The connection length L between the magnetic field line and the wall in the scrape-off layer (SOL) region in toroidal helical systems is analysed. It is found that this connection length has a logarithmic dependence on the distance from the outermost magnetic surface to the field line under consideration. Using this dependence in a fluid model, the spatial distribution of the plasma in the SOL region is studied analytically. The edge temperature scales approximately as P0.36 (P being the power flow to the SOL region), as for tokamaks. For a given value of P, the edge temperature in toroidal helical systems is lower than in tokamaks with a similar minor radius.
On the spatial structure of solitary radial electric field at the plasma edge in toroidal confinement devices