Browsing by Author "Choi, P"

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    Development of a short pulsed corona discharge ionization source for ion mobility spectrometry
    (2005) An, YA; Aliaga-Rossel, R; Choi, P; Gilles, JP
    The development of a pulsed corona discharge ionization source and its use in ion mobility spectrometry (IMS) is presented. In a point-plane electrode geometry, an electrical pulse up to 12 kV, 150 ns rise time and 500 ns pulse width was used to generate a corona discharge in air. A single positive high voltage pulse was able to generate about 1.6x10(10) ions at energy consumption of 22 mu J. Since the temporal distribution of ions is in a pulsed form, the possibility of removal the ion gate has been investigated. By purposely arranging the interface between discharge field and drift field, nearly 10(7) positive ions were drawn into the drift region with absence of the ion gate after every single discharge. The positive spectrum of acetone dimer (working at room temperature) was obtained with a resolving power of 20 by using this configuration. The advantages of this new scheme are the low power consumption compared with the dc method as well as the simplicity of the IMS cell structure. (c) 2005 American Institute of Physics.
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    Formation and dynamics of a Z pinch in a high current capillary discharge in initial vacuum
    (AMER INST PHYSICS, 2003) Wyndham, E; Favre, M; Aliaga Rossel, R; Chuaqui, H; Mitchell, I; Choi, P
    Experimental conditions are presented for the formation of a Z pinch from ablated wall material in a high current, medium diameter capillary discharge without an initial filling gas. A hollow cathode geometry is used and electron beams, initiated by a laser spark, form a transient metallic plasma in the hollow cathode volume. The laser spark plasma initiates the preionization phase, whose conditions are essential for generating a stable Z pinch during the main conduction phase. During the preionizing phase wall plasma is ablated. The main discharge is derived from a small pulsed power generator, whose rate of rise of current may be varied, at current of 120 kA for 120 ns. The capillary has an internal diameter of 5 mm and length of 6 cm. From time, spatial and energy resolution of the axial soft x-ray emission, the optimum conditions of plasma formation are found to depend on the preionizing electron beams, current and the rate of rise of main discharge current applied. The temperature is obtained from soft x-ray spectra, and attains a maximum value of 80 eV. The relevance of these results in soft x-ray lasing capillary discharges is discussed. (C) 2003 American Institute of Physics.
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    Ionization waves in electron-beam-assisted, shielded capillary discharge
    (2000) Rutkevich, I; Mond, M; Kaufman, Y; Choi, P; Favre, M
    A theory of propagation of cathode-directed ionization waves during the early stages of an electrical breakdown in a shielded, low-pressure capillary is developed. The discharge process occurs due to the ionization of the low-density gas in the capillary by an electron beam that is emanating from a hollow cathode. Due to the strong electric field in the capillary the electrons are in the fast acceleration regime. Consequently, the full momentum equation for the electrons is employed, rather than the electron drift velocity approach. The smallness of the ratio of the capillary radius to the characteristic length of the electric potential variation in the axial direction allows the construction of a quasi-one-dimensional model. The latter retains the important two-dimensional nature of the electron flow as well as the electrodynamic boundary conditions at the capillary wall and the conducting shield and results in a set of one-dimensional, time-dependent partial differential equations for the on-axis distributions of the physical quantities. It is shown that those equations admit self-similar solutions that represent ionization waves propagating with constant velocities. The resulting set of ordinary differential equations is solved numerically for various initial conditions representing a nonperturbed steady state ahead of the ionization front and the resulting features of the: ionization waves are investigated and discussed. The obtained solutions describe both ionization growth and virtual anode propagation and represent fast ionization waves in plasma waveguides, for which the maximum value of the mean electron velocity is much higher than the wave velocity. The space-charge distribution associated with the ionization waves is found in the form of plasma oscillations with a continuously increasing frequency and a solitary envelope. The calculated wave velocity increases with the gas pressure and this tendency is in agreement with corresponding experimental observations.