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1 May 2010

Volume 107, Issue 9, Articles (09xxxx)

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back to top Plasmas and Electrical Discharges

High power microwave generation from coaxial virtual cathode oscillator using graphite and velvet cathodes

Rakhee Menon, Amitava Roy, S. K. Singh, S. Mitra, Vishnu Sharma, Senthil Kumar, Archana Sharma, K. V. Nagesh, K. C. Mittal, and D. P. Chakravarthy

J. Appl. Phys. 107, 093301 (2010); http://dx.doi.org/10.1063/1.3399650 (6 pages) | Cited 5 times

Online Publication Date: 3 May 2010

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High power microwave (HPM) generation studies were carried out in KALI-5000 pulse power system. The intense relativistic electron beam was utilized to generate HPMs using a coaxial virtual cathode oscillator. The typical electron beam parameters were 350 kV, 25 kA, and 100 ns, with a few hundreds of ampere per centimeter square current density. Microwaves were generated with graphite and polymer velvet cathode at various diode voltage, current, and accelerating gaps. A horn antenna setup with diode detector and attenuators was used to measure the microwave power. It was observed that the microwave power increases with the diode voltage and current and reduces with the accelerating gap. It was found that both the peak power and width of the microwave pulse is larger for the velvet cathode compared to the graphite cathode. In a coaxial vircator, velvet cathode is superior to the graphite cathode due to its shorter turn on time and better electron beam uniformity.
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84.40.Fe Microwave tubes (e.g., klystrons, magnetrons, traveling-wave, backward-wave tubes, etc.)

Effect of resonance in external radio-frequency circuit on very high frequency plasma discharge

Shahid Rauf, Zhigang Chen, and Ken Collins

J. Appl. Phys. 107, 093302 (2010); http://dx.doi.org/10.1063/1.3406153 (6 pages) | Cited 3 times

Online Publication Date: 3 May 2010

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A fully electromagnetic plasma model for an asymmetric capacitively coupled plasma discharge is used to understand the interaction between the external radio-frequency (rf) distributed circuit and the plasma. The plasma is excited using a 150 MHz rf source connected to the top electrode, the bottom electrode is connected to a shorted transmission line, and the electrodes are separated from the chamber walls through dielectric rings. Under typical conditions, the electron density peaks in the center of the plasma chamber due to the standing electromagnetic wave and the rf current from the top electrode primarily returns through the bottom electrode. When the electrical length of the bottom transmission line is adjusted such that it presents a large (open-circuit) impedance at the plasma chamber interface, the rf return current shifts from the bottom electrode to the chamber wall. As a consequence, the peak in electron density also moves from the center of the chamber toward its outer periphery.
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52.80.Pi High-frequency and RF discharges
52.50.Dg Plasma sources
84.30.-r Electronic circuits

Transitions between corona, glow, and spark regimes of nanosecond repetitively pulsed discharges in air at atmospheric pressure

David Z. Pai, Deanna A. Lacoste, and Christophe O. Laux

J. Appl. Phys. 107, 093303 (2010); http://dx.doi.org/10.1063/1.3309758 (15 pages) | Cited 19 times

Online Publication Date: 6 May 2010

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In atmospheric pressure air preheated from 300 to 1000 K, the nanosecond repetitively pulsed (NRP) method has been used to generate corona, glow, and spark discharges. Experiments have been performed to determine the parameter space (applied voltage, pulse repetition frequency, ambient gas temperature, and interelectrode gap distance) of each discharge regime. In particular, the experimental conditions necessary for the glow regime of NRP discharges have been determined, with the notable result that there exists a minimum and maximum gap distance for its existence at a given ambient gas temperature. The minimum gap distance increases with decreasing gas temperature, whereas the maximum does not vary appreciably. To explain the experimental results, an analytical model is developed to explain the corona-to-glow (C-G) and glow-to-spark (G-S) transitions. The C-G transition is analyzed in terms of the avalanche-to-streamer transition and the breakdown field during the conduction phase following the establishment of a conducting channel across the discharge gap. The G-S transition is determined by the thermal ionization instability, and we show analytically that this transition occurs at a certain reduced electric field for the NRP discharges studied here. This model shows that the electrode geometry plays an important role in the existence of the NRP glow regime at a given gas temperature. We derive a criterion for the existence of the NRP glow regime as a function of the ambient gas temperature, pulse repetition frequency, electrode radius of curvature, and interelectrode gap distance.
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52.80.Hc Glow; corona
52.80.Mg Arcs; sparks; lightning; atmospheric electricity
51.50.+v Electrical properties (ionization, breakdown, electron and ion mobility, etc.)

