Three-probe diagnosis system suitable for SCU-PSI plasma linear device and denaturation diagnosis method

Abstract

The invention discloses a three-probe diagnosis system suitable for an SCU-PSI plasma linear device and a denaturation diagnosis method, relates to the technical field of plasmas, is developed based on a three-probe theory, and comprises a three-probe, a circuit, a data acquisition system and a transmission device, the three pins are coupled with the SCU-PSI plasma linear device through a lead and a flange; the circuit is connected with the three pins and is used for shielding electromagnetic interference from different electrical components; the data acquisition system is used for acquiring plasma parameters; and the transmission device is used for driving the probe to move in the radial direction in the cavity. The three-probe diagnosis system suitable for the SCU-PSI plasma linear device is developed and used for researching the shielding effect of the lithium vapor, the diagnosis result is reliable, and the physical mechanism of interaction between the lithium vapor and the plasma characteristics can be disclosed.

Description

Technical Field

[0001] The present invention relates to the field of plasma technology, and more particularly to a three-probe diagnostic system and a timely degeneration diagnostic method suitable for a SCU-PSI plasma linear device. Background Art

[0002] As a candidate material for future fusion reactors facing plasma, liquid lithium has the following advantages as a plasma-facing material: (1) Lithium is a low-Z material and has good compatibility with plasma; (2) Liquid lithium can withstand up to 50MW / m 2 heat load; (3) the flowing liquid lithium has a self-healing function; (4) the circulating liquid lithium continuously flows and brings out heat, which can not only effectively protect the high-Z solid wall behind it, but also effectively prevent Z impurities from contaminating the core plasma; (5) a small amount of sputtered and evaporated lithium atoms can play the role of automatic feeding even if they enter the core plasma. Studies have shown that liquid lithium will continue to evaporate, transport, form vapor clouds and reprecipitate under the irradiation of high-density D2 / T2 / He plasma. Since the Langmuir single and double probes need to repeatedly load bias to diagnose the time-varying characteristics of plasma, the Langmuir triple probe can continuously diagnose the time-varying characteristics of plasma parameters. Therefore, it is necessary to develop a Langmuir triple probe suitable for the SCU-PSI plasma linear device to reveal the shielding effect and mechanism of lithium vapor cloud on heat load and particles; and to establish and develop a physical model of lithium vapor cloud movement.

[0003] At present, the three-probe technology has been maturely applied to the timing diagnosis of various plasma characteristics. Chen Lei and others from the China Academy of Engineering Physics used the three-probe developed by themselves to study the characteristics of pulsed discharge plasma. The experimental results showed that with the increase of discharge time, the electron temperature at the high-voltage extraction interface first increased and then decreased; Hu Jiansheng and others from the Hefei Institute of Plasma Physics, Chinese Academy of Sciences, used the three-probe to track the plasma characteristics during the lithiation wall treatment process in the EAST device. The experimental results showed that the lithiation wall treatment can effectively improve the plasma characteristics; Soonwook Jung and others from the University of Illinois, USA, used the three-probe to test the electron temperature and density of plasma under different pressures, confirming that lithium vapor has a certain shielding effect on the heat load carried by the plasma in the Devex device.

[0004] Therefore, how to build a three-probe diagnostic system suitable for the SCU-PSI plasma linear device and obtain the time-varying characteristics of the plasma when the plasma interacts with the liquid lithium surface is a technical problem that technicians in this field urgently need to solve. Summary of the invention

[0005] In view of this, the present invention provides a three-probe diagnostic system and a timely degeneration diagnostic method suitable for an SCU-PSI plasma linear device, which solves the problems existing in the background technology.

[0006] In order to achieve the above object, the present invention provides the following technical solutions:

[0007] A three-probe diagnostic system suitable for SCU-PSI plasma linear device is developed based on the three-probe theory, including: three needles, circuits, data acquisition system and transmission device;

[0008] The three needles are coupled to the SCU-PSI plasma linear device through wires and flanges;

[0009] The circuit is connected to three pins to shield electromagnetic interference from different electrical components;

[0010] A data acquisition system for collecting plasma parameters;

[0011] The transmission device is used for driving the probe to move radially in the chamber.

