Flexible microwave system to measure the electron number density and quantify the communications impact of electric thruster plasma plumes

Abstract

An advanced microwave interferometric system operating in the Ku (12–18 GHz) band has been implemented for use in very large vacuum chambers to determine the effects of electromagnetic wave propagation through a plasma plume created by a space electric propulsion thruster. This diagnostic tool is used to nonintrusively obtain the local electron number density as well as provide information necessary for understanding impact to communications and other spacecraft electromagnetic systems. The use of a nonintrusive electromagnetic measurement provides highly accurate line integrated density and avoids problems caused by intrusive measurement techniques. If the plasma is symmetrical, local plasma density can also be determined accurately using well known inversion techniques. A network analyzer acts as a transmitter and receiver while a two axis positioning system maps the amplitude and phase variation of a transmitted signal over one plane of the plasma plume. The utilization of a 6 m×9 m vacuum chamber effectively minimizes plasma boundary effects, but the longer cable path lengths have required a frequency conversion circuit to reduce power loss and phase uncertainty at high frequencies. Two studies are presented: the first is a measurement of the local electron density in the plume of a 1 kW arcjet and the second is a measurement of attenuation in the plume of a stationary plasma thruster. Both the arcjet and SPT emit a steady state conical unmagnetized plasma that is radially symmetric. The arcjet peak density is 1015–1016 m−3 along centerline and the SPT peak density is 1016–1017 m−3 along centerline.