This work was supported in part by OSTIn-SRP/EDB, the National Research Foundation (Singapore), Academic Research Fund AcRF Tier 1 RP 6/16 (Singapore), and the George Washington Institute for Nanotechnology (USA). I. L. acknowledges the support from the School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology.

Here we present the protocols for the thrust calibration procedure and performance evaluation, independent thrust verification via null measurement and plume profilometry through spatial in situ data sensing.
1. Thrust calibration procedure and thrust performance evaluation
<ol>
	<li>Ensure that all components are installed in the chamber as shown in Figure 5.</li>
	<li>Test the connectivity of the diagnostic tools externally before sealing the chamber.<...

Typical Hall-type thrusters44 are relatively simple, cheap and highly efficient devices that could accelerate an ion flux to the velocities of several tens of km/s, providing thrust required for accelerating satellites and spacecraft, as well as for maneuvering, orientation, position and attitude control, and de-orbiting at the end of their operation service life. Application of Hall thrusters on satellites and other orbital payloads enhance mission lifetime, allow orbital transfer and formation/c...

Renewed interest in the space industry has in part been catalyzed by highly efficient electric propulsion systems that deliver enhanced mission capabilities at increasingly reduced launch costs1,2,3. Many different types of space electric propulsion devices have recently been proposed and tested4,5,6,7,8&#160;supported by the present-day interest in space exploration9,10. Among them, gridded ion11,12 and Hall-type thrusters13,14 are of primary interest due to their ability to reach very high efficiency of about 80%, exceeding that of any chemical thruster, including the most efficient oxygen-hydrogen systems, the efficiency of which is limited to about 5000 m/s by the principal physical laws15,16,17,18.
Comprehensive, reliable testing of miniaturized space thrusters typically requires a large complex of test facilities that include test chambers, vacuum facilities (pumps), control and diagnostics instruments, a system for measurement of plasma parameters19, and a wide range of auxiliary equipment that sustain the operation of the thruster, such as an electric power supply system, propellant supply unit, thrust measurement stand and many others20,21. Moreover, a typical space propulsion thruster consists of several units which separately influence the efficiency and service life of the whole thrust system, and therefore, could be tested both separately and as part of the thruster assembly22,23. This significantly complicates test procedures and implies long test periods24,25. Reliability of a thruster&#39;s cathode unit, as well as operation of thrusters when different propellants are used also requires special consideration26,27.
To quantify performance of an electric propulsion system, and to qualify modules for operational deployment in space missions, ground testing facilities which enable simulation of realistic space environments are needed for testing of multi-scaled propulsion units28,29,30. An example of such a system is a large scaled space environment simulation chamber located at the Space Propulsion Centre-Singapore (SPC-S, Figure 1a,b)31. When developing such a simulation environment, the following primary and secondary considerations need to be taken into account. Primary concerns are that the thus-created space environment must accurately and reliably simulate a realistic space environment, and the in-built diagnostic systems must provide precise and accurate diagnostics during performance evaluation of a system. Secondary concerns are that the simulated space environments must be highly customizable to enable rapid installation and testing of different propulsion and diagnostic modules, and the environment must be able to accommodate high throughput testing to optimize discharge and operational conditions of multiple units simultaneously.
Space environment simulators and pumping facilities
Here, we illustrate two simulation facilities at SPC-S that have been implemented for the testing of miniaturized electric propulsion systems, as well as integrated modules. These two facilities are of different scales, and primarily have different roles in the process of performance evaluation, as outlined below.
Large plasma space actuation chamber (PSAC)
The PSAC has dimensions of 4.75 m (Length) x 2.3 m (Diameter) and has a vacuum pumping suite which comprises of numerous high capacity pumps working in tandem. It is able to achieve a base pressure lower than 10-6 Pa. It has an integrated vacuum control readout and pump activation/purge system for evacuation and purging of the chamber. It is equipped with numerous customizable flanges, electrical feedthroughs and visual diagnostic portholes to provide line test facility. This, together with a full-suite of diagnostics capabilities mounted internally, allows it to be rapidly modified for multi-modal diagnostics. The scale of PSAC also allows for testing of completely integrated modules for applications in a simulated environment.
PSAC is the SPC-S flagship space environment simulation facility (Figure 1c,d). Its sheer size allows for testing of complete modules of up to a few U&#39;s mounted on a quadfilar stage. The advantage of this method would be in the real-time visualization of how the propulsion modules as mounted on different payloads may influence in situ maneuvering of payloads in space. This is simulated through the mounting and suspension of the entire payload on a proprietary quadfilar thrust measurement platform. The thruster can then be fired, and the suspended platform with the thruster and payload would be tested according to space conditions. Propellant gas feedstocks which enter the test environment via the electric propulsion modules are pumped out efficiently by the vacuum suite to ensure that the chamber&#39;s overall pressure is not altered, thus, maintaining a realistic space environment32,33,34. Furthermore, electric propulsion systems typically involve the production of plasmas and exploit the manipulation of trajectories of charged particles exiting the system in order to generate thrust35. In smaller simulation environments, the buildup of charge or plasma sheaths on the wall may affect the discharge performance through plasma-wall interactions due to its proximity to the propulsion system, especially for micropropulsion where typical thrust values are in the order of millinewtons. Therefore, special attention and emphasis must be made to account for and marginalize contributions from such factors36. The PSAC&#39;s large size minimizes plasma-wall interactions, rendering them negligible, giving a more accurate representation of discharge parameters and enabling monitoring of plume profiles in electric propulsion modules. The PSAC is typically used in full module evaluation and systems integration/optimization processes which allows for quick translation of thruster prototypes into operationally ready systems for ground testing in preparation for space qualification.
