Plasma electron density measuring device and method based on laser wavefront analysis

Abstract

The invention belongs to the field of plasma electron density measurement, and discloses a plasma electron density measurement device and method based on laser wavefront analysis. The device adopts a frequency doubling crystal to convert a fundamental frequency laser beam into a fundamental frequency light beam and a second harmonic light beam which are coaxial; the two coaxial laser beams generate different wavefront changes when passing through the plasma, and the wavefront changes are converted into focus spot centroid displacement changes through the micro-lens array; different from other measurement modes, the device needs to set probe light and reference light, does not need to compare before and after plasma, and only needs to refer to the change difference of focus spot centroids after two coaxial laser beams with different wavelengths pass through the plasma; by adopting the measuring device, the interference of neutral particles in the experiment process can be eliminated, the difference between the presence and absence of plasmas in two lasers in the experiment can be eliminated, synchronous compensation can be realized, and the problem that the measurement precision is limited as a reference light path arranged in the prior art is easily influenced by the environment is effectively solved.

Description

Technical Field

[0001] The present invention belongs to the field of plasma parameter diagnosis, and specifically relates to a plasma electron density measurement method, and in particular to a plasma electron density measurement device and method based on laser wavefront analysis. Background Art

[0002] In the field of plasma electron density measurement technology, the spatiotemporal distribution of plasma electron density is one of the core parameters. Accurate measurement of plasma electron density is crucial to understanding the characteristics and behavior of plasma, and is also the basis for further understanding of physical processes and development.

[0003] Optical diagnostic methods are usually used to measure plasma electron density. Optical diagnostic methods play an important role in plasma electron density measurement due to their high spatial and temporal resolution, as well as non-invasive characteristics. Common plasma electron density measurements require the use of laser interferometry technology, including Michelson, Mach-Zehnder and other methods, but the above methods require the setting of a reference optical path; since the set reference optical path is easily affected by environmental factors, any slight environmental change may cause changes in the reference optical path, thereby affecting the stability and accuracy of the interference signal; and the stability and accuracy of the reference optical path directly affect the phase difference of the interference signal, thereby affecting the measurement accuracy of the plasma electron density. It can be seen that if the reference optical path is disturbed or changes, errors will be introduced, reducing the reliability of the measurement results and limiting the measurement accuracy. In addition, obtaining the fringe offset by the fringe tracing method or the phase unwrapping method can only be used for part of the electron density 10 17 cm -2 ~10 19 cm -2 Parameter diagnosis of plasmas below 10 17 cm -2 Low-density plasma has limited diagnostic capabilities.

[0004] It can be seen that the existing plasma electron density measurement measures require the pre-setting of the reference light path. Since the reference light path is easily affected by the environment, the measurement accuracy is limited. Summary of the invention

[0005] The present invention provides a plasma electron density measurement device and method based on laser wavefront analysis, so as to solve the technical problem that the existing plasma electron density measurement measures need to pre-set a reference light path, and the reference light path is easily affected by the environment, resulting in limited measurement accuracy.

[0006] In order to achieve the above object, the present invention adopts the following technical contents: A plasma electron density measuring device based on laser wavefront analysis, comprising: a pulsed laser, a beam splitter, a first reflector, a collimation system, a second reflector, a third reflector, a dichroic mirror, a first microlens array, a first camera, a second microlens array, and a second camera; Wherein, the pulse laser beam emitted by the pulse laser is incident on the first reflecting mirror through the beam splitter; A frequency doubling crystal is arranged in the collimation system; the pulsed laser beam passing through the first reflector enters the frequency doubling crystal inside the collimation system and then is emitted, and is incident on the plasma through the second reflector; the laser beam passing through the plasma is split after passing through the third reflector and the dichroic mirror, the fundamental frequency beam is incident on the first lens array and the first camera, and the second harmonic beam is incident on the second lens array and the second camera.

[0007] Furthermore, the collimation system comprises a first adjustable aperture, a first lens, a second lens, and a second adjustable aperture which are coaxially arranged in sequence; and the frequency doubling crystal is located between the first lens and the second lens.

[0008] Furthermore, the frequency doubling crystal uses a frequency doubling crystal with a frequency doubling efficiency greater than 50%.

[0009] Furthermore, the intermediate connecting parts between the first microlens array and the first camera and between the second microlens array and the second camera are all subjected to light shielding treatment.

