing through the interior free space 114 of the second hollow cylinder 112 and the gas flowing through the annular space 113 both have the same flow direction. If both gas flows have opposite flow directions, this leads to a stronger turbulence of the two gas flows at the openings of the cylinder wall of the second hollow cylinder 112, which leads to a heat build-up due to poor gas flow. It is likewise advantageous if the same pressure conditions are established in the annular space 113 as in the interior of the second hollow cylinder 112, so that no mixing of the two gas flows occurs at the openings of the cylinder wall of the second hollow cylinder 112 and no particles enter the annular space 113.

[0039] The device according to the application can therefore have a first regulating circuit, by means of which the pressure in the annular space 113 can be regulated. Such a first regulating circuit can comprise, for example, a first sensor for detecting a first actual value of the pressure in the interior of the hollow cylinder 112, a second sensor for detecting a second actual value of the pressure in the annular space 113, an evaluation device and a blower or pump device for generating a cooling gas flow through the annular space 113. Here, the first actual value is compared with the second actual value by means of the evaluation device, and depending on the comparison result, the electrical power of the blower or pump device for generating the cooling gas flow is adjusted, for example. If the evaluation device determines that the pressure in the annular space 113 is too low, the electrical power of the blower or pump device can be increased, for example, until the pressure in the annular space 113 is exactly equal to the pressure in the interior of the second hollow cylinder 112. Conversely, if the evaluation device determines that the pressure in the annular space 113 is higher than the pressure in the interior of the second hollow cylinder 112, the electrical power of the blower or pump device can be reduced.

[0040] Alternatively, if the regulating circuit comprises a device for regulating the pressure inside the second hollow cylinder 112, such as a blower or pump device, the pressure inside the second hollow cylinder 112 can also be adapted to the pressure in the annular space 113 by such a regulating circuit, whereby a gas or gas mixture flow to be treated is generated through the second hollow cylinder and the blower or pump device is controlled in accordance with the pressure in the annular space 113 and the determined pressure value inside the second hollow cylinder.

[0041] As mentioned above, the pressure in the annular space 113 and the pressure inside the second hollow cylinder 112 are regulated to be the same by the first regulating circuit. However, the pressure in the annular space 113 can also be regulated to be slightly higher than the pressure inside the second hollow cylinder 112 by such a first regulating circuit, so that a small amount of cooling gas is diverted into the free space 114 inside the second hollow cylinder 112. Such a procedure avoids in any case that a possibly particle-laden hot gaseous medium flowing through the second hollow cylinder enters the annular space 113, thereby avoiding that the electron exit window 104 is subjected to additional thermal loads or contamination.

