Gas sources in fusion devices

Fusion plasmas are routinely produced from pure gas injected into the vacuum vessel of fusion devices at pressures in the range of millitorr. The large surface to volume ratio, characteristic of these fully instrumented devices, makes plasma density and impurity control a difficult task. The problem is further aggravated due to the large fluxes of energetic species produced by the imperfect confinement of particles and energy into the plasma and, additionally, due to the large efficiency of the plasma in pumping the gas released by the interaction of such particles with the vessel walls. Many of these processes have a direct impact in the control of the tritium inventory, of high relevance in the operation of future fusion reactors, and they are presently the focus of very active research.In the present work, the problems associated to the control of gas sources in fusion devices are addressed. First, the impact of the gas release on the particle and impurity control of the fusion plasmas is discussed. Secondly, the different types of sources that are present either before or during plasma formation are identified and finally, the techniques oriented to control the interchange of particles between the plasma and its surroundings are described.