[0010] Furthermore, it also includes a vacuum cavity, and the plasma is arranged inside the vacuum cavity.

[0011] A plasma electron density measurement method based on laser wavefront analysis, based on the above-mentioned plasma electron density measurement device based on laser wavefront analysis, comprises: The pulse laser emits a pulse laser beam, which passes through a beam splitter, a first reflector, a first part of a collimation system, a frequency doubling crystal, a second part of a collimation system, a second reflector, a plasma, a third reflector, and a dichroic mirror in sequence, and emits a coaxial dual-wavelength laser. The fundamental frequency beam of the coaxial dual-wavelength laser is incident on the first lens array and the first camera, and the second harmonic beam is incident on the second lens array and the second camera; Using the coaxial dual-wavelength laser incident from the first camera and the second camera to obtain the centroid coordinates of the light spot array at two wavelengths; The distribution of plasma electron density is obtained based on the centroid coordinates of the spot array at two wavelengths.

[0012] Further, wherein: A pulsed laser is used as the light source to emit a The pulsed laser beam is incident on the beam splitter; then it is incident on the first reflecting mirror through the beam splitter; The fundamental frequency beam of the pulsed laser beam passes through the first adjustable aperture and the first lens and then enters the frequency doubling crystal. The wavelength generated after passing through the frequency doubling crystal is The second harmonic of The fundamental frequency beam and the second harmonic beam of the pulsed laser beam are coaxially emitted from the frequency doubling crystal, enter the vacuum cavity and pass through the plasma after passing through the second lens, the second adjustable aperture and the second reflector; the laser beam passing through the plasma passes through the third reflector and is separated into the fundamental frequency beam and the second harmonic beam by the dichroic mirror; The fundamental frequency beam is incident on the first lens array and the first camera, and the second harmonic beam is incident on the second lens array and the second camera.

[0013] Furthermore, the specific steps of the fundamental frequency beam of the coaxial dual-wavelength laser is incident on the first lens array and the first camera, and the second harmonic beam is incident on the second lens array and the second camera are as follows: The coaxial dual-wavelength laser passing through the plasma is separated into a fundamental frequency beam and a second harmonic beam by a dichroic mirror. The fundamental frequency beam is incident on the first lens array and the first camera, and the second harmonic beam is incident on the second lens array and the second camera to generate a focal spot array image. The centroid coordinates of the focal spot array at two wavelengths are obtained according to the focal spot array image.

[0014] Furthermore, the focal spot array centroid coordinates at two wavelengths are obtained according to the focal spot array image, and the focal spot centroid position result is obtained, specifically: The first step is to traverse from the upper left corner to the lower right corner of the entire focal spot array image, obtain the pixel sum of the corresponding sub-image under the window coverage, store the pixel sum in the matrix and find the maximum value of the matrix; scan the position of the sub-image corresponding to each maximum value, extract the sub-image with overlapping parts, and calculate the sub-image with overlapping positions:

[0015] In the formula, is the light intensity of the pixel at the center of the window, Represents the light intensity of the point pixels adjacent to the center point, is an integer, and ; The second step is to use the adaptive threshold selection method to estimate the number of pixels m occupied by the light spot and the width of the sub-spot , the number of pixels occupied by the main spot diameter , the number of pixels occupied by the main spot , arrange the grayscale values ​​of all pixels of each sub-spot from large to small, for the imaging area corresponding to each lens Calculate the To The average value of gray values ​​is used as the threshold of the sub-spot area. :

[0016] In the formula, , as well as Respectively represent from the first to the nth 2 The gray value of pixels; To complete the positioning of the center of mass of the focal light spot and obtain the center of mass position result of the focal light spot.

[0017] Furthermore, the distribution of plasma electron density is obtained according to the center of mass position of the focal spot, specifically: The difference between the center of mass displacement of the focal spot at the wavelength of the fundamental beam and the wavelength of the second harmonic beam , the specific formula is as follows:

[0018] In the formula, and represent the constant refractive index and the change in particle density of neutral particles before and after plasma generation, respectively; and They represent positive ions respectively; represents the change of electrons, They represent the radial, axial and optical path directions of the plasma, respectively; is the laser wavelength; is a constant; When the laser beam passes through the plasma generated at 0≤z≤L, the kth focal spot offset in the x direction can be expressed as ,use The two different wavelengths used to solve the electron density imaging are used to effectively eliminate the interference of neutral particles, specifically:

[0019] In the formula, is the focal length of the microlens; The plasma line integral density is expressed as:

[0020] Where Ne is the integral of the spatial electron density; The plasma electron density is further expressed as: .

