Neoclassical modeling of the radial electric field and comparison with measurements in the TJ-II stellarator

The results of the radial electric field measurements by Heavy Ion Beam Probe (HIBP) in the TJ-II stellarator are compared with neoclassical transport computations. The role played by several plasma features is identified by studying a reduced analytical transport model, using both actual density and temperature profiles and representative model profiles for varying density. Additional electric field calculations are carried out numerically with the Astra code using three different expressions for the neoclassical transport coefficients in order to identify the common features characterizing E, particularly the sign reversal. For regimes with a wide variation of collisionality, a general qualitative agreement between modeling and experimental data is shown. The obtention of roots for E depends critically upon the temperature and density profiles as they determine the plasma collisional regime. It is found that the root transition (i.e. E sign reversal) occurs for a specific range of a collisionality parameter, consistent for all models employed. It is found that when density and temperature profiles give real roots of the electric field the pressure is almost constant. Discharges with high radial gradient of E are found to correlate well with high confinement regimes.