CN205751548U - Superlaser gamma-ray source based on micro-dimension near critical density plasma - Google Patents

Abstract

The utility model discloses a kind of superlaser gamma-ray source based on micro-dimension near critical density plasma, including vacuum target chamber system and the composite construction target that is arranged on described vacuum target chamber system memory, described composite construction target includes target frame and is arranged on the metal tantalum post of target frame side, it is coated with the hydrocarbon layer of low-density away from described metal tantalum post one end at described target frame, it is provided with closed cavity between described metal tantalum post and described target frame, the hydrocarbon layer thickness of described low-density is 10 μm, and average density is 5mg/cm³, described target frame is nonmetal making.This utility model other laser gamma-ray sources compared to existing technology are compared, and have that Source size is little, source brightness high, low cost and other advantages.

Description

High-energy laser gamma-ray source based on micro-sized near-critical density plasma

technical field

The utility model relates to a high-energy laser gamma ray source based on micro-sized near-critical density plasma, which can be used as a high-performance gamma ray source in the fields of high energy density physics, material state detection, cancer treatment and the like.

Background technique

The interaction of tightly focused short-pulse high-energy electron beams with high-Z metal materials can produce pulsed gamma-ray sources, which have small spatial dimensions (about tens of microns), small divergence (miradian scale), and high energy ( Photon energy can reach MeV) and other advantages, so it has great application prospects in the following aspects: 1) Positron generation; 2) Photography of high-Z elemental metal materials to detect the internal characteristics of high-Z materials. The research on this new type of ultrafast tightly focused pulsed gamma ray source has aroused great interest of scientists from all over the world.

It is generally believed that the interaction between ultra-intense laser and near-critical density plasma can generate high-energy electron beam with large charge. Electron beams generated by lasers and near-critical-density plasmas are injected into high-Z metallic converters to produce high-brightness gamma-ray sources.

Prior art [1]: A near-critical density plasma can be generated by using a specially designed small-sized supersonic nozzle jet. For example, for a nozzle with a diameter of 0.5 mm, when the back pressure of the gas is 300-400 bar, the density of the ejected helium gas is 10 ²¹ /cm ³ , and the corresponding plasma density after full ionization is 2×10 ²⁰ /cm ³ . The plasma density of helium fully ionized after the small-sized supersonic nozzle is close to the critical density. The nozzle can safely work in a high vacuum environment without putting too much load on the turbomolecular pump so as not to damage the turbomolecular pump. The density of the critical density plasma generated by the nozzle is relatively uniform, but there are also several obvious disadvantages, mainly because the nozzle and other gas distribution structures are complex and the system is huge, which is not conducive to the application in some occasions. In addition, even with a small-sized nozzle, the size of the formed near-critical density plasma is still large, several millimeters. French scientist Y. Glinec et al. injected a laser with a pulse width of 30fs and an energy of 1.3J onto a 3mm supersonic jet target (the plasma density after full ionization was 7.5×10 ¹⁸ /cm ³ ), and the focal spot size was 18μm. A high-energy electron beam with a charge of 1.6nC (energy greater than 2MeV) was obtained and injected into a tantalum column with a thickness of 2.5mm, which was placed 3mm behind the nozzle. The maximum photon energy finally obtained can exceed 140MeV.

Prior art [2]: By adjusting the parameters of the pre-pulse or spontaneous emission of large radiation to control the expansion of the plasma itself, a near-critical density plasma with a certain density gradient scale length can also be produced. Yogo et al. proved that the near-critical density plasma can be generated by adjusting the duration of spontaneous radiation in the range of 0.5-5 ns by adjusting the Pockels cell. For example, when the laser power density is 1.5×10 ¹⁹ W/cm ² and the contrast ratio of the spontaneous maximum radiation intensity at 10 ps before the main pulse is 2×10 ⁵ , the spontaneous maximum radiation incident on a polyimide film with a thickness of 7.5 μm can produce Plasma with the highest density of 2×10 ²² /cm ³ . Hui Chen from the Lawrence Livermore Laboratory in the United States used a pre-pulse with a power density of 10 ¹² - 10 ¹⁴ W/cm ² to directly act on a 1mm-thick gold disk target. In the experiment, a pre-plasma with a scale length of 20-50 μm was generated. body. After the laser main pulse and its interacted high-energy electrons enter the gold disk target, gamma photons with an energy of tens of MeV are finally generated. However, the plasma density produced by this technology is still high, about 10 times the critical density, which is not conducive to the generation of high-energy electron beams with small spatial divergence angles, and is not conducive to controlling the spatial size of the gamma-ray source.

