Fusion Energy

Nuclear fusion is a natural universal process that creates massive amounts of energy. Considered the holy grail of energy science, nuclear fusion unlocks entirely new eras in the growth of a species. Harnessing this power is like the "invention" of fire for an already technologically advanced civilization. It is cleaner than all other forms of energy while using abundant materials in their construction and operation. The energy released from fusing atoms allows for large, permanent settlements to be built in places without sunlight or geothermal energy, and it unlocks a whole new breed of space travel. During the Discovery Age of the Celestial Grove, the time of which species were expanding to all galaxies, fusion energy made up practically all of the power generation for spacecraft and anything else in the dark depths of space.

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Principles of Fusion

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Fusion energy, of course, utilizes the natural fusion reaction. In a nutshell, small atomic nuclei -smaller than Iron - fuse together and release energy. The resulting nucleus is small in mass than the two that created it. This loss of mass is found in the energy released. The excess of energy typically comes in the form of heat imparted on the product nucleus and stray partices such as photons and neutrons. Heat extracted from the walls of the reactor or magnetic pressure from pulsating plasma can be used to produce electricity through a steam turbine or thermoelectric generator.
Atoms may electrically bind to form molecules, but the electric charge of nuclei repels each other. Electrostatic repulsion between protons is incredibly strong and is nearly impossible to overcome without astronomical amounts of energy. Stars literally have astronomical amounts of energy. They use a combination of very high core temperatures and incredible pressures to fuse hydrogen. Unfortunately, fusion reactors can't achieve stellar pressure. This means only temperature can be relied upon. The necessary amount of heat in artificial reactors puts stars to shame. Temperatures in excess of hundreds of millions of degrees compared to meager tens of millions. At reactor core temps, atomic nuclei are moving so fast that they cannot avoid each and collisions prompt the atoms to fuse.

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Principles of Reactor Design

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A fusion reactor is an incredibly sophisticated machine that represents thousands of years of continual technological innovation. For any species, they are simultaneously the gateway to a post-scarcity society, but also a potential tool for its destruction. Artificial fusion uses heat to overcome the strong nuclear force of atoms, and then extracts extra energy created by the fusion reaction. Unfortunately, the heat is tremendous and could instantly vaporize any material. Reaction plasma must be kept from any and all contact with the reactor and exhaust components. This can be done via electromagnetism that suspends the fuel (magnetic confinement), or by simply mounting less fuel away from walls or other parts (inertial confinement). Alternatively, the heat and energy could be free to escape. In which case a nuclear bomb has been built. A device that can be fit in a cargo container with the capability to level cities.
While uncontrolled reactions burn through instantly, controlled reactions are very slow and deliberate. Heat input and output, fuel injection, exhaust, and energy extraction are carefully controlled by heavy and expensive components. Uncontrolled artificial fusion, such as those in nuclear devices, are meant to happen as quickly as possible without major obstacles impeding reactions. In both cases, the vessel has to be completely self-contained. Pollutants (such as dust or ambient air) have to be thoroughly removed from reactors as they can damage the reactor vessel, make reactions unstable, or stop them entirely. Confinement magnets, if present, also have to be as pure as possible and maintained so that their magnetic flux is of the correct strength and position. Improper magnet placement or quality can prevent a reaction from ever occurring.
Controlled reactors must also receive substantial amounts of evenly applied cooling. Uneven heating and cooling can cause wear and displace components such as heaters or confinement magnets. Precise reactor temperature control is absolutely vital for the longevity and safety of the unit. If a unit uses ambient or hot superconductors then they must be kept at a precise temperature range or they will "quench" and destroy the reactor and various electrical components. This is especially a concern on magnetic pressure generators. Aneutronic reactors often rely on rhythmic pulses from fusion reactions to generate heat via the containment magnets. This means units will be undergoing temperature shifts constantly.
As reactions occur, large quantities of waste material are produced. If these products are not filtered out then they could poison a reactor and reduce its efficiency until the plasma field cools to an unsustainable temperature. These products also can't be filtered out while reactions are in progress. So the reaction will need to be stopped briefly and all plasma in the reactor will have to be vented. Viable fuel still in the gas can be extracted and reused. How waste is disposed of is up to user as it can likely be resold for other uses. Alternatively, it can just be vented to atmosphere or space.

