Note from the Series Editor .................................... ix
Foreword ....................................................... xi
Preface ...................................................... xiii
Acknowledgments ................................................ XV
Chapter 1: Introduction ......................................... 1
1.1. Electric Propulsion Background ............................. 2
1.2. Electric Thruster Types .................................... 3
1.3. Ion Thruster Geometry ...................................... 6
1.4. Hall Thruster Geometry ..................................... 6
1.5. Beam/Plume Characteristics ................................. 9
References ..................................................... 11
Chapter 2: Thruster Principles ................................. 15
2.1. The Rocket Equation ....................................... 15
2.2. Force Transfer in Ion and Hall Thrusters .................. 18
2.3. Thrust .................................................... 21
2.4. Specific Impulse .......................................... 25
2.5. Thruster Efficiency ....................................... 27
2.6. Power Dissipation ......................................... 30
2.7. Neutral Densities and Ingestion in Electric Thrusters ..... 32
References ..................................................... 34
Problems ....................................................... 35
Chapter 3: Basic Plasma Physics ................................ 37
3.1. Introduction .............................................. 37
3.2. Maxwell's Equations ....................................... 38
3.3. Single Particle Motions ................................... 39
3.4. Particle Energies and Velocities .......................... 43
3.5. Plasma as a Fluid ......................................... 46
3.5.1. Momentum Conservation .............................. 46
3.5.2. Particle Conservation .............................. 48
3.5.3. Energy Conservation ................................ 51
3.6. Diffusion in Partially Ionized Gases ...................... 54
3.6.1. Collisions ......................................... 55
3.6.2. Diffusion and Mobility Without a Magnetic Field .... 60
3.6.3. Diffusion Across Magnetic Fields ................... 66
3.7. Sheaths at the Boundaries of Plasmas ...................... 71
3.7.1. Debye Sheaths ...................................... 73
3.7.2. Pre-Sheaths ........................................ 76
3.7.3. Child-Langmuir Sheaths ............................. 79
3.7.4. Generalized Sheath Solution ........................ 81
3.7.5. Double Sheaths ..................................... 84
3.7.6. Summary of Sheath Effects .......................... 86
References ..................................................... 88
Problems ....................................................... 89
Chapter 4: Ion Thruster Plasma Generators ...................... 91
4.1. Introduction .............................................. 91
4.2. Idealized Ion Thruster Plasma Generator ................... 93
4.3. DC Discharge Ion Thruster ................................ 100
4.3.1. Generalized 0-D Ring-Cusp Ion Thruster Model ...... 102
4.3.2. Magnetic Multipole Boundaries ..................... 105
4.3.3. Electron Confinement .............................. 108
4.3.4. Ion Confinement at the Anode Wall ................. 110
4.3.5. Ion and Excited Neutral Production ................ 117
4.3.6. Neutral and Primary Densities in the Discharge
Chamber ........................................... 120
4.3.7. Power and Energy Balance in the Discharge
Chamber ........................................... 124
4.3.8. Discharge Loss .................................... 126
4.3.9. Discharge Stability ............................... 133
4.3.10.Recycling Behavior ................................ 137
4.3.11.Limitations of a 0-D Model ........................ 141
4.4. Kaufman Ion Thrusters .................................... 142
4.5. rf Ion Thrusters ......................................... 148
4.6. Microwave Ion Thrusters .................................. 158
4.7. 2-D Computer Models of the Ion Thruster Discharge
Chamber .................................................. 171
4.7.1. Neutral Atom Model ................................ 172
4.7.2. Primary Electron Motion and Ionization Model ...... 176
4.7.3. Discharge Chamber Model Results ................... 179
References .................................................... 182
Problems ...................................................... 187
Chapter 5: Ion Thruster Accelerator Grids ..................... 189
5.1. Grid Configurations ...................................... 190
5.2. Ion Accelerator Basics ................................... 196
5.3. Ion Optics ............................................... 200
5.3.1. Ion Trajectories .................................. 200
5.3.2. Perveance Limits .................................. 204
5.3.3. Grid Expansion and Alignment ...................... 206
5.4. Electron Backstreaming ................................... 208
5.5. High-Voltage Considerations .............................. 216
5.5.1. Electrode Breakdown ............................... 217
5.5.2. Molybdenum Electrodes ............................. 218
5.5.3. Carbon-Carbon Composite Materials ................. 221
