Browsing by Author "Favre, M"

Now showing 1 - 5 of 5
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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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    Further statistical studies of ionization growth and breakdown formation mechanisms in the final breakdown phase of a transient hollow cathode discharge
    (2002) Moreno, J; Zambra, M; Favre, M
    A Transient Hollow Cathode Discharge (THCD) is a high-voltage low-pressure discharge, which is characterized by an axial hollow in the cathode electrode. The temporal sequence of the different parts of ionization growth in breakdown formation, which take place just before electric breakdown, are statistically studied. The von Lane formalism has been used to characterize in detail the statistical time distribution of the different processes required for electric breakdown to occur. The experiments have been performed in Hydrogen at pressure between 13.3 and 53.2 Pa, with different sizes of the cathode aperture. It has been found that the different processes of ionization growth and breakdown formation mechanisms involved in the sequence leading to breakdown are not associated with a single characteristic time. Time-shifted Gaussian distribution functions have been Identified which, when added together, reproduce the cumulative time distribution for each observed event, especially at low pressure and smaller cathode aperture, where the hollow cathode effect is less effective.
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    Ion beam emission in a low energy plasma focus device operating with methane
    (2005) Bhuyan, H; Chuaqui, H; Favre, M; Mitchell, I; Wyndham, E
    An investigation of ion beam emission from a low energy plasma focus (PF) device operating with methane is reported. Graphite collectors, operating in the bias ion collector mode, are used to estimate the energy spectrum and ion flux along the PF axis, using the time-of-flight technique. The ion beam signals are time correlated with the emission of soft x-ray pulses from the pinched focus plasma. The correlation of ion beam intensity with filling gas pressure indicates that the beam emission is maximized at the optimum pressure for focus formation at peak current. Ion beam energy correlations for operation in methane indicate that the dominant charge states in carbon ions are C+4 and C+5. The estimated maximum ion energy for H+, C+4 and C+5 are in the range of 200-400 keV, 400-600 keV and 900-1100 keV, respectively, whereas their densities are maximum for the energy range 60-100 keV, 150-250 keV and 350-450 keV, respectively. These results suggest that the ion beams are emitted from a high density, high temperature, short lived focus plasma, at a time which appears to precede the emission of soft x-ray pulses. The properties of the carbon ion beams are discussed in the context of potential applications in materials science.
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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.
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    LLAMPUDKEN: A high-current, low-impedance pulser employing an auxiliary exponential transmission line
    (1997) Chuaqui, H; Wyndham, E; Friedli, C; Favre, M
    The design and constructional aspects of a novel pulse power generator for use in dense plasma research presently under construction are presented. The generator consists of two Marx capacitor banks, each of 0.25 mu F, 480 kV, and 28.8 kJ. Each Marx generator drives a water transmission line, in which the live electrode is the central conductor. The transmission lines consist of a constant impedance section followed by a multielectrode gas line gap followed by an exponential taper to the load section. The novel feature is the use of an auxiliary exponential line coupled at the load. This line controls both the voltage and the effective impedance at the load section. In addition, by leaving this line circuit open, energy not coupled to the plasma in the initial high-impedance phase may be reflected back and deposited into the discharge, increasing the peak current by 50%. Circuit simulations using a real-time-varying load impedance show that the current pulse rises in an approximately linear way to a maximum of 1.2 MA at 250 ns. The current falls to zero in the following 250 ns. The current waveform may be flattened simply by disconnecting the auxiliary line, giving a rectangular pulse of 350 ns with a maximum value of 950 kA. The overall impedance of the entire system may be adjusted by varying the separation between the conductors. The equivalent source impedance at the load is 0.8 Omega. This low value is by virtue of the auxiliary line, which limits the voltage at the load section and reduces the insulator constraints. We present simulations of the generator under real load conditions. The model also is checked against analytical solutions of exponential line behavior and against other published models of pulse power generators.