Brian E. Beal

Plasma Properties in the Plume of a Hall Thruster Cluster

The Hall thruster cluster is an attractive propulsion approach for spacecraft requiring very high-power electric propulsion systems. Plasma density, electron temperature, and plasma potential data collected with a combination of triple langmuir probes and floating emissive probes in the plume of a low-power, four-engine Hall thruster cluster are presented. Simple analytical formulas are introduced that allow these quantities to be predicted downstream of a cluster based solely on the known plume properties of a single thruster. Nomenclature A = area of one electrode AS = surface area of sheath surrounding an electrode B = magnetic field strength E = electric field strength e = electron charge kb = Boltzmann’s constant me = electron mass mi = ion mass n = electron number density n0 = reference density Te = electron temperature Te,0 = reference electron temperature Vd2 =v oltage measured between triple probe electrodes 1 and 2 Vd3 =v oltage applied between triple probe electrodes 1 and 3 V f = floating potential γ = ratio of specific heats δ = sheath thickness λD = electron Debye length φ = plasma potential φT = thermalized potential Subscript j = contribution from an individual thruster

Methodology and historical perspective of a hall thruster efficiency analysis

to ionization processes and losses that manifest as Joule heating, and contains no information about the vector properties of the jet. Propellant efficiency incorporates losses from dispersion in the jet composition and is unity for 100% ionization to a single ion species. The effect of neutrals on dispersion of the jet velocity distribution function in propellant efficiency is introduced in the neutral-gain utilization. The beam efficiency accounts for divergence of the jet and is ideal when the ion velocity vectors are parallel to the thrust axis. Plume divergence is defined as a momentum-weighted term, and the approximation as a charge-weighted term is characterized. The efficiency architecture is derived from first principles and is applicable to all propulsion employing electrostatic acceleration, including Hall thrusters and ion thrusters. Distinctions and similarities to several past methodologies are discussed, including past ion thruster analyses, early Russian performance studies, and contemporary architectures. To illustratethepotentialforenhancedunderstandingoflossmechanismsandionizationprocesseswithanarrayoffarfield plume diagnostics, a case study is presented of low-discharge voltage operation from a 6 kW laboratory Hall thruster.

Preliminary Plume Characterization of a Low-Power Hall Thruster Cluster

In an effort to understand the technical issues related to running multiple Hall effect thrusters in close proximity to each other, testing of a cluster of four Busek BHT-200-X3 devices has begun in Chamber 6 at the Air Force Research Laboratory. Preliminary measurements have shown that the variations in the discharge currents of the four thrusters are synchronized, possibly due to cross talk through the thruster plumes. Measurements of plasma density, electron temperature, and plasma potential in the thruster plumes obtained using a triple Langmuir probe are presented. Anomalously high electron temperatures were recorded along the centerline of each thruster. Collisionless, magnetosonic shock waves induced by the ion-ion two-stream instability are proposed as a possible cause of the high temperatures. The unperturbed ion velocity distribution along the centerline of a Hall thruster is shown to be unstable and a simple geometric model is presented to illustrate the qualitative changes in plasma properties expected across the proposed shock. Estimates using this model show that relatively large changes in electron temperature are consistent with small changes in electron number density across a shock. Qualitative arguments are presented indicating that collisionless shocks are unlikely to form as a result of clustering multiple thrusters.

Air Force Research Laboratory high power electric propulsion technology development

Space solar power generation systems have a significant impact on Electric Propulsion (EP) technology development.1,2,3 Recent advances in solar cell, deployment, and concentrator hardware have led to significant reductions in component mass, thereby decreasing power generation system specific mass. Combined with maneuvering requirements for Air Force and DoD missions of interest, propulsive requirements emerge that provide direction for technology investments. Projections for near- to mid-term propulsion capabilities are presented indicating the need for thrusters capable of processing larger amounts of power (100 – 200 kW), operating at relatively moderate specific impulse (2000 – 6000 seconds) and high efficiency (≫ 60%), and having low propulsion system mass (≪ 1 kg/kW). Two technology areas are identified and discussed in the context of the above thruster constraints. Concentric channel Hall thrusters are an extension of a mature technology, offering operation over expanded power levels and lower propulsion system specific mass at state-of-the-art (SOTA) efficiencies. Field Reverse Configuration (FRC) thrusters are a specific type of pulsed inductive accelerator that have the potential to operate up to MW power levels, at propulsion system specific masses even lower than concentric channel Hall thrusters, and on a wider range of propellants. However, FRCs are currently less mature than the Hall thruster variants. Comparisons of candidate technologies are evaluated with VASIMR, a well publicized high power EP device currently under development.

Plasma properties downstream of a low-power Hall thruster

Abstract : Triple Langmuir probes and emissive probes were used to measure the electron number density, electron temperature, and plasma potential downstream of a low-power Hall thruster. The results show a polytropic relation between electron temperature and electron number density throughout the sampled region. Over a large frat ion of the plume, the plasma potential obeys the predictions of ambipolar expansion. Near the thruster centerline, however, observations show larger gradients of plasma potential than can be accounted for by this means. Radial profiles of plasma potential in the very-near-field plume are shown to contain large gradients that correspond in location to the boundaries of a visually intense plasma region. [Copyright 2005 American Institute of Physics.]

