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The prospect of nuclear fusion

2024-04-25 Update From: SLTechnology News&Howtos shulou NAV: SLTechnology News&Howtos > IT Information >


Shulou( Report--

Original title: "how to master the power of the sun?" "

For nearly a century, astronomers and physicists have recognized a process called thermonuclear fusion. This process has accompanied the sun and stars for millions or even billions of years. Since this discovery, they have dreamed of bringing this energy to Earth and using it to power the modern world.

In today's increasing climate change, this dream will only become more eye-catching. Using thermonuclear fusion and sending it to the world's power grid can make all our coal-and gas-fired power plants that emit carbon dioxide a distant memory. Fusion power plants can provide zero-carbon power day and night without worrying about the effects of wind or weather, and without the shortcomings of today's nuclear fission power plants, such as the potential risk of a catastrophic meltdown and the radioactive waste that must be isolated for thousands of centuries.

In fact, nuclear fusion is the opposite of fission; instead of splitting heavy elements such as uranium into lighter atoms, fusion generates energy by combining various isotopes of light elements such as hydrogen into heavier atoms.

For this fantasy to come true, fusion scientists must ignite fusion on the ground-which means they do not have the conditions for a high gravity field like the center of the sun. Doing this on Earth means putting these light isotopes in a reactor and finding a way to heat them to hundreds of millions of degrees Celsius-turning them into ionized "plasmas" similar to those inside lightning, but at higher temperatures and harder to control. This means finding a way to control lightning, usually by grabbing the plasma with some kind of magnetic field and holding on to it as it wiggles, twists and tries to escape like a creature.

To say the least, both challenges are daunting. In fact, it wasn't until the end of 2022 that a multibillion-dollar fusion experiment in California finally resulted in a tiny isotope sample that released more heat than ignited it. The event, which lasts only about 1/10 nanoseconds, must be triggered by the combined output of 192 of the world's most powerful lasers.

This nuclear fusion method starts with a tiny solid target filled with perylene-tritium fuel, which is hit by strong energy pulses from all directions. This can be done indirectly by surrounding the target with a small metal cylinder (left). The laser irradiates the inside of the cylinder to produce x-rays that can heat fuel pellets. The laser beam can also heat the target directly (right). Either way, the fuel particles will implode, and the resulting energy will quickly blow up the target. This indirect method is used in a nuclear fusion experiment called "break even", which produces more energy than is transmitted by a laser. However, it may take decades for this nuclear fusion method to become a practical method of power generation. At present, with the realization of more and more technological inventions, human beings are getting closer to the realization of controllable nuclear fusion. New technologies such as high-temperature superconductivity are expected to make fusion reactors look smaller, simpler, cheaper and more efficient than ever before. Better yet, decades of slow and sustained progress seems to have exceeded a tipping point, and fusion researchers now have enough experience to design plasma experiments that are almost as predicted.

"humanity is on the verge of a future where we can develop enough technology to achieve controllable fusion," said Michelle Binderbauer, CEO of TAE Technologies in Southern California.

In fact, since TAE became the first commercial fusion company in 1998, more than 40 commercial fusion companies have been established-most of them in the past five years. Among them, many companies' nuclear fusion devices are expected to be put into operation within the next decade. " Although we have done everything we can to achieve our goals, "said Andrew Holland, CEO of the Fusion Industry Association, an advocacy group he founded in Washington, D.C., in 2018." But I don't think that's enough. We're still looking for more and more companies to join our industry with different ideas. "

The development of nuclear fusion has been the focus of capital, and start-ups committed to controllable fusion technology have raised about $6 billion and continue to do so. "the combination of new technologies and capital in controlled fusion creates a synergy." Jonathan Maynard, research director at the Princeton Plasma Physics Laboratory of the Department of Energy in New Jersey.

Of course, there are good reasons to be cautious-first of all, so far, these companies have not proved that it can generate net fusion energy, even briefly, let alone achieve commercial operation of controllable fusion within a decade. "many companies even promise to achieve goals that we think are unlikely to be achieved in a limited period of time," Maynard said. " However, he added: "We are happy to be proved wrong."

At present, more than 40 companies are experimenting with controllable fusion technology, and we will soon know which one will stand out. At the same time, in order to give you a general idea of achieving the goal of controllable nuclear fusion, we will list the current challenges facing the realization of controllable nuclear fusion and the optimal designs proposed by these enterprises in order to solve these problems.

