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Operation of satellites on Very Low Earth Orbits provides numerous benefits when compared to higher altitude orbits[1]. However, such orbits induce increased spacecraft drag which requires frequent compensation through engine thrust. The Air Breathing Electric Propulsion (ABEP) concept could allow more frequent utilization of such orbits by using collected particles from the residual atmosphere as propellant in an electric thruster. However, advances in such systems are still required for ABEP effectiveness to be proven. One of the areas crucial to ABEP feasibility is propellant collection through Air Intake-Collectors(AICs)[2,3]. VKI is currently building a small-scale Low Density Facility designed to duplicate VLEO conditions and with the immediate purpose of characterizing the collection efficiency and compression ratio of ABEP intake-collectors. This work aims to introduce the facility and present the design of experiments that allow the measurement of intake performance parameters. The Low Density Facility can be briefly described as being composed of two vacuum chambers connected by a pipe housing the engineering model of the air intake collector to be tested. Flow generation is obtained using an Inductively Coupled Plasma(ICP) source with magnetic beam control. Moreover the facility presents enough pumping capacity to allow for free molecular flow conditions with continuous PFG operation. The vacuum pumping system comprises one turbomolecular pump on each chamber with enough capacity to reach 0.002Pa under typical operating conditions. Facility diagnostics include cold-cathode ionization gauges for pressure measurements, a flow meter to measure the feed to the particle flow generator, and Retarding Field Energy Analyzer(RFEA) and Faraday probe will be used to measure the current as well as the energy distribution of ions at different positions in the plume. The extraction of intake performance parameters is a challenging task due to the complexity in flow phenomena present in the facility. Firstly, the fast ions that duplicate orbital conditions neutralize upon wall collisions meaning that ion diagnostics cannot be reliably used at the intake exit. Alternatively, chamber quantities (pressure, temperature) can be measured and related to the intake performance as it is usually performed in conductance measurements [4]. However, due to the plasma source utilization efficiency, only a fraction of the feed of gas is ionized and accelerated to duplicate orbital conditions. The remaining particles exit the source with much lower velocities. Furthermore, although sufficiently rarefied for the facility to operate under free molecular flow conditions, the background gas density is high enough for a relevant flux of background particles to exist between both chambers through the intake. As a result, chamber quantities are influenced by these three flows and their interaction with the intake. If an analogous procedure to classical conductance measurements is to be used, a model that quantifies the influence of each flow to the measured quantities needs to be developed. In this work, after giving a high level overview of the facility, we develop a simplified model that relates the quantities that are measurable in the facility (pressure, temperature, ion flow rates) to the intake performance parameters (collection efficiency, compression ratio). The simplified model is based on a 0-D mass balance of each chamber. At steady-state it provides a functional form that relates the measurements of the relevant flow rates to each chamber with the intake collection efficiency. Moreover, we simulate the operation of the LDF by means of Direct Simulation Monte Carlo(DSMC) simulations performed with the SPARTA open source software[5]. We evaluate the validity of using the 0-D model by comparing its prediction to the DSMC simulation results. It is found that the 0-D model predictions of each chamber density as well as the chambers mass exchange agree well with the DSMC simulations. Therefore, the 0-D model assumptions are justified and the model can be used as a data reduction equation for the calculation of the AIC collection efficiency from the experimental quantities measured in the chambers. |
