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1 Aug 2007

Volume 102, Issue 3, Articles (03xxxx)

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Electric field and electron orbits near a triple point

Nicholas M. Jordan, Y. Y. Lau, David M. French, R. M. Gilgenbach, and P. Pengvanich

J. Appl. Phys. 102, 033301 (2007); http://dx.doi.org/10.1063/1.2764211 (10 pages) | Cited 21 times

Online Publication Date: 2 August 2007

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Triple point, defined as the junction of metal, dielectric, and vacuum, is the location where electron emission is favored in the presence of a sufficiently strong electric field. In addition to being an electron source, the triple point is generally regarded as the location where flashover is initiated in high voltage insulation, and as the vulnerable spot from which rf breakdown is triggered. In this paper, we focus on the electric field distribution at a triple point of a general geometry, as well as the electron orbits in its immediate vicinity. We calculate the orbit of the first generation electrons, the seed electrons. It is found that, despite the mathematically divergent electric field at the triple point, significant electron yield most likely results from secondary electron emission when the seed electrons strike the dielectric. The analysis gives the voltage scale in which this electron multiplication may occur. It also provides an explanation on why certain dielectric angles are more favorable to electron generation over others, as observed in previous experiments.
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79.20.Hx Electron impact: secondary emission
77.22.Jp Dielectric breakdown and space-charge effects
52.80.Vp Discharge in vacuum

Effects of gas temperature and electron temperature on species concentration of air plasmas

XinPei Lu

J. Appl. Phys. 102, 033302 (2007); http://dx.doi.org/10.1063/1.2764242 (6 pages) | Cited 3 times

Online Publication Date: 2 August 2007

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The applications of atmospheric pressure air plasma, such as absorption and reflection of electromagnetic radiation, require an electron density of 1012 cm−3 or higher. To achieve the required electron density, the electron temperature needs to reach certain level for a given gas temperature. In this manuscript, to obtain an electron density of 1012 cm−3 in one atmospheric pressure air, the required electron temperatures are studied for the given gas temperatures from 500 to 2000 K. It is found that the required electron temperatures decrease dramatically from 15 310 to 11 750 K when the gas temperature increases from 500 to 1000 K, but further increase the gas temperature to 2000 K; the electron temperature of 11 315 K is still needed to achieve an electron density of 1012 cm−3.
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52.25.Dg Plasma kinetic equations

Raman scattering measurements within a flat plate boundary layer in an inductively coupled plasma wind tunnel

Damien Studer and Pierre Vervisch

J. Appl. Phys. 102, 033303 (2007); http://dx.doi.org/10.1063/1.2768067 (8 pages) | Cited 4 times

Online Publication Date: 10 August 2007

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High temperature air chemistry is a crucial issue concerning next reusable space vehicle thermal protection system. The aim of this paper is to measure N2 and O2 densities and characteristic temperatures thanks to spontaneous Raman scattering within the boundary layer of a stainless steel flat plate cooled down at 300 K. This shear-flow test configuration is considered as a nonequilibrium air plasma test case. Vibrational and rotational temperatures are determined by comparing experimental spectra with computed ones. The density calculation is performed using the ratio of first vibrational transition intensities for both cases with and without plasma at 38 hPa. Several sections were investigated between 15 and 40 mm from the leading edge. All these sections exhibit a classical boundary layer pattern. The rotational temperature is completely in equilibrium with the plate and reaches 2500 K at the outer edge of the boundary layer. On the contrary, the vibrational temperature drops to 1500 K near of the plate and is about 5000 K in the freestream. Molecular densities are smaller than expected at equilibrium, about 60% of the equilibrium value in the freestream for N2.
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52.40.Hf Plasma-material interactions; boundary layer effects
52.70.Kz Optical (ultraviolet, visible, infrared) measurements
33.20.Fb Raman and Rayleigh spectra (including optical scattering)

Hard x-ray source for flash radiography based on a 2.5 kJ plasma focus

F. Di Lorenzo, V. Raspa, P. Knoblauch, A. Lazarte, C. Moreno, and A. Clausse

J. Appl. Phys. 102, 033304 (2007); http://dx.doi.org/10.1063/1.2767829 (5 pages) | Cited 7 times

Online Publication Date: 14 August 2007

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A compact tabletop plasma focus suitable for hard x-ray imaging applications is presented. The hard x-ray emission was characterized by means of an effective energy analysis, based on images obtained with a single shot. The effective energy of the radiation was estimated to be around 83 keV from radiographic images of metallic pieces. Numerical calculations, based on validated codes, are presented to give a quantitative interpretation of the experimental results. Experimental demonstration of the suitability of this device for introspective imaging of metallic pieces is also given.
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52.59.Px Hard X-ray sources
81.70.Ex Nondestructive testing: electromagnetic testing, eddy-current testing
52.58.Lq Z-pinches, plasma focus, and other pinch devices

Effect of preionization, fluorine concentration, and current density on the discharge uniformity in F2 excimer laser gas mixtures

D. Mathew, H. M. J. Bastiaens, K. J. Boller, and P. J. M. Peters

J. Appl. Phys. 102, 033305 (2007); http://dx.doi.org/10.1063/1.2767869 (11 pages) | Cited 4 times

Online Publication Date: 15 August 2007

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The discharge homogeneity in F2-based excimer laser gas mixtures and its dependence on various key parameters, such as the degree of preionization, preionization delay time, F2 concentration and current density, is investigated in a small x-ray preionized discharge chamber. The spatial and temporal evolution of the discharges is monitored by taking photographs of the discharge fluorescence with a fast intensified CCD camera. It is found that a preionization electron density of about 107 cm−3 bar−1 is sufficient to initiate a streamer-free homogeneous discharge in gas mixtures of helium and fluorine with multiatmospheric gas pressure. The accompanying optimum time delay between the application of the x-ray pulse and voltage across the discharge electrodes is determined to be about 20 ns. It is shown that in spite of these optimum initial conditions, a homogeneous glow discharge eventually transforms into an inhomogeneous discharge containing numerous filaments. Our experiments show that the higher the initial F2 concentration, the initial current density or the pump power density, the shorter the time interval over which the discharge stays homogeneous. By a quantitative characterization and defining a detailed measure of the observed discharge inhomogeneity we find that halogen depletion, as suggested from the theory, is responsible for the temporal instability of discharges in such laser gas mixtures, as the experimental results are in good agreement with the theory on the halogen depletion instability mechanism.
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51.70.+f Optical and dielectric properties
42.55.Lt Gas lasers including excimer and metal-vapor lasers
52.80.-s Electric discharges
51.50.+v Electrical properties (ionization, breakdown, electron and ion mobility, etc.)
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