Name: Optimization, Test and Diagnostics of Miniaturized Hall Thrusters (Video) | JoVE Journal
Uploaded: 2019-02-16T13:00:00.000+00:00
Duration: 12 min 22 s
Description: Learn automated testing methods for miniaturized Hall thrusters using integrated hardware-software systems. This protocol enables remote diagnostics and performance evaluation in simulated space environments.

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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.</li>
	<li>Use the integrated facility control to seal the chamber.</li>
	<li>Turn on the vacuum pumps in cascading order starting from the dry pumps (until the chamber reaches 1 Pa), turbo-molecular pumps (until it reaches ~5 x 10-4 Pa), and then the cryogenic pumps. 
	NOTE: PSAC is left to pump down to high vacuum (&#60; ~10-5 Pa) to simulate space environment. The protocol can be paused here.</li>
	<li>Use the developed apps to synchronize the devices with the wireless transponder in the chamber. The synchronization process is complete when the light-emitting diode (LED) on the transponder stops flashing.</li>
	<li>Once the desired vacuum has been obtained, take an initial reading (analog voltage) off the laser displacement sensor as a baseline.</li>
	<li>Use the developed app to trigger the lowering of a weight (of a precisely known and calibrated mass of copper loop) for force translation on the quadfilar stage. 
	NOTE: The mass of each copper loop depends on the intended sensitivity of the quadfilar stage being used. In this case, the mass of each copper loop was in the order of 100 mg for the extended calibration regime and 10 mg for the fine calibration regime. See the representative results for more information.</li>
	<li>Record the displacement (analog voltage) from the laser displacement sensor when it is triggered after the mass is fully lowered and its weight is translated into a horizontal force.</li>
	<li>Repeat the process (steps 1.7 and 1.8) of lowering the weights and recording of the displacement of the quadfilar stage until all the calibrations weights are expanded. All the weights will be automatically returned to the equilibrium position by the calibration unit after the sequence is completed to allow the quadfilar stage to reach an equilibrium position before thruster can be fired. Save the calibration factor ( File | Save as | &#34;Factor.txt&#34;).</li>
	<li>Draw a calibration curve to obtain the calibration factor for the system installed on the quadfilar stage, where the calibration factor (in mN/V) is the gradient of the force/voltage graph.</li>
	<li>Record a baseline analog voltage from the laser displacement sensor again before firing the thruster.</li>
	<li>Activate the in situ MATLAB program for calculating thrust instantaneously using Equation 3 (see the representative results) and input the calibration factor derived in step 1.9 ( File | Open | &#34;Factor.txt&#34;).</li>
	<li>The thrusters can then be fired again. Capture the desired parameters in real time using the in-house data acquisition program. 
	NOTE: Alternatively, an integrated app can be used to fully automate the calibration process while synchronizing the actuation sequence from the motors, and data acquisition from the sensors accordingly.</li>
</ol>
2. Null measurement protocol for independent thrust verification
<ol>
	<li>First, take a baseline (analog voltage) reading (from the laser displacement sensor) of the thruster in equilibrium position.</li>
	<li>Toggle operational parameters to desired values from the thruster control panel and fire the thruster.</li>
	<li>Once the thruster is fired, wait for the oscillations on the quadfilar pendulum to stabilize.</li>
	<li>After the quadfilar stabilizes to a steady state, use the control app for the null measurement system to trigger the lowering of weights. Readings from the laser displacement sensor are monitored simultaneously. The weights are continually lowered until the quadfilar stage is actuated back into equilibrium.</li>
	<li>Once the equilibrium position is reached, terminate the actuation sequence, and determine the force required to bring the quadfilar system back to equilibrium.</li>
	<li>Trigger a stopper block to stop the quadfilar stage from moving.</li>
	<li>Compute mass corresponding to the horizontal force required to pull the system back into equilibrium.</li>
</ol>
3. Actuation of robotic turrets for spatial in situ data sensing and plume profilometry
NOTE: During operation of the thruster, an operator may choose to actuate the system manually to desired angles to obtain plume characteristics at particular locations or trigger an automated sequence.
<ol>
	<li>Mount the thruster on a moving stage (as in the case of PSAC) before starting the experiment.</li>
	<li>Activate the stop-bar mechanism to prevent the stage from actuating during the experiment.</li>
	<li>Trigger the measurement protocol and servo motor to actuate the probe to the 0&#176; position.</li>
	<li>Acquire a measurement from the probe. 
	NOTE: Depending on the type of probes installed, the measurement processes can be varied according to the programmable sequence for obtaining complete spatial plume profiles of the discharge. (a) If a Faraday probe is mounted, a reading off a source meter is taken (where a bias of -30 V is continuously applied to the guard rings). (b) If a Langmuir probe is mounted, a sawtooth voltage waveform is supplied to the probe and the I-V characteristics are obtained and interpreted. (c) If an RPA is mounted, a sawtooth voltage waveform is applied to the discriminating grid, and the I-V characteristics are obtained and interpreted.</li>
	<li>Trigger the servo motor using the microcontroller, to move to the next angular position where the probe sequence is triggered to make a measurement again.</li>
	<li>Save the measurements in individually marked arrays in a data matrix.</li>
	<li>Repeat steps 3.5 and 3.6 until a full sweep up to 180&#176; has been performed, and the probe is brought back to 0&#176;.</li>
	<li>Analyze the saved data.</li>
</ol>

