A multi-mode TOF probe system for electric thruster pulse plume velocity measurements

Abstract

The application belongs to the technical field of plasma plume diagnosis of electric thrusters, and relates to a multi-mode TOF probe system for pulse plume velocity measurement of electric thrusters, which is a kind of diagnosis technology for the pulse plume generated by a pulse plasma thruster. The TOF probe system can realize several different working modes, and can measure the velocities of different components of the pulse plasma plume. The probe loop current signal is realized by using an indirect measurement technology, so that the problems of electrostatic suspension and isolation in the direct current measurement technology can be avoided.

Description

Technical Field

[0001] This invention belongs to the field of plasma plume diagnostic technology for electric thrusters, and relates to a multi-mode TOF probe system for measuring the velocity of pulsed plumes in electric thrusters. More specifically, it is a diagnostic technique for pulsed plumes generated by pulsed plasma thrusters. Background Technology

[0002] In recent years, pulsed plasma thrusters (PPTs) have been widely used in microsatellite space missions. Measuring and characterizing the pulsed plasma plume generated by PPT discharge is crucial for improving PPT key performance. Among these, the velocity of the pulsed plasma plume is a vital indicator of PPT's fundamental performance. The magnitude of the pulsed plasma plume velocity determines the specific impulse of the PPT, thus becoming a decisive parameter for evaluating its propellant utilization efficiency. Furthermore, the stability of the pulsed plasma plume velocity directly reflects the stability of PPT performance.

[0003] A pulsed plasma plume contains electrons, ions, and neutral particles. The ejection velocity of the neutral particles is a result of thermal expansion dynamics: the propellant is ablated by a high-temperature electric arc, forming a high-temperature, high-pressure gas pulse cluster, which expands thermally to form a pulsed plume that is ejected outwards. The electrons and ions within the plume are also subject to Ampere's force; their ejection velocity is the result of the combined effects of thermal expansion and Ampere's force acceleration. Because the mass of electrons is much smaller than that of ions, electrons have a greater ejection velocity. Therefore, the velocities of electrons, ions, and neutral particles in a pulsed plasma plume will differ.

[0004] Existing pulsed plasma plume velocity measurement methods are mostly for heavy particle velocities, and there are few mature methods for measuring electron velocities. Differentiating the velocities of different components in a pulsed plasma plume is crucial for exploring the PPT thrust generation mechanism and improving PPT performance.

[0005] The Doppler effect is a commonly used principle for measuring the velocity of flying objects or microscopic particles. For example, patent CN106018878B uses laser-induced fluorescence (LIF) to measure the Doppler frequency shift of the fluorescence spectrum of tracer particles in a flowing gas, thereby obtaining the spatial distribution of the tracer particle velocity and, consequently, the flow field distribution. As a non-contact, highly sensitive, and highly selective velocimetry technique, LIF instruments are complex in structure, expensive, and have stringent environmental requirements.

[0006] Time-of-flight (TOF) is also an important method for measuring the velocity of flying objects or microscopic particles. This method calculates the velocity by measuring the time it takes for the object to travel a known distance. TOF can be used to measure plasma velocities, and the time of flight can be measured using the luminescent or electrical properties of plasma.

[0007] Patent CN108008144A describes a method for rapidly imaging the luminescence morphology of a plasma jet, measuring the distance the plasma jet travels between two imaging intervals, and calculating the plasma jet velocity. Specifically, it involves continuously capturing images of light intensity fluctuations in the flow field using a high-speed camera, processing the time series of light intensity at each spatial sequence point using Fast Fourier Transform, comparing the extracted frequency with the phase difference of light intensity fluctuations at adjacent spatial sequence points to obtain the time difference between the two, and finally obtaining the plasma jet velocity distribution for each frame of the image.

[0008] Patent CN111060714A, based on the time-of-flight method, utilizes the electrostatic properties of plasma to measure the flight time of ions in a pulsed plasma plume, thereby obtaining the ion velocity. Specifically, it involves simultaneously applying a negative bias voltage to a concentric circular grid and a circular plate to collect primary and secondary ion currents. The peak moment of the ion current is taken as the arrival time of the ion cluster in the pulsed plasma plume, thus obtaining the ion flight time and calculating the ion velocity in the pulsed plasma thruster plume. Similarly, patent CN109521224A utilizes the conductivity of plasma to collect ion current using a dual probe composed of a metal grid. The collected ion current signal is amplified by a transformer to obtain the peak moment of the ion current. Three sets of dual probes are used to measure the ion flight time, ultimately calculating the ion velocity in the pulsed plasma plume.