[0042] Protecting the electron exit window 104 of the device according to the application from overheating and reliable operation of the device according to the application, it is important that by means of the second regulation circuit the local power density distribution, which is adapted to the hydrodynamic and thermodynamic parameters of the gaseous medium, i.e. the energy per unit of time and volume from the electrons to the gas particles at a given location, can additionally or alternatively be ensured primarily. For this purpose, the acceleration voltage applied between the second cathode 107 and the second anode 108 is regulated in accordance with the composition and the temperature of the gaseous medium flowing through the second hollow cylinder 112 and the pressure and flow profile prevailing therein. What is utilized here is that the electron range in the relevant energy range (<1 MeV) is approximately a quadratic function of the acceleration voltage and is inversely proportional to the mass density of the energy-absorbing medium. The composition of the gaseous medium flowing in the second hollow cylinder 112, in particular the relevant average mass of the gas particles, the pressure and the temperature determine its effective mass density. The higher the average particle mass and the pressure and the lower the temperature of the gaseous medium, the higher its mass density and the higher the required acceleration voltage. The flow profile, i.e. the radial velocity distribution of the gas particles, is also relevant for determining a favorable acceleration voltage. This is greatest in the center of the free space 114 and smallest at the inner wall of the second hollow cylinder 112. In order to ensure that the energy dose is transferred to the gas particles as uniformly as possible, the acceleration voltage must be set so high that the resulting electron range and electron scattering contribute to a reduction of the maximum power density distribution radially outwards in the center of the free space 114. The way in which the upper limit of the acceleration voltage is set is that the electron range must not be chosen to be greater than the diameter of the second hollow cylinder 112 in order to avoid thermal loads in the diametrically opposite region of the electron exit window 104. If, in the further course of the process, the power density distribution, which is characterized by the prevailing gas composition or average particle mass m, pressure p, temperature T and gas throughput D, is to be maintained as set by the selection of a favorable acceleration voltage as a working point, the acceleration voltage must be readjusted in accordance with the changing state variables of the gaseous medium. In an embodiment of this second regulation circuit, therefore, the actual values of the gas composition, the pressure, the temperature and the flow rate in the free space 114 are recorded continuously but always at a uniform point in time by means of corresponding sensors and processed in an evaluation device. A higher gas flow rate generally leads to a greater ratio of minimum flow rate to maximum flow rate, which is often compensated by increasing the acceleration voltage. The average particle mass m can be determined from the gas composition and then multiplied by the quotient of the actual pressure value p and the actual temperature value T. If this product decreases over time, the acceleration voltage U B is also adjusted according to a predefined functional relationship U B = U B(m, p, T, D) decreases; conversely, if the product increases over time, the acceleration voltage will increase according to the same functional relationship. If one or more of the state variables (m, p, T, D) are assumed to be independent of time, a simplified implementation of the second regulating circuit can be obtained. For example, if the gas composition, the temperature and the flow rate are known not to change during the specific process, the functional relationship for controlling the acceleration voltage simplifies to U B = U B (p), in which case the acceleration voltage increases or decreases approximately in direct proportion to the pressure.

[0043] By virtue of the stable operation of the device according to the application, it is also possible, by means of a third regulating circuit, to ensure that the energy dose E specified by the technology is transferred to the gaseous medium with the smallest possible fluctuations, by virtue of which third regulating circuit the beam current I B emitted by the second cathode 107 in the device according to the application is regulated as a function of the composition, the pressure, the temperature and the flow rate of the gaseous medium inside the second hollow cylinder 112. As shown in the description of the second regulating circuit, the advantageous acceleration voltage U B of the initial operating point and the effective mass density of the gaseous medium can be determined as a function of the gas composition, the pressure, the temperature and the flow rate. The product of the effective mass density and the average flow rate inside the second hollow cylinder 112 quantifies the integral mass flow F of the gaseous medium. In order to achieve the required physical activation and subsequent chemical conversion of the gaseous medium, a certain energy dose E is required, which is usually specified as a technology parameter. From P G = E · F, it is thus possible to calculate the power with which the accelerated electrons are converted in the gaseous medium and introduced into the free space 114. For the acceleration voltage U B set by the second regulating circuit, this can be achieved by the electron current I G = l G = P B / U B emitted by the second cathode 107. For this purpose, the beam current I V must increase by the amount of electron losses (which can be attributed to the leakage current I B ) on the path from the surface of the second cathode 107 to the inner wall of the second hollow cylinder 112: I G = I V . Such losses occur due to elastic and inelastic scattering of the accelerated electrons when they pass through the evacuatable spaces 102a and 102b, the plasma 106, the second anode 108, the electron exit window 104, the annular space 113 and the second hollow cylinder 112. In principle, the functional dependence of the leakage current Iv on the various apparatus and process parameters can be recorded in a sufficiently precise manner by analysis or by calibration measurements.

