A method for predicting the evolution of ion thruster grid corrosion morphology

Abstract

The present invention discloses a method for predicting the evolution of erosion morphology of an ion thruster grid, comprising: obtaining the size of a single-hole beam downstream of an accelerating grid through experimental measurement; selecting a pair of screen grids and an accelerating grid hole as objects of numerical simulation research; obtaining an initial plasma density upstream of the screen grid based on a relationship between beam current and plasma density upstream of the screen grid; obtaining a Debye length based on upstream plasma density calculation and selecting a size of a simulation grid; obtaining a neutral gas density at each grid node in a simulation area based on a neutral gas collision model; establishing a grid corrosion morphology evolution model, starting beam calculation, simulating and obtaining the single-hole beam size, performing feedback adjustment based on experimental values, and determining a final upstream ion density; processing the collision process between ions and the grid, starting corrosion calculation, and obtaining an accurate grid corrosion rate; reducing grid mass based on the corrosion rate, eliminating grid nodes, and realizing a change in the grid morphology.

Description

Technical Field

[0001] The present invention belongs to the field of plasma propulsion technology, and in particular relates to a method for predicting the evolution of corrosion morphology of an ion thruster grid. Background Art

[0002] Ion thrusters, with their low thrust, high specific impulse, and long lifespan, are widely used in space propulsion, such as spacecraft attitude control, position maintenance, orbital maneuvers, and interstellar flight. With the development of my country's space industry, space exploration missions are moving towards higher payloads, longer distances, and longer durations.

[0003] During ion thruster operation, energetic ions bombard components such as the hollow cathode, screen grid, and accelerator grid, causing corrosion. Because the accelerator grid has a large negative potential, ions downstream of the accelerator grid are attracted by the negative potential and cause the most severe corrosion, becoming a key factor limiting the life of ion thrusters.

[0004] Traditional ion thruster gate corrosion measurement experiments have long measurement cycles, typically thousands of hours, and even tens of thousands of hours for sputter-resistant carbon-carbon gates. Numerical simulation can simulate the extraction of gate beam ions and the sputtering corrosion of ions on the gate, making it a highly effective method for predicting the evolution of gate corrosion morphology. Existing gate corrosion model construction schemes consider the modification of field calculation boundary conditions due to changes in gate morphology. However, for sputtering corrosion, changes in gate morphology can cause changes in the local normal orientation of the gate, resulting in deviations in the angle of ion incidence on the gate, making it difficult to accurately predict the evolution of gate corrosion morphology. Summary of the Invention

[0005] In view of the shortcomings of the existing technology, the present invention proposes a method for predicting the evolution of ion thruster grid corrosion morphology. Based on basic experimental measurement results and using numerical simulation methods, the method accurately predicts the evolution of ion thruster grid corrosion morphology.

[0006] To achieve the above object, the present invention provides the following solutions:

[0007] A method for predicting the evolution of ion thruster grid corrosion morphology includes the following steps:

[0008] S1: The single-hole beam size downstream of the accelerating grid is obtained through experimental measurement;

[0009] S2: Perform numerical simulation on the cylindrical area where the screen grid and acceleration grid holes are located to obtain the axisymmetric calculation domain;

[0010] S3: in the axisymmetric calculation domain, obtaining an initial plasma density upstream of the screen grid according to a relationship between the beam current and the plasma density upstream of the screen grid;

[0011] S4: Obtain the Debye length according to the plasma density upstream of the screen, and obtain the size of the simulation grid based on the Debye length;

[0012] S5: According to the neutral gas collision model, the neutral gas density at each grid node in the simulation area is obtained;

[0013] S6: establishing a gate corrosion morphology evolution model, starting beam calculation based on the gate corrosion morphology evolution model, and simulating a single-hole beam size; based on the simulated single-hole beam size and the single-hole beam size obtained by experimental measurement in S1, feedback adjustment is performed on the initial screen grid upstream plasma density to obtain a final screen grid upstream plasma density;

[0014] S7: Processing the collision process between the ions and the gate to generate charge exchange ions; starting corrosion calculation based on the charge exchange ions, and using the center of mass and center of gravity of the gate to correct the ion collision angle to obtain the gate corrosion rate;

[0015] S8: reducing the grid quality based on the gate corrosion rate, and when the grid quality meets the preset requirements, eliminating the grid nodes to achieve the change of the gate morphology.