[0021] Compared with the prior art, the present invention has the following beneficial effects: The present invention provides a plasma electron density measuring device based on laser wavefront analysis. The device uses a frequency doubling crystal to transform a fundamental frequency laser beam into a coaxial fundamental frequency beam and a second harmonic beam. The two coaxial laser beams will produce different wavefront changes when passing through the plasma and are converted into changes in the centroid displacement of the focal light spot through a microlens array. Unlike other measurement methods, the device needs to set up a detection light and a reference light, and does not need to compare before and after the presence or absence of plasma. It only needs to refer to the difference in the change of the centroid of the focal light spot after two coaxial laser beams of different wavelengths pass through the plasma. The use of the measuring device can eliminate the interference of neutral particles in the experimental process and eliminate the difference in lasers in the two experiments with and without plasma, and can achieve synchronous compensation, which effectively solves the problem that the reference light path set in the prior art is easily affected by the environment, thereby resulting in limited measurement accuracy. The device is simple in principle, easy to implement, and has high measurement sensitivity. It has good promotion and application value.

[0022] Preferably, in the present invention, the structure of the collimation system includes a coaxially arranged adjustable aperture and lens, and the position of the frequency doubling crystal. This design ensures the collimation and stability of the laser beam, thereby improving the measurement accuracy.

[0023] Preferably, in the present invention, a frequency doubling crystal with a frequency doubling efficiency greater than 50% is used to improve the conversion efficiency of the second harmonic, so that the intensity of the second harmonic beam is strong enough to be clearly captured by the camera, further improving the sensitivity and accuracy of the measurement.

[0024] Preferably, in the present invention, by introducing a vacuum chamber to accommodate the plasma, the interference of the external environment on the plasma is effectively isolated, the stability and purity of the measurement environment are ensured, and the reliability of the measurement result is improved.

[0025] Preferably, in the present invention, the connecting parts of the first microlens array and the first camera and the second microlens array and the second camera are shielded and connected using a 30mm coaxial system, which reduces the interference of stray light, improves the imaging quality, and makes the centroid coordinates of the focal spot array more accurate.

[0026] The present invention also provides a plasma electron density measurement method based on laser wavefront analysis. Based on the above-mentioned plasma electron density measurement device based on laser wavefront analysis, firstly, the measurement method emits a coaxial dual-wavelength laser through the constructed measurement device; the camera receives the dual-wavelength laser and obtains the focal spot array centroid coordinates under two wavelengths, and finally obtains the distribution of plasma electron density through the focal spot array centroid coordinates; the method can eliminate the interference of neutral particles during the experiment and accurately measure the plasma electron density; secondly, it maintains good stability because the difference between the lasers in the two experiments with and without plasma can be eliminated, and synchronous compensation can be achieved; in addition, the coaxial laser can more accurately achieve the measurement of the same position, which is more accurate in spatial position for the already delicate measurement process. Finally, the plasma line integral density measurement sensitivity is achieved that is 1 to 2 orders of magnitude higher than ordinary interference, thereby achieving efficient, reliable and highly sensitive measurement of plasma electron density.

[0027] Preferably, in the present invention, the entire process of the pulsed laser beam from emission to passing through the plasma and being captured by the camera includes steps such as frequency doubling, optical path adjustment and separation, which help to achieve precise measurement and ensure the efficiency and accuracy of the measurement.

[0028] Preferably, in the present invention, the focal spot array centroid coordinates are obtained through the focal spot array image, which provides an accurate data basis for subsequent electron density calculation.

[0029] Further preferably, in the present invention, an adaptive threshold selection method is proposed to determine the center of mass position of the focal spot, which can automatically adapt to focal spots of different brightness and shapes, improve the accuracy and robustness of center of mass positioning, and provide reliable input data for electron density calculation.

