Fusion Science and Technology / Volume 51 / Number 2 / February 2007 / Pages 238-260
Plasma boundary control in stellarators has been shown to be very effective in improving plasma performance and, accordingly, is an important element from the very beginning of the National Compact Stellarator Experiment (NCSX) design. Studies of the magnetic field topology outside the last closed magnetic surface (LCMS) indicate the possibility of many toroidal revolutions of field lines launched within a couple of centimeters of the LCMS. Field line connection lengths, typically in the order of 100 m, should be sufficient to allow for the necessary separation of divertor and separatrix temperatures. In the top and bottom of the bean-shaped cross section (toroidal angle = 0), a field expansion of >5 is observed, which will help to spread out the heat flux on limiters and divertor plates. Plasma-facing components (PFCs) will be developed systematically according to our respective understanding of the NCSX boundary; the phased PFC development will start out with a set of limiters and has the eventual goal to develop a divertor with all the benefits of impurity and neutrals control. Neutrals calculations have been started to investigate the effect of neutrals penetration at various plasma cross sections, especially at the location of = 0 deg. Advanced wall conditioning techniques, as employed in other major fusion devices, will be incorporated in the NCSX operation.