[0016] Preferably, in S2, the axisymmetric calculation domain includes: the upstream region of the screen grid, the screen grid, the region between the grids, the acceleration grid and the downstream region of the acceleration grid.

[0017] Preferably, in said S3, the method for obtaining the initial plasma density upstream of the screen grid comprises:

[0018]

[0019] Where η is the transparency of the gate system, e is the charge of the elementary charge, k is the Boltzmann constant, and T e is the electron temperature, m i is the ion mass, and R is the radius of the computational domain.

[0020] Preferably, in S4, the method for obtaining the Debye length includes:

[0021]

[0022] Where ε0 is the dielectric constant of vacuum, k is the Boltzmann constant, and T e is the electron temperature in the plasma, n0 is the reference plasma density, and e is the charge of the elementary charge.

[0023] Preferably, in S6, the method of performing beam calculation based on the gate corrosion morphology evolution model and simulating to obtain the single-hole beam size includes:

[0024] S61: Sprinkling ions at the left boundary of the axisymmetric calculation domain;

[0025] S62: Solve the Poisson equation by successive super-relaxation iterations to obtain the electric potential and electric field magnitude at each grid node in the axisymmetric computational domain;

[0026] S63: The ions in the axisymmetric calculation domain move under the potential and electric field;

[0027] S64: Repeat the processes of S61, S62 and S63 until the number of ions in the axisymmetric calculation domain no longer changes, and obtain the single-hole beam size.

[0028] Preferably, in S7, the method for generating charge exchange ions comprises:

[0029] According to the neutral atom density on each grid node obtained in S5, the charge exchange collision process between ions and neutral atoms is processed by Monte Carlo collision to generate charge exchange ions.

[0030] Preferably, in S7, the method for obtaining the gate corrosion rate includes:

[0031] Select nine grids centered on the collision grid as a surface unit, and obtain the centroid coordinates (z m ,r m ), shape center coordinates (z c ,r c );

[0032] Based on the centroid coordinates (z m ,r m ) and the shape center coordinates (z c ,r c ), obtain the surface normal of the bombarded mesh node

[0033] Based on the surface normal of the bombarded mesh node and the charge exchange ion incident vector Obtain the angle between the ion incident direction and the local surface of the grid

[0034] Based on beam current calculation, the charge exchange ion energy is obtained;

[0035] Based on the angle between the ion incident direction and the local surface of the grid and exchanging ion energy with the charge to obtain a sputtering yield Y(E,θ) of each ion, where E is the incident ion energy;

[0036] Based on the sputtering yield Y(E,θ) of each ion, the corrosion rate N at different node positions of the gate is obtained r =∑Y(E,θ) / Δt, where Y(E,θ) is the sputtering yield of a single ion and Δt is the time when sputtering occurs.

[0037] Preferably, in S8, the method for changing the gate morphology includes:

[0038] Reduce the mesh quality based on the gate corrosion rate, when the number of lost atoms is less than the node N r <N node , the gate shape does not change;

[0039] When the number of lost atoms is greater than the number of nodes N r >N node Eliminate grid nodes to change the gate morphology.

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] The present invention adopts the latest gate corrosion morphology to correct the angle of ion bombardment on the gate, obtains the accurate gate corrosion rate, can accurately obtain the evolution process of the gate corrosion morphology through numerical simulation calculation, realizes the rapid optimization guidance of the design parameters of the ion thruster, can greatly shorten the design cycle of the thruster, and effectively improves the production efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In order to more clearly illustrate the technical solution of the present invention, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0043] Figure 1 Schematic diagram of a flow chart of a method for predicting the corrosion morphology evolution of an ion thruster grid according to an embodiment of the present invention;

[0044] Figure 2 Schematic diagram of selection of simulation objects (dashed circle) and setting of simulation calculation domain in an embodiment of the present invention;

[0045] Figure 3 Schematic diagram of the incident angle of ions bombarding the grid in an embodiment of the present invention;

[0046] Figure 4Schematic diagram of gate corrosion morphology in an embodiment of the present invention, wherein (a) is the gate corrosion morphology after 2000h, and (b) is the gate corrosion morphology after 5000h. DETAILED DESCRIPTION

[0047] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. 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 making creative efforts are within the scope of protection of the present invention.

