RU2488909C2 - Method for generation of uhf range broadband electromagnetic radiation and device for its implementation - Google Patents

Abstract

FIELD: electrical engineering.

SUBSTANCE: with the help of discharge of a HV vacuum photodiode initiated by laser radiation falling onto the photocathode at an angle a current pulse is created propagating along the interelectrode interspace at a supersonic gap. Radiation generation occurs within the discharge gap. Laser radiation propagates inside the photodiode, reflected from the electrodes surfaces. To increase the electromagnetic radiation generator power and energy, several identical photodiodes are stacked.

EFFECT: increased efficiency of electrostatic energy conversion to electromagnetic radiation energy.

2 cl, 5 dwg

Description

The invention relates to microwave technology and can be used in the development of generators of powerful broadband electromagnetic pulses (EMP) in the centimeter, millimeter and submillimeter wavelength ranges.

A known method of generating pulses of microwave radiation in a device with a virtual cathode (VK) [1] (Hwang GS, Wu MW, Song PS, Hou WS, "High power microwave generation from a tunable radially extracted vircator", J. Appl. Phys. 1991, No. 69 (3), P.1247). This generation method consists in creating a pulsed electron beam with a current above the limiting current in the diode region of the device, which is injected through the mesh anode into the drift space, where a VC is formed due to the action of the space charge of the electrons. A part of the electrons is reflected from the VC and oscillates between the real and virtual cathodes. The energy of these electrons is transferred to the electromagnetic field. The parameters and position of the VC oscillate in time and also contribute to the radiation energy. The disadvantage of this method is the low (about several percent) efficiency of converting the energy of the electron beam into radiation energy.

The closest (prototype) to the proposed method is a method of generating electromagnetic radiation of the microwave range, described in [2] (Yu.N. Lazarev, P.V. Petrov, “Generator of electromagnetic radiation of the microwave range based on superluminal source”, JETP, 1999, T.88, S.926), based on the use for the generation of EMP of a current pulse propagating with a superluminal speed that appears above the anode during the discharge of a high-voltage photodiode initiated by laser radiation. It allows you to get a powerful broadband directional pulse of electromagnetic radiation.

The disadvantage of this technical solution is the relatively low (approximately 12%) conversion efficiency of the stored electrostatic energy into electromagnetic energy. To achieve higher conversion efficiency values, a significantly higher level of radiation intensity is required.

There are two devices that implement in practice the method of generating EMR, selected as a prototype.

One of them described in [3] (Bessarab A.V., Dubinov A.E., Lazarev Yu.N. et al., Patent RU 2175154 C2, 11/15/1999; Bessarab A.V., Garanin S.G. ., Martynenko SP et al., “Generator of ultra-wideband electromagnetic radiation initiated by a picosecond laser,” Reports of the Academy of Sciences, 2006. Vol. 411, No. 5, P. 609), includes a laser, a photocathode, and a grid anode in the form of rotation paraboloids, a voltage pulse generator (GIN), a parabolic mirror for converting a laser beam into a spherically diverging wave, mounted inside the anode paraboloid coaxially and sof placing behind him. Due to the shape of the electrodes, this generator has a very low efficiency of converting electrostatic energy into electromagnetic energy ≈2% and an extremely unsuccessful radiation pattern: the generator does not radiate along its axis.

Another device described in [4] (PVPetrov, VIAfonin, DOZamuraev et al., (Experimental and Theoretical Investigation of Directional Wideband Electromagnetic Pulse Photoemission Generator ”, Book of Abstracts of EUROEM 2008 European Electromagnetics, Lausanne, Switzerland, 21 -25 July 2008, P.302; AA Kondratyev, Yu.N. Lazarev, AB Potapov et al., “Experimental study of a microwave EMP generator based on a superluminal source”, Reports of the Academy of Sciences, 2011. V.438, No. 5, S.615), includes a GIN, a pulsed laser, a flat vacuum photodiode with a mesh anode, a vacuum chamber above the anode, a LI input and EMI output system in the form of a glass window in a vacuum wall me a camera opposite the anode.