[0044] If a constant energy dose E is to be applied to the gaseous medium inside the second hollow cylinder 112, regardless of the gas throughput, the advantageous acceleration voltage must first be determined and continuously tracked in accordance with the criteria and algorithms described for the second regulation circuit, and the beam current can then also be adjusted in accordance with the gas composition and pressure, temperature and flow rate inside the second hollow cylinder 112. In one embodiment of this third regulation circuit, the actual values of the gas composition, pressure, temperature and flow rate inside the second hollow cylinder 112 are therefore recorded continuously but always at uniform times by the corresponding associated sensors and processed in an evaluation device, in particular the instantaneous mass flow of the gaseous medium calculated from the above-mentioned relationships. If this increases over time, the beam power P G in the gaseous medium introduced in the device according to the application also increases proportionally therewith, and vice versa, since the instantaneous acceleration voltage U B and the (calculated or tabulated) leakage current I V are set in accordance with the beam current I B . In a simplified embodiment of this third regulation circuit, the sensors of the second regulation circuit and the measured values provided thereby are used to calculate the setting variables of the third regulation circuit, and one or more process parameters are kept constant, similar to the design description of the second regulation circuit.

[0045] Alternatively, it is advantageous for the application if the energy dose transferred to the gaseous medium does not remain constant, but is adjusted in accordance with the chemically converted expected composition of the gaseous medium to be treated.

[0046] This is important, for example, for the purification of SO X , NO X and partially combusted hydrocarbons from engine exhaust gases. The pollutant content in the exhaust gas does not change strictly in proportion to the gas flow, but rather in accordance with its own individual characteristics for the respective pollutant species and as a function of the instantaneous engine load. In addition, the proportionate change between the pollutants also leads to a change in the energy dose required for neutralizing the pollutants. It is known that the exhaust gas purification goals achievable by means of electronic treatment are to bring all the pollutants treated below the non-critical limit values. This requires a certain energy dose. If this value is exceeded, the energy consumption of the electron source increases further, but the purification effect does not increase significantly. This means that under these conditions the energy efficiency is affected.

[0047] To avoid this, the third regulating circuit can be designed in such a way that, after the energy transfer zone in front of the electron exit window 104 and / or if the gaseous medium, which has been converted by the electron treatment, is passed through further chemical or physical treatment devices, a sensor for the gaseous components is arranged and the beam current is regulated in order to achieve a specified composition of the gaseous medium after passing through the entire arrangement of the individual process modules. Thus, in one embodiment, at least one sensor for detecting the actual value of the composition of the gaseous medium is arranged in the gas flow zone of the gaseous medium in which the gas flow has passed through the electron exit window 104, and the beam current is controlled in accordance with the actual value of this sensor.

[0048] This design of the third regulating circuit is not only important for waste gas treatment, but also, in particular, for chemical synthesis processes with gaseous media using accelerated electron treatment.

[0049] In the device according to the application, the acceleration voltage of the electron beam generator and / or the beam current of the electron beam generator can be regulated in accordance with the gas composition and / or the pressure and / or the temperature and / or the flow profile inside the second hollow cylinder 112 and / or after passing through the downstream treatment devices.

[0050] Figure 3 Fig. 3 shows a sectional view of an alternative embodiment of a device 300 according to the application. The device 300 can, for example, comprise all components described for the device 100 in Figs. 1 and 2. Thus, the device 300 also comprises an annular housing 301, an annular electron exit window 304 configured as a first hollow cylinder, a second hollow cylinder 312 made of a stainless steel mesh, which encloses a free space 314 through which a gaseous medium to be treated with accelerated electrons flows from bottom to top. Furthermore, the electron exit window 304 and the second hollow cylinder 312 delimit an annular space 313 through which a cooling gas also flows from bottom to top. All annular or cylindrical components of the device 300 are configured rotationally symmetrical about an axis 303. Figure 1 and Figure 2 Fig. 3 shows a sectional view of an alternative embodiment of a device 300 according to the application. The device 300 can, for example, comprise all components described for the device 100 in Figs. 1 and 2. Thus, the device 300 also comprises an annular housing 301, an annular electron exit window 304 configured as a first hollow cylinder, a second hollow cylinder 312 made of a stainless steel mesh, which encloses a free space 314 through which a gaseous medium to be treated with accelerated electrons flows from bottom to top. Furthermore, the electron exit window 304 and the second hollow cylinder 312 delimit an annular space 313 through which a cooling gas also flows from bottom to top. All annular or cylindrical components of the device 300 are configured rotationally symmetrical about an axis 303.

