Method for diagnosing hydromechanics instability by Thomson scattering

Abstract

The invention provides a method for diagnosing hydrodynamic instability by using Thomson scattering, and relates to the technical field of high-energy density physical diagnosis. Comprising the following steps: setting a probe light source, setting a probe light incidence system, setting a scattered light collection system, realizing one-dimensional angle resolution by using a plano-concave cylindrical lens, performing short gate width recording by using a gate control camera, calibrating a scattering angle by using a transmission grating, processing experimental data to calculate a plasma density disturbance spectrum, the scattering power corresponds to the disturbance amplitude. According to the invention, space-time resolution measurement can be carried out on micron or submicron scale electron density disturbance caused by hydromechanics instability.

Description

Technical Field

[0001] The present invention relates to the technical field of high energy density physical diagnosis, and in particular to a method for diagnosing fluid mechanics instability by utilizing Thomson scattering. Background Art

[0002] The interaction between lasers and matter to produce high-energy-density plasmas drives a variety of fluid dynamic instabilities. These instabilities are widespread in fields such as astrophysics and inertial confinement fusion, and are of great research value, thus attracting considerable attention. During the development of fluid dynamic instabilities, initially small perturbations grow over time and evolve into turbulent states, with spatial scales spanning multiple orders of magnitude. X-ray imaging is the most commonly used method for analyzing fluid dynamic instabilities, with its highest spatial resolution being at the level of several microns. However, the evolution of the flow field can produce spatial structures as small as one micron or even hundreds of nanometers, which are difficult to resolve using existing X-ray imaging methods.

[0003] Fluid dynamic instabilities can cause density fluctuations, and optical Thomson scattering can directly diagnose electron density fluctuations in plasmas. When probe light enters the plasma, electrons vibrate under the action of the laser electric field and generate radiation, which is scattered light. The coherent superposition of scattered light from a large number of electrons within the scattering volume reflects the fluctuation spectrum of the electron density. Previous researchers have used Thomson scattering to diagnose ion acoustic waves and electron Langmuir waves excited by kinetic processes such as laser plasma instabilities. However, there are fewer studies on the diagnosis of fluid instabilities. Published work cannot quantitatively determine the amplitude of the disturbance and is limited to measurements at a single moment and a single spatial point, providing limited diagnostic tools for fluid dynamics research. Summary of the Invention

[0004] In order to solve the problem that the existing technology is difficult to distinguish micron and submicron scale fluid dynamics disturbances in plasma, the present invention proposes a method for diagnosing fluid dynamics instability using Thomson scattering to diagnose smaller scale fluid dynamics disturbance spectra.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] A method for diagnosing fluid dynamics instability using Thomson scattering comprises the following steps:

[0007] Step 1: Set up the probe light source at the experimental site;

[0008] Step 2: Setting a probe light incident system on the beam path emitted by the probe light source so that the laser light is incident into the plasma in a one-dimensional line focus for subsequent imaging to achieve one-dimensional spatial resolution;

[0009] Step 3: Set up a framing imaging system in the outgoing direction of the probe light after passing through the plasma to be measured to collect the transmitted light and scattered light. The framing imaging system is composed of a collection lens, a framing module, and a rear lens arranged in sequence;

[0010] Step 4: Place a plano-concave cylindrical lens before the image plane;

[0011] Step 5: The scattered signal with one-dimensional spatial resolution and one-dimensional angular resolution reaches the image plane, and a gated camera is used for short gate-width recording and finally received by the CCD;

[0012] Step 6: Calibrate the scattering angle;

[0013] Step 7: Conduct experimental data acquisition and data processing to obtain the plasma density perturbation spectrum.

[0014] The beneficial effects of the present invention are as follows:

[0015] (1) The present invention uses Thomson scattering to diagnose hydrodynamic perturbations in the plasma. Compared with X-ray radiography, it can diagnose smaller-scale hydrodynamic perturbations. By diagnosing different scattering angles, the present invention can theoretically diagnose perturbation wavelengths in the range of 200 nanometers to 20 micrometers, and can complement the existing X-ray microscopy imaging to provide a complete diagnosis for the spatial scales spanning multiple orders of magnitude during the evolution process of hydrodynamic instability;

[0016] (2) The present invention reflects the plasma density perturbation spectrum through angular-resolved scattering diagnosis, and can directly extract the perturbation wavelength and the corresponding amplitude information;

[0017] (3) The present invention also has one-dimensional spatial resolution and time resolution at multiple moments, obtains more information in the same shot, has a high experimental efficiency, and helps to study the evolution process of the fluid. Description of the Drawings

[0018] Figure 1 is a schematic optical path diagram of the method for diagnosing hydrodynamic instability using optical Thomson scattering according to an embodiment of the present invention; wherein, a is a side view and b is a top view.

