nuclei to react in the disassembly time. At DT liquid density this would require a sphere 10-30 cm in radius and a huge release of energy. To keep the energy to initiate fusion small and the energy released manageable, a small sphere (weighing a few milligrams) must be used. This requires compression. The areal density rises during compression (at fixed mass, ρR ∞ R–2) until it reaches a substantial fraction of fusion-relevant levels (of order 3-7 g/cm2). For 3 mg of solid/liquid DT an increase in the density of order one thousand is needed.
In most ICF schemes, a shell of cryogenic deuterium and tritium fuel is accelerated inward and compressed by the reaction force from an ablating outer shell. The ablating outer shell is heated either by direct laser irradiation (called “direct drive”) or by the X-rays produced by heating a high-Z enclosure (hohlraum) that surrounds the fuel target (called “indirect drive”). The hohlraum in indirect drive schemes may be driven (heated) by lasers, particle beams, or pulsed power systems. During compression the fuel is kept as cold as possible to minimize the work needed for compression. At stagnation, a central hot spot enclosing a few percent of the total mass is heated and ignited. Ignition occurs when the alpha-particle