An electrostatic three-probe system suitable for pulsed plasma diagnosis

Abstract

The present invention discloses an electrostatic three-probe system suitable for pulse plasma diagnosis. The system comprises the following steps: introducing magnetic coupling into circuit design to achieve electrical isolation between a probe current measurement circuit and a probe drive circuit, so that a probe current measurement portion can share a common ground with a data acquisition device without affecting reliable electrical suspension of the probe drive circuit; reducing the composition and volume of the circuit suspension portion, reducing the ground capacitance of the probe circuit; and sharing a common ground with the data acquisition device, thereby facilitating the integration, automation, and instrumentation of the entire probe system. By weakening the electrostatic coupling between the two circuits, interference of pulse discharge on the current measurement circuit is effectively suppressed. A three-probe data analysis algorithm is proposed to effectively correct diagnostic result deviations caused by probe operating point voltage selection, expand the selection range of the probe operating point voltage, resolve data analysis difficulties when the three-probe operating point voltage is in a probe bias saturation range, and expand and improve the diagnostic principle of the three-probe.

Description

Technical Field

[0001] The invention belongs to the technical field of pulse plasma electrostatic probe diagnosis, and relates to an electrostatic three-probe system suitable for pulse plasma diagnosis. Background Art

[0002] Pulsed discharge plasmas typically exhibit short durations, rapid state changes, and high peak values. They are widely used in electric propulsion systems, plasma-assisted rocket ignition, and many other civilian industrial technologies. Pulsed plasmas used in electric propulsion typically last only microseconds, and parameters such as the electron temperature and ion density change rapidly during this duration. This places special demands on diagnostic technology and equipment, particularly those with ultra-high temporal resolution.

[0003] Conventional low-temperature plasma diagnostic technologies include: electrostatic probe technology and spectroscopy technology.

[0004] Although spectral diagnostic technology has the ability to analyze parameters such as plasma electron temperature, ion density, and elemental composition, its accuracy needs to be calibrated with an electrostatic probe. In addition, the response time of the spectral diagnostic system is usually long, that is, the time resolution capability is poor, and it is not suitable for high-speed real-time tracking of pulsed plasma parameters.

[0005] Electrostatic probe (also known as Langmuir probe) technology is a fundamental technology for low-temperature plasma diagnosis. It comes in various types and can diagnose the basic parameters of low-temperature plasmas in various states. For pulsed plasmas, especially short-pulse plasmas, electrostatic three-probe technology is a fundamental means of achieving real-time tracking of their state parameters. This is determined by the working principle of the electrostatic three-probe technology. The three-probe drive does not require voltage scanning, only the setting of a fixed operating point voltage. Therefore, the three-probe signal can reflect changes in plasma parameters in real time, thereby achieving real-time tracking and diagnosis. Its time resolution depends on the response rate of the probe sheath and the drive circuit.

[0006] Pulsed plasma is commonly found in electric thrusters and the ignition devices of conventional rockets. Therefore, real-time monitoring of pulsed plasma is one of the fundamental technologies in aerospace engineering. Pulsed plasma usually has the characteristics of short duration and high peak density, so the current signal formed on the current mode three-probe has the characteristics of high characteristic frequency and large peak value. The electrostatic three-probe technology is a solution for real-time tracking of the basic parameters of time-varying plasma.

[0007] Common electrostatic three-probe design schemes can be divided into voltage mode and current mode according to the working principle. Regardless of the mode, the commonly used measurement and recording methods of three-probe signals have various circuit technical difficulties due to the constraints of electrical suspension conditions, which has prevented the design of the three-probe system from being mature and qualitative, and from being commercialized. Patent CN 114245555 A proposes a technical solution that integrates electrostatic single probes, dual probes, and triple probes, in which the three probes are based on the design of the traditional voltage mode. Specifically: one of the three probes is suspended, and the other two probes apply driving voltage to form a group of fixed working point dual probes, and the suspension potential of the suspended probe and the current of the dual probes at the fixed working point are measured at the same time as the three-probe signal, and the electron temperature and ion density of the plasma are calculated based on the basic principle of the three probes. Both the suspension potential and the probe current are directly displayed by the electrical suspension instrument, and cannot be integrated with the information processing equipment.

