Basic Fusion
Fusion reactions combine lighter atoms, such as hydrogen, together to form larger ones. Generally the reactions take place at such high temperatures that the atoms have been ionized, their electrons stripped off by the heat; thus, fusion is typically described in terms of "nuclei" instead of "atoms". Nuclei are positively charged, and repel each other due to the electrostatic force. Counteracting this is the strong force that pulls nucleons together, but only at very short ranges. Thus a fluid of nuclei will generally not undergo fusion on its own – the nuclei must be forced together before the strong force can pull them together into stable collections. The amount of energy that needs to be applied to force the nuclei together is termed the Coulomb barrier or fusion barrier energy. To create needed conditions, the fuel must be heated to tens of millions of degrees, and/or compressed to immense pressures, for a long enough time. The temperature, pressure, and time needed for any given fuel to fuse is termed the Lawson criterion. Since the criterion contains both pressure and temperature, existing approaches to practical fusion power have generally worked to raise one or another of these values.
Magnetic fusion works to heat a dilute plasma (10 ions per cm) 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 works to produce extremely high densities, 10 ions per cubic cm, about 100 times the density of lead. This causes reactions to occur extremely quickly (~1 nanosecond), which causes confinement time to be extremely short, as the heat of reactions drives the plasma outward. The $3–4 billion dollar National Ignition Facility (NIF) machine at Lawrence Livermore National Laboratory (LLNL) will be a definitive test of ICF at megajoule energy levels. Both conventional methods of nuclear fusion are nearing net energy (Q>1) levels now after many decades of research, but remain far from a practical energy-producing device.
Read more about this topic: Magnetized Target Fusion
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