Development of laser beam injection system for the Edge Thomson Scattering (ETS) in ITER

Yatsuka, Eiichi; Hatae, Takaki; Suito, Satoshi*; Ohara, Miyuki*; Hagita, Koji*; Inoue, Kazunori*; Bassan, M.*; Walsh, M.*; Itami, Kiyoshi

In the ITER Edge Thomson Scattering, the penetration in the port plug allows the laser beams to be injected into the plasma. The penetration has to be as small as possible to reduce neutron streaming from the plasma-side. In ITER, multiple laser systems are needed for high availability of Thomson scattering diagnostics. A new type of beam combiner was developed to obtain collinear, i.e. smallest footprint because of the common path, and fixed polarization from multiple laser beams using a rotating half-wave-plate. The rotating half-wave-plate method does not induce misalignment even if the rotating mechanism defects. The combined beam is injected into plasma and is absorbed at the beam dump. The beam alignment system was designed to hit the laser beam onto the center of the beam dump. The beam position at the beam dump head is monitored by multiple alignment laser beams which are collinear with the diagnostic YAG laser beam and an imaging system installed outside the diagnostic port.