[0012] Optionally, the three-needle consists of a single probe and a double probe;

[0013] The single probe is a first probe suspended in the plasma and is used to measure the plasma suspension potential;

[0014] The double probe includes a second probe and a third probe, which are respectively connected to the positive and negative electrodes of a DC power supply through a voltage-dividing resistor; the second probe works in a transition region, and the third probe works in an ion saturation region.

[0015] Optionally, the three needles are composed of a tungsten wire and a ceramic tube, and the tungsten wire is fixed in the ceramic tube by ceramic glue;

[0016] The endpoints of the three needles form an equilateral triangle, and the perpendicular bisector of the equilateral triangle is parallel to the central axis of the plasma beam.

[0017] Alternatively, from the three-probe theory, the electron temperature of the plasma is:

[0018]

[0019] Where: e is the electron charge, k is the Boltzmann constant, V9 is the voltage of the second probe, V f is the voltage of the first probe, and the electron temperature of the plasma is obtained from the positive bias probe voltage and the floating potential;

[0020] Substituting the obtained electron temperature into the ion saturation current calculation formula, the electron density of the plasma is obtained:

[0021]

[0022] Where: I io is the ion saturation current, S is the probe surface area, m i is the ion mass;

[0023] Based on the measured electron temperature and electron density, the ion flux distribution and the heat load carried by the ions are obtained:

[0024]

[0025] Where: i is the ion flux distribution, q is the heat load carried by the ions, and ξ is the thermal conductivity of the plasma.

[0026] Optionally, the circuit includes: a wire with a BNC connector, a capacitor, an inductor, and voltage-dividing resistors with different resistance values; through step-by-step debugging, capacitors and inductors suitable for the current circuit range are selected, and for the saturated ion current of different plasmas, the voltage-dividing resistors used to test the saturated ion current have different sizes.

[0027] Optionally, the data acquisition system includes a data acquisition card and acquisition software, and the data acquisition card is connected to the three pins through a circuit.

[0028] Optionally, the transmission device includes: a transmission shaft, a variable frequency motor and a variable frequency software;

[0029] Frequency conversion software, used to adjust the speed of the frequency conversion motor;

[0030] The variable frequency motor is connected to the transmission shaft and is used for controlling the stepping rate of the probe in the radial direction of the chamber through the transmission shaft.

[0031] A method for diagnosing the time-varying characteristics of plasma during lithium evaporation, using a three-probe diagnostic system suitable for a SCU-PSI plasma linear device as described in any one of the above, comprising the following steps:

[0032] The three probes were coupled to the SCU-PSI plasma linear device through wires and flanges, and the shielding effect of lithium vapor on plasma was studied using the three-probe diagnostic system.

[0033] The circuit is used to shield the electromagnetic interference from different electrical components, and the transmission device drives the probe to move radially in the chamber;

[0034] The plasma parameters were collected based on the data acquisition system, and the data were exported in Excel format to obtain the relationship between the plasma characteristics and the lithium vapor concentration during lithium evaporation.

[0035] Alternatively, the spectral intensity of lithium is proportional to the lithium atomic density and electron density, specifically expressed as:

[0036]

[0037] Where: is the distribution function of the relative velocity between lithium atoms and electrons, R0 is the plasma radius approximated by a quasi-cylindrical shape, r is the vertical distance from the chamber axis, L is the axial length of the plasma, dΩ is the fixed center angle between the OES probe and the lithium vapor cloud, and n e (z,r) is the electron density at position (z,r), n Li (z,r) is the density of evaporated lithium atoms at position (z,r), <σ ei υ Li→e > e is the collision excitation rate coefficient between evaporated lithium atoms and electrons, σ ei is the electron impact excitation cross section of lithium atoms; v Li→e is the absolute value of the relative velocity between lithium atoms and electrons, that is

[0038] For the number of lithium vapor atoms as the only variable, and other influencing factors being the same in the experiment, the spectral intensity of lithium is proportional to the number of lithium vapor atoms.

