Boundary plasma physics in magnetic confinement devices

The boundary of magnetically confined plasmas is offen characterized by weak chaos (spontaneous in the Reversed Field Pinch, or produced by purpose in Tokamaks and Stellarators), which can affect ion and electron diffusion. Besides this, particle transport is not only determined by the magnetic field alone, but also by the electrostatic potential which arises from the ambipolar constraint and plasma-wall interaction (PWI). In this topic it will present a summary of experimental observations in the RFX-mod Reversed Field Pinch device, indicating that plasma pressure, floating potential, particle influx, plasma flow and turbulence structures are modulated according to the geometry of the edge magnetic field. The distortions of magnetic flux surfaces, in the toroidal and poloidal directions, are due to magnetic islands, whose X-points are characterized by a negative charge excess (electron accumulation). Ions have larger drifts with respect to magnetic surfaces and their diffusion is much more uniform. The resulting ambipolar potential tends to balance this charge excess, and therefore has the same shape as the parent magnetic island. The presence of an electrostatic potential makes the edge physics of these plasmas sensitive to particle energy, to energy-exchanging collisions and to particle recycling at the wall. The behavior of electrons near X-points is usually numerically modeled by the "connection length", which is the path along a magnetic field line that connects a point inside the plasma to the wall. The connection length is obviously modulated by the presence of magnetic islands, and near the wall "ergodic" regions and laminar flux tubes can be distinguished. These studies share a common ground between RFPs and Tokamaks, where edge chaos is produced by purpose to control plasma exhaust and power flow to the walls.