Browsing by Author "Choi, Peter"

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    Discharge formation in fast pulsed capillary discharges
    (IEEE, 2002) Favre Domínguez, Mario; Choi, Peter; Leñero Marchant, Ana María; Castillo, Francisco; Suzuki Vidal, Francisco Andrés; Chuaqui Kettlun, Hernán; Mitchell, Ian H.; Wyndham Hodder, Edmund Sydenham
    We present a study of discharge formation in a fast pulsed capillary discharge operated in Argon or Methane, in a 50 mm long, 1.6 mm internal diameter capillary, at −15 kV applied voltage. A pressure gradient is used along the capillary, with pressures between 0.2 and 1.0 Torr in the cathode region and a pressure one tenth lower in the anode side. The diagnostics include a capacitive probe array, Faraday cup and beam-target scintillator-photomultiplier detectors. It is found that following the emission of electron beams from the hollow cathode region, a fast ionization wave propagates from the cathode towards the anode, with characteristic velocities of the order of 106 to 107 m/s. The propagation of the ionization front is assisted by the electron beams, which reach a peak current of around 200 mA.
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    Experimental observation in a high current capillary discharge
    (IEEE, 2002) Wyndham, Edmund; Chuaqui, Hernán; Favre Domínguez, Mario; Mitchell, Ian H.; Aliaga R., Raúl; Choi, Peter
    Experimental observations of the soft X-ray emission from a 5 mm diameter, 60 mm length capillary discharge in an initial vacuum show the effectiveness of transient hollow cathode mechanisms in establishing the initial plasma conditions for a 120 kA Z-pinch. Hollow cathode geometry is used. A laser spark behind the cathode orifice initiates an axial electron beam when an external bias provides a current-limited preionizing current in the capillary. These beams intensify during the first few ns of the main discharge. The Z-pinch dynamics are measured using time-resolved soft X-ray spectroscopy and filtered pinhole photography. These are found to vary according to the rate of rise of the current, 1.5 and 3.0 × 1012 A/s, the polarity and level of the prionizing current. A stable pinch is formed with a preionizing current of ∼50A as an e-beam from the hollow cathode and with the higher value of di/dt. With higher values of preionizing current the e-beams show diode rather than hollow cathode origin and the stability of the following Z-pinch deteriorates notably. The pinched plasma is predominantly of wall material at ∼100 eV.
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    X-ray and plasma dynamics of an intermediate size capillary discharge
    (2002) Wyndham Hodder, Edmund Sydenham; Aliaga Rossel, Raul; Chuaqui Kettlun, Hernán; Favre Dominguez, Mario Benjamin; Mitchell, Ian H.; Choi, Peter
    A small pulsed power generator, 150 kA and 120 ns, is used to form a plasma in a 5-mm diameter alumina ceramic tube. A hollow cathode geometry is used and a preionized plasma is formed in an initial vacuum background by focussing a pulsed Nd:YAG laser onto a metallic target in the hollow cathode volume. The evolution of the preionizing plasma and its expansion into the main discharge volume may be assisted by applying a current of order Amps for a variable time before the main discharge current is applied. Strong electron beams are observed both during the preionizing stage and during the start of the main current. The plasma species and temporal evolution during the main discharge is observed using X-ray spectroscopy and X-ray pinhole imaging. On varying the rate of rise of the current in the pinching phase, the transient hollow cathode effect was found to be significant at early times in the discharge in the case of the lower value of dI/dt. Both the pinch temperature and diameter depend on varying the dI/dt from 1.5 to 3 /spl times/ 10/sup 12/ A/s. The implications of plasma injection for metal vapor capillary discharges are discussed.