[0051] The wall element 315 extends the annular space 313 in the direction of the axis 303 until the annular space opens into a gas supply conduit 316 at the lower end and into a gas discharge conduit 317 at the upper end. For example, the second hollow cylinder can be connected to a pipe by means of a flange 318, through which the gaseous medium to be treated with the device 300 is guided to the device 300 or away from the device 300 again after passing through the device 300.

[0052] Figure 4A cross-sectional view of a further alternative apparatus 400 according to the application is schematically shown in Fig. 4, in which the cooling gas no longer flows through the annular space 313 in only one direction, but the cooling gas is introduced into the annular space 313 under pressure both at the lower end of the annular space 313 through the gas supply duct 316 and at the upper end of the annular space 313 through the gas discharge duct 416. Here, the cooling gas is pressed through the openings of the second hollow cylinder 312 into the interior of the second hollow cylinder 312 and is discharged there with the flow of the gaseous medium to be treated.

[0053] The apparatus 400 according to the application described here can also preferably be used for the chemical synthesis of gaseous media. It is advantageous here to use as cooling gas a gas having at least one chemical element which acts as a reactant in the chemical conversion of the gaseous medium, and / or for the second hollow cylinder 312 to be made of at least one catalytically active material (for example nickel, yttrium-stabilized zirconium oxide or a lanthanide compound) which promotes the intended chemical reaction, or to be coated at least on its inner wall with this material.

[0054] Figure 5 A cross-sectional view of an apparatus 500 according to the application is schematically shown in Fig. 5, which apparatus 500 can have Figures 1 to 4 The features of the apparatus according to the application described in connection with Figs. 1 to 4 apply to the apparatus 500 described here as well. In addition, the apparatus 500 has an electron reflector 519 arranged inside the second hollow cylinder 312. The electron reflector 519 can for example be constructed as a hollow cylinder or alternatively be made of a solid material in the form of a rod. The electron reflector 519 can be made of a homogeneous material or a composite material, for example of one metal as a base body and another metal or compound which completely or partially covers its surface. On the outer wall of the electron reflector 519, the high-energy electrons are partially elastically backscattered back into the free space 314, which increases the energy dose applied to the gaseous medium to be treated, but most importantly makes their radial distribution more uniform.

[0055] By choosing a suitable material, in particular for the surface of the electron reflector 519, the percentage of electrons backscattered back into the free space 314 (which is related to the beam current) can be increased. Refractory metals for example have a significantly higher electron backscattering rate than light metals. In one embodiment, the surface material of the electron reflector 519 comprises a refractory metal or a light metal.

[0056] In addition, by choosing a suitable material, in particular for the surface of the electron reflector 519, the yield of secondary electrons can be increased. When high-energy beam electrons hit, secondary electrons are knocked out of the edge layer close to the surface of the electron reflector 519 and released into the free space 314. These secondary electrons have a very low energy (typically < 50 eV) and therefore a very high interaction cross section with atoms or molecules, in turn enabling in particular the excitation of chemical conversions.

[0057] It is also advantageous if the electron reflector 519 is made of or coated with a heat-resistant material and / or a catalytically active material (e.g. nickel, yttrium-stabilized zirconium oxide or a lanthanide compound) so that at least the surface of the electron reflector comprises these materials.

[0058] Depending on the intended application, it can be advantageous to cool the electron reflector 519 completely or partially. It is particularly advantageous if the heat dissipation from its surface can be set or even regulated to have a desired temperature. Due to the energy input from the electron beam, this temperature will generally be higher than the temperature of the gaseous medium provided. This elevated temperature increases the effectiveness of any catalyst applied and improves the cleanliness of the catalyst by avoiding condensation or combustion of contaminants.

