The Effect of Secondary Electron Emission on a Plasma Sheath PDF Download

Author: Peter J. Harbour
Publisher:
ISBN:
Category :
Languages : en
Pages : 27

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Author:
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ISBN:
Category :
Languages : en
Pages : 0

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Theory and experiments are presented that explore the space-charge limited regime of plasma sheaths surrounding electron emitting surfaces. The separation point technique and the inflection point probe methods for measuring the plasma potential were compared to the floating potential of a highly emissive probe in a Hall thruster plasma. The inflection point techniques' measurement of the plasma potential was ~2Te/e above the floating potential measurement, supporting fluid theory predictions. The separation point technique was inconsistent with every other technique and did not accurately measure the plasma potential. A virtual cathode was observed in a multidipole chamber plasma sheath near a grounded, metal boundary coated with a dielectric. A combination of secondary electron emission and a relatively high density of primary electrons formed the space-charge limited sheath indicated by the virtual cathode. Low neutral pressure, primary electron energy, and discharge current allowed the virtual cathode to form. The discharge current greatly affected the sheath potential, reducing it as the current increased. A kinetic theory of space-charge limited electron emitting sheaths is presented which accurately treats the loss of plasma electrons to the boundary and the velocity distribution of emitted electrons. By considering those electrons lost to the wall, the predicted sheath potential was reduced by 10%. Using a kinetic description of the emitted electrons, assuming a half-Maxwellian distribution, greatly affects the sheath potential. It is shown that the kinetic theory predicts that the sheath potential goes to zero as the plasma potential. To test this kinetic theory of emissive sheaths, time-resolved measurements of the emissive sheath potential were made in the afterglow of a capacitively coupled plasma. The results showed that as the plasma electron temperature cooled and approached the emitted electron temperature, the normalized sheath potential was drastically reduced and went to zero when the two temperatures were equal, qualitative supporting the emissive sheath kinetic theory. The emissive sheath potential was unexpectedly large when the plasma electron temperature was larger than the emitted electron temperature by a factor of 4.

Author: Lou Ann Schwager
Publisher:
ISBN:
Category :
Languages : en
Pages : 59

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Author: Stanton Jerrold Peale
Publisher:
ISBN:
Category : Antennas (Electronics)
Languages : en
Pages : 154

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In recent years there has been a growing interest in general plasma physics. Communications over long distances are affected by plasma clouds and any magnetic fields they contain. While radio sounding is currently used to explore the ionosphere, investigation of plasmas located at points far beyond the ionosphere demands that environmental sampling techniques be developed to produce reliable knowledge.

Author:
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ISBN:
Category :
Languages : en
Pages :

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The Debye sheath is shown to be unstable under general conditions. For surface materials with sufficient secondary electron emission (SEE) yields, the surface's current-voltage characteristic has an unstable branch when the bulk plasma temperature (Te) exceeds a critical value, or when there are fast electron populations present. The plasma-surface interaction becomes dynamic where the sheath may undergo spontaneous transitions or oscillations. Using particle-in-cell simulations, we analyze sheath instabilities occurring in a high Te plasma slab bounded by walls with SEE. As the plasma evolves, whenever the sheath enters an unstable state, its amplitude rapidly collapses, allowing a large flux of previously trapped electrons to hit the wall. These hot electrons induce more than one secondary on average, causing a net loss of electrons from the wall. The sheath collapse quenches when the surface charge becomes positive because the attractive field inhibits further electrons from escaping. Sheath instabilities influence the current balance, energy loss, cross-B-field transport and even the bulk plasma properties. Implications for discharges including Hall thrusters are discussed. More generally, the results show that common theories that treat emission as a fixed (time-independent) "coefficient" do not capture the full extent of SEE effects.

Author: Dan M. Goebel
Publisher: John Wiley & Sons
ISBN: 0470436263
Category : Technology & Engineering
Languages : en
Pages : 528

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Throughout most of the twentieth century, electric propulsion was considered the technology of the future. Now, the future has arrived. This important new book explains the fundamentals of electric propulsion for spacecraft and describes in detail the physics and characteristics of the two major electric thrusters in use today, ion and Hall thrusters. The authors provide an introduction to plasma physics in order to allow readers to understand the models and derivations used in determining electric thruster performance. They then go on to present detailed explanations of: Thruster principles Ion thruster plasma generators and accelerator grids Hollow cathodes Hall thrusters Ion and Hall thruster plumes Flight ion and Hall thrusters Based largely on research and development performed at the Jet Propulsion Laboratory (JPL) and complemented with scores of tables, figures, homework problems, and references, Fundamentals of Electric Propulsion: Ion and Hall Thrusters is an indispensable textbook for advanced undergraduate and graduate students who are preparing to enter the aerospace industry. It also serves as an equally valuable resource for professional engineers already at work in the field.

