reaction continues because the alpha particles released in the interior heat the fuel around it. At liquid density the alphas travel about 10 mm and thus their energy escapes the fuel. In the 0.1 mm compressed fuel, the alphas have a range of about 0.016 mm, meaning that they will stop within the fuel and heat it. In this case a "propagating burn" can be caused by heating only the center of the fuel to the needed temperature. This requires far less energy; calculations suggested 1 kJ is enough to reach the compression goal. Some method is needed to heat the interior to fusion temperatures, and do so while when the fuel is compressed and the density is high enough. In modern ICF devices, the density of the compressed fuel mixture is as much as one-thousand times the density of water, or one-hundred times that of lead, around 1000 g/cm3. Much of the work since the 1970s has been on ways to create the central hot-spot that starts off the burning, and dealing with the many practical problems in reaching the desired density. Heating concepts Early calculations suggested that the amount of energy needed to ignite the fuel was very small, but this does not match subsequent experience. Hot spot ignition The initial solution to the heating problem involved deliberate "shaping" of the energy delivery. The idea was to use an initial lower-energy pulse to vaporize the capsule and cause compression, and then a very short, very powerful pulse near the end of the compression cycle. The goal is to launch s hock waves into the compressed fuel that travel inward to the center. When they reach the center they meet the waves coming in from other sides. This causes a brief period where the density in the center reaches much higher values, over 800 g/cm3. The central hot spot ignition concept was the first to suggest ICF was not only a practical route to fusion, but relatively simple. This led to numerous efforts to build working systems in the early 1970s. These experiments revealed unexpected loss mechanisms. Early calculations suggested about 4.5x107 J/g would be needed, but modern calculations place it closer to 108 J/g. Greater understanding led to complex shaping of the pulse into multiple time intervals. Fast ignition The fast ignition approach employes a separate laser to supply additional energy directly to the center of the fuel.