Fast Ion CTS Diagnostic for ITER - State of Design

Intr oduction Plasmas for thermonuclear fusion contain a highly non-thermal population of fast ions which may carry about one third of the plasma kinetic energy. Ions are accelerated to high energies by neutral beam injection or i on cyclotron resonance heating. They are also born in the deuterium-tritium fusion reaction which is the workhorse of c-particle heating for reactors which approach or reach the break-even condition. The dynamics of the fast ions plays an increasingly central role as fusion power plant conditions are approached: Fast ions must be confined in the plasma long enough to heat the bulk plasma. However, they also drive instabilities and turbulence and may leave the plasma before they have transferred their excess energy to the bulk plasma. It is therefore essential to understand the dynamics of fast ions in fusion plasmas (1-2). Collective Thomson scattering (CTS) offers the opportunity to diagnose confined fast ions resolved in space, velocity space, and in time. The resolution and accuracy meet the ITER diagnostic requirements (1,3,4). The CTS diagnostic has been previously successfully applied in the JET experiment (5), and in the TEXTOR experiment (6). In CTS, one monitors a signal of millimetre waves resulting from scattering of a powerful beam of probe radiation on collective fluctuations in the electron distribution. The velocity distribution (given by the scattering geometry) along a chosen direction can be inferred from the spectral content of the scattered radiation (7-8). As the fast ion distribution may be highly anisotropic, the velocity distribution should optimally be resolved in at least two directions: Perpendicular and parallel to the magnetic field. The spatial location of the measuring volume and the spatial resolution is given by the overlap of the probe beam and the receiver beam collecting the scattered signal.