Hall thruster operation with externally driven breathing mode oscillations

The Hall thruster exhibits two types of large-scale coherent structures: axially propagating breathing mode (m = 0) and azimuthal, with low m, typically m = 1, spoke mode. In our previous work, it was demonstrated that axial breathing mode can be controlled via the external modulations of the anode potential. Two regimes of the thruster response, linear and nonlinear, have been revealed depending on the modulation amplitude. In this work, using the high-speed camera images and developed image-processing technique, we have investigated the response of the azimuthal mode to the external modulations. We have found that, in linear regime, at low modulation voltages, axial and azimuthal structures coexist. At larger amplitudes, in the nonlinear regime, the azimuthal mode is suppressed, and only axial driven mode remains.

The first direct experimental measurements of electron current due to rotating spokes in a modern highpower Hall thruster are presented. A segmented anode consisting of 12 equally spaced azimuthal sections has been retrofitted onto the H6 6-kW class Hall thruster and operated at power levels up to 3 kilowatts. Independent discharge current measurements on each anode segment at 1 MHz and synchronous high-speed video of the discharge at 87,500 frames per second reveal that visible rotating spoke structures in the thruster channel correspond to local electron current oscillations with amplitude approximately 30% of the mean local discharge current through each segment. Discrete Fourier transforms of discharge current oscillations on each segment reveal peaks at spoke rotation frequencies an order of magnitude larger than at the well-known breathing mode frequency. The apparent dominance of the breathing mode in traditional Hall thruster discharge current frequency spectra is revealed to be an artifact of the use of a contiguous ring-shaped anode. Based on the magnitude of local discharge current oscillations on each segment, the magnitude of plasma density oscillations are inferred to be of the order of the mean plasma density and the net discharge current carried by the spoke mechanism is calculated to be up to 50% of the total thruster discharge current.

We present a new power supply control method, which achieves constant flow Rate control for the thrust of a 20mN-class Hall thruster. First, we present observations of a 20mN-class Hall thruster with oscillation-mode-map. We make a theoretical study of the thrust and experiments on electrical characteristics of the Hall thruster, and conclude that thrust, thrust efficiency and low frequency oscillation are clearly determined by the external control parameters, anode voltage, gas flow rate, and magnetic flux density. Second, we discuss how to control the power supplies to suppress the power consumption, especially when the operation or thruster conditions change temporarily during use. The new method will be a very important guideline for Hall thruster system design and operation, in particular making it easy to manage the power consumption in a satellite by controlling the thrust resources. As a result of performance experiments for a 20mN-class Hall thruster, over 36% thrust efficiency of the Hall thruster was found to be sensitive to the anode voltage and applied magnetic flux density. The new power control method achieves constant flow rate control method of the thrust. The benefits are light weight and low cost.

The present paper describes performance evaluation of a Dimethyl Ether (DME) propellant arcjet thruster. We have proposed to apply DME to arcjet thrusters owing to its favorable characteristics for space propulsion device such as non toxicity, weak chemical reactivity and storability. Thrust measurement using a vertical-pendulum-type thrust stand showed that thrust was almost monotonously increased with mass flow rate at a given discharge current, and rose with specific power. Specific impulse was also enhanced with specific power whereas thruster efficiency was attenuated. DME propellant presented higher thrust, specific power and thruster efficiency than nitrogen propellant at a given discharge current or specific power. Thrust measurement yielded a thrust of 0.19N, a specific impulse of 330s, thruster efficiency of 0.14 at a DME flow rate of 60-mg/s with the corresponding discharge voltage, power, and specific power of 90V, 2350W, and 39MJ/kg, respectively.

A multi-institutional team of undergraduate students conducted an independent study in which they designed, fabricated, and tested a small Hall effect thruster. The study was motivated by student desire to engage in a hands-on, multidisciplinary project in the field of space propulsion. This paper represents the outcome of this educational experiment. It describes the educational framework followed; the fundamental physics behind Hall thruster operation (at an early undergraduate level); the design process followed to develop and construct the thruster including a novel additively-manufactured propellant diffuser; and testing results. The thruster did not successfully ignite during testing. The cathode/plasma source was identified as the likely point of failure; rather than design flaws in the thruster itself, thereby introducing next steps for a future study following a similar framework. The educational program presented serves as a case study for a small-team undergraduate space propulsion project with limited resources and prior knowledge as well as a technical reference and knowledge base for future teams attempting a project of similar scope.

