Magnetized target fusion
Magnetized target fusion (MTF) is a fusion power concept that combines features of magnetic confinement fusion (MCF) and inertial confinement fusion (ICF). Like the magnetic approach, the fusion fuel is confined at lower density by magnetic fields while it is heated into a plasma. As with the inertial approach, fusion is initiated by rapidly squeezing the target to greatly increase fuel density and temperature. Although the resulting density is far lower than in ICF, it is thought that the combination of longer confinement times and better heat retention will let MTF operate, yet be easier to build. The term magneto-inertial fusion (MIF) is similar, but encompasses a wider variety of arrangements. The two terms are often applied interchangeably to experiments.
In fusion, lighter atoms are fused to make heavier atoms. The easiest fuels to do this with are isotopes of hydrogen. Generally these reactions take place inside a plasma. A plasma is a heated gas, where all the electrons have been stripped away; the gas has been fully ionized. The ions are positively charged, so they repel each other due to the electrostatic force. Fusion occurs when two ions collide at high energy, allowing the strong force to overcome the electrostatic force at a short distance. The amount of energy that needs to be applied to force the nuclei together is termed the Coulomb barrier or fusion barrier energy. For fusion to occur in bulk plasma, it must be heated to tens of millions of degrees and compressed at high pressures, for a sufficient amount of time. Together, this is known as the "Triple Product". Fusion research focuses on reaching the highest triple product possible.
Magnetic fusion works to heat a dilute plasma (1014 ions per cm3) to high temperatures, around 20 keV (~200 million C). Ambient air is about 100,000 times denser. To make a practical reactor at these temperatures, the fuel must be confined for long periods of time, on the order of 1 second. The ITER tokamak design is currently being built to test the magnetic approach with pulse lengths up to 20 minutes. Inertial fusion attempts to produce much higher densities, 1025 ions per cubic cm, about 100 times the density of lead. This causes reactions to occur extremely quickly (~1 nanosecond), which allows confinement time to be extremely short, as the heat of reactions drives the plasma outward. The $3–4 billion National Ignition Facility (NIF) machine at Lawrence Livermore National Laboratory (LLNL) will be a definitive test of ICF at megajoule energy levels. As of 2015 both conventional methods of nuclear fusion are nearing net energy (Q>1) levels after many decades of research, but remain far from a practical energy-producing device.
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