. Эмитированные электроны ускоряются в поле между фотокатодом и анодом, проходят через сетчатый анод и попадают в свободное от внешнего поля пространство. Под действием пространственного заряда прошедших через анод электронов в их потоке формируется виртуальный катод. Облако инжектированных в пространство над анодом электронов распространяется вдоль поверхности анода со сверхсветовой скоростью $$\text{}с/Sin\theta =\sqrt{2}c$$

и излучает широкополосное ЭМИ в направлении, зеркальном углу падения ЛИ θ. Эффективность преобразования электростатической энергии в электромагнитную ≈9%. Интенсивность ЭМИ≈7 МВт/см².This generator operates as follows. A voltage pulse with an amplitude of up to 100 kV, a front duration of about 2 · 10 ^-9 sec, τ _FWHM ≈7 · 10 ^-9 sec, is supplied to the discharge gap. A pulsed laser generates a radiation pulse with a flat front of duration τ _FWHM ≈1 · 10 ^-12 sec, pulse energy (100-600) μJ, λ = 0.53 μm. Laser radiation passes through the mesh anode, incident on the Cs ₃ Sb photocathode at an angle θ = 45 ° and knocks electrons out of it. The photoemission process propagates along the surface of the photocathode at a speed $$\text{A.}\mathrm{from}/Sin\theta =\sqrt{2}c$$

. The emitted electrons are accelerated in the field between the photocathode and the anode, pass through the mesh anode and fall into the free space from the external field. Under the influence of the space charge of electrons passing through the anode, a virtual cathode is formed in their flow. A cloud of electrons injected into the space above the anode propagates along the surface of the anode at a superluminal speed $$\text{A.}\mathrm{from}/Sin\theta =\sqrt{2}c$$

and emits a broadband EMR in the direction specular to the angle of incidence of the LR θ. The conversion efficiency of electrostatic energy into electromagnetic energy is ≈9%. The intensity of the EMP is about 7 MW / cm ² .

. В первом случае T_p>T₀, во втором - Т_р<Т₀. При T₀/T_p→∞ амплитуда поля излучения, монотонно возрастая, стремится к некоторому предельному значению, а плотность потока энергии при сравнительно небольших значениях ускоряющего поля Е₀=φ₀/L≈10⁶ В/см может достигать величины ~0.5·10⁸ Вт/см².According to existing concepts, an electromagnetic wave arising above the anode has an amplitude H ^out = tgθ · ε / eλ in the source region, here θ is the angle of incidence of the laser radiation (LI) initiating the discharge of the photodiode, s is the maximum energy of electrons emanating from the anode, λ = cT , T is the characteristic time of the photodiode discharge process. Depending on the irradiation conditions, the size of the discharge gap L and the value of the initial potential difference φ _{0, the} time T is equal to either the time of formation of the space charge T _p or the time of flight of the discharge gap by the electron - $$T_0=L\sqrt{2m/e{\varphi }_0}$$

. In the first case, T _p > T ₀ , in the second - T _p <T ₀ . At T ₀ / T _p → ∞, the amplitude of the radiation field, monotonically increasing, tends to a certain limiting value, and the energy flux density at relatively small values of the accelerating field E ₀ = φ ₀ / L≈10 ⁶ V / cm can reach ~ 0.5 · 10 ⁸ W / cm ² .

It is known that for an inclined incident LI and T ₀ / T _p → ∞, the maximum energy of electrons emitted from the discharge gap depends on the angle of incidence LI θ: ε = eφ0Cos ² θ, which leads to ∝Cos ² θ decreasing the wave amplitude and ∝Cos ² θ decrease in the density of electromagnetic energy flux above the anode (A.V. Soldatov, A.A. Soloviev, M.S. Terekhina, Plasma Physics, 2007, V.33, P.795).

A significant decrease in the energy of electrons emitted from the anode at θ≥45 ° indicates that a rather strong electromagnetic wave is generated inside the photodiode, which slows down the electrons. The amplitude of this wave should be comparable with the magnitude of the electric field E ₀ applied to the discharge gap. Since E ^out > E ₀ , the energy flux in such a wave is much larger than the electromagnetic energy flux above the anode. Consequently, if we use not the external but the internal region of the photodiode to generate electromagnetic radiation, then a much more powerful source of electromagnetic radiation can be obtained than with the known generation method.

The objective of the present invention is to create a method that allows to obtain pulses of a broadband EMP microwave range with significantly higher values of the radiation intensity and, as a result, a higher efficiency of the conversion of electrostatic energy into EMP energy.

