Tokamak plasma boundary and power exhaust

Abstract

‘By doubting we come to questioning, and by questioning we perceive the truth.’ P. Abelard (c. 1120) Having examined the equilibrium, stability and transport properties of magnetized plasmas, we are finally ready to tackle our ultimate aim, i.e. the exhaust of particles and power from the plasma itself and the limits imposed by the resulting plasma wall loads and thermo-mechanical material constraints on the reactor performance. Geographically, this question naturally leads us towards the plasma boundary, commonly known as the scrape-off layer (SOL), defined as the region of open field lines between the last closed flux surface (LCFS) and the vessel wall, which is the only place in a fusion reactor where the stellar world of hot (keV) plasmas meets the earthly world of cold (sub-eV) solids. As the name indicates the LCFS represents the transition between the regions of closed and open magnetic field lines, i.e. between the edge and SOL regions. This surface can be created either by inserting a solid object, known as the limiter , into the plasma, or by shaping the poloidal magnetic field with external current carrying coils to create a poloidal field null, or X-point, and a magnetic separatrix , and thus to divert the SOL plasma into a specifically designed structure, known as the divertor , see Section 7.1.3. Since the location and degree of plasma contact with the wall is determined by transport processes in the SOL, the understanding of these processes is critical for translating a plasma load limit (per component surface area) into a plasma exhaust limit (per plasma surface area at the LCFS), recall (1.16).