2. Electric Propulsion Overview & Hall Thruster What is Electric Propulsion?
“The acceleration of gases for propulsion by electrical heating and/or by electrical and magnetic body forces.” (Physics of Electric Propulsion, Robert G. Jahn, 1968)
For Ve above 10,000 m/s, a propulsion system needs; Extremely high temperature of an insulated gas stream or Direct acceleration by applied body forces
EP thrusters are very “fuel efficient” due to high exhaust
velocity Ve. 2 How to accelerate?
Electrothermal Electrostatic Electromagnetic
Arcjet thruster (5-10km/s） MPD thruster (50-100km/s)
Hall thruster Ion thruster
(15-25km/s） (30-50km/s） 3 How much mass reduction expected?
History of Himawari-5 (Metrological Sat.) 1995 Mar. Launched 2000 Mar. Life-time (5 years)Expired 2003 May Taken over by retired GOES-9 2005 Feb. Himawari-6 2006 Feb. Himawari-7
Hydrazine monopropellant thruster Momo ⇒ Ion Engine Propellant for NSSK 207kg(28%) ⇒ 21kg ‘Whether satellite Himawari-5’ Propellant for OTV 402kg(54%) ⇒ 40kg Total mass 747kg ⇒ 199kg 4 How Large ∆V mission is possible?
Large ∆V = long-range missions (interplanetary flights) long-time missions (maintenance of satellite)
Ex. Orbit to Mars ∆V = 14 km/s
Typical ion thruster Ve = 30 km/s
mi m fexp V V e 1.6
Cf. Typical chemical Ve = 3 km/s
mi m fexp V V e 106
5 Electric Propulsion vs Chemical Propulsion
Chemical Ve = 4.5 km/s Electric Ve = 30 km/s
6 Power Supply – The higher the Ve is, The Better?
All EP thrusters must have electric power supply on-board the spacecraft.
The weight of power supply scales monotonically with the power level , which is directly related to Ve.
There exists an optimal Ve for a given ∆V and Total Mass vs. Ve (Isp). thrust level. 7 Transfer time
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 power. (AIAA-95-2513) 8 Optimum exhaust velocity for OTV by Electric Propulsion (1)
2 mVprop e Thrust Efficiency th Ps: Available Power 2Ps
Specific power of solar cell panels
α=Ps/mpanel (W/kg)
β= mpanel /Ps (kg/W) Typical β=0.05 kg/W
Propellant consumption rate
mmprop prop : Transfer Time 9 Optimum exhaust velocity for OTV by Electric
Propulsion (2) 1 ΔV/V0=0.05 0.9 0.1 mmpay P prop 1 s 0.8 m m m 0.2 i i i 0.7 0.3 mprop Ps 0.6
11 ratio mmi prop 0.5 0.5 2 0.4
Payload V Ve 1 1 exp 1 0.3 0.7 Ve2 th 0.2 2 VV Ve 0.1 exp 1 exp VVe 2 th e 0 0.1 1 10 Ve/Vo VVe,opt 0 th Payload ratio and exhaust velocity
V 10km/s for 30 days, 33km/s for 1 year. e,opt 10 History of Electric Propulsion Brief History of EP – It’s An Old Idea. In 1906, R. H. Goddard expressed informally many of the key physical concepts of EP. In 1911, Konstantin Tsiolkovskiy proposed similar concepts as Goddard. In 1929, Herman Oberth included a chapter on EP in his classic book “Man into Space.” In 1950s, Ernst Stuhlinger summarized his studies in his book “Ion Propulsion for Space Flight.” In 1960s, EP became a major component of space propulsion development. Ion thrusters in US Hall thrusters in the former Soviet Union
12 In the United states in ’90s
PAS-5
Telstar
Telstar (‘93-’95) 24 Arcjets Intelsat (GE Aerospace-Lockeed Martin) PanAmSat (‘90-’00) Intelsat(’93-’96) :40 Resistjets 60 Ion thrusters (Ford Aerospace-SS/Loral) (Hughes-Boeing)
Aerojet 550W Arcjet
Boeing NSTAR Ion engine 13 In Russia and Europe
Russia
SPT100 D-55
More than 100 Hall thrusters on Cosmos & Meteor series from ’70s.