A prerequisite for fusion the first challenge of controlled fusion is ignition, so to speak: nuclear fusion devices must mix all kinds of nuclear fuels, that is, mixed isotopes, and make nuclear contact, fusion and release energy.

This means literally "contact": nuclear fusion is a contact movement, and the reaction does not begin until the nucleus collides head on. What makes this problem tricky is that each nucleus contains protons with positive charges, which repel each other. So the only way to overcome the repulsive force is to make the nuclei move quickly so that they collide and fuse before they deflect.

As a result, the fusion process requires a plasma temperature of at least 100 million degrees Celsius, which is only the temperature of the fuel mixture of hydrogen's two heavy isotopes, deuterium and tritium. Mixtures of other isotopes get hotter-which is why the "deuterium-tritium fuel mixture" (DmurT) is still the fuel of choice in most reactor designs.

In fusion reactors, light isotopes fuse to form heavy isotopes and release energy in the process. Here are four examples of reactor fuel. The first is Dmurt, which combines two kinds of heavy hydrogen (deuterium and tritium). This mixing is the most common because it begins fusion at the lowest temperature, but tritium is radioactive and the resulting neutrons make the reactor radioactive. The reaction between the two deuterons (Dmurd) is slow and requires a higher temperature. Mixtures of deuterium-helium-3 are also less common, in part because helium-3 is rare and expensive. Perhaps the most striking is the mixture of proton and boron-11 (Pmur11B). Both isotopes are non-radioactive and rich, but their fusion products are stable and easy to capture for energy extraction. The challenge of this method will be to bring the mixture to a fusion temperature of more than 1 billion degrees Celsius. But no matter what fuel is used in nuclear fusion, the key to achieving the fusion temperature lies in the competition between the energy input from the external environment and the plasma ions: researchers try to input energy from external sources such as microwaves or high-energy beams of neutral atoms, while plasma ions try to radiate energy as quickly as they receive energy.

The ultimate goal is to let the plasma exceed the "ignition" temperature, at which point the fusion reaction will begin to generate enough internal energy to make up for the radiated energy, as well as power to one or two cities. But this leads to a second challenge: once the fire is ignited, any applied reactor must keep the process going-that is, keeping these overheated nuclei close enough to maintain a reasonable collision rate for long enough to generate a useful energy flow.

In most reactors, this means protecting the plasma in an airtight chamber because the missing air molecules cool the plasma and extinguish the reaction. But it also means keeping the plasma away from the cavity wall, which is much colder than the plasma, and even the slightest contact can disrupt the reaction. The problem is that if you try to use a non-physical barrier (such as a strong magnetic field) to keep the plasma away from the wall, the flow of ions will soon be distorted by the current and magnetic field inside the plasma and become useless. Unless you have designed the distribution of the field in the device very carefully-this is also the most significant difference between different fusion schemes.

Finally, practical reactors must include methods to extract fusion energy and convert it into stable electricity. Although engineers are never short of ideas for this last challenge, the key to the process depends on the fuel mixture used in the reactor.

In the case of deuterium-tritium fuel, for example, most of the energy generated by the reaction is in the form of high-speed particles called neutrons, which are not limited by a magnetic field because they are not charged. Because there is no charge, neutrons can pass not only through the magnetic field, but also through the reactor wall. Therefore, the plasma chamber must be surrounded by a "blanket": a thick layer of heavy materials such as lead or steel that can absorb neutrons and convert their energy into heat. The heat can then be transferred to the boiler unit and generated by the same type of steam turbine used in conventional power plants.

Nuclear fusion power plants can use one of several different reactor types, but it will convert fusion energy into electricity in the same way as fossil fuel power plants or nuclear fission reactors: heat from energy boils water to produce steam, which flows through steam turbines, which transmit electricity to the grid by rotating generators. Many DT reactor designs also require the addition of some lithium to the blanket material so that neutrons react with lithium to produce new tritium nuclei. This step is crucial: because each DT fusion consumes a tritium nucleus, and the isotope is radioactive and does not exist in nature, the reactor will quickly run out of fuel if it is not fueled.

The complexity of DT fuel is enough to allow some bolder fusion startups to choose other fuel mixtures that produce better results. Binderbauer's TAE, for example, aims to develop an ultimate fusion fuel: a mixture of protons and boron-11. These two components are not only stable, non-toxic and rich, but their only reaction products are three positively charged helium-4 nuclei, whose energy is easily captured by a magnetic field and does not need to be wrapped in a blanket.

