Publications

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

Abstract not provided.

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

Abstract not provided.

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

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Fierro, Andrew S.; Barnat, Edward V.; Moore, Christopher H.; Yee, Benjamin T.; Hopkins, Matthew M.

Abstract not provided.

Campione, Salvatore; Warne, Larry K.; Yee, Benjamin T.; Cartwright, Keith C.; Basilio, Lorena I.

This report details the comparison of ATLOG modeling results for the response of a finite-length dissipative aerial conductor interacting with a conducting ground to a measurement taken November 2016 at the High-Energy Radiation Megavolt Electron Source (HERMES) facility. We use the ATLOG time-domain method based on transmission line theory. Good agreement is observed between simulations and experiments. Intentionally Left Blank

Hopkins, Matthew M.; Fierro, Andrew S.; Moore, Christopher H.; Yee, Benjamin T.

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

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Fierro, Andrew S.; Moore, Christopher H.; Yee, Benjamin T.; Hopkins, Matthew M.

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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.

Manginell, Ronald P.; Sillerud, Colin H.; Hopkins, Matthew M.; Yee, Benjamin T.; Moorman, Matthew W.; Schwindt, Peter S.; Anderson, John M.; Pfeifer, Nathaniel B.

The temporal evolution of spectral lines from microplasma devices (MD) was studied, including impurity transitions. Long-wavelength emission diminishes more rapidly than deep UV with decreasing pulse width and RF operation. Thus, switching from DC to short pulsed or RF operation, UV emissions can be suppressed, allowing for real-time tuning of the ionization energy of a microplasma photo-ionization source, which is useful for chemical and atomic physics. Scaling allows MD to operate near atmospheric pressure where excimer states are efficiently created and emit down to 65 nm; laser emissions fall off below 200 nm, making MD light sources attractive for deep UV use. A first fully-kinetic three-dimensional model was developed that explicitly calculates electron-energy distribution function. This, and non-continuum effects, were studied with the model and how they are impacted by geometry and transient or DC operation. Finally, a global non-dimensional model was developed to help explain general trends MD physics.

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

Abstract not provided.

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

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

Abstract not provided.

Physics of Plasmas

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

The form of a sheath near a small electrode, with bias changing from below to above the plasma potential, is studied using 2D particle-in-cell simulations. When the electrode is biased within Te/2e below the plasma potential, the electron velocity distribution functions (EVDFs) exhibit a loss-cone type truncation due to fast electrons overcoming the small potential difference between the electrode and plasma. No sheath is present in this regime, and the plasma remains quasineutral up to the electrode. The EVDF truncation leads to a presheath-like density and flow velocity gradients. Once the bias exceeds the plasma potential, an electron sheath is present. In this case, the truncation driven behavior persists, but is accompanied by a shift in the maximum value of the EVDF that is not present in the negative bias cases. The flow moment has significant contributions from both the flow shift of the EVDF maximum, and the loss-cone truncation.

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

Abstract not provided.

Fierro, Andrew S.; Moore, Christopher H.; Yee, Benjamin T.; Hopkins, Matthew M.

Abstract not provided.

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

Abstract not provided.

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

Abstract not provided.

Physics of Plasmas

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

In this paper, we provide insight into the role and impact that a positively biased electrode (anode) has on bulk plasma potential. Using two-dimensional Particle-in-Cell simulations, we investigate the plasma potential as an anode transitions from very small ("probe" mode) to large ("locking" mode). Prior theory provides some guidance on when and how this transition takes place. Initial experimental results are also compared. The simulations demonstrate that as the surface area of the anode is increased transitions in plasma potential and sheath polarity occur, consistent with experimental observations and theoretical predictions. It is expected that understanding this basic plasma behavior will be of interest to basic plasma physics communities, diagnostic developers, and plasma processing devices where control of bulk plasma potential is important.

Physics of Plasmas

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

Electron sheaths are commonly found near Langmuir probes collecting the electron saturation current. The common assumption is that the probe collects the random flux of electrons incident on the sheath, which tacitly implies that there is no electron presheath and that the flux collected is due to a velocity space truncation of the electron velocity distribution function (EVDF). This work provides a dedicated theory of electron sheaths, which suggests that they are not so simple. Motivated by EVDFs observed in particle-in-cell (PIC) simulations, a 1D model for the electron sheath and presheath is developed. In the model, under low temperature plasma conditions (Te 蠑 Ti), an electron pressure gradient accelerates electrons in the presheath to a flow velocity that exceeds the electron thermal speed at the sheath edge. This pressure gradient generates large flow velocities compared to what would be generated by ballistic motion in response to the electric field. It is found that in many situations, under common plasma conditions, the electron presheath extends much further into the plasma than an analogous ion presheath. PIC simulations reveal that the ion density in the electron presheath is determined by a flow around the electron sheath and that this flow is due to 2D aspects of the sheath geometry. Simulations also indicate the presence of ion acoustic instabilities excited by the differential flow between electrons and ions in the presheath, which result in sheath edge fluctuations. The 1D model and time averaged PIC simulations are compared and it is shown that the model provides a good description of the electron sheath and presheath.

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

Abstract not provided.

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

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Yee, Benjamin T.

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Barnat, Edward V.; Hopkins, Matthew M.; Yee, Benjamin T.; Laity, George R.

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

Abstract not provided.