Europe
PPS1350 SMART-1
Astrium and Alcatel are competing and cooperating for Hall thrusters in 2000s. 14 In Japan
MELCO Ion thruster ISAS Ion Thruster ETS-series On Hayabsa explorer
Ion thrusters No. Operation Satellite Type NASA NSTAR Ring Cusp 1 16kh DS1 XIPS13 52 55kh BS601HP Boeing Ring Cusp XIPS25 24 14kh BS702 UK10 Kaufman 2 0.7kh Artemis Astrium RIT10 RF 3 7.7kh Artemis, EURECA MELCO IES Kaufman 8 0.2kh ETS6, COMETS ISAS/NEC μ10 Microwave 4 40 kh HAYABUSA 15 Renewed interests in EP in recent years More on-board electric power In-space propulsion for manned asteroid/planet exploration All Electric satellites
Courtesy NASA Boeing satellite 16 Summary of EP overview So, Do We Really Want an EP System? In general, large ∆V missions are appropriate for using EP systems. Factors in determining a suitable propulsion system:
Ve, exhaust velocity EP Thrust CP Efficiency EP Power subsystem CP Propellant management subsystem ---- Mission trajectory and time CP/EP Interface with other subsystems CP Reliability (heritage, redundancy) CP Operation of spacecraft (orbit, trajectory) CP/EP Cost CP (advantage for) EP: electric propulsion CP: chemical propulsion 18 Operation of a GEO Satellite
MBSAT: DBS communications satellite for Japan and Korea built by SS/L, 2004. The first US-built satellite with SPT-100.
Orbit Control Maneuvering of chemical and Hybrid GEO satellites
Propulsion NSSK EWSK NSSK Cycle EWSK Cycle System time/firing time/firing
20-N Bi-prop 1 per two 2 per two A few minutes A few seconds (x 2 units) weeks weeks
SPT-100/Bi- 12 hrs. 2 per day A few seconds 2 per day prop by SPT-100 by SPT-100 by a Bi-prop by a Bi-prop
19 Operation of a GEO Satellite (2)
• For the bi-prop system, 3 maneuvers in 2 weeks. • For the EP/Bi-prop hybrid system, 56 maneuvers in 2 weeks. • The hybrid system requires orbit control maneuvering every day due to its low thrust SPT-100. • One NSSK maneuver takes a few minutes for the bi-prop system and 12 hours for the hybrid system. • Orbit control ∆V: 95% for NSSK, 5% for EWSK • Large electric power (~ 2 kW) from both the solar cells and batteries is supplied for the hybrid system. Complex battery charging management, especially during the eclipse periods in spring and autumn.
20 Cost of EP systems (1)
As a simple comparison, the number of orbital maneuvers for a GEO communications satellite by the hybrid system is 18 times that by the bi-prop system. An orbital maneuver is proceeded by a ranging, orbit determination, and control planning and followed by another ranging, orbit determination, and assessment. A rough estimate of cost increase in orbit control operations (with one operator + one orbital engineer) is ~ $8 million in 15 years (orbital lifetime).
21 Cost of EP systems (2) MBSAT carries the same number of bi-prop thrusters as the satellite that does not carry SPT-100s. MBSAT: 12 bi-prop thrusters + 4 SPT-100 thrusters Chemical propulsion satellite: 12 bi-prop thrusters ~ $20 million cost increase due to four SPT-100 thrusters plus gimbals, PPU, and PFS Trade-off between the bi-propellant mass and the mass of SPT-related hardware and xenon. The decrease in launch cost is minimal. Having an EP system, with less propellant mass and longer mission lifetime, would NOT REALLY save the total cost when considering actual satellite procurement and operations!!