[0009] The luminescence characteristics can also be used to measure the velocity of pulsed plasma plumes. However, due to the short duration and rapid decay of luminescence intensity of pulsed plasma plumes, the luminescence intensity varies greatly at different spatial locations along the plume path. In particular, the luminescence is very weak in the far field region of the plume, resulting in a low signal-to-noise ratio for signal collection. Therefore, the measurable range of the spatial distribution of velocity is limited.

[0010] Currently, methods for measuring the velocity of pulsed plasma plumes using the electrostatic and electrical properties of plasma all have certain shortcomings:

[0011] (1) There are factors that interfere with the collection of ion current. For example, ions in pulsed plasma plumes are collected by applying a negative voltage to the grid structure, but the grid structure cannot completely eliminate the interference of the grid structure on the collection of electrons in the plume, which causes the measured ion current to deviate from the actual ion current and affects the judgment of the ion arrival time.

[0012] (2) Negatively biased grids accelerate ions. Since the entire grid structure requires a negative bias to attract ions and repel electrons, it accelerates the ion components in the pulsed plasma plume, resulting in a higher measured ion velocity.

[0013] (3) Low spatial resolution. Whether using the conductive or electrostatic properties of the plume, the current collected by the acquisition device is very small. In order to improve the measurement accuracy, a grid with a sufficiently large surface area must be used, resulting in low spatial resolution. It is usually used to measure the axial distribution of PPT pulsed plasma plume velocity. It is difficult to measure the radial distribution of PPT plume velocity, and it is impossible to accurately characterize the complete spatial distribution of PPT plume velocity.

[0014] (4) The velocity of neutral particles in a pulsed plasma plume cannot be measured. The conductivity of plasma is due to the function of its charged components, which do not involve neutral particles. Therefore, the obtained velocity of the pulsed plasma plume cannot reflect the velocity of neutral particles. Summary of the Invention

[0015] In view of the pulse characteristics and the differences in velocity of each component of the PPT plume, this invention proposes a time-of-flight probe (TOF probe) scheme for multi-mode velocities based on the time-of-flight principle. The main contents of the invention include: (1) This TOF probe scheme can realize several different working modes and can measure the velocity of different components of the pulsed plasma plume. (2) The probe loop current signal is realized by indirect measurement technology, which can avoid the electrostatic levitation and isolation problems in direct current measurement technology. Based on the time-of-flight (TOF) measurement principle, this invention proposes a technical solution with multiple working modes, which can measure the velocity of different components of the pulsed plasma plume.

[0016] The technical solution of the present invention is as follows:

[0017] A multi-mode TOF probe system for measuring the velocity of pulsed plumes in electric thrusters includes a TOF probe assembly, a three-dimensional positioning device for the probe assembly, a drive circuit, and a data acquisition device.

[0018] The TOF probe assembly consists of two sets of dual probe assemblies.

[0019] The probe assembly three-dimensional positioning device includes a probe base, a vertical adjustment mechanism, and a horizontal positioning device. The probe base is an insulating material disk with multiple evenly distributed circular holes. The dual probe assemblies are inserted into the circular holes, and the flight distance between the two sets of dual probe assemblies is changed according to different positions. The probe base is fixed to the top of the vertical adjustment mechanism. The horizontal positioning device has multiple evenly distributed circular holes, and the bottom end of the vertical adjustment mechanism is inserted into the circular holes. The vertical adjustment mechanism is used to change the vertical position of the TOF probe assembly in the pulsed plasma plume, and the horizontal positioning device is used to change the horizontal plane position of the TOF probe assembly in the pulsed plasma plume, that is, the radial and axial positions in the pulsed plasma plume.

[0020] The driving circuit includes an adjustable DC power supply, a buffer capacitor, and a charging current-limiting resistor. One terminal of the buffer capacitor is connected to the adjustable DC power supply, and the other terminal is grounded. A charging current-limiting resistor is connected in series between the buffer capacitor and the adjustable DC power supply to limit the peak charging current and charging time. The adjustable DC power supply is used to charge the buffer capacitor. The buffer capacitor is connected in parallel across the two ends of the dual-probe component in the TOF probe assembly to provide a driving voltage for the TOF probe. Different driving voltages enable the TOF probe assembly to enter different operating modes.

[0021] The two channels of the data acquisition device are connected to the positive terminal of the buffer capacitor to acquire and record the electrical signals of the TOF probe assembly in the working state, and transmit the signals to the computer for calculation to obtain the pulsed plasma plume velocity.

