Browsing by Author "Mitchell, Ian H."

Now showing 1 - 14 of 14
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    Comparative studies of ion emission characteristics in a low energy Plasma Focus operating with different gas fillings
    (IEEE, 2004) Bhuyan, Heman; Favre Domínguez, Mario; Kauffmann, Karla; Chuaqui Kettlun, Hernán; Mitchell, Ian H.; Wyndham Hodder, Edmund Sydenham
    The results of an investigation of ion emission from a low energy Plasma Focus (PF), operating at 20 kV, with 1.8 kJ stored energy, using several different gas fillings, including hydrogen, nitrogen, argon and methane, are presented. 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 focus plasma. Characteristic duration of the ion beam pulses is found to be in the 10s to 100 s of nanoseconds. Characteristic ion energy is found to be in the 50 to 500 keV range, depending on the operating gas. Ion beam energy correlations for operation in methane indicate that the dominant charge states in carbon ions are C+4 and C+5.
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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 investigation of ionization growth in the pre-breakdown phase of fast pulsed capillary discharges
    (2003) Favre Domínguez, Mario; Chuaqui, Hernán; Mitchell, Ian H.; Wyndham, Edmund
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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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    Investigation of the Plasma Jet Formation in X-Pinch Plasmas Using Laser Interferometry
    (2000) Mitchell, Ian H.; Chuaqui, Hernán; Favre Domínguez, Mario; Wyndham, Edmund
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    Laser initiated hollow gas-embedded Z-pinch
    (IEEE, 2004) Chuaqui Kettlun, Hernán; Aliaga Rossel, Raul; Veloso Espinosa, Felipe; Pavez Morales, Cristian Arturo; Favre Domínguez, Mario; Mitchell, Ian H.; Wyndham Hodder, Edmund Sydenham
    Results of new optical method of generating a precursor plasma for a shell gas embedded Z-pinch are presented. Experiments were performed on a pulse power generator using a peak current up to 150 kA with a rise time of 70 ns, 120 ns pulse length. The optical precursor plasma was generated by using a Nd-Y AG laser, 200 ml, 8 ns at 1.06 μm. Two different optical schemes were used, one consists of a combination of lenses capable of producing a hollow beam; while the other uses an axicon to generate the hollow beam. In both cases the hollow beam was focused at the cathode surface where metallic ring plasma, of either 2 or 6 mm diameter, is created. The annular preionization is created immediately before or during the first 30 ns after the line voltage is applied. The discharge was carried out in a chamber filled with hydrogen gas at 1/3 atm. Flat electrodes were used with 10 mm separation. The anode has a 6 mm diameter central hole to allow the passage of the preionizing laser. Optical diagnostics (schlieren, shadowgraphy and interferometry) were performed using the second harmonics of the same laser used to preionize. Preionizing using an axicon results in better formed hollow discharges.
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    Observations of Ablation Dynamics in Wire-Array Z-Pinch Experiments on the Llampüdkeñ Generator
    (2012) Veloso Espinosa, Felipe; Suzuki Vidal, Francisco Andres; Molina, Francisco; Mitchell, Ian H.; Chuaqui Kettlun, Hernán; Favre Dominguez, Mario Benjamin; Wyndham Hodder, Edmund Sydenham
    Wire-array Z-pinch experiments have been performed on the Llampüdkeñ generator (~470 kA in ~410 ns) [Chuaqui et al., Laser Part. Beams 15, 241 (1997)] using an overmassed load of aluminum wires. Time-resolved optical laser schlieren and extreme ultraviolet imaging were used in order to study the plasma ablation dynamics. The results show the formation and subsequent ablation of plasma from the wires, followed by the appearance of a precursor plasma column on the axis. According to these results and to the rocket model of wire ablation, an ablation velocity of (1.7-2.6) × 105 m/s is estimated, which is larger than those obtained in faster generators. Additionally, the main resemblances and differences with previous experiments are presented, allowing in this way to open the study of this configuration in smaller scale machines.
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    Plasma Jet Emission in Fast-Pulsed Capillary Discharges
    (2005) Caballero, L.; Chuaqui, Hernán; Favre Domínguez, Mario; Mitchell, Ian H.; Wyndham, Edmund
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    Simple holographic polarogram
    (2008) Inestrosa Izurieta, María José; Chuaqui, Hernán; Aliaga, R. Raúl; Favre Domínguez, Mario; Mitchell, Ian H.; Wyndham, Edmund
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    Transient hollow cathode effects and Z-pinch formation in a high current capillary discharge with a metal plasma
    (IEEE, 2004) Wyndham, Edmund; Favre Domínguez, Mario; Aliaga R., Raúl; Chuaqui, Hernán; Mitchell, Ian H.
    The results from a series of experiments are presented whose purpose is to explore different schemes which may lead to the formation of pure metal plasmas in a capillary discharge with parameters appropriate for X ray lasing. The experiments were performed in ceramic wall capillary discharges at currents of up to 120 kA, with an available ID of between 3 and 8 mm and with lengths from 60 to 100 mm. Initial plasma conditions in the capillary exploit transient hollow cathode effects in a preionizing discharge. A laser focused onto the back surface of the cathode initiates both beam activity in the capillary volume and plasma injection. To promote metal ablation into the pinch channel of elements other than the ceramic wall material, a number of graded ring schemes have been tried. The plasma is observed axially using both time and energy resolved soft X-ray pin hole images as well as from time resolved soft X-ray spectra. By varying the rate of rise of the current of the main discharge, and the preionizing conditions the diameter and the stability of the Z-pinch column are seen to be affected. The ratio of the species from the ablated wall material to the plasma formed from the graded ring structure is found to depend both on the capillary dimensions as well as the other operating conditions.
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    X ray emission from X pinch experiments on the Llampudken Generator
    (IEEE, 2002) Mitchell, Ian H.; Aliaga Rossel, Raul; Gómez Luna, Jorge Antonio; Chuaqui Kettlun, Hernán; Favre Domínguez, Mario; Wyndham Hodder, Edmund Sydenham
    The results from the first plasma physics experiments on the Llampüdkeñ Generator (1MA, 250 ns) are presented. X Pinch experiments have been undertaken at current levels of 400 kA with a rise time of ∼250 ns. X pinches were produced mainly from aluminium wires of different diameters and with varying numbers of wires. Results from X-ray diagnostics characterising the emitted radiation are presented. The diagnostics include filtered PIN diodes and a pinhole and slit-wire camera. Radiation of energy greater then 2.5 keV was emitted from hot spots in timescales of a few nanoseconds. Using the results from the slit-wire camera, the diameter of the hot spots is shown to be less than 5 µm.
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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.
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    X-Ray Emission From 125 Mu M Diameter Aluminium Wire X-Pinches At Currents of 400 Ka
    (2005) Mitchell, Ian H.; Chuaqui, Hernán; Favre Domínguez, Mario; Wyndham, Edmund