[0008] Patent CN 112415250 A is a conventional current mode three-probe solution. The so-called current mode three-probe can be regarded as two sets of dual probes driven by different working point voltages, one of which is shared by the two sets of dual probes. In the above patent, the two current signals are measured by connecting a sampling resistor. The plasma electron temperature and electron density are calculated based on the basic principle of the three-probe according to the working voltage of the two sets of probes and the measured current signal. Although the sampling resistor solution used therein can realize the measurement and display of current signals, it is difficult to realize high-speed automatic acquisition of digital information equipment because the information equipment is difficult to connect to the probe circuit. Similarly, the probe current measurement method proposed in patent CN 114236222 A is also based on the sampling resistor. The signal on the resistor is digitized by the data acquisition module after passing through the differential amplifier circuit, thereby realizing integration with the digital information processing system. However, this solution has the following disadvantages: (1) It increases the circuit complexity and increases the chance of introducing noise. In particular, the effective signal of the sampling resistor is generally much smaller than the voltage of the sampling resistor to the ground. The slight fluctuation of the voltage to the ground will become a significant interference noise to the effective signal. (2) The introduction of the differential amplifier circuit increases the capacitance of the probe circuit to ground. Its application in three-probe current measurement will introduce an AC grounding point for the three probes, reducing the electrical suspension ability of the three probes.

[0009] When an electrostatic three-probe system is used for short-pulse plasma diagnosis, the probe system's suspended potential relative to the plasma changes rapidly, and therefore the probe system's potential relative to ground also changes rapidly. This short-pulse signal becomes a potential noise source for the probe circuit. However, the voltage-mode three-probe's noise immunity is significantly weaker than the current-mode one. This is because, in voltage mode, the suspended probe potential of the three-probe system must be measured. The electromagnetic radiation from the short-pulse plasma can cause fluctuations in the suspended probe potential, making accurate measurement difficult. Therefore, the current-mode three-probe system is more suitable for real-time tracking and diagnosis of short-pulse plasma parameters.

[0010] The plasma used in many aerospace applications is short-pulse plasma, which places an urgent need for mature and reliable three-probe technology. Current-mode three-probe current signal measurement methods are crucial. Currently, the current-mode three-probe solutions used in patents and academic papers still have various shortcomings, the most significant of which are as follows:

[0011] As an integral part of the probe circuit, the probe current measurement circuit introduces a large probe-to-ground capacitance due to the cumbersome measurement circuit, which reduces the electrical levitation performance of the probe system. For example, in the commonly used sampling resistor current measurement scheme, since the effective signal on the resistor is generally very small, it needs to be amplified by an amplifier circuit before it can be reliably collected. However, the probe current measurement circuit and auxiliary devices, such as oscilloscopes or other digital information acquisition systems, will significantly increase the probe system's capacitance to ground. Short pulse probe currents will be shunted through the ground capacitance, making the probe current measurement inaccurate. Currently, the commonly used probe current measurement technology must rely on the overall suspension of the circuit to maintain the electrical suspension of the probe system. Therefore, it cannot be directly connected to the common ground of the digital acquisition system and the computer terminal, making it difficult to achieve informatization, automation, and integration of the probe system.

[0012] Current three-probe schemes, such as those described in the paper "Instantaneous Direct-Display System of Plasma Parameters by Means of Triple Probe," require that their operating point voltage be within the transition range of the probe bias voltage. If the operating point falls within the saturation region of the probe bias voltage, the electron temperature and ion density calculated using the commonly used data analysis methods described in the paper will exhibit significant deviations. Therefore, the choice of operating point voltage for conventional three-probe measurements must be restricted. This constraint often leads to measurement errors in three-probe measurements due to improper operating point voltage settings. Summary of the Invention

[0013] In view of the characteristics and diagnostic requirements of pulsed plasma used in aerospace, this paper proposes an electrostatic three-probe system suitable for pulsed plasma diagnosis. The three-probe circuit and analysis algorithm are also provided to achieve the following goals: (1) solve the problem of incomplete suspension between the probe circuit and the pulsed plasma. (2) break through the selection constraints of the probe operating point voltage.

