On the particle fluxes and density profiles in helical systems

Quasilinear and neoclassical particle fluxes and their relation to the density profiles in helical plasmas are discussed, by taking the Large Helical Device (LHD) [O. Motojima et al., Nucl. Fusion 43, 1674 (2003)] as a representative example. Various magnetic configurations can be realized in the LHD experiments by changing the coil currents. The neoclassical particle flux shows strong configuration dependence through the significant change of effective helical ripple, while it is insensitive to the change of density profiles. On the other hand, the quasilinear flux shows weak dependence on the magnetic configurations while it strongly depends on the density profiles. As a result of the particle balance in the steady state in the no-source region, a qualitative explanation for the hollow density profiles, which are often observed experimentally in LHD, is proposed. In the appendixes, properties of quasilinear particle flux in a simple model tokamak are discussed, and expressions for neoclassical fluxes in the 1∕ν regime in a coordinate system are given.