Tokamak: Making Fusion possible.
Table of contents:
- Recap
- The chemistry behind Fusion
- What is required to attain Fusion on Earth?
- ITER- The components of the machine and the purpose they serve
- The magnets
- Toroidal Field Systems
- Poloidal Field Systems
- Central Solenoid
- Correction coils
- Magnet Feeders
- In-vessel coils
- The vacuum vessel
- In-wall shielding
- Thermal shielding
- The blanket
- First wall panels
- Shield blocks
- Divertor
- Divertor Targets
- Cryostat
- The tokamak: How do all the components work together to provide Fusion?
- How is this energy harvested?
- When will the ITER tokamak be up and running?
Recap
Fusion is a thermonuclear process which occurs at millions of degrees. In these extreme temperatures, the electrons split from the atom creating a plasma where nuclei and electrons bounce around freely. Nuclei are all positively charged, meaning they tend to repel each other. In order to overcome this repulsion, the particles move very fast and collide. This collision at a high speed causes the nuclei to merge and fuse, releasing energy in the process. It is this energy release that scientists are trying to achieve to create a new generation of powerplants.
The Chemistry Behind Fusion
We know that Fusion energy is released when two nuclei merge and fuse to make a heavier nucleus. Now let’s take a look at Fusion in more detail. Deuterium and tritium are isotopes of hydrogen, which are used in Fusion reactions. This is the current best fuel because:
- It is easy to produce and store and is available in mass quantities (making up more than 70% of the mass of the entire universe)
- The deuterium-tritium fuel releases more energy, as well as reaches fusion conditions at considerably lower temperatures than other fuels
All isotopes of hydrogen have one proton. In addition to that, deuterium also has one neutron and tritium has two neutrons. Their ion masses are heavier than protium, the isotope of hydrogen with no neutrons. The fusion of deuterium and tritium creates a helium nucleus (which has two protons and two neutrons) and also releases an energetic neutron along with great amounts of energy.
What is required to attain Fusion on Earth?
To achieve fusion in a laboratory, three conditions must be fulfilled:
- High temperatures (to provoke high-energy collisions)
- Sufficient plasma particle density (to increase the likelihood that collisions occur)
- Sufficient confinement time (to hold the plasma, which has a natural tendency to expand, within a defined volume).
A tokamak is a machine that confines plasma using a donut-shaped magnetic field called a torus. This machine is capable of meeting the conditions listed above and achieving fusion.
ITER- the components of the machine and the purpose they serve
ITER will be the world’s largest tokamak, with a plasma radius (R) of 6.2 m and a plasma volume of 840 m³. Its magnet system will be the largest and most integrated superconducting magnet system ever built, weighing 23000 tonnes, reaching temperatures up to 150 million degrees Celsius, and providing an output of 500 megawatts of electricity.
Let’s break this down to understand what a tokamak is and look at the different components that make up this machine. The main components of the Tokamak are: The magnets, the vacuum vessel, the blanket, the divertor and the cryostat. We must understand what purpose all these components serve in the Tokamak, to get a bigger, final and detailed understanding of how fusion occurs, which is explained at the very end.
THE MAGNETS
The ITER magnet system will be the largest superconducting magnet system ever built, with a combined stored magnetic energy of 51 Gigajoules. These magnetic fields will initiate, confine, shape and control the ITER plasma.
The main parts of the ITER magnet are:
- Toroidal Field Systems
- Poloidal Field Systems
- Central Solenoid
- Correction coils
- Magnet feeders
- In-vessel coils.
Toroidal Field Systems
Eighteen “D”-shaped toroidal field magnets are placed around the vacuum vessel, wound in layers of spiraled conductor embedded in radial plates and encased in large stainless steel structures. They produce a magnetic field whose primary function is to confine the plasma particles. Weighing 360 tonnes each, and measuring 9 x 17 m, these magnets are the largest components of the ITER machine.
The toroidal field coils are designed to produce a total magnetic energy of 41 gigajoules and a maximum magnetic field of 11.8 tesla, with the total weight of 3,400 tonnes.
Poloidal Field Systems
Six ring-shaped poloidal field coils are situated outside of the toroidal field magnet structure to shape the plasma and contribute to its stability by keeping it away from the walls. The poloidal field coils are designed to produce a total magnetic energy of four gigajoules and a maximum magnetic field of sixtesla. The largest coil has a diameter of 24 meters; the heaviest is 400 tonnes.
Central Solenoid
The central solenoid is referred to as the “backbone” of ITER’s magnet system because it allows a powerful current to be induced the plasma, which is maintained during long plasma pulses. It is thirteen meters tall (eighteen meters, with structure), four meters wide and one thousand tonnes, made of six independent coil packs wound from niobium-tin superconducting cable.
