Evaluation system of nanosecond pulse hollow cathode electron beam current

Abstract

The invention discloses a nanosecond pulse hollow cathode electron beam current evaluation system, which aims at the problem of limited capacity of a current satellite communication channel, researches a nanosecond pulse high-current electron beam for driving a terahertz radiation source, and measures electron beam current and energy density distribution. The device is fixed on a four-axis motion platform through a Faraday cup with a slit hole, and the four-axis platform comprises an x axis, a y axis, a z axis and an R axis. Under the driving of the four-axis platform, the nanosecond time response Faraday cup can rotate around the z axis and sweep the section of the electron beam at a certain rotation angle in the direction perpendicular to the slit, and accordingly parallel projections of the electron beam at different angles are obtained. And current density distribution of a certain section of the electron beam is obtained through a filtering back projection algorithm program of the computer. On the basis, the three-dimensional density distribution of different energy electrons is obtained by changing the axial distance and the electron energy screening voltage, and then the time-space domain distribution of the electron beam energy density is obtained.

Description

Technical Field

[0001] The present invention belongs to the technical field of terahertz radiation sources, and in particular relates to an evaluation system for a nanosecond pulse hollow cathode electron injection beam, which measures the spatial distribution of current density of a nanosecond pulse electron beam based on the principle of tomography, and further obtains the electron beam energy density distribution by screening the electron beam energy. Background Art

[0002] With the development of satellite systems into constellation systems, increasing the capacity and bandwidth of communication channels has become an urgent need. As the communication frequency band is developing towards higher frequency bands, millimeter wave communication has become an inevitable way to meet current needs. Therefore, millimeter wave and terahertz communication technology has become an important research hotspot in the field of electronic information. One of the key technologies to achieve terahertz communication is the development of terahertz radiation sources. Terahertz radiation sources can be divided into those based on optical effects and those based on electronic effects based on their generation mechanism. Among them, the electronic effect of terahertz microwaves generated by the interaction between high-energy electron beams and metal gates under vacuum has the characteristics of high power and high conversion rate, and has attracted widespread attention.

[0003] Most traditional microwave vacuum electronic devices use hot cathodes as electron emission sources, which have the disadvantages of requiring continuous high-temperature heating, low lifespan, and large size. Hollow cathode pulsed electron beams, represented by pseudospark high-current pulsed electron beams, have become ideal electron emission sources for terahertz vacuum devices due to their characteristics of working at room temperature, fast response time, high emission current density, and high emission efficiency. Literature points out that a pseudospark discharge pulsed electron beam at 30kV can generate microwave radiation with a power of 20W and a frequency of 186GHz-202GHz at a gas pressure of 15Pa when acting with a backward wave oscillator. Therefore, the study of terahertz radiation driven by hollow cathode high-current pulsed electron beams has become a hot research field.

[0004] Measuring the characteristic parameters of hollow cathode high-current pulsed electron beams, such as beam spot size, beam current, beam current density, and electron energy distribution, is of great significance to the generation of terahertz wave radiation. However, the unique properties of hollow cathode pulsed electron beams pose great challenges to the measurement of electron beam performance parameters.

[0005] For a continuous electron beam, the electron beam current density distribution and its acceleration voltage can be considered constant within a certain time range. For a pulsed electron beam, the electron beam current density distribution and acceleration voltage will change over time, so the measurement of beam current density and energy requires a more complex measurement system and measurement method. For a nanosecond pulsed electron beam (hollow cathode pulsed electron beam), the beam spot size is only a few millimeters, and the electron beam current and energy will also change dramatically within tens of nanoseconds, which places high demands on the time response of the measurement system and the measurement solution. Summary of the invention

[0006] The purpose of the present invention is to provide an evaluation system for nanosecond pulse hollow cathode electron injection beam current, which can measure the beam current and energy density spatial distribution of a nanosecond pulse electron beam whose current density distribution and electron energy both change rapidly with time.

[0007] To solve the above problems, the technical solution of the present invention is:

[0008] A nanosecond pulse hollow cathode electron beam flow evaluation system, comprising: a high voltage grid assembly, a multi-axis motion platform, a Faraday cup, and a data acquisition and electron beam energy density distribution imaging system;

[0009] The Faraday cup is fixed on a multi-axis motion platform and driven by the multi-axis motion platform to sweep the electron beam cross section along the direction perpendicular to the slit at a certain speed and at a preset rotation angle to obtain parallel projections of the electron beam at different angles; the slit is provided on the upper cover plate of the Faraday cup;

[0010] The electron beam energy density distribution imaging system is used to realize the closed-loop position control of the linear axis of the multi-axis motion platform and the rotation control of the rotating axis, and quickly capture and collect the parallel projection signal of the electron beam, process the parallel projection signal, convert it into a sinusoidal graph, and obtain the two-dimensional beam density distribution of the electron beam cross section through a filtered back-projection algorithm; synthesize the three-dimensional density distribution by measuring the two-dimensional beam density distribution of each cross section at different heights of the electron beam;

[0011] The high-voltage grid assembly is arranged between the electron beam emission source and the Faraday cup, and is used to screen the electron beam energy; the electron beam energy density distribution imaging system obtains the current density distribution of electrons with different energies according to the various negative voltages loaded on the high-voltage grid assembly, and then obtains the energy density distribution of the pulsed electron beam.