[0014] The specific technical solutions of the present invention are as follows:

[0015] An electrostatic three-probe system suitable for pulsed plasma diagnosis, comprising an electrostatic three-probe system and a data acquisition and analysis system;

[0016] The electrostatic three-probe comprises an electrostatic three-probe body and an electrostatic three-probe circuit;

[0017] The electrostatic three-probe body includes three probes and a porous ceramic tube. The three probes are placed in the ceramic tube holes and are positioned and supported by the tube holes. The three probes are marked as P1, P2, and P3, respectively, where P1 is a common probe; the electrostatic three-probe circuit includes two loops, each loop includes a magnetic induction current sensor, a DC power supply, a buffer capacitor and two probes; the magnetic induction current sensor includes a primary coil, a secondary coil and a magnetic core, and the primary coil and the secondary coil are respectively tightly wound on the magnetic core; the buffer capacitors in the two loops are both connected in parallel to both ends of the DC power supply, the positive pole of the DC power supply is connected to the common probe P1, and the negative pole of the DC power supply is connected in series with the primary coil of the magnetic induction current sensor and then connected to probes P2 and P3 respectively.

[0018] The number of turns in the primary coil of a magnetic induction current sensor is determined by predicting the probe loop current amplitude and rate of change and calculating the induced voltage of the primary coil. The number of turns required for the induced voltage of the primary coil to be less than one-tenth of the probe drive voltage is sufficient to ensure that the impact of the primary coil voltage on the probe operating point voltage is negligible. The turns ratio of the primary and secondary coils of a magnetic induction current sensor can affect the voltage ratio between the primary and secondary coils, and the number of turns of the secondary coil can be determined based on signal amplification requirements. The capacitance of the buffer capacitor must be selected to ensure that the total charge flowing through the probe loop during a single pulse is significantly less than the charge stored in the buffer capacitor.

[0019] The magnetic core material must ensure maximum magnetic coupling efficiency, and its shape must be optimized to minimize the coupling capacitance between the primary and secondary coils. The core material's rated operating frequency should be higher than the characteristic frequency of the current pulse signal, and its magnetic permeability should be as high as possible and its electrical conductivity as low as possible. The core shape should ensure maximum spacing between the primary and secondary coils wound around the core.

[0020] The data acquisition and analysis system includes a data acquisition module and a data processing and analysis system;

[0021] The data acquisition module consists of a preamplifier and a data acquisition card. The preamplifier is directly connected to the secondary coil of the magnetic induction current sensor. The induced voltage signal of the secondary coil of the current sensor is amplified and noise suppressed by the preamplifier. The data acquisition card completes data acquisition and digitization and transmits it to the data processing and analysis system. To ensure the overall time resolution capability of the three-probe system, the sampling rate of the data acquisition card must be higher than the response rate of the electrostatic three-probe circuit.

[0022] The data processing and analysis system first calculates the probe current based on the induced voltage of the secondary coil of the magnetic induction current sensor recorded by the data acquisition module. The specific calculation method is as follows:

[0023] When the electrostatic three-probe circuit is placed in a pulsed plasma, the current signal formed in the loop of any electrostatic three-probe circuit is i(t). Then, the secondary coil of the current sensor outputs an induced voltage u(t) under the excitation of the current i(t). The relationship between the induced voltage u(t) of the secondary coil of the current sensor and the loop current of the electrostatic three-probe circuit at the primary coil end is:

[0024]

[0025] Where M is the mutual inductance of the primary and secondary coils.

[0026] The loop current of the electrostatic three-probe circuit can be calculated by integrating the induced voltage u(t) of the current sensor secondary coil:

[0027]

[0028] Then, the data processing and analysis system calculates the current I2 and I3 of the probes P2 and P3 and the working point voltage V 12 、V 13 Calculate the electron temperature and ion density of the plasma.

[0029] The specific data analysis algorithm is as follows:

[0030] (a) The three-probe operating point voltage V 12 、V 13 The larger value V 13 The data analysis method for the probe bias voltage in the saturation range is:

[0031] The currents I1 and I2 of probes P1 and P2 are expressed as:

[0032]

[0033]

[0034] in,

[0035]

[0036]

[0037] Where, J e is the electron saturation current density, V1 is the negative bias voltage of probe P1 relative to the plasma potential, V2 is the negative bias voltage of probe P2 relative to the plasma potential, J i (V1) is the ion saturation current density of probe P1, J i (V2) is the ion saturation current density of probe P2, S is the probe surface area, k is the Boltzmann constant, e is the electron charge, m e is the mass of the electron, T e is the electron temperature, n e is the electron density and π is the circumference of a circle.

