Ion temperature measurements in RFX-mod

A Neutral Particle Analyzer (NPA) of the ACORD series, developed by theIOFFE Institute of the Russian Academy of Sciences has been put in operation in the RFX-mod device. It resolves the energy distribution of the neutral hydrogen particles produced by charge exchange (CX) processes and leaving the plasma. A first estimate of the ion temperature Ti is deduced from the logarithmic slope of collected spectra. A more detailed calculation of Ti is performed according to the method suggested in Schlatter, PPCF 2006.. The method gives information on the ion temperature profile matching the experimental and the calculated spectrum of particle flux, taking also into accountthe effect on the neutral spectrum of the attenuation processes. The density and the energy of both first generation and energetic neutrals are calculated with the MonteCarlo code Nenè The analyses show a dependence of the ion temperature on the plasma density. At low density (ne<= 3e-19m-3) the calculated Ti values are lower than the electron temperatureTe, as expected in the framework of a collisional transfer of energy from electrons to ions. When the density increases Ti becomes equal to Te (ne=4-5e-19m-3), and then it raises up to three times higher than Te (ne>1e-20m-3) reaching 600-700eV, suggesting the presence of an anomalous ion heating mechanism. It is worth noting that in this density regime the measurements of NPA refer to the external region (r/a ~0.8). The temperature of impurities is estimated from Doppler broadening spectroscopic measurements. A good agreement is found between the hydrogen and the CV and BV (associated radial positions averaged on the emissivity profile r/a~0.8-0.9 and r/a~0.6-0.7) ion temperatures at low density, while at higher density the impurity temperature is weakly decreasing with the density and never exceeds 300 eV. Further insight is expected from the measurements of neutron flux that will be performed during discharges in deuterium which are planned in the next experimental campaign in RFX-mod.