[0030] Further preferably, in the present invention, a calculation formula for the plasma electron density is derived based on the focal spot centroid position result, and the difference between the focal spot centroid displacements of two different wavelengths is used to eliminate the interference of neutral particles, thereby achieving accurate measurement of the plasma electron density; this enables the method to have higher measurement accuracy and anti-interference ability. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 A schematic structural diagram of a plasma electron density measurement device based on laser wavefront analysis provided by an embodiment of the present invention; Figure 2 A schematic structural diagram of another plasma electron density measuring device based on laser wavefront analysis provided by an embodiment of the present invention; Figure 3A flowchart of a plasma electron density measuring device based on laser wavefront analysis provided in an embodiment of the present invention.

[0032] Reference numerals: 1. Pulse laser; 2. Pulse laser beam; 3. Beam splitter; 4. First reflector; 5. First adjustable aperture; 6. First lens; 7. Frequency doubling crystal; 8. Second lens; 9. Second adjustable aperture; 10. Second reflector; 11. Plasma; 12. Third reflector; 13. Dichroic mirror; 14. First microlens array; 15. First camera; 16. Second microlens array; 17. Second camera; 18. Vacuum chamber. DETAILED DESCRIPTION

[0033] In order to make the technical problems, technical solutions and beneficial effects solved by the present invention more clearly understood, the present invention is further described in detail in the following specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0034] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings here can be arranged and designed in various different configurations.

[0035] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention claimed for protection, but merely represents selected embodiments of the present invention. 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.

[0036] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, further definition and explanation thereof is not required in subsequent drawings.

[0037] In the description of the embodiments of the present invention, it should be noted that if the terms "upper", "lower", "horizontal", "inner", etc. indicate an orientation or positional relationship based on the orientation or positional relationship shown in the drawings, or the orientation or positional relationship in which the product of the invention is usually placed when in use, it is only for the convenience of describing the present invention and simplifying the description, and does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention. In addition, the terms "first", "second", etc. are only used to distinguish the description, and cannot be understood as indicating or implying relative importance.

[0038] In addition, if the term "horizontal" appears, it does not mean that the component must be absolutely horizontal, but can be slightly tilted. For example, "horizontal" only means that its direction is more horizontal than "vertical", which does not mean that the structure must be completely horizontal, but can be slightly tilted.

[0039] In the description of the embodiments of the present invention, it is also necessary to explain that, unless otherwise clearly specified and limited, the terms "set", "install", "connect", and "connect" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal connection of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0040] The present invention is further described in detail below in conjunction with the accompanying drawings: Example 1 As described in the background technology above, common plasma electron density measurements require the use of laser interferometry technology, including Michelson, Mach-Zehnder and other methods, but the above methods require the setting of a reference light path; since the set reference light path is easily affected by environmental factors, any slight environmental change may cause changes in the reference light path, thereby affecting the stability and accuracy of the interference signal; and the stability and accuracy of the reference light path directly affect the phase difference of the interference signal, thereby affecting the measurement accuracy of the plasma electron density. It can be seen that if the reference light path is disturbed or changes, errors will be introduced, reducing the reliability of the measurement results and resulting in limited measurement accuracy. In addition, obtaining the fringe offset by the fringe tracing method or the phase unwrapping method can only be used for part of the electron density 10 17 cm -2 ~10 19 cm -2 Parameter diagnosis of plasmas below 10 17 cm -2 Low-density plasma has limited diagnostic capabilities.

[0041] In order to solve the above problems, this embodiment provides a plasma electron density measurement device based on laser wavefront analysis. The use of this device can solve the problems of low sensitivity, narrow measurement range and great influence of laser quality in interferometric measurement of plasma electron density.

[0042] This embodiment provides a plasma measurement device based on fiber-coupled dispersion interferometry, based on the following principles: The light is aligned to be incident on the microlens array in front of the wavefront sensor, and a single focal spot is formed on the CMOS camera sensor at the focal plane of the microlens array. If the wavefront is planar, all focal spots are directly located at the center position behind the corresponding lens, and these focal spot positions are called reference focal spot positions; the wavefront of lasers of different wavelengths is distorted after passing through the plasma, and the focal spot will deviate from the reference position on the camera sensor; using the focal length of the lens and the relative position of the focal spot to the reference focal spot, the propagation angle of this part of the beam can be found, thereby obtaining the local shape of the wavefront, and the wavefront shape across the entire cross-section of the beam is calculated by the two-dimensional integral of all focal spot displacements, thereby inverting the line integral density of plasma electrons (i.e., electron density).