[0059] However, the overall advantage of this arrangement is that the energy transfer from the accelerated electrons to the gaseous medium no longer takes place over the entire diameter of the circle formed by the second hollow cylinder 312, but can take place only over the distance between the second hollow cylinder 312 and the electron reflector 519. Assuming a moderate flow rate in the annular space 313 (much less than the speed of sound), and thus negligible density differences of the gaseous medium flowing around the electron reflector, it is possible to significantly reduce the required electron range and thus significantly reduce the acceleration voltage to be applied. This reduces both the cost of the beam source and its supply unit and the effort required to shield against parasitic X-rays.

[0060] The device 500 can optionally comprise a power supply device 520 by which a voltage is provided and which is connected between the second hollow cylinder 312 and the electron reflector 519. The voltage provided by the power supply device 520 can be a direct voltage or an alternating voltage. In particular, in one embodiment, the power supply device 520 can generate voltage pulses, the energy storage of which can be increased by passive energy accumulators (coils or capacitors) that can be recharged during the pause of the pulses.

[0061] In a preferred embodiment, the second hollow cylinder 312 has the ground potential of the device 500. Figure 5 In the illustration, one pole of the power supply device 520 is electrically connected to the wall element 315, which is in electrically conductive connection with the second hollow cylinder 312, so that this pole of the power supply device is also in electrically conductive connection with the second hollow cylinder 312. By this voltage between the second hollow cylinder 312 and the electron reflector 519, the beam electrons that have penetrated into the free space 314, as well as the secondary electrons that they release when decelerated in ionizing collisions, experience an additional acceleration, which, under suitable geometric and electrical dimensions, can lead to a further electron multiplication in the sense of a non-self-sustaining gas discharge. The energy absorbed by all the electrons can then also be transferred into the gaseous medium and can be used in an economically advantageous way to increase the overall effective energy dose.

[0062] Of particular importance for the energy efficiency (defined as the ratio of the theoretical reaction enthalpy to the actual energy consumption required to actually achieve the intended chemical conversion) in the chemical conversion processes excited by gas discharges is the reduced electric field strength (defined as the quotient of the electric field strength E in the discharge space and the particle density n).

[0063] The interaction between high-energy electrons and gaseous media leads to ionization, dissociation, electronic excitation states and intramolecular vibrations of its atoms or molecules. The manifestation and existence of the individual interaction effects (professionally referred to as interaction cross sections) depend in a characteristic and material-specific manner on the electron energy, which in turn depends on the reduced electric field strength.

[0064] In particular, molecular reactions (decomposition or synthesis) can be stimulated by exciting the degrees of freedom of intramolecular vibrations. The energy provided by the discharge device in the vibration excitation primarily serves to overcome the energy barriers that inhibit the chemical conversion processes, and in the case of endothermic reactions, effectively feeds the absorbed energy into the reaction products, with only a slight increase in the gas temperature, and the energy efficiency of the conversion processes is increased.

[0065] On the other hand, the ions, radicals and electronically excited species formed by the impact of high-energy electrons are of particular importance for the decomposition of pollutants in waste gases.

[0066] In this way, the efficiency of the intended chemical conversion processes (not only in terms of energy consumption, but also in terms of the degree of conversion that can be achieved) can be increased by selectively transferring energy to the mechanisms that primarily promote the intended reactions. The specific way in which this is achieved is by selecting the reduced electric field strength of the gas discharge such that it corresponds to the maximum of its interaction cross section, while at this value no or only slight excitation of other interaction processes occurs.

[0067] In the case of a given particle density n and a given geometric distance d between the electrodes driving the gas discharge, the intended reduced electric field strength E / n D can be set by appropriately selecting the supply voltage U SOLL of the discharge. Here, the simple example of a plate capacitor is chosen to illustrate the dimensioning principle, for which it follows that U D = n · d E / n SOLL .

