Diagnostics of fusion plasmas with fast lithium beams

Lithium beam diagnostic systems have been installed at the experiments ASDEX-Upgrade (AUG) and WENDELSTEIN 7 AS (W7 AS) of IPP Garching, Germany. A lithium atom beam injector has been improved, now delivering about 1.1 mA of equivalent neutral particle current at 35 keV beam energy. By means of lithium induced impact excitation spectroscopy (Li-IXS), electron density profiles have been measured at AUG and W7AS. In addition, at W7 AS the neutral particle density outside the plasma could be measured. Evaluation of the density decay-lengths for different plasma modes on AUG shows a steeper gradient in the high confinement mode (H-mode) than in the low confinement mode. Effects of edge-localised modes appearing in H-modes on the density profiles have been studied, showing an increase in the electron density by a factor of four in the outermost radial range, connected with a dramatic increase in the decay-length. A method has been introduced for combining line density- and Li-IXS-measurements to obtain more reliable density profiles on AUG. When applied to discharges with neon gas puffing for enhanced radiation power losses, this method delivers a peaking of the electron density in the plasmacentre. On both experiments the measured electron density profiles have been compared with diagnostic data from independent techniques (Thomson scattering, laser interferometry, reflectometry, electron cyclotron emission), giving a generally satisfactory agreement. (author)