[0043] like Figure 1 As shown, this embodiment provides a plasma measurement device based on fiber-coupled dispersion interferometry, comprising: Pulse laser 1, beam splitter 3, first reflector 4, first adjustable aperture 5, first lens 6, frequency doubling crystal 7, second lens 8, second adjustable aperture 9, second reflector 10, third reflector 12, dichroic mirror 13, first microlens array 14, first camera 15, second microlens array 16, second camera 17 and vacuum chamber 18.

[0044] Among them, a pulse laser 1 is used as a light source, and the emitted pulse laser beam 2 is incident on a beam splitter 3; it is incident on a first reflector 4 from the beam splitter 3; it is incident on a frequency doubling crystal 7 after passing through a first adjustable aperture 5 and a first lens 6; the frequency doubling crystal 7 is emitted through a second lens 8, a second adjustable aperture 9, a second reflector 10, and then passes through a plasma 11; the laser light passing through the plasma passes through a third reflector 12 and a dichroic mirror 13, and the fundamental frequency light beam passes through the dichroic mirror 13 and then enters a first lens array 14 and a first camera 15, and the doubled frequency light (second harmonic light beam) enters a second lens array 16 and a first camera 17.

[0045] The first adjustable diaphragm 5, the first lens 6, the second lens 8 and the second adjustable diaphragm 9 constitute a collimation system, which ensures the collimation and stability of the laser beam, thereby improving the accuracy and repeatability of the measurement.

[0046] In this embodiment, the frequency doubling crystal 7 is a KTP crystal, which has a frequency doubling efficiency greater than 50%.

[0047] like Figure 2 As shown, in this embodiment, plasma 11 is arranged inside vacuum chamber 18. Vacuum chamber 18 is sealed with steel plate and evacuated to vacuum with vacuum pump; plasma 11 is generated by applying pulse current to wire array load, the peak value of pulse current is 450kA, and the rising edge is 400ns; the pulse width is ns level or less, so as to ensure the time resolution of measurement.

[0048] In this embodiment, the intermediate connecting parts of the first microlens array 14 and the first camera 15 as well as the second microlens array 16 and the second camera 17 are all light-shielded and connected using a 30 mm coaxial system; the interference of stray light is reduced, the imaging quality is improved, and the centroid coordinates of the focal spot array are more accurate.

[0049] It can be seen that the present embodiment provides a plasma electron density measurement device based on laser wavefront analysis, wherein the present device transforms the fundamental frequency laser beam into a coaxial fundamental frequency laser beam and a second harmonic laser beam through a nonlinear frequency doubling crystal, wherein the two coaxial laser beams will produce different wavefront changes when passing through the plasma and are converted into changes in the displacement of the center of mass through the microlens array. This device is different from other measurement methods in that it requires the setting of detection light and reference light, and does not require the comparison before and after the presence or absence of plasma. It only requires the reference difference in the change of the center of mass of the focal spot of the two coaxial laser beams of different wavelengths after passing through the plasma. First, the measurement method can eliminate the interference of neutral particles during the experiment and accurately measure the plasma electron density; secondly, it maintains good stability because it can eliminate the difference in lasers in the two experiments with or without plasma, and can achieve synchronous compensation; in addition, the coaxial laser can more accurately achieve the measurement of the same position, which is more accurate in spatial position for the already delicate measurement process. Finally, the plasma line integral density measurement sensitivity is 1-2 orders of magnitude higher than that of ordinary interference, thereby achieving efficient, reliable and highly sensitive measurement of plasma electron density.

[0050] Example 2 like Figure 3 As shown, this embodiment provides a plasma electron density measurement method based on laser wavefront analysis, based on the plasma electron density measurement device based on laser wavefront analysis provided in Example 1, specifically comprising: S1, build the wavefront measurement system: Based on the above-mentioned plasma electron density measurement device based on laser wavefront analysis, a pulse laser 1 is used as a light source to emit a pulse laser beam 2 with a wavelength of , which is incident on a beam splitter 3; the beam splitter 3 is incident on a first reflector 4; the fundamental frequency beam of the pulse laser beam 2 passes through a first adjustable aperture 5 and a first lens 6 and then enters a frequency doubling crystal 7; the wavelength generated after passing through the frequency doubling crystal 7 is The fundamental frequency beam and the second harmonic beam of the pulsed laser beam 2 are coaxially emitted from the frequency doubling crystal 7, and then pass through the second lens 8, the second adjustable aperture 9, and the second reflector 10, and then enter the vacuum chamber 18 and pass through the plasma 11; the laser passing through the plasma 11 passes through the third reflector 12 and is separated from the fundamental frequency beam and the second harmonic beam by the dichroic mirror 13, and the fundamental frequency beam is incident on the first lens array 14 and the first camera 15, and the second harmonic beam is incident on the second lens array 16 and the second camera 17. The coaxial dual-wavelength laser measurement method using the frequency doubling crystal can effectively control the target position error problem in the measurement.

