For proper confinement, this toroidal field must have superimposed upon it a perpendicular field component a poloidal field. The result is a magnetic field with force lines following spiral helical paths that confine and control the plasma. There are several types of toroidal confinement system, the most important being tokamaks, stellarators and reversed field pinch RFP devices. In a tokamak, the toroidal field is created by a series of coils evenly spaced around the torus-shaped reactor, and the poloidal field is created by a system of horizontal coils outside the toroidal magnet structure.
A strong electric current is induced in the plasma using a central solenoid, and this induced current also contributes to the poloidal field. In a stellarator, the helical lines of force are produced by a series of coils which may themselves be helical in shape. Unlike tokamaks, stellarators do not require a toroidal current to be induced in the plasma. RFP devices have the same toroidal and poloidal components as a tokamak, but the current flowing through the plasma is much stronger and the direction of the toroidal field within the plasma is reversed.
In tokamaks and RFP devices, the current flowing through the plasma also serves to heat it to a temperature of about 10 million degrees Celsius. Beyond that, additional heating systems are needed to achieve the temperatures necessary for fusion.
In stellarators, these heating systems have to supply all the energy needed. The tokamak toroidalnya kamera ee magnetnaya katushka — torus-shaped magnetic chamber was designed in by Soviet physicists Andrei Sakharov and Igor Tamm.
Tokamaks operate within limited parameters outside which sudden losses of energy confinement disruptions can occur, causing major thermal and mechanical stresses to the structure and walls.
Nevertheless, it is considered the most promising design, and research is continuing on various tokamaks around the world. Research is also being carried out on several types of stellarator. Lyman Spitzer devised and began work on the first fusion device — a stellarator — at the Princeton Plasma Physics Laboratory in Due to the difficulty in confining plasmas, stellarators fell out of favour until computer modelling techniques allowed accurate geometries to be calculated.
Because stellarators have no toroidal plasma current, plasma stability is increased compared with tokamaks. Since the burning plasma can be more easily controlled and monitored, stellerators have an intrinsic potential for steady-state, continuous operation. The disadvantage is that, due to their more complex shape, stellarators are much more complex than tokamaks to design and build. RFP devices differ from tokamaks mainly in the spatial distribution of the toroidal magnetic field, which changes sign at the edge of the plasma.
The RFX machine in Padua, Italy is used to study the physical problems arising from the spontaneous reorganisation of the magnetic field, which is an intrinsic feature of this configuration. In inertial confinement fusion, which is a newer line of research, laser or ion beams are focused very precisely onto the surface of a target, which is a pellet of D-T fuel, a few millimetres in diameter.
This heats the outer layer of the material, which explodes outwards generating an inward-moving compression front or implosion that compresses and heats the inner layers of material. The core of the fuel may be compressed to one thousand times its liquid density, resulting in conditions where fusion can occur. The energy released then would heat the surrounding fuel, which may also undergo fusion leading to a chain reaction known as ignition as the reaction spreads outwards through the fuel.
The time required for these reactions to occur is limited by the inertia of the fuel hence the name , but is less than a microsecond. So far, most inertial confinement work has involved lasers. Recent work at Osaka University's Institute of Laser Engineering in Japan suggests that ignition may be achieved at lower temperature with a second very intense laser pulse guided through a millimetre-high gold cone into the compressed fuel, and timed to coincide with the peak compression.
This technique, known as 'fast ignition', means that fuel compression is separated from hot spot generation with ignition, making the process more practical. In the UK First Light Fusion based near Oxford is researching inertial fusion energy IFE with a focus on power driver technology using an asymmetric implosion approach.
As well as power generation, the company envisages material processing and chemical manufacturing applications. It focuses powerful laser beams into a small target in a few billionths of a second, delivering more than 2 MJ of ultraviolet energy and TW of peak power. A completely different concept, the 'Z-pinch' or 'zeta pinch' , uses a strong electrical current in a plasma to generate X-rays, which compress a tiny D-T fuel cylinder.
Magnetized target fusion MTF , also referred to as magneto-inertial fusion MIF , is a pulsed approach to fusion that combines the compressional heating of inertial confinement fusion with the magnetically reduced thermal transport and magnetically enhanced alpha heating of magnetic confinement fusion. A range of MTF systems are currently being experimented with, and commonly use a magnetic field to confine a plasma with compressional heating provided by laser, electromagnetic or mechanical liner implosion.
As a result of this combined approach, shorter plasma confinement times are required than for magnetic confinement from ns to 1 ms, depending on the MIF approach , reducing the requirement to stabilize the plasma for long periods. Conversely, compression can be achieved over timescales longer than those typical for inertial confinement, making it possible to achieve compression through mechanical, magnetic, chemical, or relatively low-powered laser drivers.
Due to the reduced demands on confinement time and compression velocities, MTF has been pursued as a lower-cost and simpler approach to investigating these challenges than conventional fusion projects. Fusion can also be combined with fission in what is referred to as hybrid nuclear fusion where the blanket surrounding the core is a subcritical fission reactor.
The fusion reaction acts as a source of neutrons for the surrounding blanket, where these neutrons are captured, resulting in fission reactions taking place.
