Diode laser-induced fluorescence velocimetry of clustered hall thruster plumes

Summary form only given. We present laser-induced fluorescence (LIF) measurements of singly-ionized xenon (5d 2F7/2) in the plume of a cluster of four 600-Watt (BHT-600) Hall thrusters. Two 834.7 nm beams from a TUI Optics TA-100/830 tapered-amplifier diode laser are focused on orthogonal optical axes onto a submillimeter interrogation volume, from which we collect LIF at 541.9 nm. Since high ion velocities produce large Doppler shifts (50 GHz), we use repeatable frequency markings from a 2-GHz confocal etalon to patch together extended scans from several mode-hop-free segments (each 10-GHz). Isotopic shifts and lower-state (5d 2F7/2) nuclear-spin constants estimated from optogalvanic spectra permit Fourier-transform deconvolution of ion velocity estimates from LIF spectra along the axial and radial beam directions. Transformation of deconvolved velocity distributions to energy space permits comparison of far-field LIF to two-dimensional energy distributions measured with a top-hat electrostatic analyzer (TOPAZ)