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

Abstract

The present 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 uses a frequency doubling crystal to convert a fundamental frequency laser beam into a coaxial fundamental frequency beam and a second harmonic beam. The two coaxial laser beams produce different wavefront changes when passing through plasma, and these changes are converted into changes in the centroid displacement of a focal light spot through a microlens array. Unlike other measurement methods that require the provision of probe light and reference light, the device does not require a comparison before and after the presence of plasma, and only requires a reference to the difference in the change in the centroid of the focal light spot after two coaxial laser beams of different wavelengths pass through the plasma. The use of the measurement device can eliminate the interference of neutral particles during the experiment and eliminate the difference in laser light in two experiments with and without plasma, thereby achieving synchronous compensation and effectively solving the problem that the reference light path provided in the prior art is easily affected by the environment, thereby limiting the measurement accuracy.

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 more particularly 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 for measuring 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 setting 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 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 plasma at the order of magnitude, and for plasma 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 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 to solve the technical problem that the existing plasma electron density measurement measures require presetting of a reference light path, which is easily affected by the environment and leads to limited measurement accuracy.

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

[0007] A plasma electron density measuring device based on laser wavefront analysis, comprising:

[0008] 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;

[0009] The pulse laser beam emitted by the pulse laser is incident on the first reflecting mirror through the beam splitter;

[0010] A frequency-doubling crystal is provided in the collimation system. The pulsed laser beam passing through the first reflector enters the frequency-doubling crystal inside the collimation system and is then emitted and incident on the plasma through the second reflector. The laser beam passing through the plasma is split by 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.

[0011] Furthermore, the collimation system includes a first adjustable aperture, a first lens, a second lens, and a second adjustable aperture that are coaxially arranged in sequence; and the frequency doubling crystal is located between the first lens and the second lens.

[0012] Furthermore, the frequency doubling crystal uses a frequency doubling crystal with a frequency doubling efficiency greater than 50%.

[0013] Furthermore, intermediate connecting parts between the first microlens array and the first camera, and between the second microlens array and the second camera are all light-shielded.

[0014] Furthermore, a vacuum cavity is included, and the plasma is arranged inside the vacuum cavity.

[0015] 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, comprising:

[0016] The pulse laser emits a pulsed laser beam, which passes through the beam splitter, the first reflector, the first part of the collimation system, the frequency doubling crystal, the second part of the collimation system, the second reflector, the plasma, the third reflector, and the dichroic mirror in sequence, and emits a coaxial dual-wavelength laser.

[0017] 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;

[0018] Using the coaxial dual-wavelength laser incident on the first camera and the second camera, the centroid coordinates of the light spot array at the two wavelengths are obtained;

[0019] The distribution of plasma electron density is obtained based on the centroid coordinates of the spot array at two wavelengths.

[0020] Further, wherein:

[0021] Pulsed laser is used as the light source to emit a wavelength of The pulsed laser beam is incident on the beam splitter; then it is incident on the first reflecting mirror;

[0022] 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

[0023] The fundamental frequency beam and the second harmonic beam of the pulsed laser beam are coaxially emitted from the frequency doubling crystal, pass through the second lens, the second adjustable aperture, and the second reflector, enter the vacuum chamber, and pass through the plasma. The laser beam that has passed through the plasma passes through the third reflector and is separated into the fundamental frequency beam and the second harmonic beam by a dichroic mirror.

[0024] 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.

[0025] Furthermore, the specific steps of the coaxial dual-wavelength laser fundamental frequency beam incident on the first lens array and the first camera, and the second harmonic beam incident on the second lens array and the second camera are as follows:

[0026] The coaxial dual-wavelength laser passing through the plasma is separated into the fundamental frequency beam and the 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, generating a focal spot array image. The centroid coordinates of the focal spot array at two wavelengths are obtained based on the focal spot array image.

[0027] Furthermore, the coordinates of the center of mass of the focal spot array at two wavelengths are obtained according to the focal spot array image, and the center of mass position of the focal spot is obtained, specifically:

[0028] The first step is to traverse the entire focal spot array image from the upper left corner to the lower right corner, 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-images with overlapping parts, and calculate the sub-images with overlapping positions:

[0029]

[0030] Where, is the light intensity of the pixel at the center of the window, Represents the light intensity of the pixels adjacent to the center point, is an integer, and ;

[0031] The second step is to use the adaptive threshold selection method to estimate the number of pixels m occupied by the 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 light 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 the grayscale values ​​is used as the threshold of the sub-spot area :

[0032]

[0033] Where, , as well as Respectively represent from the first to the nth 2 The grayscale value of each pixel;

[0034] 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.