[0012] According to an embodiment of the present invention, the high-voltage grid assembly, the multi-axis motion platform and the Faraday cup are arranged in a vacuum chamber, the grounded grid in the high-voltage grid assembly is connected to the inner wall of the vacuum chamber, and the negative high-voltage grid in the high-voltage grid assembly is connected to an external negative high-voltage power supply through a high-voltage flange of the vacuum chamber; when the negative high-voltage grid is loaded with a negative high voltage, the Faraday cup can only receive electrons with an energy greater than that obtained by the negative high voltage;

[0013] The distance d between the high voltage grid component and the nanosecond pulse electron beam anode is 1 Smaller than the inter-electrode spacing d of nanosecond pulsed electron beam 0 The high-voltage gate assembly uses the same trigger source as the nanosecond pulse electron beam, ensuring that the pulse electron beam is emitted first and the gate voltage is loaded later, with a time interval of nanoseconds. To avoid discharge breakdown of the high-voltage gate, the gate is generally loaded with a pulsed negative high voltage.

[0014] According to an embodiment of the present invention, the Faraday cup includes a housing, an insulating layer, an electron collecting plate, a graphite resistor, an insulating sleeve, a signal line, a flange and a BNC connector;

[0015] A rectangular slit is provided on the top of the shell along the axis of the Faraday cup as an electron beam entrance; the cross section of the rectangular slit is knife-edge shaped, which increases the collection efficiency of backscattered electrons.

[0016] The insulating layer is arranged between the electron collecting plate and the housing, and the electrons received by the electron collecting plate flow into the graphite resistor and then generate a voltage signal with the flange, and the voltage signal is transmitted to the BNC connector through the signal line;

[0017] The graphite resistor is isolated from the signal line by the insulating sleeve. The signal line adopts a variable diameter scheme, that is, the signal line has different diameters inside the insulating sleeve and in the air (or vacuum) medium in the flange cavity, so that the electron beam voltage signal transmission path has a constant characteristic impedance of 50 ohms everywhere.

[0018] According to an embodiment of the present invention, the width of the rectangular hole slit is 1 / 20 to 1 / 10 of the size of the electron beam spot, and the length of the rectangular hole slit is much larger than the maximum size of the electron beam spot cross section;

[0019] The rectangular hole slit is made of metal tungsten or hard alloy material and embedded in the shell body to ensure dimensional accuracy and stability under long-term pulse high-current electron beam irradiation.

[0020] According to an embodiment of the present invention, the multi-axis motion platform includes an x-axis, a y-axis, a z-axis, and an R-axis rotating around the z-axis, wherein the x-axis and the y-axis are servo-controlled by a linear motor, the z-axis is driven by a servo electric cylinder, and the R-axis is driven by a stepper motor;

[0021] The x-axis, y-axis and z-axis collect position signals through gratings to achieve position closed-loop control;

[0022] The x-axis, y-axis, z-axis, and R-axis motors, grating sensors, and other electromagnetic sensitive device installation modules in a vacuum environment are all designed with 1-2mm thick stainless steel metal shielding shells, and the shell openings are all designed to be cutoff waveguide modes. The motor is cooled by a constant flow water cooling system.

[0023] According to an embodiment of the present invention, a zero position switch is provided on the R axis to realize zero return of the R axis and eliminate the accumulated angle error;

[0024] The Faraday cup is coaxially mounted on the R-axis, and a sub-millimeter-sized insulating groove structure is arranged between the bottom surface of the Faraday cup and the R-axis rotating disk to prevent arc flash along the surface, and the depth-to-width ratio of the groove is 2:1.

[0025] According to an embodiment of the present invention, the electron beam energy density distribution imaging system realizes position control of three linear axes of the multi-axis motion platform through an Ethercat bus;

[0026] The electron beam energy density distribution imaging system realizes the rotation angle control of the rotation axis of the multi-axis motion platform through a digital quantity board and a stepper motor driver;

[0027] The multi-axis motion platform adopts a high-frequency electromagnetic pulse shielding design combining electromagnetic pulse absorption and discharge, high surge current suppression, and low-pass filter at the information interface entering the computer data sampling imaging system.

