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Influence of Photon and Ion Induced Secondary Yields on Transient Plasma Formation

Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV

Fierro, Andrew S.; Barnat, Edward V.; Moore, Christopher H.; Clem, Paul G.; Hopkins, Matthew M.

The influence of different quantum yields for photons and secondary emission yields for ions striking a surface is investigated. Using a one-dimensional particle-in-cell simulation, these secondary emission coefficients are varied to observe the impact on discharge current. The discharge is assumed to occur in pure helium gas at a pressure of 75 torr. To handle binary particle interactions, the Direct Simulation Monte Carlo (DSMC) method is utilized. The model includes electron-neutral interactions, neutral-neutral interactions, and photon-neutral interactions. It is observed that the discharge current in the early stages of discharge is heavily dependent upon the quantum yield due to photon impact. In the later stages of discharge, the current depends on both the quantum yield and secondary emission coefficient for ion impact.

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TYPE Conference Poster YEAR 2018
ScopusOSTIDOI

Detailed Characterization of Surface Structure and Influence on Field Emission

Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV

Hopkins, Matthew M.; Smith, Sean S.; Clem, Paul G.; Berg, Morgann B.; Scrymgeour, David S.; Moore, Christopher H.; Bussmann, Ezra B.; Ohta, Taisuke O.

In most models of vacuum breakdown, there is some initial emission of electrons from the cathodic surface, usually employing some form of Fowler-Nordheim emission. While this may be correct for 'textbook' surfaces, it is generally unreliable for real surfaces and fitted parameters are often used. For example, the beta employed is generally unphysical based on usual definitions (e.g., it incorporates more, but unexplained, physics than just a geometry-based field concentration effect). In this work, we describe experimental efforts to better characterize which surface structure parameters influence the vacuum field emission current.

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TYPE Conference Poster YEAR 2018
ScopusOSTIDOI

Three-dimensional kinetic modeling of streamer propagation in a nitrogen/helium gas mixture

Plasma Sources Science and Technology

Fierro, Andrew S.; Moore, Chris; Yee, Ben; Hopkins, Matthew M.

A fully resolved kinetic model (particle-in-cell and direct simulation Monte Carlo for particle/photon collisions) of a near atmospheric pressure ionization wave is presented here. Fully resolving the required numerical spatial (sub-μm) and temporal scales (tens of fs) for atmospheric pressure discharges in three-dimensions is still a challenging task on modern super computers. To keep the overall problem tractable, the total number of elements are reduced by only simulating a 10° wedge rather than a full 360° geometry. The ionization wave is generated in a needle-plane configuration with a gap size of 250 μm and a background of nitrogen and helium gas. A voltage of 1500 V is applied to the anode and an initial electron and ion density of 109 cm-3 is seeded in a region near the anode electrode tip and extending towards the cathode. As these initial electrons are swept away, photoionization and photoemission create new electrons and allow the ionization front to propagate towards the cathode. Results from the 90% N2, 10% He discharge indicate that photoionization has minimal impact on plasma formation processes and cathode photoemission is the dominant mechanism for new electrons. In the 90% He, 10% N2 discharge case, however, photoionization likely has an impact as the observed locations of photoionization occur far enough away from the ionization front to allow for sufficient avalanche processes that contribute to the propagation of the ionization wave. Additionally, the electron energy distribution functions in the 90% He, 10% N2 case indicate that there is less energy loss to the low lying molecular N2 electronic states as well as the vibrational and rotational modes. This leads to higher electron energies and faster plasma development times of ∼0.4 ns for the 90% He, 10% N2 case, and ∼1.5 ns for the 90% N2, 10% He case. In addition to analysis of the ionization wave results, the overall challenges associated with simulations near atmospheric pressure discharges in three-dimensions are discussed, including the limitations of the 10° wedge that produces, at least qualitatively, minimal 3D effects.

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TYPE Journal Article YEAR 2018
ScopusOSTIDOI
Results 51–75 of 193
Results 51–75 of 193