Two-Beam-Combined 7.4 J, 50 Hz Q-switch Pulsed YAG Laser System Based on SBS Phase Conjugation Mirror for Plasma Diagnostics

Japanese Journal of Applied Physics (Impact Factor: 1.13). 08/2004; 43(8). DOI: 10.1143/JJAP.43.L1038


The phase conjugation of the optically nonlinear stimulated Brillouin scattering (SBS) process using heavy-fluorocarbon materials effectively compensated thermal degradation at two amplifier lines, resulting in an output energy of 7.4 J at 50 Hz. When combining two beams, the beat wave generation caused by the temporal coherence between the two beams can be prevented because the two beams leave at intervals longer than the coherence length of the pulse. The use of different Brillouin frequencies of reflection from different SBS substances can also be effective in preventing the interference effect caused by the spatial coherence between the two beams.

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    ABSTRACT: In order to improve the measurement performance of incoherent Thomson scattering diagnostics, a high performance phase conjugate mirror based on stimulated Brillouin scattering (SBS-PCM) is applied to a Thomson scattering system for the first time in the JT-60U tokamak. We have demonstrated that a SBS-PCM which uses heavy-fluorocarbon liquid showed a high reflectivity of 95% at a high input-power of 145 W. Using the SBS-PCM, two newly developed methods were employed to increase the amount of scattered light. In the first method, we first developed a new optical design to provide a double-pass scattering scheme with the SBS-PCM. In this new optical design, a laser beam passing through the plasma is reflected by the SBS-PCM, and the reflected beam is returned via the same path by means of the phase conjugate effect, and is then passed through the plasma again, in order to increase the scattered light. A double-pass Thomson scattering scheme using the SBS-PCM was demonstrated in JT-60U ohmic plasma, resulting in an increase of the scattered light by a factor of 1.6, and the reduction of relative error by 2/3 for electron temperature measurement in contrast to single-pass scattering. A multi-pass Thomson scattering scheme is also proposed based on the results of double-pass scattering. It is estimated that multi-pass scattering allows the generation of several times the amount of scattered light, and the reduction of the relative error for electron temperature measurement by 37% in contrast to single-pass scattering. Regarding the second method, a high average-power of YAG laser system was developed by applying the SBS-PCM to a existent diagnostic laser. As a result, the average-power was increased by over 8 times in contrast to the average power of the original system, achieving up to 368 W (7.4 J × 50 Hz).
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