The problem is solved in that, in contrast to the known method for generating electromagnetic radiation of the microwave range, including oblique irradiation of the photocathode with laser radiation passing through the anode, electron emission from the cathode into the evacuated volume, electron acceleration between the photocathode and anode, electron passage through the anode, electron motion above the anode with the formation of a virtual cathode, in the proposed method, laser radiation enters the photocathode, either passing through the end boundary of the interelectrode transits, either through the anode, or through both. In this case, the region of EMR generation is the region between the electrodes, in which the EMR intensity is more than an order of magnitude higher than above the anode due to the locking of the EMR between the reflecting electrodes and the increase in the interaction time between the EMR and the electrons.

Since the region above the anode is not regarded as a source of electromagnetic radiation, the electrons that have passed through the anode can move in any way possible and, in particular, be absorbed. Therefore, the size of the region above the anode can be changed if necessary.

Irradiation of the photodiode through the end allows you to abandon the mesh anode, expands the possibilities of irradiating a stack of identical photodiodes, since laser radiation can irradiate the photocathode as a result of reflection from the electrodes. Multiple reflection of the LI from the electrodes provides a more efficient use of the LI energy, allows increasing the length of the photodiode in the direction of LI propagation, so that the amplitude of the generated electromagnetic wave is close to saturation.

The technical result of the proposed method is to obtain a much more intense generation of EMR due to the fact that the electromagnetic wave that occurs inside the discharge gap more efficiently selects the energy of electrons than the wave that appears above the anode. As a result, there is a higher efficiency of converting electrostatic energy into electromagnetic energy.

In a particular device, this result is achieved due to the fact that the proposed EMR generator, like the prototype [4], includes a pulsed or pulsed-periodic laser, a flat vacuum photodiode connected to a pulsed voltage generator, a LI input system, an EMR output system, a vacuum chamber for accelerated electrons passing through the anode, but unlike the known one, it uses the interelectrode space of the photodiode to generate electromagnetic radiation, where the amplitudes of the generated fields are much higher. Since the interelectrode gap is small, to increase the area of the radiating aperture, several photodiodes together with vacuum chambers are connected in a stack. This became possible due to the fact that the LI enters the photodiode through its end surface and propagates inside the photodiode, reflected from the electrodes. Thus, the LI input system is located at one end of the photodiodes, and the EMR output system is located at the opposite ends. As a result, by connecting the photodiodes to the stack, it is possible to create the same area of the emitting aperture as that of the prototype, and due to higher intensity values of the generated EMR, obtain an EMR source with significantly higher power and energy values at the same input parameters as the prototype. The role of laser radiation remains the same. It provides the formation of the required number of electrons and the synchronization of radiation, in this case emitted by different photodiodes.

The physical basis of the proposed invention is explained below.

Since the dimensions of the source under consideration are much larger than the characteristic radiation wavelength, the study of such a source reduces to the study of the discharge of a planar photodiode.

An approximate solution of the Maxwell equations inside a planar photodiode.

. Ось z перпендикулярна плоскостям электродов, θ - угол падения лазерного излучения. Компонента электрического поля Е_х внутри диода является решением следующей задачи:Consider an infinite along x, y, flat photodiode $$\left(T_0/T_p>>\mathrm{one},ƛ=\mathrm{<figure-callout\; id="2"\; label="L"\; filenames="00000029.png,00000031.png"\; state="\{\{state\}\}">L</figure-callout>}\sqrt{2/(\gamma -\mathrm{one})},\gamma -\mathrm{one}=e{\varphi }_0/m{c}^2\right)$$

. The z axis is perpendicular to the planes of the electrodes, θ is the angle of incidence of the laser radiation. The electric field component E _x inside the diode is a solution to the following problem:

At T ₀ / T _p > 1, the characteristic time of the processes is T ₀ .

, то запаздыванием можно пренебречь и, приближенноThen if $$2LCos\theta /c{T}_o=Cos\mathrm{<figure-callout\; id="2"\; label="\theta "\; filenames="00000029.png,00000031.png"\; state="\{\{state\}\}">\theta </figure-callout>}\sqrt{2/(\gamma -\mathrm{one})},<<\mathrm{one}$$

, then the delay can be neglected and, approximately

с точностью до знака производная по времени плотности дипольного момента внутри диода.Here $${\dot{P}}_z^{in}=-{\displaystyle \underset{0}{\overset{L}{\int }}dz{j}_z}$$

up to the sign, the time derivative of the density of the dipole moment inside the diode.