Scaled plasma space environment simulator (PSEC)
The PSEC has dimensions of 65 cm x 40 cm x 100 cm and has a vacuum pumping suite which comprises of six high capacity pumps working in tandem (dry vacuum pump, turbomolecular and cryo vacuum pumps). It is able to achieve a base pressure lower than 10-5 Pa when the whole pumping system is operating (all pumps are in use). Pressure and propellant flows are monitored in real-time through integrated mass flow readout boxes and pressure gauges. The PSEC is primarily employed in endurance testing of thrusters. Thrusters are fired for extended periods of time to evaluate effects of plasma damage on discharge channels and on its lifetime. Additionally, as shown in Figure 2,&#160;a complex gas flow controller network in this facility enables quick connection of other feedstock propellants to the cathode and anodes to test compatibility of thrusters with novel propellants and effects of the latter on thruster performance. This is of increased interest to research groups working on &#34;air-breathing&#34; electric thrusters utilizing novel propellants during operation.
Integrated diagnostic facilities (multi-modal diagnostics)
Different integrated diagnostic facilities, equipped with automated integrated robotic systems (AIRS-&#181;S)19,23, have been developed for the two systems in PSEC and PSAC to cater for diagnostics at different scales and purposes.
Integrated diagnostics in PSEC
The diagnostic tools in PSEC essentially hinge on real-time monitoring of discharge through extended operations. The quality management system monitors residual gas in the facility for contaminant species that arise from sputtering of material during a discharge. These trace amounts are quantitatively monitored over time to evaluate erosion rates of the discharge channel and electrodes of the thruster to estimate the thruster&#39;s lifetime. The optical emissions spectrometer (OES) complements this procedure by monitoring spectral lines corresponding to electronic transitions of contaminant species due to erosion, such as copper from the electronics. OES also enables non-invasive plasma diagnostics and active monitoring of plume profile which qualitatively evaluates performance of the thruster. Finally, a robotic Faraday probe which can be controlled remotely, or set to fully autonomous mode, is used to derive quick sweeps of the plume profile to optimize collimation of beam through parametrically varying discharge conditions (Figure 3).
Integrated diagnostics in PSAC 
The luxury of physical space in the PSAC enables installation of multiple thruster systems at various locations due to its modular design, allowing for plug-and-play-like installation for various diagnostics simultaneously. Figure 4 shows the internal cross-section of the PSAC in various configurations, with the fully suspended quadfilar thrust measurement platform being its most notable and permanent fixture. Turret systems, controlled autonomously or wirelessly via Android apps using microcontrollers and Bluetooth modules, can then be mounted in a modular manner facing the thruster to obtain characteristics of the plume through the installation of various probes such as Faraday, Langmuir and Retarding Potential Analyzer (RPA). Also shown in Figure 4 is the ability of the PSAC to allow for configurable mounting of thruster systems for rapid simultaneous diagnostics of various plasma parameters. The thrusters can be mounted vertically in a single column and tested rapidly, one after another to avoid interactions between the different thruster systems. It has been verified that efficient evaluation of up to 3 different modules at a single instance is possible, thus significantly reducing the downtime during evacuation and purging processes required otherwise when testing systems individually. On the other hand, this system is a valuable opportunity for testing the thruster assemblies that should operate in a bunch, on the same satellite. The thrusters can be mounted vertically in a single column and tested rapidly, one after another to avoid interactions between the different thruster systems. It has been tested to be effective in the evaluation of up to 3 different modules at a single instance, significantly reducing downtime during evacuation and purging processes required otherwise when testing systems individually.
It is vital to determine the thrust in micropropulsion systems accurately so that parameters such as efficiency, &#951;eff&#160;and the specific impulse Isp, are accurate, thus, giving a reliable representation of the dependence of thruster performance on various input parameters such as the propellant flow, and power supplied to the different terminals of the thrusters as shown in Equations 1 and 2. Explicitly, performance evaluation of micropropulsion systems typically revolves around the measurement of thrust generated from the system at various operating parameters. Therefore, performance evaluation systems need to be calibrated according to a set of standards before being installed into the space environment for use in diagnostics and testing to ensure their reliability and accuracy19.
<img alt="Equation 1" src="/files/ftp_upload/58466/58466eq1.jpg" />
<img alt="Equation 2" src="/files/ftp_upload/58466/58466eq2.jpg" />
Typical systems employ force calibration externally before thrust measurement units are installed into the test environment38. However, such systems do not account for the space environments affecting the material properties of the calibration standards, and for electrical, vacuum and thermal influences on the degradation of the calibrated standards over the dynamic course of performance evaluation of the thrusters. The automated wireless calibration unit shown in Figure 5, on the other hand, allows for in situ calibration of the system in the simulated environment before the thruster is operational. This accounts for the dynamic effects of the test environment on the measurement stage, and allows for rapid re-calibration of the system prior to firing of thrusters. The system also features a symmetric modular null thrust verification unit which verifies the thrust independently. It is operated while the thruster is operational for in situ analysis of the derived thrusts from given discharge conditions. The entire process is done via MATLAB apps, allowing users to focus on optimization of hardware and design of propulsion systems, and expedites testing of such systems. Details of this method would be elaborated in the following subsection.