[0035] Furthermore, the distribution of plasma electron density is obtained according to the center of mass position of the focal spot, specifically:

[0036] The difference in the centroid 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:

[0037]

[0038] Where, and represent the constant refractive index and the change in particle density of neutral particles before and after plasma generation, respectively; and represent positive ions respectively; represents the change of electrons, represent the radial, axial and optical directions of the plasma, respectively; is the laser wavelength; is a constant;

[0039] 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:

[0040]

[0041] Where, is the focal length of the microlens;

[0042] The plasma line integral density is expressed as:

[0043]

[0044] Where Ne is the integral of the spatial electron density;

[0045] The plasma electron density is further expressed as:

[0046] .

[0047] Compared with the prior art, the present invention has the following beneficial effects:

[0048] The present invention provides a plasma electron density measurement device based on laser wavefront analysis. This device uses a frequency-doubling crystal to convert a fundamental laser beam into a coaxial fundamental beam and a second harmonic beam. The two coaxial laser beams produce different wavefront changes when passing through the plasma, which are converted into changes in the center of mass displacement of the focal spot through a microlens array. Unlike other measurement methods that require probe and reference light, and do not require a comparison before and after the presence of plasma, this device only requires the difference in the center of mass displacement of the focal spot after the two coaxial laser beams of different wavelengths pass through the plasma. This measurement device eliminates interference from neutral particles during the experiment and eliminates the difference in laser light between experiments with and without plasma, enabling synchronous compensation. This effectively solves the problem in existing technologies where the reference light path is easily affected by the environment, thereby limiting measurement accuracy. The device has a simple principle, is easy to implement, and has high measurement sensitivity. It has excellent application value.

[0049] Preferably, in the present invention, the structure of the collimation system includes a coaxially arranged adjustable aperture and lens, as well as the position of the frequency doubling crystal. This design ensures the collimation and stability of the laser beam, thereby improving the measurement accuracy.

[0050] 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.

[0051] 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 results is improved.

[0052] Preferably, in the present invention, the connection parts between the first microlens array and the first camera, and between the second microlens array and the second camera are shielded from light and connected using a 30mm coaxial system, thereby reducing the interference of stray light, improving the imaging quality, and making the centroid coordinates of the focal spot array more accurate.

[0053] The present invention also provides a plasma electron density measurement method based on laser wavefront analysis. Based on the aforementioned plasma electron density measurement device based on laser wavefront analysis, this measurement method firstly emits a coaxial dual-wavelength laser beam through a constructed measurement device; a camera receives the dual-wavelength laser beam and obtains the centroid coordinates of the focal spot array at the two wavelengths, and finally obtains the distribution of plasma electron density through the centroid coordinates of the focal spot array. This 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 laser beams between experiments with and without plasma, allowing for synchronous compensation. In addition, the coaxial laser beam can more accurately measure the same position, making the already delicate measurement process more precise in terms of spatial position. Ultimately, it achieves a plasma line integrated density measurement sensitivity that is 1 to 2 orders of magnitude higher than that of conventional interferometry, thereby achieving efficient, reliable, and highly sensitive measurement of plasma electron density.

[0054] 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. The above steps help to achieve accurate measurement and ensure the efficiency and accuracy of the measurement.

[0055] Preferably, in the present invention, the focal spot array centroid coordinates are obtained through the focal spot array image, providing an accurate data basis for subsequent electron density calculation.

[0056] Further preferably, in the present invention, an adaptive threshold selection method is proposed to determine the center of mass position of the focal spot. This method 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.

[0057] Further preferably, in the present invention, a calculation formula for the plasma electron density is derived based on the results of the focal spot center of mass position, and the difference between the focal spot center of mass 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

[0058] Figure 1 A schematic structural diagram of a plasma electron density measurement device based on laser wavefront analysis provided in an embodiment of the present invention;

[0059] Figure 2 A schematic structural diagram of another plasma electron density measurement device based on laser wavefront analysis provided by an embodiment of the present invention;

[0060] Figure 3 This is a flowchart of a plasma electron density measurement device based on laser wavefront analysis provided by an embodiment of the present invention.

[0061] Reference numerals:

[0062] 1. Pulsed laser; 2. Pulsed 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

[0063] 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 intended to limit the present invention.

[0064] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of 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. Generally, the components of the embodiments of the present invention described and shown in the drawings herein can be arranged and designed in various different configurations.

[0065] 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 as claimed, but rather merely represents selected embodiments of the present invention. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without creative effort are intended to fall within the scope of protection of the present invention.

[0066] 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, it does not need to be further defined or explained in subsequent drawings.

[0067] In the description of the embodiments of the present invention, it should be noted that if the terms "upper," "lower," "horizontal," "inner," etc. appear, the orientation or positional relationship indicated is based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship in which the inventive product is typically placed when in use. These terms are merely for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or component referred to must have a specific orientation, be constructed, or operate in a specific orientation. Therefore, they should not be construed as limitations on the present invention. In addition, the terms "first," "second," etc. are used only to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0068] 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", and does not mean that the structure must be completely horizontal, but can be slightly tilted.