The fulfillment of a weaker condition is practically sufficient. $$\sqrt{2/(\gamma -\mathrm{one})}\cdot Cos\theta <\mathrm{one.}$$

Analysis of the solution.

Let us compare the energy flux density of an electromagnetic wave inside a photodiode

$$W_x^{in}=-c/\mathrm{four}\pi E_z^{in}{H}_y^{in}$$

and above the anode

. $$W^{out}=W_{\theta }^{out}\frac{S_{\perp }}{S}=W_{\theta }^{out}Cos\theta =\frac{c}{\mathrm{four}\pi }{\left(H_y^{out}\right)}^{2}Cos\theta$$

.

According to equation (4)

$$\frac{\partial E_z^{in}}{\partial \xi }=-\mathrm{four}\pi j_z+\frac{\mathrm{four}\pi }{L}\frac{Si{n}^2\theta }{Co{s}^2\theta }{\dot{P}}_z^{in}\left(\xi \right).(5)$$

It follows from the Maxwell equation that

$$-\frac{Sin\theta }{c}\frac{\partial H_y^{in}}{\partial \xi }=\frac{\mathrm{four}\pi }{c}{j}_z+\frac{\mathrm{one}}{c}\frac{\partial E_z^{in}}{\partial \xi }.(6)$$

Substituting (5) into (6), we obtain

$$\frac{\partial H_y^{in}}{\partial \xi }=-\frac{\mathrm{four}\pi }{L}\frac{Sin\theta }{Co{s}^2\theta }{\dot{P}}_z^{in}\left(\xi \right).(7)$$

Since the amplitude of the density of the dipole moment depends only on the maximum electron energy

$${\left(P_z^{in}\right)}_{\max }=\epsilon /\mathrm{four}\pi e=E_0\text{A.}LCo{s}^2\theta /\mathrm{four}\pi ,$$

then the following estimate of the magnetic field amplitude takes place

$${\left(H_y^{in}\right)}_{max}=E_{0}Sin\theta .(8)$$

If we assume that the main wave propagating along the x axis is excited inside the photodiode, then

$${\left(E_z^{in}\right)}_{max}={\left(H_y^{in}\right)}_{max}=E_{0}Sin\theta$$

and therefore

$${\left(W_x^{in}\right)}_{\max }=c/\mathrm{four}\pi E_0^2{\mathrm{<figure-callout\; id="2"\; label="sin"\; filenames="00000029.png,00000031.png"\; state="\{\{state\}\}">sin</figure-callout>}}^2\theta \stackrel{\theta \to \pi /2}{\to }c/\mathrm{four}\pi E_0^2.(9)$$

Above the anode

, $$H_y^{out}=-tg\theta \frac{\epsilon }{c{T}_o}Co{s}^2\theta =-Sin\theta Cos\theta \sqrt{\frac{\gamma -\mathrm{one}}{2}}{E}_0$$

,

$$W^{out}=Si{n}^2\theta Co{s}^3\theta \left(\gamma -\mathrm{one}\right)\frac{c}{8\pi }{E}_0^2$$

The maximum value of W ^{out is} achieved at Cos ² θ = 0.6

$$W_{max}^{out}=0.\mathrm{one}2\sqrt{0.6}\left(\gamma -\mathrm{one}\right)\frac{c}{\mathrm{four}\pi }{E}_0^2.(\mathrm{one}0)$$

Comparing (9) and (10), we obtain

$$\frac{{\left(W_x^{in}\right)}_{max}}{W_{max}^{out}}\approx \frac{\text{eleven}}{\gamma \text{-one}}.(\mathrm{eleven})$$

плотность потока электромагнитной энергии внутри фотодиода в 50-100 раз больше плотности потока электромагнитной энергии над анодом.It follows from (11) that for $$\gamma -\mathrm{one}\simeq 0.1-0.2$$

the electromagnetic energy flux density inside the photodiode is 50-100 times higher than the electromagnetic energy flux density above the anode.

