Active control of laser beam direction for LHD YAG Thomson scattering

We have developed a YAG Thomson scattering (TS) system for the measurements of electron temperature and density profiles on the large helical device (LHD). The LHD-TS has four YAG lasers, and flexible operational modes are possible by using them. For example, (1) high-energy mode: The pulse energy can be increased up to four times by firing the four lasers simultaneously. In this mode, the data quality can be improved for low-density plasmas. (2) High repetition mode: When firing the lasers at intervals of 5 ms, the lasers work as a 200 Hz laser. The laser beams are guided to the LHD by seven steering mirrors. The first mirror is real-time feedback controlled for precise beam transport. The beam pointing stability is improved successfully from 200 mu rad to below 4 mu rad with the feedback-control system. We describe the details of the laser system for the LHD-TS. (C) 2001 American Institute of Physics.