Large field of view centripetal irradiation high energy concentrator and its design method

Abstract

The present invention relates to a large-field centripetal irradiation high-energy concentrator based on a high-damage threshold laser material and a design method thereof, which can be used in a cone target ICF experiment to solve the problem of limited incident angle of irradiated laser light due to the cone target structure. The device has a side surface and two front and rear spherical refractive surfaces, and the two spherical refractive surfaces are coaxial. The concentrator is made of a crystal with a high damage threshold. In the cone target ICF experiment, a light beam that is originally outside the solid angle of the cone target space and cannot be incident on the target capsule due to the limitation of the maximum incident angle can meet the requirements of the cone target for the incident angle after passing through the large-field centripetal irradiation high-energy concentrator, and still be incident on the target capsule in a centripetal irradiation manner. While ensuring the centripetal and irradiation uniformity of the light beam, the problem of limited incident angle of the irradiated light beam in the cone target ICF experiment is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of concentrators, in particular to a large-field centripetal irradiation high-energy concentrator suitable for use in the field of high-power lasers and a design method thereof. Background Art

[0002] Laser inertial confinement fusion (ICF) is of great significance to high-energy physics research and the development of future clean energy. In ICF, high-energy laser pulses are focused onto a target pellet to compress the pellet and achieve the high temperature and pressure conditions required for fusion reactions. During laser irradiation of the pellet, the resulting plasma corona interacts with the incident laser, dissipating the laser energy and reducing the energy coupling efficiency between the laser and the pellet.

[0003] Previous studies have shown that compared to conventional planar targets and spherical targets, conical targets can effectively improve the coupling efficiency between the laser and the target pellet. Since the conical target has a certain solid angle, only the light beam within the solid angle of the cone target can enter the cone target, and the light beam outside the solid angle of the cone target cannot enter the cone target. It can be seen that the structure of the cone target limits the incident angle of the irradiation beam, so that the number of beams that ultimately act on the target pellet is limited, which in turn limits the laser energy injected into the target pellet, resulting in the output capacity of the ICF laser device cannot be fully utilized. In order to improve the beam-target coupling efficiency while giving full play to the output capacity of the laser device and injecting as much energy as possible into the target pellet, it is very necessary to find a solution to the problem of limited laser incident angle in the cone target ICF experiment without changing the existing target chamber laser arrangement and without affecting the beam centripetal and irradiation uniformity. Summary of the Invention

[0004] The purpose of the present invention is to provide a design method for a large-field centripetal irradiation high-energy concentrator to solve the problem that the incident angle of the irradiation beam is limited by the solid angle of the cone target in the cone target ICF experiment.

[0005] The technical solution of the present invention is:

[0006] A large-field centripetal irradiation high-energy concentrator, characterized in that:

[0007] The device has two coaxial spherical refractive surfaces (such as Figure 1 (a), where the first refractive surface 1 is a convex spherical surface, the second refractive surface 2 is a concave spherical surface, and the two spherical surfaces are coaxial. When the light beam enters the concentrator, it is refracted by the first surface 1, causing the light beam originally outside the cone target solid angle to enter the cone target solid angle. Refracted by the second surface 2, the incident light beam is finally able to meet the cone target's requirements for the beam incident angle, thereby increasing the laser energy ultimately injected into the target pellet.

[0008] Furthermore, the device does not change the focus position of the light beam while changing the injection angle of the light beam, so that the light beam still maintains its original centripetal state after adjustment;

[0009] Furthermore, considering the high energy in the ICF experiment, the present invention adopts materials with a high damage threshold, such as LBO crystal.