Simulations of direct-current air glow discharge at pressures ∼ 1 Torr: Discharge model validation

Shankar Mahadevan and Laxminarayan L. Raja

J. Appl. Phys. 107, 093304 (2010); http://dx.doi.org/10.1063/1.3374711 (11 pages) | Cited 4 times

Online Publication Date: 6 May 2010

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Computational simulations of air glow discharge phenomena in the pressure range typical of plasma actuator applications for high speed flow control are presented. The model is based on a self-consistent, multispecies, and multitemperature continuum description of the plasma. A reduced air plasma model suitable for multidimensional simulations with 11 species and 21 gas phase chemical reactions is validated against experimental results in the literature. The discharge model predicts experimentally observed glow mode discharge operation, the current-voltage characteristics of the discharge, and spatial profiles of the electron temperature and positive ion number densities. For pressures of order 1 Torr, O2+ and N2+ are the dominant positive ion species in the discharge, and the concentration of O− negative ion is comparable to electron concentration. The two-dimensional structure of the discharge is predicted by the model is found to be in agreement with qualitative observations from the experiments.
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52.80.Hc Glow; corona
52.65.-y Plasma simulation
82.33.Xj Plasma reactions (including flowing afterglow and electric discharges)
52.20.Hv Atomic, molecular, ion, and heavy-particle collisions
52.25.-b Plasma properties
52.20.Fs Electron collisions

H-atom interaction with amorphous hydrocarbon films: Effect of surface temperature, H flux and exposure time

A. Erradi, R. Clergereaux, and F. Gaboriau

J. Appl. Phys. 107, 093305 (2010); http://dx.doi.org/10.1063/1.3369286 (6 pages) | Cited 5 times

Online Publication Date: 7 May 2010

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In the present paper, we study the interaction between atomic hydrogen generated in a microwave afterglow with amorphous hydrogenated carbon films. A simple surface model is described and compared with the experimental results. Erosion rate is time dependent and exhibits a transient regime before reaching a constant value. Estimate of the modified film thickness by ellipsometry shows that thickness increases with time and becomes constant and equal to 1.4 nm when reaching the permanent regime. In addition, this limit is independent on the conditions, e.g., on hydrogen flux and temperature. Erosion rate depends linearly on hydrogen flux arriving at the surface and shows an exponential increase with surface temperature. A simple model proposed in the paper is in good agreement with the experimental data and allows giving an estimate of the erosion activation energy Ea = 0.2 eV. This value is in agreement with the energy involved in the reaction between hydrogen atom and carbon atom in sp3 hybridization.
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81.05.U- Carbon/carbon-based materials
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces
81.40.Pq Friction, lubrication, and wear
62.20.Qp Friction, tribology, and hardness
68.55.jd Thickness
52.80.Hc Glow; corona

Ion energy distribution near a plasma meniscus with beam extraction for multi element focused ion beams

Jose V. Mathew, Samit Paul, and Sudeep Bhattacharjee

J. Appl. Phys. 107, 093306 (2010); http://dx.doi.org/10.1063/1.3369287 (5 pages) | Cited 4 times

Online Publication Date: 7 May 2010

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An earlier study of the axial ion energy distribution in the extraction region (plasma meniscus) of a compact microwave plasma ion source showed that the axial ion energy spread near the meniscus is small ( ∼ 5 eV) and comparable to that of a liquid metal ion source, making it a promising candidate for focused ion beam (FIB) applications [ J. V. Mathew and S. Bhattacharjee, J. Appl. Phys. 105, 96101 (2009) ]. In the present work we have investigated the radial ion energy distribution (IED) under the influence of beam extraction. Initially a single Einzel lens system has been used for beam extraction with potentials up to −6 kV for obtaining parallel beams. In situ measurements of IED with extraction voltages upto −5 kV indicates that beam extraction has a weak influence on the energy spread (±0.5 eV) which is of significance from the point of view of FIB applications. It is found that by reducing the geometrical acceptance angle at the ion energy analyzer probe, close to unidirectional distribution can be obtained with a spread that is smaller by at least 1 eV.
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52.50.Dg Plasma sources
41.75.Cn Negative-ion beams
52.59.-f Intense particle beams and radiation sources

Micron-focused ion beamlets

Abhishek Chowdhury and Sudeep Bhattacharjee

J. Appl. Phys. 107, 093307 (2010); http://dx.doi.org/10.1063/1.3371688 (5 pages) | Cited 3 times

Online Publication Date: 10 May 2010

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A multiple beam electrode system (MBES) is used to provide focused ion beamlets of elements from a compact microwave plasma. In this study, a honeycomb patterned plasma electrode with micron size apertures for extracting ion beamlets is investigated. The performance of the MBES is evaluated with the help of two widely adopted and commercially available beam simulation tools, AXCEL-INP and SIMION, where the input parameters are obtained from our experiments. A simple theoretical model based upon electrostatic ray optics is employed to compare the results of the simulations. It is found that the results for the beam focal length agree reasonably well. Different geometries are used to optimize the beam spot size and a beam spot ∼ 5–10 μm is obtained. The multiple ion beamlets will be used to produce microfunctional surfaces on soft matter like polymers. Additionally, the experimental set-up and plans are presented in the light of above applications.
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29.25.Ni Ion sources: positive and negative
41.85.Ar Particle beam extraction, beam injection
52.40.Mj Particle beam interactions in plasmas
52.65.-y Plasma simulation
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