Hall Thruster for Space Applications: Advanced Concepts and Research Challenges Yevgeny Raitses Princeton Plasma Physics Laboratory Princeton, New Jersey 16th International Conference on Ion Sources, New York City, NY, August 28, 2015

Outline Hall thruster Status quo Frontiers applications Two big issues unresolved for 50 years: Wall erosion Anomalous electron cross-field transport Potential solutions New challenges: facility effects

Hall thruster (HT) E =-ve  B e << L << i e Diameter ~ 1-100 cm B ~ 100-200 Gauss Working gases: Xe, Kr Pressure ~ 0.1 mtorr Vd < 1 kV Power ~ 0.1- 50 kW Thrust ~ 10-3 - 1N Isp ~ 1000-3000 sec Efficiency ~ 6-70% E =-ve  B e << L << i Unlike ion thruster, HT is not space-charge limited. Thrust density is higher than in ion thrusters. Limited by the magnetic field pressure, B2/2.

Hall thruster spaceflight heritage 2010 First flight of US Hall thruster , 4kW, on operation GEO satellite mission. Status quo: highly efficient 0.5-5 kW Hall thrusters used for station keeping, drag compensation, orbit transfer, moon mission. Future: rendezvous with asteroids and comets, interplanetary missions.

Frontiers applications for Hall thrusters Future space applications will take Hall thruster technology beyond and above its current status. - low power < 500 W small and micro satellites. - very low power < 10 W nano satellites. - very high power 100 ‘s kW to support human exploration missions. A NASA concept of a 300 kW SEP vehicle for human NEO missions. The wingspan of the solar arrays is 66 m. A 100 kW class nested-channel Hall thruster. (NASA-AFRL-Univ. Michigan) NASA Near-Earth-Object rendezvous microspacecraft: 7 kg, 0.2m x 0.3m x 0.3m. A 50 W cylindrical Hall thruster. (PPPL)

Issue # 1 (for all power levels): ion-induced channel and cathode erosion limiting thruster lifetime 1.35-kW SPT-100 New 1.35-kW SPT-100 5,700 Hrs Operational Cathode 7 mm Non-operational cathode Courtesy: L. King F. Taccagona Ceramic channel, 10 cm OD diameter

Issue # 2: anomalously high electron cross-field current limiting thruster efficiency Efficiency reduces with increasing electron current across the magnetic field. Note: - High power Hall thruster (> 1kW) are highly efficient ~ 50% - 70%, - Lower power thrusters are much less efficient, 7-40% and have shorter lifetime- in a great part due to enhanced electron transport. Classical collisional mechanism can not explain the discharge current measured for Hall thrusters. Enhanced cross-field current is usually attributed to anomalous fluctuation-induced (Bohm-type) diffusion and near-wall conductivity.

Hall thruster plasma research challenges Development of predictive modeling capabilities for designing of the next generation Hall thrusters requires understanding: Mechanisms of anomalous electron cross-field transport. Plasma-wall interactions in the thruster and their effects: sputtering, electron emission, sheath instabilities, wall-induced electron transport. Facility effects for high power thrusters vs. space environments. Scaling laws for power/size/performance/lifetime. Modeling needs to be multidimensional, multiscale fully kinetic (for electrons, ions, neutrals), and… validated.

Hall thruster plasma Median Anode Wall Exit Magnetic Pole Neutral density ~ 1012-1013 cm3 Plasma density ~ 1011-1012 cm-3 Highly ionized flow: ion/n > 100% Electron temperature ~ 20-60 eV Ion temperature ~ 1 eV Ion kinetic energy ~ 102-103 eV Collisionless, partially ionized, non-equilibrium plasma with magnetized electrons and non-magnetized ions in non-uniform E ×B fields, and pressure gradients.

Electron-induced secondary electron emission (SEE) plays a very important role in Hall thruster operation For ceramic materials, SEE yield is higher and approaches 100% at lower energies than for graphite and metals. Use of conductive channel walls can lead to short-circuit current (across magnetic field) increasing power losses.

SEE can strongly enhance electron flux from plasma to the wall Fluid Approach w(x) scs Te When i SEE turns sheath to space-charge limited regime (SCL). In SCL, wall acts as effective heat sink for plasma increasing power losses. When e see Hobbs and Wesson, Plasma Phys. (1967)

Wall material effect on discharge characteristics Segments of different SEE materials drastically can change V-I characteristics High SEE material - Very low SEE material Phys. Plasmas (2006)

Wall material effect on electric field in plasma High-SEE Very low-SEE With very low SEE channel walls, the electric field is not far a fundamental limit for quasi neutral plasma ~ Te/D. IEEE Trans. Plasma Sci. (2010)

Simulations predict strong kinetic effects for collisionless Hall thruster plasma Hall thruster plasma, 2D-EVDF Isotropic Maxwellian plasma, 2D-EVDF Anisotropic EVDF with beams of SEE electrons travelling between walls. Sydorenko et al, Kaganovich et al., Phys. Plasmas (2005, 2006, 2007 2009), Ahedo, Phys. Plasmas (2005)

SEE-induced electron cross-field transport Exchange of primary magnetized electrons by non-magnetized SEE electrons induces so called near-wall conductivity across magnetic field. during the flight time H/ubx The displacement , , gives SEE-induced cross-field current E B Kaganovich, Raitses, Sydorenko. Smolyakov, Phys. Plasmas (2007)

Surface-architectured materials to mitigate SEE effects on thruster plasma Carbon velvet BN Quartz Macor Dendritic Re/W, Re/Mo Graphite Carbon Velvet Surface-architectured materials reduce the effective SEE yield by trapping SEE electrons between surface features.

