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The effectiveness of collimated Faraday probes in obtaining the ion current density profile in Hall thrusters is investigated. A collimated probe design is attractive because it offers the possibility of obtaining the true ion current density profile regardless of the facility pumping speed. The collimator is intended to act primarily as a filter f...
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... of these variations resulted in any changes to the measured current density. Table 4 presents the thruster operating conditions that were investigated. The thruster was operated at 300 and 500 V and 4.5 and 10 A. Biased and floating operation was actually collected at all conditions, but not all of the biased collimator or floating nude results are presented. ...
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Current gradient-index (GRIN) lens based proximal-driven intracoronary optical coherence tomography (ICOCT) probes consist of a spacer and a GRIN lens with large gradient constant. This design provides great flexibility to control beam profiles, but the spacer length should be well controlled to obtain desired beam profiles and thus it sets an obst...
Citations
... First of all, the cup design of the collector limits the additional current carried by secondary electrons emitted from the collector surface due to ion bombardment. Additionally, the collimating geometry of the probe allows capturing ions from a well-defined solid angle [8,9]. Moreover, the collimated FC is less sensitive to low energy charge exchange ions, the density of which depends heavily on the chamber background pressure [10]. ...
A prototype of krypton Hall-effect thruster (HET) of 0.5 kW nominal power and a dedicated diagnostic system for ion current collection were designed in the laboratory of plasma space propulsion (PlaNS) of the Institute of Plasma Physics and Laser Microfusion (IPPLM) in Warsaw. The diagnostic system consisting of a collimated Faraday Cup (FC) and a Planar Probe with Guarding Ring, named also Faraday Probe (FP) was intended to capture both temporal and spatial ion current variation, allowing to analyze not only ion current dynamics locally but also to estimate the total ion current value and a plasma jet divergence. Reliable engine operation provided by stability of plasma in a discharge channel of the thruster is unambiguously reflected in the oscillations of the discharge current. The so-called breathing mode, categorized as ionization instability with frequencies in the range of 10–30 kHz, is commonly recognized in the HET’s discharge current. Rising of instabilities makes it difficult to increase the specific impulse effectively by just simply attempting to operate thruster in high-voltage regime because it may result in very irregular thruster functioning and often to ceasing of plasma. Discharge current oscillations should also be strongly reflected in the ion current. Indeed, a similar to discharge current behavior was observed in the recorded FC and FP ion current signals. By changing thruster operating conditions, like discharge voltage and magnitude of B-field, transitions between smooth and oscillating current regimes were examined. Studying the ion current dynamics seems particularly important, since it is predicted that the control of discharge instabilities may be crucial to improve the performance of HETs in the future.
... The current coming from the side of the probe can be isolated from coming to the planar collector. In addition, variations caused by the plasma in the gap between the collector and the outer casing can be corrected by simply applying a correction factor [15,16]. ...
Characterization of IEC tight jet mode is achieved through Faraday probe measurements. Preliminary results indicate that the tight jet is a highly energetic electron beam with scattering of secondary ions and electrons, which result from electron beam impact ionization. A novel analytical model is proposed to evaluate this non-Maxwellian plasma beam, including a compensation of the secondary electron emission effect on the probe’s surface. The results show prominent features on the extracted jet’s kinetic energy and the respective (electron) current of IEC. In addition, they demonstrate a practical method to characterise non-Maxwellian plasma jets through Faraday probes.
... The Faraday probe is a simple plasma diagnostic used to measure ion current density in the HET plume1011121314. The probe is based on the Jet Propulsion Laboratory nude Faraday probe design [11]. ...
... Biasing the collector and guard ring to the same potential reduces edge effects by creating a uniform sheath potential around the collector. Twenty volts was chosen based on prior experience indicating that ion saturation is reached without substantial sheath growth for this type of Faraday probe [11,12]. Voltage measurements across a 1.417 kΩ, 0.5 W resistor placed in series with the collector line are read by an Agilent 34980A data acquisition unit. ...
The T-220HT Hall-effect thruster was tested with the external cathode at different azimuthal locations to determine the effect of an external cathode on the far-field plume. The cathode is mounted in two configurations: 1)on top of the thruster, perpendicular to the plane of plume measurements, and 2)in the plane of plume measurements. The thruster was tested at discharge voltages of 150-300V at a constant discharge current of 9A on krypton propellant. The vacuum facility operating pressure was below 9.2x10-6 torr center dot Kr for all operating conditions. Noticeable changes in the plume ion current density were measured that indicate plume asymmetry. With the cathode in the plane of the probes, the ion current density peak increases by up to 43% and the divergence angle decreases by up to 2.6deg. However, there is less than a 5V shift in the ion energy distribution, and the distribution shape remains constant. The results indicate that an external cathode alters the plume symmetry and creates a region of increased ion flux. An analysis of the cathode generated species densities indicates that the increase in ion current density is caused by nonuniform ingestion of cathode neutrals and nonuniform electron density near the cathode. This phenomenon can affect thruster plume measurements as well as the orientation of thrusters for satellite integration.