But alternative fuels face different challenges. For example, TAE must increase the fusion temperature of the proton-boron-11 mixture to at least 1 billion degrees Celsius, about 10 times the DT threshold.

Tokamak devices ignite plasma, maintain fusion reactions and extract energy-the basic principles of these three challenges have been studied since the early days of nuclear fusion development. By the 1950s, researchers focused on nuclear fusion devices had begun to come up with many solutions to these problems-but most of them were shelved after Soviet physicists released a class of designs called tokamaks in 1968.

Like several early reactor concepts, the tokamak is characterized by the fact that the plasma chamber is a bit like a hollow doughnut-a shape that allows ions to circulate endlessly without hitting anything-and controls plasma ions through a magnetic field generated by a current-carrying coil that surrounds the circulation.

But the tokamak also has a new set of coils that allow the current to rotate around the confinement ring in the plasma, like a circular lightning. This current rotates the magnetic field to a certain extent and plays an important role in stabilizing the plasma. Although the first tokamak still could not reach the temperature and constraint time required for the power reactor, it was much better than the previous design. So after that, almost all of the researchers turned to tokamaks.

Both the tokamak reactor (left) and the associated star simulator reactor (right) use magnetic fields (purple) generated by electromagnetic coils (blue and red) to limit superhot plasma (yellow). For the most common reactor tokamaks, these coils also start the current to flow through the plasma, which helps keep the reaction stable. The design of the star simulator also limits the plasma to a sealed ring cavity, but eliminates the need for circulating current in the ring cavity by using a more complex set of external coils (blue) to control the plasma. Since then, more than 200 tokamaks of different designs have been built around the world, and physicists have gained a better understanding of tokamak plasma, and they can confidently predict the performance of machines in the future. It is because of this confidence that a financial group of private equity funds is willing to spend more than $20 billion to build ITER (Latin for "road"): to enlarge the tokamak to the size of a 10-story building. ITER has been under construction in southern France since 2010 and is expected to start experiments with deuterium-tritium fuel in 2035. Physicists are confident that when it is implemented, ITER will be able to maintain and study the burning fusion plasma for a few minutes at a time, providing a unique treasure trove of data that is expected to play a role in the construction of electric reactors.

ITER is not only an experimental fusion power generation, but also an experimental device with more instruments and functions than conventional nuclear power plants-the huge cost of ITER has prompted two start-ups focused on the commercialization of nuclear fusion to compete to develop smaller, simpler and cheaper tokamak reactors.

The first to stand out is Tokamak Energy, a British company founded in 2009. Over the years, the company has raised about $250 million in venture capital to develop a "spherical tokamak"-based reactor, a particularly pocket variant that looks more like an apple with a core than a doughnut.

But Massachusetts's Federal Fusion system (Commonwealth Fusion Systems), a branch of the Massachusetts Institute of Technology, was not launched until 2018. Although Federal's tokamak design uses a more traditional circular structure, the company has raised nearly $2 billion through MIT's extensive fundraising network.

Both companies were the first to use high temperature superconductor (HTS) cables to generate magnetic fields. These materials were discovered in the 1980s, but have only recently appeared in the form of cables, which can transmit current without resistance even at relatively high temperatures of 77 Kelvin (- 196 degrees Celsius), a temperature high enough to be achieved with liquid nitrogen or liquid helium. This makes HTS cables easier to cool and cheaper than those used by ITER.

But more importantly, HTS cables generate stronger magnetic fields in smaller spaces than cryogenic cables, which means both companies are able to shrink their power plant designs to a fraction of ITER.

Although the tokamak has always been dominant, most fusion startups today do not use this design. They are reviving old alternatives that are smaller, simpler and cheaper to work than tokamaks.

The main example of these revival designs is a fusion reactor based on a smoke ring plasma vortex called field inversion structure (FRC). The FRC whirlpool is like a fat hollow cigar, spinning around its axis like a top, and it holds itself together with its own internal current and magnetic field-which means that the FRC reactor does not need to keep its ions circulating in a circular plasma chamber. At least in theory, the eddy current will remain stable in a straight cylindrical cavity, requiring only a light touch of the external field to keep it stable. This means that FRC-based reactors can discard most of the expensive, power-consuming external magnetic field coils and make them smaller, simpler and cheaper than tokamaks or almost anything else.