[0038] Probe P3 is at voltage V 13 The electron flow part of the probe current I3 is directly ignored and simplified to:

[0039] I3=SJ i (V3) (7)

[0040] Where V3 is the negative bias voltage of probe P3 relative to the plasma potential, J i (V3) is the ion saturation current density of probe P3.

[0041] The three probe currents satisfy the relationship:

[0042] I1=I2+I3 (8)

[0043] The working point voltage of the three probes and the probe potential satisfy the relationship:

[0044] V 12 =V2-V1 (9)

[0045] V 13 =V3-V1 (10)

[0046] Assume that the changes in the saturation currents of the three probe ions caused by the probe potential are negligible, that is:

[0047] J i (V1) = J i (V2) = J i (V3) (11)

[0048] By combining the above expressions (3) to (10), we can derive the equation for the electron temperature:

[0049]

[0050] According to equation (12), the electron temperature of the plasma can be calculated as T e , unit is K.

[0051] The current I3 of probe P3 is the ion saturation current, and the ion saturation current density formula is:

[0052] J i =n i ev i (13)

[0053] Among them, n i is the ion density at the interface between the sheath and the pre-sheath, v i is the corresponding ion velocity, according to the Bohm sheath principle:

[0054]

[0055]

[0056] Where n0 is the ion density in the plasma, m i is the ion mass.

[0057] The simultaneous equations (7), (13), (14), and (15) can be used to obtain the equation for ion density:

[0058]

[0059] According to equation (16), the ion density n0 in the plasma can be calculated, and its unit is m -3 .

[0060] (b) When the three-probe operating point voltage V 12 、V 13 When both are in the transition range of the probe bias voltage, the electron temperature and ion density of the plasma can be calculated according to the commonly used triple-probe data analysis method in the paper "Instantaneous Direct-Display System of Plasma Parameters by Means of TripleProbe". Specifically:

[0061] The currents I1, I2, and I3 of the three probes P1, P2, and P3 are expressed as:

[0062]

[0063]

[0064]

[0065] Among them, Je is the electron saturation current density, V1 is the negative bias voltage of probe P1 relative to the plasma potential, V2 is the negative bias voltage of probe P2 relative to the plasma potential, V3 is the negative bias voltage of probe P3 relative to the plasma potential, J i (V1) is the ion saturation current density of probe P1, J i (V2) is the ion saturation current density of probe P2, J i (V2) is the ion saturation current density of probe P2, S is the probe surface area, k is the Boltzmann constant, e is the electron charge, m e is the mass of the electron, T e is the electron temperature, n e is the electron density, π is the circumference of the circle, is formula (5), J e It is formula (6).

[0066] The equation for electron temperature can be derived from equations (5), (6), (8) to (11), and (17) to (19):

[0067]

[0068] The electron temperature T can be calculated using equation (20): e .

[0069] According to equations (18), (19) and (11), the ion saturation density J can be derived i for:

[0070]

[0071] According to equation (21), equations (13) to (15) and equations (9) and (10), the equation for ion density can be derived:

[0072]

[0073] The ion density n0 can be calculated according to equation (22).

[0074] According to the above two data analysis methods where the three-probe operating point voltage is in different intervals, the specific process of the algorithm is as follows:

[0075] Since it is impossible to predict whether the three-probe working point voltage will enter the saturation range of the probe bias when selecting the three-probe working point voltage, it is necessary to use the measured probe current signals I2, I3 and the three-probe working point voltage V 12 、V 13 , use the data analysis method in (b) to preliminarily calculate the electron temperature and ion density; then compare the calculated electron temperature with the larger of the three probe working point voltages V13 Compare the values. If V 13 If the difference between the value of V and the electron temperature is not large, the current settlement result is output directly; if V 13 When it is greater than three times the electron temperature (this multiple can be appropriately increased), it is necessary to use the data analysis method in (a) to enter the probe bias saturation range when the three-probe operating point voltage enters the electron temperature and ion density to recalculate the electron temperature and ion density; then, output the recalculated electron temperature and ion density.

[0076] Beneficial effects of the present invention:

[0077] (1) Magnetic coupling is introduced into the circuit design to achieve electrical isolation between the probe current measurement circuit and the probe drive circuit, so that the probe current measurement part can share the same ground with the data acquisition device without affecting the reliable electrical suspension of the probe drive circuit. This greatly reduces the composition and volume of the circuit suspension part and significantly reduces the ground capacitance of the probe circuit. The common ground of the probe current measurement circuit part and the data acquisition device facilitates the integration, automation, and instrumentation of the entire probe system.

