Laser-induced Fluorescence Velocimetry of Xe II in the 30-cm NSTAR-type Ion Engine Plume

We present laser-induced fluorescence (LIF) measurements of Xe II (4D7/2) in the FMT2 ion engine plume. Three 605.1 nm beams from a ring dye laser are focused through a single lens into a submillimeter interrogation volume, from which we collect LIF at 529.2 nm. Fourier-transform deconvolution of the LIF spectra returns ion velocity distribution estimates along the three laser beam directions. From this, we calculate axial, radial and vertical components of the mean ion velocity and temperature from 1 mm to 300 mm downstream of the accelerator grid. The error propagation endemic to this off-axis multiplex LIF technique obscures the location of the neutralization plane in axial profiles of axial velocity, while radial sweeps of axial and vertical velocity show no discernable trend. Radial profiles of radial velocity, however, show increasing divergence with radial position.