Results from a double Li-beam technique for measurement of both radial and poloidal components of electron density fluctuations using two thermal beams

Determination of both the radial and poloidal components of electron density fluctuation parameters in a tokamak edge plasma was achieved by operation of two narrow Li-beams (FWHM ≈10 mm at a distance of 100 mm from the source), which were separated by 50 mm. Beams of thermal lithium atoms were generated using a newly developed oven, which provides a long operation time and a high reliability. The lithium atoms, with a mean velocity of ≈1.7 km s−1, were injected radially into the plasma edge, where they were excited and ionized by the plasma. The line radiation (resonance transition 2s–2p, λ = 670.8 nm) of the excited atoms was observed, which enables determination of the electron density profiles. The measurements are performed with high temporal (Δt ≃ 2 µs) and spatial (Δr ≃ 1 mm in the radial and Δp ≃ 10 mm in the poloidal direction) resolution, thus also allowing determination of the fluctuations. This new diagnostic permits studies of both radial and poloidal fluctuation parameters like the correlation length, wavenumber, frequency spectrum, propagation velocity and lifetime. The method can be applied continuously over long discharges without disturbing the plasma.