Cylindrical Langmuir probe measurements in an ion thruster plume

Summary form only given. Cylindrical Langmuir probe measurements have been made in the exhaust plume of a 15 cm diameter three-grid ion thruster. Xenon gas was used as a propellant. The experiments were performed in a large vacuum chamber with background pressure values of 2/spl times/10/sup -4/ Torr and 3/spl times/10/sup -6/ Torr. A 250 um diameter tungsten probe 2.78 mm long was used in the experiments. The axis of the probe was kept approximately parallel to the ion flow direction in order to minimize the probe surface exposed to the ion jet. The probe was referenced to a 1 m/spl times/1 m graphite plate placed 3 m downstream of the thruster and perpendicular to the plume. The graphite plate provided sufficient ion collection for reliable measurement in the electron saturation pan of the probe characteristic curve. Both the Langmuir probe and the reference electrode were electrically isolated with respect to the test chamber and the ion thruster. The probe and the thruster were positioned on perpendicular translation stages which permitted axial and radial displacement of the probe relative to the thruster. The data collected for radial and axial scans were analyzed to provide measurements of electron temperature, plasma potential and electron density. A method of data analysis was developed which includes an estimation of the collected ion current. This method will be compared with a numerical particle simulation model for calculation of the ion current component.