[0039] It can be seen from the above technical solutions that, compared with the prior art, the present invention discloses a three-probe diagnostic system and a timely degeneration diagnostic method suitable for the SCU-PSI plasma linear device, which has the following beneficial effects:

[0040] Based on the three-probe theory, a three-probe diagnostic system suitable for SCU-PSI plasma linear devices was developed and used to study the shielding effect of lithium vapor, and the diagnostic results were reliable. The three-probe diagnostic system was used to detect the time-variability of plasma characteristics during lithium evaporation. As the liquid lithium begins to evaporate, the lithium vapor cloud can rapidly reduce the electron temperature, ion flux and heat flux. When the evaporation rate of lithium tends to be flat, the shielding effect of the lithium vapor cloud tends to equilibrium. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0042] Figure 1 A structural diagram of a three-probe diagnostic system suitable for a SCU-PSI plasma linear device provided by the present invention;

[0043] Figure 2Schematic diagram of the electrostatic three-probe structure and SCU-PSI linear device provided by the present invention;

[0044] Figure 3 This is a three-probe stability test diagram provided by the present invention;

[0045] Figure 4 The argon plasma characteristic test diagram provided by the present invention;

[0046] Figure 5 A helium plasma characteristic test diagram provided by the present invention;

[0047] Figure 6 A schematic diagram of the relationship between the plasma characteristics measured by three probes and the time during the lithium evaporation process provided by the present invention;

[0048] Figure 7 A schematic diagram of the relationship between the electron density, electron temperature, ion flux, heat load carried by the plasma and lithium vapor concentration provided by the present invention;

[0049] Figure 8 A schematic diagram of the lithium evaporation process provided by the present invention;

[0050] Figure numerals: 1-first probe, 2-wire, 3-chamber, 4-insulating flange, 5-data acquisition card, 6-voltage-dividing resistor, 7-DC power supply, 8-PC terminal, 9-second probe, 10-third probe, A-suspended potential, B-voltage on probe, C-radial movement. DETAILED DESCRIPTION

[0051] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0052] In order to obtain the time-varying properties of plasma when the plasma interacts with the liquid lithium surface, the present invention designs and establishes a three-probe diagnostic system in the SCU-PSI linear plasma device. Based on the three-probe theory, the circuit design and data processing method are optimized, and the radial distribution of the electron temperature, density, ion flux and heat load of argon and helium plasma in the linear device is preliminarily diagnosed, and the time-varying properties of plasma during lithium evaporation are tested, which will help reveal the physical mechanism of the interaction between lithium vapor and plasma properties.

[0053] 1. Introduction of three-probe system

[0054] Specifically, the embodiment of the present invention discloses a three-probe diagnostic system suitable for a SCU-PSI plasma linear device, such as Figure 1 As shown, it is developed based on the three-probe theory, including: three needles, circuits, data acquisition system and transmission device;

[0055] The three needles are coupled to the SCU-PSI plasma linear device through the wire 2 and the insulating flange 4;

[0056] The circuit is connected to three pins to shield electromagnetic interference from different electrical components;

[0057] A data acquisition system for collecting plasma parameters;

[0058] Transmission device, used to drive the probe to move radially in the chamber 3 ( Figure 1 C).

[0059] Furthermore, the three probes are composed of a single probe and a double probe; wherein the single probe is a first probe 1 suspended in the plasma, which is used to measure the plasma suspension potential ( Figure 1 A); the dual probe includes a second probe 9 and a third probe 10, which are respectively connected to the positive and negative electrodes of a DC power supply 7 through a voltage divider resistor 6; a suitable DC voltage makes the second probe 9 work in the transition zone and the third probe 10 work in the ion saturation zone.

[0060] Furthermore, from the three-probe theory, it can be known that the electron temperature of the plasma is:

[0061]

[0062] Where: e is the electron charge, k is the Boltzmann constant, V9 is the voltage of the second probe 9, V f is the voltage of the first probe 1, and the electron temperature of the plasma is obtained from the positive bias probe voltage and the floating potential;

[0063] Substituting the obtained electron temperature into the ion saturation current calculation formula, the electron density of the plasma is obtained:

[0064]

[0065] Where: I io is the ion saturation current, S is the probe surface area, m i is the ion mass;

[0066] Based on the measured electron temperature and electron density, the ion flux distribution and the heat load carried by the ions are obtained:

[0067]

[0068] q=kTe Γ i ξ

[0069] Where: i is the ion flux distribution, q is the heat load carried by the ions, and ξ is the thermal conductivity of the plasma, which is generally between 6 and 7.

