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  • Other Sources  (66)
  • 1
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    Magnetic Shielding of the Channel Walls in a Hall Plasma Accelerator (2011)
    In:  Other Sources
    Details
    Publication Date: 2018-06-11
    Description: In a qualification life test of a Hall thruster it was found that the erosion of the acceleration channel practically stopped after approx 5,600 h. Numerical simulations using a two-dimensional axisymmetric plasma solver with a magnetic field-aligned mesh reveal that when the channel receded from its early-in-life to its steady-state configuration the following changes occurred near the wall: (1) reduction of the electric field parallel to the wall that prohibited ions from acquiring significant impact kinetic energy before entering the sheath, (2) reduction of the potential fall in the sheath that further diminished the total energy ions gained before striking the material, and (3) reduction of the ion number density that decreased the flux of ions to the wall. All these changes, found to have been induced by the magnetic field, constituted collectively an effective shielding of the walls from any significant ion bombardment. Thus, we term this process in Hall thrusters "magnetic shielding."
    Keywords: Spacecraft Propulsion and Power
    Type: Physics of Plasmas; Volume 118; Issue 3; 033501
    Format: text
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  • 2
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    Electric propulsion research and development at JPL (2005)
    In:  Other Sources
    Details
    Publication Date: 2018-06-11
    Description: In this paper, we will describe the electronic propulsion technologies of interest and our role in developing and interjecting these technologies into JPL missions.
    Keywords: Spacecraft Propulsion and Power
    Type: 2005 AIAA Joint Propulsion Conference; Tucson, AZ; United States
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  • 3
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    Variable specific impulse high power ion thruster (2005)
    In:  Other Sources
    Details
    Publication Date: 2018-06-11
    Description: The power, Isp and thrust of ion thrusters are constrained by ther fixed grid gap in the ion accellerator, which limits performance and life to a limited range in Isp and thrust.
    Keywords: Spacecraft Propulsion and Power
    Type: 2005 AIAA Joint Propulsion Conference; Tucson, AZ; United States
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    NASA TECHNICAL REPORTS
  • 4
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    Potential Fluctuations and Energetic Ion Production in Hollow Cathode Discharges (2007)
    In:  Other Sources
    Details
    Publication Date: 2018-06-11
    Description: Ions with energies significantly in excess of the applied discharge voltage have been reported for many years in hollow cathode discharges. Models of dc potential hills downstream of the cathode and instabilities in postulated double layers in the cathode orifice have been proposed to explain this, but have not been substantiated. Measurements of the dc and rf plasma density and potential profiles near the exit of hollow cathodes by miniature fast-scanning probes suggests that turbulent ion acoustic fluctuations and ionization instabilities in the cathode plume significantly increase the energy of the ions that flow from this region. Increases in the discharge current and/or decreases in the cathode gas flow enhance the amplitude of the fluctuations and increase the number and energy of the energetic ions, which increases the erosion rate of the cathode electrodes. The transition from the quiescent 'spot mode' to the noisy 'plume mode' characteristic of these discharges is found to be a gradual transition of increasing fluctuation amplitudes.
    Keywords: Energy Production and Conversion
    Type: Physics of Plasmas; Volume 14; Issue 10
    Format: text
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    NASA TECHNICAL REPORTS
  • 5
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    Mechanisms for Pole Piece Erosion in a 6-kW Magnetically-Shielded Hall Thruster (2016)
    In:  Other Sources
    Details
    Publication Date: 2019-07-20
    Description: The mechanisms contributing to the erosion of the inner magnetic pole of a 6-kW magnetically shielded Hall thruster are experimentally and numerically investigated. Sputtering from ion bombardment is believed to be the cause of this wear, but the nature and source of the responsible ions remains unknown. Laser induced fluorescence and translating and surface-mounted probes are employed to characterize the flux and energy distribution of ions incident at the inner pole. It is found that the bulk population is comprised of ions originating from the cathode plume and the near field immediately adjacent to the inner pole. These ions have a low average kinetic energy, 〈 5 eV kinetic, but a high thermal spread in velocity, Ti = 10 eV. Calculations for the erosion at the pole from experimental measurements indicate that the bulk population only accounts for 10-25% of the observed erosion. It is postulated that the remainder of the erosion results from a separate population of ions with sufficiently high energy (〉 150 V) to cause high sputtering but too low density to be detected experimentally by the plasma diagnostics in this region. The existence for this high energy population is shown to be a consequence of the high plasma potentials downstream of the thruster exit plane that are characteristic of a magnetically-shielded topography. A simulation validated by experimental measurements of the thruster provides further evidence that a high energy ion populations exists with sufficient density to explain the observed erosion.
    Keywords: Spacecraft Propulsion and Power; Quality Assurance and Reliability
    Type: JPL-CL-16-3070 , AIAA Joint Propulsion Conference; Jul 25, 2016 - Jul 27, 2016; Salt Lake City, UT; United States
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  • 6
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    Electron Backstreaming Determination for Ion Thrusters (2008)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: Electron backstreaming in ion thrusters is caused by the random flux of beam electrons past a potential barrier established by the accel grid. A technique that integrates this flux over the radial extent of the barrier reveals important aspects of electron backstreaming phenomena for individual beamlets, across the thruster beam, and throughout thruster life. For individual beamlets it was found that over 99% of the electron backstreaming occurs in a small annulus at the center of the beamlet that is less than 20% the area of the beamlet at the potential barrier established by the accel grid. For the thruster beam it was found that over 99% of the backstreaming current occurs inside of r = 6 cm for the over 28 cm diameter NSTAR grid. Initial validation against ELT data shows that the technique provides the correct behavior and magnitude of electron backstreaming limit, V(sub ebs). From the sensitivity analyses it is apparent that accel grid chamfering may be the dominant mechanism contributing to the sharp rise in the absolute value of V(sub ebs) observed in the ELT but does not explain the rise in ion transparency. Grid gap change also contributes to the absolute value of V(sub ebs) rise and large rises in ion transparency with thruster life for the center gridlet. Screen grid erosion contributes generally to rises in the absolute value of V(sub ebs) and ion transparency, but for the assumptions used herein, it appears to not have as much of an effect chamfering or grid gap change. Overall, it is apparent that accel grid chamfering, grid gap change, and screen grid erosion are important to the increase in electron backstreaming observed during the ELT.