[0068] While the voltage of the additional gas discharge is suitably determined in accordance with the above optimization criteria, the energy dose transferred to the gaseous medium can be set or changed by appropriately selecting the discharge current, for which, in turn, a regulation circuit can be integrated to ensure the desired composition of the treated gaseous medium.

[0069] The duration of the energy transfer to the raw material also plays a decisive role for many chemical conversions. A variation of the duration of the energy transfer offers the opportunity to suppress undesired recombination or secondary reactions of the reaction products formed.

[0070] For this reason, a pulsed energy transfer is particularly advantageous and can be influenced very specifically. The pulsed voltage establishes a favorable reduction in the electric field strength, the pulse duration determines the duration of the energy transfer, and the pulsed current finally determines the energy dose transferred. In order to ensure the desired composition of the gaseous medium after the treatment, all of the above-mentioned parameters can be used individually or in combination as manipulated variables in the regulating circuit.

[0071] The pulse sequence of the power supply device 520 can be designed such that the overall required energy dose is delivered as a sum of the individual pulses. Alternatively, the sequence can also be associated with the flow rate of the gaseous medium, such that a new pulse is only generated when the previously applied volume of gas has completely passed through the energy transfer zone in front of the electron exit window 304 and has left again.

[0072] The implementation of passive energy stores (coils or capacitors) that release their stored energy into the pulses and are then recharged with moderate power during the pause of the pulses makes it possible to increase the cost efficiency of the power supply device 520 for the gas discharge.

[0073] In order to further increase the efficiency, it is advantageous to release (i.e. pulse) the electron beam consisting of accelerated electrons that maintains and stabilizes the gas discharge only shortly before and during the intended pulse duration of the pulse of the power supply device 520 and to synchronize this pulse with the pulse sequence of the gas discharge. Thus, in one embodiment, the electron beam formed by the accelerated electrons is generated in the form of pulses. Optionally, the pulses of the electron beam can be synchronized with the voltage pulses of the voltage generated by the power supply device 520.

[0074] Figure 6 Fig. 6 shows a cross-sectional view of a device 600 according to the present application, which differs from the device 500 in Figure 5 Fig. 6 shows a cross-sectional view of a device 600 according to the present application, which differs from the device 500 in Figure 5 In contrast to the rod-shaped electron reflector 519 in Fig. 5, the electron reflector 619 has a droplet shape and provides less flow resistance and less vortex formation for the gaseous medium flowing from bottom to top that is to be treated with accelerated electrons.

[0075] In one embodiment, the electron reflector 519 or 619 has at least one opening in the lower region through which a (gaseous or liquid) chemical agent 621 can be injected axially downward into the free space 314, thereby mixing with the gaseous medium and then passing upward through the radiation zone with the gas flow.

[0076] Alternatively, the chemical reagent can also be injected primarily radially inward into the free space 314 by at least one feed device separate from the electron reflector 519 or 619, wherein the opening of the feed device is advantageously placed directed so as to deliver the chemical reagent injected into the free space 314 to the gas stream of the gaseous medium treated with accelerated electrons, before the gas stream passes through the treatment space in front of the electron exit window 304. In this way, compounds that are subjected to accelerated electrons and thus activated, ionized or split can form new compounds with the injected chemical reagent.

[0077] Finally, Figure 7 Fig. 6 shows a cross-sectional view of a device 600 according to the present application, which has Figure 6 Fig. 6 shows a cross-sectional view of a device 600 according to the present application, which has Figure 6 Fig. 6 shows a cross-sectional view of a device 600 according to the present application, which has

[0078] In this embodiment, the physical activation and subsequent chemical reaction of the injected reagent 721 with the gaseous medium is caused by a non-self-sustained gas discharge between the electron reflector 719 and an additional electrode 722, which is electrically insulated from ground potential and in particular from the potential of the second hollow cylinder 312. The additional electrode 722 is arranged in the gas stream of the gaseous medium to be subjected to accelerated electrons above the electron exit window 304 and in a ring-shaped configuration. The voltage for this non-self-sustained gas discharge is provided by a power supply device 720. The advantage of the non-self-sustained gas discharge above the electron exit window 304 is that the additional thermal load associated with the gas discharge does not act on the second hollow cylinder 312 or the electron exit window 304. Advantageously, the additional electrode 722 is cooled.