[0028] According to an embodiment of the present invention, when the electron beam energy density distribution imaging system measures the two-dimensional distribution of current density in the cross section of the electron beam spot, the multi-axis motion platform drives the Faraday cup to move along the x-axis direction to reach the x-axis of the electron beam spot. 0 At this point, the current measured by the Faraday cup is the electron beam cross section x 0 The integral of the current in the y direction is denoted as ∫f(x 0 )dy; the frequency of the nanosecond pulse electron beam is f, k waveforms are measured at the same position, and the projection value of the position is

[0029]

[0030] The slit of the Faraday cup is swept across the entire electron beam cross section at a preset speed v(f) determined by f under the movement of the multi-axis motion platform, and the total waveform of the electron beam projected in the y direction at different x coordinates when the angle is zero is obtained as the parallel projection of the electron beam at zero degree;

[0031] The rotation angle θ of the rotating axis driving the multi-axis motion platform 1 The slit of the Faraday cup sweeps across the entire electron beam cross section at a preset speed under the movement of the multi-axis motion platform, and the angle θ is obtained. 1 The total waveform of the electron beam projected in the y direction at different x coordinates is taken as θ 1 Parallel projection of the electron beam at an angle.

[0032] According to one embodiment of the present invention, after the electron beam energy density distribution imaging system obtains parallel projections of multiple electron beam pulses on the entire time axis, it selects a waveform composed of data at the same time point as the parallel projection under a single electron beam pulse, which is used to measure the beam current density distribution of the electron beam.

[0033] According to an embodiment of the present invention, after obtaining the two-dimensional beam density distribution of a certain cross section of the electron beam, the electron beam energy density distribution imaging system controls the multi-axis motion platform to move the Faraday cup in the z-axis direction by a distance to reach z1, and measures the two-dimensional beam density distribution of the electron beam cross section at z1; updates the position of the Faraday cup in the z-axis direction, measures the two-dimensional beam density distribution at the position, and obtains the two-dimensional beam density distribution at different cross sections in the z-axis direction, thereby obtaining the three-dimensional spatial distribution of the beam density;

[0034] By changing the voltage of the high-voltage grid, the electrons in the electron beam are selectively transmitted, and the three-dimensional energy density spectrum measurement of the electron beam is achieved.

[0035] Due to the adoption of the above technical solution, the present invention has the following advantages and positive effects compared with the prior art:

[0036] The nanosecond pulse hollow cathode electron injection beam flow evaluation system in one embodiment of the present invention measures the pulse electron beam flow and energy distribution in response to the problem of limited capacity of current satellite communication channels. A Faraday cup with a slit hole is fixed on a four-axis motion platform, and the four-axis platform includes x, y, z and R axes. Driven by the four-axis platform, the Faraday cup with nanosecond time response can rotate around the z-axis and sweep the electron beam cross section along the direction perpendicular to the slit at a certain angle, thereby obtaining the parallel projection of the electron beam at different angles; the current density distribution of a certain cross section of the electron beam is obtained through the computer's filtered back-projection algorithm program. On this basis, by changing the axial distance and the electron energy screening voltage, the three-dimensional density distribution of electrons with different energies is obtained, and then the time-space domain distribution of the electron beam energy density is obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is a schematic diagram of an evaluation system for nanosecond pulse hollow cathode electron injection beam in one embodiment of the present invention;

[0038] Figure 2 is a schematic diagram of a Faraday cup in one embodiment of the present invention;

[0039] Figure 3 The figure is a flow chart of measuring electron beam energy density distribution in one embodiment of the present invention. DETAILED DESCRIPTION

[0040] The following is a further detailed description of a nanosecond pulse hollow cathode electron beam evaluation system proposed by the present invention in conjunction with the accompanying drawings and specific embodiments. The advantages and features of the present invention will become more apparent from the following description and claims.

[0041] The present embodiment provides an evaluation system for a nanosecond pulse hollow cathode electron beam, comprising: a high-voltage grid assembly, a multi-axis motion platform, a Faraday cup, and an electron beam energy density distribution imaging system; wherein the high-voltage grid assembly is disposed between an electron beam emission source and the Faraday cup, and is used to screen the electron beam energy; the electron beam energy density distribution imaging system obtains the current density distribution of electrons with different energies according to each negative voltage loaded on the high-voltage grid assembly, and then obtains the energy density distribution of the pulsed electron beam.

[0042] The distance d between the high voltage grid assembly and the nanosecond pulse electron beam anode 1 Smaller than the inter-electrode spacing d of nanosecond pulsed electron beam 0 The high-voltage gate assembly uses the same trigger source as the nanosecond pulse electron beam, ensuring that the pulse electron beam is emitted first, the gate voltage is loaded later, and the time interval is in the nanosecond level.