Author:
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ISBN:
Category :
Languages : en
Pages :

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Abstract: The properties of a collisionless plasma sheath are investigated by using a fluid model in which two species of positive ions and secondary electrons are taken into account. It is shown that the positive ion speeds at the sheath edge increase with secondary electron emission (SEE) coefficient, and the sheath structure is affected by the interplay between the two species of positive ions and secondary electrons. The critical SEE coefficients and the sheath widths depend strongly on the positive ion charge number, mass and concentration in the cases with and without SEE. In addition, ion kinetic energy flux to the wall and the impact of positive ion species on secondary electron density at the sheath edge are also discussed.

Author: Marlene Idy Patino
Publisher:
ISBN:
Category :
Languages : en
Pages : 219

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An improved understanding of plasma inter-particle and particle-wall interactions is critical to the advancement of plasma devices used for space electric propulsion, fusion, high-power communications, and next-generation energy systems. Two interactions of particular importance are (1) ion-atom collisions in the plasma bulk and (2) secondary electron emission from plasma-facing materials. For ion-atom collisions, interactions between fast ions and slow atoms are commonly dominated by charge-exchange and momentum-exchange collisions that are important to understanding the performance and behavior of many plasma devices. To investigate this behavior, this work developed a simple, well-characterized experiment that accurately measures the effects of high energy xenon ions incident on a background of xenon neutral atoms. By comparing these results to both analytical and computational models of ion-atom interactions, we discovered the importance of (1) accurately treating the differential cross-sections for momentum-exchange and charge-exchange collisions over all neutral background pressures, and (2) commonly overlooked interactions, including ion-induced electron emission and neutral-neutral ionization collisions, at high pressures. Data provide vital information on the angular scattering distributions of charge-exchange and momentum-exchange ions at 1.5 keV relevant for ion thrusters, and serve as canonical data for validation of plasma models. This work also investigates electron-induced secondary electron emission behavior relevant to materials commonly considered for plasma thrusters, fusion systems, and many other plasma devices. For such applications, secondary electron emission can alter the sheath potential, which can significantly affect device performance and life. Secondary electron emission properties were measured for materials that are critical to the efficient operation of many plasma devices, including: graphite (for tokamaks, ion thrusters, and traveling wave tubes), lithium (for tokamak walls), tungsten (the most promising material for future tokamaks such as ITER), and nickel (for plasma-enhanced chemistry). Measurements were made for incident electron energies up to 1.5 keV and angles between 0 and 78i . The most significant results from these measurements are as follows: (1) first-ever measurements of naturally-forming tungsten fuzz show a more than 40% reduction in secondary electron emission and an independence on incidence angle; (2) original measurements of lithium oxide show a 2x and 6x increase in secondary electron emission for 17% and 100% oxidation; and (3) unique measurements of Ni(110) single crystal show extrema in secondary electron emission when incidence angle is varied and an up to 36% increase at 0i over polycrystalline nickel. Each of these results are important discoveries for improving plasma devices. For example, from (1), the growth of tungsten fuzz in tokamaks is desirable for minimizing adverse secondary electron emission effects. From (2), the opposite is true for tokamaks with lithium coatings which are oxidized by typical residual gases. From (3), secondary electron emission from Ni(110) catalysts in plasma-enhanced chemistry may facilitate further reactions.

Author: H. Bruining
Publisher: Elsevier
ISBN: 1483149870
Category : Science
Languages : en
Pages : 199

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Physics and Applications of Secondary Electron Emission provides a survey of the physics and applications of secondary electron emission. It is part of a series of monographs that aim to report on research carried out in electronics and applied physics. The monographs are written by specialists in their own subjects. Wherever it is practical the monographs will be kept short in length to enable all those interested in electronics to find the essentials necessary for their work in a condensed and concentrated form. The book begins with a discussion of secondary electrons. Separate chapters cover methods for measuring secondary electron emission; numerical results on the secondary electron emission yield of both metals and metal compounds; the influence of externally adsorbed foreign atoms and ions on secondary electron emission; and the mechanism of secondary electron emission. The final three chapters deal with the application side. These include the applications of electron multiplication; the elimination of disturbing effects due to secondary electrons; and ""storage"" devices in which information on electrical charges is written on an insulating surface, often by making use of secondary electron emission.

Author: Charles K. Birdsall
Publisher:
ISBN:
Category :
Languages : en
Pages : 27

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Ion acceleration in the plasma source sheath, with an initial ion velocity where it is found analytically that ions leave the source with drift velocity greater than the velocity of sound obviating the need for pre-sheath acceleration in the bulk plasma, preceding a collector sheath; and (2) Plasma wall physics -- theory and simulation. Abstracts are provided on: Collector and source sheaths of a finite ion temperature plasma, Effects of secondary electron emission on the collector and source sheaths of a finite ion temperature plasma, Effects of ion reflection on the collector and source sheaths of a finite ion temperature plasma, and Vortex dynamics and transport to the wall in a crossed field plasma sheath.