Plasma Physics And Nuclear Fusion
In magnetic-confinement fusion, a mixture of hydrogen isotopes is brought to extremely high temperatures (100 million degrees) in a toroidal chamber (tokamak). At these temperatures, the hydrogen is completely ionized and forms a plasma of positive ions and negative electrons. As no material container could sustain such extreme conditions the plasma must imperatively be confined well inside the toroidal chamber, without reaching its internal walls. This is achieved by means of powerful magnetic fields which constrain the charged particles to travel approximately along the field lines. Although the magnetic field is carefully engineered so as to optimize the efficiency of the confinement, a fraction of the plasma inevitably escapes. This can be due to different causes, among which random collisions between the particles, various instabilities, and plasma turbulence. For the particles that escape the confinement, some appropriate treatment is necessary. Indeed, these particles are still very energetic and could damage the tokamak vessel if not properly managed. In modern tokamaks, this issue is addressed by a special configuration of the magnetic field, whose field lines lie on a series of nested toroidal surfaces. In the tokamak core, these magnetic surfaces are closed, so that the particles are mostly confined. Near the tokamak edge the field lines are open and direct the plasma towards a device (divertor) specifically designed to optimize the interaction with energetic charged particles. The constant bombardment of energetic ions and neutral particles can damage and erode the divertor surface, releasing impurities that can deteriorate the plasma confinement. In order to keep the erosion within acceptable limits, it is important to estimate the plasma characteristics in the region in contact with the wall, particularly the heat and particles fluxes. Further applications of plasma-wall interactions come from probe theory. Probes are routinely used for tokamak edge measurements, although their results are notoriously difficult to interpret, because the very presence of the probe can perturb the ambient plasma. It is therefore of paramount importance to assess the properties of the plasma-probe interaction in order to relate the quantities measured by the probe to those of the unperturbed plasma. Plasma-wall interaction studies are also important for other fields. For instance, in plasma-assisted surface treatment, low-pressure plasmas are often utilized to achieve specific technological goals, such as surface coating. Finally, plasma-wall interactions play a relevant role in space physics, as man-made satellites are constantly exposed to energetic ions originating from the solar wind.