[0043] The Faraday cup is fixed on a multi-axis motion platform and driven by the multi-axis motion platform. It scans the electron beam cross section along the direction perpendicular to the slit at a preset angle at a nanosecond time response speed to obtain parallel projections of the electron beam at different angles; the slit is opened on the upper cover of the Faraday cup.

[0044] The electron beam energy density distribution imaging system is used to realize the closed-loop position control of the linear axis of the multi-axis motion platform and the rotation control of the rotating axis, as well as to quickly capture and collect the parallel projection signal of the electron beam, process the parallel projection signal, convert it into a sinusoidal graph, and obtain the two-dimensional beam density distribution of the electron beam cross section through the filtered back projection algorithm; the three-dimensional density distribution is synthesized through the two-dimensional beam density distribution of each cross section.

[0045] The operation of the electron beam energy density distribution imaging system for measuring the electron beam energy density distribution with a beam spot diameter of about r is divided into two parts: measurement and calculation, which are specifically divided into the following steps:

[0046] AMeasurement

[0047] (1) The linear axis of the multi-axis motion platform returns to zero, and the R axis of the rotation axis returns to zero. The slit of the Faraday cup is now perpendicular to the x-axis, and the z-axis moves to the appropriate position.

[0048] (2) Alignment of the Faraday cup. When the beam source is turned on and a pulsed electron beam is generated, the x-axis of the multi-axis motion platform moves, driving the Faraday cup to move so that the slit sweeps across the electron beam cross section and records the x-axis coordinate x at the moment when the electron beam current is maximum. 0 ; The Faraday cup rotates 90 degrees, the y-axis moves, drives the Faraday cup slit to sweep the electron beam cross section along the y direction, and records the y-axis position y at the time when the electron beam current is maximum 0 The center position of the electron beam is (x 0 ,y 0 ).

[0049] (3) Negative high voltage gate connected to high voltage -V i .

[0050] (4) The axis of the multi-axis motion platform rotates through an angle θ = 0 degrees.

[0051] (5) The multi-axis motion platform moves to the coordinate x=x 0 +3rcosθ,y=y 0 The position of +3rsinθ.

[0052] (6) The multi-axis motion platform moves back and forth in a direction perpendicular to the slit.

[0053] (7) The oscilloscope collects waveforms and stores them in a computer.

[0054] (8) The multi-axis motion platform moves to the next angle θ = θ 1 .

[0055] (9) Repeat steps (5)-(8) until the multi-axis motion platform rotates 180 degrees. You can get energy greater than eV i Parallel projections of the electron beam at different angles.

[0056] (10) The z-axis moves at different distances z j , obtain projection waveform data at different axial positions.

[0057] (11) The voltage value connected to the negative high-voltage grid is changed, and the sinusoidal graphs of the electron beam current density distribution at different energies and different z positions are measured.

[0058] B Calculation

[0059] (1) The electron beam parallel projection obtained at the same z position under the same screening voltage is (x 0 ,y 0 ) is used as the center to perform filtered back projection calculation and obtain the two-dimensional density distribution of the beam in this cross section.

[0060] (2) Under the same screening voltage, the two-dimensional beam density distribution at different positions is superimposed according to the z coordinate to obtain the three-dimensional beam density distribution.

[0061] (3) The distribution of beam current density under different screening voltages is calculated to obtain the distribution of electron beam energy density.

[0062] The energy density distribution of the pulsed electron beam with any irregular distribution can be measured by the evaluation system of the hollow cathode nanosecond pulsed electron beam for the terahertz radiation source.

[0063] The following is a specific example to introduce the evaluation system of nanosecond pulse hollow cathode electron injection beam for measuring pseudospark electron beam energy density distribution.

[0064] like Figure 1 As shown, the system is mainly composed of a pseudospark electron beam source 1, a vacuum chamber 2, a high-voltage grid assembly 3, a fast-response Faraday cup 4, a multi-axis motion platform 5, a control and data acquisition system 6 and the like.

[0065] The high-voltage grid assembly 3 for electron beam energy screening includes a negative high-voltage grid assembly 31, a grounding grid 32 and a current limiting resistor 33. The grounding grid 32 is connected to the inner wall of the vacuum chamber through a wire to achieve grounding. The negative high-voltage grid 31 is connected to the current limiting resistor 33 through a high-voltage flange on the vacuum chamber, and the other end of the current limiting resistor 33 is connected to a negative high-voltage power supply (not shown). When the negative high-voltage power supply outputs a certain high voltage V x When the energy of the electron beam current is lower than eV x The electrons will be blocked by the grid and cannot be collected by the Faraday cup, thus filtering the electron beam energy. Where e represents the electron charge.

