Since my country’s artificial sun keeps breaking records, why can’t controlled nuclear fusion light up a lamp?

At the beginning of this year, the Chinese Academy of Sciences' artificial sun experimental device in Hefei reported another good news: it achieved long-pulse high-parameter operation for 1056 seconds, breaking its own world record that it had repeatedly broken, and taking the lead in the world.

The artificial sun is not a small sun artificially created to illuminate the world, but a way to use the energy generation mechanism inside the sun to create and use energy on Earth. The real scientific name is "controlled nuclear fusion". In order to obtain this energy that may change the course of human civilization, humans have spared no effort and spared no cost for decades.

But I want to pour some cold water here: So far, the sun has been shining brightly on the solar system every day, but the artificial little sun has not been able to light a lamp.

Why do humans spare no effort to study controlled nuclear fusion?

Einstein discovered the mystery of the mass-energy equation 100 years ago. Humanity has made a huge breakthrough in understanding the relationship between the mass and energy of matter. We know that mass and energy are equivalent and interchangeable. Any substance contains enormous energy. As long as the energy can be released, it will be earth-shaking.

The formula of the mass-energy equation is: E=MC^2. Here E represents energy, M represents the mass of any substance, and C represents the speed of light. The meaning is that as long as 1kg (kilogram or kilogram) of matter is converted into energy, 9*10^16J (joule) of energy can be obtained, which is equivalent to 25 billion kilowatt-hours of electricity.

However, it is very difficult to convert the energy in matter. Methods such as combustion can only stay at the level of chemical reaction, and the mass-energy conversion rate is very low. For example, the heat energy obtained from burning coal is only equivalent to one-third of the mass-energy conversion rate, and the burning of crude oil is about one-second.

Nuclear fusion is the method that humans have discovered so far that can achieve the greatest mass-energy conversion, with a mass-energy conversion rate of about 7/1000. The endless energy radiation from the core of the sun is released by nuclear fusion. This process is the process of 4 hydrogen nuclei fusing into 1 helium nucleus.

The atomic weight of 4 hydrogen nuclei is 4.03136, and the atomic weight of 1 helium nucleus is 4.0026. We can see that the atomic weight is reduced by 0.0294 after nuclear fusion, which is about 0.718% of the atomic weight loss. In this way, the mass loss of 1 kilogram of hydrogen nuclear fusion reaches 7.18 grams.

Where does this lost mass go? According to Einstein's mass-energy conversion theory, this mass is converted into energy and radiated. Every second, 600 million tons of hydrogen in the sun participate in nuclear fusion, and about 4.3 million tons of mass loss is converted into energy and radiated into space. This energy is 3.87*10^26J, equivalent to 100 trillion kWh.

The sun has been burning like this for 5 billion years and can continue to burn for another 5 billion years. Our earth receives 1/2.2 billion of the sun's energy, which is about 1.76*10^17 J/s of energy, equivalent to 49 billion kilowatt-hours of electricity per second, which is the total power generation of 10 million Three Gorges Dams. This energy seems huge, but it is actually the energy converted from less than 2 kg of matter.

Humans are inspired by the huge energy released by stellar nuclear fusion. If we can master this way of obtaining energy, it will completely alleviate the energy crisis in the development of human civilization. Because the fuel for solar nuclear fusion is hydrogen, hydrogen is a very common element on Earth and its reserves are very abundant.

Water is composed of hydrogen and oxygen. The earth's seawater reserves reach 136 trillion tons. If the hydrogen in each liter of seawater is used as nuclear fusion fuel, the energy it produces is equivalent to 300 liters of gasoline. Therefore, if the nuclear fusion method of the sun can be successfully tested on the earth, the energy crisis of mankind will not exist for a long time, and human civilization will enter a new period of rapid development.

Since the last century, people have obtained the energy of nuclear fusion, which is the hydrogen bomb explosion. However, the hydrogen bomb explosion is only a momentary explosion. Apart from being used in war and deterrence, it is of little help to human life. How to control this power and let it release slowly like the sun? This is the inherent motivation for humans to spare no effort to research and develop controlled nuclear fusion technology.

Difficulties of controlled nuclear fusion

The sun itself is a huge spherical high-temperature plasma. This plasma is a process in which energy is continuously released due to nuclear fusion in the core, and is continuously transferred to the surface through radiation and convection, and then released into space. All stars like the sun have the same energy generation mechanism.

Gravity will pull all celestial bodies of huge mass into a spherical shape and continuously shrink towards the core. The mass of the sun is about 2*10^30kg. The centripetal contraction pressure of this huge mass creates a pressure of 300 billion atmospheres and a temperature of 15 million degrees in the core of the sun, thus igniting the nuclear fusion of the sun, which will continue.

The huge radiation pressure of nuclear fusion resists the gravitational contraction pressure caused by mass, forming a balance, which is the main sequence stage of the sun, and this stage can last for about 10 billion years. The nuclear fusion in the core of the sun is continuously carried out under the constraint of huge gravitational contraction pressure. This nuclear fusion constraint method is called gravity confinement.

It is this combination of huge pressure and high temperature that causes the hydrogen atoms in the core of the sun to be stripped of their outer electrons, exposing their nuclei. The constant collision and fusion of nuclei leads to continuous nuclear fusion. If people want to imitate the nuclear fusion of the sun on Earth, they cannot create such huge pressure. The highest pressure on Earth is in the core, where the pressure is only 3.6 million atmospheres, which is only about one-tenth of the pressure in the core of the sun. How can hydrogen expose its nucleus?

