Diagnosis of energetic ions and ion composition in fusion plasmas by collective Thomson scattering of mm-waves

Summary form only given. In fusion plasmas, the dominant heating source will be fusion generated energetic ions slowing down in the plasma. The same ions can also drive waves and instabilities in the plasma. Their distribution in velocity and in space has major impact on plasma dynamics, and plasma dynamics in turn affects the energetic ion distributions. The dynamics of energetic ions is thus important to measure in order to understand fusion plasmas, and important to monitor as part of input to plasma control. The collective Thomson scattering of millimeter waves has proven to be a valuable means of diagnosing energetic ion distributions in fusion plasmas
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. A beam of mm-waves with a diameter of 5-10 cm and a power of 150-600 kW is sent through the plasma, and radiation scattered from this probe beam by the microscopic fluctuations in the plasma is detected. These microscopic fluctuations are in part induced by the ion motion and the fluctuations and hence the scattered radiation is thus sensitive to the ion distribution. This permits the fast ion distribution to be inferred from the detected scattered radiation. Dynamics of the fast ions is measured, and phenomena related to plasma instabilities observed. For scattering in the plane near perpendicular to the magnetic field confining the plasma, the microscopic fluctuations are dominated by the cyclotron motion of the ions and the dielectric response of the plasma near ion Bernstein waves. Both render the scattered radiation strongly sensitive to the ion composition of the plasma. It has been demonstrated that this can be exploited to measure relative concentrations of the dominant ion species in the plasma. This holds the perspective of providing information on the ratio of fuel ion densities, of deuterium and tritium, in the experimental fusion reactor ITER, currently under construction. In this presentation we summarize the principles of the techniques of CTS for diagnosing fast ions and ion composition and illustrate the capabilities with experimental results.