neutrons being produced by the fusion reactions. This is normally accomplished through the use of a breeding blanket, a layer of material containing large amounts of lithium. In order to capture most of the neutrons, the blanket has to be about 1 to 1.5 meters thick, which moves the magnets away from the plasma and therefore requires them to be more powerful than those on experimental machines where they line the outside of the vacuum chamber directly. This is normally addressed by scaling the machine up to extremely large sizes, such that the ~10 centimetre separation found in smaller machines is linearly scaled to about 1 meter. This has the effect of making the machine much larger, growing to impractical sizes. Designs with smaller aspect ratios, which scale more rapidly, would address this effect to some degree, but designs of such systems, like ARIES-CS, are enormous, about 8 meters in radius with a relatively high aspect ratio of about 4.6. The stellarator's complex magnets combine together to produce the desired field shape. This demands extremely high positioning tolerances which drive up construction costs. It was this problem that led to the cancellation of the US's National Compact Stellarator Experiment, or NCSX, which was an experimental low-aspect design with a ratio of 4.4. To work properly, the maximum deviation in placement across the entire machine was . As it was assembled this was found to be impossible to achieve, even the natural sagging of the components over time was more than the allowed limit. Construction was cancelled in 2008, throwing the future of the PPPL into doubt. Finally, stellarator designs are expected to leak around 5% of the generated alpha particles, increasing stress on the plasma-facing components of a reactor. Plasma heating There are several ways to heat the plasma (which must be done before ignition can occur). Current heating The plasma is electrically conductive, and heats up when a current is passed through it (due to electrical resistance). Only used for initial heating, as the resistance is inversely proportional to the plasma temperature. High-frequency electromagnetic waves The plasma absorbs energy when electromagnetic waves are applied to it (in the same manner as food in a microwave). Heating by neutral particles A neutral particle beam injector makes ions and accelerates them with an electric field. To avoid being affected by the Stellarator's magnetic field, the ions must be neutralised. Neutralised ions