[0069] In the description of the embodiments of the present invention, it should be noted that, unless otherwise expressly specified or limited, the terms "disposed," "installed," "connected," and "connected" should be understood in a broad sense. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections; they can refer to direct connections or indirect connections through an intermediate medium; and they can refer to internal connections between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on specific circumstances.

[0070] The present invention is described in further detail below with reference to the accompanying drawings:

[0071] Example 1

[0072] 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 plasma at the order of magnitude, and for plasma below 10 17 cm -2 Low-density plasma has limited diagnostic capabilities.

[0073] To solve the above problems, this embodiment provides a plasma electron density measurement device based on laser wavefront analysis. 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.

[0074] This embodiment provides a plasma measurement device based on fiber-coupled dispersion interferometry, based on the following principles:

[0075] The light is aligned to be incident on the microlens array in front of the wavefront sensor, forming a single focal spot on the CMOS camera sensor at the focal plane of the microlens array. If the wavefront is planar, all focal spots are located directly at the center behind the corresponding lens. These focal spot positions are called reference focal spot positions. After lasers of different wavelengths pass through the plasma, the wavefront is distorted, 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 portion of the beam can be found, thereby deriving the local shape of the wavefront. 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., the electron density).

[0076] like Figure 1 As shown, this embodiment provides a plasma measurement device based on fiber-coupled dispersion interferometry, comprising:

[0077] Pulsed 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.

[0078] Among them, the pulse laser 1 is used as the light source, and the emitted pulse laser beam 2 is incident on the beam splitter 3; then it is incident on the first reflector 4 from the beam splitter 3; after passing through the first adjustable aperture 5 and the first lens 6, it is incident on the frequency doubling crystal 7; the frequency doubling crystal 7 is emitted through the second lens 8, the second adjustable aperture 9, the second reflector 10, and then passes through the plasma 11; the laser light passing through the plasma passes through the third reflector 12 and the dichroic mirror 13, and the fundamental frequency beam passes through the dichroic mirror 13 and is incident on the first lens array 14 and the first camera 15, and the doubled frequency light (second harmonic beam) is incident on the second lens array 16 and the first camera 17.

[0079] The first adjustable iris 5 , the first lens 6 , the second lens 8 and the second adjustable iris 9 constitute a collimation system, which ensures the collimation and stability of the laser beam, thereby improving the accuracy and repeatability of the measurement.

[0080] In this embodiment, the frequency doubling crystal 7 is a KTP crystal, which has a frequency doubling efficiency greater than 50%.

[0081] like Figure 2 As shown, in this embodiment, plasma 11 is disposed within vacuum chamber 18. Vacuum chamber 18 is sealed with steel plates and evacuated to a vacuum using a vacuum pump. Plasma 11 is generated by applying a pulsed current to a wire array load. The pulsed current has a peak value of 450 kA and a rise time of 400 ns. The pulse width is on the order of nanoseconds or less to ensure the time resolution of the measurement.

[0082] In this embodiment, the intermediate connecting portions between 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 shielded and connected using a 30 mm coaxial system. This reduces the interference of stray light, improves the imaging quality, and makes the centroid coordinates of the focal spot array more accurate.

[0083] Thus, this embodiment provides a plasma electron density measurement device based on laser wavefront analysis. This device uses a nonlinear frequency-doubling crystal to convert a fundamental laser beam into a coaxial fundamental laser beam and a second harmonic laser beam. The two coaxial laser beams produce different wavefront changes when passing through the plasma, which are converted into changes in the center of mass displacement through a microlens array. Unlike other measurement methods that require probe and reference light, and do not require a comparison before and after the plasma is present, this device only requires the difference in the center of mass change of the focal spot of two coaxial laser beams of different wavelengths after passing through the plasma. First, this measurement method can eliminate interference from neutral particles during the experiment and accurately measure the plasma electron density. Second, it maintains excellent stability because it can eliminate the difference in laser light between the two experiments with and without plasma, allowing for synchronous compensation. In addition, the coaxial laser beams can more accurately measure the same position, further enhancing the spatial precision of the already delicate measurement process. Ultimately, the plasma line integrated density measurement sensitivity is 1-2 orders of magnitude higher than that of conventional interferometry, thereby achieving efficient, reliable, and highly sensitive measurement of plasma electron density.

