Kinetic ion induced electron emission model applied to Z-pinch electrode plasmas

As nuclear fusion devices scale to higher energies, ion induced electron emission (IIEE) begins playing an important role in altering plasma sheath dynamics. We develop a novel ion induced electron emission model that accurately captures the secondary electron yield (SEY) and emitted electron energy spectrum for proton, deuteron, and triton impacts on metallic surfaces. We implement this model as a boundary condition in the continuum kinetic code Gkeyll. We perform 1X-1V simulations of an unmagnetized Z-pinch plasma doubly bounded between copper electrodes for varying electric potential bias cases. We find that the SEY increases with bias potential and is significantly higher at the cathode compared to the anode. The emitted electrons are accelerated into the domain by the sheath potential barrier. The now high energy emitted electrons collide with the bulk domain plasma increasing the electron temperature leading to a larger plasma potential. This increases the ion acceleration and sheath effects, counter to previous work examining how thermionic and electron induced electron emission decrease the sheath effects. These works showed that sufficiently strong SEY can yield space-charge limited (SCL) or inverse sheaths. In our simulations, however, a classical sheath forms despite the SEY being greater than 1 for some cases because the emitted electron particle flux does not reach the space charge saturation limit. In addition, there has been a discrepancy in the experimental current and that of emissionless sheath theory. The simulation current with emissions is found to be greater than predicted by theory and most closely matches experiment at a bias potential of 4 kV. Our unmagnetized simulations act as upper bound estimates of the current with the azimuthal Z-pinch magnetic field expected to reduce the effective electron emission. Future work will consider the effects of the magnetic field, electron induced electron emission, and electrode oxidation on sheath formation and resulting current.