Current status of optically pumped 385 μm D2O laser system for fusion plasma diagnostics

As the method for determining the ion temperature and electron temperature of the plasma in which local thermal equilibrium is maintained, the method of determining the ion temperature from the frequency shift of Thomson scattering has such features that it does not depend on impurity atoms, and that local temperature can be obtained from wide angle scattering components. There are several requirements for the submillimeter wave laser required for irradiating on plasma, and as a light source of submillimeter waves satisfying these requirements, light-excited 385 μm D2O laser is regarded as most important at present. The light-excited 385 μm D2O laser of output energy 5 J and pulse with 3 μs was reported by Princeton University, and in MIT, that of relatively low output 100 kW and 1 μs was applied to a tokamak, and the ion temperature was able to be measured. In this paper, the principle of the measurement of plasma ion temperature by Thomson scattering method, the mechanism of oscillation of the 385 μm D2O laser, AC Stark effect and the present state of the development of 385 μm D2O laser are described. (Kako, I.)