These fission reactions would also produce more neutrons, thereby assisting further fission reactions in the blanket. The concept of hybrid fusion can be compared with an accelerator-driven system ADS , where an accelerator is the source of neutrons for the blanket assembly, rather than nuclear fusion reactions see page on Accelerator-driven Nuclear Energy.
The blanket of a hybrid fusion system can therefore contain the same fuel as an ADS — for example, the abundant element thorium or the long-lived heavy isotopes present in used nuclear fuel from a conventional reactor could be used as fuel. The blanket containing fission fuel in a hybrid fusion system would not require the development of new materials capable of withstanding constant neutron bombardment, whereas such materials would be needed in the blanket of a 'conventional' fusion system.
A further advantage of a hybrid system is that the fusion part would not need to produce as many neutrons as a non-hybrid fusion reactor would in order to generate more power than is consumed — so a commercial-scale fusion reactor in a hybrid system does not need to be as large as a fusion-only reactor. A long-standing quip about fusion points out that, since the s, commercial deployment of fusion power has always been about 40 years away.
While there is some truth in this, many breakthroughs have been made, particularly in recent years, and there are a number of major projects under development that may bring research to the point where fusion power can be commercialised. Much research has also been carried out on stellarators. It is being used to study the best magnetic configuration for plasma confinement. At the Garching site of the Max Planck Institute for Plasma Physics in Germany, research carried out at the Wendelstein 7-AS between and is being progressed at the Wendelstein 7-X, which was built over 19 years at Max Planck Institute's Greifswald site and started up at the end of In the USA, at Princeton Plasma Physics Laboratory, where the first stellarators were built in , construction on the NCSX stellerator was abandoned in due to cost overruns and lack of funding 2.
There have also been significant developments in research into inertial fusion energy IFE. Both are designed to deliver, in a few billionths of a second, nearly two million joules of light energy to targets measuring a few millimeters in size. Between and , the initial designs were drawn up for an International Thermonuclear Experimental Reactor ITER, which also means 'a path' or 'journey' in Latin with the aim of proving that fusion could produce useful energy.
The four parties agreed in to collaborate further on engineering design activities for ITER. Canada and Kazakhstan are also involved through Euratom and Russia, respectively.
The envisaged energy gain is unlikely to be enough for a power plant, but it should demonstrate feasibility. In , the USA rejoined the project and China also announced it would join. The deal involved major concessions to Japan, which had put forward Rokkasho as a preferred site. India became the seventh member of the ITER consortium at the end of The total cost of the MW ITER comprises about half for the ten-year construction and half for 20 years of operation.
Site preparation works at Cadarache commenced in January First concrete for the buildings was poured in December Experiments were due to begin in , when hydrogen will be used to avoid activating the magnets, but this is now expected in The first D-T plasma is not expected until ITER is large because confinement time increases with the cube of machine size. The vacuum vessel will be 19 m across and 11 m high, and weigh more than tonnes.
The goal of ITER is to operate with a plasma thermal output of MW for at least seconds continuously with less than 50 MW of plasma heating power input. No electricity will be generated at ITER. It is focused on the divertor structure to remove helium, testing the durability of tungsten materials used. A 2 GW Demonstration Power Plant, known as Demo, is expected to demonstrate large-scale production of electrical power on a continual basis.
The conceptual design of Demo was expected to be completed by , with construction beginning around and the first phase of operation commencing from It has since been delayed, with construction now planned for after JET is the largest tokamak operating in the world today.
JET produced its first plasma in , and became the first experiment to produce controlled fusion power in November , albeit with high input of electricity. Up to 16 MW of fusion power for one second and 5 MW sustained has been achieved in D-T plasmas using the device, from 24 MW of power injected into its heating system, and many experiments are conducted to study different heating schemes and other techniques.
JET has been very successful in operating remote handling techniques in a radioactive environment to modify the interior of the device and has shown that the remote handling maintenance of fusion devices is realistic. It has been significantly upgraded in recent years to test ITER plasma physics and engineering systems.
Further enhancements are planned at JET with a view to exceeding its fusion power record in future D-T experiments. MAST Upgrade is focused on designing a plasma exhaust system or divertor that would be able withstand the intense power loads created in commercial-sized fusion reactors. It achieved first plasma in October The technical objectives of STEP are: to deliver predictable net electricity greater than MW; to exploit fusion energy beyond electricity production; to ensure tritium self-sufficiency; to qualify materials and components under appropriate fusion conditions of neutron flux; and to develop a viable path to affordable life-cycle costs.
STEP is scheduled for completion in Tokamak Energy in the UK is a private company developing a spherical tokamak, and hopes to commercialize this by The company grew out of Culham laboratory, home to JET, and its technology revolves around high temperature superconducting HTS magnets, which allow for relatively low-power and small-size devices, but high performance and potentially widespread commercial deployment.
It produced plasma temperatures of 15 million degrees Celsius in and after the commissioning of further magnetic coils.