[0084] Example 2

[0085] 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:

[0086] S1, build the wavefront measurement system:

[0087] 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 and second harmonic beams of pulsed laser beam 2 coaxially emerge from frequency-doubling crystal 7, pass through second lens 8, second adjustable aperture 9, and second reflector 10, enter vacuum chamber 18, and pass through plasma 11. The laser light passing through plasma 11 passes through third reflector 12, and is separated into the fundamental and second harmonic beams by dichroic mirror 13. The fundamental beam enters first lens array 14 and first camera 15, while the second harmonic beam enters second lens array 16 and second camera 17. Using a frequency-doubling crystal to implement a coaxial dual-wavelength laser measurement method can effectively control target position errors during measurement.

[0088] S2, obtain the focus spot array image:

[0089] 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.

[0090] S3, calculate the centroid displacement of the focal spot:

[0091] 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, an adaptive focal spot centroid detection method is designed as follows:

[0092] 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. The sum of the pixels is stored in the matrix and the maximum value of the matrix is ​​calculated. For the actual focal spot image, there may be several sub-images with the same pixel sum value near a focal spot. In this case, 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:

[0093] (1)

[0094] Where, is the light intensity of the pixel at the center of the window, Represents the light intensity of the pixels adjacent to the center point, is an integer, and ;

[0095] Secondly, the centroid of the spot focus is located. The spot intensity in the target image is Gaussian distributed, and the pixel area occupied 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 the grayscale values ​​is used as the threshold of the sub-spot area :

[0096] (2)

[0097] Where, , as well as Respectively represent from (m+1) to n 2 The grayscale value of each pixel;

[0098] The difference in the center of mass displacement of the focal spot at the fundamental wavelength and the second harmonic wavelength is obtained by calculation. .

[0099] S4, calculate the electron line integral density:

[0100] The difference in the centroid 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:

[0101] (3)

[0102] In the formula and Represents a constant refractive index (unit is m 3 ) and the change in particle density between neutral particles before and after plasma generation (unit: m −3 );same and represent positive ions respectively; represents the change of electrons, represent the radial, axial and optical 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 );

[0103] 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:

[0104] (4)

[0105] Where, is the focal length of the microlens (m);

[0106] The plasma line integral density can be expressed as

[0107] (5)

[0108] In the formula, Ne(m -3 ) is the integral of the spatial electron density;

[0109] The plasma electron density can be further expressed as:

[0110] (6)

[0111] 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 methods:

[0112] 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 measurement accuracy, but also effectively eliminates interference from neutral particles, thereby improving the reliability of measurement results.

[0113] Second, high-efficiency 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 utilization efficiency of the laser and the sensitivity of the measurement.

[0114] 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.

[0115] Fourth, advanced imaging technology: Using a combination of a microlens array and a camera, the laser passing through the plasma is imaged. 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.

[0116] 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.

[0117] 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 applications in these fields.

[0118] 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 this 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 pulsed 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 reflector (4) via the beam splitter (3); A frequency doubling crystal (7) is provided 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, is emitted, and is incident on the plasma (11) through the second reflector (10); the laser beam 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 collimating 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 linking 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 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 includes a vacuum cavity (18), and the plasma (11) is arranged inside the vacuum cavity (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 the 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 the beam splitter (3); and then incident on the first reflecting mirror (4) through 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); after passing through the frequency doubling crystal (7), a wavelength of The second harmonic of The fundamental frequency beam and the second harmonic beam of the pulsed laser beam (2) are coaxially emitted from the frequency doubling crystal (7), pass through the second lens (8), the second adjustable aperture (9), and the second reflector (10), enter the vacuum chamber (18), and pass through the plasma (11); 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 the coaxial dual-wavelength laser fundamental frequency beam incident on the first lens array (14) and the first camera (15), and the second harmonic beam 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 by a dichroic mirror (13), the fundamental frequency beam is incident on a first lens array (14) and a first camera (15), and the second harmonic beam is incident on a second lens array (16) and a second camera (17), thereby generating a focal spot array image; and the centroid coordinates of the focal spot array at two wavelengths are obtained according to the focal 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 coordinates of the focus spot array centroid at two wavelengths are obtained, and the focus spot centroid position result is obtained, specifically: The first step is to traverse the entire focal spot array image from the upper left corner to the lower right corner, 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-images with overlapping parts, and calculate the sub-images with overlapping positions: Where, is the light intensity of the pixel at the center of the window, Represents the light intensity of the 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 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 light 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 the grayscale values ​​is used as the threshold of the sub-spot area : Where, , as well as Respectively represent from (m+1) to n 2 The grayscale value of each pixel; 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, specifically: The difference in the centroid 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: Where, , , Represents a constant refractive index coefficient, which is a constant; among them, , is the laser wavelength, is a constant; , , They represent the changes in particle density of neutral particles, ions, and electrons before and after the generation of plasma; represent the radial, axial and optical directions of the plasma, respectively; 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: Where, 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: ； P represents the spacing between sub-lenses in the lens array.

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