[0022] By adjusting the height of the vertical adjustment mechanism and its installation position on the horizontal positioning device, as well as the installation position of the TOF probe assembly on the probe base, the two sets of dual-probe assemblies are positioned at different locations along the pulsed plasma plume path, allowing for the measurement of the three-dimensional distribution of the pulsed plasma plume velocity. When the pulsed plume passes through the dual-probe assemblies, the plume plasma causes the dual-probe assemblies to conduct, and under the action of the driving voltage, a current is generated between the probes. The current pulses are measured and recorded by the data acquisition device to obtain the time when the pulsed plasma plume arrives at the dual probes. The time difference between the arrival times of the plume at the two sets of dual probes is calculated, which is the flight time of the pulsed plasma plume.

[0023] Furthermore, the two sets of dual-probe structures in the TOF probe assembly are exactly the same size.

[0024] Furthermore, each dual-probe assembly in the TOF probe kit includes two probes, a dual-hole ceramic tube, a single-hole alumina ceramic tube, and a shielded signal line. The lower part of the dual-hole ceramic tube is inserted into the single-hole alumina ceramic tube, and the lower parts of the two probes are inserted into the two holes of the dual-hole ceramic tube. The top of the probe is located outside the dual-hole ceramic tube, and the bottom of the probe is located outside the dual-hole ceramic tube and inside the single-hole alumina ceramic tube. The bottom of the probe is connected to the shielded signal line. The alumina single-hole ceramic tube and the dual-hole ceramic tube provide fixed support for the probes and shield against electromagnetic interference from external signals. The exposed length of the TOF probe assembly can be adjusted by adjusting the length of the probes protruding from the dual-hole ceramic tube. The probe gap can be adjusted by changing the spacing between the two holes of the dual-control ceramic tube.

[0025] Furthermore, the vertical adjustment mechanism has threads at both ends for connecting to the probe base and the horizontal positioning device.

[0026] Furthermore, the tip of the TOF probe assembly is copper-plated to facilitate the soldering of shielded signal lines; and an insulating layer is used to encapsulate the soldering area to prevent the solder joints from being exposed.

[0027] The method for calculating the velocity v of a pulsed plasma plume based on the time-of-flight method is as follows:

[0028]

[0029] Wherein, ΔL is the straight-line distance between the two sets of dual probes in the TOF probe assembly, i.e. the flight distance. By recording the times t1 and t2 when the pulsed plasma plume arrives at the two sets of dual probe assemblies, the flight time can be calculated as t2-t1, and the velocity of the pulsed plasma plume can be calculated according to formula (1).

[0030] The method for measuring the arrival times t1 and t2 of the pulsed plasma plume is as follows:

[0031] The arrival time of a pulsed plasma plume is measured by directly measuring the current pulse within the probe gap, with the peak value of the current pulse taken as the arrival time. Typical direct current measurement schemes require the measuring end to be suspended, leading to complex interface design challenges with subsequent digital acquisition circuits. This invention employs a method of measuring the discharge voltage waveform of a buffer capacitor and then calculating the discharge current: since one electrode of the buffer capacitor can be grounded, the data acquisition device can be directly connected to the buffer capacitor to measure and record its discharge voltage waveform, and then the discharge current waveform is obtained through numerical calculation. This allows the current measurement section and the data acquisition device to share a common ground, facilitating the integration, automation, and instrumentation of the entire probe system.

[0032] Specifically, when the plume pulse reaches the dual probes, the probe gap will conduct in different modes depending on the probe driving voltage, causing the charged buffer capacitor to discharge and form a discharge current. The discharge voltage waveform of the buffer capacitor is recorded by the data acquisition device, and then the discharge current waveform can be obtained by performing differentiation using formula (2). The peak time of this discharge current waveform represents the moment when the main body of the pulsed plasma plume arrives at the dual probes, and this is regarded as the start and end time of the pulsed plasma plume flight.

[0033] The discharge formula for the buffer capacitor is derived from equation (2):

[0034]

[0035] Where V is the voltage of the buffer capacitor and C is the capacitance value.

