RU2572104C1 - Generator of electromagnetic pulses - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: invention is referred to method of electromagnetic pulses (EMP) generation and may be used in pulse detection and ranging and pulsed filed testing of radio electronic equipment. The device includes photocathode and mesh anode with their operating surfaces made as surfaces of rotation bodies are connected to voltage source, pulse or pulse-periodic light source, converter converting radiation of the light source into spherically diverging wave of optical, ultraviolet or X-ray radiation, which radiation centre matches focus of the photocathode. The mesh anode is placed at equidistance from the photocathode, and operating surface of the photocathode is represented by nonsymmetrical cutout from paraboloid of rotation deformed by shifting its points from the peak and along radius of rotation in regard to radius of rotation. Additional distinctions may include the fact that nonsymmetrical cutout from paraboloid of rotation may be made as a circular or oval cylinder or rectangular prism. The converter may be made as parabolic mirror with metal or dielectric multilayer coating or as flat target of spherical, conical or flat shape.

EFFECT: improved directivity and increased intensity of the generated electromagnetic emission, which allows expanding its scope of application in radar detecting and ranging and tests methods for pulsed electromagnetic effects.

3 cl, 5 dwg

Description

The invention relates to techniques for generating electromagnetic pulses (EMP) and can be used in pulsed radar and in testing electronic equipment for exposure to pulsed fields.

Known EMP generator [Bessarab A.V. et al., “Superluminal source of electromagnetic radiation initiated by a short X-ray pulse”, Proceedings of the RFNC-VNIIEF, Scientific publication - Sarov: RFNC-VNIIEF, issue 7, 2004, pp. 218-228], containing a voltage source, a flat photocathode and a parallel grid anode, a pulsed light source in the form of a laser, a converter that converts the laser radiation into a spherically diverging x-ray wave. This generator operates as follows. A voltage is applied to the gap between the photocathode and the anode. A pulsed laser produces a subnanosecond light pulse, which is directed at some target to create a layer of laser plasma near its surface that converts the light pulse into an x-ray pulse. If you first orient the photocathode and anode so that the x-ray radiation illuminates the photocathode at a certain angle φ <90 °, then an electron emission wave will run along the surface of the photocathode with a speed of ν = c / sin (ϕ)> s. The emitted electrons, accelerating in the gap “photocathode-anode”, pass through the mesh anode and fall into the equipotential half-space free from an external electric field. The wave of electron injection into the half-space, traveling along the anode grid also with superluminal speed, is a source of broadband EMR, and the directivity of electromagnetic radiation is provided by the Cherenkov character of the formation of the interference pattern of radiation.

Taking into account that the laser plasma is actually a point source of X-ray radiation, the angle of incidence φ of the X-ray quanta on the photocathode in its various sections is different, therefore, the direction of the Cherenkov radiation changes as the injection wave propagates.

Thus, the main disadvantage of the analogue is the low directivity of the radiation and the low intensity of the generated electromagnetic radiation, which limits its use, for example, in pulsed radar.

Structurally close to the claimed device is an EMP generator [Bessarab A.V., Dubinov A.E., Lazarev Yu.N. and others, "Generator of electromagnetic pulses", Patent RU No. 2175154, priority 15.11.1999, publ. BI No. 29, 2001], containing a pulsed or pulsed-periodic light source in the form of a laser, a photocathode and a mesh anode, working surfaces that are made in the form of surfaces of revolution bodies, and which are connected to a voltage source, a converter that converts the radiation of the light source into spherically a diverging wave of optical, ultraviolet or X-ray radiation, the center of radiation of which coincides with the focus of the photocathode. The operating principle of the well-known EMR generator is based on the following sequence of processes: generation of a sequence of light pulses of a subnanosecond range of duration using a light source, conversion of light radiation into a spherically diverging wave of optical radiation, illumination of the photocathode by this wave in order to initiate a surface wave of photoemission of electrons traveling along the photocathode in the direction from its axis, the acceleration of electrons in the gap "photocathode-anode" and their subsequent injection through the retina fifth anode inside covered anode. Then, an electron injection wave traveling along the anode grid, which is the source of electromagnetic radiation, is excited on the outer surface of the anode. According to the described patent, the RFNC-VNIIEF developed an experimental model (EO) of a UWB EMR generator [Bessarab A.V., Garanin S.G., Martynenko S.P., Prudka N.A., Soldatov A.V., Terekhin V .A., Academician Trutnev Yu.A. Generator of ultra-wideband electromagnetic radiation (UWB EMR) initiated by a picosecond laser, Reports of the Academy of Sciences, 2006, v. 411, No. 5, p. 1-4], where the characteristics of EO are given and which is subsequently selected as the closest analogue. Design parameters of the parabolic generator: focal length 5 cm, the distance between the working surface of the photocathode and the inner surface of the mesh anode (interelectrode gap) 2 cm, the total length of 65 cm. However, this generator has a number of disadvantages:

1. For an observer located on the longitudinal axis of the paraboloid, the distribution of radiating currents over the surface of the paraboloid is radial (in Fig. 4, the arrows schematically show the directions of local dipole moments). In this case, each elementary radiating current can be associated with the same current in magnitude, but mirror symmetric. Each such pair emits signals of the same amplitude, but in antiphase. As a result of interference, the radiation in the direction of the axis of the paraboloid is completely suppressed - an area free of electromagnetic fields is formed, and the radiation appears as a side lobe with an increased aperture angle, but a much smaller amplitude of the emitted signal. The irradiation zone has the form of a ring and does not allow focusing the radiation energy on the target. The dependence of polarization on the azimuthal angle makes it impossible to use a generator for radar purposes.

Consider the situation from the point of view of an observer located at the maximum radiation pattern of this generator. For convenience, we choose an observation point so that it lies in a plane defined by the axis of symmetry of the paraboloid and the axis Y (see Fig. 4).

It follows from symmetry considerations that the electric field strength at the observation point has only the Y component. And only sources with a predominant polarization along the Y axis contribute to it. Sources with a predominant polarization along the X axis practically do not affect the parameters of electromagnetic radiation. Thus, already initially only half the area of the mirror is used to form radiation. The second phenomenon, which reduces the efficiency of using the area of the paraboloid, is due to the fact that radiation at the maximum of the radiation pattern is formed by subtracting radiation from the diametrically opposite halves of the paraboloid. This gives a loss of at least 50% of the paraboloid area. Thus, no more than 25% of the area of the paraboloid is used to form radiation.

2. Due to the fact that the electrons in the accelerating gap “photocathode-grid” propagate along trajectories whose directions do not coincide with the longitudinal axis of the paraboloid, an additional phase error arises that worsens the directivity of the radiation. Since the electrons propagate along the lines of the smallest distance, a path difference appears between different sections of the paraboloid. The origin of this difference is illustrated in FIG. 2.

The surface of the photocathode is described by the equation

y ² = 4sz,

where y is the distance from a given point of the paraboloid to the axis of rotation; z is the distance from a given point of the paraboloid to the plane perpendicular to the axis of rotation and passing through the vertex of the paraboloid, s is the focal length of the paraboloid.

When irradiated from focus, the path difference between an arbitrary point of the parabolic photocathode and its peak is

,

,

The graph of the dependence of the difference in stroke introduced by the accelerating gap using the example of the prototype is shown in FIG. 3 (graph a). As follows from the graph, the path difference between the top of the paraboloid and its periphery of the parabolic diode in this case reaches ≈1.5 cm.

The path difference introduced by the accelerating gap leads to a violation of the synchronization of elementary radiators in the high-frequency part of the spectrum, which is equivalent to cutting off the high-frequency part of the spectrum of the emitted electromagnetic radiation. The limiting stroke difference, which does not affect the operation of the EMP generator, is

,

,

where c = 3 · 10 ⁸ m / s is the speed of light; f _up , λ _up is the upper cutoff frequency in the spectrum of the pulse and the corresponding wavelength, respectively.

If we approximate the main peak of the radiation pulse by the function,

,

,

.Where

.

then its spectral density can be represented as

,

,

.Where

.

. Для ЭО генератора СШП ЭМИ, характерная длительность пика излучаемого импульса составляет τ≈300 пс. Верхняя граничная частота в спектре равна

. Тогда предельная допустимая разность хода равна

.The relationship of the time shape of the pulse and its frequency spectrum is illustrated in FIG. 5. As follows from the data presented, the bulk of the pulse energy is concentrated in the frequency range from 0 to

. For the EO of the UWB EMR generator, the characteristic peak duration of the emitted pulse is τ≈300 ps. The upper cutoff frequency in the spectrum is

. Then the maximum allowable stroke difference is

.

Thus, the actual stroke difference between different sections of the paraboloid is approximately 3 times higher than the permissible one. This leads to erosion of the shape of the pulse and a decrease in its amplitude, which entails a deterioration in directivity and a decrease in the intensity of electromagnetic radiation in the irradiation zone.

The technical result achieved in the proposed technical solution is to improve the directivity and increase the intensity of the generated electromagnetic radiation, which will expand the scope of its application in radar and testing techniques for pulsed electromagnetic effects.