[0048] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0049] The ion thruster grid is a two-layer (screen grid, accelerator grid) or three-layer (screen grid, accelerator grid, decelerator grid) mesh structure with gaps less than 1mm between layers. The plasma upstream of the screen grid is accelerated by the bias voltage on the grid and is extracted from the grid aperture to form a high-energy ion beam. Because some unionized neutral atoms exist in the grid region, high-energy ions collide with low-energy neutral atoms, exchanging charge to form charge-exchange ions. The charge-exchange ions are attracted by the negative voltage of the accelerator grid downstream, and they begin to decelerate and eventually form backflow ions, which accelerate and bombard the accelerator grid. Due to the high energy of the backflow ions, atoms on the grid surface are collided and stripped from the grid, causing grid corrosion and morphological changes.

[0050] Example 1

[0051] like Figure 1 As shown, a method for predicting the evolution of ion thruster grid corrosion morphology includes the following steps:

[0052] S1: The single-hole beam size downstream of the accelerating grid is obtained through experimental measurement;

[0053] S2: Perform numerical simulation on the cylindrical area where the screen grid and acceleration grid holes are located to obtain the axisymmetric calculation domain;

[0054] S3: In the axisymmetric calculation domain, the initial plasma density upstream of the screen grid is obtained based on the relationship between the beam current and the plasma density upstream of the screen grid;

[0055] S4: Obtain the Debye length according to the plasma density upstream of the screen, and obtain the size of the simulation grid based on the Debye length; the size of the simulation grid is generally smaller than the Debye length;

[0056] S5: According to the neutral gas collision model, the neutral gas density at each grid node in the simulation area is obtained;

[0057] S6: establishing a gate corrosion morphology evolution model, starting beam calculation based on the gate corrosion morphology evolution model, and simulating the single-hole beam size; based on the simulated single-hole beam size and the single-hole beam size obtained by experimental measurement in S1, feedback adjustment is performed on the initial screen grid upstream plasma density to obtain the final screen grid upstream plasma density;

[0058] S7: Processing the collision process between ions and the gate to generate charge exchange ions; based on the charge exchange ions, corrosion calculation is started, and the gate center of mass and gravity are used to correct the ion collision angle to obtain the gate corrosion rate;

[0059] S8: Reduce the mesh quality based on the gate corrosion rate. When the mesh quality meets the preset requirements (when the mesh quality is ≤ 0), eliminate the mesh nodes to achieve the change of the gate morphology.

[0060] In this embodiment, in S1, the Faraday probe is placed in the downstream area of ​​the center hole of the grid through the Faraday probe diagnostic experiment to obtain the single hole beam size I downstream of the accelerating grid. b , as the input parameters of the numerical simulation model.

[0061] In this embodiment, a cylindrical region where a pair of screen grids and an accelerating grid aperture are located is selected as the object of numerical simulation research. Due to the symmetry of the grid aperture beam parameters, an axisymmetric calculation domain is used to simplify the three-dimensional problem into a two-dimensional calculation, which greatly reduces the amount of calculation and improves the calculation efficiency. Among them, the grid aperture beam parameters include ion density, velocity, and potential distribution, which have a spatial symmetry.

[0062] like Figure 2 As shown, in S2, the axisymmetric calculation domain includes: the upstream area of ​​the screen grid, the screen grid, the area between the gates, the acceleration grid and the downstream area of ​​the acceleration grid.

[0063] In this embodiment, in S3, the method for obtaining the initial plasma density upstream of the screen grid includes:

[0064] The number of ions leaving the right boundary per unit time downstream can be obtained from the beam size of a single hole downstream of the grid. The upstream ion density can be approximately estimated based on the grid transmittance and the saturation current formula. The specific expression is:

[0065]

[0066] Where η is the transparency of the gate system, e is the charge of the elementary charge, k is the Boltzmann constant, and T e is the electron temperature, m iis the ion mass, and R is the radius of the computational domain.

[0067] In this embodiment, in S4, the method for obtaining the Debye length includes:

[0068]

[0069] Where ε0 is the dielectric constant of vacuum, k is the Boltzmann constant, and T e is the electron temperature in the plasma, n0 is the reference plasma density, and e is the charge of the elementary charge.