[0070] Furthermore, the circuit includes: a wire 2 with a BNC connector, a 2200μF capacitor, a 0.55mH inductor, and voltage-dividing resistors 6 with different resistance values; the wire 2 with a shielded wire and a BNC connector, the capacitors in parallel, and the inductors in series are all used to shield electromagnetic interference from different electrical components. Through step-by-step debugging, the capacitors and inductors suitable for the current circuit range are selected. For the saturated ion current of different plasmas, the voltage-dividing resistors 6 used to test the saturated ion current have different sizes, namely 20Ω (Ar), 100Ω (He), and 20kΩ (H).

[0071] Furthermore, the data acquisition system includes a data acquisition card 5 and acquisition software. The data acquisition card 5 is connected to the three pins through a circuit, and the data acquisition card 5 is connected to the PC terminal 8. The characteristics of this data acquisition system are: (1) high acquisition accuracy, with a resolution generally above 1.22mV; (2) a frequency of up to 50kHz; (3) a full-scale error within 10mV; (4) data can be exported in Excel format.

[0072] Furthermore, the transmission device includes: a transmission shaft, a variable frequency motor and a variable frequency software; wherein the variable frequency software is used to adjust the speed of the variable frequency motor; the variable frequency motor is connected to the transmission shaft and is used to control the radial stepping rate of the probe in the chamber 3 through the transmission shaft (such as Figure 1 B), the speed is 0.1-10mm / s.

[0073] Figure 2 Schematic diagram of the electrostatic three-probe structure and SCU-PSI linear device. Figure 2 As shown in (1), the probe in the chamber 3 is composed of a tungsten wire with a diameter of 0.3 mm and a ceramic tube with an inner diameter of 0.5 mm, an outer diameter of 1 mm, and a length of 150 mm. The tungsten wire is fixed in the ceramic tube by high-temperature ceramic glue, and the length of the tungsten wire exposed to the plasma is 5-10 mm. Figure 2(2) is a schematic diagram of the position of the three probes in the plasma. The distance between the single probe and the two probes of the double probe is 2.5 mm, and the distance between the two probes of the double probe is 3.53 mm. The endpoints of the three probes form an equilateral triangle, and the perpendicular bisector of the equilateral triangle is parallel to the central axis of the plasma beam. The three probes are well coupled to the SCU-PSI linear device through the existing diagnostic window on the chamber 3 through the wire 2 and the insulating flange 4. The plasma parameters of the SCU-PSI linear device of this embodiment are shown in Table 1. In addition to being used for the diagnosis of single cathode source plasma characteristics, the three probes will also be used for the study of triple cathode source, hollow cathode source and high heat load pulse plasma characteristics.

[0074] Table 1 Plasma parameters

[0075]

[0076] 2. Experimental results and analysis

[0077] 2.1 Three-probe reliability test results

[0078] like Figure 3 The figure shows the results of the three-probe reliability test, where (1) is the ion saturation current, (2) is the electron temperature, (3) is the electron density, and (4) is the evolution of the ion flux with the discharge time; the discharge time is 160s, and the plasma discharge conditions are He gas flux 2slm, current 90A, and magnetic field 0.1T. Figure 3 From (1) in the experiment, we can see that the relative error of the ion saturation current is 1.3% during the experimental time. Figure 3 From (2) and (3) in the figure, we can see that the electron temperature and electron density are 0.42-0.43 eV and 1.57×10 18 ~1.60×10 19 m -3 The electron temperature first decreases and then increases, and the electron density first increases and then decreases. The relative errors are both 1.3%. Under the same experimental conditions, the order of magnitude is consistent with the helium plasma characteristics measured by the Langmuir double probe. Figure 3 From (4) in the figure, we can see that the ion flux calculated by the measured electron temperature and electron density is 3.64×10 21 ~3.68×10 21 m -2 s -1 The stability test experiment not only shows that the three probes developed have good stability, but also shows that the plasma stability in the SCU-PSI plasma linear device is good.

