  The drift kinetic equation is solved for low density TJ-II plasmas employing
slowly varying, time-dependent profiles. This allows to simulate density
ramp-up experiments and describe from first principles the formation and
physics of the radial electric field shear layer. The main features of the
transition are perfectly captured by the calculation, and good quantitative
agreement is also found. The results presented here, that should be valid for
other non-quasisymmetric stellarators, provide a fundamental explanation for a
wealth of experimental observations connected to the shear layer emergence in
TJ-II. The key quantity is the neoclassical viscosity, which is shown to go
smoothly to zero when the critical density is approached from below. This makes
it possible for turbulence-related phenomena, and particularly zonal flows, to
arise in the neighborhood of the transition.
