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Feb 2013

Volume 20, Issue 2, Articles (02xxxx)

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Phys. Plasmas 20, 022303 (2013); http://dx.doi.org/10.1063/1.4790639 (12 pages)

Julio J. Martinell and Diego del-Castillo-Negrete
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back to top Low-Temperature Plasmas, Plasma Applications, Plasma Sources, Sheaths

Combined effects of gas pressure and exciting frequency on electron energy distribution functions in hydrogen capacitively coupled plasmas

E. Abdel-Fattah and H. Sugai

Phys. Plasmas 20, 023501 (2013); http://dx.doi.org/10.1063/1.4789611 (9 pages) | Cited 1 time

Online Publication Date: 4 February 2013

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The combined effects of the variation of hydrogen pressure (40–400 mTorr) and exciting frequency (13.56–50 MHz) on the electron energy probability function (EEPF) and other plasma parameters in capacitively coupled hydrogen H2 discharge at fixed discharge voltage were investigated using rf-compensated Langmuir probe. At a fixed exciting frequency of 13.56 MHz, the EEPF evolved from Maxwellian-like distribution to a bi-Maxwellian distribution when the H2 pressure increased, possibly due to efficient vibrational excitation. The electron density largely increased to a peak value and then decreased with the increase of H2 pressure. Meanwhile, the electron temperature and plasma potential significantly decrease and reaching a minimum at 120 mTorr beyond, which saturated or slightly increases. On the other hand, the dissipated power and electron density markedly increased with increasing the exciting frequency at fixed H2 pressure and voltage. The electron temperatures negligibly dependent on the driving frequency. The EEPFs at low pressure 60 mTorr resemble Maxwellian-like distribution and evolve into a bi-Maxwellian type as frequency increased, due to a collisonless (stochastic) sheath-heating in the very high frequency regime, while the EEPF at hydrogen pressure ≥ 120 mTorr retained a bi-Maxwellian-type distribution irrespective of the driving frequency. Such evolution of the EEPFs shape with the driving frequency and hydrogen pressure has been discussed on the basis of electron diffusion processes and low threshold-energy inelastic collision processes taking place in the discharge. The ratio of stochastic power to bulk power heating ratio is dependent on the hydrogen pressure while it is independent on the driving frequency.
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52.70.Ds Electric and magnetic measurements
02.50.Cw Probability theory
52.80.Pi High-frequency and RF discharges
52.40.Kh Plasma sheaths
52.25.Fi Transport properties
52.20.Fs Electron collisions

Two-dimensional quasi-double-layers in two-electron-temperature, current-free plasmas

Mario Merino and Eduardo Ahedo

Phys. Plasmas 20, 023502 (2013); http://dx.doi.org/10.1063/1.4789900 (11 pages) | Cited 1 time

Online Publication Date: 5 February 2013

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The expansion of a plasma with two disparate electron populations into vacuum and channeled by a divergent magnetic nozzle is analyzed with an axisymmetric model. The purpose is to study the formation and two-dimensional shape of a current-free double-layer in the case when the electric potential steepening can still be treated within the quasineutral approximation. The properties of this quasi-double-layer are investigated in terms of the relative fraction of the high-energy electron population, its radial distribution when injected into the nozzle, and the geometry and intensity of the applied magnetic field. The two-dimensional double layer presents a curved shape, which is dependent on the natural curvature of the equipotential lines in a magnetically expanded plasma and the particular radial distribution of high-energy electrons at injection. The double layer curvature increases the higher the nozzle divergence is, the lower the magnetic strength is, and the more peripherally hot electrons are injected. A central application of the study is the operation of a helicon plasma thruster in space. To this respect, it is shown that the curvature of the double layer does not increment the thrust, it does not modify appreciably the downstream divergence of the plasma beam, but it increases the magnetic-to-pressure thrust ratio. The present study does not attempt to cover current-free double layers involving plasmas with multiple populations of positive ions.
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52.40.Kh Plasma sheaths
52.25.Fi Transport properties

Atmospheric-pressure plasma jets: Effect of gas flow, active species, and snake-like bullet propagation

S. Wu, Z. Wang, Q. Huang, X. Tan, X. Lu, and K. Ostrikov

Phys. Plasmas 20, 023503 (2013); http://dx.doi.org/10.1063/1.4791652 (7 pages)

Online Publication Date: 11 February 2013

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Cold atmospheric-pressure plasma jets have recently attracted enormous interest owing to numerous applications in plasma biology, health care, medicine, and nanotechnology. A dedicated study of the interaction between the upstream and downstream plasma plumes revealed that the active species (electrons, ions, excited OH, metastable Ar, and nitrogen-related species) generated by the upstream plasma plume enhance the propagation of the downstream plasma plume. At gas flows exceeding 2 l/min, the downstream plasma plume is longer than the upstream plasma plume. Detailed plasma diagnostics and discharge species analysis suggest that this effect is due to the electrons and ions that are generated by the upstream plasma and flow into the downstream plume. This in turn leads to the relatively higher electron density in the downstream plasma. Moreover, high-speed photography reveals a highly unusual behavior of the plasma bullets, which propagate in snake-like motions, very differently from the previous reports. This behavior is related to the hydrodynamic instability of the gas flow, which results in non-uniform distributions of long-lifetime active species in the discharge tube and of surface charges on the inner surface of the tube.
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52.75.-d Plasma devices
52.80.Tn Other gas discharges
47.60.Dx Flows in ducts and channels
52.40.-w Plasma interactions (nonlaser)

