With the increase of laboratory laser power, aspheric lenses and mirrors have been pushed to provide extremely high intensity.
Around the world, nuclear research laser facilities increasingly use powerful, amplified and ultrashort duration coherent optical pulses to provide huge energy density on a given target. Synchronous transmission of these high-power pulses creates extreme conditions in which scientists can carry out state-of-the-art nuclear research.
It also allows the study of conditions that exist only in distant and exotic environments, Manufacturer of spherical lens such as the core of our sun and beyond. These facilities also provide opportunities for researchers to improve their knowledge in the fields of materials science, astrophysics and particle and nuclear physics. The resulting applications range from medicine to nuclear energy.
The national ignition device (NIF) of Lawrence Livermore National Laboratory was put into use and built 10 years ago. The use of nuclear energy through self-sustaining fusion reaction will help to solve the global energy crisis, but this has not been achieved. However, there is no doubt that NIF and other laser facilities around the world have enabled scientists to carry out groundbreaking research. This expands the understanding that laser-induced fusion is essential for the future.
Remarkable achievements have been made in laser technology. However, Manufacturer of spherical lens these advances would not have been possible without the same advances in the design and manufacture of transmission and reflection optics. One of the biggest determinants of the quality of the transmitted beam is the quality of the optical elements.
High power laser system
Generally, nuclear research laser system consists of a series of beam lines. Through these, the beam propagates and is expanded, power amplified, and then spatially filtered before reaching the focusing unit. The focusing unit is located near the vacuum chamber, which contains diagnostic instruments and target components.
Since the target assembly contains the fuel tank responsible for initiating the nuclear reaction, it is a key component. The assembly consists of a cylindrical cell called a cavity, a spherical fuel tank containing the selected fuel, and the fuel itself.
Usually, beam propagation and expansion are achieved through a series of telescope optical elements. On the other hand, power amplification is usually achieved by a pile of neodymium doped phosphate glass plates and various powerful flash lamps. Neodymium atoms are excited to a higher energy state by a flash lamp. This means that when the low-energy laser pulse from the injection laser system passes through the slab, the excess energy stored in the neodymium atom will be released into the laser pulse. This energy is released in the form of highly coherent light of a specific wavelength.
When each beam passes back and forth through the same set of Optics and glass plates, the pulse power amplification will be repeated many times. This is called a multichannel scheme. By using a separate but identical beam propagation subsystem dedicated to each beam, all beam lines are amplified in parallel.
The goal is to simultaneously transmit densely focused coherent light points from each beam line in the vacuum chamber where the fuel tank is located. It is very important that all generated light spots arrive at the same time, time and space with very high accuracy. It is precisely this precise coincidence of dense focal points that means a symmetrical force can be applied to the fuel tank. This also means that the maximum energy density can be transmitted to the target. Both of these things meet the basic requirements of initiating fusion reaction conditions.
The neodymium glass laser of nif produces 1053 nm (1) in the NIR region ω) However, before focusing it on the target, the laser is converted to its third harmonic 351 nm (3 ω)。 Frequency conversion is achieved using two nonlinear crystal plates made of potassium dihydrogen phosphate (KDP). Conversion is necessary because inertial confinement fusion targets absorb ultraviolet rays more effectively than longer wavelengths. They also have better performance at shorter wavelengths.
After the ultraviolet ray is effectively absorbed, the temperature around the fuel tank will rise significantly due to the generation of secondary X-rays. This extreme local environment will initially lead to the temperature rise and ablation of the fuel tank surface. The shock wave generated during the ablation process makes the capsule implode rapidly. This rapid implosion compresses DT (deuterium and tritium) fuel to a very high density and heats it to a high temperature.
As mentioned earlier, although the force distribution in the target is not completely dependent, it largely depends on the synchronization and accuracy of the transmitted beam. Successful beam transmission of high-density energy may lead to symmetrical implosion and compression, so as to raise the temperature to a sufficient level (more than 100 million degrees) At these temperatures, the fusion reaction equals or exceeds the laser energy deposited in the target. This is a condition called Lawson criterion or ignition.
However, the precise application of symmetrical shock waves to achieve the temperature and pressure required to start a self-sustaining fusion reaction is still technically elusive. A technology called "rapid ignition" relaxes some of these limitations.
This is achieved by providing a very short (picosecond or femtosecond) energy burst directly on one side of the core fuel after the explosion reaches the maximum density. At present, rapid ignition is being explored as a feasible option to improve the performance of inertial confinement fusion reaction.
High power facilities
The proposed European high power laser energy facility (HiPER) is an example of the use of fast ignition to start self sustained fusion. It is expected that the facility will use less energy than NIF during operation and generate a significantly higher fusion gain (thermonuclear gain of 100). However, the commissioning roadmap is still uncertain before funding is obtained.
The Eli beamlines laser research center in the Czech Republic uses a variety of complete laser systems. These include L1 alegra,Manufacturer of spherical lens L2 Amos, L3 hapls and L4 Aton with ultrashort laser pulses. These systems include high power and high repetition rate options and the highest power single emission (10-pw) configuration.