<table border="0" cellpadding="0" cellspacing="0" style="width:912px;" width="911">
	<tbody>
		<tr height="31">
			<td height="31" style="height:31px;width:429px;">Arduino Microcontroller</td>
			<td style="width:159px;">Arduino</td>
			<td style="width:260px;">Arduino Uno Rev 3</td>
			<td style="width:65px;"></td>
		</tr>
		<tr height="31">
			<td height="31" style="height:31px;">Bluetooth communication device</td>
			<td>SG Botic</td>
			<td>WIR-02471</td>
			<td></td>
		</tr>
		<tr height="31">
			<td height="31" style="height:31px;">Cryogenic Pump</td>
			<td>ULVAC</td>
			<td>CRYO-U12HLE&#160;</td>
			<td></td>
		</tr>
		<tr height="31">
			<td height="31" style="height:31px;">Digital Oscilloscope</td>
			<td>Yokogawa</td>
			<td>DLM 2054</td>
			<td></td>
		</tr>
		<tr height="31">
			<td height="31" style="height:31px;">Dry Pump</td>
			<td>Agilent</td>
			<td>Triscroll-600</td>
			<td></td>
		</tr>
		<tr height="31">
			<td height="31" style="height:31px;">High resolution laser displacement sensor</td>
			<td>Micro-Epsilon</td>
			<td>optoNCDT ILD-1420-50</td>
			<td></td>
		</tr>
		<tr height="31">
			<td height="31" style="height:31px;">Mass Flow Controller</td>
			<td>MKS</td>
			<td>MKS M100B</td>
			<td></td>
		</tr>
		<tr height="31">
			<td height="31" style="height:31px;">Optical Emission Spectrometer</td>
			<td>Avantes</td>
			<td>AvaSpec-ULS2048XL-EVO</td>
			<td></td>
		</tr>
		<tr height="31">
			<td height="31" style="height:31px;">Servo Motor</td>
			<td>Tower Pro</td>
			<td>Servo Motor SG90</td>
			<td></td>
		</tr>
		<tr height="31">
			<td height="31" style="height:31px;">Stepper Motor</td>
			<td>Oriental Motor</td>
			<td>PKP213D05A</td>
			<td></td>
		</tr>
		<tr height="31">
			<td height="31" style="height:31px;">Turbomolecular Pump</td>
			<td>Pfeiffer</td>
			<td>ATH-500M</td>
			<td></td>
		</tr>
	</tbody>
</table>

