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«TRANSPORT-PROPERTY AND MASS SPECTRAL MEASUREMENTS IN THE PLASMA EXHAUST PLUME OF A HALL-EFFECT SPACE PROPULSION SYSTEYM by Lyon Bradley King A ...»

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TRANSPORT-PROPERTY AND MASS SPECTRAL MEASUREMENTS

IN THE PLASMA EXHAUST PLUME OF A

HALL-EFFECT SPACE PROPULSION SYSTEYM

by

Lyon Bradley King

A dissertation submitted in partial fulfillment

of the requirements for the degree of

Doctor of Philosophy

(Aerospace Engineering)

Doctoral Committee

Associate Professor Alec Gallimore

Professor James Driscoll Associate Professor Brian Gilchrist Professor Tamas Gombosi

ACKNOWLEDGMENTS

Although the spine of this book bears my name, the work reported here would not have been possible without the help of a great number of people.

First and foremost, I would like to thank my advisor, Prof. Alec Gallimore, for stepping forward when I required guidance, and stepping back when things were going well. Prof. Gallimore has created a research environment in which students are free to pursue their own unique interests and creativity flourishes.

Additionally I would like to thank my dissertation committee comprised of Prof.

Jim Driscoll, Prof. Brian Gilchrist, and Prof. Tamas Gombosi for their time in evaluating this thesis; any errors that remain are my own. I am also indebted to the talented students that I had the fortune to work alongside. I am richer for the many hours spent swearing, wiring, contemplating, fixing, taping (that’s right, Sang, taping), and occasionally celebrating with Colleen, Frank, James, Matt, Sang, and more recently George and Shane.

A hardware-intensive endeavor, such as this research proved to be, required the assistance of skilled technicians. I would like to thank Warren Eaton, Terry Larrow, Tom Griffin, Gary Gould, and Dave McLean for assistance with all things metal.

ii Financially, this research benefited from the generous support of the Air Force Office of Scientific Research (AFOSR) represented by Dr. Mitat Birkan, the NASA-Lewis Research Center with equipment grants administered by Mr. John Sankovic, and support from the NASA-Johnson Space Center under the direction of Mr. Richard Barton. The unique opportunity to evaluate a state-of-the-art thruster was made available by a generous equipment loan from Mr. Mike Day of the Space Systems/Loral company. This support is gratefully acknowledged.

On a personal level I would like to thank my parents for emphasizing the importance of education in life. Moreover, the opportunities afforded to me as a result of their support and guidance were designed to produce nothing less than complete success in my every endeavor.

Finally, I would like to express my heartfelt thanks to my wife, who bore the brunt of my frustrations throughout my studies. Pursuit of the Ph.D. has consisted of months of pure boredom punctuated by moments of sheer panic;

she was with me through every peak and valley. I would like to thank her for keeping faith in me and for being my biggest fan.

–  –  –

This thesis represents a broad study to characterize the heavy-particle structure of the exhaust plume produced from a 1.5-kW-class Hall thruster. The goal of this study was to provide an extensive data base of plasmadynamic quantities to be used as an input to plasma-surface interaction models.

Additionally, conclusions drawn from analysis of these quantities yielded insight regarding basic thruster performance mechanisms.

The plume characterization study employed the use of a variety of classic plasma diagnostic techniques including Langmuir probes, retarding potential analyzers (RPAs), and Faraday probes. Novel probes were also conceived of and tested to evaluate previously un-obtained information regarding the plasma components. These techniques included the development of a neutral particle flux probe (NPF) to quantify the existence of high-energy neutral atoms and the application of a heat-flux probe technique in the determination of ion and neutral densities.

To complement the in-situ probe data, a unique molecular beam mass spectrometer (MBMS) was designed and used to provide great insight into the plasma species and energy structure of the Hall thruster plume. This system provided simultaneous mass and energy measurement through the use of an electrostatic energy analyzer in a time-of-flight mode. The MBMS data enabled

–  –  –

dependent ion energy distribution functions useful for evaluation of basic thruster acceleration mechanisms.