The Eli project has many objectives. One of the main objectives is to generate particles and radiation from super relativistic and relativistic interactions using the single shot, ultra short peak power laser option. This will enable scientists to examine the behavior of matter in the super relativistic state, resulting in a state of matter called quantum plasma.
It is estimated that the 10-pw laser in the facility will provide basic scientific research projects with focusing intensity up to 1024w / cm2 at a higher dose rate. The ultrashort pulse source is not to pursue the progress of nuclear energy, but to meet the needs of basic research.
Figure 1. History of laser intensity with different laser matter interaction cycles.
Relativistic and sub relativistic regimes have been studied in the past 20 years 1,2,3. Recent efforts include Eli 4 and Gemini laser facilities, as well as possible Hiper in the future. The purpose of these efforts is to solve super relativistic systems or quantum states with expected focusing intensity > 1023w / cm2 (Fig. 1). CPA (chirped pulse amplification) developed in 1985 In the 1990s, the development of solid-state lasers made it possible to produce ultrafast picosecond and femtosecond pulses. These developments are key milestones in the pursuit of quantum plasma and relativistic state.
In CPA technology, a pair of gratings are used to stretch ultrashort laser pulses in time before introducing ultrashort laser pulses into granular media. The arrangement of gratings makes the varying frequency components of laser pulses propagate through unequal paths. This process is called pulse broadening. Then, the intensity of each part of the spectrum can be reduced and extended to a level sufficient for amplification. This is By overloading the pulse when it passes without damaging the grain medium.
Figure 2. Schematic diagram of CPA technology and amplification stage.
Finally, the stretched and amplified laser pulse is recompressed to its original pulse width. This is achieved by reversing the stretching process. The peak power realized by the system is several orders of magnitude higher than that generated by the laser system before CPA (Fig. 2).
Focusing optics
Focusing optics is a key aspect of all laser based nuclear research projects. It successfully transmits energy with high uniformity and high density distribution to the target. One of the most important roles is focusing optics. Aspheric mirrors (such as off-axis parabola with large off-axis angle) can be used Or transmissive aspherical focusing lens, or a combination of long and short pulses of both 5 may be used to achieve successful delivery. This depends on the specific operational requirements and design of each laser facility.
Since very short pulses tend to diffuse significantly when transmitted through glass, lasers that produce ultrashort pulses need reflective optical elements. However, longer pulses do not show significant expansion, so the use of transmission elements such as lenses is not a feasible option.
M. Rosete Aguilar and his colleagues have extensively solved the influence of optical materials on the time propagation of optical pulses. This study focuses on glasses with different dispersion characteristics and analyzes their expansion as a wavelength function under different pulse widths.
Compared with longer pulses (e.g., pulse length > > 100 fs), the effect of glass on ultrashort pulse time broadening is significantly more serious. This study confirms this phenomenon. In addition, when the wavelength is close to ultraviolet (e.g., 351 nm) The effect seems to be more obvious. This behavior is expressed to varying degrees in all types of glass. In glass research, fused quartz seems to show the best behavior.
Considering the above factors, the decision whether to use transmission or optical components depends largely on whether ultrashort pulses are used in the experiment. For example, in the case of Eli, the reflection solution should be selected because a single ultrashort pulse is used. However, in other cases, such as NIF, the transmission solution is preferred because multiple longer pulses are used.
Fig. 3. Pulse expansion through 1 to 10 mm thick blocks (a) and through 10 to 100 mm thick blocks (b).
The data processed by Rosete Aguilar and colleagues show that the pulse expansion increases when the beam propagates through glass with different thicknesses. Figure 3 shows the ultrashort pulse in the case of lens not suitable for fused quartz, with a pulse range of 50 to 100 fs and a wavelength of 800 nm. For thin glass blocks (e.g., 10 mm thick) , this result is negligible, which shows that it is acceptable to use thinner optical devices (including optical windows) even in ultrashort pulses.
However, it is necessary to provide in-phase, Manufacturer of spherical lens aberration free, high-precision and high-density light spots on the focal plane, which means that users must use optical elements with short focal length and larger aperture. These requirements can be met only by using highly aspherical mirrors and lenses with natural spherical aberration free.
Figure 4. The relationship between focal length and focal spot diameter for a given input beam diameter. The blue line represents a scene similar to NIF operation requirements, in which multiple 351 nm short pulse beams converge to the target. The orange line represents a scene similar to Eli, in which a single high peak power ultrashort beam is provided at 800 nm.
Fig. 5. Gaussian beam propagation theory applied to a beam passing through a focusing element.
Selecting the correct optical elements helps to provide a target spot with a diameter of only a few microns at the target (Fig. 4). The calculation assumes perfect optics, a perfect Gaussian beam with a 300 mm diameter aperture, and the Gaussian beam propagation theory (Fig. 5) An aspherical focusing lens with a focal length of 1 ~ 3 M can be considered a suitable choice for nuclear laser facilities. This depends on the number of beam lines used, chamber diameter and other design parameters. However, the most suitable solution is an off-axis paraboloid mirror with a very short focal length. Many other reasons not detailed here also make it the most suitable choice.