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Important Reactions

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Deuterium-Deuterium

Deuterium fusion (DD) is a high temperature, low gain fusion reaction used mainly for the synthesis of rare hydrogen and helium isotopes. The deuterium isotope of hydrogen reacts with itself and has a 50-50 chance to produce either a low-energy neutron with helion (³H), or a proton with tritium. In both cases, the output energy is very low compared to the required temp, but the products of the reaction are valuable which makes up for the operation cost.

Deuterium-Tritium

The deuterium-tritum (DT) reaction is a low-temp, very high energy output reaction. It is easily the most efficient fusion reaction and even outperforms natural stellar hydrogen fusion. However, DT reactions emit most their energy in neutrons. These heavy particles, with no electric charge, do not interact with magnetic fields while having the highest penetrating power of all forms of radiation. Consequently, DT reactors need to be exceptionally heavy. Beyond that, atoms capturing waste neutrons can become "activated". Activated nuclei are atomically unstable and thus are radioactive themselves. This can be used to breed tritium in the chamber walls, but the walls need to be disposed of often.

Deuterium-Helion

Deuterium-helion (D³H) reactions output about as much energy as DT reactions but they occur at much higher temperature thresholds. The support infrastructure for these types of reactors tend to be more massive to create conditions for D³H reactions. A major benefit of this type of reactor is that the weight can be much lower than the weight of radiation shielding required in DT reactors. The reason why there is no need for shielding is that D³H reactions are aneutronic - they produce no neutrons. Instead, radiation is produced in the form of photons. Gamma rays are not nearly as problematic as high-energy neutrons because they have less penetration power, are unable to activate nuclei, and can be captured by the reactor's magnetic field. Harnessing photons for waste is comparatively more efficient as interactions in the reactor wall are actually controllable.

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Applications

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Electrical Generation

Fusion for power generation is one of the most popular forms of creating electricity. Second only to solar where it is available. Outside of sunlight, fusion provides power to the rest of civilization and powers almost all spacecraft. Commercial power plants most often use DT reactors for their high energy efficiency. Distasteful for spacecraft, the very heavy shielding required for the neutronic reaction of deuterium and tritium is not an issue for any static object. Spacecraft find the use of D³H reactors much more appetizing. Spacecraft fusion reactors are much smaller than their public supply counterparts, and they likely will come with thermal regeneration systems that harness as much heat as possible.
Antimatter catalyzed fusion jets are a rare breed of engine that are remarkebly energy efficient but are incredibly expensive. Instead of relying on intense compressive forces to ignite plasma in a stream, antimatter reactions provide energy. Primitive fusion engines might have used an antimatter ignition source. Even once tritium fusion was possible without a catalyst, D³H fuel still required the intensely powerful reactions of antimatter to start a reaction. Although such engines were very niche regardless. Once helion ignition was possible with simple fusion jets, catalyzed fusion jets were phased out from interstellar markets.

Rare Isotope Production

Reactors that are built solely to fuse deuterium with itself are common, just not for energy extraction. These reactors are what keep the other fusion reactors alive. Tritium and Helium-3, isotopes used in energy production and spacecraft propulsion, are quite rare in the universe with tritium itself being unstable. Thankfully, DD fusion produces equal amounts of both elements. Industrial reactors supply large volumes of rare isotopes required for all other reactors and thus keep the industry fueled.

Propulsion

Nuclear fusion for spacecraft propulsion is incredibly common. The common fusion engine breed is the fusion jet. These systems are reliable and use no moving parts while balancing thrust and fuel consumption. The design of these engines developed a lot through time, albeit always behind large reactor tech. Magnetic compression can achieve the necessary temperatures for fusion, but on a smaller scale. D³H fusion wasn't possible for a while even when it could be fused in large toroidal reactors. Eventually they replaced DT jets for weight savings and D³H became dominant.

