Non-invasive in situ measurement of the near-wall ion kinetic energy in a magnetically shielded Hall thruster

The ion velocity distribution functions in the near-wall region of a high-power, 9-kW class magnetically shielded Hall thruster are experimentally characterized to quantify the degree to which the discharge chamber walls are shielded from ion bombardment. Ion velocity measurements are performed at a discharge voltage of 300 V and a discharge current of 15 A using laser-induced fluorescence along the centerline and discharge chamber walls. The centerline results show that the location of the acceleration region exists downstream of the exit plane of the thruster, which is consistent with previous measurements on other magnetically shielded thrusters. Measurements performed in the near-wall region show that ions are moving approximately parallel to the walls up to the exit of the thruster; however, a small but finite maximum mean velocity of 0.2 km s−1 is directed toward the discharge chamber walls. A sheath analysis is performed to define a critical electron temperature below which shielding will be maintained given these near wall ion velocities. This value is found to be in excess of predicted temperatures in this region. Taken together, these results confirm that all criteria for magnetic shielding of Hall thrusters have been satisfied.