[0036] The initial voltage applied to the TOF probe by the buffer capacitor is called the driving voltage of the TOF probe. Different driving voltages allow the TOF probe to operate in different modes. The operating modes of the TOF probe include breakdown mode and conduction mode. The breakdown mode includes positive breakdown mode and negative breakdown mode. The working principle is as follows:

[0037] (1) Positive Breakdown Mode. When a sufficiently high positive driving voltage is applied to the TOF probes, breakdown conduction occurs in the probe gap, which is the positive breakdown mode. At this time, the probe gap is at a positive potential, which attracts electrons from the pulsed plasma plume and repels ions. The breakdown of the probe gap is achieved by attracting the enriched electrons and inducing avalanche ionization. The peak moment of the buffer capacitor discharge current characterizes the moment when the electron cluster in the pulsed plasma plume arrives at the dual probes, and the flight time calculated from this is the flight time of the electron cluster in the pulsed plasma plume.

[0038] (2) Negative Breakdown Mode. When a sufficiently high negative driving voltage is applied to the TOF probes, the probe gap can also undergo breakdown and conduction, which is the negative breakdown mode. At this time, the probe gap is at a negative potential, which attracts ions in the pulsed plasma plume and repels electrons. The breakdown of the probe gap is achieved by the enriched ions as seed charges: by bombarding the probe surface, secondary electron emission occurs, and then avalanche ionization occurs due to collisions with neutral particles, thereby inducing the breakdown of the probe gap. The peak moment of the buffer capacitor discharge current characterizes the moment when the ion cluster in the pulsed plasma plume arrives at the dual probes, and the flight time calculated from this is the flight time of the ion cluster in the pulsed plasma plume.

[0039] This invention also allows for the evaluation of neutral particle velocities in pulsed plasma plumes using a negative breakdown mode. In positive breakdown mode, electrons enriched within the probe gap require only a small number of neutral particles to achieve avalanche ionization. However, in negative breakdown mode, ion bombardment of the TOF probe electrode surface induces secondary electron emission, generating a small number of seed electrons, which then undergo avalanche ionization before finally triggering probe gap breakdown. Due to the low electron density generated by secondary electron emission, the probe gap requires a sufficient density of neutral particles to induce avalanche ionization-induced breakdown. Therefore, in negative breakdown mode, the neutral gas cloud and ion cloud must arrive at the probe gap synchronously. The peak discharge current of the buffer capacitor also represents the arrival time of the main neutral gas cloud, and the calculated velocity reflects the velocity of neutral particles in the pulsed plasma plume.

[0040] (3) Conductive Mode. When the TOF probe driving voltage is low, regardless of whether it is positive or negative, the TOF probe operates in conductive mode. In this mode, the probe gap does not ionize; it is only connected by charged particles in the pulsed plasma plume. Therefore, the conductivity of the probe gap is weak, the buffer capacitor discharges slowly, and the duration is longer. Due to the bipolar diffusion effect of electrons and ions in the pulsed plasma plume, they generally reach the probe gap at similar speeds. This is a measurement of the average velocity of the two types of charged particles, independent of neutral particles in the pulsed plasma plume. In conductive mode, the low TOF probe driving voltage has almost no effect on the flight speed of the pulsed plasma plume, and the obtained velocity better reflects the velocity of the charged components in the plume.

[0041] Based on the above analysis, different operating modes can be used to distinguish the flight time and velocity of different components in a pulsed plasma plume. Specifically, the positive breakdown mode can measure the velocity of electron clusters in the pulsed plume; the negative breakdown mode can measure the velocity of ion clusters in the pulsed plume; the conduction mode can measure the average velocity of charged particles in the pulsed plume; and the negative breakdown mode can also be used to evaluate the velocity of neutral particles in the pulsed plasma plume.

[0042] The beneficial effects of this invention are:

[0043] 1. This invention improves the current measurement method by providing an indirect current measurement scheme. Besides solving the problem of complex interface design between the measurement terminal and the acquisition circuit, which is required in direct current measurement schemes, it also avoids the use of current sensors commonly used in direct current measurement schemes, thus avoiding errors introduced by current sensors in the measurement results.

[0044] 2. Secondly, the TOF probe of the present invention has high sensitivity and high signal-to-noise ratio in the breakdown mode, and can achieve high spatial resolution even when the probe volume is small.

[0045] 3. This invention uses a digital acquisition module instead of an oscilloscope for data acquisition, which is easier to integrate and miniaturize compared to an oscilloscope. Furthermore, this module can integrate data acquisition and processing into software, facilitating automation and promoting the commercial development of TOF probes. Attached Figure Description

[0046] Figure 1 A schematic diagram showing the connection between the TOF probe assembly and the probe assembly 3D positioning device;

[0047] Figure 2(a) and Figure 2(b) are respectively internal and overall schematic diagrams of a set of dual probe components in the TOF probe assembly of the present invention;

[0048] Figure 3 A schematic diagram of the TOF probe assembly, its driving circuit structure, and the measurement method;

[0049] Figure 4 This is a measurement schematic diagram of a specific embodiment of the present invention;

[0050] Figure 5(a) shows the discharge voltage waveform of the buffer capacitor obtained in the positive breakdown mode in the embodiment, and Figure 5(b) shows the discharge current waveform of the buffer capacitor obtained in the positive breakdown mode in the embodiment.