[0070] In this embodiment, the neutral gas flow in the ion thruster belongs to the rarefied flow field. A direct Monte Carlo collision method is used to obtain the neutral gas density at each grid node in the simulation area. In the subsequent simulation process, it is assumed that the density at each grid node remains unchanged.

[0071] In this embodiment, in S6, based on the gate corrosion morphology evolution model, beam current calculation is performed to simulate and obtain the single-hole beam current size, including:

[0072] S61: ion ejection at the left boundary of the axisymmetric computational domain;

[0073] S62: Solve the Poisson equation by successive super-relaxation iterations to obtain the electric potential and electric field magnitude at each grid node in the axisymmetric computational domain;

[0074] S63: Ions in the axisymmetric computational domain move under the force of potential and electric field;

[0075] S64: Repeat the processes of S61, S62 and S63 until the number of ions in the axisymmetric calculation domain no longer changes. At this time, it is considered that the beam calculation has reached stability and the single-hole beam size is obtained.

[0076] In this embodiment, feedback adjustment is performed through the experimental value of S1, the ion density upstream of the screen grid is accurately adjusted, and the beam calculation is restarted. When the difference between the simulated beam size and the experimental one is within 1%, it is considered that the thruster is operating under normal experimental conditions, and the upstream ion density at this time is the final ion density.

[0077] In this embodiment, in S7, the method for generating charge exchange ions includes:

[0078] Based on the neutral atom density at each grid node obtained in S5, a Monte Carlo collision method is used to process the charge exchange collision process between ions and neutral atoms to generate charge exchange ions. Charge exchange ions are the main particles that cause gate sputtering corrosion.

[0079] In this embodiment, in S7, the method for obtaining the gate corrosion rate includes:

[0080] The gate corrosion rate can be calculated using the sputtering yield. The sputtering yield Y is a function of the charge exchange ion energy and the incident angle. The incident angle of each charge exchange ion to the gate surface changes greatly with the change of the gate morphology.

[0081] Select nine grids centered on the collision grid as a surface unit, and obtain the centroid coordinates (z m ,r m ), shape center coordinates (z c ,r c );

[0082] Based on the centroid coordinates (z m ,r m ) and the shape center coordinates (z c ,r c ), obtain the surface normal of the bombarded mesh node

[0083] Based on the surface normal at the bombarded mesh node and the charge exchange ion incident vector Obtain the angle between the ion incident direction and the local surface of the grid like Figure 3 As shown;

[0084] Based on beam current calculation, the charge exchange ion energy is obtained;

[0085] Based on the angle between the ion incident direction and the local surface of the grid and charge exchange ion energy to obtain the sputtering yield of each ion Y(E,θ), where E is the incident ion energy;

[0086] Based on the sputtering yield Y(E,θ) of each ion, the corrosion rate N at different node positions of the gate is obtained r =∑Y(E,θ) / Δt, where Y(E,θ) is the sputtering yield of a single ion and Δt is the time when sputtering occurs.

[0087] In this embodiment, in S8, the method for changing the gate morphology includes:

[0088] Reduce the mesh quality based on the gate corrosion rate, when the number of lost atoms is less than the node N r <N node , the gate shape does not change;

[0089] When the number of lost atoms is greater than the number of nodes N r >N nodeEliminate the grid nodes to change the gate morphology. Figure 4 (a) and Figure 4 (b) shown.

[0090] In this embodiment, in order to further improve the calculation accuracy, when the gate shape changes, the new gate shape is used to re-perform the beam calculation and corrosion calculation.

[0091] The embodiments described above are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by persons skilled in the art should fall within the scope of protection defined by the claims of the present invention.

