A Projectile Probe for Measuring the Electric Field Inside a Spacecraft Plasma Sheath

A self contained electrically floating spherical electric field probe has been developed use in volume regions where spatial field variation occurs on a length scale larger than the sphere diameter. In plasma sheath, the sphere floats to the local plasma potential, minimizing field perturbation, and provides a value of field magnitude found from the Laplacian spherical harmonic field solution. The sphere is also capable of monitoring the amount of charge collected on its own surface--a quantity that can in principle be used to infer properties of the ambient plasma. The 15 cm battery operated aluminum sphere contains six miniature 800 Hz field mill sensors, a synchronous detection data acquisition system and A/D converter, and 50 MHz digital fm transmitter. Data is sent at 1200 BAUD to a nearby receiver and computer for processing in real time. Fields as low as 50 V/m can be detected with a sampling time of about 1 second. At present the probe works well in air and vacuum, but has been only partially successful plasma. Keywords: Electric field measurements; Plasma; Sheath; Sensors; Probe; Surface; Charge; Sphere; Projectile; Floating.