22 EP CAN…. can have an advantage with their low thrust level. Extremely small attitude disturbance by their maneuvering. Suitable for satellites with flexible structures. Suitable for missions with fine maneuvering requirement such as co-location and formation flying. can appeal to missions of extremely high ∆V requirements. Impractical to carry the necessary propellant mass of a chemical rocket system Interplanetary missions Deep-space probes can continue to evolve. Higher reliability Lower cost
Greater variety of the combinations of thrust, Isp, and power levels. 23 Hall Thruster Hall Thruster Principle: Overview Acceleration Electrostatic acceleration Propellant channel (Xe)
Axial E and radial B Xe+
Cathode as electron emitter Hall current Design criteria:
(Larmor radius) Anode E electron B Cathode (Hall parameter)
(Ion mean free path)
: Channel length 25 Hall Thruster Principle: Electric Field
100 ~ 1000 V DC E field Plasma is highly Plasma conductive → Electric field is shielded Anode r B field side Cathode z Channel wall Radial magnetic field to confine electron motion Φ B Electron gyroradius must r be much smaller than channel length
Electron Larmor radius Anode Channel exit Cathode
26 Hall Thruster Principle: Hall Current
E x B drift movement of E x B direction charged particles B electron motion E Electron-induced Hall current Electron Hall current
For effective confinement of B electron motion, electron E Hall parameter must be much larger than 1 Collisions contribute to cross B-field transports (Hall parameter) = (Gyrofrequency) x (mean free time) Electron motion with collisions
27 Hall Thruster Principle: Closed Annular Cavity
Hall current B
E
Operation of Hall thruster
Linear Hall-effect ion source1
1https://www.ulvac.co.jp/ 28 Hall Thruster Principle: Ion Generation
Energy loss: Energy gain: Electrons are very lightweight Ionization, Joule heating Wall loss, etc. Φ Te Electrons are heated to Ionization 20 eV = 232,000 K region
Collisional ionization Anode Channel exit Cathode First ionization energy [eV]: + Argon: 15.8 Xe Krypton: 14.0 Xenon: 12.1 Electron 29 Hall Thruster Principle: Ion Acceleration
Ions are smoothly accelerated by electric field Ion Little disturbance by magnetic field or collisions Anode r z Channel wall Ion Larmor radius Ion mean free path
Exhaust velocity estimation
Discharge voltage: Acceleration voltage: If 90% of is ionized, Ion mass (Xe): Elemental charge: 30 Hall Thruster Principle: Recap Acceleration Electrostatic acceleration Propellant channel (Xe)
Axial E and radial B Xe+
Cathode as electron emitter Hall current Design criteria:
(Larmor radius) Anode E electron B Cathode (Hall parameter)
(Ion mean free path)
31 Magnetic Layer vs. Anode Layer Magnetic-layer type Channel length > channel width Dielectric material (BN) to protect ferromagnetic pole pieces Anode-layer type Very short channel length, so that no need for magnetic pole protection Conducting metal at the cathode potential
32 Stationary Plasma Thruster (SPT)
SPT series specifications
Parameter SPT-50 SPT-70 SPT-100 SPT-140 Slot diameter, mm 50 70 100 140 Thrust input power, W 350 700 1350 5000 Average Isp, s 1100 1500 1600 1750 Thrust, mN 20 40 80 300 Thrust efficiency, % 35 45 50 >55 Lifetime, h 1500 3000 9000 >7000 Status Flight Flight Flight Qualified
33 Thruster with anode layer (TAL)
TAL series specifications Parameter D-38 D-55 D-100 D-110 D-150 Slot diameter, mm 38 55 100 110 150 Thrust input power, W 1500 2500 7500 15000 17500 Average Isp, s 2000 2000 2100 1750 2300 Thrust, mN 100 120 340 240 800 Thrust efficiency, % 70 55 60 60 60
34 Thrust Performances of SPT and TAL
Outer diam.: 100mm Outer diam.: 55mm Power : 1.35 kW Power : 2.5 kW I : 2000 sec Isp : 1600 sec sp Thrust : 80 mN Thrust : 120 mN Efficiency : 50 % Efficiency : 55 %
Magnetic layer type: SPT-100 Anode layer type: D-55
35 Global Exploration Roadmap (GER) GER: Post-ISS international cooperative missions High-power EP (In-space propulsion) is a key tech.