Claims

1. A method for predicting the evolution of ion thruster grid corrosion morphology, characterized in that: The following steps are involved: S1: The single-hole beam size downstream of the accelerating grid is obtained through experimental measurement; S2: Perform numerical simulation on the cylindrical area where the screen grid and acceleration grid holes are located to obtain the axisymmetric calculation domain; S3: in the axisymmetric calculation domain, obtaining an initial plasma density upstream of the screen grid according to a relationship between the beam current and the plasma density upstream of the screen grid; S4: Obtain the Debye length according to the plasma density upstream of the screen, and obtain the size of the simulation grid based on the Debye length; S5: According to the neutral gas collision model, the neutral gas density at each grid node in the simulation area is obtained; S6: establishing a gate corrosion morphology evolution model, starting beam calculation based on the gate corrosion morphology evolution model, and simulating a single-hole beam size; based on the simulated single-hole beam size and the single-hole beam size obtained by experimental measurement in S1, feedback adjustment is performed on the initial screen grid upstream plasma density to obtain a final screen grid upstream plasma density; S7: Processing the collision process between ions and the gate to generate charge exchange ions; Based on the charge exchange ions, corrosion calculation is started, and the gate center of mass and gravity center are used to correct the ion collision angle to obtain the gate corrosion rate; S8: reducing the grid quality based on the gate corrosion rate, and when the grid quality meets the preset requirements, eliminating the grid nodes to achieve the change of the gate morphology.

2. The method for predicting the evolution of ion thruster grid corrosion morphology according to claim 1, characterized in that: In the above S2, the axisymmetric calculation domain includes: the upstream region of the screen grid, the screen grid, the region between the grids, the acceleration grid and the downstream region of the acceleration grid.

3. The method for predicting the evolution of ion thruster grid corrosion morphology according to claim 1, characterized in that: In said S3, the method for obtaining the initial plasma density upstream of the screen grid comprises: Where η is the transparency of the gate system, e is the charge of the elementary charge, k is the Boltzmann constant, and T e is the electron temperature, m i is the ion mass, and R is the radius of the computational domain.

4. The method for predicting the evolution of ion thruster grid corrosion morphology according to claim 1, characterized in that: In S4, the method for obtaining the Debye length includes: Where ε0 is the dielectric constant of vacuum, k is the Boltzmann constant, and T e is the electron temperature in the plasma, n0 is the reference plasma density, and e is the charge of the elementary charge.

5. The method for predicting the evolution of ion thruster grid corrosion morphology according to claim 1, characterized in that: In S6, based on the gate corrosion morphology evolution model, beam current calculation is performed to simulate and obtain the single-hole beam current size, including: S61: Sprinkling ions at the left boundary of the axisymmetric calculation domain; S62: Solve the Poisson equation by successive super-relaxation iterations to obtain the electric potential and electric field magnitude at each grid node in the axisymmetric computational domain; S63: The ions in the axisymmetric calculation domain move under the potential and electric field; S64: Repeat the processes of S61, S62 and S63 until the number of ions in the axisymmetric calculation domain no longer changes, and obtain the single-hole beam size.

6. The method for predicting the evolution of ion thruster grid corrosion morphology according to claim 1, characterized in that: In said S7, the method for generating charge exchange ions comprises: According to the neutral atom density on each grid node obtained in S5, the charge exchange collision process between ions and neutral atoms is processed by Monte Carlo collision to generate charge exchange ions.

7. The method for predicting the evolution of ion thruster grid corrosion morphology according to claim 2, characterized in that: In said S7, the method for obtaining the gate corrosion rate includes: Select nine grids centered on the collision grid as a surface unit, and obtain the centroid coordinates (z m ,r m ), shape center coordinates (z c ,r c ); Based on the centroid coordinates (z m ,r m ) and the shape center coordinates (z c ,r c ), obtain the surface normal of the bombarded mesh node Based on the surface normal of the bombarded mesh node and the charge exchange ion incident vector Obtain the angle between the ion incident direction and the local surface of the grid Based on beam current calculation, the charge exchange ion energy is obtained; Based on the angle between the ion incident direction and the local surface of the grid and exchanging ion energy with the charge to obtain a sputtering yield Y(E,θ) of each ion, where E is the incident ion energy; Based on the sputtering yield Y(E,θ) of each ion, the corrosion rate N at different node positions of the gate is obtained r =∑Y(E,θ) / Δt, where Y(E,θ) is the sputtering yield of a single ion and Δt is the time when sputtering occurs.

8. The method for predicting the evolution of ion thruster grid corrosion morphology according to claim 1, characterized in that: In S8, the method for changing the gate morphology includes: Reduce the mesh quality based on the gate corrosion rate, when the number of lost atoms is less than the node N r <N node , the gate shape does not change; When the number of lost atoms is greater than the number of nodes N r >N node Eliminate grid nodes to change the gate morphology.

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