и вдоль радиуса вращения к оси вращения на величину

, где z - расстояние от точки недеформированного параболоида вращения до плоскости перпендикулярной оси вращения и проходящей через вершину параболы; Δ - расстояние между рабочей поверхностью фотокатода и внутренней поверхностью сетчатого анода; s - фокусное расстояние.The claimed technical result is achieved due to the fact that in the electromagnetic pulse generator, which includes a photocathode and a mesh anode, the working surfaces of which are made in the form of surfaces of revolution bodies and which are connected to a voltage source, a pulsed or pulse-periodic light source, a converter, converting radiation of a light source into a spherically diverging wave of optical, ultraviolet, or X-ray radiation, the center of radiation of which coincides with the focus of the photocathode, is new It is that the grid anode located equidistant from the photocathode, the photocathode and the working surface is an asymmetrical clipping of the paraboloid of revolution deformed by shifting its points from the top along a rotational axis by

and along the radius of rotation to the axis of rotation by

where z is the distance from the point of the undeformed rotation paraboloid to the plane perpendicular to the axis of rotation and passing through the top of the parabola; Δ is the distance between the working surface of the photocathode and the inner surface of the mesh anode; s is the focal length.

Additional differences are that the asymmetric notch of rotation paraboloid can be made with a circular or oval cylinder or a rectangular prism. And the converter can be made in the form of a parabolic mirror with a metal or with a dielectric multilayer coating or as a flat target of a spherical, conical or flat shape.

Placing the mesh anode equidistant to the photocathode provides the same transit time of electrons emitted from any point of the photocathode, which creates the conditions necessary to obtain a directed flow of electromagnetic radiation.

The implementation of the working surface of the photocathode in the form of an asymmetric notch from the paraboloid of rotation deformed, as described above, eliminates the suppression of radiation in the direction of the axis of the paraboloid and compensates for the travel difference introduced by the accelerating gap between the photocathode and the anode, which improves directionality, increases both the intensity and the frequency spectrum of electromagnetic radiation.

Performing an asymmetric cut from a paraboloid of revolution with a circular or oval cylinder or a rectangular prism allows you to choose the optimal design option for the generator.

The implementation of the converter in the form of a parabolic mirror with a metal or with a dielectric multilayer coating allows you to convert optical and ultraviolet radiation into a diverging wave.

The design of the converter as a point target of a spherical, conical or flat shape allows you to convert the radiation of a light source into diverging x-ray radiation.

An example of the proposed generator EMR is illustrated by the following drawings:

FIG. 1 - design of an EMP generator;

FIG. 2 is a diagram of generating and compensating for a synchronization error;

FIG. 3 is a graph of the dependence of the stroke difference for an ultra-wideband electromagnetic pulse generator (UWB EMP) and the generator proposed in this invention from the distance to the top of the paraboloid;

FIG. 4 is a diagram of a distribution of radiating currents of a prototype for an observer located on the axis of a paraboloid;

FIG. 5 - the relationship of the temporal shape of the pulse and its frequency spectrum.

In the figures, the positions indicated: 1 - photocathode; 2 - mesh anode; 3 - pulse or pulse-periodic light source; 4 - converter, 5 - axis of rotation of the paraboloid; 6 - section of the photocathode before deformation and 7 - after deformation; 8 - trajectory of rotation (displacement) of a surface point of a paraboloid during its deformation; 9 - focus of the paraboloid of rotation; Δ is the width of the interelectrode gap; δ is the synchronization error; ϕ is the angle between the beam emanating from the focus and the axis of rotation; β is the angle between the normal to the surface of the photocathode and the plane perpendicular to the longitudinal axis of the paraboloid; a - dependence of the path difference on the distance of the point of the photocathode to the top of the paraboloid along the axis of rotation, b - the same for the photocathode, the shape of which is adjusted according to the invention.

As the light source 3, it is possible to use, for example, a neodymium laser operating at the second harmonic (λ = 0.53 μm), or an ultraviolet laser. In the first case, possible materials for photocathode 1: a coating with negative electron affinity based on GaAs doped with cesium, or Cs ₂ Te; in the second case, coatings based on metal oxides of the W-Zr-O type are applicable. If the EMP generator is supposed to be used in conditions of constant illumination, for example, daylight, it is recommended to use an ultraviolet laser in conjunction with a photocathode made of materials like Cs ₂ Te or Rb ₂ Te, which are insensitive to illumination by visible light. The light source 3 is placed behind the plane of the photocathode so that its beam is perpendicular to the plane of the photocathode and passes through the optical center of the converter. The mesh anode 2 can be made of a thin metal wire, for example, of tungsten or tantalum, achieving transparency> 90%. This will reduce the loss of reflected light and accelerated electrons to insignificant. Converter 4 can be made either in the form of a parabolic mirror, which can be made either with a metal or with a dielectric multilayer coating (odd layers of a material with a high refractive index - zinc or antimony sulfide, oxides of titanium, zirconium, hafnium, thorium, lead, and even layers — from materials with a low refractive index — magnesium fluoride, strontium, silicon dioxide), or as a flat target from a material with a large atomic number (gold) and a size of 1 mm that converts laser radiation into ultraviolet E or X-ray radiation.