Magnetically-shielded Hall thruster to mitigate adverse plasma-wall interaction effects Low electron temperature near the wall  low sheath potential 1 Low energy of ions impinging the walls  low wall erosion  longer thruster lifetime.1 Oblique magnetic field may prevent SEE electrons from flowing to the plasma   no near-wall conductivity. Unshielded Shielded I. Mikellides et al, Appl. Phys. Lett. (2013) R. Hofer, Michigan (2013) NASA JPL Hall Thruster with magnetic shield.

Cylindrical Hall thruster (CHT) Diverging magnetic field topology. Operation involves closed EB drift. Electrons are confined in the hybrid magneto-electrostatic trap. Ions are accelerated in a large volume- to-surface area channel. (potentially lower erosion). Cathode 100 W 2.6 cm CHT Phys. Plasmas 8, (2001)

Unusual focusing of the plasma flow in diverging magnetic field due to rotating electrons Ion current in plume LIF measurements Force balance parallel to the magnetic field surface requires the parallel electric field component Pressure gradient Centrifugal force on E×B rotating electrons Fisch et al., Plasma Phys. Control. Fusion (2011) Spektor et al., Phys. Plasmas (2010) Raitses at al., Appl. Phys. Lett. (2007)

Plasma non-uniformities (spoke) in Hall thruster 12 cm diameter, 2kW Hall thruster Unfiltered high speed imaging . Xenon operation Spoke frequency ~ 10 kHz 10’s times slower than E/B Spoke frequency >> ci

From probes and camera, strongest m=1 mode spoke near the anode where B~0.5 kGauss High speed images Langmuir probes to measure spoke in CHT Direction: ExB Velocity: 1.2-2.8 km/s E/B: 30 km/s Via 1-3 km/s Size: 1.0-1.5 cm Local wavenumber-frequency spectrum J. Parker, Y. Raitses, N. J. Fisch, Appl. Phys. Lett., (2010)

More than 50% of the discharge current is conducted to the anode through the spoke . Segmented anode The evaluation of the segment current Ellison, Raitses, Fisch, Phys. Plasmas 19 (2012)

Phenomenology of - current through the spoke Br E0z + - E0z×B Eθ Eθ×B Initial density perturbation, Only electrons undergo azimuthal drift motion, Eθ generated across the perturbation, Eθ×B drift across the magnetic field, to the anode. Correlated density and E fluctuations would explain enhanced electron transport. Possible instability mechanisms: modified Simon-Hoh, Kelvin-Helmholtz, ionization instabilities. 23

Can spoke be suppressed and controlled? Resistors attached between each anode segment and the thruster power supply The feedback resistors, Rf, are either 1, 100 , 200 , or 300 Spoke increases the current through the segment leading to the increase the voltage drop across the resistor attached the segment. This results in the reduction of the voltage between the segment voltage and the cathode.

Spoke suppression with the feedback control Feedback off Feedback on The suppression of the spoke leads to a reduction in the total discharge current due to the anomalous current that is carried by the spoke.

Facility effects in on weakly collisional Hall plasma Background pressure affect on CHT performance and plume. Spoke frequency increases wit the background pressure in a 28 m3 vacuum vessel. No spoke above ~ 510-5 torr. Spoke suppression was also obtained with cathode overrun and anode feedback control. Spoke suppression is accompanied with excitation of fast oscillations in discharge current – could be a mode transition of the electron transport from mezo-scale to micro-scale. Raitses et al., AIAA 2010-6775 Parker et al., Appl. Phys. Lett. 97 (2010) Griswold et al., Plasma Sour Sci. Technol. 23 (2014) 26

Concluding remarks Hall thruster technology is seemingly mature but two big issues remained unresolved for 50 years: wall erosion and anomalous transport. Anomalous electron cross-field transport due to wall conductivity and low frequency spoke instability affect the electric field in Hall thrusters. Need better understanding of spoke and near-wall conductivity: -need 3D PIC simulations, -theory of instabilities -experiments Reduction of anomalous transport by minimizing SEE effects and suppression of spoke instability was demonstrated. New challenges: Facility effects on weakly collisional thruster plasma.

Acknowledgement The research presented here is the result of multi-year studies of Hall thruster physics conducted by the Hall Thruster Experiment (HTX) group at PPPL. Experiments: Artem Smirnov, David Staack, Alexander Dunaevsky, Jeffery Parker, Lee Ellison, Martin Griswold. Theory and simulations: Nat Fisch, Igor Kaganovich, Dmytro Sydorenko, Andrey Smolyakov. Diagnostic development: Ahmed Diallo, Vincent Donelly, Panos Svarnos, Rostislav Spektor, Ivan Ramadanov. The research was supported by AFOSR and US DOE.