Far forward scattering from plasma fluctuations using a detector array and coherent signal processing at 10 μm

A far forward scattering experiment with a CO² laser is described which uses a linear array of five photoconductive detector elements allowing the simultaneous measurement of the amplitude and phase of the scattered signals in one single shot. The direction of propagation of fluctuations can be determined from the spatial phase profile of the scattered light. The method described here is equivalent to a time resolved holographic diagnostic of electron density fluctuations. The plasma source used is a traveling wave experiment. Broad frequency and wavenumber spectra of electron density fluctuations associated with the excitation of strong nonlinear compressional Alfvén waves were measured. Two representative regimes of operation with low and high axial magnetic fields were investigated with wavenumber spectra ranging from k = 2 to 60 cm^-1. The orientation of the array allowed observation of waves propagating either parallel or perpendicular to the magnetic field.