[0066] like Figure 2 As shown, the Faraday cup 4 is composed of a Faraday cup housing 41, a Teflon insulating layer 42, an electron collecting plate 43, a graphite resistor 44, a Teflon insulating sleeve 45, a signal line 46, a BNC connector mounting flange 47 and a BNC connector 48. Among them, a rectangular hole slit 49 is opened along the axis of the Faraday cup at the entrance of the electron beam on the Faraday cup housing 41. The length of the slit 49 exceeds the size of the electron beam. The width of the slit 49 is 1 / 20 to 1 / 10 of the size of the electron beam spot, such as a width of 0.4 mm and a length of 16 mm. The slit 49 is made of metal tungsten or cemented carbide material and embedded in the housing body to ensure dimensional accuracy and stability under long-term pulsed high-current electron beam irradiation.

[0067] The Faraday cup shell 41 made of copper metal has the function of shielding external interference signals and absorbing undetected electrons in the electron beam. The electrons received by the electron collecting plate 43 flow into the graphite resistor 44 to form a voltage signal between the BNC connector mounting flange 47, which is output by the signal line 46 through the BNC connector 48. The graphite resistor 44 adopts a coaxial design and is separated from the signal line 46 by a Teflon insulating sleeve 45 to ensure a characteristic impedance of 50 ohms. It is worth noting that the signal line 46 has different diameters inside the BNC connector mounting flange 47 (not shown in the figure), ensuring that the characteristic impedance between the signal line 46 and the inner wall of the flange 47 is also 50 ohms. This allows the Faraday cup to have a fast response of nanoseconds.

[0068] like Figure 1The fast response Faraday cup shown is mounted and fixed on the multi-axis motion platform 5. The multi-axis motion platform 5 consists of three linear axes x, y, and z axes and an R axis that can rotate 360 ​​degrees around the z axis. The x and y axes are driven by linear motors, and the z axis is driven by a servo electric cylinder. The three linear axes all measure their positions through gratings to form a closed-loop position control, which can achieve a positioning accuracy of 1um. The rotating shaft R axis is driven by a stepper motor, and the minimum resolution can be 0.1 degrees. A zero position switch is installed on the R axis to return the R axis to zero and eliminate the accumulated angle error.

[0069] The Faraday cup 4 is coaxially mounted on the R-axis, and a sub-millimeter-sized insulating groove structure is arranged between the bottom surface of the Faraday cup and the R-axis turntable to prevent arc flash along the surface, and the depth-to-width ratio of the groove is 2:1.

[0070] like Figure 1 The hardware part of the entire evaluation system shown is controlled by a control and data acquisition system 6. Among them, the industrial computer system 62 realizes the control of the multi-axis motion platform 5 through a digital quantity board 63 and an Ethercat bus 64. The digital quantity board 63 is connected to a stepper motor driver 65 and a zero position switch (not shown) on the R axis to control the rotation of the R axis. The Ethercat network cable is connected in series with an x-axis motor driver 66, a y-axis motor driver 67 and a z-axis motor driver 68 in sequence to control the position of the linear axis of the multi-axis motion platform 5 in a closed loop. The signal of the Faraday cup 4 is quickly responded to, and is led out through the coaxial flange on the vacuum chamber and connected to an oscilloscope 61 with a sampling frequency of GHz to complete the rapid capture and collection of pseudospark electron beam signals. The signal collected by the oscilloscope can be transmitted to the industrial computer system 62 through a serial port and stored through relevant software.

[0071] like Figure 1 The industrial computer system shown is programmed in C language and MATLAB to write a data acquisition imaging software system. The software system includes a multi-axis control unit, a data acquisition unit, a data processing unit, and a display unit. The multi-axis control unit controls the rotation of the turntable R through a digital quantity board, and realizes closed-loop position control of the linear axis through the Ethercat bus. The data acquisition unit automatically collects the parallel projection signal of the nanosecond pulse pseudospark electron beam captured by the oscilloscope, numbers the data, and stores it according to a certain rule. The data processing unit mainly realizes the arrangement and conversion of the electron beam waveform signal into a sine graph, and executes the core filtering back-projection algorithm to obtain the beam current density distribution of the electron beam cross section.

[0072] The filtered back-projection algorithm for reconstructing the two-dimensional distribution of electron beam current is as follows: The first step is to perform one-dimensional Fourier transform on the measured projection sinusoidal graphs of the electron beam at various angles within 180° to obtain the corresponding frequency domain signals. The second step is to perform SL filtering on the frequency domain signals after projection transformation. The third step is to perform Fourier inverse transform on the filtered frequency domain signals, and then superimpose the obtained time domain signals to obtain the beam current distribution of the electron beam.

