Kinetic simulation of secondary electron emission effects in Hall thrusters

Abstract

The particle-in-cell code has been developed for kinetic simulations of Hall thrusters with a focus on plasma-wall interaction. It is shown that the effect of secondary electron emission on wall losses is different from predictions of previous fluid and kinetic studies. In simulations, the electron velocity distribution function is strongly anisotropic, depleted at high energy, and nonmonotonic. Secondary electrons form two beams propagating between the walls of a thruster channel in opposite radial directions. The beams produce secondary electron emission themselves depending on their energy at the moment of impact with the wall, which is defined by the electric and magnetic fields in the thruster as well as by the electron transit time between the walls. The condition for the space-charge-limited secondary electron emission depends not only on the energy of bulk plasma electrons but also on the energy of beam electrons. The contribution of the beams to the particles and energy wall losses may be much larger than that of the plasma bulk electrons. Recent experimental studies may indirectly support the results of these simulations, in particular, with respect to the electron temperature saturation and the channel width effect on the thruster discharge.

Absence of Debye Sheaths due to Secondary Electron Emission

“…Electrons travel unimpeded to the walls. The plasma in this new regime is dramatically different than in past EDIPIC HT simulations [12,13,14,15,16] with smaller drift energy.EDIPIC (electrostatic direct implicit particle-in-cell code) simulates a planar E×B xenon plasma bounded by floating walls made of boron-nitride ceramics (B.N.C. ), see Fig.…”

Absence of Debye Sheaths Due to Secondary Electron Emission

“…Electrons travel unimpeded to the walls. The plasma in this new regime is dramatically different than in past EDIPIC simulations [13,14,15,16,17] with smaller drift energy.…”

Instability, collapse, and oscillation of sheaths caused by secondary electron emission

“…al 27 . The applicability of the model for HT's is justified elsewhere 28,29,25 . The main control parameters are the uniform applied fields E z and B x , neutral gas density n a , plasma density n 0 , plasma width H and turbulent collision frequency ν turb .…”

“…1(b) taken early in the simulation has a rather flat region in the middle because the interior ions have not yet reacted to the sheath formation. υ(x) in a true equilibrium looks (qualitatively) parabolic 28 . Interestingly, it was found in Ref.…”