[0051] Figure 6(a) shows the discharge voltage waveform of the buffer capacitor obtained in the negative breakdown mode in the embodiment, and Figure 6(b) shows the discharge current waveform of the buffer capacitor obtained in the negative breakdown mode in the embodiment.

[0052] Figure 7(a) shows the discharge voltage waveform of the buffer capacitor obtained in the conduction mode in the embodiment, and Figure 7(b) shows the discharge current waveform of the buffer capacitor obtained in the conduction mode in the embodiment.

[0053] Among them, 1-dual probe assembly, 1a-two probes, 1b-double-hole ceramic tube, 1c-insulating layer, 1d-single-hole alumina ceramic tube, 1f-shielded signal line, 2-probe base, 3-vertical adjustment mechanism, 4-horizontal positioning device, 5-drive circuit, 6-data acquisition module, 7-computer integrated software system, 8-vacuum chamber, 9-external DC power supply, 10-pulsed plasma thruster, 11-pulsed plasma plume. Detailed Implementation

[0054] The present invention will be further described below with reference to the accompanying drawings and specific embodiments:

[0055] like Figure 4 This is a measurement schematic diagram of a specific embodiment of the present invention.

[0056] The measurement object in this embodiment is the pulsed plasma plume 11 generated by the discharge of the parallel plate structure pulsed plasma thruster 10. The external DC power supply 9 has an output range of 0-2000V, the duration of the plume is on the order of microseconds, the maximum discharge current can reach tens of thousands of amperes, the maximum PPT injection energy is 6J, and a reliable signal can be measured at a distance of up to 250mm along the plume path. The main components of the pulsed plasma plume 11 are electrons, ions, and neutral particles with different velocities.

[0057] The device system of this embodiment includes a TOF probe assembly, a probe assembly three-dimensional positioning device, a drive circuit 5, a data acquisition device 6, and a computer integrated software system 7. The probe assembly three-dimensional positioning device includes a probe base 2, a vertical adjustment mechanism 3, and a horizontal positioning device 4.

[0058] like Figure 1 This is a schematic diagram showing the connection relationship between a set of dual probe components 1, probe base 2, vertical adjustment mechanism 3 and horizontal positioning device 4 in a TOF probe assembly.

[0059] The dual-probe assembly structure of the dual-probe assembly 1 is shown in Figures 2(a) and 2(b). Probe 1a consists of two cylindrical tungsten needles with a diameter of 0.3 mm, connected to the shielded signal line 1f by soldering. An insulating layer 1c is used to encapsulate the solder joints to prevent them from being exposed. The probe size selection must avoid excessive current collection leading to arcing and damage, and also avoid insufficient current collection affecting the signal-to-noise ratio. In this embodiment, by adjusting the length of probe 1a exposed above the double-hole ceramic tube 1b and the spacing between the two holes, the exposed probe length is set to 5 mm and the probe gap to 1 mm, achieving high spatial resolution. For the pulsed plasma plume 11 mentioned in this embodiment, in breakdown mode, the peak probe current is approximately 30 A. Based on the probe size, the calculated surface area is approximately 5 mm². 2 The peak probe current density is approximately 6 A / mm². 2 The double-hole ceramic tube 1b is fixedly supported by a single-hole alumina ceramic tube 1d with an inner diameter of 2mm, an outer diameter of 3mm, and a length of 20mm, using epoxy resin adhesive.

[0060] The dual-probe assembly 1 is fixed to the probe base 2 via a single-hole alumina ceramic tube 1d and connected to the horizontal positioning device 4 via a vertical adjustment mechanism 3. Simultaneously, the dual-probe assembly 1 is connected to the external drive circuit 5 of the vacuum chamber 8 via a shielded signal line 1f. The probe base 2 has multiple circular holes for adjusting the relative positions of the two sets of dual probes in the dual-probe assembly 1, thereby changing the flight distance ΔL of the pulsed plasma plume 11. The vertical adjustment mechanism 3 has a diameter of 6mm and a length of 40mm, with a threaded section at the bottom 20mm for adjusting the longitudinal height of the dual-probe assembly 1, i.e., changing the vertical position of the dual-probe assembly 1 within the pulsed plasma plume 11. The horizontal positioning device 4 is used to change the horizontal position of the dual-probe assembly 1, i.e., its radial and axial positions within the pulsed plasma plume 11. By adjusting the vertical adjustment mechanism 3 and the horizontal positioning device 4, the probes can traverse a certain range of the pulsed plasma plume space.

