3D PIC simulation of the rotating spoke in a Hall thruster

The 3 dimensional Particle-in-Cell code STOIC¹ was applied to simulate the rotating spoke in a 100 W CHT thruster. The model utilizes an equidistant Cartesian grid which explicitly assures momentum conservation and zero self forces. The simulation treats electrons, Xe⁺ ions and neutral Xenon atoms. All relevant collisional processes are included in the model: Coulomb collisions between charged particles; electron-neutral elastic, ionization and excitation collisions; ion-neutral momentum transfer and charge exchange collisions. The dynamics of the background neutral gas is self-consistently resolved with direct simulation Monte Carlo. The computational domain is extended beyond the thruster channel and includes the near-field plume region. In the simulation a spoke rotating with the velocity of about 1.8 km/s was observed in the thruster channel. Both the spoke and non-spoke regimes of the CHT operation were achieved in the simulation. In agreement with the experiment, the transition from spoke to nonspoke regime in the simulations was observed with an increase of the electron current from the cathode. The influence of the rotating spoke on the ion flow in the thruster was studied. The simulations have demonstrated that the presence of the spoke leads to strong decrease (factor 3.5) of the average potential drop inside the thruster channel, which results in almost twice lower average ion velocity at the thruster exit.In the spoke regime the ion flow outside of the thruster exhibits visibly higher divergence as compared with the non- spoke regime. These results agree quite good with LIF measurements of the ion velocity in CHT plume. The simulations have revealed that a rotating spoke causes the rotating asymmetry in the ion flow in the CHT thruster.