Rafael A. Martinez

Numerical and Experimental Investigations of Crossover Ion Impingement for Subscale Ion Optics

A coordinated experimental and modeling study is carried out to investigate the crossover ion impingement of the accelerator grid for a seven-aperture gridlet. Simulations from a new three-dimensional global gridlet model reveal the differences in beamlet profile and accelerator-grid impingement pattern between the center aperture and an edge aperture on the accelerator grid at near-crossover operating conditions. It is shown that simulation predictions of the accelerator-grid impingement-current characteristics and the crossover limits agree well with those from measurements.

Current?voltage characteristics of a cathodic plasma contactor with discharge chamber for application in electrodynamic tether propulsion

This paper focuses on the net electron-emission current as a function of bias voltage of a plasma source that is being used as the cathodic element in a bare electrodynamic tether system. An analysis is made that enables an understanding of the basic issues determining the current?voltage (C?V) behaviour. This is important for the efficiency of the electrodynamic tether and for low impedance performance without relying on the properties of space plasma for varying orbital altitudes, inclinations, day?night cycles or the position of the plasma contactor relative to the wake of the spacecraft. The cathodic plasma contactor considered has a cylindrical discharge chamber (10?cm in diameter and ?11?cm in length) and is driven by a hollow cathode. Experiments and a 1D spherical model are both used to study the contactors C?V curves. The experiments demonstrate how the cathodic contactor would emit electrons into space for anode voltages in the range of 25?40?V, discharge currents in the range of 1?2.5?A, and low xenon gas flows of 2?4?sccm. Plasma properties are measured and compared with (3?A) and without net electron emission. A study of the dependence of relevant parameters found that the C?V behaviour strongly depends on electron temperature, initial ion energy and ion emission current at the contactor exit. However, it depended only weakly on ambient plasma density. The error in the developed model compared with the experimental C?V curves is within 5% at low electron-emission currents (0?2?A). The external ionization processes and high ion production rate caused by the discharge chamber, which dominate the C?V behaviour at electron-emission currents over 2?A, are further highlighted and discussed.

Ion Impingement Limits of Sub-scale Ion Optics: Comparisons of Simulation and Experiment

Experiments and simulations are carried out to investigate direct impingement on accel grid using sub-scale ion optics. Good agreement is observed between measurements and simulations of the cross-over limit. When the efiects from charge-exchange current and leakage current are taken into account, modeling reproduces almost exactly the measured impingement current behavior.

Numerical and Experimental Investigations of Cross-Over Ion Impingement for Sub-Scale Ion Optics

A coordinated experimental and modeling study is carried out to investigate ion optics accelerator grid impingement current and the cross-over limit using a 7-aperture gridlet. Simulations from a new three-dimensional global gridlet model reveal the difierences in beamlet proflle and accelerator grid impingement pattern between the center aperture and an edge aperture on the accelerator grid at low beamlet currents near cross-over conditions. It is shown that the simulation model correctly predicts the cross-over limit behavior and reproduces almost exactly the measured accelerator grid impingement current characteristics.

Evaluation of Sub-Scale NEXIS Ion Optics and Strategies for Performing Accelerated Wear Testing

Sub-scale ion optics assemblies (gridlets) are used to investigate the operational limits of the NEXIS thruster on beamlet (or per hole) ion current. Gridlet data are presented over wide ranges of net and total voltage, grid-to-grid spacing, and accelerator hole diameter. A discussion of a technique that can be used for performing accelerated wear tests (at rates that are 50x higher than standard tests) is also presented that may be useful for quickly and inexpensively evaluating the life time of the NEXIS accelerator grid for missions requiring long (~100 khr) thruster firing times. The technique is based on exposing the accelerator grid surface (through a mask) to an ion beam that is capable of trenching pit and groove patterns on the downstream face and eroding side-wall and etching upstream surfaces of apertures. The status of this work is summarized, and beam current profiles and related facility details are presented. Data on a specially designed power supply with ultra-low stored energy features are also presented. Low stored energy power supplies are critical for high specific impulse ion thrusters that utilize carbon-based ion optics systems in order to avoid arc damage of the accelerator grid surface when the thruster is operated at high total accelerating voltages.