Through an evaluation of the probe-based data in addition to the MBMS results a collisional analysis of the ionic portion of the plasma plume was performed. Through models and concepts developed in this thesis the products of both elastic momentum transfer and inelastic charge-exchange collisions were directly identified within the measured ion energy distributions. These results confirmed the existence of both single- and multiple-electron transfers between plume ions and parasitic neutral gas due to ground-test facility interactions in addition to momentum transfer collisions between propellant ionic species.

–  –  –

ACKNOWLEDGMENTS

PREFACE

LIST OF FIGURES

LIST OF TABLES

NOMENCLATURE

CHAPTER





1. INTRODUCTION

1.1. Overview of Electric Propulsion Concepts

1.2. Hall-effect Thrusters: Research History

1.3. Physics of Hall Thruster Operation

1.4. Contributions of Research

1.5. Description of Experimental Facilities

2. PRACTICAL THEORY OF IN-SITU PROBES

2.1. Introduction

2.2. Retarding Potential Analyzer

2.3. Langmuir Probe

2.4. Faraday Probe

2.5. Heat-Flux Probe

2.6. Neutral Particle Flux Probe

3. A PROBE-BASED STUDY OF THE HALL THRUSTER

–  –  –

3.2. Probe Data and Results

3.2.1. Langmuir Probe

3.2.2. Retarding Potential Analyzer

3.2.3. Faraday Probe

3.2.4. Heat-Flux Probe

3.2.5. NPF Probe

3.3. Interpretation of Probe Results and Derived Quantities

3.3.1. Ion Energy Structure

3.3.2. Density Distribution

3.3.3. Neutral-Particle Properties

3.3.4. Ion-Neutral Charge Exchange Processes

4. MOLECULAR BEAM MASS SPECTROMETER DESIGN................ 70

4.1. Motivation Behind MBMS Research

4.2. Configuration of Apparatus

4.3. Electrostatic Energy Analyzer

4.3.1. Theory of Operation

4.3.2. Description of Apparatus

4.4. Time-of-flight Mass Analyzer

4.4.1. Theory of Operation

4.4.2. Design Principles

4.4.3. Practical Design Considerations

4.4.4. Description of Apparatus

5. MBMS ENERGY DIAGNOSTICS

5.1. Experimental Set-up

5.2. Ion Energy Measurements in an SPT-100 at 0.5 m

5.3. Ion Energy Measurements in an SPT-100 at 1.0 m

5.4. Discussion of Ion Voltage Distributions

5.4.1. Comparison with RPA

5.4.2. Ion Temperature

5.4.3. Most Probable Voltage

5.4.4. Two-stream Instability Analysis

5.4.5. Multiple Peak Structure

6. MASS SPECTROMETRIC STUDIES

6.1. Experimental Set-up

6.2. Measurements in an SPT-100

vii

6.3. Discussion of Mass Spectral Measurements

6.3.1. Minor Species Analysis

6.3.2. Propellant Ionization

7. PLUME ION COLLISION ANALYSIS

7.1. Overview of Collision Processes

7.1.1. Ion Collision Probabilities

7.1.2. Elastic Collision Signatures

7.1.3. CE Collision Signatures

7.2. Collision Evidence in SPT-100 Plume

7.3. Ion-Electron Recombination

7.4. Discussion of Plume Collisions

8. CONCLUSIONS

8.1. Overall Hall Thruster Plume Structure

8.2. Ion Energy Structure

8.3. Facility Interaction Considerations

8.4. Plume Asymmetry

8.5. Suggestions for Future Work

–  –  –

FIGURE 1-1. FINAL NET MASS DELIVERED TO GEO AS A FUNCTION OF LEO-TO-GEO TRIP TIME FOR VARIOUS

PROPULSION OPTIONS BASED ON A 1550 KG ATLAS IIAS-CLASS PAYLOAD WITH 10 KW ON-BOARD

POWER.