Prior art [3]: using ns long pulse to ablate bulk foam. Generally, a frequency-doubled or triple-frequency ns neodymium glass laser is used to ablate bulk foam materials to generate near-critical plasma. For example, multiple frequency tripled laser beams smoothed by KPP beams are used to inject into C ₁₅ H ₂₀ O ₆ foam, and the diameter of the foam target is 2.5 mm, and the length is 500 μm. The average laser energy per beam is 200-400J, the pulse length is 3ns, and the plasma electron temperature obtained according to the local heat balance model (LTE) is about 1.3keV. For the way of ns long pulse ablation bulk foam, the aperture of the beam is larger, so the required ns laser energy is still larger.

The near-critical density plasma formed by the above three prior technologies is either too large in size or too high in density, which is not conducive to the stable transmission of laser light in it, which in turn is not conducive to the generation of high-brightness tightly focused short-pulse gamma-ray sources.

Utility model content

The purpose of the utility model is to provide a high-energy laser gamma ray source based on micro-sized near-critical density plasma.

The utility model is realized through the following technical solutions:

A high-energy laser gamma ray source based on micro-sized near-critical density plasma, including a vacuum target chamber system and a composite structure target arranged inside the vacuum target chamber system, and the composite structure target includes a target frame and a target arranged on the target The metal tantalum column on one side of the frame is covered with a low-density hydrocarbon layer at the end of the target frame away from the metal tantalum column, and a closed cavity is provided between the metal tantalum column and the target frame . In the present invention, the thickness of the low-density hydrocarbon layer is preferably 10 μm, the average density is preferably 5 mg/cm ³ , and the target frame is made of non-metal.

In the utility model, before the main laser pulse arrives, the laser pre-pulse will be absorbed in a large amount within the skin depth of the low-density hydrocarbon layer, and the ionized material will be ablated by means of collision absorption and parametric absorption. The utility model adopts an ultra-thin low-density hydrocarbon layer, and because the average density of the foam is low, the ablation rate is high. Before the arrival of the main pulse, the ablation wave generated by the laser pre-pulse with a duration of nanoseconds fully ionizes the ultra-thin hydrocarbon layer to form plasma. Under the action of the ablation wave, the entire hydrocarbon layer is almost heated to the temperature of KeV. At such high temperatures, the plasma expands toward the vacuum surrounding the low-density hydrocarbon layer. The expanded plasma density is close to the critical density and enters the confined space formed by the target frame and the metal tantalum pillar. Therefore, a small-sized near-critical density plasma can be formed before the metal tantalum pillar.

After the main laser pulse arrives, it will interact with the expanded near-critical density plasma. Since the laser main pulse power density is greater than the relativistic self-transparency threshold. The laser beam will undergo relativistic self-focusing and qualitative dynamic self-focusing. When the Gaussian laser beam is transmitted in the plasma, the mass power of the laser radially pushes the electrons away from the middle area, forming a plasma channel. In the plasma channel, because the electrons are pushed away by the mass force, a quasi-static radial charge separation field is formed, and the direction of the electric field is radial. The returning electrons along the channel walls will generate a quasi-steady angular magnetic field. The relativistic electrons are confined in the plasma channel by the quasi-static electromagnetic field and move forward along the axis with the laser, while the electrons oscillate at the frequency ωp/2 in the transverse direction. Energy is resonantly exchanged between the laser and the electrons, the electrons surf in the laser field, and the electrons with the proper phase will continuously extract the average oscillation energy from the laser field. After experiencing multiple resonances, the electrons will obtain a very high energy gain, forming a collimated high-energy electron beam with a large amount of charge, and the average temperature of the electron beam can reach several MeV.

The collimated beam of high-energy electrons then enters a metal tantalum column behind the target frame, where it is converted into gamma-ray photons by bremsstrahlung. Photon energies are mainly on the order of MeV. Considering the quasi-static electromagnetic field confinement, the incident electron beam has a small spatial divergence angle, resulting in a tightly focused gamma-ray source.

Since the image spatial resolution of point source projection photography depends to a large extent on the size of the source, the generation of gamma rays in the utility model comes from the bremsstrahlung of high-energy electrons. Considering the instability of the laser in the plasma transmission process, it is necessary to obtain a small-sized near-critical density plasma. Compared with the prior art [1], the size of the plasma is greatly reduced, which suppresses the instability of laser transmission, and is beneficial to control the size of the electron beam and thus the size of the gamma ray source. In addition, the confinement effect of the quasi-static electromagnetic field further reduces the divergence angle of high-energy electrons and the spatial size of the gamma-ray source.