[0079] 2.2 Preliminary test results and analysis

[0080] Figure 4This is the spatial distribution of argon plasma characteristics measured by three probes. The discharge conditions are Ar_90A, 2slm, and 0.1T. Figure 4 (1) and (2) are the spatial distributions of electron temperature and electron density, respectively. As can be seen from the figure, both electron temperature and electron density are Gaussian distributions. In the axial range of 0 to 35 mm, the electron temperature varies from 0.75 to 0.35 eV, and the electron density varies from 5.5×10 19 ~1.5×10 19 m -3 ; Figure 4 (3) and (4) are the spatial distribution of ion flux and ion heat load, respectively. In the axial range of 0 to 35 mm, the ion flux varies in the range of 6.3×10 22 ~1.2×10 22 m -2 s -1 The heat load brought by ions varies from 70 to 6 kWm -2 . Figure 4 The curves (1) and (2) are the axial scans of the movable three-probe for 160 s, containing 8×10 5 At the test point, within the adjustable range of the step rate, the radial distribution test of the plasma can be completed in a few seconds, which is faster and more accurate than the dual probe.

[0081] Figure 5 This is the spatial distribution of helium plasma characteristics measured by three probes. The discharge conditions are He_90A, 2slm, and 0.1T. Figure 5 (1) and (2) are the spatial distributions of electron temperature and electron density, respectively. As can be seen from the figure, unlike argon plasma, the highest value of electron temperature does not appear at the geometric center of the chamber, ranging from 0.69 to 0.44 eV, while the electron density presents a Gaussian distribution, ranging from 5.0×10 18 ~0.5×10 18 m -3 Between. Figure 5 As shown in (3) and (4) in the figure, the ion flux and the heat load carried by the ions gradually decrease with the increase of the distance from the center, and the range of variation is 1.2×10 22 ~0.1×10 22 m -2 s -1 and 6~0.5kWm -2 Compared with argon plasma, the difference is about one order of magnitude, which is consistent with the result measured by the double probe. The triple probe can be used to diagnose the influence of lithium vapor on plasma characteristics, and the test effect is good.

[0082] 2.3 Three-probe diagnosis of time-varying plasma parameters during lithium evaporation

[0083] This embodiment proposes a method for diagnosing the time-varying characteristics of plasma during lithium evaporation, using the above-mentioned three-probe diagnostic system suitable for the SCU-PSI plasma linear device, including the following steps:

[0084] The three needles are coupled to the SCU-PSI plasma linear device through the wire 2 and the insulating flange 4, and the shielding effect of lithium vapor on plasma is studied using the three-probe diagnostic system;

[0085] The circuit is used to shield the electromagnetic interference from different electrical components, and the transmission device drives the probe to move radially in the chamber 3;

[0086] The plasma parameters were collected based on the data acquisition system, and the data were exported in Excel format to obtain the relationship between the plasma characteristics and the lithium vapor concentration during lithium evaporation.

[0087] Specifically, in the PSI plasma linear device, the shielding effect of lithium vapor on plasma was studied through the interaction between plasma and liquid lithium. The experimental results are as follows: Figure 6 shown. Figure 6 (1) in shows the temporal variation of the plasma electron temperature characteristics. As the evaporated lithium atoms collide with the incident ions, the kinetic energy of the incident ions decreases, and the electron temperature gradually decreases with the increase of lithium vapor concentration. However, the electron density increases slightly, as Figure 6 As shown in (2), due to the low first ionization energy of lithium atoms, the ionization of lithium atoms can increase the electron density. According to the sheath theory, the ion flux is a function of the electron density and the electron temperature. Figure 6 (3) in shows that the ion flux decreases linearly with the experimental time. The heat load carried by the plasma is a function of the ion flux and the electron temperature, such as Figure 6 As shown in (4), during the increase of lithium spectrum intensity, the heat load carried by the plasma decreases linearly with time, which indicates that lithium vapor has a certain shielding effect on the heat load.

