Thrust measurements of the RIT-10 thruster with DLR's new DEPB thrust balance

Abstract

To study new electric propulsion devices, accurate long-time measurements of the low thrust forces produced by these devices are necessary. For this purpose, a new, lightweight thrust balance has been established to be used at DLR’s STG-ET facility – the DLR Electric Propulsion Thrust Balance (DEPB). Several advancements in comparison to former thrust balances have been established. To evaluate the performance of this thrust balance, it has been operated to perform measurements of the well-studied RIT 10 gridded-ion thruster. Within this work, the new thrust balance and its performance shall be demonstrated and a first estimate on accuracy and long-term stability shall be given. To overcome some of the issues of previous thrust balance designs, some new approaches have been applied in the development and production of the new DEPB thrust balance. While the thrust balance itself is implemented as -inverted pendulum, these new design choices include the use of a commercial weighing-cell, a quartz-glass bearing of the thrust table and an eddy-current brake, all within a more compact design. The commercial weighing-cell, mounted in a vertical position, externalizes the issue of the thrust measurement mechanism to a self-contained highly accurate device. In principle it consists of an optical sensor and a voice coil for active compensation of appearing deflections by external forces. The quartz-glass bearing is optimized to be less sensitive to thermal gradients within the thrust balance while also being able to carry a high thruster mass of up to 50 kg and giving still a relatively high sensitivity to the thrust measurements. The eddy-current brake can effectively suppress oscillations of the weighing table. Additionally, the thrust balance itself can be heated or cooled using water piping integrated into the supporting structure. While the setup has been fully manufactured and calibrated, a final test using an electric thruster is still necessary. Since the RIT-10 thruster is well-studied, it has been selected for the final test of the thruster within DLRs small MT vacuum chamber. To study the behavior of the thrust balance, the RIT-10 thruster is operated for extended intervals on the DEPB thrust balance. The long-term stability in terms of thermal drift, effects of the fluid and electric interface and effects of the vacuum and plasma environment onto the thrust balance are studied. The results of the measurements and its performance in terms of accuracy and stability will be presented.