Extracted Current, Bias Voltage, and Ion Production of Cathodic Hollow-Cathode-Driven Plasma Contactors

Plasma properties on extracted electron current–bias voltage characteristics are presented for three selected plasma contactor configurations: a hollow-cathode-only concept, a motive discharge chamber concept, and a passive discharge chamber concept. Measurements were used to demonstrate how one could achieve low impedance performance without being affected by space plasma properties or by consuming significant propellant mass and power. A one-dimensional model was applied to describe the plasma expansion process that occurs downstream of a cathodic contactor. The model matched well with experimental trends and indicated that the plasma ion production rate within and nearby the plasma contactor dominated the emission-bias behavior of the devices. High ion production rate at a given total mass flow resulted in high propellant utilization and low discharge loss. However, plasma potential measurements showed that an anode sheath limited the maximum propellant unitization to less than ∼75% and led to a foldba...

Electric Field Breakdown Characteristics of Molybdenum and Carbon-Based Electrodes under Conditions Where Ions are being Extracted

Measurements of the enhanced electric field at breakdown for perforated flat plates fabricated from carbon-carbon composite, Poco graphite, pyrolytic graphite, and molybdenum are presented. The perforated flat plates represent electrodes used in ion sources and ion thrusters and measurements are made with and without ion beamlet extraction through the perforations. A ranking of the materials is presented of their suitability in ion source applications. The electrical breakdown characteristics of candidate materials for ion optics systems used in satellite propulsion systems are of importance for several reasons, including (1) the necessity to specify the maximum applied electric field to reduce the probability of arcing and/or frequency of arcing, (2) the need to specify the maximum tolerable charge transfer level to avoid damage to the grids that would reduce their voltage standoff capability, (3) the avoidance of incessant arcing between the grids that would result in unacceptable electromagnetic interference levels, and (4) the impact that incessant arcing would have on the power production, power processing, and power distribution system

Non-invasive Hall current distribution measurement in a Hall effect thruster

A means is presented to determine the Hall current density distribution in a closed drift thruster by remotely measuring the magnetic field and solving the inverse problem for the current density. The magnetic field was measured by employing an array of eight tunneling magnetoresistive (TMR) sensors capable of milligauss sensitivity when placed in a high background field. The array was positioned just outside the thruster channel on a 1.5 kW Hall thruster equipped with a center-mounted hollow cathode. In the sensor array location, the static magnetic field is approximately 30 G, which is within the linear operating range of the TMR sensors. Furthermore, the induced field at this distance is approximately tens of milligauss, which is within the sensitivity range of the TMR sensors. Because of the nature of the inverse problem, the induced-field measurements do not provide the Hall current density by a simple inversion; however, a Tikhonov regularization of the induced field does provide the current density distributions. These distributions are shown as a function of time in contour plots. The measured ratios between the average Hall current and the average discharge current ranged from 6.1 to 7.3 over a range of operating conditions from 1.3 kW to 2.2 kW. The temporal inverse solution at 1.5 kW exhibited a breathing mode frequency of 24 kHz, which was in agreement with temporal measurements of the discharge current.

Electric Field Breakdown Properties of Materials Used in Ion Optics Systems

Measurements are presented of the electric field at breakdown for perforated flat plates fabricated from carbon-carbon (CC) composite, Poco graphite, pyrolytic graphite (PG), and molybdenum. The perforated flat plates represent electrodes used in ion sources and ion thrusters and measurements are made with and without ion beamlet extraction through the perforations. A ranking of the materials is presented of their suitability in ion source applications in terms of their electrical breakdown characteristics. For effective use in space missions, materials for ion optics systems must be capable of withstanding moderate electric field stress for long periods of time. In this regard, a simple analysis is presented where thrust density is shown to vary with the square of the electric field. This result suggests that a 50% increase in electric field will result in 125% higher thrust density and a thruster area reduction factor of 0.44. The reduction in thruster area would enable a commiserate decrease in thruster and gimbal specific mass. Experimental data are presented on the field emission onset, electric field enhancement factor, and electrical breakdown properties of the materials listed above as a function of conditioning state, grid spacing, and charge transfer level per arc. Tests results are presented for both beginning of life electrodes and for electrodes that have been heavily worn.