[0061] The horizontal positioning device 4 is 15mm thick and 340mm in diameter. It has multiple threaded holes with a diameter of 6mm and a spacing of 20mm between the threaded holes. It is used to fix the dual probe assembly 1 to achieve spatial resolution. It can measure the plume velocity up to 250mm after the pulsed plasma plume 11 exits the electrode plate.

[0062] In this embodiment, a measurement point is selected to verify the basic function of the TOF probe. By adjusting the position of the TOF probe component 1 and setting the flight distance ΔL of the plume to 30mm, the velocity of the pulsed plasma plume 11 100mm downstream of the pulsed plasma thruster 10 is measured.

[0063] like Figure 3 This is a schematic diagram of the TOF probe assembly and its driving circuit structure and measurement method. The driving circuit 5 includes: an adjustable DC power supply, a buffer capacitor, and a charging current-limiting resistor. The four electrostatic probes in the two sets of dual-probe assemblies 1 are labeled P1, P2, P3, and P4, respectively. Probes P1 and P4 are connected to the positive terminal of the adjustable DC power supply, while probes P2 and P3 are grounded. Buffer capacitors, labeled C1 and C2, are connected in parallel across the two sets of dual probes of the TOF probe assembly to drive the current pulse between the TOF probe assemblies. In this embodiment, a buffer capacitor value of 57nF is selected. Experiments show that in breakdown mode, when the main body of the pulsed plasma plume 11 passes through the two sets of dual probes, the buffer capacitor can discharge rapidly. The half-width at half-maximum (FWHM) of the discharge pulse is less than 1 microsecond, far less than the duration of the pulsed plasma plume 11, achieving high time resolution and high sensitivity. Charging current-limiting resistors, labeled R1 and R2, are connected in series between the buffer capacitor and the positive terminal of the adjustable DC power supply to limit the charging current and charging time of the buffer capacitor. Both resistors have a resistance value of 1MΩ.

[0064] In this embodiment, the Analog Discovery 2 module from Digilent Inc. is selected as the data acquisition device 6 to record the discharge voltage waveform of the buffer capacitor. The measurement method is as follows: Figure 3 As shown, the Analog Discovery 2 is a USB multifunction module. Using its oscilloscope function, the positive terminals of the cache capacitors C1 and C2 are connected to two channels of the Analog Discovery 2. The Analog Discovery 2 is connected to a computer via USB, and the module's driver software displays the discharge voltage waveform of the stored cache capacitors.

[0065] The discharge voltage data of the buffer capacitor collected by the data acquisition device 6 is imported into the Origin software. According to the calculation of equation (2), combined with the buffer capacitor value of 57nF, the discharge current waveform of the buffer capacitor can be obtained. The peak time of the discharge current of the buffer capacitor is used to characterize the time when the pulsed plasma plume 11 arrives at the dual probe. The breakdown time of the energy storage capacitor of the PPT prototype 10 is taken as the zero point of time and marked as t1 and t2 respectively.

[0066] Regarding the probe size in this embodiment, when the probe driving voltage is set to be greater than 200V, the TOF probe can operate in positive breakdown mode. In this embodiment, the probe driving voltage is set to 370V. Figure 5(a) shows the waveform of the buffer capacitor discharge voltage acquired by the data acquisition device 6 in positive breakdown mode. The waveform of the buffer capacitor discharge current is shown in Figure 5(b). The half-width at half-maximum (WHM) of the buffer capacitor discharge current pulse is approximately 0.4μs, which is much smaller than the duration of the pulsed plasma plume. At the same time, the peak value of the current pulse is approximately 30A, which is much larger than the amplitude of the noise signal. In breakdown mode, the TOF probe has high time sensitivity, and the measurement signal has a high signal-to-noise ratio even when the probe volume is small.

[0067] From Figure 5(b), we can obtain t1 = 2.54 μs and t2 = 3.35 μs. The flight time of the 11-electron cluster in the pulsed plasma plume can be calculated as follows:

[0068] t2-t1=0.81μs

[0069] The straight-line distance ΔL between the two sets of dual probes is 30 mm. According to formula (1), the velocity of the electron cluster in the pulsed plasma plume is finally obtained.