36 In-Space Propulsion in US and Europe
NASA’s 12.5-kW HERMeS thruster1 Snecma’s 5-kW PPS®50002 Thrust efficiency : ~ 63% Specific impulse : ~ 3000 s
1Kamhawi et al., IEPC-2015-07, 2015. 2Duchemin et al., IEPC-2015-127, 2015. 37 RAIJIN: All-Japan Collaboration Thruster Head, PPU Plume Interaction
Project RAIJIN
High-Current Neutralizer Mission & Orbit Analyses
38 Development of UT-58 Anode-Layer Thruster
UT-58: 2 kW-level TAL developed in UT
Magnetic Field Analysis
UT-58 3D Model Thermal Analysis
39 Performance of UT-58
Xenon Argon Operation of UT-58 with Xenon and Argon Maximum value Xenon (~3.0 Aeq) Argon (~8.0 Aeq) Input power, kW 1.92 2.81 Thrust, mN 92.2 51.8 Anode efficiency, % 73.0 23.5 Exhaust velocity, km/s 23.0 18.6 40 Comparison: UT-58 and D-series
Compared with Russia’s D-series, UT-58 achieved 70% of anode eff. with 1-order small input power
Anode efficiency vs. Input power of UT-58 and D=series 41 Ion Energy Loss to Wall and Wall Erosion Equipotential lines Ion collision with wall causes energy losses of ionization
and acceleration Anode Channel wall erosion by high- r energy ion sputtering limits z Channel wall thruster lifetime Ion Sputtering
Channel wall
Magnetic pole piece Simulation of channel wall recession 42 B Field Optimization: Plasma Lens Focusing
Highly conductive E field Plasma
Metal
B field Electric field lines surrounding Plasma lens focusing conductor
43 B Field Optimization: Actual Designs
Magnetic field geometry using plasma lens focusing ATON thruster
44 B Field Optimization: Numerical Simulations
330 h 600 h
Calculated space potential distribution. Begin of life and end of life.
1Cho et al., 48th Joint Propulsion Conference, AIAA 2012-4016, 2012. 45 Ion Attracting Physics: Presheath
Acceleration in presheath Bohm criterion
Channel wall
Outer channel wall Outer channel wall Equipotential line
Ion-loss region
Inner channel wall Inner channel wall
Convex-shaped magnetic field and Deflected equipotential lines and ideal ion acceleration direction ion-loss region by presheath 46 Magnetic Shielding: Theory
Magnetic Shielding:
Very concave magnetic field High Te geometry Strong presheath Channel wall parallel to magnetic lines
Φ Low T Te e Weak presheath
Z Anode Channel exit Cathode 47 Magnetic Shielding: Numerical Simulation
Unshielded configuration1
Magnetic shielding configuration1 Calculated space potential distribution2
1 2 Hofer et al., Journal of Applied Physics, 2014. Mikellides et al., Journal of Applied Physics, 2014. 48 Magnetic Shielding: JPL Experiments
Measured erosion rate of unshielded (US) and magnetic shielding (MS) configurations1 1Hofer et al., Journal of Applied Physics, 2014. 49 UT Experiment: Magnetic Shielding in TAL
gap = 14 mm Guard-ring Ion wall current = current ratio Discharge current
Better 14 Current trade- New trade-off gap = 19 mm 12 off relation relation
10 gap=14 8
6 gap=19 ring current ratio, % ratio, current ring gap = 24 mm - 4 gap=24
Strong magnetic shielding magnetic Strong 2 Guard Better 0 20 30 40 50 60 70 Anode Wall Anode efficiency, % Magnetic and wall geometry of UT-58 50 Simulation: Magnetic Shielding in TAL
Weak magnetic shielding Strong magnetic shielding
Calculated space potential distribution in UT-58
51 Simulation: Magnetic Shielding in TAL
All ions Ions entering channel wall
Calculated ion production rate distribution of magnetically shielded UT-58 52 Discharge Current Oscillation
Inherent oscillation types:
1. Ionization: 10 kHz - 100 kHz
2. Transit-time: 100 kHz - 1 MHz
3. Electron-drift: 1 MHz - 10 MHz
4. Electron-cyclotron: 1 GHz
5. Langmuir: 0.1 GHz - 10 GHz
Discharge oscillation causes
Heavy power processing unit Time history of discharge current Electromagnetic noise
53 Ionization Oscillation
Frequent Ionization rate ionization
Neutral “Predator-Prey” model depletion
Infrequent Ion, Electron n Neutral, n e n ionization
Neutral
recovery Number density Number Time Principle of ionization oscillation 54 Synchronous Breathing-mode Oscillation
Oscillation behavior of plasma1.
1Yamamoto et al., Trans. Japan Soc. Aero. Space Sci. (48), 2005. 55 Electron Diffusion and Discharge Oscillation
Classical Transition Anomalous diffusion region diffusion Transition Classical region diffusion
0.4
0.01 Anomalous diffusion
Typical tendency of discharge current and oscillation vs. magnetic flux density
56 Electron Diffusion and Discharge Oscillation
Xenon Argon Transition and anomalous diffusion region Classical diffusion 24 60 region
1 10
57 Summary: Hall Thruster
Principle: Hall current and electrostatic acceleration
High-power thruster for In-space propulsion
B-field optimization and magnetic shielding
Discharge current oscillation
58