... Its use has been well documented. 2,[8][9][10][11] Figure 3 shows a picture and electrical schematic of the Faraday probe used in this work. The probe consists of a tungsten-coated, stainless-steel collection electrode with a stainless-steel guard ring surrounding it, with a 0.12 cm gap between. ...
The T-220HT is a 10-kW class Hall effect thruster developed as the primary propulsion system for satellites. In-channel electrodes and additional magnetic coils are added to study ion focusing to decrease energy losses from ion-wall neutralization and plume divergence in order to increase the thrust-to-power ratio. In this study, electrically-biased graphite electrode rings are embedded in the discharge channel walls to repel radial ions. The thruster is tested from 125-300 V at 9 A discharge, with the electrodes either floating, biased to 10 V or 30 V. The mass flow rate was varied from 9.8-10.4 mg/s to maintain constant current. Maximum chamber pressure was 1.5e-5 Torr-Xe. Performance measurements on xenon show a maximum increase in thrust-to-power ratio of 4.84 mN/kW, 15.3 mN thrust, 206 s I sp , and 8% anode efficiency. The plume ion current density, ion energy distribution function, and plasma potential is characterized and indicates a collimation of the ion beam and a increase in ion number density without an increase in propellant neutrals, which results in an increase in mass utilization. The different electrode currents and ion energy distribution functions at 10 V compared to 30 V electrodes leads to the idea of different modes of operation with different electrode biases.
... 1 High-energy exhaust particles interact with the neutral background particles through charge exchange collisions (CEX). 3,4 In the plume, the effects of CEX products are most evident in the perimeter, where they lead to an increase in the measured current density. At elevated background pressures, residual gas particles can be entrained into the thruster discharge region, artificially increasing engine thrust. ...
A technique for calibrating a vacuum chamber in terms of pressure to account for elevated back pressure while testing Hall thrusters was presented. As a first step in this process, a neutral gas background pressure map of the Large Vacuum Test Facility (LVTF) was created at a series of cold anode flow rates. The experimental results were used to validate a numerical model of LVTF with a cold-flow thruster. The computational model employs the direct simulation Monte Carlo method that includes the chamber walls and cryopumps.
... The numerical solutions from Magnet and measurements using a Gaussmeter agreed to within ±10%. The ion current density measurements used a nude Faraday probe identical to the one used in Ref. [228] and similar to the probes described in section 6.3.2. The goal of these experiments was to determine whether the focal length and plume divergence of the P5 could be improved by altering the inclination of the magnetic field lines. ...
... The first experiments with the NASA-173Mv1 were reported in Ref. [169,228]. During these experiments, the thruster design was still evolving. ...
... 9 shows the ion current density measured in the plume of the NASA-173Mv1 during the experiments described in Ref.[228]. The thruster was operated at 300 and 500 V, at mass flow rates of 5.3 and 10.0 mg/s, and only the inner and outer coils were energized. ...
This dissertation presents research aimed at extending the efficient operation of 1600 s specific impulse Hall thruster technology to the 2000 to 3000 s range. Motivated by previous industry efforts and mission studies, the aim of this research was to develop and characterize xenon Hall thrusters capable of both high-specific impulse and high-efficiency operation. During the development phase, the laboratory-model NASA 173M Hall thrusters were designed and their performance and plasma characteristics were evaluated. Experiments with the NASA-173M version 1 (v1) validated the plasma lens magnetic field design. Experiments with the NASA 173M version 2 (v2) showed there was a minimum current density and optimum magnetic field topography at which efficiency monotonically increased with voltage. Comparison of the thrusters showed that efficiency can be optimized for specific impulse by varying the plasma lens. During the characterization phase, additional plasma properties of the NASA 173Mv2 were measured and a performance model was derived. Results from the model and experimental data showed how efficient operation at high-specific impulse was enabled through regulation of the electron current with the magnetic field. The electron Hall parameter was approximately constant with voltage, which confirmed efficient operation can be realized only over a limited range of Hall parameters.
... 1 High-energy exhaust particles interact with the neutral background particles through charge exchange collisions (CEX). 2 In the plume, the effects of CEX products are most evident in the perimeter, where they lead to an increase in the measured current density. Thruster operation and performance are dependent on the background pressure of the facility. ...
A neutral background pressure map of the Large Vacuum Test Facility (LVTF) at the University of Michigan is presented. The LVTF was mapped at hot anode (i.e., discharge on) flow rates of 5.25 and 10.46 mg/s, corresponding to P5 Hall thruster operating conditions of 1.5 and 3.0 kW. The chamber pressure was mapped at nominal xenon pumping speeds of 140,000 and 240,000 l/s, corresponding to base pressures of 1.3x10-6 Torr and 7.3x10-7 Torr, respectively. The pressure map was performed with a rake consisting of five calibrated Bayard-Alpert (BA) hot-cathode ionization gauges. Analysis of axial pressure profiles on the LVTF's centerline shows that the plume pressure decreases from a maximum at the thruster exit plane down to the facility background pressure at approximately 2 m downstream of the exit plane. In addition, a neutral gas background pressure map of Vacuum Facility 12 (VF-12) at the NASA Glenn Research Center is presented. VF-12 was mapped at a series of cold anode flow rates corresponding to P5 Hall thruster operating conditions of 1.5, 3.0 and 9.0 kW. The chamber pressure was mapped at a nominal xenon pumping speed of 282,000 l/s with a rake consisting of the same five BA hot-cathode ionization gauges used in the LVTF. The cold flow pressure in VF-12 rapidly decreases out to approximately 2 m downstream of the thruster exit plane. From this point, the pressure slowly, but continually drops as the cryosurfaces are approached. Cryopump location affects the neutral density gradients within a vacuum facility and should be carefully considered when analyzing plume and performance data or designing a new Hall thruster facility. The hot flow neutral density profiles of each facility will be used to help correct performance and plume measurements for facility effects.