FIGURE 1-2. BASIC HALL THRUSTER COMPONENTS SHOWING LAYOUT OF ELECTRIC DISCHARGE BETWEEN

ANODE AND CATHODE, APPLIED MAGNETIC FIELD CIRCUITRY, AND ORIENTATION OF DOMINANT

ELECTRIC AND MAGNETIC FIELDS WITHIN DISCHARGE VOLUME.

FIGURE 1-3. SCHEMATIC OF HALL THRUSTER COMPONENTS SHOWING A COMPARISON BETWEEN THE MAGNET

LAYER TYPE, SUCH AS THE SPT-100, AND THE ANODE LAYER TYPE, SUCH AS THE D-55

FIGURE 1-4. SCHEMATIC OF THE 9 X 6 M VACUUM CHAMBER. THE POSITION OF THE THRUSTER FOR THE

–  –  –

0.5 M AND 1.0 M RADIUS SHOWN FOR PERSPECTIVE. ALSO SHOWN IS THE POSITION OF THE THRUSTER

RELATIVE TO THE MOLECULAR BEAM MASS SPECTROMETER (MBMS) AS REPORTED IN CHAPTERS 5

AND 6.

FIGURE 2-1. SCHEMATIC OF THREE-GRID RPA UTILIZED TO OBTAIN ION ENERGY DISTRIBUTION FUNCTION. 24

FIGURE 2-2. FARADAY PROBE USED TO MEASURE THE ION CURRENT DENSITY.

FIGURE 2-3. HEAT-FLUX SENSOR SHOWING BOTH TOTAL- AND RADIANT-HEAT-FLUX TRANSDUCERS MOUNTED

IN A WATER-COOLED PROBE HOUSING

FIGURE 2-4. NEUTRAL PARTICLE FLUX PROBE SHOWING ELECTROSTATIC REPULSION GRIDS AND IONIZATION

PRESSURE SENSOR USED AS A DETECTOR.

FIGURE 3-1. LAYOUT OF APPARATUS FOR IN-SITU PROBE BASED STUDY OF THE SPT-100 SHOWING PROBE

MOUNTS TO AUTOMATED TRANSLATION SYSTEM AND COORDINATE SYSTEM SIGN CONVENTIONS.........39

FIGURE 3-2. TYPICAL LANGMUIR PROBE CURRENT-VOLTAGE CHARACTERISTIC. THIS TRACE WAS TAKEN AT

1.0 M FROM THE SPT-100 AT 20 DEGREES OFF THRUSTER CENTERLINE

FIGURE 3-3. PLASMA POTENTIAL IN THE PLUME OF THE SPT-100 AT 0.5 M AND 1.0 M RADIUS FROM THE THRUSTER EXIT PLANE

–  –  –

VOLTAGE IS LESS THAN 10%.

FIGURE 3-5. ION ENERGY DISTRIBUTION CURVES MEASURED WITH THE RPA IN THE PLUME OF THE SPT-100

–  –  –

FIGURE 3-7. ION ENERGY DISTRIBUTION MEASURED WITH THE RPA IN THE PLUME OF THE SPT-100 AT 0.5 M

RADIUS AT A POSITION -60 DEGREES OFF THRUSTER CENTERLINE SHOWING HIGH LEVELS OF

–  –  –

FIGURE 3-8. ION ENERGY DISTRIBUTION FUNCTION MEASURED WITH THE RPA IN THE PLUME OF THE SPT-100

AT 1.0 M RADIUS FROM THE THRUSTER FOR POSITIONS WITHIN 50 DEGREES OF THRUSTER CENTERLINE.48

FIGURE 3-9. ION CURRENT DENSITY MEASUREMENTS IN THE SPT-100 AND D-55 AT RADIAL POSITIONS OF 0.5

–  –  –

POSITION IS 3 DEGREES.

FIGURE 3-10. RADIANT AND TOTAL HEAT FLUX MEASUREMENTS IN THE SPT-100 AT 0.5 M RADIUS FROM THE

–  –  –

POSITION IS 3 DEGREES.

