Numerical investigations of a cylindrical Hall thruster

1. Numerical investigations of a cylindrical Hall thruster K. Matyash, R. Schneider, O. Kalentev Greifswald University, Greifswald, D-17487, Germany Y. Raitses, N. J. Fisch Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543, USA

2. Cylindrical Hall Thruster Similar to conventional HTs, the operation involves closed EB electron drift. Fundamentally different from conventional HTs in the way the electrons are confined and the ion space charge is neutralized: Electrons are confined in the hybrid magneto-electrostatic trap Ions are accelerated in a large volume-to-surface area channel (potentially lower erosion) Raitses and Fisch, Phys. Plasmas 8, 2579 (2001)

3. 2d3v RZ Particle in Cell simulation of 2.6 cm CHT anomalous electron transport due to Bohm diffusion is included via scattering of electron perpendicular velocity with Electron density profile Potential profile K. Matyash, R. Schneider, O. Kalentev, Y. Raitses, N. J. Fisch, Annual meeting of APS-DPP, Nov. 2010 Although the simulated plasma parameters were in overall agreement with the experiment, the simulation did not reproduce the changes due to enhanced cathode emission: the model for anomalous electron transport with constant KBohm is too simplistic. 3D model, resolving the azymuthal dynamics is necessary

4. 3 dimensional Particle in Cell code with Monte-Carlo collisions Kinetic treatment of all plasma species Cartesian geometry and the regular mesh (X,Y,Z) guarantees conservation of momentum and absence of self forces in the PIC algorithm Neutral dynamics self-consistently resolved with direct simulation Monte Carlo (DSMC) All relevant collisions are included e- + Xe → Xe+ + 2e- ionization e- + Xe → Xe* + e- total excitation e- + Xe → Xe + e- elastic scattering Xe+ + Xe → Xe+ + Xe elastic scattering Xe + Xe+ → Xe+ + Xe charge exchange e-, Xe+ Coulomb collisions Geometry scaling Scaling factor f = 0.1 is used in the present simulations Monte-Carlo secondary electron emission (SEE) model at the dielectric surface In the present simulations no SEE at the dielectric walls was accounted ( g = 0 ) 60x60x80 grid is used

5. Simulation geometry Axial cross-section Top view Rectangular Hall thruster is simulated

6. The plasma cloud in the annular part is rotating in direction of ExB drift with v ~ 1.8 km/s Strong oscillations of the azimuthal E-field and the azymuthal depletion of neutrals are associated with its rotation

7. Rotating spoke in the CHT experiments The spoke rotating at 15-35 kHz, corresponding to a speed of 1.2 – 2.8 km/s in direction of ExB drift was observed experimentally in CHT Leland Ellison, Yevgeny Raitses and Nathaniel J. Fisch, IEPC-2011-173

8. Dependence of the spoke position on the cathode placement In the simulation the spoke position is defined by the cathode placement The further investigations are necessary to study dependence on other asymmetry sources (neutral gas injection, magnetic field, …)

9. Plasma dynamics inside the spoke Oscillations of azimuthal E-field with E ~ 100 V/cm, f ~ 10 MHz and l ~ 5 mm are responsible for the electron transport toward the anode in the simulations Such oscillations were not observed experimentally in CHT, possibly due to frequency bandwidth limitations

10. Conclusions • Full 3D PIC MCC model for CHT is developed • The model is able to resolve the anomalous electron transport due to azimuthal E-field oscillations • The spoke rotating with v ~ 1.8 km/s is observed in the simulations • Spoke rotation is associated with azimuthal depletion of the neutral gas and strong azimuthal E-field oscillations with E ~ 100 V/cm and f ~ 10 MHz • Further joint simulation and experiment efforts are necessary for clarification of the phenomena underlying the spoke formation and the dynamics as well as electron transport inside the spoke Funding by DLR is kindly acknowledged

11. Thank you for your attention !