[0078] (2) The loop current signal of the electrostatic three-probe circuit is obtained by time integration of the output voltage signal of the current sensor. As for the noise of the signal, the integration operation can reduce the noise amplitude and improve the signal-to-noise ratio of the probe current signal.

[0079] (3) A three-probe data analysis algorithm is proposed to effectively correct the diagnostic result deviation caused by the selection of the probe working point voltage, expand the selection range of the probe working point voltage, basically remove the restrictions on the selection of the probe working point, solve the data analysis difficulties when the three-probe working point voltage is in the probe bias saturation range, and expand and improve the diagnostic principle of the three-probe. BRIEF DESCRIPTION OF THE DRAWINGS

[0080] FIG1( a ) is a schematic diagram of the probes in the electrostatic three-probe body of the present invention, and FIG1( b ) is a schematic diagram of the overall structure of the electrostatic three-probe body of the present invention;

[0081] Figure 2 Schematic diagram of the assembly of the electrostatic three-probe and supporting machinery;

[0082] Figure 3 This is a schematic diagram of the electrostatic three-probe circuit of the present invention;

[0083] Figure 4 Schematic diagram of the magnetic induction current sensor structure and probe current measurement method;

[0084] Figure 5 Schematic diagram of a measuring device according to an embodiment of the present invention;

[0085] Figure 6This is a flow chart of the three-probe analysis algorithm of the present invention;

[0086] Figure 7 For this implementation, the spark plug discharge voltage and current waveforms;

[0087] Figure 8 : is a typical signal waveform of the magnetic induction current sensor of this embodiment;

[0088] Figure 9 is the typical three-probe current waveform of this embodiment;

[0089] Figure 10 This is an example of the electron temperature and density calculated using conventional methods and the experimental calculation of the electron temperature using the three-probe data analysis algorithm of the present invention;

[0090] In the figure: electrostatic three-probe body 1, adjustable probe base and bracket 2, probe horizontal movement platform 3, data acquisition module 7, computer 8, probe circuit 9, vacuum chamber 10, spark plug power supply 11, spark plug 12, spark plug pulse plasma plume 13; 1a tungsten needle, 1b copper sleeve, 1c three-core signal wire, 1d porous ceramic tube. DETAILED DESCRIPTION

[0091] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0092] Figure 5 The test system of this embodiment is shown, comprising: an electrostatic three-probe body 1, an adjustable probe base and bracket 2, a probe horizontal motion platform 3, a data acquisition module 7, a computer 8 (with a built-in data processing and analysis system), a probe circuit 9, a vacuum chamber 10, a spark plug power supply 11, a spark plug 12, and a spark plug pulse plasma plume 13. The measurement target of this embodiment is the pulsed plasma plume 13 generated by the ignition spark plug 12 of the PPT.

[0093] The structure of the three-probe body 1 is shown in Figure 1(a) and Figure 1(b), wherein Figure 1(a) is a cylindrical tungsten needle with a diameter of 1mm and a length of 20mm. The end of the tungsten needle 1a is inserted into the copper sleeve 1b, and the copper sleeve 1b is squeezed to make it in close contact with the tungsten needle 1a. The copper sleeve 1b is connected to the three-core signal line 1c by soldering, solving the problem of difficulty in directly welding the needle tip material and the signal line. The three-probe support body is a porous ceramic tube 1d. The three tungsten needles 1a and the copper sleeve are placed in the ceramic tube hole and are positioned and fixed by the tube hole. The hole spacing is 3mm. By adjusting the length of the tungsten needle 1a exposed from the ceramic tube hole, the exposed length of the tungsten needle 1a is set to 10mm, that is, the probe tip size is: 1mm in diameter and 10mm in length. According to the probe size, the surface area of ​​the probe is calculated to be approximately 32mm. 2 , for the spark plug discharge pulse plasma plume 13 mentioned in this embodiment, according to the spark plug discharge plasma density of about 1011 cm -3 The three tungsten needles of the three-probe body 1 are marked as probes P1, P2, and P3 respectively. The assembly of the electrostatic three-probe and the supporting mechanism is as follows: Figure 2 shown.

