Profiling the Beam of the T5 Ion Engine

Current density profiles in the beam of a xenon ion engine are measured using a long tungsten wire probe to collect ions while making radial traverses through the beam. Between traverses the probe is moved along the axial coordinate to sample the profile at distances of 5-50 cm from the exit plane. Abel inversion of the data provides flux contour maps and integrated current maps as functions of beam voltage and accelerator voltage. Data for the British T5 engine are compared with previous experiments using a thrust balance and an angle-resolving Faraday probe. Introduction Studies of the T5 ion thruster in our laboratory have demonstrated the use of angle-resolving Langmuir and Faraday probes, an electrostatic deflection energy analyzer, and a time-of-flight mass and charge analyzer. I-3 These diagnostics measure the ion flux, electron density, electron characteristic energy, floating potential, plasma potential, ion beam energy, and charge-state distribution as functions of position within the plume. The resulting information on plume properties and discharge chamber erosion is useful for developing mathematical models of lifelimiting factors and spacecraft interactions. Although the basic properties of the T5 plume are now fairly well documented, there are still important questions to be answered. The angleresolved ion flux exhibits a complex dependence on thrust level and beam energy that is not fully understood.3 We also would like to determine what traction of the beam power supply current is due to electrons penetrating the grids and plasma screen rather than to the ejection of high velocity ions. Thrust balance data for the T5 are at least 2%-4% lower than the thrust values derived from the supply current after correcting for divergence and multiply-charged ions.’ Also suggestive of a thrust loss are angular distributions measured with a Faraday probe that give an integrated beam current about 20% less than the supply current.3 Finally, we would like to map the ion flux in the region within 50 cm from the exit plane as a function of the voltages applied to the screen and accelerator grids, for comparison with recent Faraday probe data measured further downstream.5 The experiment described here begins to address these questions by recording beam profiles for the T5 over a wider range of operating points and spatial locations than in previous tests. Copyright