[0051] S2, obtain the focus spot array image: At the focal position of the microlens, two laser beams of different wavelengths will present a focal spot array on the camera image. Due to the different wavelengths, the refractive index will also change after passing through the plasma, resulting in a focal spot array image.

[0052] S3, calculate the centroid displacement of the focal spot: Based on the characteristics that the intensity of the central pixel of the focal spot in the collected image is the highest and the surrounding light intensity gradually decreases, the adaptive focal spot centroid detection method is designed as follows: First, select the window and design a window slightly larger than the focal spot size. The window traverses from the upper left corner to the lower right corner of the entire image to obtain the pixel sum of the corresponding sub-image covered by the window, store the pixel sum in the matrix and find the maximum value of the matrix. For the actual focal spot image, there may be several sub-images with the same pixel sum value near a focal spot. At this time, it is necessary to scan the positions of the sub-images corresponding to the maximum values, extract the sub-images with overlapping parts, and calculate the sub-images with overlapping positions: (1) In the formula, is the light intensity of the pixel at the center of the window, Represents the light intensity of the point pixels adjacent to the center point, is an integer, and ; Secondly, the centroid of the spot focus is located. The spot intensity in the target image is Gaussian distributed, and the occupied pixel area is small, and the overall grayscale value is relatively concentrated. Therefore, the adaptive threshold selection method is used: the number of pixels occupied by the sub-spot is estimated to be m, and the sub-spot width is ,in, is the laser wavelength, is the focal length of the microlens, is the diameter of the microlens. The number of pixels occupied by the main spot diameter is , then the number of pixels occupied by the main light spot is , arrange the grayscale values ​​of all pixels of each sub-spot from large to small, for the imaging area corresponding to each lens Calculate the To The average value of gray values ​​is used as the threshold of the sub-spot area. : (2) In the formula, , as well as Respectively represent from (m+1) to n 2 The gray value of pixels; The difference in the centroid displacement of the focal spot at the fundamental wavelength and the second harmonic wavelength is obtained by calculation. .

[0053] S4, calculate the electron line integral density: The difference between the center of mass displacement of the focal spot at the wavelength of the fundamental beam and the wavelength of the second harmonic beam , the specific formula is as follows: (3) In the formula and Represents a constant refractive index (in m 3 ) and the particle density change of neutral particles before and after plasma generation (in m −3 );same and They represent positive ions respectively; represents the change of electrons, They represent the radial, axial and optical path directions of the plasma, respectively; is the laser wavelength (in m); is a constant, ,in is the electron charge (C), The speed of light (ms −1 ), is the mass of the electron (kg), is the vacuum dielectric constant ( F m −1 ); When the laser beam passes through the plasma generated at 0 ≤z ≤L (m), the kth focal spot offset in the x direction can be expressed as (m) , using The two different wavelengths used to solve the electron density imaging are used to effectively eliminate the interference of neutral particles, specifically: (4) In the formula, is the focal length of the microlens (m); Then the plasma line integral density can be expressed as (5) In the formula, Ne(m -3 ) is the integral of the spatial electron density; The plasma electron density can be further expressed as: (6) In summary, the present invention provides a plasma electron density measurement device and method based on laser wavefront analysis, which has the following advantages over existing measurement measures: First, high-precision measurement: The device uses laser wavefront analysis technology and a coaxial dual-wavelength (fundamental frequency beam and second harmonic beam) measurement method to accurately obtain the distribution of plasma electron density; dual-wavelength measurement not only improves the measurement accuracy, but also effectively eliminates the interference of neutral particles, thereby improving the reliability of the measurement results.

[0054] Second, efficient frequency doubling system: Using a frequency doubling crystal with a frequency doubling efficiency greater than 50%, the laser can efficiently generate second harmonics after passing through the frequency doubling crystal, thereby enhancing the laser utilization efficiency and measurement sensitivity.