[0019] Among them, the reference numerals are: probe light laser beam 1, first lens 2, plano-convex cylindrical lens 3, second lens 4, third lens 5, first beam splitter 6, second beam splitter 7, fourth lens 8, plano-concave cylindrical lens 9, attenuation sheet 10, gated camera 11, high-power nanosecond laser irradiated target 12. Detailed Embodiments

[0020] In order to make the objectives, technical solutions and advantages of the present invention more clear and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and 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. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0021] As Figure 1 shown is a schematic optical path diagram of a diagnostic system used in the method for diagnosing hydrodynamic instability by optical Thomson scattering provided by an embodiment of the present invention. The diagnostic system includes a probe light laser beam 1, a probe light incident system composed of a first lens 2, a plano-convex cylindrical lens 3, and a second lens 4, a framing imaging system composed of a third lens 5, a first beam splitter 6, a second beam splitter 7, and a fourth lens 8, an angular resolution module composed of a plano-concave cylindrical lens 9 and an attenuation sheet 10, a gated camera 11, and the diagnostic object is a plasma generated by irradiating a target 12 with a high-power nanosecond laser. Figure 1 a in it is a side view, that is, the longitudinal direction described below, Figure 1 b in it is a top view, that is, the transverse direction described below.

[0022] The following presents the specific embodiments of the present invention with reference to the accompanying drawings.

[0023] The plasma to be measured is generated by irradiating a hydrocarbon plastic target 12 with a high-power nanosecond laser. The laser wavelength is 351 nanometers, the power density reaching the target is W / cm², and the duration is 3 nanoseconds; the non-uniformity on the target surface becomes the seed of the Rayleigh-Taylor instability, exciting density perturbations in the plasma; the longitudinal size of the plasma that can be used for optical scattering diagnosis is about 1 millimeter; the diagnostic system performs imaging longitudinally and angular resolution diagnosis on the scattered light at different angles transversely;

[0024] Step 1: Set the probe light source at a suitable position according to the experimental site. The incident laser source needs to meet the following conditions: In order to diagnose a higher-density plasma region, the laser wavelength needs to reach 351 nanometers or shorter; since the Thomson scattering cross-section is very small, the laser needs to have a high power; the laser pulse width is in the order of nanoseconds to diagnose the process of fluid evolution;

[0025] Preferably, the probe light source is a pulsed laser, which is about 2 meters away from the target point. The probe light laser beam 1 is the probe light emitted by the laser, which is in the same horizontal plane as the plasma to be measured. The pulse intensity is greater than 5 joules, the pulse width is 3 nanoseconds, and the wavelength is 266 nanometers;

[0026] Step 2: Set up a probe light incident system on the beam path of the probe light source, so that the laser is focused and incident into the plasma in a one-dimensional line for subsequent imaging to achieve one-dimensional spatial resolution; the divergence angle of the beam should be small enough to enable the diagnostic system to have sufficient angular resolution; the longer the wavelength to be diagnosed, the higher the requirement for angular resolution. For plasma perturbations with a wavelength of about 10 microns, the F number of the probe light focus needs to be greater than 40.

[0027] Preferably, the first lens 2 and the second lens 4 form a beam expander system. Longitudinally, the laser beam is not affected by the plano-convex cylindrical lens 3 and exits parallel after passing through the first lens 2 and the second lens 4, with a width of 5 mm; transversely, after the combination of the first lens 2, the plano-convex cylindrical lens 3, and the second lens 4, the beam is focused at the diagnostic position, with a cone angle of 1.1 degrees and the transverse width of the focal spot less than 200 microns.

[0028] Step 3: Set up a framing imaging system in the exit direction of the probe light after passing through the plasma to be measured to collect transmitted light and scattered light, which includes a collection lens, a framing module, and a rear lens. The receiving solid angle range of the system determines the diagnostic instability wavelength range. The F number of the collection lens needs to be less than 8 to obtain a wide enough perturbation wavelength spectrum range; the collection lens and the rear lens form a 4F imaging system, and a framing module is added to the parallel light part between the two lenses. Signal lights with different time delays are arranged in sequence in space.

[0029] Preferably, the collection lens is the third lens 5, with a diameter of 10.16 cm, an F number of 5, and a maximum collection angle of 11.5 degrees; the rear lens is the fourth lens 8, with an F number of 10. The third lens 5 and the fourth lens 8 form an image transfer system for imaging with a magnification of 2 times; both the first beam splitter 6 and the second beam splitter 7 are beam splitters with a transmission-to-reflection ratio of 1:9. The two beam splitters form a framing module with a spacing of 12 cm. The beam is transmitted and reflected back and forth between them to form multiple signal lights with a time interval of 0.8 ns. The second beam splitter has a slight inclination longitudinally to separate the beams with different time delays longitudinally.

[0030] Step 4: Place a plano-concave cylindrical lens 9. The focal length of the plano-concave cylindrical lens 9 is 5 cm, and it is placed in front of the image plane of the light collection system to make the focal plane coincide with the image plane; the beam is not affected by the cylindrical lens longitudinally and becomes parallel light with a total width of 5.1 mm transversely, so that light rays with different scattering angles reach different positions on the image plane.

[0031] Step 5: Use a gated camera 11 for short gate width recording, with an integration time of 200 picoseconds.