Stored magnetic energy of 6.4 GJ in the central solenoid will initiate and sustain a plasma current of fifteen MA for durations of 300–500 seconds, reaching the maximum field of thirteen tesla will be reached in the center of the stacked modules.
In order to maintain the structural integrity of the central solenoid assembly, the support structure will have to withstand forces in the range of 60 meganewtons, or over 6,000 tonnes of force. In comparison, the force behind the thrust of a Space Shuttle lift off is about 30 meganewtons.
Correction Coils
There are eighteen superconducting correction coils inserted between the toroidal and poloidal field coils, which will compensate for any minor field errors. These coils measure eight meters in width, which causes particular challenges for assembly and installation.
Magnet Feeders
For the ITER tokamak, magnet feeders are very important because they regulate the cryogenic liquids to cool and control the temperature of the magnets, connecting the magnets to their power supplies. In total, 31 superconducting feeders will relay electrical power and cryogens through the warm-cold barrier to the ITER magnets. Superconducting busbars, made out of steel conduit containing niobium-titanium superconductor cable, are designed to absorb the large temperature variations during the cool-down of the machine.
In-vessel coils
Two non-superconducting coil systems inside of the ITER vacuum vessel provide additional plasma control capabilities. Two vertical stability coils installed above and below the machine’s mid-plane provide fast vertical stabilization of the plasma.
THE VACUUM VESSEL
The vacuum vessel is the main component of the ITER tokamak. All fusion reactions will take place inside this vessel, which is a sealed steel container and a first safety containment barrier. In its donut-shaped chamber, or torus, the plasma particles spiral around continuously without touching the walls.
The ITER vacuum vessel, with an interior volume of 1,400 m³, will measure 19.4 meters across (outer diameter), 11.4 meters high, and weigh approximately 5,200 tonnes.
The vacuum vessel provides a high-vacuum environment for the plasma, and improves radiation shielding and plasma stability. The vessel also provides a confinement barrier for radioactivity, and provides support for in-vessel components such as the blanket and the divertor, which will be explained later in this article. Cooling water circulates outside the vessel’s double steel walls, which will remove the heat generated during the fusion reaction.
44 ports or openings in the vacuum vessel provide for:
- Accessing remote handling operations (which can be used to conduct inspections, or repair any of the tokamak components in the activated area, which otherwise would be impossible)
- Diagnostics (to provide the measurements necessary to control, evaluate and optimize plasma performance in ITER)
- Heating (through neutral beam injection and two sources of high-frequency electromagnetic waves)
- Vacuum (to eliminate all sources of organic molecules prior to starting the fusion reaction, that would otherwise be broken up in the hot plasma)
The inner surfaces of the vessel will have a blanket module lining to provide shielding from the high-energy neutrons produced by the fusion reactions.
In wall shielding
Modular blocks, weighing up to 500 kg each, will occupy approximately 55 percent of the space between the double walls of the vacuum vessel.
The modular blocks are made of stainless steel, which is borated and ferromagnetic. The blocks will provide shielding from neutron radiation for components situated outside of the vacuum vessel (such as the magnets) and contribute to plasma performance by limiting toroidal field ripple.
Thermal shielding
Two layers of thermal shielding are interposed between the vacuum vessel and the cryostat, which will be explained later in this article. The shielding is important because it minimizes heat loads transferred by thermal radiation and conduction from warm components to other components (such as the magnets).
The thermal shield consists of stainless steel panels that are cooled by helium gas flowing inside of a cooling tube welded on the panel surface. During plasma operation, the temperature of the helium gas ranges from 80 K and 100 K.
One layer of thermal shield will be installed between the vacuum vessel and the superconducting magnets; another will be installed between the magnets and the cryostat. The thermal shield covers a surface area of approximately 10,000 m². Once assembled, it will stand 25 meters at its highest point.
THE BLANKET
The blanket modules completely cover the inner walls of the vacuum vessel, protecting the steel structure and the superconducting toroidal field magnets from the heat produced by the fusion reactions. As the neutrons are slowed in the blanket, their kinetic energy is transformed into heat energy and collected by the water coolant. In a fusion power plant, this energy will be used for electrical power production.
The ITER blanket covers a surface of 600 m² and is one of the most critical and technically complex components in ITER because it directly faces the plasma along with the divertor.
Beryllium has been chosen as the element to cover the first wall due to its unique physical properties. High copper and stainless steel will make up the rest of the blanket modules.
First Wall Panels
The first wall panels are the front-facing elements of the blanket, which are designed to sustain the intense heat of millions of degrees of the plasma. These are made of beryllium tiles bonded with a copper alloy and 316L (N) stainless steel.
