Dynamic laser alignment control for Thomson scattering on ASDEX Upgrade

Thomson scattering, as a very direct diagnostic method to measure electron density ne and temperature Te of a plasma, has become a standard tool in plasma physics. Reliable Nd:YAG lasers have been developed for industrial applications, the accuracy of scattering measurements is sufficiently high to measure profile gradients of Te and ne with millimeter spatial resolution down to electron densities of 1018 m−3 with reasonable error bars. As the accuracy of the measurement increased steadily, a new category of errors has been revealed, which is not very likely to be detected when only one laser is used. Inhomogeneities of the scattered power density within the entrance slit of a polychromator cause systematic errors, thus the precise alignment of the scattering lasers is critical. Such an alignment is now continuously maintained during diagnostic operation by a new closed-loop control system implemented with a simple and effective optoelectronic balance technique.