optimization, test and diagnostics of miniaturized hall thrusters

Miniaturized spacecraft and satellites require smart, highly efficient and durable low-thrust thrusters, capable of extended, reliable operation without attendance and adjustment. Thermochemical thrusters which utilize thermodynamic properties of gases as a means of acceleration have physical limitations on their exhaust gas velocity, resulting in low efficiency. Moreover, these engines demonstrate extremely low efficiency at small thrusts and may be unsuitable for continuously operating systems which provide real-time adaptive control of the spacecraft orientation, velocity and position. In contrast, electric propulsion systems which use electromagnetic fields to accelerate ionized gases (i.e., plasmas) do not have any physical limitation in terms of exhaust velocity, allowing virtually any mass efficiency and specific impulse. Low-thrust Hall thrusters have a lifetime of several thousand hours. Their discharge voltage ranges between 100 and 300 V, operating at a nominal power of &lt;1 kW. They vary from 20 to 100 mm in size. Large Hall thrusters can provide fractions of millinewton of thrust. Over the past few decades, there has been an increasing interest in small mass, low power, and high efficiency propulsion systems to drive satellites of 50-200 kg. In this work, we will demonstrate how to build, test, and optimize a small (30 mm) Hall thruster capable of propelling a small satellite weighing about 50 kg. We will show the thruster operating in a large space environment simulator, and describe how thrust is measured and electric parameters, including plasma characteristics, are collected and processed to assess key thruster parameters. We will also demonstrate how the thruster is optimized to make it one of the most efficient small thrusters ever built. We will also address challenges and opportunities presented by new thruster materials.

Thrust calibration procedure and thrust performance evaluation
Evaluation of thrust values from the quadfilar thrust measurement stage comes in two phases. The first phase is through obtaining calibration factors from the automated wireless calibration unit shown to the right of Figure 5. In this calibration process, fine weights are lowered across a smooth polytetrafluoroethylene b...

Watch this Scientific Journal Video about Optimization, Test and Diagnostics of Miniaturized Hall Thrusters at JoVE.com

Optimization, Test and Diagnostics of Miniaturized Hall Thrusters

Here, we present a protocol to test and optimize space propulsion systems based on miniaturized Hall-type thrusters.

Miniaturized spacecraft and satellites require smart, highly efficient and durable low-thrust thrusters, capable of extended, reliable operation without ...