Reactive species created by a microhollow cathode discharge (MHCD) in He+H2O mixtures were investigated at 160 Torr using the molecular beam mass spectrometry. Ion currents of He+, HeH+, O+, OH+, H+(H2O), and H+(H2O)2 were measured as functions of H2O relative concentration (0.24-14%) and discharge current (5-15 mA). When the concentration exceeds 6%, most of the ions were decreased, but H+(H2O)2 (cluster ion) significantly increased at discharge current of 5 mA, whose ion current was very low compared with those of other ions. With increasing the discharge current, the cluster ion showed a sharp decrease, while the other ions were almost constant. These features were reasonably explained by the increase of the gas temperature and the plasma rarefaction due to the Joule heating of the working gas.

A series of numerical simulations and experiments have been performed to assess the effectiveness of magnetic shielding in a Hall thruster operating in the discharge voltage range of 300–700 V (Isp ≈ 2000–2700 s) at 6 kW, and 800 V (Isp ≈ 3000) at 9 kW. At 6 kW, the magnetic field topology with which highly effective magnetic shielding was previously demonstrated at 300 V has been retained for all other discharge voltages; only the magnitude of the field has been changed to achieve optimum thruster performance. It is found that magnetic shielding remains highly effective for all discharge voltages studied. This is because the channel is long enough to allow hot electrons near the channel exit to cool significantly upon reaching the anode. Thus, despite the rise of the maximum electron temperature in the channel with discharge voltage, the electrons along the grazing lines of force remain cold enough to eliminate or reduce significantly parallel gradients of the plasma potential near the walls. Computed maximum erosion rates in the range of 300–700 V are found not to exceed 10−2 mm/kh. Such rates are ∼3 orders of magnitude less than those observed in the unshielded version of the same thruster at 300 V. At 9 kW and 800 V, saturation of the magnetic circuit did not allow for precisely the same magnetic shielding topology as that employed during the 6-kW operation since this thruster was not designed to operate at this condition. Consequently, the maximum erosion rate at the inner wall is found to be ∼1 order of magnitude higher (∼10−1 mm/kh) than that at 6 kW. At the outer wall, the ion energy is found to be below the sputtering yield threshold so no measurable erosion is expected.

Several forms of exercise are adopted to increase muscular strength and power. The amplitude of the electromyogram (EMG) increases with the training loads and is maximal for levels near maximal voluntary contraction. Therefore, strength training requires use of heavy loads. Lately, an alternative form of exercise characterized by the superimposition of vibration has been suggested as an effective methodology to improve muscle strength and power performance. In this paper, a prototype for muscular conditioning is presented and validated. It uses a controlled electrical motor to generate a loading force that can be modulated at any frequency up to 40 Hz. We refer to this training modality as force modulation (FM). Modulation amplitude and frequency can be independently determined by computer. We evaluated the activation and co-activation of biceps and triceps muscles during isometric exercise with and without 28-Hz FM. Twelve healthy volunteers performed isometric elbow flexion and extension trials at different force levels in two conditions: force modulation [FM] and normal loading [C]. EMG activity of biceps and triceps muscles was simultaneously measured and assessed by estimation of the EMG root mean square (RMS). The RMS was estimated by power spectrum integration excluding the harmonics generated by movement artifacts due to FM. The results revealed a significant RMS increase especially when FM was applied at higher force levels. During elbow flexion, the EMG activity of biceps and triceps was respectively 38% and 26% higher than C, while during elbow extension, their activity was 28% and 98% higher than C, respectively. This study shows that FM exercise produces an increase of the activation and coactivation of biceps and triceps. This training modality seems therefore suitable for several applications, ranging from sports to rehabilitation.KeywordsElectromyographyvibration exercisemuscle activationforce modulation

A constant frequency zero-voltage-switching (ZVS) control strategy with a minimum root mean square (rms) value of inductor current is proposed in this letter for the four-switch buck-boost (FSBB) dc-dc converter that is used as a 48 V intermediate bus pre-regulator in distributed power systems. The quadrilateral inductor current modulation is adopted to achieve ZVS operation. The four control time intervals are optimally selected such that the inductor current rms value is minimized, thus reducing the conduction loss. The proposed closed-loop ZVS control scheme can be implemented without sensing the load current, although the rms minimum current algorithm needs the input and output voltages and load current at given a steady-state condition. The control scheme is verified in a 300 W dc-dc prototype of the ZVS FSBB converter. The input voltage and load transition tests are provided to verify the closed-loop characteristics without sensing load current.

