Plasma dynamics of two-wire Z-pinches

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Title: Plasma dynamics of two-wire Z-pinches
Authors: Ruiz-Camacho, Jose G.
Item Type: Thesis or dissertation
Abstract: This thesis is concerned with a series of experiments on wire z-pinches to study the merging of the plasma from two parallel wires and its dynamics as a function of wire separation and material. The pulsed power generator (IMP) is capable of providing a maximum peak current of 160kA with a rise time of 65ns. Two parallel IS^im aluminium and 7.5|am tungsten wires were used as load. The separation between electrodes was 20mm. Three different separations between wires were used (0.3, 0.6 and 1.5mm) to study mainly: (i) the plasma expansion, (ii) dynamic of instabilities, (iii) formation of the central plasma column and (iv) x-ray emission. The plasma was optically probed by schlieren photography in two orthogonal directions: face-on and side-on (perpendicular and parallel to two-wire plane respectively). A 7 ns (FWHD) pulse of Ruby laser (694.3//m) was used with a 520//m circular stop, the angle detection limits for this probing were from 1.3 to 45.0 mrad. It allowed detecting electron densities in the range of ^3.0x10'®-1.8x10^° j [cm"^]. In the optical emission regime, a four-frame optical camera, with an exposure time of 1 ns (inter-frame time of 15 ns and 5 ns), and an optical streak camera in radial mode (perpendicular to z-pinch axis) were used. The x-ray emission was detected using a time-integrated pinhole camera and a couple of filtered PIN diodes. It was observed that two separated plasmas were formed, localised at: (1) surrounding each of the two wires and (2) a central plasma column at midway between the wires, which is wire separation dependent. Magnetohydrodynamic (MHD) instabilities appear in both plasmas. For aluminium wires, the localised coronal plasma around wires is m = 0 unstable. The expansion of the coronal plasma is asymmetric, being faster in the interwire region compared with the expansion in the external region. Spatial correlation along the wire axis of coronal plasma instabilities was observed for both wires, which is associated with the existence of a global magnetic field surrounding the wires. The central plasma column, confined in the plane between wires, shows a m = 1 -MHD instability. For tungsten wires, the coronal instabilities were spatially correlated later compared with aluminium. An important feature of 1.5mm separation tungsten discharges was a series of jets along Z-axis direction streaming towards the centre. These jets were observed at 70ns after current start. In addition, experiments on single-wire z-pinches are reported. Tungsten wires of various diameters (5, 7.5, 10, 13 andl8//m) were used and results are compared with 15//m aluminium wire. The expansion of the pinch is studied as a function of wire diameter and material. Schlieren observations show that the coronal plasma of tungsten pinches is independent of wire size, it expanding with a mean velocity of (9.4±1.0)xl0'm/s. The aluminium pinch expands with a velocity of(2.4±0.2)xl0'' m/s, which is in a factor of 2.5 faster than tungsten one. The m = 0 - MHD instability appear at about 8 ns for aluminium compared with 20 ns for the tungsten pinch. The instability wavelength and pinch diameter increases with time for both types of wires, and relatively faster for the aluminium pinch. A 2D(r,z) resistive MHD code was used with the addition of a modified Thomas-Fermi equation of state and dense plasma transport model to allow the phase transitions from solid to plasma. The coronal and core expansions from the code can be compared directly to experimental results obtained by schlieren photography. The simulation and experimental results are compared showing a rapid expansion of the coronal plasma during the first 17ns. The initial coronal expansion and then contraction is evident in the simulation. The delay with reference to current start, of the core expansion and instability development often referred as the "dwell time", is reproduced in simulation and is associated with the self-pinching time of the corona.
Content Version: Open access
Date Awarded: 2002
Supervisor: Haines, Professor Malcolm G.
Sponsor/Funder: CONICIT (Venezuelan research council); CENAMEC Foundation (Venezuela).
Department: Department of Physics
Publisher: Imperial College London
Qualification Level: Doctoral
Qualification Name: Doctor of Philosophy (PhD)
Author Permission: Permission granted
Appears in Collections:University of London awarded theses - Imperial authors

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