[0094] Figure 3 The schematic diagram of the electrostatic three-probe circuit is shown. The electrostatic three-probe circuit includes two loops. Each loop includes a magnetic induction current sensor, a DC power supply, a buffer capacitor and two probes. The structure of the magnetic induction current sensor is as follows: Figure 4 As shown, it includes the primary coil 4, the secondary coil 6 and the magnetic core 5. The buffer capacitors in the two loops are connected in parallel at both ends of the DC power supply. The positive pole of the DC power supply is connected to the common probe P1. The negative pole of the DC power supply is connected in series with the primary coil of the magnetic induction current sensor and then connected to probes P2 and P3 respectively. Set the DC power supply output voltage V 12 is 3V, V 13 The current sensor has a primary coil of 30 turns and a secondary coil of 100 turns.

[0095] The secondary coil of the magnetic induction current sensor is directly connected to a data acquisition module 7, which records the induced voltage signal from the magnetic induction current sensor at a sampling rate of 1 MSPS. This module is connected to a computer 8 via a USB cable. The computer's data processing and analysis system reads, displays, and processes the induced voltage signal from the secondary coil of the magnetic induction current sensor, ultimately calculating the plasma electron temperature and ion density.

[0096] The magnetic core of the magnetic induction current sensor is a rectangle with two characteristic dimensions: width d and length L. The length L is 30 mm and the width d is 15 mm. The primary coil and the secondary coil are tightly wound around the two short sides of the rectangular core. Increasing the length L of the rectangular core can adjust the spacing between the primary and secondary coils and reduce the coupling capacitance between the primary and secondary coils. The smaller the capacitance, the weaker the AC coupling between the primary and secondary coils, the more complete the isolation, and the more conducive to the electrostatic suspension of the probe drive circuit.

[0097] In determining the capacitance of the buffer capacitor and the number of turns of the primary coil of the magnetic induction current sensor, assuming that the probe current is 2 mA on average during the pulse holding time and the holding time is 20 μs, the total charge flowing through the probe circuit during one pulse is 4×10 -8 C, the stored charge when the buffer capacitor voltage is 3V is 3×10 -3C is much larger than the total charge flowing in the circuit. The buffer capacitor can ensure the voltage stability of the DC power supply. The inductance of the magnetic induction current sensor with 30 turns of the primary coil is 18μH using an impedance analyzer. The density of the pulse plasma generated by the spark plug increases rapidly in the early stage. Assuming a 6V voltage drive, the rate of change of the probe current in the early stage of the pulse, di(t) / dt, can reach a maximum of 5×10 4 After estimation, the induced voltage of the primary coil of the magnetic induction current sensor reaches a maximum of 0.9V. During most of the pulse duration, the induced voltage of the primary coil of the magnetic induction current sensor is far less than 0.9V, maintaining approximately between 0.1 and 0.5V, which has little impact on the voltage between the probes. The secondary coil of the magnetic induction current sensor has 100 turns, and the voltage transformation ratio between the primary and secondary coils is approximately 3 times. The output voltage signal at the secondary coil end is approximately between 0.3 and 1.5V.

[0098] The three probes are moved by the probe moving platform 3 to a position 20 mm downstream of the spark plug end face, and the electron temperature and ion density of the spark plug plume 13 at this position are measured. Figure 7 The figure shows the typical spark plug discharge voltage and current waveforms of this embodiment. The spark plug discharge maintenance time is about 17μs and the peak current is 200A. Figure 8 The figure shows the induced voltage signal of the secondary coil of the magnetic induction current sensor recorded by the data acquisition module 7 of this embodiment. The voltage signal is integrated according to formula (2) to obtain Figure 9 As shown in the probe currents I2 and I3, the current peak value of probe P2 reaches 1.5mA, and the current peak value of probe P3 reaches 4mA.