Since T ₀ is the characteristic time of the change in the density of the dipole

moment and its derivatives, the electromagnetic wave generated inside the photodiode can be amplified on a spatial scale of ~ λ = cT ₀ . Therefore, for l _{x, the} following estimate holds:

l _x ~ λSinθ.

, совпадающие с аналогичными характеристиками фотодиодов стопки, и площадь электродов, совпадающую с площадью излучающей поверхности стопки фотодиодов.Consider the source of electromagnetic radiation (Fig. 4, 5), which is a stack of N >> 1 flat photodiodes with rectangular electrodes l _x × L _y (l _x << L _y , L _y >> cT ₀ ) and interelectrode gap L. According to the result obtained above, such a source can radiate tens of times more energy than a photodiode in which an electromagnetic wave is emitted by a dipole layer formed above the anode and which has the characteristics of a discharge gap $$(\left(\gamma -\mathrm{one,}L)\right)$$

coinciding with the similar characteristics of the photodiodes of the stack, and the area of the electrodes coinciding with the area of the radiating surface of the stack of photodiodes.

The results of numerical calculations of the generation of electromagnetic radiation inside a planar photodiode.

In a numerical study of the discharge dynamics inside a planar photodiode in a two-dimensional formulation, the Maxwell equations and the equations of electron motion were solved. Two cases were considered.

In the first, it was assumed that the laser irradiates the photocathode, passing through a transparent anode for the laser radiation and electrons. We studied the characteristics of the electromagnetic wave (l _x , W _x ) inside the photodiode for three values of the LI incidence angle corresponding to tgθ = 0.5, 1, 2. The length of the photodiode varied from L _x = 0.2 cm to L _x = 5 cm, the interelectrode gap was L = 0.1 cm, φ (z = -L) = - 100 kV, φ (z = 0) = 0. The calculation data in comparison with the corresponding analytical results are presented in Figs. 1, 2.

In the second case, it was assumed that the photodiode is irradiated from the end. We considered a plane photodiode, infinite along the y axis, with a height of 2L (L = 0.1 cm) and a length along the x axis: L _x = 1.6 cm, tanθ = 0.5, 1, 2, 4. It was assumed that the anode is transparent to electrons. At z = ± L φ = -100 kV, at the anode (z = 0) φ = 0. Approximately, as described above, a typical element of a stack of photodiodes may look like.

попадает на соседний отрезок той же длины и так далее. ЭМИ проходит отрезок длиной 2Ltgθ за время $$\Delta t_{ЭМИ}\text{}\text{}=\frac{2Ltg\theta }{c}$$

. При используемых исходных данных Δt_ЛИ-Δt_ЭМИ<<T₀, a L_x>>cT₀Sinθ~2Ltgθ.Laser radiation is incident on a segment of the photocathode (z = -L) of length 2Ltgθ. Reflected from the photocathode and from the anode, it after a period of time $$\Delta t_{L\mathrm{AND}}\text{A.}\text{A.}=\frac{2L}{c\cdot Cos\theta }$$

falls on an adjacent segment of the same length and so on. EMP passes a segment of length 2Ltgθ in time $$\Delta t_{EM\mathrm{AND}}\text{A.}\text{A.}=\frac{2Ltg\theta }{c}$$

. With the initial data used, Δt _LI -Δt _EMP << T ₀ , a L _x >> cT ₀ Sinθ ~ 2Ltgθ.

These conditions mean that the addition of electromagnetic waves generated by different regions of the photodiode occurs in approximately the same way as with continuous irradiation of the photocathode, considered in the first case. And since the length of the diode exceeds the length at which the electromagnetic wave reaches the limiting amplitude, the calculated values of the amplitude of the energy flux density at the output of the photodiode should be close to theoretical estimates, which is shown in the graph shown in Fig. 3. The conversion efficiency of electrostatic energy into electromagnetic energy is approximately 43% at tgθ = 4 (θ≈76 °).

The results of analytical and numerical studies of the electromagnetic field arising inside a planar photodiode, the discharge of which is initiated by a planar flux of radiation, obliquely incident on the photocathode, show that

- when θ → π / 2, the amplitude of the electromagnetic wave inside the photodiode reaches a value comparable to the value of the initial electric field applied to the photodiode;

- the density of the flux of electromagnetic energy inside the photodiode is tens or more times higher than the density of the flux of electromagnetic energy in the space above the anode.

