Density Dependence of Ion Temperature Measured by Active Charge-Exchange Spectroscopy in ECRH Plasmas of the TJ-II Stellarator

First impurity ion temperature profiles obtained using an active diagnostic system, recently installed on the TJ-II stellarator, are presented. This diagnostic consists of a multichannel spectrometer and a compact diagnostic neutral beam injector system optimized for performing charge-exchange recombination spectroscopy. Here, after summarizing the experimental setup, details of the system alignment and calibration, as well as the data analysis method adopted, are presented. Next, impurity ion temperature profiles, determined from C VI emission line widths (at 529.06 nm), are presented for a range of plasma conditions (different densities plus two injected electron cyclotron resonance heating powers) in order to highlight the system capabilities. Then, the comportment of core impurity ion temperature for an electron density scan (4 × 10¹⁸ to 9 × 10¹⁸ m^-3) is examined. It reveals a clear minimum between <n_e> = 6 × 10¹⁸ and 8 × 10¹⁸ m^-3 that coincides with the values for the transition from the electron-to-ion root of the radial electric field. Finally, these results are compared with ion temperatures determined by passive methods to evaluate the system performance, and the physics behind the observed impurity ion temperature behavior is examined.