Application of FIR heterodyne detection to collective scattering measurements of tokamak microturbulence

Multichannel collective scattering of far-infrared (FIR) laser radiation is employed on the TEXT tokamak to investigate small-scale turbulent density fluctuations. A twin-frequency optically pumped FIR laser operating at 1222 μm (≂20 mW) and a difference frequency ΔωIF/2π≂1 MHz is used for the source and local oscillator beams. This permits heterodyne detection of the frequency-shifted scattered radiation which is necessary to determine wave propagation direction of plasma density fluctuations. The need for heterodyne versus homodyne detection is demonstrated. Such a diagnostic capability has allowed preliminary identification of the ion pressure-gradient-driven instability (whose most notable signature is that of turbulent fluctuations propagating in the ion diamagnetic drift direction) which is potentially responsible for saturation of the global energy confinement time with increasing density in tokamak plasmas.