Nuclear Weapons

One of the earliest applications of nuclear fusion, especially by the more aggressive advanced species, are those of nuclear weapons. Hydrogen gas placed in warhead can be used to boost a thermonuclear bomb's yield, or be used as the main stage in a lighting bomb. Lightning bombs are common warheads placed in anti-ship missiles because even near-miss detonations can severely damage a target. EM and electron emissions induce electric currents that can readily create electric arcs, hence the bomb's name. These types of warheads can't be detonated using traditional methods. A fissile nuclear core is always needed to produce the necessary conditions for fusion. Nuclear bombs utilize the inertial confinement principle of fusion and their own unique method of instigating fusion by exploding a uranium core.

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Confinement Methods

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Magnetic Confinement

Magnetic confinement fusion controls plasma via electromagnetism. Superconducting magnets shape and control the movement of plasma in a reactor. There are multiple are multiple confinement shapes such as linear or toroidal. Linear confinement is highly useful in propulsion. The design compresses fuel as it travels into the reactor and controls exhaust to prevent it from damaging the ship. The shape is produced via rows of magnets arranged in rings around the path of hydrogen and fusion products. Toroidal confinement arranges the plasma in a donut shape. They are very common in power plant reactors as it allows fuel to more readily mix. Adequate mixing increases efficiency. Unfortunately, such a shape is not as stable when compared to other methods of magnetic confinement so it requires massive superconducting magnets. Typically there is a solenoid in the middle, confinement rings around the fusion chamber, and additional stabilization around the confinement rings.

Inertial Confinement

Inertial confinement uses quick pulses of energy to confine plasma in a small point and produce fusion conditions. This method of confinement is now only used in explosive devices, but was used in early fusion reactors. Methods like this lack any magnetic field instabilities, because well, there are no magnetic fields. Confinement is accomplished via the inertia of moving atoms as they are forced into an incredibly small volume. Power to drive the fusion fuel comes from radiation emmisions such as laser light or x-rays. Energy is focused evenly around the fuel, causing the surface to explode in all directions which compresses the core to supercriticality.

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Heating and Ignition Methods

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Ionic Heating

Ionic heating uses two electromagnetic frequencies to increase the energy of electrons and atomic nuclei in a plasma field. Radio waves tuned to the resonance frequencies of electrons and the fusion isotopes interact with the particles in the plasma and impart energy on them. This energy gets transferred to others via collisions in the reactor chamber thus heating the plasma. This process may be slow, but it is decently efficient. Ionic heating is the primary heating method for toroidal reactors and ignition method for linear reactors.

Magnetic Compression

Magnetic compression heats plasma via cyclic compression of fusion fuel in linear reactors. Energy is introduced into the system via superconducting electromagnets. Slight pulses of energy through the magnets alter the magnetic field, squeezing nuclei and electrons, forcing them to collide which in turns produces large amounts of heat. The squeezing action also forces the gas to flow very quickly. The compression gradually increases as plasma temperature increases until it reaches near supercriticality.

Antimatter Catalyst Reaction

Antimatter reactions are incredibly energetic. All of the potential energy of electrons, protons, and their polar opposites are converted into very light subatomic particles and kinetic energy. So much energy in fact, that these particles travel very near to the speed of light. Energy from the particles can be used to super-heat hydrogen gas and very quickly instigate fusion processes.

Inertial Compression

Inertial compression is the technical term for how inertial confinement fusion actually works. A driver, such as a laser, electron beam, or any other source that could cause a radiation capsule to emit X-rays evenly radiates a small pellet of frozen fuel. Evenly distributed energy is key here. Uneven distribution will most likely just cause the fuel to explode without actually fusing. Radiation capsules help smooth high mode laser asymmetries by taking the high mode energy and simply emitting X-rays evenly across its entire surface. Nuclear bombs also use this method to ignite fusion cores with the interior wall also being a radiation capsule, but instead of a laser power source, it is an exploding uranium core producing energy for fusion.