The horizontal lines on the wall of the image above represent the first wall panels, which are attached to a structural beam that serves as the backbone of each panel and also houses the cooling water channels. The panels will also be attached to the structural shield block by special studs.
Shield Blocks
The shield blocks provide nuclear shielding for the vacuum vessel and coil systems as well as support for the first wall panels. Cooling water will run to and from the shield blocks through manifolds and branch pipes to remove the high heat load expected during ITER operation.
Each shield block will be bolted directly to the vacuum vessel, which will be challenging to assemble, as they must be aligned with tolerances of approximately 10 mm globally and with nominal gap requirements between adjacent modules (both vertically and horizontally) of 4 mm.
DIVERTOR
The divertor is situated at the bottom of the vacuum vessel and extracts heat and ash produced by the fusion reaction. It also protects the surrounding walls from thermal and neutronic loads and minimizes plasma contamination.
The divertor has a supporting structure in stainless steel and three plasma-facing components: the inner vertical targets, the outer vertical target and the dome.
The inner and outer vertical targets are positioned at the intersection of magnetic field lines where particle bombardment will be particularly intense in ITER. As the high-energy plasma particles strike the vertical targets, their kinetic energy is transformed into heat which is removed by active water cooling.
Divertor Targets
- The plasma-facing components of the ITER divertor will be exposed to ten times the higher amounts of heat than that of a spacecraft re-entering Earth’s atmosphere, therefore is is manufactured with extreme care to withstand these temperatures.
- Sections of the plasma-facing components undergo testing in a high-heat flux test facility specially conceived and built for ITER, to demonstrate that the components can withstand the demanding thermal conditions.
CRYOSTAT
The ITER cryostat is the largest stainless steel high-vacuum pressure chamber ever built (16,000 m³) and it provides a high vacuum, ultra-cool environment for the ITER vacuum vessel and the superconducting magnets.
The cryostat is nearly 30 meters wide, with an internal diameter of 28 meters. It is the largest components in the ITER tokamak and has the two largest poloidal field coils. It is manufactured from stainless steel, and weighs 3,850 tonnes.
The cryostat has 23 penetrations to allow access for maintenance. It also has over 200 penetrations, some of which are as large as four meters in size that provide access for cooling systems, magnet feeders, auxiliary heating, diagnostics, and the removal of blanket sections and parts of the divertor.
THE TOKAMAK: HOW DO ALL THE COMPONENTS WORK TOGETHER TO PROVIDE FUSION ENERGY?
The heart of the Tokamak is its donut-shaped vacuum chamber, in which plasma (a hot, electrically charged gas) is formed under the influence of extreme heat and pressure. The massive magnetic coils around the vessel are used to shape and control the charged particles of plasma. This important property is used to confine the hot plasma away from the vessel walls to prevent meltdowns of the equipment.
To start the process, air and other impurities are evacuated from the chamber. Then, the magnet systems are charged up and the gaseous fuel is introduced, which will help to confine and control the plasma. A powerful electrical current is then run through the vessel, causing the gas to break down electrically, becoming ionized and causing the electrons to separate from their nuclei, forming plasma.
The plasma constantly heats up as the particles become energized and begin to collide. These extreme high temperatures (between 150 and 300 million °C) cause the nuclei to overcome their natural electromagnetic repulsion to fuse, releasing huge amounts of energy.
How is this energy Harvested?
The helium nucleus carries an electric charge, which will be undergo the magnetic fields of the tokamak and remain confined with the plasma, contributing to its heating. However, about 80 percent of the created energy is carried away from the plasma by the neutron, which is unaffected by magnetic fields due to the absence of the electric charge. The neutrons will be absorbed by the surrounding walls of the tokamak, where their kinetic energy will be transferred to the walls as heat.
In the International Thermonuclear Experimental Reactor (ITER for short), the heat will be captured by cooling water circulating the vessel walls and will be spread through cooling towers. In the near future, the heat will be used to produce steam, causing the turbines and alternators to rotate, producing electricity.
When will the ITER tokamak be up and running?
ITER is scheduled to be up and running by the year 2025. It is a revolutionary piece of technology and will start a new generation of powerplants, providing energy for billions, without any greenhouse gas emissions or environmental impacts.
In reference to DOE explains…Deuterium-Tritium Fusion Reactor Fuel, inspired by Office of Science.
In reference to What is a Tokamak? inspired by ITER.org.
In reference to Magnets, inspired by ITER.org.
In reference to Vacuum Vessel, inspired by ITER.org.
In reference to Blanket, inspired by ITER.org.
In reference to Divertor, inspired by ITER.org.
In reference to Cryostat, inspired by ITER.org.
In reference to Machine, inspired by ITER.org.
In reference to Making it Work, inspired by ITER.org.