Transport properties of multicomponent thermal plasmas: Grad method versus Chapman-Enskog method

P. Porytsky, I. Krivtsun, V. Demchenko, U. Reisgen, O. Mokrov, A. Zabirov, S. Gorchakov, A. Timofeev, and D. Uhrlandt

Phys. Plasmas 20, 023504 (2013); http://dx.doi.org/10.1063/1.4790661 (12 pages)

Online Publication Date: 21 February 2013

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Transport properties (thermal conductivity, viscosity, and electrical conductivity) for multicomponent Ar-Fe thermal plasmas at atmospheric pressure have been determined by means of two different methods. The transport coefficients set based on Grad's method is compared with the data obtained when using the Chapman-Enskog's method. Results from both applied methods are in good agreement. It is shown that the Grad method is suitable for the determination of transport properties of the thermal plasmas.
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52.25.Fi Transport properties
52.25.Kn Thermodynamics of plasmas

Spatial measurements of electron energy distribution and plasma parameters in a weakly magnetized inductive discharge

Young-Do Kim, Young-Kwang Lee, Hyo-Chang Lee, and Chin-Wook Chung

Phys. Plasmas 20, 023505 (2013); http://dx.doi.org/10.1063/1.4790664 (5 pages)

Online Publication Date: 21 February 2013

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Spatial characteristics of plasma parameters such as electron temperature, plasma density, plasma potential, and electron energy distribution (EED) were studied in inductively coupled plasma with an axial dc magnetic field. With dc magnetic field, the measured EEDs in the total electron energy scale are spatially coincided except cutting of the low electron energy part indicating the conserved non-local electron kinetics in an axial direction, even though the dc magnetic field is applied. Spatial distributions of the plasma densities at axial positions have almost same trends with various magnetic field strengths. We also discuss the reduction of the ambipolar potential along the axial direction as the applied magnetic field increased.
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52.70.Ds Electric and magnetic measurements
52.80.-s Electric discharges
52.25.-b Plasma properties
52.25.Fi Transport properties
52.25.Xz Magnetized plasmas
52.50.Dg Plasma sources

Enhanced transportation of energetic electrons in dual-frequency atmospheric microplasmas

H. C. Kwon, H. Y. Kim, I. H. Won, H. Wk. Lee, H. K. Shin, and J. K. Lee

Phys. Plasmas 20, 023506 (2013); http://dx.doi.org/10.1063/1.4793728 (7 pages)

Online Publication Date: 26 February 2013

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A comparative study of electron kinetics between single-frequency (SF) microplasmas and their equivalent dual-frequency (DF) microplasmas with matching effective frequencies in atmospheric-pressure helium discharges was performed using particle-in-cell simulation with a Monte Carlo collision. The effective-frequency concept helps in analyzing DF microplasmas in a fashion similar to SF microplasmas with effective parameters. In this study, the plasma characteristics such as the plasma potential, density, and electron energy probability functions of the SF microplasma and its DF counterpart were almost the same. However, the oscillating sheath edge was pushed further into the electrode for a substantial fraction of the time and the sheath width decreased in DF microplasmas. As a result, the transportation of the energetic electrons (ε > 4 eV) usable for tailoring the surface chemistry in atmospheric microplasmas is enhanced in DF microplasmas as compared to SF microplasmas.
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52.25.Fi Transport properties
52.40.Kh Plasma sheaths
52.65.Pp Monte Carlo methods
52.80.-s Electric discharges
52.20.Fs Electron collisions

Effect of the annular region on the performance of a cylindrical Hall plasma thruster

Mihui Seo, Jongsub Lee, Jongho Seon, Hae June Lee, and Wonho Choe

Phys. Plasmas 20, 023507 (2013); http://dx.doi.org/10.1063/1.4793741 (4 pages)

Online Publication Date: 26 February 2013

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Performance characteristics of a cylindrical Hall thruster depending on the depth of the annular region (La) in front of the anode were investigated. The effect of the annular region was examined by operating thrusters corresponding to four different values of La (0, 4, 6, and 10 mm) and a fixed length of the cylindrical region (25 mm). Various measurements such as electron and ion currents, thrust, anode efficiency, current and propellant utilizations, and ion energy distribution functions were performed. Such measurements lead to an interpretation that (1) a considerable potential difference may exist between the anode and the ionization region, which is presumably located near the end of the annular region where magnetic field lines converge; (2) this potential difference increases with respect to increasing La; and (3) the presence of the annular region near the anode reduces the specific impulse and anode efficiency for the examined thrusters.
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52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.25.Fi Transport properties
52.75.-d Plasma devices
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