The laser-assisted pulsed plasma thruster is considered a promising propulsion system to support the tasks of microsatellites because of its high specific impulse and low volume. Different from the traditional pulsed plasma thruster, laser-assisted pulsed plasma thruster uses the laser to replace the spark plug for ignition, which can avoid ignition failure and remove the side effect of carbon deposition. Both the thrust efficiency and impulse bit are expected to increase after the plasma flow produced by laser ablation is further ionized and accelerated. Since there are a few macro-performance prediction models in laser-assisted pulsed plasma thrusters, this paper develops a model based on the laser ablation model and electromagnetic acceleration model to capture macro-performances of laser-assisted pulsed plasma thrusters. In this model, the initial velocity and mass of plasma flow can be obtained from the ablation model, and the acceleration model is utilized to describe the electromagnetic acceleration process of plasma flow. With this combined model, the discharge current, voltage, impulse bit, specific impulse, and thrust efficiency can be estimated. The deviation between the predicted results and experimental results was less than 10%, verifying the correctness of the developed model. The effects of different parameters on the performance are further investigated with this model.

The interaction of the plasma discharge with the ceramic walls of a Hall thruster leads to plasma recombination, energy losses, and extra electron collisionality. These three phenomena are included in a one-dimensional axial model of the discharge through source terms obtained from an auxiliary model of the radial dynamics. Spatial solutions are presented for different discharge voltages and wall materials, and agree satisfactorily with experimental data. The parameters related to wall effects are investigated extensively. The energy balance among Joule heating, wall-losses cooling, and heat conduction shapes the temperature profile; three different profile types are identified depending on the wall material and the discharge voltage. For long chambers, the main source of energy losses is the plasma interaction with the walls, even for zero secondary electron emission. By contrast, wall collisionality due to primary/secondary exchanges of electrons is negligible always. The current utilization is related directly to the total energy losses. The propellant utilization is set by the balance between gas ionization and wall recombination in the acceleration region. The rate of wall recombination suggested by the axial solution is much lower than the values given by radial models based on a Maxwellian electron distribution function.

The BPT-4500 and method of tuning the acceleration by focusing a plasma lens is described. Test results show that overall thruster efficiency and plume shape is improved by adjusting the curvature of magnetic field lines in the acceleration zone. Thrust efficiency is improved to 50% from 21% at 150 V and from 58% to 62.5% at 350 V using this technique. At 125 V, the BPT-4500 operates at 45% thrust efficiency and 1030 s specific impulse yielding a thrust/power ratio only 10% less than a 600 s hydrazine arcjet. Introduction Multi-mode Hall thruster capability enables one thruster to serve constant power applications needing the highest possible thrust for orbit transfer and high specific impulse for station keeping. Towards this objective, we have evaluated a GENERAL DYNAMICS (formerly PRIMEX Aerospace Company) approach for tuning the BPT-4500 for the best possible performance over a range of accelerating potentials. This paper describes our initial exploratory efforts to improve thruster efficiency by actively changing magnetic field shape. Thruster Description The BPT-4500 is a 4.5 kW Hall field plasma accelerator based on an earlier 4000 watt device''. These thrusters employ a short discharge to minimize contact with the insulators and pole pieces to shape the curvature of the magnetic field in the ion creation zone. As is well known in Hall type accelerators, the magnetic field lines become nearly equipotential lines defining the acceleration vector of the ions. Also, the technique of adjusting electric potential shape by magnetized electrons in a magnetic field is commonly called a plasma lens. By virtue of the discharge properties in the BPT and shaping these fields, we have attempted to Copyright (c) 2001 by GENERAL DYNAMICS. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. create a plasma lens with adjustable focus. Plasma Lens Focusing Morozov in the Soviet Union has shown that using two sets of electromagnets to change the gradient of the radial magnetic field can have a significant effect on the behavior of ion acceleration in Hall type discharges. Gavryushin and Kim have shown that the axial gradient of the radial magnetic field regulated by varying the degree of screening of the accelerator channel. These accelerators are characterized by relatively long acceleration lengths in channels that typically have a length to width of 2:1. Insofar as these accelerators had a long discharge, the curvature of the field was not emphasized in the description of this early work. Instead, the researchers discuss primarily the axial gradient of Curvature of ion path changed by Anode focal length Anode potential contour Cathode potential contour Figure 1 Hall Acceleration with Plasma Lens Concept (c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. the magnetic field. The BPT discharge is configured to take place in a very short channel, typically with a length to width of 1:2. As such, physical evidence from a variety of geometry studies suggests that the BPT discharge creates ions in a thin, -1-2 mm layer, occurring approximately 3-4 mm downstream of the anode, schematically illustrated in Fig 1. As such, we hypothesize the BPT discharge embodies Hall acceleration within a plasma lens. Our innovation is to exploit the features of this short discharge by adjusting the focal length of a plasma lens using a trim or focusing coil. Variation of the curvature of the fields in this layer has the effect of changing the initial trajectory of ions. Changes in the shape downstream of the ionization zone are minimized by the fact that magnetic fields are manipulated inside the primary magnetic structure. The BPT-4500 thruster shown in Fig 2 is composed of a primary magnetic structure with four outer coils and an inner coil. The BPT-4500 has a 13 cm mid-diameter and a 2 cm annulus providing slightly more discharge area than the BPT-4000. The schematic cross section of the thruster is shown in Fig. 3. This shows the anode is surrounded by a secondary magnetic structure to add or subtract from the magnetic field radius of curvature in the annulus between insulator rings. Figure 2 BPT-4500 Laboratory Style Thruster Flight Version A flight capable version of the thruster is under development in the Lockheed Martin funded Hall Thruster Propulsion System (HTPS) program. This flight thruster is referred to as the BPT-4000. This thruster has a fixed magnetic field and plasma lens taken from the optimal configuration found here. GENERAL DYNAMICS is on track to deliver a flight qualified thruster, power processor and xenon flow control system in 2003.

