Cesium contact ion microthruster experiment aboard applications technology satellite /ATS/ IV.

On 10 August 1968, ATS-IV was launched into a low-altitude parking orbit, remaining attached to the Centaur stage, which failed to achieve a second burn. Onboard were two ion microthruster systems, and a spacecraft potential monitor that employed one of the gravitygradient booms as a Langmuir probe. The booms were deployed just prior to the last of five ion engine test periods. The large ram ion currents available from the relatively dense ambient plasma precluded the achievement of appreciable neutralizer emission current except for a few brief periods. During the last test, emission currents up to roughly half the beam current were obtained and spacecraft potentials as great as — 133v were measured. Notable achievements, in addition to providing the first successful orbital test of an ion engine, include demonstration of the absence of detectable electromagnetic interference (EMI) and the satisfactory operation of the zero-g feed system.