[0079] The device and the method according to the present application can be used, for example, for applying accelerated electrons to exhaust gases originating from internal combustion engines, industrial facilities or thermal power plants in order to decompose compounds that can be contained in the exhaust gases and are harmful to the environment and, optionally, to convert them into compounds that are less harmful or compounds that are easily retained in downstream dust filters or wet-chemical gas scrubbers by adding chemical reagents.

[0080] Another possible application is in the field of molecular chemistry and the plasma-chemical synthesis of gases and liquids.

[0081] Due to the cooling gas between the electron exit window and the gaseous medium to be treated with accelerated electrons, hot exhaust gases or synthesis gases can also be guided through the ring source according to the application. It is also advantageous that only one electron beam source is required, and only one associated peripheral equipment, such as a vacuum pump, a high-voltage power supply and a controller for the process parameters to be set.

Claims

1. An apparatus for applying accelerated electrons to a gaseous medium, comprising at least one electron beam generator having at least one cathode (107) for emitting electrons and at least one electron exit window (104; 304). Its features are, a) The at least one cathode (107) is annular, and the at least one electron emission window (104; 304) is constructed as an annular first hollow cylinder, wherein the annular electron emission window (104; 304) constructed as a first hollow cylinder forms the inner wall of the annular shell (101; 301) of the electron beam generator, wherein electrons emitted by the cathode (107) can be accelerated toward the annular axis (103; 303) of the annular shell (101; 301); b) An annular second hollow cylinder (112; 312) is arranged inside an electron exit window (104; 304) constructed as a first hollow cylinder, which defines an annular space (113; 313) between the first hollow cylinder and the second hollow cylinder (112; 312). c) Cooling gas flows through the annular space (113; 313) between the first hollow cylinder and the second hollow cylinder (112; 312); and d) The gaseous medium to which the accelerated electrons are to be applied flows through the second hollow cylinder (112; 312).

2. The device according to claim 1, characterized in that, The gaseous medium to which the accelerating electrons are to be applied is exhaust gas from an internal combustion engine, industrial facility, or thermoelectric facility.

3. The device according to claim 1, characterized in that, The gaseous medium to which accelerated electrons are applied is a raw material used in chemical synthesis.

4. The device according to any one of claims 1 to 3, characterized in that, The second hollow cylinder (112; 312) has multiple openings in its column wall.

5. The device according to claim 4, characterized in that, The second hollow cylinder (112; 312) has a grid structure or is constructed as a wire mesh on its column wall.

6. The device according to any one of claims 1 to 3, characterized in that, The walls of the second hollow cylinder (112; 312) are made of at least one material with a melting point higher than 1250°C.

7. The device according to any one of claims 1 to 3, characterized in that, The second hollow cylinder contains, at least on its inner wall, a material that acts as a catalyst in the chemical reaction of the gaseous medium.

8. The device according to claim 7, characterized in that, The catalytic material is nickel, yttrium-stabilized zirconium oxide, or a lanthanide compound.

9. The device according to any one of claims 1 to 3, characterized in that, The cooling gas and the gaseous medium to which accelerated electrons are applied have the same flow direction.

10. The device according to claim 4, characterized in that, The cooling gas flows into the free space from the annular space (113; 313) through multiple openings in the second hollow cylinder.

11. The device according to any one of claims 1 to 3, characterized in that... A first regulating circuit for regulating the internal pressure of the annular space (113; 313) includes a first sensor for detecting a first actual value of the internal pressure of the second hollow cylinder (112; 312), a second sensor for detecting a second actual value of the internal pressure of the annular space (113; 313), an evaluation device, and a blower or pump device for generating a cooling airflow through the annular space (113; 313).