The overall goal of this presentation, is to present the methodology involved in the testing of specialized electric propulsion thrusters in a ground based space environment simulation facility. The methods involved feature automated systems which incorporate hardware-software integration to realize smart systems for automated or remote diagnostics and performance evaluation of propulsion modules and other payloads in space. The space propulsion center, Singapore, is a research center at the National Institute of Education, Nanyang Technological University, Singapore.The test environments developed here include two space environment simulation facilities for different purposes. The scaled space environment simulator is primally employed in lifetime testing of thrusters. Thrusters are fired for extended periods of time in this facility to evaluate effects of plasma damage on distress channels.and subsequently to infer the lifetime of the thrusters. The quadfilar suspended stage allows for proper visualization of how the propulsion modules as mounted on different payloads may influence in situ maneuver in space. This is simulated through the mounting and suspension of the entire payload on the suspended system.The thrusters can then be fired and the suspended platform where the modules are mounted on are actuated according to conditions in space. The quadfilar pendulum also expedites the process of leveling, calibration and installation of thrusters and modules for testing in the space environment. With this setup only one operator is required in the test facility to utilize frequency modulation on the torsional wires, to level and calibrate the entire system.In the large scale space environment simulator apart from the quadfilar suspended thrust stage that enables the in situ derivation of thrust, Modular spatially actuated robotic probes can also be customized for mounting through configurable attachment units. It is also worth noting that the large Space Environment Facility features numerous mounting points and vacuum grip electronic filters to allow for the installation of multiple thrusters and diagnostic equipment for simultaneous performance evaluation. This reduces the downtime incurred when the chamber is evacuated and pumped during access installation and reconfiguration processes if testing were to be done individually.We will now go through the procedures for installation and calibration of the quadfilar pendulum before testing of propulsion units. First, ensure that all components are installed in the chamber as required for subsequent testing. Test the connectivity of the diagnostic tools externally before sealing the chamber.Use the integrated facility control to seal the chamber. Turn on the vacuum pumps in cascading order starting from the dry pumps, turbo molecular pumps and then the cryogenic pumps. Use the developed apps to synchronize your devices with the wireless transponders in the chamber.The synchronization process is complete, when the flashing LED on the transponders stop flashing. Once the desired vacuum has been obtained and initial reading is taken off the laser displacement sensor as a baseline. Use the developed app to trigger the lowering of a calibrated weight for force translation on the quadfilar stage.Record the displacement from the laser displacement sensor. Repeat the process of lowering the weights and recording of the displacement quadfilar stage until all the calibration weights are expended. Draw a calibration curve to obtain the calibration factor for the system installed on the quadfilar stage.The thrusters can then be fired and the desired parameters can be captured in real time by data acquisition program written by in house researchers. Alternatively an integrated app can be used to fully automate the calibration process while syncing the actuation sequence from the motors and data acquisition from the sensors accordingly. We will now go through the procedures for independently verifying the obtained thrust parameters to a null measurement and how a spatially actuated proxy can be triggered to obtain plume profiles after thrust measurements have been made.First, take a baseline reading of the quadfilar pendulum in the equilibrium position. Toggle operational parameters to desired values from the thruster control panel and fire the thruster. Once the trust is fired, wait for the oscillations on the quadfilar pendulum to stabilize.After stabilization, use the control app for the null measurement system to trigger the lowering of weights. The weights are continually lowered until the quadfilar stage is actuated back into equilibrium. Once the equilibrium position is reached, the actuation sequence is terminated and the force required to bring the quadfilar system back to equilibrium is determined.A stopper block is then triggered to stop the quadfilar stage from moving. A sweeping sequence is then performed on the spatial measurement probe mount. A synchronized sequence is looped to acquire data from the probe at each spatial location and stored in an array to be analyzed accordingly.Other probes can be customized to be mounted on the modular attachment to use spatial information on the plume profiles. In this section we will go through typical results obtained from a calibration sequence, as well as typical plume profiles obtained through a faraday probe sweep. Calibration of the quadfilar thrust to measurement stage is done through the employment of that probe motor driven for the translation system.In order to derive the calibration factors required for derivation of thrust during experimental tasking. A sequence it triggered by an operator of an automated program to lower fine calibration weights that act vertically and translates horizontally to simulate actuation when the thruster is fired. Readings from a high resolution laser displacement sensor are taken at each interval and a calibration curve is then drawn to obtain the calibration factor for subsequent measurements on the system.In this figure, we see a typical calibration curve drawn during a single automated calibration process. As can be seen, the proper alignment and setup of the quadfilar stage results in a very linear calibration plot yielding a calibration factor of 27.65 milli newtons per volt. In a standardized set up for thrust measurements over a wide range of forces.The setup can also be modified to fit in calibration weights for extended regimes as shown in this calibration plot. The torsional wires are adjusted for sensitivity and both fine and course calibration weights are included to yield a calibration plot that is linear in both regimes. A sample of the in situ measurements for derived thrust is shown in this figure.This figure shows how an operator is able to monitor the dependence of thrust on discharge voltage. During the course of the experiment, until the discharge is extinguished. Using the quadfilar thrust measurements stage, we were able to measure the thrust produced by the whole thruster at various input power given by the discharge current and applied voltage.Through these information, the variation of efficiency and specific impulse with respect to input power can be obtained. These figures show how the thrust and specific impulse vary with input power at four different mass flow rates. And this figure shows how the efficiency depends on the input power.The results show that the thruster has been optimized to work at input power is below 100 watts, where low flow rates have resulted in efficiencies of almost 30%After the thruster is fired a null measurement sequences is triggered to independently verify the trust obtained from the system. When the thrust is fired, the stage displaces according to the magnitude of propulsion derived out of the system. The null measurement unit is a symmetric system mounted opposite calibration unit, which utilizes a similar force translation system to actuate the stage back into equilibrium.A laser displacement sensor actively monitors the displacement throughout the measurement and triggers the activation system to activate a sequence which terminates only when baseline equilibrium is achieved. An operator is also able to visualize the in situ plume profiles as shown in this figure. This figure shows how the discharge power influences the magnitude of the peak iron current density and the full width at half maximum accordingly.Physical processes that are intrinsic to plasma's are known to drive and control self organization and self assembly during material synthesis. at QUT in collaboration with plasma sources and application center, we study how these building blocks are formed, shaped and delivered to surfaces on the different plasma conditions. We hope that by understanding how these plasma form nano structures behave we will be able to design processes that ensure the timely and efficient delivery and incorporation only to add the site where the repair is actually needed.Providing us with a plasma propulsion systems that are longer serving and more efficient. In this presentation we have presented an overview of the considerations made when designing a facility for testing of propulsion systems and deployable modules in a simulated space environment. Additionally, we have shown the versatility and strengths of using micro controller based systems for in situ data acquisition and analytics, which can be rapidly adapted to perform other modes of evaluation depending on operational mission demands accordingly.

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