Advanced impurity measurement for deuterium–tritium-burning plasmas using pulsed CO₂ laser collective Thomson scattering

Knowledge of the dynamics and concentration of impurities in the plasma core is crucial to the development of a deuterium–tritium (DT) tokamak fusion reactor. In this article, we first describe an advanced diagnostic technique to measure the impurities, thermalized helium density, and the deuterium-to-tritium density ratio in DT-burning plasmas. The measurement is made by small-angle collective Thomson scattering using a high-power pulsed CO₂ laser and heterodyne receiver system. The component of scattered laser power nearly perpendicular to the magnetic field gives rise to ion cyclotron modulation of the scattered spectrum. Scattered spectrum of expected impurities (Ar, Be, and He ash) are presented for the International Thermonuclear Experimental Reactor plasmas. The method for the determination of the helium ash density and deuterium-to-tritium ratio are also proposed. Modulation spectra as a function of frequency for different magnetic-field angles are confirmed. Experimental requirements to measure a scattered spectrum with fine details resulting from ion cyclotron oscillations are also discussed. © 2000 American Institute of Physics.