[0073] The two-dimensional distribution of each cross section is used to synthesize the three-dimensional distribution, and the current density distribution of electrons with different energies is obtained according to the negative voltage loaded on the negative high-voltage grid 31. On this basis, the energy density distribution of the pulsed electron beam is obtained through a certain algorithm.

[0074] When the data acquisition unit collects projection data, it is assumed that the rotation angle of the turntable is zero in the initial state, and the slit on the upper cover of the Faraday cup is perpendicular to the x-axis of the platform. The Faraday cup fixed on the multi-axis motion platform moves a certain distance along the x-direction under the drive of the platform to reach the x-axis of the electron beam spot. 0 Position, at this moment the current measured by the Faraday cup is the electron beam cross section x 0 Specifically, when the electron beam energy density distribution imaging system measures the two-dimensional distribution of current density in the electron beam spot cross section, the multi-axis motion platform drives the Faraday cup to move along the x-axis direction to reach the x-axis of the electron beam spot. 0 At this point, the current measured by the Faraday cup is the electron beam cross section x 0 The integral of the current in the y direction is denoted as ∫f(x 0 )dy; the frequency of the nanosecond pulse electron beam is f, k waveforms are measured at the same position, and the projection value of the position is

[0075]

[0076] The slit of the Faraday cup is swept across the entire electron beam cross section at a preset speed v(f) determined by f under the movement of the multi-axis motion platform, and the total waveform of the electron beam projected in the y direction at different x coordinates when the angle is zero is obtained as the parallel projection of the electron beam at zero degrees. Then the turntable is driven to rotate through a certain angle θ 1 , repeat the above steps to obtain θ 1 Parallel projection of the electron beam at an angle.

[0077] When the data processing unit samples the collected data, the pulse width of the pulsed electron beam is in the microsecond or nanosecond level compared to the continuous electron beam. If we blindly pursue completing a scan within the duration of the pulse, the platform needs to have a very high speed. However, due to the large mass and inertia of the platform itself, excessive speed or acceleration will cause the platform to vibrate. For this reason, the scanning time needs to be much longer than the electron beam pulse width. At this time, the parallel projection of the electron beam is the parallel projection of multiple electron beam pulses on the entire time axis. Therefore, it is necessary to select a series of waveforms composed of data sampling points at the same time point on the waveform to form the parallel projection under a single electron beam pulse.

[0078] The measurement principle of the data processing unit for the three-dimensional distribution of electron beam current density is as follows: after obtaining the beam current density distribution J of a certain cross section of the electron beam, z0 After (x, y), the computer controls the Faraday cup to move a certain distance in the z direction to z1, and measures the density distribution J of the beam at this section. z1 (x,y). Repeat this step to obtain the beam density distribution J at different cross sections in the z direction. zi (x,y)(i=0,1,2……), we can get the spatial distribution of beam density J(x,y,z).

[0079] The data processing unit measures the energy density distribution of the pulsed electron beam in the following way: Compared with the continuous electron beam, the energy of the pulsed electron beam generally changes with time. At this time, a high-voltage grid assembly needs to be installed to screen the energy of the electron beam electrons. 0 The Faraday cup can only receive energy greater than eV 0 The electrons, at this time, are measured by current density to obtain J E>eV0 (x,y). By changing the voltage applied to the high-voltage grid from high to low, the spatial distribution of electron beam currents of different energies can be obtained. By synthesizing the beam current measurement results at different energies, the energy density distribution ρ(x,y,z) of the electron beam can be obtained.

[0080] Specifically, according to Figure 3 The measurement flow chart shown measures the electron beam energy density distribution. Before all steps, the vacuum system must first be evacuated to achieve a certain vacuum degree in the vacuum chamber 2. Before the measurement begins, the voltage gradient V[i] to be loaded on the negative high-voltage grid 31 should be designed according to the maximum energy of the pseudospark electron beam (for example, V[i] = 20kV, 15kV, 10kV, 5kV... (i = 1, 2, 3...)), and the angle increment α for current density measurement should be determined, for example, α = 7.2° (180 / 25). After the measurement starts, air begins to enter the vacuum chamber 2. When the system reaches a certain air pressure, the pseudospark discharge is adjusted to a value higher than the breakdown voltage V 1The discharge is carried out under the condition of triggering at a frequency of 10Hz. After the multi-axis motion system 5 is returned to zero and aligned according to the method described above (x 0 ,y 0 ), the negative high voltage gate 31 is loaded with the initial high voltage value V 1 , the R axis drives the fast response Faraday cup 4 to rotate through the specified angle α 0 (In the initial state, α 0 =0). Driven by the x- and y-axis linear motors, the fast-response Faraday cup 4 moves at a speed of 1 mm / s in the direction perpendicular to the slit 49, and the slit 49 vertically sweeps the electron beam cross section. During the time when the slit sweeps the electron beam cross section, discharge has occurred several times, and the oscilloscope 61 collects a series of waveforms. From this series of waveforms, data points corresponding to the same time on the electron beam waveform are collected. After simple processing of these points, the horizontal coordinate is the beam spot coordinate, the vertical coordinate is the electron beam projection current, and the beam current at α 0 Then change the R axis rotation angle α by an increment of angle α. 0 , repeat the above steps to obtain different α 0 According to the parallel projection Figure 3 The process shown further obtains different screening voltages V i , the electron beam parallel projection sequence (sinusoidal diagram) at different z-positions can be used to obtain the three-dimensional distribution of the electron beam current density and the distribution of the energy density ρ(x,y,z) using the filtered back projection algorithm.

