target in order to compress it. In an H-bomb, this is provided by a nuclear fission explosive. In non-fission devices, energy sources include laser and particle beams. The target is a small spherical pellet containing a few milligrams of fusion fuel, typically a mix of deuterium (D) and tritium (T), as this composition has the lowest ignition temperature. Multiple laser beams heat the surface of the pellet into a plasma, which explodes away from the surface. The rest of the pellet is driven inward on all sides, into a small volume of extremely high density. The surface explosion creates shock waves that travel inward. At the center of the fuel, a small volume is further heated and compressed. When the temperature and density are high enough, fusion reactions occur. The energy must be delivered quickly and spread extremely evenly across the target's outer surface in order to compress the fuel symmetrically. The reactions release high-energy particles, some of which, primarily alpha particles, collide with unfused fuel and heat it further, potentially triggering additional fusion. At the same time, the fuel is also losing heat through x-ray losses and hot electrons leaving the fuel area. Thus the rate of alpha heating must be greater than the loss rate, termed bootstrapping. Given the right conditions—high enough density, temperature, and duration—bootstrapping results in a chain reaction, burning outward from the center. This is known as ignition, which fuses a significant portion of the fuel and releases large amounts of energy. As of 1998, most ICF experiments had used laser drivers. Other drivers have been examined, such as heavy ions driven by particle accelerators. Design System As of 2004, NIF used the indirect drive method of operation, in which the laser heats a small metal cylinder surrounding the capsule inside it. The heat causes the cylinder, known as a hohlraum (German for "hollow room", or cavity), to re-emit the energy as even higher frequency X-rays, which are still more evenly distributed and symmetrical. Experimental systems, including the OMEGA and Nova lasers, validated this approach. The NIF's high power supports a much larger target; the baseline pellet design is about 2 mm in diameter. It is chilled to about 18 kelvin (−255 °C) and lined with a layer of frozen deuterium–tritium (DT) fuel. The hollow interior contains a small amount of DT gas. In a typical experiment, the laser generates 3 MJ of infrared laser