R. Aliaga-Rossel

The dynamics of wire array Z-pinch implosions

Wire array Z-pinch dynamics are studied in experiments with 16-mm diameter arrays of between 8 and 64, 15-μm diameter aluminum wires, imploded in 200–260 ns by a 1.4-MA current pulse. Side-on laser probing shows early development of noncorrelated m=0-like instabilities with an axial wavelength ∼0.5 mm in individual wires. End-on interferometry (r-θ plane) shows azimuthal merging of the plasma with a density of 1017 cm−3 in 90–65 ns for 8–64 wires, respectively. At the same time low-density plasma reaches the array axis and forms a precursor pinch by 120–140 ns. At 0.7–0.85 of the implosion time a global m=0 instability with a wavelength of 1.7–2.3 mm was detected in soft x-ray gated images, laser probing, and optical streaks. The time when the instability reaches the observable level corresponds to the number of e-foldings for the growth of the classical Rayleigh–Taylor instability of ∫γ dt∼5.6–7. The scaling of this number with the number of wires is consistent with the instability growth from the seed l...

Experimental observations of the spatial anisotropy of the neutron emission in a medium energy plasma focus

The results of experiments carried out on the DPF-78, a 60 kV, 28 kT plasma focus device are presented. The primary objective of these experiments was to investigate the spatial anisotropy of the neutron emission and to correlate the total neutron yield with hard X-ray emission and the presence of hot spots. To influence the plasma parameters, gas fillings of deuterium with doping of neon, argon, and krypton were used. An admixture of gases with an equivalent mass density of 5 mbar of D/sub 2/ was employed throughout the experiment. A novel technique for neutron detection was used, which allowed the recording of neutron signals at close distance to the focus (65 cm), with minimal dispersion of the signal due to time of flight and yet high X-ray rejection. A set of these detectors was placed at different angular positions to investigate the polar distribution in the neutron emission. It was identified for the first time that in a medium energy plasma focus device, the neutron emission is composed of two periods, similar to that reported in high energy devices. It was found that the first period occurs immediately before the maximum compression and lasts less than 50 ns while the second period lasts between 150 and 200 ns. It was found that the polar distribution of the neutron emission shows a strong anisotropy during the second period and depends on the Z of the doping gas. No correlation was found between the total neutron yield (about 10/sup 10/) and the hard X-ray emission or appearance of hot spots.

Investigation of the plasma jet formation in X-pinch plasmas using laser interferometry

A two-frame Mach–Zender interferometer is used to investigate the dynamics of X pinches formed from two 10 μm aluminum wires at current levels of 100 kA. Particularly, the columns of plasma that form on the interelectrode axis of the X pinch are studied quantitatively. It is demonstrated that the plasma which forms these columns does not come solely from expansion of the corona from the limbs of the X pinch but rather predominantly from the crossing point region. The results suggest that the plasma column is indeed a jet which consists of several components.

Time-resolved energy measurement of electron beams in fiber Z-pinch discharges

The emission of hard x rays with energies much greater than the applied anode-cathode voltage is a common feature of Z-pinch plasmas. Here time-resolved measurements of such emission from fiber Z pinches at the mega-ampere current level are reported. The x-ray spectrum measured by an array of detectors is used to calculate the energy of the electron beam producing the emission. Pulses of between 20 and 100 ns duration were observed and electron-beam energy measurements of around 2 MeV obtained. It is thought that these high-energy beams are generated by the plasma becoming resistive at the time of x-ray emission.

Optical probing of fiber z-pinch plasmas

An experimental study of optical probing of a dense z-pinch plasma using the MAGPIE (mega-ampere generator for plasma implosion experiments) generator [I. H. Mitchell et al., Rev. Sci. Instrum. 67, 1533 (1996)] is reported. The generator was operated with a peak current of 1.1 MA rising in 150 ns (10%–90%). The loads were 33 μm diam carbon fibers. Faraday rotation was used to investigate the distribution of the current flowing in the plasma. A measurable Faraday rotation angle was observed only in a time window from 50 to 60 ns after the current start, due to the fact that this effect depends on a combination of the magnetic-field strength and electron number density. A new type of self-referencing cyclic radial shear interferometer was used to evaluate the plasma density profiles which are necessary for the reconstruction of the current distribution. It was calculated that ∼110 kA was flowing in the plasma at 52 ns after the current start. Shadowgraphy was used to study the dynamics of the plasma and to ...

Coronal plasma behavior of the Z pinch produced from carbon and cryogenic deuterium fibers

A series of fiber pinch experiments has been carried out on the MAGPIE (mega-ampere generator for plasma implosion experiments) generator (1.8 MA, 150 ns) [Mithell et al., Rev. Sci. Instrum. 67, 1533 (1996)] to study the temporal evolution of the coronal plasma. Analysis of schlieren photographs, axial streak images and gated x-ray photographs gives the radial and axial motion of the coronal plasma. The influence of a current pulse (prepulse) of 30 kA applied 200 ns before the main discharge was also studied. Radial expansion velocities of 5.5×106 cm/s for carbon fiber shots without prepulse and 3.6×106 cm/s for carbon fibers with prepulse were measured. Axial wavelengths (λz) of dominant instabilities in the corona were between 0.05 and 0.2 cm corresponding to ka∼10–20. Comparison of the results obtained with carbon fibers with and without current prepulse and cryogenic deuterium fibers are presented.

The dynamics of bifurcating bright-spots in fiber Z-pinch plasmas

Results are presented from the diagnosis of the optical and x-ray emission from “bright-spots” in carbon fiber Z-pinch experiments using the MAGPIE (Mega-Ampere Generator for Plasma Implosion Experiments) generator [I. H. Mitchell et al., Rev. Sci. Instrum. 67, 1533 (1996)]. Inhomogeneities evolve very rapidly within the plasma with bright-spots becoming detectable after 15–20 ns. After a short (∼4 ns) duration formation phase, these bright-spots exhibit highly dynamic behavior. Bifurcation of the bright-spots is observed giving rise to rapid axial motion at 1–3×105 ms−1. The post-bifurcation bright-spots persist for up to 40 ns. Analysis of cross-filtered, time integrated, x-ray pinhole images yield bright-spot parameters during the formation phase (diameter ∼80 μm, temperature 250–300 eV, ion number densities ∼2×1026 m−3). With a spatial resolution of 175 μm, the strong temperature and density gradients within the post-bifurcation spots can be resolved in gated x-ray images with 2 ns exposure times. Aft...

Investigation of electron and ion beams in mega-ampere fiber pinch plasmas

The latter stages of fiber Z-pinch discharges at current levels up to 1.4 MA with a rise time of 150 ns have been investigated on the MAGPIE generator. Carbon fibers with a diameter of 33 /spl mu/m and deuterated polyethylene (CD/sub 2/) fibers with diameters between 50 and 200 /spl mu/m were used as loads. The detection of hard X-rays and neutrons provide evidence for MeV electrons and ions. The hard X-rays occur around 120-200 ns into the discharge and typically last for between 20 and 100 ns. Beam target neutrons have also been detected at this time with neutron energies of up to 5.2 MeV. Optical and X-ray diagnostics indicate that the dynamic hot spot phase of the pinch is over by this time and that no rapid pinching or expansion of dense plasma is occurring. The pinch is seen to be composed of high density regions along the axis interspersed by tenuous regions where the density is at least two orders of magnitude lower. A conceptual model which is consistent with the experimental data is proposed to explain the energy and duration of the X-ray and neutron pulses.

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.

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.