12. The device according to claim 10, characterized in that... Electron reflectors (519; 619; 719) are arranged in the free space.

13. The device according to claim 12, characterized in that, The surface material of the electron reflector comprises refractory metals or light metals.

14. The device according to claim 12 or 13, characterized in that, The surface of the electron reflector contains nickel, yttrium-stabilized zirconium oxide, or lanthanide compounds.

15. The device according to claim 12 or 13, characterized in that... The power supply has one pole electrically connected to an electron reflector arranged in the free space, and the other pole electrically connected to the second hollow cylinder.

16. The device according to claim 12 or 13, characterized in that... The power supply device has one pole electrically connected to an electron reflector arranged in the free space, and the other pole electrically connected to an electrode (722) arranged in the gaseous medium but electrically insulated from the second hollow cylinder.

17. A method for applying accelerated electrons to a gaseous medium by means of an electron beam generator, said electron beam generator having at least one cathode (107) for emitting electrons and at least one electron exit window (104; 304). Its features are, a) At least one cathode (107) is constructed in an annular shape, and at least one electron emission window (104; 304) is constructed as an annular first hollow cylinder, wherein the annular electron emission window (104; 304) constructed as a first hollow cylinder forms the inner wall of the annular shell (101; 301) of the electron beam generator, wherein electrons emitted by the cathode (107) are accelerated toward the annular axis (103; 303) of the annular shell (101; 301); b) An annular second hollow cylinder (112; 312) is arranged within an electron exit window (104; 304) constructed as a first hollow cylinder, which defines an annular space (113; 313) between the first hollow cylinder and the second hollow cylinder (112; 312). c) Guiding the cooling gas through the annular space (113; 313) between the first hollow cylinder and the second hollow cylinder (112; 312); and d) Guide the gaseous medium through which the accelerated electrons are to be applied to flow through the second hollow cylinder (112; 312).

18. The method according to claim 17, characterized in that, The accelerating voltage of the electron beam generator is adjusted according to the gas composition and / or pressure and / or temperature and / or flow distribution inside the second hollow cylinder (112; 312).

19. The method according to claim 17 or 18, characterized in that, The beam current of the electron beam generator is adjusted according to the gas composition and / or pressure and / or temperature and / or flow rate inside the second hollow cylinder (112; 312).

20. The method according to claim 17 or 18, characterized in that, The gas used as a cooling gas contains at least one chemical element that undergoes chemical transformation as a reactant in the gaseous medium.

21. The method according to claim 17 or 18, characterized in that, The cooling gas flows into the free space from the annular space (113; 313) through multiple openings in the second hollow cylinder, and an electron reflector (519; 619; 719) is arranged in the free space.

22. The method according to claim 21, characterized in that, One pole of the power supply is electrically connected to an electron reflector arranged in the free space, while the other pole is electrically connected to the second hollow cylinder.

23. The method according to claim 21, characterized in that, One pole of the power supply is electrically connected to an electron reflector disposed in the second hollow cylinder, while the other pole is electrically connected to an electrode (722) disposed in the gaseous medium but electrically insulated from the second hollow cylinder.

24. The method according to claim 17 or 18, characterized in that, An electron beam, formed by accelerated electrons, is generated in pulses.

25. The method according to claim 22 or 23, characterized in that, The power supply device is used to generate DC voltage, AC voltage, or voltage pulses.

26. The method according to claim 25, characterized in that, The pulses of the electron beam are synchronized with the voltage pulses of the power supply device.

27. The method according to claim 17 or 18, characterized in that, At least one sensor for detecting the actual value of the composition of the gaseous medium is arranged in the gas flow zone of the gaseous medium, in which the gas flow has passed through the electron exit window and the beam is controlled according to the actual value of the sensor.

28. The method according to claim 21, characterized in that, The electron reflectors (519; 619; 719) are cooled.

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