FUSION - THE ULTIMATE ENERGY SOLUTION
In contrast to nuclear fission, where a large, unstable nucleus is divided into smaller parts, fusion occurs when the nuclei of a lightweight element, usually hydrogen, collide with sufficient force to combine and create a heavier element. During this process, some mass is released and converted into energy, as explained by Albert Einstein's famous equation: E = mc2. Fusion energy is abundant in the universe, with the sun and other stable stars being fuelled by thermonuclear fusion. However, the task of initiating and controlling a self-sustaining fusion reaction and harnessing its power is widely considered the most challenging engineering endeavor ever undertaken by humans.
In order to merge hydrogen nuclei, engineers working on terrestrial reactors must discover methods to overcome the mutual repulsion of positively charged ions, known as the Coulomb force, and bring them close together to form bonds through the strong nuclear force. Many techniques involve extremely high temperatures, much hotter than the sun's core temperature of 15 million °C, at which matter can only exist as plasma. In this state, electrons separate from their atomic nuclei and move freely within cloud-like gas formations.
A high-energy-density plasma is known for its instability and difficulty to control. It moves and struggles to break free, moving towards the boundaries of the containment field where it rapidly cools down and dissipates. The main challenges in fusion energy revolve around the plasma, such as how to heat it, contain it, shape it, and control it. There are two main methods: magnetic confinement and inertial confinement. Magnetic-confinement reactors aim to maintain the plasma's stability within a reactor using strong magnetic fields. In contrast, inertial-confinement approaches utilize lasers to compress and collapse the plasma quickly enough to keep it in place for the reaction to initiate.
Magnetic-Confinement Fusion (MCF)
The concept involves using strong magnetic fields to contain and heat plasma in a torus-shaped device known as a tokamak. Over 200 operational tokamaks have been constructed since the 1960s, and the basic principles of plasma physics are widely understood.
Inertial-Confinement Fusion (ICF)
The concept involves using intense laser or ion beams to compress a small fuel pellet, creating high densities. This generates a shock wave that quickly heats the plasma. However, there are concerns about the forces acting on the fuel pellet, which can cause laser-plasma instabilities. These instabilities produce high-energy electrons that scatter and heat the fuel, preventing successful fusion.
Magnetized Target Fusion (MTF)
A hybrid method known as magneto-inertial fusion (MIF) combines magnetic fields to contain a less dense plasma, similar to magnetic-confinement fusion, and then applies techniques like lasers or pistons to heat and compress it, similar to inertial-confinement fusion. However, the challenge lies in the fact that scientists have not yet achieved a high enough plasma density or maintained it for a sufficient duration to achieve significant fuel fusion.
Field-Reversed Configuration (FRC)
The concept behind an FRC reactor involves the confinement of plasma within its own magnetic field by inducing an electric current in a cylindrical plasma. Inside the reactor, the direction of the magnetic field is reversed in relation to an externally applied field due to eddy currents in the plasma. To achieve this, the reactor utilizes plasma guns to accelerate two plasmas towards each other, which are then heated using particle beams. However, it is important to note that despite FRC machines being more stable compared to other magnetic-confinement methods, no laboratory has successfully demonstrated a functional FRC reactor capable of generating plasma with sufficient density and stability.
Energy for the Future
Significant advancements are required in fusion materials, magnets, and heating and current drive actuators. Technology progress is necessary to withstand the extreme conditions that will be encountered in future fusion reactors and to effectively utilize fusion energy and produce fuel. Alongside advancing research on current facilities like linear plasma devices and in-pile fission irradiation, there is a need to enhance resources to promptly address critical design issues for fusion power plants.
Immediate action is necessary due to the long-time frames involved in facility development and research. It is also important to increase investment in theory and simulation to support research on these facilities. Emphasis is placed on developing materials and components for plasma-facing, structural, and functional purposes, as well as fusion blanket and fuel cycle elements for a fusion plant. Furthermore, advancements in diagnostics are needed to better understand how materials interact with the fusion environment.
Magnetic fusion configurations must include magnets, thus it is desirable to produce magnets with greater fields, operating temperatures, and dependability. It is also desirable to develop magnets with simplified manufacturing processes and lower production costs. Each of these elements helps fusion plants operate more efficiently and/or more affordable. Government programs should complement and, when possible, work in conjunction with private business initiatives that have made substantial strides in developing the required magnet technology, especially high-temperature superconducting magnets.