The concept of a linear reactor is shown here, which is based on a particularly stable plasma vortex that combines with its own internal current and magnetic field. It is called the field inversion structure (FRC) and is formed by the merging of two simpler vortices fired by a plasma gun from both ends of the reaction chamber. The fresh fuel beam entering from the side keeps the FRC hot and rotates quickly. Unfortunately, in practice, the first experiment with FRC in the 1960s found that they always seemed to get out of control within a few hundred microseconds, which is why this approach was mostly put aside in the age of tokamaks.

However, the simplicity of FRC reactors has always been attractive. In fact, the FRC device is stable when the plasma reaches extremely high temperatures-which is why TAE chose FRC as its nuclear fusion device in 1998, when the company began to seek to take advantage of the proton-boron-11 fusion reaction that took place at 1 billion degrees Celsius.

Binderbauer and the late physicist Norman Rostoker, co-founder of TAE, proposed a scheme that can stabilize and maintain the FRC vortex indefinitely: as long as a fresh fuel beam is fired along the outer edge of the vortex, the plasma can maintain high temperature and high spin rate.

By the mid-2010s, the TAE team had shown that as long as the beam injector was energized, the beam entering from the side did indeed keep the FRC rotating and stable. These can be extracted from burning proton-boron-11 reactors. By 2022, they have proved that their FRCs can maintain this stability above 70 million degrees Celsius.

The next field reversal reactor, the 30-meter-long Copernicus, is scheduled to be completed in 2025. TAE hopes to actually achieve combustion conditions of more than 100 million degrees. The reactor can provide the necessary data for the TAE team to design higher-temperature fusion reactors.

Plasma vessels meanwhile, General Fusion, based in Vancouver, Canada, is working with the British Atomic Energy Authority to build a demonstration reactor based on perhaps the most novel principle, the magnetized directional fusion device. This 1970s concept is equivalent to firing a plasma eddy current into a metal jar and then crushing the jar. If the speed is fast enough, the captured plasma will be compressed and heated to fusion conditions. Keep doing this, and a more or less continuous burst of fusion energy will come back, and you will have a power reactor.

In the current concept of General Fusion, the metal tank will be replaced by a molten lead-lithium mixture, which is held by centrifugal force on both sides of a cylindrical container rotating at a speed of 400 rpm. At the beginning of each reactor cycle, a downward pointing plasma gun will inject a whirlpool of ionized deuterium-tritium fuel-a "magnetized target"-which will briefly turn the rotating metal-lined container into a miniature spherical tokamak. Next, compressed air pistons arranged outside the container will push the lead-lithium mixture into the eddy current, compressing it from 3 meters in diameter to 30 centimeters in about 5 milliseconds and raising deuterium-tritium to the fusion temperature.

Magnetized target fusion is a method in the 1970s, which is equivalent to emitting plasma eddies into metal cans and then compressing the cans. Shown here is a modern version in which the metal can is replaced by a molten lead-lithium mixture, which is fixed on both sides of the rotating container under the action of centrifugal force. The plasma gun sends the eddy current of the deuterium-tritium plasma into the hollow interior of the container, while the pistons arranged on the outside of the container push the lead-lithium mixture inward, crushing the plasma and causing fusion. The shock wave pushed the molten lead-lithium mixture out and restarted the system. The resulting shock wave will impact the molten lead-lithium mixture, push it back to the rotating cylinder wall, and reset the system for the next cycle-about a second later. At the same time, on a slower time scale, the pump will steadily circulate the molten metal to the outside so that the heat exchanger can collect the fusion energy it absorbs, while other systems can remove tritium from the neutron-lithium interaction.

All of these moving parts require some complex choreography, but as long as everything goes according to the simulation, the company hopes to build a full deuterium-tritium-fired power plant in the 1930s.

No one knows when (or whether) the special reactor concept mentioned here will become a real commercial power plant, or whether the first reactor to enter the market will be one of the many alternative reactor designs being developed by more than 40 other fusion companies.

However, few of these companies think that the exploration of nuclear fusion energy is a competitive or zero-sum game. Many of them describe their competition as fierce but largely friendly-mainly because there are broad commercial prospects for developing multiple types of fusion reactors in a world where any form of carbon-free energy is urgently needed.

"I would say my idea is better than theirs. But if you ask them, they may tell you that their idea is better than mine," said Michel Laberge, a physicist and founder and chief scientist of General Fusion. "most of them are serious researchers and there are no fundamental flaws in their plans." Because there are more possibilities, the chances of actual success of nuclear fusion will increase, he said. "We really need fusion on this planet, very much."

Author: M. Mitchell Waldrop

Translation: depth

Revision: machine 7

Original link: Pursuing fusion power

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop), author: M. Waldrop

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