Thus, the electromagnetic radiation source that uses the internal space of the photodiode to generate EMP has tens of times higher energy characteristics than the well-known technical solution in which the EMP is generated by a dipole layer above the anode. As such a source, one can imagine a stack (stack) of flat photodiodes. When E ₀ = φ ₀ / L≈l0 ⁶ V / cm, L = 0.1 cm, the power taken from 1 cm ^{2 of the} radiating aperture of the source will be ~ 3 · 10 ⁹ W.

Figure 1 shows comparative data for the length at which the electromagnetic wave inside the planar photodiode reaches its maximum amplitude • - numerical calculation, ―― - l _x ≈2.7 cT ₀ Sinθ.

Figure 2 shows the dependence of the amplitude of the energy flux density inside a flat photodiode on the angle of incidence of laser radiation on the photocathode when irradiated through the anode (continuous irradiation).

Figure 3 shows the results of calculating the amplitude of the energy flux density inside a planar photodiode for different angles of incidence of laser radiation on the photocathode when irradiated from the end of the photodiode.

Figures 4 and 5 show examples of the implementation of a broadband EMR generator using the proposed generation method. The generator is a stack of identical flat photodiodes irradiated from the end by a plane-front LI pulse. Laser radiation synchronizes the electromagnetic radiation of individual photodiodes of the stack in the direction of LI propagation. 1 - photocathode, 2 - anode, 3 - vacuum chamber, 4 - mirror, arrows indicate the progress of the PI.

These examples are not the only possible ones, but they clearly demonstrate the possibility of achieving the essential characteristics of the desired result by the given sets.

Many elements of the prototype can be used in the proposed devices. You can use the same GIN, laser radiation with a wavelength of λ≈0.53 μm and antimony-cesium photocathodes. The anode 2 can be made of foil, for example, scandium. Mirror 4 can be made of any metal with a sufficiently high conductivity.

The operation of the EMP generator, for example, shown in Fig. 4, begins with the supply of a voltage pulse to the stack of photodiodes. After the voltage across the discharge gaps reaches a maximum of about 100 kV, they begin to irradiate with an LI pulse of a duration of the order of a picosecond or less. A flow of linear-plane lasers propagating perpendicular to the end surface of a stack of photodiodes falls on a system of mirrors and is divided into parallel beams irradiating the photocathodes at an angle θ relative to the normal to their surfaces. LI quanta knock out electrons from the photocathode, which are accelerated in the electric field between the photocathode and the anode and emit electromagnetic radiation. Inside each stack photodiode, the LI beam, successively reflected from the photocathode and anode, creates a chain of such sources growing with speed c · Sinθ. The closer the angle θ to 90 °, the closer the growth rate of the source chain to the speed of light, the greater the length of the chain at which the sources emit along the axis of the photodiode almost synchronously and at which the field amplitudes from different sources add up, reaching a value close to the magnitude of the initial field in the discharge gap. Since the optical lengths for the stack photodiodes are the same, the EMR emitted by the stack of photodiodes will, like the LI, have a flat front parallel to the LI front. In this case, the power taken from a unit area of the radiating aperture of the generator in question will be much larger than the same value for the prototype, since the field amplitude in the interelectrode gap is almost an order of magnitude larger than the field amplitude above the anode.

Claims (2)

1. A method of generating electromagnetic radiation (EMP) of the microwave range, namely, that a voltage pulse is applied to the electrodes of the photodiode, the photocathode is obliquely irradiated with pulsed laser radiation (LI), as a result of which electrons are emitted from the cathode, which are accelerated in the evacuated interelectrode gap, which differs the fact that, in order to increase the efficiency and intensity of the EMP, an anode and a cathode are used, which reflect back the EMP generated in the interelectrode gap, which is led out through a window transparent to the EMP. 2. An electromagnetic pulse generator (EMP), which includes a flat vacuum photodiode, a laser radiation input system (LI), which provides an inclined incidence of the LI onto the photocathode, an EMR output system, a vacuum chamber located above the anode, a pulsed or pulse-periodic laser, a generator voltage pulses, characterized in that it contains several identical elements, consisting of a flat photodiode with reflecting EMR electrodes and a vacuum chamber above it, connected to a stack, the LI input and EMI output systems are located on opposite ends of the photodiode.

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