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Henry Parker
Henry Parker

Fusion [REPACK]


The National Network of Fusion Centers is the hub of much of the two-way intelligence and information flow between the federal government and our State, Local, Tribal and Territorial (SLTT) and private sector partners. The fusion centers represent a shared commitment between the federal government and the state and local governments who own and operate them. Individually, each is a vital resource for integrating information from national and local sources to prevent and respond to all threats and hazards. The enhanced collaboration between the federal government, SLTT and private sector partners represents the new standard through which we view homeland security.




Fusion



Fusion power is a proposed form of power generation that would generate electricity by using heat from nuclear fusion reactions. In a fusion process, two lighter atomic nuclei combine to form a heavier nucleus, while releasing energy. Devices designed to harness this energy are known as fusion reactors. Research into fusion reactors began in the 1940s, but as of 2022, only one design, an inertial confinement laser-driven fusion machine at the US National Ignition Facility, has in any sense produced a positive fusion energy gain factor, i.e. more power output than input.[1][2][3]


Fusion processes require fuel and a confined environment with sufficient temperature, pressure, and confinement time to create a plasma in which fusion can occur. The combination of these figures that results in a power-producing system is known as the Lawson criterion. In stars, the most common fuel is hydrogen, and gravity provides extremely long confinement times that reach the conditions needed for fusion energy production. Proposed fusion reactors generally use heavy hydrogen isotopes such as deuterium and tritium (and especially a mixture of the two), which react more easily than protium (the most common hydrogen isotope), to allow them to reach the Lawson criterion requirements with less extreme conditions. Most designs aim to heat their fuel to around 100 million degrees, which presents a major challenge in producing a successful design.


As a source of power, nuclear fusion is expected to have many advantages over fission. These include reduced radioactivity in operation and little high-level nuclear waste, ample fuel supplies, and increased safety. However, the necessary combination of temperature, pressure, and duration has proven to be difficult to produce in a practical and economical manner. A second issue that affects common reactions is managing neutrons that are released during the reaction, which over time degrade many common materials used within the reaction chamber.


Fusion researchers have investigated various confinement concepts. The early emphasis was on three main systems: z-pinch, stellarator, and magnetic mirror. The current leading designs are the tokamak and inertial confinement (ICF) by laser. Both designs are under research at very large scales, most notably the ITER tokamak in France, and the National Ignition Facility (NIF) laser in the United States. Researchers are also studying other designs that may offer cheaper approaches. Among these alternatives, there is increasing interest in magnetized target fusion and inertial electrostatic confinement, and new variations of the stellarator.


Fusion reactions occur when two or more atomic nuclei come close enough for long enough that the nuclear force pulling them together exceeds the electrostatic force pushing them apart, fusing them into heavier nuclei. For nuclei heavier than iron-56, the reaction is endothermic, requiring an input of energy.[4] The heavy nuclei bigger than iron have many more protons resulting in a greater repulsive force. For nuclei lighter than iron-56, the reaction is exothermic, releasing energy when they fuse. Since hydrogen has a single proton in its nucleus, it requires the least effort to attain fusion, and yields the most net energy output. Also since it has one electron, hydrogen is the easiest fuel to fully ionize.


An atom loses its electrons once it is heated past its ionization energy. An ion is the name for the resultant bare nucleus. The result of this ionization is plasma, which is a heated cloud of ions and free electrons that were formerly bound to them. Plasmas are electrically conducting and magnetically controlled because the charges are separated. This is used by several fusion devices to confine the hot particles.


A reaction's cross section, denoted σ, measures the probability that a fusion reaction will happen. This depends on the relative velocity of the two nuclei. Higher relative velocities generally increase the probability, but the probability begins to decrease again at very high energies.[5]


The Lawson criterion shows how energy output varies with temperature, density, speed of collision for any given fuel. This equation was central to John Lawson's analysis of fusion working with a hot plasma. Lawson assumed an energy balance, shown below.[6]


In contrast, inertial confinement systems approach useful triple product values via higher density, and have short confinement intervals. In NIF, the initial frozen hydrogen fuel load has a density less than water that is increased to about 100 times the density of lead. In these conditions, the rate of fusion is so high that the fuel fuses in the microseconds it takes for the heat generated by the reactions to blow the fuel apart. Although NIF is also large, this is a function of its "driver" design, not inherent to the fusion process.