New materials are required for the launching structures used in radio-frequency plasma heating and current drive actuators. These materials need to be able to withstand the harsh neutron and plasma environment, have built-in steady-state cooling, and offer improved long-pulse reliability. Furthermore, it is essential to develop more efficient sources, transmission systems, and plasma coupling methods to enhance the competitiveness of fusion plants. The international effort to develop fusion energy is heavily dependent on the development of materials and technology that can withstand the unique conditions of a fusion reactor.
THE SR55 REACTOR PROJECT
The majority of devices currently in existence are tokamak reactors, which have been extensively studied and show the closest resemblance to ignition conditions.
In order to create a magnetic field enclosure for a tokamak reactor, three magnetic fields are needed. The first is a circular field generated by external coils. The second is the field created by a current flowing through the plasma, which causes the field lines to become helical. This twisting of the field lines and the formation of magnetic surfaces are crucial for containing the plasma. The third field, which is vertical, serves to stabilize the position of the current within the plasma.
The plasma current in a reactor is typically created by a transformer coil. As a result, the reactor operates in pulsed mode because the transformer can only generate an increasing current in the primary winding for a limited period of time, allowing a current to be induced in the plasma. Afterwards, the transformer needs to be discharged and the current restarted. To enable continuous operation in a future nuclear fusion power plant, researchers are exploring alternative methods of generating a continuous current, such as using high-frequency waves.
As the temperature rises, materials undergo a series of changes, transitioning from solid to liquid and then to gas. When the temperature continues to increase, a new state of matter called plasma is obtained. Plasma is known as the fourth aggregate state of matter.
Plasma is formed when the atoms in a gas break apart into their individual parts, such as electrons and nuclei. Examples of plasma include the glowing column in a neon tube, an electric spark, or the streak of lightning. Unlike regular gases, plasma has unique properties. It can conduct electricity and its movement can be controlled by electric and magnetic fields. This quality is utilized in fusion devices, where the hot plasma is contained within a so-called magnetic field cage to prevent contact with the surrounding walls.
Similar to a wood fire, a fusion fire also requires specific ignition conditions before it can combust on its own.
To sustain a burning plasma, a significant amount of particles must interact with each other frequently and intensely. For this to happen, the magnetic field enclosure must confine a satisfactory quantity of particles, and their thermal energy should not dissipate rapidly into the surrounding environment. Therefore, the plasma's density, temperature, and thermal insulation must meet the following criteria:
- A temperature of a minimum of 100 million degrees in the plasma;
- A two-second energy confinement period - this thermal insulation measurement indicates the length of time until the thermal energy injected into the plasma by heating systems is again lost;
- A plasma density of over 1000 particles per cubic centimeter, which is approximately 250,000 times thinner than the air mantle of the earth. Due to its extremely low density, burning fusion plasma offers a power density that is hardly greater than that of a standard light bulb, despite its tremendous temperature.
A fusion plasma cannot be directly stored in material containers because of its great temperature. The thin gas would instantly re-cool upon contact with a wall. Magnetic fields are used to confine and thermally insulate the fuel, overcoming the situation by keeping it away from the vessel walls.
Charged particles, including ions and electrons, are propelled into helical and circular orbits around magnetic field lines by a magnetic field. Therefore, the particles are connected to the field lines. They may, however, move around freely along the lines' longitudinal direction. Therefore, it is attainable to contain plasma and keep it away from material barriers in a cage with a magnetic field that is suitably formed.
The plasma must be heated externally before ignition. Several approaches may be used for this:
When an electric current is applied to the conductive plasma, heat is produced due to the resistance, similar to how a hotplate heats up. However, this method is mainly effective for initial heating because the resistance decreases as the temperature rises.
When electromagnetic waves with the right frequency are directed towards the plasma, the particles in the plasma absorb energy from the wave's field and pass it on to other particles by colliding with them. The ions and electrons move in circular paths along the magnetic field lines, which creates favorable resonances. The ions orbit at frequencies ranging from 10 to 100 megahertz, while the electrons, which are lighter, orbit at frequencies ranging from 60 to 150 gigahertz.
Neutral Particle Heating
High-energy particles that are introduced into the plasma transfer their energy to the plasma particles by colliding with them and heating them. In a neutral particle injector, ions are initially generated in an ion source and then accelerated using an electric field. In order to enable the fast ions to enter the plasma without being impeded by the magnetic field enclosure, they need to be re-neutralized. Once neutralized, these particles are directed into the plasma where they collide with the plasma particles and transfer their energy to them.