[0010] The present invention also provides a method for designing a light concentrating device, comprising the following steps:

[0011] (1) According to the actual use, determine the angle between the incident edge ray and the outgoing edge ray and the optical axis, as well as the curvature radius of the first refractive surface 1 and the distance between its extreme point and the focus;

[0012] (2) Calculating the propagation direction of light inside and after passing through the concentrator by ray tracing method, and obtaining the relationship between the distance between the two refractive surface poles on the optical axis and the curvature radius of the second refractive surface 2;

[0013] (3) Using the relationship between the angles of the light before and after refraction on the second refractive surface 2 as a constraint condition, the distance between the two refractive surface poles on the optical axis and the curvature radius of the second refractive surface 2 are finally determined.

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

[0015] 1. By designing the two surface parameters of the concentrator, the present invention can adjust the focusing angle of the light beam without changing the focusing position;

[0016] 2. By using materials with high damage threshold (such as LBO crystal), the present invention can withstand higher laser power and can be applied to high-power laser systems;

[0017] 3. In the cone target ICF experiment, the light beam that was originally outside the solid angle of the cone target space and could not enter the cone target can enter the cone target, solving the problem of limited laser incident angle in the cone target ICF experiment without changing the target room layout. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 Schematic diagram (a) and three-view drawing (b) of the large-field centripetal irradiation high-energy concentrator of the present invention;

[0019] Figure 2 is a schematic diagram of the ray tracing method used in the present invention;

[0020] Figure 3 Schematic diagram of the incident angle of the irradiation beam in the cone target ICF experiment due to the cone target structure.

[0021] Figure 4 Solve the results of the position and curvature radius of the second refractive surface 2 for numerical calculation;

[0022] Figure 5 This is an effect diagram of an embodiment of the present invention.

[0023] In the figure: 1 the first refractive surface of the concentrator; 2 the second refractive surface of the concentrator; 3 the side surface of the concentrator; 4 the cone target; 5 the maximum incident angle allowed by the cone target; 6 the light beam within the solid angle of the cone target space; 7 the light beam outside the solid angle of the cone target space and unable to enter the cone target; 8 the light beam refracted by the first refractive surface of the concentrator; 9 the light beam refracted by the second refractive surface of the concentrator, that is, the outgoing light beam. DETAILED DESCRIPTION

[0024] The large-field centripetal irradiation high-energy concentrator and its design method will be introduced in detail below with reference to the drawings in the specification.

[0025] In ICF experiments, to ensure the energy coupling efficiency between the laser and the target pellet, the uniformity of the light field irradiated on the target pellet must be strictly controlled. To ensure that the uniformity of laser irradiation is not affected, the light beam entering the cone target after passing through the concentrator should meet the following two conditions as much as possible: (1) The transmission direction of the light beam remains centripetal, that is, the focus position of the light beam is consistent with the original focus position to ensure that the irradiation direction of the light beam points to the vertex of the cone target; (2) After passing through the concentrator, the spatial distribution of the light beam should be as uniform as possible to ensure the uniformity of laser irradiation.

[0026] In the design process of the concentrator, light is used to replace the actual light beam, and the edge meridian ray tracing method is used to calculate the surface parameters of the concentrator. The ray tracing method is as follows Figure 2 As shown, the symbols of the parameters are defined as follows: Assuming that the light is incident from the left and the focal point is located on the right side of the concentrator. Taking the pole of the refractive surface as the coordinate origin, when the intersection of the light and the optical axis is on the right side of the refractive surface, the distance between it and the coordinate origin is positive, otherwise it is negative; when the center of curvature is on the right side of the pole of the refractive surface, the radius of curvature is positive, otherwise it is negative; if the optical axis rotates less than 90 degrees clockwise around the intersection of the light and the optical axis to coincide with the light, then the angle between the light and the optical axis is positive, otherwise it is negative; if the incident (or refracted) light rotates less than 90 degrees around its intersection with the refractive surface to coincide with the normal (i.e., the radius of curvature of the refractive surface), then the angle of incidence (or refraction) is positive, otherwise it is negative.