[0088] 2.4 Effect of lithium atom concentration on plasma characteristics during lithium evaporation

[0089] In order to study the effect of lithium atomic concentration on plasma characteristics, Figure 7 The relationship between the spectral intensity of lithium and the plasma characteristics is shown. The spectral intensity of lithium is proportional to the lithium atom density and electron density, which can be expressed as:

[0090]

[0091] Where: is the distribution function of the relative velocity between lithium atoms and electrons, R0 is the plasma radius approximated by a quasi-cylindrical shape, r is the vertical distance from the chamber axis, L is the axial length of the plasma, dΩ is the fixed center angle between the OES probe and the lithium vapor cloud, and n e (z,r) is the electron density at position (z,r), n Li (z,r) is the density of evaporated lithium atoms at position (z,r), <σ ei υ Li→e > e is the collision excitation rate coefficient between evaporated lithium atoms and electrons, σ ei is the electron impact excitation cross section of lithium atoms; v Li→e is the absolute value of the relative velocity between lithium atoms and electrons, that is However, for the only variable being the number of lithium vapor atoms, other influencing factors being the same in the experiment, the spectral intensity of lithium is proportional to the number of lithium vapor atoms.

[0092] To investigate how the lithium spectral intensity changes the plasma properties, Figure 7 The relationship between plasma characteristics and lithium vapor concentration during lithium evaporation measured by three probes. It can be seen that the electron density increases only slightly with the increase of lithium vapor concentration, and fluctuates greatly when lithium just starts to evaporate; the electron temperature decreases sharply with the increase of lithium vapor concentration and then gradually stabilizes, and the final stable value is 94.6% of the initial value; the ion flux decreases sharply with the increase of lithium vapor concentration and then gradually stabilizes, and the final stable value is 97.7% of the initial value; the heat load carried by the plasma decreases sharply with the increase of lithium vapor concentration and then gradually stabilizes, and the final stable value is 92.0% of the initial value.

[0093] Depend on Figure 7 It can be seen that, except for the electron density, the decrease in the electron temperature, ion flux and heat load carried by the plasma accounted for 62.5%, 70.0% and 55.6% of the total decrease respectively when the relative value of the lithium vapor concentration was 1; and when the relative value of the lithium vapor concentration increased from 1 to 5.5, these physical parameters gradually tended to be stable, that is, the evaporated lithium atoms in the measured area remained basically unchanged after the relative value reached 1. The reason for this phenomenon is that under certain experimental conditions, the concentration of lithium atoms will reach saturation, as explained in detail below. Figure 8 shown.

[0094] When a lithium sample is exposed to plasma, it will heat and melt. Figure 8 As shown in (2) in , when lithium reaches a certain temperature, lithium begins to evaporate and forms a lithium vapor cloud. During this period, the shielding effect of the lithium vapor cloud increases rapidly, and the electron temperature, ion particles, and heat flow decrease rapidly; Figure 8As shown in (3) in , when the evaporated lithium atoms reach saturation concentration, the shielding effect of the lithium vapor cloud tends to be stable, so there is no obvious change in the electron temperature, ion particles, and heat flux; Figure 8 As shown in (4), after the lithium atom concentration reaches saturation, the subsequent evaporation of lithium atoms will not affect the shielding effect of the lithium vapor cloud.

[0095] Based on the experimental results, it was found that the shielding efficiency of lithium vapor is not high because the concentration of lithium atoms is very low. Next, the two reasons will be explained in detail. The Lindehard model using incident low-energy ions must take into account the electron cloud excitation and ionization of lithium vapor, as well as the elastic Coulomb collisions between ions and nuclei in lithium vapor;

[0096] The stopping power of electrons is as follows:

[0097]

[0098] In the formula, C Lss is a constant related to the incident ions and lithium vapor and is expressed as:

[0099]

[0100] When the incident particle energy is very low, elastic Coulomb collisions between the incident ions and lithium vapor are the main energy loss method.