[0055] Third, optimize the optical path design: The collimation system ensures the collimation and stability of the laser beam through coaxially arranged adjustable apertures and lenses, reducing losses and interference in the optical path; at the same time, the use of a vacuum cavity further reduces the impact of the external environment on the plasma measurement process.

[0056] Fourth, advanced imaging technology: a combination of a microlens array and a camera is used to image the laser passing through the plasma. By calculating the centroid coordinates of the focal spot array, the changes in the plasma electron density can be accurately reflected. This imaging technology not only improves the spatial resolution of the measurement, but also makes the measurement process more intuitive and convenient.

[0057] Fifth, intelligent data processing: In terms of data processing, through the adaptive threshold selection method and the focal spot centroid positioning algorithm, useful information can be automatically and accurately extracted from the focal spot array image, and the distribution of plasma electron density can be calculated; this intelligent data processing method not only improves the degree of measurement automation, but also reduces the impact of human errors.

[0058] Sixth, broad application prospects: This device and method have broad application prospects in the fields of plasma physics, nuclear fusion research, laser processing, etc.; by accurately measuring the plasma electron density, it can provide important experimental data and theoretical support for research and application in these fields.

[0059] The above embodiment is only one of the implementation methods that can realize the technical solution of the present invention. The scope of protection claimed by the present invention is not limited only to this embodiment, but also includes changes, replacements and other implementation methods that can be easily thought of by any technician familiar with the technical field within the technical scope disclosed by the present invention.

Claims

1. A plasma electron density measuring device based on laser wavefront analysis, characterized in that: include: A pulse laser (1), a beam splitter (3), a first reflector (4), a collimation system, a second reflector (10), a third reflector (12), a dichroic mirror (13), a first microlens array (14), a first camera (15), a second microlens array (16), and a second camera (17); The pulse laser beam (2) emitted by the pulse laser (1) is incident on the first reflection mirror (4) via the beam splitter (3); A frequency doubling crystal (7) is arranged in the collimation system; the pulsed laser beam (2) passes through the first reflector (4) and enters the frequency doubling crystal (7) inside the collimation system before being emitted and incident on the plasma (11) through the second reflector (10); the laser light passing through the plasma (11) is split after passing through the third reflector (12) and the dichroic mirror (13), the fundamental frequency beam is incident on the first lens array (14) and the first camera (15), and the second harmonic beam is incident on the second lens array (16) and the second camera (17).

2. The plasma electron density measuring device based on laser wavefront analysis according to claim 1, characterized in that: The collimation system comprises a first adjustable diaphragm (5), a first lens (6), a second lens (8), and a second adjustable diaphragm (9) which are coaxially arranged in sequence; the frequency doubling crystal (7) is located between the first lens (6) and the second lens (8).

3. The plasma electron density measuring device based on laser wavefront analysis according to claim 1, characterized in that: The frequency doubling crystal (7) uses a frequency doubling crystal with a frequency doubling efficiency greater than 50%.

4. The plasma electron density measuring device based on laser wavefront analysis according to claim 1, characterized in that: The intermediate connecting parts of the first microlens array (14) and the first camera (15) and the second microlens array (16) and the second camera (17) are all subjected to light shielding treatment.

5. The plasma electron density measuring device based on laser wavefront analysis according to any one of claims 1 to 4, characterized in that: It also comprises a vacuum chamber (18), wherein the plasma (11) is arranged inside the vacuum chamber (18).

6. A plasma electron density measurement method based on laser wavefront analysis, characterized in that: The plasma electron density measuring device based on laser wavefront analysis according to any one of claims 1 to 5 comprises: The pulse laser (1) emits a pulse laser beam (2), which passes through a beam splitter (3), a first reflector (4), a first part of a collimation system, a frequency doubling crystal (7), a second part of a collimation system, a second reflector (10), a plasma (11), a third reflector (12), and a dichroic mirror (13) in sequence, thereby emitting a coaxial dual-wavelength laser. The fundamental frequency beam of the coaxial dual-wavelength laser is incident on the first lens array (14) and the first camera (15), and the second harmonic beam is incident on the second lens array (16) and the second camera (17); Using the coaxial dual-wavelength laser incident on the first camera (15) and the second camera (17), the centroid coordinates of the light spot array at two wavelengths are obtained; The distribution of plasma electron density is obtained based on the centroid coordinates of the spot array at two wavelengths.

