The United States National Ignition Facility (NIF), the world's largest laser, known as the "artificial sun," is getting closer and closer to its goal, showing that a sustainable fusion reactor is becoming a reality from dreams. However, due to fundamental physics research and engineering problems, there are still significant obstacles to be overcome in order for nuclear fusion to reach a level at which energy can be exported stably.
Middle East oil, South Africa's gold ... The energy crisis has become one of the major problems that plague human development. According to the research, fossil fuels such as oil, coal and natural gas accumulated over the world for hundreds of thousands of years are generally available to mankind for about 100 years. The current use of nuclear energy because of its huge security risks, sustainability is not favored. As a result, fusion has become the best way for human beings to entertain an inexhaustible supply of "inexhaustible" energy.
According to the principle of laser fusion, the so-called "artificial sun," the United States National Ignition Device (NIF) recently came the news, fusion devices in the world a landmark breakthrough.
Controllable nuclear fusion harsh conditions
In the 1940s, German nuclear physicists Otto & Hahn and Lize-Meitner discovered nuclear fission, but neither the nuclear weapon or the nuclear power plant made by this reaction was caused by the fuel itself and the reaction products Huge radiation, so scientists have been treated with extreme caution.
In contrast, the enormous energy released by the sun is generated by the fusion reaction of two hydrogen atoms into one helium nucleus. Compared to fission, the energy released by each nucleus, that is, more energy released per kilogram of fusion fuel, and enriched reserves of fusion fuel on the earth, more importantly, the radiation produced by fuel and fusion is much less than the fission .
However, all the nuclei are positively charged. The closer they are to each other, the stronger electrostatic repulsion will be. The most important condition to accomplish fusion is to overcome this force. The sun's nuclear fusion takes place and continues due to its central temperature of up to 20 million degrees Celsius and the super-high pressure conditions created by its own powerful gravitational pull. To complete this process on Earth, gravity is too small, the pressure is not enough, nuclear fusion needs to be carried out at a higher temperature (under such conditions the material is in a plasma state). This is also why controlled nuclear fusion is so difficult.
Obviously, in order to meet the harsh conditions of controlled nuclear fusion, we must first input a large amount of energy. When the energy released by the nuclear fusion reaction is greater than the input energy, this critical condition is called ignition and has the value of energy application. In fact, since the energy consumed to create a fusion condition is typically three times greater than its thermal energy, the equivalent gain or loss can only be achieved if the energy gain factor equals three.
Wang Xiaofang, professor of physics at University of Science and Technology of China, told the author that just as in daily life it ignites firewood, it no longer needs outside heating and ignition after it is ignited. The heat released by the burning of firewood is enough to burn new firewood and sustain such heat release. Is the real ignition. Firewood is equivalent to the nuclear fusion in the plasma state of matter.
In 1957, the British scientist Lawson put forward the general conditions to achieve ignition, also known as the pros and cons of the conditions, that Lawson criterion. For deuterium and tritium fusion, in order to achieve easier, the plasma temperature is required to reach 100 million degrees Celsius.
The American ignition plan is in trouble
On Earth, nuclear fusion was first produced in large quantities in hydrogen bombs. In hydrogen bombs, the high temperature and pressure generated by the detonating atomic bombs squeeze together the fusion fuel in the hydrogen bombs. Due to the inertia of the material, a large amount of fusion (also called inertial confinement fusion) occurs before scattering. However, the hydrogen bomb explosion is so powerful that humans can not control it.
60s of last century, the use of the principle, the Soviet scientists proposed and proved that the laser can deuterium tritium fusion occurs. Until 2009, the $ 3.5 billion National Firing Device (NIF) finally enabled scientists to see the possibility of laser fusion. The human being hopes to get from this laboratory "an inexhaustible, Exhausting "clean nuclear energy.
The world's largest laser fusion machine is housed in a large "warehouse" at Lawrence Livermore National Laboratory in California. Inside the device, the laser generates 192 laser beams that strike a hydrogen-spherical target containing deuterium and tritium, crashing it and generating a high temperature of about 100 million degrees Celsius, triggering the fusion of hydrogen atoms and releasing large amounts of energy. The collision between the laser and the hydrogen target is extremely brief, lasting only a few nanoseconds (1 nanosecond equals 1 billionth of a second). In order to reach the critical point or to light the reactor, the laser has a design energy of 1.8 megajoules.
As early as last year, according to the report of Nature, the laser emitted by the United States National Ignition Facility (NIF), known as the "artificial sun," has reached 2 MJ and is the first step the laser has taken toward nuclear fusion energy .
Recently, BBC News reported on October 7 that in a fusion experiment conducted in late September, the energy released by the fusion reaction exceeded the energy absorbed by the hydrogen fuel ball - a milestone breakthrough in fusion devices around the world. However, the reporter has not yet seen the news on Lawrence Livermore National Laboratory's official website.
In fact, the NIF project was not easy and the NIF research team's ignition target was pushed back a bit. According to a report by Scientific American, an interim report by the panel of experts of the National Academy of Sciences last year showed that NIF laser-triggered nuclear fusion methods are not very promising.
Wang Xiaofang told the author that the laser emission repetition rate is still very low, can not continue fusion capacity. "This is because the glass magnifying media currently used in lasers are not capable of delivering more shots per unit of time while still guaranteeing the quality of the laser beam."
At present, NIF lasers can only be fired several times a day. Only when three or four or more nuclear fusion occurs per second and continues to flow continuously, and the energy gain per fusion reaches 10 to 100 times, can it be put into practical use.
"In order to improve the repetition rate of laser emission, scientists are also developing new types of lasers, such as semiconductor laser pumps and fiber lasers." However, Wang Xiaofang said these lasers can not yet be made in sufficient size and laser output is not enough energy Achieve fusion ignition. "At present, we have not found a way to improve the laser emission repetition rate to continue fusion capacity."
It is understood that, recently, the NIF research team has been laser alignment of the real fuel ball, the experiment went further, but the ignition target ball in the extreme temperature and pressure repeatedly premature rupture. It is not hard to see that the troubles of the United States' national ignition system have always been in line with the new progress.
Nuclear fusion studies look forward to new breakthroughs
In fact, except used to
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