We are performing an experimental feasibility study on down-scaling HEMP (High Efficiency Multistage Plasma) thrusters in order to gain a deeper understanding of the influence of design parameters and to test the possibility of a working µN HEMP thruster. This includes building up prototypes and probing the operation space as well as measuring the divergence angle of the plume and ion acceleration voltage by means of Faraday Cups and a Retarding Potential Analyzer. From these parameters thrust and specific impulse can be calculated using simple models.
Due to the importance of the thrust value and uncertainties of the models it is desirable to measure the thrust and thrust noise directly with a thrust balance. Building up such a device is under progress.
Down-Scaling of HEMP Thrusters
While LISA Pathfinder currently considers both cold gas and FEEP (Field Emission Electric Propulsion) as possible micro-propulsion systems, the new system baseline for NGO was kept compatible with all kinds of precision propulsion technologies, including cold gas, FEEP, and also µRIT (Radio Ion Thruster). A further interesting alternative could be the use of a down-scaled HEMP (High Efficiency Multistage Plasma) thruster. The permanent magnet assembly inside the thruster leads to a low erosion of thruster parts and the gridless design results in a simple system.
Main test device as depicted below is improved from precursor with respect to temperature stability and reliability. This thruster was built with different housing materials which differ with respect to magnetic and electric properties: insulating ceramics, paramagnetic aluminum and ferromagnetic steel.
The Operation space is dependent on housing material. Lowest calculated thrust of 70µN was achieved with ceramics housing (corresponding specific impulse 125 s) while for higher thrust values and other housing materials (150 – 430 µN) points are between 300 – 580 seconds. These values are calculated with measured divergence efficiencies (see next section).
With Big Mac test facility at university of Gießen the three thruster variations were characterized via an array of Faraday Cups and a Retarding Potential Analyzer. It is possible to measure ion current density and ion acceleration voltage angular dependent.
The results of Faraday Cups measurements are shown in Figure 2. The paramagnetic ceramics and aluminum housing shows side lobes with a maximum of ion emission near 60° while the ferromagnetic steel housing has a broad maximum at 0°. This indicates that the plume geometry depends on the magnetic property of housing material and is independent of electric characteristics. All thrusters have approximately the same divergence efficiency of 0.5.
Using the Retarding Potential Analyzer it is possible to measure the potential difference the ions have passed. The results of the ceramics housing thruster for two angles are shown in Figure 3. At 60° main part of ions passed the whole potential difference while at 0°they passed only a small fraction of potential. Since the current density at 60°is much larger, this is the predominant case and hence the acceleration efficiency is very high. From these measurements it is not possible to derive the kinetic ion energy because the charge state of the ions is unknown.
For thrust measurement, a balance is in assembly which is shown in Figure 4. Goal is to measure thrust and thrust noise in the µN range directly. It consists of two highly symmetric pendulums for common mode suppression of seismic noise and an optical readout via a picometer heterodyne interferometer. Electric connections are carried out over the spring so that no wire changes the movement of pendulum. An electrostatic actuator can be used for closed loop operation and for spring tuning (negative spring constant).
After manufacturing the reference pendulum and the interferometer, first characterization measurements can be performed.
A systematic test campaign with the HEMP thrusters is planned to gain a deeper understanding of design parameters. All relevant parameters will be systematically varied and operation space and plume properties measured. The thrust balance has to be characterized without thruster operation (noise floor) and calibrated in open loop mode.