Only by raising the temperature to 100 million degrees Celsius, the electrons outside the hydrogen nucleus will be stripped away, and the hydrogen nuclei can form continuous fusion. This raises several difficult problems: first, how to reach such a high temperature; second, what container to use to contain and confine this ultra-high temperature flame (plasma); and third, how to make this high temperature generate electricity.

You should know that the highest temperature that all substances on our earth can withstand is only a few thousand degrees. The single metal that is known to withstand the highest temperature is tungsten, with a melting point of 3410°C. The alloy that can withstand the highest temperature is hafnium alloy, with a melting point of 4215°C. 100 million°C is 23,000 times the melting point of hafnium alloy.

The sun relies on gravity to compress the high temperature of nuclear fusion reaction within a radius of 1/4. How can the earth control the high temperature of 100 million degrees of nuclear fusion in a container? None of the 118 elements that have been discovered can do this. After repeated experiments and research, scientists have found a non-material method, which is magnetic confinement and inertial confinement.

Magnetic confinement is commonly used now (we will not talk about inertial confinement today).

The so-called magnetic confinement is because plasma is charged and will interact with the magnetic field. Scientists use this physical principle to create a magnetic field trap with energy, and confine the high-temperature plasma in a circular trap formed by the magnetic field. This is like a cage with no support on all four sides, so that the high-temperature plasma formed by nuclear fusion is in this cage and is not close to any parts or inner walls of the equipment.

But it is very difficult to achieve this, especially to keep the high temperature plasma confined for a longer period of time. The so-called confinement time is the continuous improvement and breakthrough of these experiments.

However, being able to produce high-temperature plasma and confine it does not mean that nuclear fusion energy has been utilized. This is only the first step in a long journey. Because a large amount of energy is required to produce high-temperature nuclear fusion and a magnetic trap to confine high-temperature plasma, how to make the output energy greater than the input energy and conduct the confined nuclear fusion energy for power generation is still a difficult issue.

What are the indicators that put China at the forefront of the world?

China's nuclear fusion research institute is located in Hefei, and its official name is "Hefei Institutes of Physical Science, Chinese Academy of Sciences". There is a device with independent intellectual property rights in China, called the Experimental Non-Circular Section All-Superconducting Tokamak, or EAST for short, which is currently in a leading position in the world.

This device has established three major goals: 1 megaampere plasma current, 100 million degree high temperature plasma, and 1000 seconds of operation time.

The results that have been achieved in stages are: in 2012, a high-parameter divertor plasma at 20 million °C was obtained for 411 seconds; in 2017, a steady-state long-pulse high-confinement plasma was achieved, operating at 50 million °C for 101.2 seconds; in 2018, the electron temperature in the center of the plasma reached 100 million °C for the first time; in 2020, it maintained operation at a high temperature of 100 million °C for 10 seconds, and in the first half of 2021, this time will be extended to 101 seconds, and the temperature will be raised to 120 million °C.

On December 30, 2021, two days ago, it was achieved that the long-pulse high-parameter plasma conditions of nearly 70 million degrees Celsius were maintained for 1056 seconds, and the injected energy reached 1.73 GJ. So far, EAST has discharged more than 100,000 times in total, achieving the three major scientific conditions of 1 megaampere plasma current, 100 million degrees Celsius high-temperature plasma confinement, and 1000 seconds of operation time.

According to relevant parties, the experiment solved three major scientific problems: completely non-inductive current drive, recycling and impurity control, heat and particle discharge, etc. These are very professional terms, as a general science popularization, we will not go into them in depth.

From my country's continuous breakthroughs in the field of controlled nuclear fusion research, we can see the growth of China's scientific and technological strength, and we can also understand the difficulty of controlled nuclear fusion research: now the entire advancement is still calculated in "seconds", and there is still a long way to go in the future.

Improving the output energy ratio is the most critical difficulty and focus that needs to be broken through

Based on the breakthrough in the effective confinement of nuclear fusion plasma, how to conduct the energy of nuclear fusion from the magnetic trap and put it into civilian use will be the difficulty and focus of future controlled nuclear fusion research.

The most important indicator is the ratio of input energy to output energy, which is called Q value. There are two types of Q value indicators: QP, which refers to the ratio of energy required for ignition to output energy; and QT, which refers to the ratio of input energy to output energy of the entire device. The difference between these two ratios is very large.

For example, in the experiment, the total input energy is 400MW, the energy used for ignition is only 50MW, and the output energy is 250MW. In this way, the QP value is Q=5, indicating that the output energy is 5 times the input energy, which seems good. However, the actual QT value is only 0.625, and the energy input is still greater than the output, which has no commercial value.

Some experimental data do not disclose the QP and QT values, so it is unknown what level they have reached. According to some available information, the world's highest Q-value controlled nuclear fusion experiments include the British JET, with a Q value of ≈ 0.67, and the Japanese JT-60 experiment, whose Q value theoretically reached 1.25, but it is said to be only a theoretical value and difficult to achieve in practice. Later, JT-60 was also dismantled.

According to the development roadmap of magnetic fusion disclosed by my country, it is planned to achieve 3000-second steady-state long-pulse plasma with Q value = 5 in 2025, steady-state plasma combustion with Q value > 10 in 2030, and grid-connected power generation in 2050. It is not specified what kind of Q value this expected Q value refers to.

Some studies believe that due to the large power losses in the process of output, transmission and conversion of controlled nuclear fusion energy, the Q value generally needs to reach 30 to have commercial value. At the same time, there must be a complete tritium circulation loop and qualified radiation-resistant materials. These studies will still take some time.

It can be seen that the research on controlled nuclear fusion is still a long way to go. However, Chinese scientists are full of confidence, and they have only one goal: to light the first lamp of nuclear fusion energy in China.

Let's look forward to it. Welcome to discuss, thank you for reading.

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