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Theory and simulation of anode spots in low pressure plasmas

Physics of Plasmas

Scheiner, Brett; Barnat, Edward V.; Baalrud, Scott D.; Hopkins, Matthew M.; Yee, Benjamin T.

When electrodes are biased above the plasma potential, electrons accelerated through the associated electron sheath can dramatically increase the ionization rate of neutrals near the electrode surface. It has previously been observed that if the ionization rate is great enough, a double layer separates a luminous high-potential plasma attached to the electrode surface (called an anode spot or fireball) from the bulk plasma. Here, results of the first 2D particle-in-cell simulations of anode spot formation are presented along with a theoretical model describing the formation process. It is found that ionization leads to the build-up of an ion-rich layer adjacent to the electrode, forming a narrow potential well near the electrode surface that traps electrons born from ionization. It is shown that anode spot onset occurs when a quasineutral region is established in the potential well and the density in this region becomes large enough to violate the steady-state Langmuir condition, which is a balance between electron and ion fluxes across the double layer. A model for steady-state properties of the anode spot is also presented, which predicts values for the anode spot size, double layer potential drop, and form of the sheath at the electrode by considering particle, power, and current balance. These predictions are found to be consistent with the presented simulation and previous experiments.

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TYPE Journal Article YEAR 2017
ScopusOSTIDOI

Radiation transport in kinetic simulations and the influence of photoemission on electron current in self-sustaining discharges

Journal of Physics D: Applied Physics

Fierro, Andrew S.; Moore, Chris; Scheiner, Brett; Yee, Benjamin T.; Hopkins, Matthew M.

A kinetic description for electronic excitation of helium for principal quantum number n 4 has been included into a particle-in-cell (PIC) simulation utilizing direct simulation Monte Carlo (DSMC) for electron-neutral interactions. The excited electronic levels radiate state-dependent photons with wavelengths from the extreme ultraviolet (EUV) to visible regimes. Photon wavelengths are chosen according to a Voigt distribution accounting for the natural, pressure, and Doppler broadened linewidths. This method allows for reconstruction of the emission spectrum for a non-thermalized electron energy distribution function (EEDF) and investigation of high energy photon effects on surfaces, specifically photoemission. A parallel plate discharge with a fixed field (i.e. space charge neglected) is used to investigate the effects of including photoemission for a Townsend discharge. When operating at a voltage near the self-sustaining discharge threshold, it is observed that the electron current into the anode is higher when including photoemission from the cathode than without even when accounting for self-absorption from ground state atoms. The photocurrent has been observed to account for as much as 20% of the total current from the cathode under steady-state conditions.

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TYPE Journal Article YEAR 2017
ScopusOSTIDOI
Results 76–100 of 193
Results 76–100 of 193