In the event that the laser pulse is converted into an X-ray pulse, ordinary metals can be used as photocathode 1: steel, nickel, aluminum, etc.

Before starting the operation of the EMP generator, a voltage, for example, 100 kV, is supplied to the “photocathode-anode” gap by means of a voltage source. Next, the EMP generator operates as follows. A pulsed or pulsed-periodic light source 3 is triggered, which generates powerful pulses of light lasting, for example, 20-100 ps, which are sent to converter 4. Converter 4, in turn, converts the laser beam into a spherically diverging wave of optical, ultraviolet or X-ray radiation. A spherical wave, expanding, illuminates the photocathode and initiates a surface electron emission wave traveling along the photocathode in the direction from the photocathode point located at a minimum distance from the center of the spherical wave. The emitted electrons are accelerated in the “photocathode-anode” gap and then injected through the grid anode 2. Then, an electron injection wave is generated on the outer surface of the anode, traveling along the anode grid in the direction from the anode point located at a minimum distance from the center of the spherical wave. This wave of electron injection is the source of the electromagnetic wave.

In order to compensate for the travel difference introduced by the accelerating gap, the shape of the photocathode must be adjusted. As follows from the second equation (see p. 2-3), the farther the paraboloid point is from the vertex, the greater the difference in travel (delay) introduced by the accelerating gap. Therefore, to compensate for each point of the paraboloid that does not coincide with its vertex, it is necessary to turn around the focus so that it moves along the axis of rotation by a distance equal to the introduced travel difference (indicated by δ in Fig. 2).

The new coordinates of each point of the photocathode will be equal to:

.

.

When deriving these relations, the movement around the circle was replaced by the movement along the tangent.

;

, имеемTaking into account that

;

, we have

,

,

That is, we move each point of the photocathode with different coordinates at different distances. Although the proposed correction of the shape of the paraboloid is not ideal, because, firstly, the circumferential shift is replaced by the tangential shift, and secondly, when the deformation is changed, the normal direction to the photocathode surface changes, the error of such correction for real structures does not play a significant role. The calculation results of the path difference for the emitter with the corrected shape of the photocathode on the example of the prototype are shown in Fig. 3 (graph b). The calculation was carried out according to the formula

.

.

As the calculation results show, the maximum path difference with the corrected photocathode does not exceed 1.3 mm, which is much less than the maximum allowable path difference for the existing time parameters of the emitted pulses.

In the proposed generator, the radiating currents in different sections of the anode have almost the same polarization, which allows you to effectively use the area of the emitter. Given that the prototype effectively uses no more than 25% of the area of the emitter, the present invention allows for the same transverse dimensions to increase the effective opening of at least 4 times, which is equivalent to an increase in power density of more than 16 times.

Thus, all new features ensure the achievement of a technical result, namely, an improvement in directivity and an increase in the intensity of the generated EMR.

Claims (3)

1. An electromagnetic pulse generator, including a photocathode and a grid anode, the working surfaces of which are made in the form of surfaces of bodies of revolution and which are connected to a voltage source, a pulsed or pulsed-periodic light source, a converter that converts the light source radiation into a spherically diverging wave of optical, ultraviolet or x-ray radiation, the center of radiation of which coincides with the focus of the photocathode, characterized in that the mesh anode is located equidistant to the working surface photocathode, and the working surface of the photocathode is an asymmetric notch from a rotation paraboloid deformed by shifting its points from the vertex along the axis of rotation by

and along the radius of rotation to the axis of rotation by

where z is the distance from the point of the undeformed rotation paraboloid to the plane perpendicular to the axis of rotation and passing through the top of the parabola; Δ is the distance between the working surface of the photocathode and the inner surface of the mesh anode; s is the focal length. 2. The generator according to claim 1, characterized in that the asymmetric notch of the paraboloid of revolution is made by a circular or oval cylinder or a rectangular prism. 3. The generator according to claim 1, characterized in that the converter is made in the form of a parabolic mirror with a metal or with a dielectric multilayer coating or in the form of a flat target of a spherical, conical or flat shape.

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