Analysis of the expected thrust misalignment of Kaufman thrusters

In the design of a spacecraft using solar powered electric propulsion, it is important to know accurately the expected misalignment of the thrust vector from the thruster, so that the misalignment can be adequately compensated. This paper reports the results of an analytical study to determine the causes and magnitudes of thrust misalignment from a Kaufman-type electron bombardment ion thruster. The effect of perturbations in the spacing and transverse alignment of the accelerating electrodes on the ion beam direction and magnitude was determined by means of a digital computer program. This analysis included the detailed changes in plasma sheath position which accompany electrode shifts and changes in current. Expected electrode distortion due to thermally induced stresses was calculated. Since the perturbations considered are small, it is valid to assume that the thrust variations are linear over the range. The results are therefore presented in terms of linear coefficients from which the expected thrust vector change can be determined. The effect of electrode misalignment is found to be most significant in high perveance flow; for instance the rate of change of axial thrust with axial electrode spacing at 3.4 X 10 ~ electron pervs is 1.63 X 10~ Ib/mil, while at 0.985 X 10~ electron pervs it decreases to 0.33 X 10-lb/mil.