If the plasma density is too high or too low, the average temperature of the electron beam will be affected, and then the energy of the gamma rays will be affected. According to the theory of interaction between intense laser and near-critical density plasma, when the plasma density is ne=0.1-1×10 ²¹ /cm ³ , when the laser is transmitted in the plasma, the group velocity of the laser and the movement of electrons are taken into account. The speed is similar, therefore, the laser can accelerate electrons to very high energy through Betatron resonance, and the generated high-energy electrons can reach tens of nC. It can be expected that the conversion rate of electrons to photons is constant, and the number of gamma photons is greatly increased compared to the prior art [1], that is, the source brightness is higher.

Compared with the prior art [1-3], the utility model greatly reduces the construction cost and operation of the gamma ray source because it does not require huge and complicated supersonic gas nozzles and gas distribution devices, nor does it require a separate ns laser beam. cost.

Further, in order to better realize the utility model, the diameter of the metal tantalum column is 0.5 mm, and the thickness is 0.5 mm.

Further, in order to better realize the utility model, the thickness of the target frame is 100-200 μm.

Further, in order to better realize the utility model, the vacuum target chamber system includes a lead shielding layer and a laser beam focusing device arranged inside the lead shielding layer, and the composite structure target is located in the lead shielding layer. inside the shield.

Further, in order to better realize the utility model, an electron beam deflection magnetic ring is arranged under the composite structure target, and a sample stage is arranged under the electron beam deflection magnetic ring.

Further, in order to better realize the utility model, a ray source point projection imaging recording device is arranged outside the lead shielding layer, and the composite structure target, the sample stage, the center line of the ray source point projection imaging recording device, etc. high.

Further, in order to better realize the utility model, a telephoto aiming system is provided on one side of the lead shielding layer.

A method for producing a high-energy laser gamma ray source based on a micro-sized near-critical density plasma, comprising the following steps:

S1: Install the composite structure target inside the vacuum target chamber system, so that the low-density hydrocarbon layer with a thickness of 10 μm and an average density of 5mg/ ^cm3 passes through the target frame to cover the metal tantalum column to form a composite structure target, so that the metal tantalum column There is a closed cavity between it and the target frame;

S2: Install a laser beam focusing device inside the vacuum target chamber system to focus the laser beam;

S3: A laser pre-pulse with a power density of 10 ¹⁴ W/cm ² is used, and the laser beam focusing device is used to focus the laser on the low-density hydrocarbon layer, so that the laser pre-pulse is largely absorbed at the critical surface. The absorption method ablates the ionized material. For the place where the density is greater than the critical surface, the ablation wave formed by the pre-pulse ionizes the hydrocarbon to form a plasma through the electronic heat conduction;

S4: Use the main laser pulse with a power density of 10 ¹⁹ -10 ²⁰ W/cm ² , use the laser beam focusing device to focus the main laser pulse on the low-density hydrocarbon layer, transmit it inside the plasma to form a plasma channel, and finally generate Gamma ray source.

The duration of the laser pre-pulse in step S3 is 1 ns.

The density of ions in step S3 is _ne = 0.1-1×10 ²¹ /cm ³ .

Compared with the prior art, the utility model has the following beneficial effects:

Compared with other laser gamma ray sources in the prior art, the utility model has the advantages of small source size, high source brightness, low cost and the like.

Description of drawings

Accompanying drawing 1 is the schematic diagram of the layout of the laser tightly focused gamma ray source device;

Accompanying drawing 2 is the front view of composite structure target;

Accompanying drawing 3 is the plan view of composite structure target;

Accompanying drawing 4 is the sectional view of accompanying drawing 3A-A.

Reference signs: 101. Vacuum target chamber system, 102. Lead shielding layer, 103. Laser beam focusing device, 104. Composite structure target, 105. Telescopic aiming system, 106. Electron beam deflection magnetic ring, 107. Sample stage, 108. Ray source point projection imaging recording equipment, 201. Low-density hydrocarbon layer, 202. Target frame, 203. Metal tantalum column.

detailed description

The utility model will be further described in detail below in conjunction with specific examples, but the implementation of the utility model is not limited thereto.

Example:

As shown in Figures 1-4, the radiation source of the present invention includes a vacuum target chamber system 101, a lead shielding layer 102, a laser beam focusing device 103, a target control system and a composite structure target 104, a telescopic aiming system 105, and an electron beam deflection system. Magnetic ring 106, sample stage 107. The laser beam and its focusing device 103, the composite structure target 104, the sample stage 107, the radiation source point projection imaging recording equipment 107, and the center line are all at the same height. Composite structure target 104, imaging and recording device 108 are coaxial. The composite structure target is composed of ultra-thin low-density hydrocarbon layer 201, target frame 202, metal tantalum column 203 and other components. The ultra-thin low-density hydrocarbon layer has a thickness of 10 μm and an average density of 5 mg/cm ³ . The entire ultra-thin low-density hydrocarbon layer is covered in front of the metal tantalum pillar through the target frame, and the material of the target frame is non-metal. The metal tantalum column has a diameter of 0.5 mm and a thickness of 0.5 mm.