The one-dimensional hydrodynamic model of a stationary plasma thruster (SPT) previously described by Morozov and Savelyev [in Reviews of Plasma Physics, Vol. 21, Consultants Bureau, New York, 2000, pp. 360-369] is solved using the space-time conservation element/solution element (CE/SE) method. The hydrodynamic model consists of three equations describing the mass conservation of ions and neutral particles and the momentum of the ions. These equations are coupled nonlinearly through source terms describing the ionization of neutrals, and a non-local term describing current through the plasma. This model is one of the simplest to produce the low-frequency oscillatory behavior (on the order of 10 kHz) known as the “breathing mode” that is observed in ground testing of SPTs. Numerical solutions to the model are challenging due to the highly coupled, nonlinear nature of the equations. The simulation outlined here was successful, but displays some of the Courant number sensitivity noted in other CE/SE simulations.

Time-resolved electrostatic probe measurements were performed in the near field of a SPT100-ML Hall effect thruster in order to investigate electron properties changes on a microsecond time scale. Such measurements allow one to monitor the electron temperature Te, the electron density ne, as well as the plasma potential Vp during a time period that corresponds to one cycle of a breathing-type plasma oscillation with f≈15–30kHz. Although Te(t) stays constant in time, ne(t) and Vp(t) oscillate with the discharge current waveform frequency. The observed time delay between ne and anode discharge current (Ida) waveforms, which is of approximately 7μs, is linked to the ion transit time from the ionization layer to the probed near-field region. The same time gap is measured between Vp(t) and Ida(t), however Vp(t) and ne(t) are in phase opposition. The electron density reaches its highest value at the very moment ions are ejected out of the thruster discharge chamber, which also corresponds to the instant the cathode potential is the most negative. Such a behavior images the need for ion beam neutralization. Further, it is shown that there is a strong correlation between the electron dynamics and the presence of high frequency (HF) plasma oscillations in the megahertz range: HF fluctuations are the strongest when ne is the highest.

The heat flux and the associated space charge in a gas-discharge plasma can lead to a change in the properties of the corresponding electronic devices: a change in the kinetic coefficients and the temporal instability of their parameters. In devices with small transverse dimensions, the surface charge field, which significantly affects the space charge in the plasma, can also affect the current in the quasineutral plasma, changing the magnitude of the electric field component, which depends on the heat fluxes of the plasma. Knowledge of the analytical dependences of the component of the longitudinal electric field that provides the discharge current on the parameters of the plasma, the heat flux of electrons and its geometry makes it possible to estimate in detail the parameters of real plasma devices. Purpose. Obtain and analyze analytical expressions for the longitudinal electric field in a confined plasma in the ambipolar diffusion mode, taking into account the heat fluxes of electrons, the value of which depends on the frequency of collisions of electrons with atoms when approximated as a power-law velocity function. Results. The equations of electron balance are obtained in integral form for a bounded flat plasma with allowance for the heat flux of electrons. Practical significance. The calculation results make it possible to evaluate the effect of the heat flux of electrons of a low-temperature flat plasma with light and heavy ions on their electrical properties.