THE EFFECTS OF CLUSTERING MULTIPLE HALL THRUSTERS ON PLASMA PLUME PROPERTIES

Clusters of Hall thrusters have been proposed as a means of achieving electric propulsion systems capable of operating at very high power levels. To facilitate testing in existing vacuum facilities, initial tests have focused on a cluster of low-power Busek BHT-200-X3 Hall thrusters. A combination of triple Langmuir probes and floating emissive probes is used to study the effects of multi-thruster operation on the electron number density, electron temperature, and plasma potential in the plasma plume. The resultant number density is shown to be a result of linear superposition of the plumes of individual thrusters, while the electron temperature in the cluster plume is measured to be slightly higher than that caused by operation of a single thruster. The plasma potential downstream of the cluster is shown to obey the Boltzmann relation. In the region between the thrusters, the plasma potential increases as a function of downstream distance and may result in reflection of some low-energy charge exchange ions back toward the cluster. A mechanism that may lead to slightly reduced ion beam divergence through focusing of ions directed toward the thruster centerline is discussed.

Development of an Annular Helicon Source for Electric Propulsion Applications

Abstract : The performance of typical electrostatic propulsion systems, such as the Hall thruster, is limited in part by inefficiencies in the electron bombardment ionization process. These limitations become especially pronounced at the operating conditions required to achieve high thrust-to-power ratios. One approach for achieving significant increases in efficiency at such operating conditions is to replace the typically-employed DC ionization mechanism with a helicon source, which is widely regarded as an efficient method for creating a high-density, low-temperature plasma. Standard cylindrical helicons, however, are not amenable to straightforward integration with annular Hall thrusters. A rigorous mathematical treatment of helicon wave physics has been completed to establish the boundary conditions required to create an annular helicon source for both the m=0 and m=1 azimuthal modes. This analysis reveals no fundamental barriers to creation of an annular helicon source so long as the radial boundary conditions are set appropriately.

DEVELOPMENT OF THE LINEAR GRIDLESS ION THRUSTER

The design of the Linear Gridless Ion Thruster, or LGIT, is presented in detail. The LGIT is a hybrid thruster that combines an ionization stage similar to those normally found on gridded ion engines with the acceleration mechanism of a Hall effect thruster. This thruster also features a linear geometry, which simplifies the design of the magnetic circuit while making the LGIT particularly well suited to future work on clustering and, perhaps, thrust vectoring by varying the magnetic fields in the acceleration zone. Initial testing with the thruster operating in a single-stage, unoptimized mode resulted in a specific impulse of 1400 seconds and an anode efficiency of 12%. The low efficiency is believed to be due in large part to operating the thruster in a single-stage mode, rather than the two-stage mode for which it was designed, and to setting the acceleration stage magnets to less than half their design value.

Improved analysis techniques for cylindrical and spherical double probes

A versatile double Langmuir probe technique has been developed by incorporating analytical fits to Laframboises numerical results for ion current collection by biased electrodes of various sizes relative to the local electron Debye length. Application of these fits to the double probe circuit has produced a set of coupled equations that express the potential of each electrode relative to the plasma potential as well as the resulting probe current as a function of applied probe voltage. These equations can be readily solved via standard numerical techniques in order to determine electron temperature and plasma density from probe current and voltage measurements. Because this method self-consistently accounts for the effects of sheath expansion, it can be readily applied to plasmas with a wide range of densities and low ion temperature (T(i)/T(e) ≪ 1) without requiring probe dimensions to be asymptotically large or small with respect to the electron Debye length. The presented approach has been successfully applied to experimental measurements obtained in the plume of a low-power Hall thruster, which produced a quasineutral, flowing xenon plasma during operation at 200 W on xenon. The measured plasma densities and electron temperatures were in the range of 1 × 10(12)-1 × 10(17) m(-3) and 0.5-5.0 eV, respectively. The estimated measurement uncertainty is +6%∕-34% in density and +∕-30% in electron temperature.

The effects of cathode configuration on hall thruster cluster plume properties

Abstract : Clusters of Hall thrusters may be used to produce electric propulsion systems capable of operating at power levels in excess of the current state of the art. One of the key factors to be considered in determining the optimum cluster architecture is the configuration of the electron-emitting cathode(s). This work presents experimentally determined plume properties and discharge current characteristics obtained with multiple thrusters coupled to a single cathode. Spatially resolved plasma density, electron temperature, and plasma potential data are presented during both single thruster and cluster operation. Measurements taken in this configuration are compared to previously published data obtained with each thruster coupled to its own independent cathode. Critical plasma parameters in the cluster plume are shown to be strongly influenced by the location of the hollow cathode.