FIGURE 3-11. RADIAL AND TOTAL HEAT FLUX MEASUREMENTS IN THE SPT-100 AT 1.0 M RADIUS FROM THE

–  –  –

POSITION IS 3 DEGREES.

FIGURE 3-12. NPF PROBE CHECK-OUT OPERATION IN THE PLUME OF THE SPT-100 AT 0.5 M RADIUS AND 15

–  –  –

FIGURE 3-13. NEUTRAL PARTICLE FLUX PROBE MEASUREMENTS IN THE SPT-100 AT 0.5 M AND 1.0 M.

UNCERTAINTY IN ANGULAR POSITION IS 3 DEGREES.

–  –  –

INDICATE THE MAGNITUDE OF THE UNCERTAINTY INHERENT TO ALL DATA POINTS

FIGURE 3-15. SCHEMATIC REPRESENTATION OF MECHANISM CAUSING HIGH-ENERGY ANNULUS AND LOW

–  –  –

EQN. 3-1 AND THE HEAT-FLUX BASED VALUE IS CALCULATED VIA EQN. 3-2.

FIGURE 3-17. COMPARISON OF CALCULATED PARTICLE DENSITY IN THE PLUME OF THE SPT-100 AT 1.0 M

–  –  –

EQN. 3-1 AND THE HEAT-FLUX BASED VALUE IS CALCULATED VIA EQN. 3-2.

FIGURE 3-18. NEUTRAL PARTICLE FLUX IN THE PLUME OF THE SPT-100 MEASURED WITH THE NPF PROBE AT

0.5 M AND 1.0 M RADIUS FROM THE THRUSTER EXIT.

FIGURE 3-19. CALCULATED NEUTRAL PARTICLE CONVECTIVE HEATING IN THE PLUME OF THE SPT-100 AT 0.5 M AND 1.0 M RADIAL POSITIONS FROM THE THRUSTER EXIT PLANE.

FIGURE 4-1. SCHEMATIC OF OVERALL CONFIGURATION OF MBMS APPARATUS SHOWING ORIENTATION TO

MAIN VACUUM CHAMBER, THRUSTER MOUNT, AND SCALE SIZE.

FIGURE 4-2. TOP-VIEW PHOTOGRAPH OF MBMS INSTRUMENT SHOWING ELECTROSTATIC ANALYZER

CHAMBER AND ELECTRON MULTIPLIER DETECTOR PORT.

FIGURE 4-3. SIDE-VIEW PHOTOGRAPH OF MBMS SHOWING OIL DIFFUSION PUMPS.

FIGURE 4-4. SCHEMATIC OF 45-DEGREE ELECTROSTATIC ION ENERGY ANALYZER. CONSTANT ELECTRIC

FIELD IS FORMED BY APPLYING REPELLING VOLTAGE TO TOP PLATE WITH BOTTOM PLATE GROUNDED.

FIELD CORRECTION PLATES ARE BIASED WITH A VOLTAGE DIVIDER TO FORCE BOUNDARY CONDITIONS

AT MID-PLANES TO PREVENT FIELD DISTORTION DUE TO SURROUNDING GROUND POTENTIAL...............79

–  –  –

FIGURE 4-6. ILLUSTRATION OF TOF SPECTROMETER CONCEPTS SHOWING GATE REGION AND FIELD-FREE

DRIFT REGION

FIGURE 4-7. ALLOWABLE GATE LENGTH TO DRIFT LENGTH RATIOS FOR TOF MASS SPECTROMETER............ 93

FIGURE 4-8. ELECTROSTATIC BEAM GATE LAYOUT FOR TOF ANALYSIS SHOWING RELEVANT PARAMETERS. 95

FIGURE 4-9. THE INCOMING ION BEAM IS CONSIDERABLY DIVERGENT AFTER PASSING THROUGH THE

–  –  –

TO STEER THE BEAM OUT OF THE COLLIMATING APERTURE DOWNSTREAM



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