The authors declare no competing financial or other interests.

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 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'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'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'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'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, 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 "air-breathing" electric thrusters utilizing novel propellants during operation.
Integrated diagnostic facilities (multi-modal diagnostics)
Different integrated diagnostic facilities, equipped with automated integrated robotic systems (AIRS-µS)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'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, ηeff 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="Efficiency equation: η_eff = T²/(2mP), formula in scientific research analysis." src="/files/ftp_upload/58466/58466eq1.jpg" data-altupdatedat="1788250015000" data-alt="Equation 1">
<img alt="Specific impulse equation \( I_{sp} = \dfrac{T}{\dot{m}g} \); propulsion efficiency calculation." src="/files/ftp_upload/58466/58466eq2.jpg" data-altupdatedat="1788250015000" data-alt="Equation 2">
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><tbody><tr><td>Arduino Microcontroller</td><td>Arduino</td><td>Arduino Uno Rev 3</td><td></td></tr><tr><td>Bluetooth communication device</td><td>SG Botic</td><td>WIR-02471</td><td></td></tr><tr><td>Cryogenic Pump</td><td>ULVAC</td><td>CRYO-U12HLE&amp;nbsp;</td><td></td></tr><tr><td>Digital Oscilloscope</td><td>Yokogawa</td><td>DLM 2054</td><td></td></tr><tr><td>Dry Pump</td><td>Agilent</td><td>Triscroll-600</td><td></td></tr><tr><td>High resolution laser displacement sensor</td><td>Micro-Epsilon</td><td>optoNCDT ILD-1420-50</td><td></td></tr><tr><td>Mass Flow Controller</td><td>MKS</td><td>MKS M100B</td><td></td></tr><tr><td>Optical Emission Spectrometer</td><td>Avantes</td><td>AvaSpec-ULS2048XL-EVO</td><td></td></tr><tr><td>Servo Motor</td><td>Tower Pro</td><td>Servo Motor SG90</td><td></td></tr><tr><td>Stepper Motor</td><td>Oriental Motor</td><td>PKP213D05A</td><td></td></tr><tr><td>Turbomolecular Pump</td><td>Pfeiffer</td><td>ATH-500M</td><td></td></tr></tbody></table>

optimization, test and diagnostics of miniaturized hall thrusters

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...

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 ...

optimization-test-and-diagnostics-of-miniaturized-hall-thrusters

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Learn automated testing methods for miniaturized Hall thrusters using integrated hardware-software systems. This protocol enables remote diagnostics and performance evaluation in simulated space environments.

Optimization, Test and Diagnostics of Miniaturized Hall Thrusters

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