Chief executive of Tokamak Energy David Kingham said: "The ST40 is designed to achieve million degrees C and get within a factor of ten of energy break-even conditions. The funds will contribute to core development work on high temperature superconducting HTS magnets and plasma exhaust system divertor technologies.
The divertor must handle high levels of heat and particle bombardment while removing impurities and waste from the system. It aims to have a prototype delivering electricity to the grid by It is a pilot device for ITER, and involves much international collaboration.
The tokamak with 1. Its first stage of development to was to prove baseline operation technologies and achieved plasma pulses of up to 20 seconds. Fission happens quite easily — and is used to generate electricity in conventional nuclear power stations. Fusion on the other hand, is the process of sticking together light nuclei typically hydrogen -like nuclei. The larger nuclei again needs less energy to hold it together — so energy is released.
This is what happens in the Sun and stars, and research on how to harness fusion energy on Earth is being carried out in devices such as tokamaks and stellarators. You might say, in fact, that our world revolves around the sun. The first person in recorded history to say that our world revolves around the sun, literally and not just metaphorically, was the Greek astronomer Aristarchus of Samos, who lived during the 3rd century BC.
Around the same time, another Greek astronomer and philosopher, Anaxagoras, suggested that the sun was not, in fact, the chariot of Helios and was instead a giant ball of flaming metal that orbited the Earth people did not like being told this.
Around the same time, Erastothenes of Cyrene, the Greek mathematician renowned for calculating the circumference of the Earth with astonishing precision, also calculated the distance from the sun to the Earth as being about million kilometers about 94 million miles. The sun is, in fact, million kilometers away from the Earth at the closest point in our orbit and million kilometers at the farthest point. Subrahmanyan Chandrasekhar and Hans Bethe developed the theoretical concept of what Eddington had proposed, now known as nuclear fusion, and calculated how the nuclear fusion reactions that power our sun worked.
As soon as we understood the nuclear furnace resting in the heart of our sun, which was in fact a giant ball of incandescent mostly hydrogen gas and not, as Anaxagoras had surmised, a fiery metal orb good guess, though!
Nuclear fusion is one of the simplest, and yet most powerful, physical processes in the universe. Two very excited, very hot, very energetic atoms collide with each other and turn into one atom, releasing a few leftover subatomic particles and leftover energy in the process. Eventually, these tiny particles began to attract each other and bond, turning quarks into electrons, neutrons, and protons—the fundamental building blocks of matter.
The hot, dense soup of the universe began to cool and curdle as it expanded, forming little lumps of hydrogen gas. For a while, the universe was nothing but hydrogen, the simplest element. But gravity slowly began to pull some of these gas clouds closer together, and as the hydrogen atoms zipping around gained more energy in their increasingly-dense, increasingly-hot environment, they began to fuse with each other to form helium, the second-lightest element.
Many of these gas clouds became stars just like our sun—massive balls of hydrogen and helium plasma. And in the dense cores of these stars, hydrogen and helium continued to fuse until they formed heavier and heavier elements.
It takes a great deal of energy to induce nuclear fusion. Atomic nuclei, which contain positively-charged protons and neutral neutrons, do not want to come near each other under normal circumstances. The Coulomb force , which describes how like charges repel each other and opposite charges attract as with the north and south poles of a magnet, for example , keeps these two atomic nuclei from colliding with each other.
If you set two atoms on a direct collision course with the intention of making their nuclei smash into each other and stick together, you will need to accelerate them to very high speeds so that when they collide, the nuclear force, which compels protons to stick to neutrons, overcomes the repulsive Coulomb force.
Nuclear binding energy is the minimum amount of energy it takes to break apart an atomic nucleus. The denser the element, the more energy it takes to break its nucleus apart. When we cause nuclear fission or fusion, the nuclear binding energy can be released. This is how nuclear fission and fusion can be used to produce electricity.
For heavier elements, fusion does not release energy. But for lighter elements, such as hydrogen and helium, when two atoms combine, the resultant third atom is filled with excess energy and an extra neutron or two in its nucleus that is making it unstable.
No atom ever wants to be unstable, and so it seeks to return to the nearest point of stability by releasing all that excess. It relieves itself by tossing out the extra neutron s , with its leftover energy released as well.
In the sun, the nuclear fusion process occurs mainly between hydrogen and helium, since that is the bulk of its composition. Our sun is a medium-sized star around the midpoint of its life cycle, having formed from a cloud of gas about five billion years ago. Outside of its core, roiling layers of superheated plasma give off heat and light which travel through the abyss of space to warm all of the planets and not-quite-planets sorry, Pluto in our solar system.
Eventually, about five billion years from now, the sun will exhaust the once-ample supply of hydrogen and helium in its core by fusing it all together into heavier elements. When that happens, the sun will violently shed what remains of its outer layers and leave behind a small gaseous core of carbon and other heavy elements.
No longer massive enough to force these heavy elements to fuse, this remaining white dwarf will rest, inert, in the center of an expanding cloud of gas until it cools to become a black dwarf. In its core, the sun fuses over million tons of hydrogen every second. It takes such a great deal of energy to produce nuclear fusion that in our modern and mature universe, nuclear fusion will only occur naturally inside stars like our sun.
0コメント