[0099] The three-probe analysis algorithm process of the present invention is as follows Figure 6 shown. Figure 10 This figure shows the electron temperature and ion density of a pulsed plasma sample obtained using a commonly used calculation method and an experimental calculation of the electron temperature using the data processing algorithm of the present invention. This example focuses only on the high-density portion of the pulse plume. For the delayed ablation of the pulse plume tail, where the plasma is relatively thin and the probe signal is small, the electron temperature and ion density are directly set to zero during the calculation. The figure shows that the electron density rises rapidly in the early stages of the pulse, reaching a peak of 4.8×10 11 cm -3 , then rapidly decreases. There is a low-density lag phase in the late pulse. When the three probes are moved to 40 mm downstream of the spark plug end face using the probe moving platform 3 and measured again, the peak electron density of the spark plug plume drops to 2.9×10 11 cm -3The electron temperature calculated by the conventional method in the figure can reach 3-4eV in the early stage of the pulse, and maintains around 2eV in the middle and late stages of the pulse. The electron temperature fluctuates to a certain extent during the pulse maintenance time. The figure also shows the experimental calculation results of the electron temperature during the pulse maintenance time using the data processing algorithm of the present invention. In order to show the effect of the data processing algorithm of the present invention, V is set in the program. 13 When it is greater than twice the electron temperature calculated by the commonly used method, the electron temperature is recalculated according to the data analysis method when the three-probe working point voltage enters the probe bias saturation interval. It can be seen that when the electron temperature is 3-4V in the early stage of the pulse, the data processing algorithm of the present invention does not recalculate the electron temperature. When the electron temperature is maintained at around 2eV in the middle and late stages of the pulse, the algorithm starts the data analysis part of the three-probe working point voltage entering the saturation interval. The recalculated electron temperature is significantly lower than the electron temperature obtained by the commonly used method. The electron temperature is related to the electron flow in the probe current. In the commonly used three-probe analysis method, the probe current includes both electron flow and ion flow. When one of the three probes is at the saturation bias, its electron flow is extremely small, or even tends to zero. In the commonly used method, the electron flow part of the probe current is still considered, which will cause the electron temperature obtained by the commonly used method to be too high. Figure 10 The electron temperature calculated by the data processing algorithm of the present invention is reduced to a certain extent.

