Fig. 8 schematic diagram of "fast ignition" laser nuclear fusion principle
(1) The traditional central hot spot laser nuclear fusion is similar to the ignition process of diesel engines; (b) Fast-ignition laser nuclear fusion is similar to the ignition process of gasoline engines. The concept of fast-fire laser nuclear fusion involves many strong-field physical interaction processes related to high-intensity ultrashort pulses [8], including the interaction between ultrashort pulse intense laser and high-density plasma, the generation and transmission of high-intensity superheated electron current in high-density plasma, the generation of harmonics near the critical density surface, and the generation and function of super-strong magnetic field. Effects related to mass dynamics, relativistic self-focusing and filamentation, "drilling" and "tunneling" effects of ultra-short pulse intense laser beams, etc. Several main physical processes of the "quick ignition" scheme are shown in Figure 9. Firstly, the hollow target pill filled with deuterium and tritium gas is highly symmetrically compressed by nanosecond long pulse laser beam. After compression, the density of deuterium and tritium gas in the center of the target pill will reach 1000 times of its solid density. Step 2, the pressed high-density target pellets are irradiated with a laser beam with a pulse width of about 100ps and a focused light intensity of 10 18Wcm-2. This focused laser beam will further press the critical density surface of the target pill toward the center and form holes in the high density target pill. Then, the target core is quickly ignited by a laser beam with a pulse width of about 10ps and a focused light intensity of 1020Wcm-2. The ignited laser beam interacts with the high-density plasma with a large density gradient in the target core, generating a large number of superheated electrons with an energy of MeV [9], which flow into the highly compressed target pellets and are deposited in the fuel of the target core, and the local temperature of the fuel near the target core is rapid.
Fig. 9 schematic diagram of several main physical processes of rapid-fire laser nuclear fusion.
(a) High compression ratio explosion; (b) "drilling" the laser beam; ? Ignition laser beam; (IV) Energy conversion in the process of "quick ignition" Actually, as shown in Figure 8(b), the laser pulse of 100ps used in the second step and the laser pulse of 10ps used in the third step in the "quick ignition" scheme are a shaped laser pulse in the actual experiment. The laser pulse consists of a leading edge of 100 picosecond and a peak value of 100 picosecond. Using this shaped laser pulse can greatly reduce the experimental difficulty. Fusion separates compression and ignition, so the requirements for explosion symmetry and driving energy can be greatly reduced. In the "fast ignition" scheme, the initial compression period only requires high density, not high temperature, so the requirement for "smoothness" of long pulse compression laser is greatly reduced. The interaction between ultra-short pulse intense laser and compressed high-density plasma can effectively convert laser energy into MeV-level superheated electrons. Then the target core is efficiently heated to realize ignition, which can greatly reduce the demand for driving energy. At present, the theoretical calculation shows that the "fast ignition" scheme can achieve high-gain nuclear fusion with only 65438+ million joules of laser energy, which is 10 times lower than the traditional central ignition scheme. Of course, there are still many physical and technical problems to be discussed and solved in the current "rapid fire" scheme. Because "rapid shooting" occurs in the late stage of high compression and high density of the target pill, there is a high requirement for the actual laser device. Therefore, at this stage, the conditions for judging the feasibility of the "rapid fire" scheme by overall experiments are temporarily unavailable. In this case, it is an important goal in the field of international laser plasma research that how to carry out reasonable decomposition experiments on the "rapid fire" scheme and study many physical processes and technical problems to judge the feasibility of this scheme. At present, the decomposition experiment of "fast ignition" scheme is mainly composed of the following physical problems: (1) Symmetrical compression of target pellets by long pulse laser. In this regard, due to the research foundation of laser nuclear fusion for many years, rich research experience has been accumulated. It is generally believed that the density required to compress the target pill to the first stage of the "fast ignition" scheme can be realized by the current laser technology and target-making technology. (2) The absorption of ultrashort pulsed intense laser in high density plasma. This seemingly simple problem actually involves extremely complicated physical processes. The absorption process of laser with different intensity and pulse width in plasma with different density is completely different. Fortunately, this problem has aroused widespread concern and interest, and hundreds of experimental and theoretical papers in the world have conducted extensive research on it. (3) generating overheated electrons in the "quick ignition" scheme. This question is related to the second question to some extent, but it also includes many other contents. For example, the influence of plasma density and temperature gradient on the yield and energy spectrum distribution of superheated electrons, the vacuum heating of plasma interface, and the acceleration of electrons by high-field laser plasma. (4) The problem of electron overheating in plasma in "fast ignition" scheme is very complicated. This is a hot spot in the research of laser plasma physics in the world. (5) The generation and function of super-strong magnetic field in ultrafast laser plasma. The interaction between ultra-short pulse intense laser and plasma will produce a strong magnetic field, which has attracted much attention because of its great influence on the absorption of laser energy and the generation and transmission of superheated electrons. But at present, the understanding of this problem is not enough. (6) The problem of "drilling holes" in the "rapid fire" scheme is a very important and concerned issue. (7) The energy deposition of ultra-hot electrons in the high-density plasma in the "rapid-fire" scheme is one of the key issues that determine the success or failure of the "rapid-fire" scheme. In this scheme, MeV is carried. Beta particles are electrons, because beta rays are electron currents) play the same role in the explosion of hydrogen bombs. However, due to the fundamental difference between electrons and alpha particles, the unique physical problems of superheating electrons in high-density plasma are brought about. (8) Nuclear reaction in the "rapid fire" scheme. At present, Livermore National Laboratory, Rutherford Laboratory and Osaka University of Japan are stepping up the last five decomposition experiments. In order to demonstrate the feasibility of this scheme experimentally as soon as possible and play a guiding role in the construction planning of NIF, the phenomenon of ultra-short pulse laser drilling holes in plasma was confirmed by experiments using the method of optical diagnosis [10]. Great progress has also been made in the study of the acceleration and transport process of superheated electrons generated by the interaction between ultra-short pulse laser and solid target in high-density plasma [1 1] China's high-tech development plan of laser nuclear fusion also attaches great importance to the "fast ignition" scheme, and related research projects have been started.
5 concluding remarks