[0032] Step 6: Calibrate the scattering angle. The probe light is diffracted to different angles by using a transmission grating. The diffraction angles are calculated theoretically and corresponding one by one to the light spots on the CCD, so as to determine the corresponding relationship between the scattering angle and the CCD pixel position. In the experiment, the intensity of the transmitted light is several orders of magnitude higher than that of the scattered light. After calibration, an attenuation sheet is added to the optical path of the transmitted light to make the intensities of the direct-through light and the scattered light collected in the experiment at a similar order of magnitude.

[0033] Preferably, a transmission grating with 300 lines per millimeter is placed at the target point. The probe light 1 is focused on the target point through the first lens 2 for probe light incidence, the plano-convex cylindrical lens 3, and the second lens 4, and is diffracted by the grating to 0°, 4.6°, and 9.2°. Then it reaches the image plane through the framing imaging system and is recorded by the CCD, so as to determine the corresponding relationship between the scattering angle and the CCD pixel position. In the diagnostic experiment, the intensity of the transmitted light is 4 orders of magnitude higher than that of the scattered light. After calibration, a 4OD attenuation sheet 10 is added before the image plane to only block the beam of the 0th-order diffraction, so that the intensities of the transmitted light and the scattered light collected in the experiment are at a similar order of magnitude.

[0034] Step 7: Collect experimental data and process the data to obtain the plasma density perturbation spectrum. The angular resolution direction is the direction of the scattering spectrum. The instability wavelength λ corresponding to the scattering angle θ is calculated according to where is the wavelength of the probe light, 266 nm; the perturbed electron density corresponding to the instability wavelength is calculated according to where is the power of the scattered light at the θ angle, is the power of the transmitted light, is the classical electron radius m, and is the length of the scattering region.

Claims

1. A method for diagnosing hydrodynamic instability using Thomson scattering, characterized in that, It includes the following steps: Step 1: Set a probe light source at the experimental site; Step 2: Set a probe light incident system on the light beam path emitted by the probe light source, so that the laser is incident on the plasma interior in a one-dimensional line focus for subsequent imaging to achieve one-dimensional spatial resolution; Step 3: Set a framing imaging system in the outgoing direction after the probe light passes through the plasma to be measured to collect the transmitted light and scattered light. The framing imaging system is composed of a collection lens, a framing module, and a rear lens arranged in sequence; Step 4: Place a plano-concave cylindrical lens in front of the image plane; Step 5: The scattered signal with one-dimensional spatial resolution and one-dimensional angular resolution reaches the image plane, and a gated camera is used for short gate width recording and finally received by the CCD; Step 6: Calibrate the scattering angle; Step 7: Collect and process the experimental data to obtain the plasma density perturbation spectrum.

2. The method for diagnosing hydrodynamic instability by Thomson scattering according to claim 1, wherein In the said Step 1, the laser wavelength is less than or equal to 351 nanometers.

3. A method for diagnosing hydrodynamic instability using Thomson scattering according to claim 1, characterized in that, In the said Step 1, the laser pulse width is 1 to 10 nanoseconds.

4. A method for diagnosing hydrodynamic instability by Thomson scattering according to claim 1, characterized in that, In the said Step 3, the F number of the collection lens is less than 8 to obtain a wide enough perturbation wavelength spectrum range.

5. A method for diagnosing hydrodynamic instability using Thomson scattering according to claim 1, characterized in that, In the said Step 3, the collection lens and the rear lens form a 4F imaging system, and a framing module is added to the parallel light part between the two lenses, and the signal lights with different time delays are arranged in sequence in space.

6. A method for diagnosing hydrodynamic instability using Thomson scattering according to claim 1, characterized in that In the said Step 4, the light beam does not change in the dimension of imaging, and the originally converged light is changed into parallel light in the dimension where spatial resolution is not required, so that the scattered lights at different angles are separated in space.

7. A method for diagnosing hydrodynamic instabilities using Thomson scattering according to claim 1, characterized in that, In the said Step 6, the probe light is diffracted to different angles by a transmission grating, the diffraction angle is theoretically calculated and corresponding to the light spots on the CCD one by one, and then the corresponding relationship between the scattering angle and the CCD pixel position is determined.

8. A method for diagnosing hydrodynamic instability using Thomson scattering according to claim 7, characterized in that, After calibration, an attenuation sheet is added to the optical path of the transmitted light so that the CCD can record the transmitted light and the scattered light simultaneously.

9. A method for diagnosing hydrodynamic instability using Thomson scattering according to claim 1, characterized in that In the step 7, the angular resolution direction is the direction of the scattering spectrum, and the instability wavelength λ corresponding to the scattering angle θ is calculated according to where is the wavelength of the probe light.

10. A method for diagnosing hydrodynamic instabilities using Thomson scattering according to claim 9, characterized in that, The perturbed electron density corresponding to the instability wavelength According to Calculate, where Is the scattered light power at the θ angle, Is the power of the transmitted light, Is the classical electron radius Meters, Is the length of the scattering region.

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