[0070]

[0071] When the probe driving voltage is set to be greater than -300V, the probe gap can also undergo breakdown and conduction; this is the negative breakdown mode of the TOF probe. In this embodiment, the probe driving voltage is set to -370V. Figure 6(a) shows the discharge voltage waveform of the buffer capacitor acquired by the data acquisition device 6 under the negative breakdown mode. The discharge current waveform of the buffer capacitor is shown in Figure 6(b).

[0072] From Figure 6(b), we can obtain t1 = 2.90 μs and t2 = 3.89 μs. The flight time of the 11 ions in the pulsed plasma plume can be calculated as follows:

[0073] t2-t1=0.99μs

[0074] The velocity of the ion cluster in the pulsed plasma plume is finally obtained according to formula (1):

[0075]

[0076] Meanwhile, in negative breakdown mode, the neutral gas cloud and ion cloud must arrive synchronously to induce breakdown in the probe gap. The time corresponding to the peak discharge current of the buffer capacitor also characterizes the arrival time of the neutral gas cloud bulk; therefore, the calculated velocity reflects the velocity of neutral particles in the pulsed plasma plume.

[0077] When the absolute value of the probe drive voltage is set below 120V, the TOF probe will operate in conduction mode regardless of whether it is positive or negative. In this embodiment, the probe drive voltage is set to approximately 100V. Figure 7(a) shows the discharge voltage waveform of the buffer capacitor acquired by the data acquisition device 6 in conduction mode. The breakdown time of the energy storage capacitor of the pulsed plasma flow thruster 10 is taken as the zero point of time.

[0078] From Figure 7(b), we can obtain t1 = 2.50 μs and t2 = 3.73 μs. The calculated flight time of the entire charged particles in the pulsed plasma plume 11 is:

[0079] t2-t1=1.23μs,

[0080] The straight-line distance ΔL between the two sets of dual probes is 30 mm. According to formula (2), the average velocity of electrons and ions in the pulsed plasma plume is finally obtained:

[0081]

Claims

1. A multi-mode TOF probe system for electric thruster pulse plume velocity measurements, characterized by, The multi-mode TOF probe system for measuring the pulse plume velocity of an electric thruster comprises a TOF probe assembly, a three-dimensional positioning device for the probe assembly, a driving circuit and a data acquisition device; The TOF probe assembly is composed of two groups of double-probe assemblies; The three-dimensional positioning device for the probe assembly comprises a probe base, a vertical adjusting mechanism and a horizontal positioning device; the probe base is a disc of insulating material, and a plurality of round holes are uniformly distributed on the disc, the double-probe assemblies are inserted into the round holes, and the flying distance between the two groups of double-probe assemblies is changed according to different positions; the probe base is fixed to the top end of the vertical adjusting mechanism; a plurality of round holes are uniformly distributed on the horizontal positioning device, and the bottom end of the vertical adjusting mechanism is inserted into the round holes; the vertical adjusting mechanism is used to change the vertical position of the TOF probe assembly in the pulse plasma plume, and the horizontal positioning device is used to change the horizontal plane position of the TOF probe assembly in the pulse plasma plume, i.e. the radial and axial positions in the pulse plasma plume; The driving circuit comprises a voltage-adjustable DC power supply, a buffer capacitor and a charging current-limiting resistor; one pole of the buffer capacitor is connected to the voltage-adjustable DC power supply, and the other pole is grounded; the buffer capacitor and the voltage-adjustable DC power supply are connected in series with the charging current-limiting resistor for limiting the charging current peak value and the charging time; the voltage-adjustable DC power supply is used to charge the buffer capacitor; the buffer capacitor is connected in parallel to the two ends of the double-probe assemblies in the TOF probe assembly to provide a driving voltage for the TOF probe; different driving voltages make the TOF probe assembly enter different working modes; The two channels of the data acquisition device are connected to the positive pole of the buffer capacitor for collecting and recording the electrical signals of the TOF probe assembly in the working state, and transmitting the signals to a computer for calculation to obtain the pulse plasma plume velocity; By adjusting the height of the vertical adjusting mechanism, the mounting position on the horizontal positioning device and the mounting position of the TOF probe assembly on the probe base, the two groups of double-probe assemblies are placed at different positions in the path of the pulse plasma plume to measure the three-dimensional distribution of the pulse plasma plume velocity; when the pulse plume passes through the double-probe assemblies, the plume plasma makes the double-probe assemblies conductive; under the action of the driving voltage, an electric current is generated between the probes; the current pulse is measured and recorded by the data acquisition device to obtain the time when the pulse plasma plume reaches the double-probe assemblies, and the time difference when the plume reaches the two groups of double-probe assemblies, i.e. the flight time of the pulse plasma plume; wherein the velocity of the pulsed plasma plume is calculated based on a time-of-flight method The method is as follows: (1) ； where Δ L is the linear distance between the two double probe assemblies in the TOF probe assembly, i.e. the flight distance; by recording the time instants t1 and t2 at which the pulsed plasma plume reaches the two double probe assemblies, the flight time is calculated as t2-t1, and the velocity of the pulsed plasma plume is calculated according to equation (1); The measurement method of the arrival time t1 and t2 of the pulse plasma plume is as follows: The measurement of the arrival time of the pulse plasma plume by using the electrical characteristics of the pulse plasma plume is realized by directly measuring the current pulse in the probe gap, and the peak time of the current pulse is taken as the arrival time of the plume; the discharge current is calculated by measuring the discharge voltage waveform of the buffer capacitor, i.e. the data acquisition device is directly connected to the buffer capacitor to measure and record the discharge voltage waveform, and the discharge current waveform is obtained by numerical calculation; the specific method is as follows: When the plume pulse reaches the double-probe assembly, the probe gap is turned on in different modes according to the different probe driving voltages, so that the charged buffer capacitor is discharged to form a discharge current; the discharge voltage waveform of the buffer capacitor is recorded by the data acquisition device, and then the formula (2) is used for differential operation to obtain the discharge current waveform, the peak time of which represents the time when the main body of the pulse plasma plume reaches the double-probe assembly, which is regarded as the starting and ending time of the flight of the pulse plasma plume; The discharge formula of the buffer capacitor is derived from equation (2): (2) ； wherein, V is the cache capacitor voltage, C is the capacitance value.