[0101] Nuclear stopping power is

[0102]

[0103]

[0104]

[0105] The total stopping power of a particle is the electronic stopping power plus the nuclear stopping power:

[0106]

[0107] Where: Z eff is the incident ion effective charge, N A is Avogadro's constant, ρ Li is the lithium vapor cloud density, A Li is the atomic weight of the lithium vapor cloud, Z Li is the average charge number of lithium vapor plasma.

[0108] According to the above theory, since the temperature of lithium vapor electrons is very low, the vapor cloud only spreads a few centimeters. Therefore, most of the incident ions reach the target surface. In order to further understand the shielding effect of lithium vapor, it is decided to calculate the lithium atomic concentration based on the following assumptions:

[0109] (1) The ideal gas state equation can be used to analyze the physical properties of lithium atoms in the gas phase and is given as:

[0110] P=nRT

[0111] (2) The velocity v of lithium atoms in the gas phase can be given by Maxwell's velocity distribution law as follows:

[0112]

[0113] (3) Since the electron temperature in this experiment is low, the effect of sheath potential on lithium evaporation can be ignored. According to the experimental data and results analysis, when the lithium temperature is between the melting point and the boiling point, the relationship between the saturated vapor pressure and temperature is:

[0114]

[0115] Based on the three formulas in (1)-(3), the evaporation equation of the extreme ideal state can be obtained:

[0116]

[0117] Where: is the evaporation rate of lithium, T S is the temperature of lithium, M Li is the atomic mass of lithium;

[0118] When the surface temperature of lithium reaches 453K, lithium begins to evaporate, and after 50 seconds, the lithium vapor reaches saturation. The evaporation formula of lithium is as follows:

[0119]

[0120] So the concentration of lithium atoms is as follows:

[0121] Where: V c is the volume of the rectilinear chamber, Δs is the surface area of ​​the plasma, T S It is a function of time t, and the temperature function is fitted: T S =0.45t+393(K). Therefore, the density of lithium atoms is 1.3×10 14 / m 3 , this value is several orders of magnitude less than the calculated value by the traditional method, so the shielding efficiency is very low. However, when the lithium is exposed to a large enough plasma area, such as a liquid lithium limiter and a liquid lithium diverter, the lithium can evaporate to form a thick enough lithium vapor cloud. At this time, the lithium vapor cloud can greatly reduce the heat flux facing the plasma material and protect the solid wall material from damage by the plasma.

[0122] In summary, based on the three-probe theory, this embodiment develops a three-probe diagnostic system suitable for SCU-PSI plasma linear devices, and uses it to study the shielding effect of lithium vapor. This embodiment specifically introduces the structure, materials and dimensions of the three-probe diagnostic system. The three-probe diagnostic system is used to detect the characteristics of argon plasma and helium plasma. The results are consistent with the results of the Langmuir double-probe diagnostic system, indicating that the results of the three-probe diagnostic system are reliable; the three-probe diagnostic system is used to detect the time-variability of plasma characteristics during lithium evaporation. As liquid lithium begins to evaporate, the lithium vapor cloud can quickly reduce the electron temperature, ion flux and heat flux. When the evaporation rate of lithium tends to be flat, the shielding effect of the lithium vapor cloud tends to balance; based on the shielding effect of lithium vapor, this embodiment calculates the concentration of lithium atoms and finds that the concentration of lithium atoms is very low. Therefore, increasing the area of ​​lithium can increase the shielding effect and protect the solid wall material from damage by plasma.

[0123] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

[0124] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A three-probe diagnostic system suitable for SCU-PSI plasma linear device, characterized in that: Developed based on the three-probe theory, it includes: three needles, circuit, data acquisition system and transmission device; The three needles are coupled to the SCU-PSI plasma linear device through wires and flanges; The circuit is connected to three pins to shield electromagnetic interference from different electrical components; A data acquisition system for collecting plasma parameters; The transmission device is used for driving the probe to move radially in the chamber.

2. A three-probe diagnostic system suitable for SCU-PSI plasma linear device according to claim 1, characterized in that: The three needles consist of a single probe and a double probe; The single probe is a first probe suspended in the plasma and is used to measure the plasma suspension potential; The double probe includes a second probe and a third probe, which are respectively connected to the positive and negative electrodes of a DC power supply through a voltage-dividing resistor; the second probe works in a transition region, and the third probe works in an ion saturation region.