[0081] The above scheme can be used to measure the energy density distribution of pulsed electron beams with any irregular distribution. The system has a simple structure and is easy to operate. The measurement results can be used to study the relevant characteristics of electron beams, or be applied to fields such as electron beam material processing to guide electron beam processing practices.

[0082] The embodiments of the present invention are described in detail above in conjunction with the accompanying drawings, but the present invention is not limited to the above embodiments. Even if various changes are made to the present invention, if these changes fall within the scope of the claims of the present invention and their equivalents, they still fall within the protection scope of the present invention.

Claims

1. A nanosecond pulse hollow cathode electron beam evaluation system, characterized in that: include: High-voltage grid components, multi-axis motion platform, Faraday cup, and data acquisition and electron beam energy density distribution imaging system; The Faraday cup is fixed on a multi-axis motion platform and driven by the multi-axis motion platform to sweep the electron beam cross section along the direction perpendicular to the slit at a certain speed and at a preset rotation angle to obtain parallel projections of the electron beam at different angles; the slit is provided on the upper cover plate of the Faraday cup; The electron beam energy density distribution imaging system is used to realize the closed-loop position control of the linear axis of the multi-axis motion platform and the rotation control of the rotating axis, and quickly capture and collect the parallel projection signal of the electron beam, process the parallel projection signal, convert it into a sinusoidal graph, and obtain the two-dimensional beam density distribution of the electron beam cross section through a filtered back-projection algorithm; synthesize the three-dimensional density distribution by measuring the two-dimensional beam density distribution of each cross section at different heights of the electron beam; The high-voltage grid assembly is arranged between the electron beam emission source and the Faraday cup, and is used to screen the electron beam energy; the electron beam energy density distribution imaging system obtains the current density distribution of electrons with different energies according to the various negative voltages loaded on the high-voltage grid assembly, and then obtains the energy density distribution of the pulsed electron beam.

2. The nanosecond pulse hollow cathode electron beam evaluation system according to claim 1, characterized in that: The high-voltage grid assembly, the multi-axis motion platform and the Faraday cup are arranged in a vacuum chamber, the grounded grid in the high-voltage grid assembly is connected to the inner wall of the vacuum chamber, and the negative high-voltage grid in the high-voltage grid assembly is connected to an external negative high-voltage power supply through a high-voltage flange of the vacuum chamber; when the negative high-voltage grid is loaded with a negative high voltage, the Faraday cup can only receive electrons with an energy greater than that obtained by the negative high voltage; The distance d1 between the high-voltage gate assembly and the nanosecond pulse electron beam anode is smaller than the nanosecond pulse electron beam anode spacing d0; the high-voltage gate assembly uses the same trigger source as the nanosecond pulse electron beam to ensure that the pulse electron beam is emitted first, the gate voltage is loaded later, and the time interval is nanosecond level. In order to avoid discharge breakdown of the high-voltage gate, the gate is loaded with pulsed negative high voltage.

3. The nanosecond pulse hollow cathode electron beam evaluation system according to claim 1, characterized in that: The Faraday cup includes a shell, an insulating layer, an electron collecting plate, a graphite resistor, an insulating sleeve, a signal line, a flange and a BNC connector; A rectangular slit is provided on the top of the shell along the axis of the Faraday cup as an electron beam entrance; the cross section of the rectangular slit is knife-edge-shaped to increase the collection efficiency of backscattered electrons; The insulating layer is arranged between the electron collecting plate and the housing, and the electrons received by the electron collecting plate flow into the graphite resistor and then generate a voltage signal with the flange, and the voltage signal is transmitted to the BNC connector through the signal line; The graphite resistor is isolated from the signal line by the insulating sleeve. The signal line adopts a variable diameter scheme, that is, the signal line has different diameters inside the insulating sleeve and in the air or vacuum medium in the flange cavity, so that the electron beam voltage signal transmission path has a constant characteristic impedance of 50 ohms everywhere.