2. A multi-mode TOF probe system for electric thruster pulse plume velocity measurements according to claim 1, characterized in that, The two double-probe assembly structures in the TOF probe assembly are completely the same in size.

3. A multi-mode TOF probe system for electric thruster pulse plume velocity measurements according to claim 1 or 2, characterized in that, Each double-probe assembly in the TOF probe assembly includes two probes, a double-hole ceramic tube, a single-hole alumina ceramic tube, and a shielding signal line, the lower part of the double-hole ceramic tube is inserted into the single-hole alumina ceramic tube, the lower part of the two probes is inserted into the two holes of the double-hole ceramic tube, the top end of the probe is located outside the double-hole ceramic tube, the bottom end of the probe is located outside the double-hole ceramic tube and inside the single-hole alumina ceramic tube, and the bottom end of the probe is connected with the shielding signal line; the alumina single-hole ceramic tube and the double-hole ceramic tube realize the fixed support of the probe, and shield the electromagnetic interference caused by external signals; the exposed length of the TOF probe assembly is adjusted by adjusting the length of the probe exposed from the double-hole ceramic tube; the probe gap can be adjusted by changing the distance between the two holes of the double-hole ceramic tube.

4. A multi-mode TOF probe system for electric thruster pulse plume velocity measurements according to claim 1 or 2, characterized in that, The vertical adjusting mechanism is attached with threads at both ends for connection with the probe base and the horizontal positioning device.

5. A multi-mode TOF probe system for electric thruster pulse plume velocity measurements according to claim 3, wherein, The vertical adjusting mechanism is attached with threads at both ends for connection with the probe base and the horizontal positioning device.

6. A multi-mode TOF probe system for electric thruster pulse plume velocity measurements according to claim 1 or 2 or 5, characterized in that, The end of the probe tip of the TOF probe assembly is plated with copper, which facilitates the welding of the shielding signal line; and an insulating layer is used to encapsulate the welding area to prevent the welding point from being exposed.

7. A multi-mode TOF probe system for electric thruster pulse plume velocity measurements according to claim 3, characterized in that, The end of the probe tip of the TOF probe assembly is plated with copper, which facilitates the welding of the shielding signal line; and an insulating layer is used to encapsulate the welding area to prevent the welding point from being exposed.

8. A multi-mode TOF probe system for electric thruster pulse plume velocity measurements according to claim 4, characterized in that, The end of the probe tip of the TOF probe assembly is plated with copper, which facilitates the welding of the shielding signal line; and an insulating layer is used to encapsulate the welding area to prevent the welding point from being exposed.

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