... 4,7 Two technical approaches to developing high power Hall propulsion systems have been considered: building a large monolithic, high-power thruster of the desired power level or building an array of smaller thrusters that collectively operate at the desired power level. 12,13 Due to ground testing constraints, 14,15 likely spacecraft architectures and reliability/redundancy issues a multiple thruster approach would likely be preferred, especially at megawatt power levels. However, the maximum size for any single thruster has not been adequately investigated, so it is yet unclear what number of thrusters would be required for high power missions. ...
The NASA Hall thruster program currently supports a number of tasks related to high power thruster development for a number of customers including the Energetics Program (formerly called the Space-based Program), the Space Solar Power Program, and the In-space Propulsion Program. In program year 2002, two tasks were central to the NASA Hall thruster program: 1) the development of a laboratory Hall thruster capable of providing high thrust at high power-, and 2) investigations into operation of Hall thrusters at high specific impulse. In addition to these two primary thruster development activities, there are a number of other on-going activities supported by the NASA Hall thruster program. These additional activities are related to issues such as high-power power processor architecture, thruster lifetime, and spacecraft integration.
... Since efficiency scales with the thrust squared, the ensuing drop in thrust causes the I sp and efficiency to fall as well. A minimum in the discharge current is later shown, and the performance correspondingly 7 AMERICAN INSTITUTE OF AERONAUTICS AND ASTRONAUTICS continues to plummet when the current later increases past the minimum. ...
Investigations of high-voltage Hall thrusters have indicated a peak in the efficiency versus voltage characteristic that limited the maximum efficiency to specific impulses of less than 3000 s. This peak is believed to be primarily a trait of modern magnetic field design, which is optimized for discharge voltages of 300 V. The NASA-173M has been operated at 300-1000 V and 5 mg/s to investigate whether performance improvements could be realized through in situ variation of the magnetic field topography through the use of an auxiliary trim coil. Without the trim coil, a peak in the efficiency characteristic was observed at 600 V. The results with the trim coil energized indicate there is always some performance benefit to altering the magnetic field topography. Above 400 V, efficiencies were maintained at >50% and above 900 V, specific impulses >3000 s were demonstrated while using the trim coil. The largest gains in performance were observed at 1000 V, where the thrust, specific impulse, and efficiency improved by 10 mN, 200 s, and 5.5%, respectively, to 165 mN, 3360 s, and 51.5%. The results demonstrate that the peak in the efficiency characteristic observed without the trim coil can be mitigated when the magnetic field topography is tailored for high-voltage operation. Analysis of the magnetic field from numerical simulations has identified several important factors contributing to the performance benefits with trim coil operation.
... 1 Highenergy exhaust particles interact with the neutral background particles through charge exchange collisions (CEX). 2 In the plume, the effects of CEX products are most evident in the perimeter, where they lead to an increase in the measured current density. Thruster operation and performance are dependent on the background pressure of the facility. ...
A neutral gas background pressure map of the Large Vacuum Test Facility (LVTF) at the Universit y of Michigan is presented. The LVTF was mapped at a series of cold anode flow rates corresponding to P5 Hall thruster operating conditions of 1.5, 3.0, and 9.0 kW. The chamber pressure was mapped at nominal xenon pumping speeds of 140,000 and 240,000 l/ s with a rake consisting of five calibrated Bayard -Alpert (BA) hot -cathode ionization gauges. The cold flow results were used to validate a full 3 -D numerical model of the LVTF with a cold -flowing Hall thruster. This computational facility model was buil t with MONACO, a 3D unstructured direct simulation Monte Carlo method (DSMC) code that includes asymmetric features such as the gridded chamber floor and cryopumps. This investigation is intended to begin initial development of the facility effects charac terization being pursued by the Plasmadynamics and Electric Propulsion Laboratory (PEPL) at the University of Michigan. The measured axial pressure profiles on the thruster centerline indicate that the plume expands to the facility background pressure in a pproximately 1.7 m. The plume expansion appears to be independent of anode flowrate and facility background pressure. The experimental and computational data exhibit the same general trends. However, there is a discrepancy of a factor ranging between 4 and 6 between the measured data and the computed results. This discrepancy is thought to stem from the choice of neutral -to - cryosurface sticking coefficient and a high computational cryosurface temperature. The next steps include modifying the model, m ak ing a similar cold -flow map of a NASA vacuum facility, and making a hot -flow map of the LVTF.
