Ian H. Mitchell

Ion beam emission in a low energy plasma focus device operating with methane

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.

Experimental investigation of ionization growth in the pre-breakdown phase of fast pulsed capillary discharges

We have investigated the pre-breakdown ionization processes in a pulsed capillary discharge using a capacitive probe array to measure ionization growth with time and space resolution. The experimental results indicate that pre-breakdown processes in shielded capillary discharges are characterized by the formation of a fast ionization wave. Depending on voltage polarity, the ionization wave can be associated with a mobile virtual anode with characteristic speed 105xa0mxa0s−1, in the case of positive polarity, or with the propagation of a high speed potential wave, of characteristic speed 107xa0mxa0s−1, in negative polarity case. The time and space evolution of the ionization waves is closely related with the formation of high energy electron beams, which originate due to the hollow cathode geometry of the open end capillary. A qualitative model based on the hollow cathode effect is proposed to explain the initial formation and later time evolution of the observed ionization waves.

Formation of hexagonal silicon carbide by high energy ion beam irradiation on Si (1 0 0) substrate

We report the investigation of high energy ion beam irradiation on Si (1 0 0) substrates at room temperature using a low energy plasma focus (PF) device operating in methane gas. The unexposed and ion exposed substrates were characterized by x-ray diffraction, scanning electron microscopy (SEM), photothermal beam deflection, energy-dispersive x-ray analysis and atomic force microscopy (AFM) and the results are reported. The interaction of the pulsed PF ion beams, with characteristic energy in the 60–450 keV range, with the Si surface, results in the formation of a surface layer of hexagonal silicon carbide. The SEM and AFM analyses indicate clear step bunching on the silicon carbide surface with an average step height of 50 nm and a terrace width of 800 nm.

X-ray emission from 125 µm diameter aluminium wire x-pinches at currents of 400 kA

Results obtained from aluminium wire x-pinch experiments at a current level of ~400 kA, 260 ns risetime, are presented. The x-pinches were made from two 125 µm diameter wires. The x-pinches typically emitted 15 J of K-shell x-rays in nanosecond duration pulses from hot spots of diameters of ~10 µm or less. Frequently several hot spots were formed in a single discharge. Spectroscopic measurements estimate an electron temperature of about 600 eV. Spatial resolution of typically 10 µm was obtained in radiographic images. Details of the dynamics of the pinch were obtained from time resolved soft x-ray frames, showing formation of the plasma jet due to the coalescence of the expanding corona plasmas from the x-pinch limbs and ejection of plasma in the direction perpendicular to the x-pinch axis when hot spots were formed.

X ray emission from X pinch experiments on the Llampüdkeñ Generator

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.

X-ray and plasma dynamics of an intermediate size capillary discharge

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.

Observations of plasma dynamics in a gas-embedded compressional Z-pinch

A series of experiments carried out in a gas embedded compressional Z-pinch are presented. A dc micro discharge of 150 /spl mu/A between two conical sharp edged electrodes is established to produce a hollow cylindrical discharge. A few nanoseconds before the application of the main voltage, a pulsed laser is focused through the anode onto the cathode. With this preionization scheme an initial coaxial current structure is established. H/sub 2/ and D/sub 2/ at a pressure of 1/3 atm were used as a working gas. The experiments have been carried out using a pulse power generator capable of delivering current of up to I/spl sim/200 kA with a dI/dt>10/sup 12/ A/s. The use of H/sub 2/ and D/sub 2/ allows the study of discharges with the same electrical properties, but with different dynamics. At early times this preionization scheme produces a coaxial double column pinch, which as current rises, coalesces into a single column becoming a gas embedded compressional Z-pinch. Diagnostics used are current and voltage monitors, single frame holographic interferometry and shadowgraphy, visible streak camera, and single frame image converter camera. Electron density, line density, pinch radius, and plasma motion are obtained from the optical diagnostics. It was found that the maximum electron density achieved on axis is greater than twice the expected value according with the filling pressure used in the discharges, which contrasts with a traditional gas embedded pinch in which the density is lower than the expected value from filling pressure. The expansion rate of the plasma column is reduced to a third of the observed value for the single channel laser initiated gas embedded pinch. These measurements agree with the existence of a central current channel in this new configuration of gas embedded pinch. The experimental results clearly show that compression is achieved with the composite preionization scheme.

Laser initiated hollow gas-embedded Z-pinch

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.

Optical measurements of plasma dynamics in carbon fiber Z-pinches

A series of experiments has been carried out on the Mega Ampere Generator for Plasma Implosion Experiments (MAGPIE) generator in order to study the dynamics of carbon fiber Z-pinches. The generator was operated at 1.4 MV, with a peak current of 1 MA, and a rise time of 150 ns. In some shots, a current prepulse of about 30 kA was provided to study its influence on the dynamics of the fiber pinch. Carbon fibers of 7, 33, and 300 /spl mu/m diameter were used during these experiments. The diagnostics employed were a self-referencing interferometer, a two-frame Schlieren system, an optical streak camera, and a four-frame X-ray framing camera. A novel feature of these measurements is the employment of an optical streak camera with a set of four slits arranged along the fiber axis and displaced in the radial direction. This permitted the study of the temporal evolution (axial and radial) of the plasma regions emitting in the visible part of the spectra. Correlation between these regions of the plasma and the location of X-ray hot spots is discussed. In carbon fibers of 33 pm diameter, the radial expansion velocity measured from Schlieren images was 3.6/spl times/10/sup 6/ cm/s and 5.5/spl times/10/sup 6/ cm/s for shots with and without prepulse, respectively. The dominant axial wavelengths of instabilities in the coronal plasma were between 0.05 and 0.2 cm, which correspond to ka values between 10 and 20, where k is the wavenumber of the instability and a is its amplitude. The dynamics of carbon fibers of different diameters are compared.

Anisotropy of Ion Emission from a Low Energy Plasma Focus

We have investigated the ion flux, ion energy and anisotropy of carbon ion emission, at different angular positions, from a low energy Plasma Focus (PF) device, operating in methane, at 20 kV, with 1.8 kJ stored energy. A detector array is used to measure simultaneously the ion beams at five different angles with respect to the PF axis (0°, 10°, 15°, 20° and 90°), at a distance of 77 cm from the ion source. Ion beam energy correlations for operation in methane indicate that the dominant charge states on the detector are H+, C+4 and C+5. The correlation of ion beam intensity with filling gas pressure indicates that the beam emission maximizes at the optimum pressure for focus formation at peak current. Estimated ion fluxes are maximum for the energy range of 50 – 100 keV, 100 – 200 keV and 300 – 400 keV, respectively. Measurements of the angular distribution of ions reveal a strong anisotropy. It is observed that the flux of hydrogen ions is maximum near the axis of the PF whereas the flux of carbon ions i...