5.5.4. Pyrolytic Graphite ................................ 223
5.5.5. Hold-off and Conditioning in Ion Thrusters ........ 224
5.6. Ion Accelerator Grid Life ................................ 225
5.6.1. Grid Models ....................................... 227
5.6.2. Barrel Erosion .................................... 230
5.6.3. Pits-and-Grooves Erosion .......................... 232
References .................................................... 235
Problems ...................................................... 240
Chapter 6: Hollow Cathodes .................................... 243
6.1. Introduction ............................................. 243
6.2. Cathode Configurations ................................... 248
6.3. Thermionic Electron Emitter Characteristics .............. 251
6.4. Insert Region Plasma ..................................... 256
6.5. Orifice Region Plasma .................................... 270
6.6. Hollow Cathode Thermal Models ............................ 281
6.7. Cathode Plume-Region Plasma .............................. 283
6.8. Hollow Cathode Life ...................................... 292
6.8.1. Dispenser Cathodes in Insert Plasmas .............. 293
6.8.2. Cathode Insert Temperature ........................ 296
6.8.3. Barium Depletion Model ............................ 298
6.8.4. Bulk-Material Insert Life ......................... 302
6.8.5. Cathode Poisoning ................................. 304
6.9. Keeper Wear and Life ..................................... 306
6.10.Hollow Cathode Operation ................................. 309
References .................................................... 315
Problems ...................................................... 321
Chapter 7: Hall Thrusters ..................................... 325
7.1. Introduction ............................................. 325
7.2. Thruster Operating Principles and Scaling ................ 329
7.2.1. Crossed-Field Structure and the Hall Current ...... 330
7.2.2. Ionization Length and Scaling ..................... 334
7.2.3. Potential and Current Distributions ............... 337
7.3. Hall Thruster Performance Models ......................... 341
7.3.1. Hall Thruster Efficiency .......................... 341
7.3.2. Multiply Charged Ion Correction ................... 345
7.3.3. Dominant Power Loss Mechanisms .................... 347
7.3.4. Plasma Electron Temperature ....................... 357
7.3.5. Hall Thruster Efficiency (Dielectric Walls) ....... 359
7.3.6. TAL Hall Thruster Efficiency (Metallic Walls) ..... 363
7.3.7. Dielectric-Wall Versus Metallic-Wall Comparison ... 364
7.4. Channel Physics and Numerical Modeling ................... 365
7.4.1. Hybrid Hall Thruster Models ....................... 366
7.4.2. Steady-State Modeling Results ..................... 372
7.4.3. Oscillations in Hall Thrusters .................... 376
7.5 Hall Thruster Life ........................................ 379
References .................................................... 384
Problems ...................................................... 389
Chapter 8: Ion and Hall Thruster Plumes ....................... 393
8.1. Introduction ............................................. 393
8.2. Plume Physics ............................................ 395
8.2.1. Plume Measurements ................................ 395
8.2.2. Flight Data ....................................... 396
8.2.3. Laboratory Plume Measurements ..................... 398
8.3. Plume Models ............................................. 400
8.3.1. Primary Beam Expansion ............................ 400
8.3.2. Neutral Gas Plumes ................................ 407
8.3.3. Secondary-Ion Generation .......................... 408
8.4. Spacecraft Interactions .................................. 410
8.4.1. Momentum of the Plume Particles ................... 412
8.4.2. Sputtering and Contamination ...................... 413
8.4.3. Plasma Interactions with Solar Arrays ............. 415
8.5. Interactions with Payloads ............................... 418
8.5.1. Microwave Phase Shift ............................. 418
8.5.2. Plume Plasma Optical Emission ..................... 419
References .................................................... 422
Problems ...................................................... 424
Chapter 9: Flight Ion and Hall Thrusters ...................... 429
9.1. Introduction ............................................. 429
9.2. Ion Thrusters ............................................ 429
9.3. Hall Thrusters ........................................... 440
References .................................................... 443
Appendices
A: Nomenclature .......................................... 447
B: Gas Flow Unit Conversions and Cathode Pressure
Estimates ............................................. 463
C: Energy Loss by Electrons .............................. 467
D: Ionization and Excitation Cross Sections for Xenon .... 471
E: Ionization and Excitation Reaction Rates for Xenon
in Maxwellian Plasmas ................................. 475
F: Electron Relaxation and Thermalization Times .......... 479
G: Clausing Factor Monte Carlo Calculation ............... 483
Index ......................................................... 487
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