[0027] like Figure 2As shown in (a), the refractive indices of air and the material used for the concentrator are n0 and n1, respectively. The angle β1 between the incident light and the optical axis is known. The angle β′2 between the final outgoing light and the optical axis is set according to actual needs, and the beam convergence ratio M is defined as β1 / β′2. A1 is the pole of the first refractive surface 1. In the refraction process of the light on the first refractive surface 1, A1 is the coordinate origin, P1 is the intersection of the incident light and the first refractive surface 1, B1 is the intersection of the incident light and the optical axis, B1′ is the intersection of the light after refracting through the first refractive surface 1 and the optical axis, and the length corresponding to A1B1 is defined as L1, that is, the distance between the pole of the first refractive surface 1 and the focal point, the length corresponding to A1B′1 is L′1, C1 is the center of curvature of the first refractive surface 1, then P1C1 is the normal at P1 on the first refractive surface 1, the length is r1, the incident angle and the refraction angle are α1 and α′1 respectively, the angle between the refracted light and the optical axis is β′1, for the refraction process occurring on the first refractive surface 1, it can be found that in ΔP1C1B1, the sine theorem can be used to obtain

[0028]

[0029] Using the law of refraction, we can get that the incident angle α1 and the refraction angle α′1 satisfy

[0030] n0 sinα1=n1 sinα′1 (2)

[0031] Using the triangle angle relationship, we can get

[0032] β′1=β1+α1-α′1 (3)

[0033] In ΔP1B′1C1, we can get

[0034]

[0035] The light ray is refracted by the first refractive surface 1 and reaches the second refractive surface 2, where it undergoes a second refraction. Figure 2 As shown in (b), with the pole A2 of the second refractive surface as the origin, the distance between the two poles of the refractive surface on the optical axis is d, C2 is the center of curvature of the second refractive surface 2, and r2 is its radius of curvature. With A2 as the origin, the light refracted by the first refractive surface 1 intersects the second refractive surface 2 at P2 and the optical axis at B'1, with an angle of β2. The length of A2B'1 is defined as L2. The intersection of the outgoing light and the optical axis is still at B1, with an angle of β'2 with the optical axis. The length of C2B1 is defined as L'2. The angles between the incident light and the refracted light and the normal P2C2 are α2 and α'2 respectively. Once again using the relationship between the triangle formed by the light, the normal, and the optical axis, the following relationship can be obtained for the refraction process of the light on the second refractive surface:

[0036]

[0037] n1 sinα2=n0 sinα′2 (6)

[0038] β2=β′2+α′2-α2 (7)

[0039]

[0040] Using the transfer equation, we can get the following relationship between the parameters of the two surfaces:

[0041] β′1=β2 (9)

[0042] L2=L′1-d (10)

[0043] Since the focus position of the light beam passing through the condenser does not change compared with the original light beam,

[0044] L1=L′2+d (11)

[0045] Using (1)-(6) and (8)-(11), we can get

[0046]

[0047] Since d is unknown, the relationship between r2 and r1 cannot be directly calculated using Equation (7). However, by using the numerical solution method, the values of r2 and d after setting L1 and r1 can be obtained by solving the equation group composed of Equations (7) and (12).

[0048] The following describes a design example to illustrate the design process of the large-field centripetal irradiation high-energy concentrator of the present invention.

[0049] In the ICF experiment, multiple laser beams are used to irradiate the target pellet from different directions simultaneously, such as Figure 3 As shown, the structure of the cone target 4 restricts the incident direction of the light beam. The spatial solid angle corresponding to the cone target determines the maximum incident angle 5 allowed for the light beam. Light beams 6 within the cone target's spatial solid angle meet the incidence conditions and can be directly incident into the target pellet without being affected by the target wall. However, light beams 7 with an incident angle greater than the maximum incident angle 5 allowed by the cone target cannot enter the cone target. The large-field-of-view centripetal irradiation high-energy concentrator of the present invention can adjust the beam angle, thereby solving this problem.