    Keywords: Spacecraft Propulsion and Power
    Type: AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit; Jul 20, 2008 - Jul 23, 2008; Hartford, CT; United States
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  • 7
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    Time-Dependent Erosion of Ion Optics (2008)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: The accurate prediction of thruster life requires time-dependent erosion estimates for the ion optics assembly. Such information is critical to end-of-life mechanisms such as electron backstreaming. CEX2D was recently modified to handle time-dependent erosion, double ions, and multiple throttle conditions in a single run. The modified code is called "CEX2D-t". Comparisons of CEX2D-t results with LDT and ELT post-tests results show good agreement for both screen and accel grid erosion including important erosion features such as chamfering of the downstream end of the accel grid and reduced rate of accel grid aperture enlargement with time.
    Keywords: Optics; Computer Programming and Software; Numerical Analysis; Spacecraft Propulsion and Power
    Type: AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit; Jul 21, 2008 - Jul 23, 2008; Hartford, CT; United States
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  • 8
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    Co-Flow Hollow Cathode Technology (2011)
    In:  CASI
    Details
    Publication Date: 2019-07-12
    Description: Hall thrusters utilize identical hollow cathode technology as ion thrusters, yet must operate at much higher mass flow rates in order to efficiently couple to the bulk plasma discharge. Higher flow rates are necessary in order to provide enough neutral collisions to transport electrons across magnetic fields so that they can reach the discharge. This higher flow rate, however, has potential life-limiting implications for the operation of the cathode. A solution to the problem involves splitting the mass flow into the hollow cathode into two streams, the internal and external flows. The internal flow is fixed and set such that the neutral pressure in the cathode allows for a high utilization of the emitter surface area. The external flow is variable depending on the flow rate through the anode of the Hall thruster, but also has a minimum in order to suppress high-energy ion generation. In the co-flow hollow cathode, the cathode assembly is mounted on thruster centerline, inside the inner magnetic core of the thruster. An annular gas plenum is placed at the base of the cathode and propellant is fed throughout to produce an azimuthally symmetric flow of gas that evenly expands around the cathode keeper. This configuration maximizes propellant utilization and is not subject to erosion processes. External gas feeds have been considered in the past for ion thruster applications, but usually in the context of eliminating high energy ion production. This approach is adapted specifically for the Hall thruster and exploits the geometry of a Hall thruster to feed and focus the external flow without introducing significant new complexity to the thruster design.
    Keywords: Spacecraft Propulsion and Power
    Type: NPO-47901 , NASA Tech Briefs, October 2011; 26
    Format: application/pdf
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  • 9
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    Plasma Perturbations in High-Speed Probing of Hall Thruster Discharge Chambers: Quantification and Mitigation (2015)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: An experimental investigation is presented to quantify the effect of high-speed probing on the plasma parameters inside the discharge chamber of a 6-kW Hall thruster. Understanding the nature of these perturbations is of significant interest given the importance of accurate plasma measurements for characterizing thruster operation. An array of diagnostics including a high-speed camera and embedded wall probes is employed to examine in real time the changes in electron temperature and plasma potential induced by inserting a high-speed reciprocating Langmuir probe into the discharge chamber. It is found that the perturbations onset when the scanning probe is downstream of the electron temperature peak, and that along channel centerline, the perturbations are best characterized as a downstream shift of plasma parameters by 15-20% the length of the discharge chamber. A parametric study is performed to investigate techniques to mitigate the observed probe perturbations including varying probe speed, probe location, and operating conditions. It is found that the perturbations largely disappear when the thruster is operated at low power and low discharge voltage. The results of this mitigation study are discussed in the context of recommended methods for generating unperturbed measurements of the discharge chamber plasma.
    Keywords: Spacecraft Propulsion and Power
    Type: AIAA/SAE/ASEE Joint Propulsion Conference; Jul 27, 2015 - Jul 29, 2015; Orlando, FL; United States
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    NASA TECHNICAL REPORTS
  • 10
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    Update on the Nested Hall Thruster Subsystem for the NextSTEP XR-100 Program (2018)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Under the NextSTEP program led by Aerojet Rocketdyne in collaboration with NASA Glenn Research Center the University of Michigan, and the Jet Propulsion Laboratory, the XR-100 100 kW Electric Propulsion system is being developed to Technology Readiness Level (TRL) 5. As part of this program, the X3, a Nested Hall Thruster (NHT) designed to operate at powers up to 200kW, is being further developed through parallel modeling and experimental efforts with the ultimate goal of supporting a 100kW-100hr system test in the final year of the NextSTEP program. Recent developments for the X3 subsystem are presented including a summary of testing and modeling results and design updates in anticipation of a risk reduction test scheduled for the summer of 2018.
    Keywords: Spacecraft Propulsion and Power
    Type: GRC-E-DAA-TN58707 , AIAA/SAE/ASEE Joint Propulsion Conference; Jul 09, 2018 - Jul 11, 2018; Cincinnati, OH
    Format: application/pdf
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