The power density of the laser main pulse is 10 ¹⁹ -10 ²⁰ W/cm ² , the contrast between the laser pre-pulse and the laser main pulse is 10 ⁵ , and the corresponding laser pre-pulse power density is 10 ¹⁴ W/cm ² . The laser light is focused onto the hydrocarbon layer of the composite structure target through an off-axis parabolic mirror in the target chamber.

The laser pre-pulse will be largely absorbed at the critical surface, and the ionized material will be ablated by means of collision absorption and parametric absorption.

For places where the density is greater than the critical surface, the ablation wave formed by the pre-pulse ionizes hydrocarbons to form plasma through electron heat conduction.

Due to the use of ultra-thin low-density hydrocarbon layer, the average density of the hydrocarbon layer is low, only 5mg/cm ³ , and the temperature in the plasma drops very slowly, which can ensure that the laser lasts on the order of nanoseconds before the main pulse arrives. The pre-pulse fully ionizes the ultra-thin hydrocarbon layer.

The entire hydrocarbon layer material is heated to the temperature of KeV under the action of the ablation wave to form plasma. At such high temperatures, the plasma expands towards the vacuum around the hydrocarbon layer. For the hydrocarbon foam layer, the pre-pulse lasts for 1 ns, and the plasma can expand about 100 μm forward and backward, and the expanded plasma density is close to the critical density (the final plasma density is about _ne = 0.1-1×10 ²¹ /cm ³ ) . The expanded plasma enters the closed space formed by the target frame (the thickness is taken as 200 μm) and the metal tantalum pillar.

After the main laser pulse arrives, it is transmitted inside the plasma to form a plasma channel.

In the plasma channel, because the electrons are pushed away by the mass force, a quasi-static radial charge separation field is formed, and the direction of the electric field is radial. The returning electrons along the channel walls will generate a quasi-steady angular magnetic field. The relativistic electrons are confined in the plasma channel by the quasi-static electromagnetic field and move forward along the axis with the laser, while the electrons oscillate at the frequency ωp/2 in the transverse direction. Energy is resonantly exchanged between the laser and the electrons, the electrons surf in the laser field, and the electrons with the proper phase will continuously extract the average oscillation energy from the laser field. After experiencing multiple Betatron resonances, the electrons will obtain a very high energy gain, the average temperature of the electron beam can reach 10MeV, and the charge is 15nC.

Furthermore, the magnitude of the quasi-steady-state magnetic field present in the plasma is tens of megagauss, which is sufficient to collimate the generated electron beams down to a divergence angle of a few degrees. The resulting gamma ray source has a spot size of less than 100 μm.

Claims (7)

1. A high-energy laser gamma ray source based on micro-sized near-critical density plasma, characterized in that: comprising a vacuum target chamber system (101) and a composite structure target (104) arranged on the inside of the vacuum target chamber system (101) ), the composite structure target (104) includes a target frame (202) and a metal tantalum column (203) arranged on one side of the target frame (202), and the target frame (202) is far away from the metal tantalum One end of the column (203) is covered with a low-density hydrocarbon layer (201), and a closed cavity is provided between the metal tantalum column (203) and the target frame (202). 2. The high-energy laser gamma ray source based on micro-sized near-critical density plasma according to claim 1, characterized in that: the metal tantalum column (203) has a diameter of 0.5mm and a thickness of 0.5mm. 3. The high-energy laser gamma ray source based on micro-sized near-critical density plasma according to claim 1 or 2, characterized in that: the thickness of the target frame (202) is 100-200 μm. 4. The high-energy laser gamma ray source based on micro-sized near-critical density plasma according to claim 1 or 2, characterized in that: the vacuum target chamber system (101) includes a lead shielding layer (102) and a set In the laser beam focusing device (103) inside the lead shielding layer (102), the composite structure target (104) is located inside the lead shielding layer (102). 5. The high-energy laser gamma ray source based on micro-sized near-critical density plasma according to claim 4, characterized in that: an electron beam deflection magnetic ring (106) is arranged below the composite structure target (104) , a sample stage (107) is arranged below the electron beam deflection magnetic ring (106). 6. the high-energy laser gamma ray source based on micro-sized near-critical density plasma according to claim 5, characterized in that: a ray source point projection imaging recording device ( 108), the composite structure target (104), the sample stage (107), and the center line of the ray source point projection imaging recording device (108) are at the same height. 7. The high-energy laser gamma ray source based on micro-sized near-critical density plasma according to claim 4, characterized in that: a telescopic aiming system (105) is arranged on one side of the lead shielding layer (102).