7. The plasma electron density measurement method based on laser wavefront analysis according to claim 6, characterized in that: in: A pulsed laser (1) is used as a light source to emit a pulse with a wavelength of The pulsed laser beam (2) is incident on a beam splitter (3); and then incident on a first reflection mirror (4) from the beam splitter (3); The fundamental frequency beam of the pulsed laser beam (2) passes through the first adjustable aperture (5) and the first lens (6) and then enters the frequency doubling crystal (7); the wavelength generated after passing through the frequency doubling crystal (7) is The second harmonic of (8); The fundamental frequency beam and the second harmonic beam of the pulsed laser beam (2) are coaxially emitted from the frequency doubling crystal (7), and enter the vacuum chamber (18) and pass through the plasma (11) after passing through the second lens (8), the second adjustable aperture (9), and the second reflector (10); the laser light passing through the plasma (11) passes through the third reflector (12) and is separated into the fundamental frequency beam and the second harmonic beam by the dichroic mirror (13); The fundamental frequency light beam is incident on the first lens array (14) and the first camera (15), and the second harmonic light beam is incident on the second lens array (16) and the second camera (17).

8. The plasma electron density measurement method based on laser wavefront analysis according to claim 6, characterized in that: The specific steps of causing the fundamental frequency beam of the coaxial dual-wavelength laser to be incident on the first lens array (14) and the first camera (15), and the second harmonic beam to be incident on the second lens array (16) and the second camera (17) are as follows: The coaxial dual-wavelength laser passing through the plasma (11) is separated into a fundamental frequency beam and a second harmonic beam through a dichroic mirror (13); the fundamental frequency beam is incident on a first lens array (14) and a first camera (15); the second harmonic beam is incident on a second lens array (16) and a second camera (17), thereby generating a focal light spot array image; and the focal light spot array centroid coordinates at two wavelengths are obtained based on the focal light spot array image.

9. The plasma electron density measurement method based on laser wavefront analysis according to claim 8, characterized in that: According to the focus spot array image, the focus spot array centroid coordinates at two wavelengths are obtained, and the focus spot centroid position result is obtained, which is specifically: The first step is to traverse from the upper left corner to the lower right corner of the entire focal spot array image, obtain the pixel sum of the corresponding sub-image under the window coverage, store the pixel sum in the matrix and find the maximum value of the matrix; scan the position of the sub-image corresponding to each maximum value, extract the sub-image with overlapping parts, and calculate the sub-image with overlapping positions: In the formula, is the light intensity of the pixel at the center of the window, Represents the light intensity of the point pixels adjacent to the center point, is an integer, and ; The second step is to use the adaptive threshold selection method to estimate the number of pixels m occupied by the light spot and the width of the sub-spot , the number of pixels occupied by the main spot diameter , the number of pixels occupied by the main spot , arrange the grayscale values ​​of all pixels of each sub-spot from large to small, for the imaging area corresponding to each lens Calculate the To The average value of gray values ​​is used as the threshold of the sub-spot area. : In the formula, , as well as Respectively represent from (m+1) to n 2 The gray value of pixels; To complete the positioning of the center of mass of the focal light spot and obtain the center of mass position result of the focal light spot.

10. The plasma electron density measurement method based on laser wavefront analysis according to claim 9, characterized in that: The distribution of plasma electron density is obtained according to the center of mass position of the focal spot, which is: The difference between the center of mass displacement of the focal spot at the wavelength of the fundamental beam and the wavelength of the second harmonic beam , the specific formula is as follows: In the formula, and represent the constant refractive index and the change in particle density of neutral particles before and after plasma generation, respectively; and They represent positive ions respectively; represents the change of electrons, They represent the radial, axial and optical path directions of the plasma, respectively; is the laser wavelength; is a constant; When the laser beam passes through the plasma generated at 0≤z≤L, the kth focal spot offset in the x direction can be expressed as ,use The two different wavelengths used to solve the electron density imaging are used to effectively eliminate the interference of neutral particles, specifically: In the formula, is the focal length of the microlens; The plasma line integral density is expressed as: Where Ne is the integral of the spatial electron density; The plasma electron density is further expressed as: 。

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