[0050] The material of the large field of view centripetal irradiation high energy concentrator is designed to be LBO crystal with a high damage threshold (in the 350nm band, the pulse duration is 7ns, and it can withstand more than 0.14GW / cm 2Taking the laser power density of LBO crystal as an example (but not limited to LBO crystal), the maximum refractive index is n when the laser wavelength is 351nm. z =1.6439, so in practical applications, the polarization direction of the light beam should be adjusted so that the light beam corresponds to the maximum refractive index.

[0051] During the design process, the marginal rays on the meridian plane are used as the object of ray tracing, and the parameters of the second refractive surface 2 are numerically calculated when the parameters of the first refractive surface 1 are determined. The parameters L1 = 30mm, the radius of curvature of the first refractive surface 1 r1 = 21mm, the angle between the incident ray and the optical axis β1 = 30°, and the angle between the outgoing ray and the optical axis β′2 = 15° are set. The relationship between r2 and d that satisfies equation (12) is calculated as follows: Figure 4 As shown by the solid line in (a), through numerical solution, the curve of the difference between the angle between the final emitted light and the optical axis and the set angle is obtained when d takes different values. Figure 4 As shown in (b), the horizontal coordinate corresponding to the point where the difference is zero in the curve is the value of d that satisfies the conditions given by equations (7) and (12), corresponding to Figure 4 The ordinate of the curve in (a) is the value of r2. Through numerical solution, we find that under the given conditions, d = 22.08 mm and r2 = 2.106 mm.

[0052] A design example of the present invention is given below.

[0053] Based on the parameters calculated in the above process, the concentrator is designed and ray tracing is performed for light with incident angles of ±10°, ±20° and ±30°. The incident angles of the light and the maximum incident angle allowed by the cone target are as follows: Figure 4 shown. Figure 5 The ray tracing results of the light with different incident angles after passing through the large-field centripetal irradiation concentrator are given. It can be seen that the light beam 7, which was originally outside the solid angle of the cone target and could not be injected into the cone target, can be injected into the interior of the cone target after adjustment by the large-field centripetal irradiation concentrator. At the same time, the light after being incident on the cone target has good centripetality and uniformity in spatial distribution, which solves the problem of limited beam irradiation angle in the cone target ICF experiment.

Claims

1. A large field of view centripetal irradiation high energy concentrator, characterized in that: The concentrator has two coaxial spherical refractive surfaces, wherein the first refractive surface (1) is a convex spherical surface and the second refractive surface (2) is a concave spherical surface. The curvature radius r2 of the second refractive surface (2) and the distance d between the poles of the first refractive surface (1) and the second refractive surface (2) satisfy the following conditions and formulas: β1′＝β2′+α2′-α2； Where L1 is the distance between the pole of the first refractive surface and the focus, r1 is the radius of curvature of the first refractive surface, β1, β1′ and β2′ are the angles between the incident marginal ray and the optical axis, the angle between the ray refracted by the first refractive surface (1) and the optical axis, and the angle between the outgoing marginal ray and the optical axis, respectively; α2 and α2′ are the incident angle and refraction angle of the ray on the second refractive surface, respectively; n0 and n1 are the refractive indices of air and the material used for the concentrator, respectively.

2. A method for designing a large-field centripetal radiation high-energy concentrator according to claim 1, comprising the following steps: (1) Based on the incident angle and the exit angle of the actual light beam, determine the angle β1 between the incident marginal ray and the optical axis, the angle β2′ between the exit marginal ray and the optical axis, the curvature radius r1 of the first refractive surface 1, and the distance L1 between the first refractive surface pole and the focus; (2) Calculate the propagation direction of the light inside and after passing through the concentrator by ray tracing method, and obtain the relationship between the distance d between the two refractive surface poles on the optical axis and the curvature radius r2 of the second refractive surface; (3) Taking the relationship between the angles of the light before and after refraction on the second refractive surface as a constraint, determine the distance d between the two refractive surface poles on the optical axis and the curvature radius r2 of the second refractive surface.

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