3. A three-probe diagnostic system suitable for SCU-PSI plasma linear device according to claim 1, characterized in that: The three needles are composed of tungsten wire and ceramic tube, and the tungsten wire is fixed in the ceramic tube by ceramic glue; The endpoints of the three needles form an equilateral triangle, and the perpendicular bisector of the equilateral triangle is parallel to the central axis of the plasma beam.

4. A three-probe diagnostic system suitable for SCU-PSI plasma linear device according to claim 2, characterized in that: According to the three-probe theory, the electron temperature of the plasma is: Where: e is the electron charge, k is the Boltzmann constant, V9 is the voltage of the second probe, V f is the voltage of the first probe, and the electron temperature of the plasma is obtained from the positive bias probe voltage and the floating potential; Substituting the obtained electron temperature into the ion saturation current calculation formula, the electron density of the plasma is obtained: Where: I io is the ion saturation current, S is the probe surface area, m i is the ion mass; Based on the measured electron temperature and electron density, the ion flux distribution and the heat load carried by the ions are obtained: q=kT e C i x Where: Γ i is the ion flux distribution, q is the heat load carried by the ions, and ξ is the thermal conductivity of the plasma.

5. A three-probe diagnostic system suitable for SCU-PSI plasma linear device according to claim 1, characterized in that: The circuit includes: a wire with a BNC connector, a capacitor, an inductor and voltage-dividing resistors with different resistance values; through step-by-step debugging, the capacitor and inductor suitable for the current circuit range are selected, and the voltage-dividing resistors used to test the saturated ion current are different in size for the saturated ion current of different plasmas.

6. A three-probe diagnostic system suitable for SCU-PSI plasma linear device according to claim 1, characterized in that: The data acquisition system includes a data acquisition card and acquisition software, and the data acquisition card is connected to the three pins through a circuit.

7. A three-probe diagnostic system suitable for SCU-PSI plasma linear device according to claim 1, characterized in that: The transmission device includes: transmission shaft, variable frequency motor and variable frequency software; Frequency conversion software, used to adjust the speed of the frequency conversion motor; The variable frequency motor is connected to the transmission shaft and is used for controlling the stepping rate of the probe in the radial direction of the chamber through the transmission shaft.

8. A method for diagnosing the time-varying characteristics of plasma during lithium evaporation, using a three-probe diagnostic system suitable for a SCU-PSI plasma linear device as described in any one of claims 1 to 7, characterized in that: The following steps are involved: The three probes were coupled to the SCU-PSI plasma linear device through wires and flanges, and the shielding effect of lithium vapor on plasma was studied using the three-probe diagnostic system. The circuit is used to shield the electromagnetic interference from different electrical components, and the transmission device drives the probe to move radially in the chamber; The plasma parameters were collected based on the data acquisition system, and the data were exported in Excel format to obtain the relationship between the plasma characteristics and the lithium vapor concentration during lithium evaporation.

9. The method for diagnosing the time-varying characteristics of plasma during lithium evaporation according to claim 8, characterized in that: The spectral intensity of lithium is proportional to the lithium atomic density and electron density, which can be expressed as: Where: is the distribution function of the relative velocity between lithium atoms and electrons, R0 is the plasma radius approximated by a quasi-cylindrical shape, r is the vertical distance from the chamber axis, L is the axial length of the plasma, dΩ is the fixed center angle between the OES probe and the lithium vapor cloud, and n e (z,r) is the electron density at position (z,r), n Li (z,r) is the density of evaporated lithium atoms at position (z,r), <σ ei v Li→e > e is the collision excitation rate coefficient between evaporated lithium atoms and electrons, σ ei is the electron impact excitation cross section of lithium atoms; v Li→e is the absolute value of the relative velocity between lithium atoms and electrons, that is For the number of lithium vapor atoms as the only variable, and other influencing factors being the same in the experiment, the spectral intensity of lithium is proportional to the number of lithium vapor atoms.

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