The inertial laser-driven fusion method, where the fuel pellet is first compressed by laser beams and then the fusion reaction is ignited by an extremely short and intense laser pulse directed at the compressed centre of the pellet. The compression occurs either directly, by laser beams aimed at the surface of the pellet, or indirectly, by X-rays generated in a cavity called the hohlraum after the laser beams impact. This is followed by heating the compressed pellet with a picosecond laser pulse. In order to direct the heating beam right into the compressed core, a cone is sometimes inserted into the pellet, which allows the laser to penetrate directly into the core, but also disturbs the symmetry of the pellet. The separation of compression and heating allows the use of lower power laser beams than other inertial fusion methods.