Measurements of fireball onset

Scheiner, Brett; Barnat, Edward V.; Hopkins, Matthew M.; Yee, Benjamin

doi:10.1063/1.5026869

Cited by 7 publications

(15 citation statements)

References 21 publications

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“…Application of LCIF has considerably advanced understanding of fireballs because it provides a non-invasive (optical) and well-resolved measurement of electric field, electron density and electron temperature. A simulation of an experiment studying fireball onset and stabilization in a 100 mTorr helium discharge was found to show agreement with the measured electric field and density profiles, as well as the onset dynamics [257]; see figure 29. This provided evidence confirming the importance of the establishment of a potential well for electrons in the anode glow region prior to onset.…”

Section: Fireball Onsetsupporting

confidence: 61%

“…A depletion of the measured saturation current indicates a loss of the highest energy electrons at the center of the fireball and a decrease of density due to a reduction of the ionization rate. PIC simulations of transient behavior after the application of a stepped electrode bias suggest that the fireball collapse may be due to a raise in the bulk plasma potential, reducing the double layer potential and thus the ionization rate [257]. An experimental study of fireballs in a hollow cathode geometry reached a similar conclusion that the double layer potential is associated with the MU fireball collapse [242].…”

Section: Hysteresismentioning

confidence: 99%

“…LCIF measurements of the electric field magnitude, 590 nm plasma emission, electron density and metastable population during fireball onset. Figure reprinted from reference[257].…”

mentioning

confidence: 99%

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Interaction of biased electrodes and plasmas: sheaths, double layers, and fireballs

Baalrud

Scheiner

Yee

et al. 2020

Plasma Sources Sci. Technol.

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Biased electrodes are common components of plasma sources and diagnostics. The plasma-electrode interaction is mediated by an intervening sheath structure that influences properties of the electrons and ions contacting the electrode surface, as well as how the electrode influences properties of the bulk plasma. A rich variety of sheath structures have been observed, including ion sheaths, electron sheaths, double sheaths, double layers, anode glow, and fireballs. These represent complex self-organized responses of the plasma that depend not only on the local influence of the electrode, but also on the global properties of the plasma and the other boundaries that it is in contact with. This review summarizes recent advances in understanding the conditions under which each type of sheath forms, what the basic stability criteria and steady-state properties of each are, and the ways in which each can influence plasma-boundary interactions and bulk plasma properties. These results may be of interest to a number of application areas where biased electrodes are used, including diagnostics, plasma modification of materials, plasma sources, electric propulsion, and the interaction of plasmas with objects in space.

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Section: Fireball Onsetsupporting

confidence: 61%

Section: Hysteresismentioning

confidence: 99%

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Interaction of biased electrodes and plasmas: sheaths, double layers, and fireballs

Baalrud

Scheiner

Yee

et al. 2020

Plasma Sources Sci. Technol.

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“…The formation of this "anode spot" is a highly dynamic, nonlinear process and represents a challenging test for both experimental diagnostics and models. The comparison of the two are the subject of [57] which was motivated by the initial modeling and theory from [58].…”

Section: Anode Spot Dynamicsmentioning

confidence: 99%

Challenges and opportunities in verification and validation of low temperature plasma simulations and experiments

et al. 2021

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This paper describes the verification and validation (V&V) framework developed for the stochastic Particle-in-Cell, Direct Simulation Monte Carlo code Aleph. An ideal framework for V&V from the viewpoint of the authors is described where a physics problem is defined, and relevant physics models and parameters to the defined problem are assessed and captured in a Phenomena Identification and Ranking Table (PIRT). Numerous V&V examples guided by the PIRT for a simple gas discharge are shown to demonstrate the V&V process applied to a real-world simulation tool with the overall goal to demonstrably increase the confidence in the results for the simulation tool and its predictive capability. Although many examples are provided here to demonstrate elements of the framework, the primary goal of this work is to introduce this framework and not to provide a fully complete implementation, which would be a much longer document. Comparisons and contrasts are made to more usual approaches to V&V, and techniques new to the low temperature plasma community are introduced. Specific challenges relating to the sufficiency of available data (e.g., cross sections), the limits of ad hoc validation approaches, the additional difficulty of utilizing a stochastic simulation tool, and the extreme cost of formal validation are discussed.

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“…1A. Upon increasing the electrode bias, the fireball onset is abrupt lasting on the order of a microsecond once a critical electrode bias is exceeded 9,11 . This critical bias is determined by the neutral gas pressure, electrode area, and the electron-impact ionization cross section of the neutral gas 12 .…”

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confidence: 99%

On the hysteresis in fireball formation and extinction

Scheiner

Beving

2019

Physics of Plasmas

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A model is proposed to explain hysteresis observed in fireball formation and extinction as electrode bias is varied in partially ionized plasmas. Formation is predicted after a sufficiently deep potential well is established in the electron sheath of the electrode. Under the experimental conditions considered, once the fireball forms the plasma potential rapidly increases, resulting in electrons being only lost to the electrode.Previous predictions suggest that once formed the fireball double layer must maintain a potential close to the ionization potential of the neutral gas to remain in a steady state. In this paper, it is predicted that changes in electrode bias after formation results in a corresponding change in fireball size and plasma potential. This change in plasma potential allows the double layer potential to be maintained at biases both above and below the electrode bias at onset. The fireball extinguishes when the required double layer potential can no longer be maintained with balance of current loss of the bulk plasma. These predictions are tested experimentally and are found to be in good agreement with the measurements.Fireballs are a discharge phenomenon that can occur near electrodes biased above the plasma potential by an amount greater than the neutral gas ionization potential 1 . They are characterized by a secondary plasma whose plasma potential is near the electrode potential and at its boundary is separated from the bulk plasma by a double layer. Typically, the fireball plasma is several hundred Debye lengths, much larger than the initial sheath scale. Recent experimental studies of fireballs have focused on their stability 2-5 and properties as ion sources 6-8 . One of the most common observations is hysteresis in the current-voltage (I-V) traces between the upswing and downswing of the electrode bias 9,10 , see Fig. 1A. Upon increasing the electrode bias, the fireball onset is abrupt lasting on the order of a microsecond once a critical electrode bias is exceeded 9,11 . This critical bias is determined by the neutral gas pressure, electrode area, and the electron-impact ionization cross section of the neutral gas 12 . After onset, increased current collection is observed primarily due to the greater electron collection by the surface area of the fireball compared to that of the initial electron sheath, but also due to an increased ionization rate within the fireball. Upon decrementing the electrode bias below the critical bias, the fireball persists and the increased current collection relative to the prefireball formation value continues to be observed. The fireball eventually collapses after continued decrementation of the electrode bias and the electron current collection returns to pre-fireball levels.When the plasma chamber is small enough, fireball formation result in a state of global non-ambipolar flow where electrons are only lost to the fireball surface area 13 . In this state, the bulk plasma potential is locked to a fixed offset of the potential of the electrode due to the ...

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Measurements of fireball onset

Cited by 7 publications

References 21 publications

Interaction of biased electrodes and plasmas: sheaths, double layers, and fireballs

Interaction of biased electrodes and plasmas: sheaths, double layers, and fireballs

Challenges and opportunities in verification and validation of low temperature plasma simulations and experiments

On the hysteresis in fireball formation and extinction

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