Claims

1. An electrostatic three-probe system suitable for pulsed plasma diagnosis, characterized in that: The electrostatic three-probe system includes three electrostatic probes and a data acquisition and analysis system; The electrostatic three-probe comprises an electrostatic three-probe body and an electrostatic three-probe circuit; The electrostatic three-probe body includes three probes and a porous ceramic tube. The three probes are placed in the ceramic tube holes and are positioned and supported by the tube holes. The three probes are marked as P1, P2, and P3, respectively, where P1 is a common probe. The electrostatic three-probe circuit includes two loops, each loop includes a magnetic induction current sensor, a DC power supply, a buffer capacitor, and two probes. The magnetic induction current sensor includes a primary coil, a secondary coil, and a magnetic core, and the primary coil and the secondary coil are respectively tightly wound on the magnetic core. The buffer capacitors in the two loops are connected in parallel to both ends of the DC power supply, the positive pole of the DC power supply is connected to the common probe P1, and the negative pole of the DC power supply is connected in series with the primary coil of the magnetic induction current sensor and then connected to probes P2 and P3 respectively. The number of turns of the primary coil of the magnetic induction current sensor is determined by predicting the amplitude and rate of change of the probe loop current and calculating the induced voltage of the primary coil. The number of turns is sufficient when the induced voltage of the primary coil is less than one-tenth of the probe drive voltage. The number of turns of the secondary coil is determined according to the needs of signal amplification. The capacitance of the buffer capacitor is selected to ensure that the total charge flowing through the probe loop during a pulse is much smaller than the charge stored in the buffer capacitor. The material of the magnetic core should be selected to ensure that the magnetic coupling efficiency is as high as possible, and the rated operating frequency of the magnetic core material should be higher than the characteristic frequency of the current pulse signal; The data acquisition and analysis system includes a data acquisition module and a data processing and analysis system; The data acquisition module consists of a preamplifier and a data acquisition card. The preamplifier is directly connected to the secondary coil of the magnetic induction current sensor. The induced voltage signal of the secondary coil of the current sensor is amplified and noise suppressed by the preamplifier. The data is then collected and digitized by the data acquisition card and transmitted to the data processing and analysis system. To ensure the overall time resolution capability of the three-probe system, the sampling rate of the data acquisition card is higher than the response rate of the electrostatic three-probe circuit. The data processing and analysis system first calculates the probe current based on the induced voltage of the secondary coil of the magnetic induction current sensor recorded by the data acquisition module. The specific calculation method is as follows: When the electrostatic three-probe circuit is placed in a pulsed plasma, the current signal formed in the loop of any electrostatic three-probe circuit is i(t). Then, the secondary coil of the current sensor outputs an induced voltage u(t) under the excitation of the current i(t). The relationship between the induced voltage u(t) of the secondary coil of the current sensor and the loop current of the electrostatic three-probe circuit at the primary coil end is: Where M is the mutual inductance of the primary and secondary coils; The loop current of the electrostatic three-probe circuit is calculated by integrating the induced voltage u(t) of the current sensor secondary coil: Then, the data processing and analysis system calculates the current I2 and I3 of the probes P2 and P3 and the working point voltage V 12 、V 13 Calculate the electron temperature and ion density of the plasma; The specific data analysis algorithm is as follows: (a) The three-probe operating point voltage V 12 、V 13 The larger value V 13 The data analysis method for the probe bias voltage in the saturation range is: The currents I1 and I2 of probes P1 and P2 are expressed as: in, Where, J e is the electron saturation current density, V1 is the negative bias voltage of probe P1 relative to the plasma potential, V2 is the negative bias voltage of probe P2 relative to the plasma potential, J i (V1) is the ion saturation current density of probe P1, J i (V2) is the ion saturation current density of probe P2, S is the probe surface area, k is the Boltzmann constant, e is the electron charge, m e is the mass of the electron, T e is the electron temperature, n e is the electron density, π is the circumference of the circle; Probe P3 is at voltage V 13 The electron flow part of the probe current I3 is directly ignored and simplified to: I3=SJ i (V3) (7) Where V3 is the negative bias voltage of probe P3 relative to the plasma potential, J i (V3) is the ion saturation current density of probe P3; The three probe currents satisfy the relationship: I1=I2+I3 (8) The voltage at the working point of the three probes and the probe potential satisfy the relationship: V 12 =V2-V1 (9) V 13 =V3-V1 (10) Assume that the changes in the saturation currents of the three probe ions caused by the probe potential are negligible, that is: J i (V1) = J i (V2) = J i (V3) (11) By combining expressions (3) to (10), we can derive the equation for electron temperature: The electron temperature T of the plasma is calculated according to equation (12): e , unit is K; The current I3 of probe P3 is the ion saturation current, and the ion saturation current density formula is: j i =n i ev i (13) Where n i is the ion density at the interface between the sheath and the pre-sheath, v i is the corresponding ion velocity, according to the Bohm sheath principle: Where n0 is the ion density in the plasma, m i is the ion mass; Combining equations (7), (13), (14), and (15) yields the equation for ion density: The ion density n0 in the plasma is calculated according to equation (16) and its unit is m -3 ; (b) When the three-probe operating point voltage V 12 、V 13 When both are in the transition range of the probe bias, the data analysis method for calculating the plasma electron temperature and ion density is: The currents I1, I2, and I3 of the three probes P1, P2, and P3 are expressed as: Among them, J e is the electron saturation current density, V1 is the negative bias voltage of probe P1 relative to the plasma potential, V2 is the negative bias voltage of probe P2 relative to the plasma potential, V3 is the negative bias voltage of probe P3 relative to the plasma potential, J i (V1) is the ion saturation current density of probe P1, J i (V2) is the ion saturation current density of probe P2, J i (V3) is the ion saturation current density of probe P3, S is the probe surface area, k is the Boltzmann constant, e is the electron charge, m e is the mass of the electron, T e is the electron temperature, n e is the electron density, π is the circumference of the circle, is formula (5), J e is formula (6); The equation for the electron temperature is derived from equations (5), (6), (8) to (11), and (17) to (19): The electron temperature T is calculated using equation (20) e ; According to equations (18), (19) and (11), the ion saturation current density J is derived i for: The equation for ion density is derived from equation (21), equations (13) to (15), and equations (9) and (10): The ion density n0 is calculated according to equation (22); According to the above two data analysis methods where the three-probe operating point voltage is in different intervals, the specific process of the algorithm is as follows: Since it is impossible to predict whether the three-probe working point voltage will enter the saturation range of the probe bias voltage when selecting the three-probe working point voltage, the probe current signals I2, I3 and the three-probe working point voltage V 12 、V 13 , use the data analysis method in (b) to preliminarily calculate the electron temperature and ion density; then compare the calculated electron temperature with the larger of the three probe working point voltages V 13 Compare the values. If V 13 If the difference between the value of V and the electron temperature is not large, the current settlement result is output directly; if V 13 When it is greater than three times the electron temperature, it is necessary to use the data analysis method in (a) to enter the probe bias saturation range of the three-probe operating point voltage to recalculate the electron temperature and ion density; then, output the recalculated electron temperature and ion density.

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