Applications of phase conjugate mirror to Thomson scattering diagnostics (invited)

A high performance phase conjugate mirror based on stimulated Brillouin scattering (SBS-PCM) has been applied to the Thomson scattering system in the JT-60U tokamak for the first time in order to improve the measurement performance. A SBS-PCM realized a high reflectivity of 95% at a high input power of $145W$ (⁠$2.9J$⁠, $50Hz$⁠). Using the SBS-PCM, two methods have been developed to increase the intensity of scattered light. For the first method, we have developed a new optical design to provide a double-pass scattering method with the SBS-PCM. A laser beam passing through the plasma is reflected by the SBS-PCM. The reflected beam passes the plasma again along the same path by means of the phase conjugation of the optically nonlinear stimulated Brillouin scattering process. The double-pass Thomson scattering method using the SBS-PCM has demonstrated an increase of the scattered light by a factor of 1.6 compared with the single-pass scattering method in JT-60U. A multipass Thomson scattering method in which the laser beam can be confined between a couple of SBS-PCMs is also proposed. It is estimated that the multipass scattering method generates the scattered light more than several times as large as that of the single-pass scattering method. For the second method, a high-average-power yttrium aluminum garnet (Nd:YAG) laser system has been developed using the SBS-PCM. The SBS-PCM effectively compensated thermal degradation at two amplifier lines, and the average power was increased by a factor of $>8$ from $45W$ (⁠$1.5J$⁠, $30Hz$⁠) to $373W$ (⁠$7.46J$⁠, $50Hz$⁠). A Nd:YAG laser (⁠$5J$⁠, $100Hz$⁠) for the edge Thomson scattering in International Thermonuclear Experimental Reactor (ITER) has been designed based on the result.