Multiple approaches have been proposed to capture the energy that fusion produces. The simplest is to heat a fluid. The commonly targeted D-T reaction releases much of its energy as fast-moving neutrons. Electrically neutral, the neutron is unaffected by the confinement scheme. In most designs, it is captured in a thick "blanket" of lithium surrounding the reactor core. When struck by a high-energy neutron, the blanket heats up. It is then actively cooled with a working fluid that drives a turbine to produce power.


Another design proposed to use the neutrons to breed fission fuel in a blanket of nuclear waste, a concept known as a fission-fusion hybrid. In these systems, the power output is enhanced by the fission events, and power is extracted using systems like those in conventional fission reactors.[11]


Designs that use other fuels, notably the proton-boron aneutronic fusion reaction, release much more of their energy in the form of charged particles. In these cases, power extraction systems based on the movement of these charges are possible. Direct energy conversion was developed at Lawrence Livermore National Laboratory (LLNL) in the 1980s as a method to maintain a voltage directly using fusion reaction products. This has demonstrated energy capture efficiency of 48 percent.[12]


A deep reinforcement learning system has been used to control a tokamak-based reactor. The AI was able to manipulate the magnetic coils to manage the plasma. The system was able to continuously adjust to maintain appropriate behavior (more complex than step-based systems). In 2014, Google began working with California-based fusion company TAE Technologies to control the Joint European Torus (JET) to predict plasma behavior.[49] DeepMind has also developed a control scheme with TCV.[50]


The diagnostics of a fusion scientific reactor are extremely complex and varied.[57] The diagnostics required for a fusion power reactor will be various but less complicated than those of a scientific reactor as by the time of commercialization, many real-time feedback and control diagnostics will have been perfected. However, the operating environment of a commercial fusion reactor will be harsher for diagnostic systems than in a scientific reactor because continuous operations may involve higher plasma temperatures and higher levels of neutron irradiation. In many proposed approaches, commercialization will require the additional ability to measure and separate diverter gases, for example helium and impurities, and to monitor fuel breeding, for instance the state of a tritium breeding liquid lithium liner.[58] The following are some basic techniques.


Electrostatic confinement fusion devices use electrostatic fields. The best known is the fusor. This device has a cathode inside an anode wire cage. Positive ions fly towards the negative inner cage, and are heated by the electric field in the process. If they miss the inner cage they can collide and fuse. Ions typically hit the cathode, however, creating prohibitory high conduction losses. Fusion rates in fusors are low because of competing physical effects, such as energy loss in the form of light radiation.[76] Designs have been proposed to avoid the problems associated with the cage, by generating the field using a non-neutral cloud. These include a plasma oscillating device,[77] a magnetically shielded-grid,[78] a penning trap, the polywell,[79] and the F1 cathode driver concept.[80]


The reactant neutron is supplied by the D-T fusion reaction shown above, and the one that has the greatest energy yield. The reaction with 6Li is exothermic, providing a small energy gain for the reactor. The reaction with 7Li is endothermic, but does not consume the neutron. Neutron multiplication reactions are required to replace the neutrons lost to absorption by other elements. Leading candidate neutron multiplication materials are beryllium and lead, but the 7Li reaction helps to keep the neutron population high. Natural lithium is mainly 7Li, which has a low tritium production cross section compared to 6Li so most reactor designs use breeder blankets with enriched 6Li.


The neutron flux expected in a commercial D-T fusion reactor is about 100 times that of fission power reactors, posing problems for material design. After a series of D-T tests at JET, the vacuum vessel was sufficiently radioactive that it required remote handling for the year following the tests.[85]


This reaction is also common in research. The optimum energy to initiate this reaction is 15 keV, only slightly higher than that for the D-T reaction. The first branch produces tritium, so that a D-D reactor is not tritium-free, even though it does not require an input of tritium or lithium. Unless the tritons are quickly removed, most of the tritium produced is burned in the reactor, which reduces the handling of tritium, with the disadvantage of producing more, and higher-energy, neutrons. The neutron from the second branch of the D-D reaction has an energy of only 2.45 MeV (0.393 pJ), while the neutron from the D-T reaction has an energy of 14.1 MeV (2.26 pJ), resulting in greater isotope production and material damage. When the tritons are removed quickly while allowing the 3He to react, the fuel cycle is called "tritium suppressed fusion".[86] The removed tritium decays to 3He with a 12.5 year half life. By recycling the 3He decay into the reactor, the fusion reactor does not require materials resistant to fast neutrons. 041b061a72


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