Improvement of Thomson Scattering System Using SBS Phase Conjugation Mirrors in JT-60U
T. Hatae1, M. Nakatsuka2, H. Yoshida2, O. Naito1, S. Kitamura1, T. Kashiwabara1, T. Sakuma1, Y. Taki1, T. Hamano1, and Y. Tsukahara1
1Naka Fusion Research Establishment, Japan Atomic Energy Research Institute (JAERI), Naka-machi, Naka-gun, Ibaraki, 311-0193, Japan
2Institute of Laser Engineering (ILE), Osaka University, Suita, Osaka 565-0871, Japan
Abstract.
Thomson scattering diagnostics is one of standard diagnostics for electron temperature and density measurement in magnetic fusion devices. Since the Thomson scattering light is very weak, an increasing of scattered light improves the measurement accuracy. Increasing the incident laser energy in order to increase the scattered light without decreasing the measurement accuracy is most effective way. Methods to increase the laser energy and scattered light can be carried by the following items. (1) An improvement of the existing laser gives higher energy output from laser device. (2) Beams from more than one laser device are bundled into one beam. (3) An incident laser beam is made to go back and forth in the plasma, that can double a scattered energy. Since these improvements need generally a large amount of R&D cost, a long-term research period and the laser system becomes larger, then new development of optical technology have been required. This paper focused on items (1) and (3), and a high performance phase conjugation mirror (PCM) by stimulated Brillouin scattering (SBS) using a liquid fluorocarbon is introduced to the existing YAG laser Thomson scattering system at JT-60U.
For the item (1), improvement of the output energy of YAG laser system using the SBS- PCM was achieved. The phase conjugation of the optically nonlinear SBS process compensated perfectly a thermal effect of power amplifiers, and an average power increased from 1-2 J in 30-Hz repetition to 2-3J at 50-Hz drive as a result by the implementation of SBS-PCM. The beam quality was also recovered without a wave front aberration and depolarization to have a transfer-limited divergence with good flattop pattern in a near field.
For the item (2), a double path Thomson scattering method with SBS-PCM was newly proposed. In this new optical design, a laser beam passing through the plasma is reflected by the SBS-PCM in place of a beam dumper, and a reflected beam returned back through the really same path as coming one by the phase conjugation effect, and passed through the plasma again. Returning laser beam reached to the laser system that was removed by a Faraday isolator. This method enable to obtain two times larger scattered light with an alignment-free operation in contrast with a conventional single path design. From the preliminary results using the double path scattering, the scattered light became 1.6 times larger and relative error reduced to 2/3 of that for single path scattering. Both approaches promise Thomson scattering diagnostics with higher accuracy in JT-60U.