Christine Charles

Initial Experiments on a Dual-Stage 4-Grid Ion Thruster for Very High Specific Impulse and Power

A new and innovative type of gridded ion thruster, the “Dual-Stage 4-Grid” or DS4G concept, has been proven under laboratory conditions under a preliminary research, development and test programme. The DS4G concept is able to operate at very high specific impulse, power and thrust density values well in excess of conventional 3-grid ion thrusters at the expense of a higher power-to-thrust ratio. A small low-power experimental laboratory model was designed and built, and its performance was measured during an extensive test campaign. The principal goals of the initial laboratory experiment were to prove the practical feasibility of the overall concept, demonstrate the performance predicted by analytical and simulation models, and investigate critical design parameters and technological challenges. In the present paper, the basic concept of the DS4G ion thruster is presented, along with the design, operating parameters and measured performance obtained from the first and second phases of the experimental campaign. Finally, the implications of these findings for future gridded ion thruster developments and future mission applications are addressed.

Hydrogen contamination in Ge-doped SiO2 thin films prepared by helicon activated reactive evaporation

Germanium-doped silicon oxide thin films were deposited at low temperature by using an improved helicon plasma assisted reactive evaporation technique. The origins of hydrogen contamination in the film were investigated, and were found to be H incorporation during deposition and postdeposition water absorption. The H incorporation during deposition was avoided by using an effective method to eliminate the residual hydrogen present in the deposition system. The microstructure, chemical bonds, chemical etch rate, and optical index of the films were studied as a function of the deposition conditions. Granular microstructures were observed in low-density films, and were found to be the cause of postdeposition water absorption. The granular microstructure was eliminated and the film was densified by increasing the helicon plasma power and substrate bias during deposition. A high-density film was shown to have no postdeposition water absorption and no OH detected by using a Fourier-transform infrared spectrometer.