4. The nanosecond pulse hollow cathode electron beam evaluation system according to claim 3, characterized in that: The width of the rectangular hole slit is 1 / 20 to 1 / 10 of the size of the electron beam spot, and the length of the rectangular hole slit is much larger than the maximum size of the electron beam spot cross section; The rectangular hole slit is made of metal tungsten or hard alloy material and embedded in the shell body to ensure dimensional accuracy and stability under long-term pulse high-current electron beam irradiation.

5. The nanosecond pulse hollow cathode electron beam evaluation system according to claim 1, characterized in that: The multi-axis motion platform includes an x-axis, a y-axis, a z-axis, and an R-axis rotating around the z-axis, wherein the x-axis and the y-axis are servo-controlled by a linear motor, the z-axis is driven by a servo electric cylinder, and the R-axis is driven by a stepper motor; The x-axis, y-axis and z-axis collect position signals through gratings to achieve position closed-loop control; The x-axis, y-axis, z-axis, and R-axis motors, grating sensors, and other electromagnetic sensitive device installation modules in a vacuum environment are all designed with 1-2mm thick stainless steel metal shielding shells, and the shell openings are all designed to be cutoff waveguide modes. The motor is cooled by a constant flow water cooling system.

6. The nanosecond pulse hollow cathode electron beam evaluation system according to claim 5, characterized in that: A zero position switch is provided on the R axis to realize the zero return of the R axis and eliminate the accumulated angle error; The Faraday cup is coaxially mounted on the R-axis, and a sub-millimeter-sized insulating groove structure is arranged between the bottom surface of the Faraday cup and the R-axis rotating disk to prevent arc flash along the surface, and the depth-to-width ratio of the groove is 2:

1.

7. The nanosecond pulse hollow cathode electron beam evaluation system according to claim 1, characterized in that: The electron beam energy density distribution imaging system realizes position control of the three linear axes of the multi-axis motion platform through the Ethercat bus; The electron beam energy density distribution imaging system realizes the rotation angle control of the rotation axis of the multi-axis motion platform through a digital quantity board and a stepper motor driver; The multi-axis motion platform adopts a high-frequency electromagnetic pulse shielding design combining electromagnetic pulse absorption and discharge, high surge current suppression, and low-pass filter at the information interface entering the computer data sampling imaging system.

8. The nanosecond pulse hollow cathode electron beam evaluation system according to claim 1, characterized in that: When the electron beam energy density distribution imaging system measures the two-dimensional distribution of current density in the electron beam spot cross section, the multi-axis motion platform drives the Faraday cup to move along the x-axis direction to the x0 position of the electron beam spot. At this moment, the current measured by the Faraday cup is the integral of the current in the x0 position of the electron beam cross section in the y direction, which is recorded as ∫f(x0)dy; the frequency of the nanosecond pulse electron beam is f, k waveforms are measured at the same position, and the projection value of this position is The slit of the Faraday cup is swept across the entire electron beam cross section at a preset speed v(f) determined by f under the movement of the multi-axis motion platform, and the total waveform of the electron beam projected in the y direction at different x coordinates when the angle is zero is obtained as the parallel projection of the electron beam at zero degree; The rotating axis of the multi-axis motion platform is driven to rotate by an angle θ1. The slit of the Faraday cup sweeps across the entire electron beam cross section at a preset speed under the movement of the multi-axis motion platform. The total waveform of the electron beam projected in the y direction at different x coordinates when the angle is θ1 is obtained as the parallel projection of the electron beam at the angle θ1.

9. The nanosecond pulse hollow cathode electron beam evaluation system according to claim 8, characterized in that: After obtaining parallel projections of multiple electron beam pulses on the entire time axis, the electron beam energy density distribution imaging system selects a waveform composed of data at the same time point as a parallel projection under a single electron beam pulse to measure the electron beam current density distribution.

10. The nanosecond pulse hollow cathode electron beam evaluation system according to claim 1, characterized in that: After obtaining the two-dimensional beam density distribution of a certain cross section of the electron beam, the electron beam energy density distribution imaging system controls the multi-axis motion platform to move the Faraday cup in the z-axis direction by a distance to reach position z1, and measures the two-dimensional beam density distribution of the electron beam cross section at z1; updates the position of the Faraday cup in the z-axis direction, and measures the two-dimensional beam density distribution at the position, so as to obtain the two-dimensional beam density distribution at different cross sections in the z-axis direction, and thus obtain the three-dimensional spatial distribution of the beam density; By changing the voltage of the high-voltage grid, the electrons in the electron beam are selectively transmitted, and the three-dimensional energy density spectrum measurement of the electron beam is achieved.

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