The evolution of an array — the gradual ablation of wire cores into coronal plasma followed by an implosion that snowploughs this coronal plasma to the axis — has been the subject of much scrutiny. Here we present measurements of the formation and dynamics of the snowplough sheath in an array, observe how the snowplough stagnates on axis and investigate how this is related to X‐ray emission. Introducing a reversing in the direction of the radial electric field at the wires of an array changed the size of the wire cores produced during initiation of the array. With a smaller than normal core size, the rate of ablation at the wires was reduced, and the dependence of implosion and X‐ray production on ablation could be explored.

A new technique is presented for the formation of annular plasmas on a metal surface with a high-power laser using a combination of axicon and converging lenses. The annular plasma formed on a titanium target in a chamber of hydrogen gas was investigated using schlieren imaging and Mach Zehnder interferometry. Expansion of the plasma was shown to be anisotropic with velocities of ∼103–104m∕s. Electron densities of 1018cm−3 were measured with radial profiles that confirm the presence of a hollow structure. The interferometric observations also show the presence of an inward shock wave traveling to the center of the annular plasma, which compresses the background neutrals, reaching a density around 18 times initial gas density, at 95ns after the initial annular plasma is produced.

The formation of plasma in wire-array Z-pinch experiments was found to depend upon the polarity of the radial-electric field near the wires. Reversing the radial-electric field midway along the length of an array resulted in the ablation rate of one-half of the array being reduced by 50%, significantly delaying the start of its implosion and altering its acceleration towards the axis. The observed phenomena cannot be explained by the standard magnetohydrodynamic models of array behavior, suggesting that effects such as electron emission may be important, especially during wire initiation.

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.

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 x 10(12) A/s. The implications of plasma injection for metal vapor capillary discharges are discussed.

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.

Llampüidkeñ [1] is a pulsed power generator designed to deliver a 1 MA, 250 ns risetime current pulse into a dense plasma load. The main novel feature of this generator is the two auxiliary transmission lines which transmit the energy not absorbed by the load, reflect it at the open end of the line and deliver it to the load when the energy from the main lines is decreasing. With the auxiliary lines an increase of 30% on the current as well as a decrease of the voltage at the load is obtained. To date Llampüidkeñ has been operated up to the 400 kA level, into both short circuit and plasma loads. Details of actual performance of the pulse power generator are presented and compared with simulations.

Plasma formation in a x-pinch-formed from two 10 μm Al wires at a current of 100 kA- was studied by using a two frame Mach Zender interferometer. It was observed that the plasma column that forms on the axis of the x-pinch, does not come entirely from the coronal plasma from the limbs but rather from the plasma formed at the crossing point region. The results obtained suggest that the plasma column is in fact a jet which consist of two components. One component is due to the expansion of the coronal plasma from the crossing point and the other is due to ejection of material caused by the pinching of the plasma in that region.

A small pulsed power generator is used to drive a capillary discharge in a 5mm diameter alumina tube. A hollow cathode geometry is used and a metallic vapor formed by a laser focused onto a target within the hollow cathode volume. The ensuing discharge is studied using a set of X-ray diagnostics, and a submillimeter diameter pinch is seen to form on axis. Spectroscopic observations show the plasma is predominantly of the cathode material. The dynamics of the plasma are studied under different preionization schemes, and the rate of rise of the main current is found to be important. Transient Hollow Cathode effects are found assist the formation of a well formed and small diameter pinch.

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.

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 level determined by the averaging of uncorrelated density perturbations in individual wires. Preliminary results from a 4×4 array permit the simultaneous observation by laser probing of the characteristic bubble and spike structure of the magneto Rayleigh–Taylor instability.

The implosion of aluminum wire array z pinches driven by a 1.4 MA, 240 ns current pulse is studied. Plasma evolution is measured in the r-theta plane for the first time by an end-on laser interferometer. Merging of coronal plasmas with density of 10(17) cm(-3) occurs in 90-65 ns for 16 mm diam arrays with 8-64 wires. Early uncorrelated instabilities (wavelength lambda similar to 0.5 mm) are observed in individual wires with later development of a global m = 0 instability (lambda similar to 2 mm). The number of Rayleigh-Taylor instability e foldings is 5.6-7 when the m = 0 instability reaches an observable level and increases with the number of wires.

Summary form only given. A series of experiments have been carried out on MAGPIE (Mega Ampere Generator for Plasma Implosion Experiments) to study the plasma formation in wire array z-pinches. The generator was operated at peak currents up to 1.4 MA with a rise time of 150 ns. Arrays of 16 mm diameter composed of between 8 and 32 aluminium and tungsten wires were used. Wire diameters were between 4 and 10 ym for tungsten and between 15 and 25 /spl mu/m for aluminium. Arrays with equally spaced wires as well as with variable inter-wire separation were studied. To study the dynamics of the plasma, an extensive set of diagnostic techniques were used which include schlieren, interferometry, optical streak camera, 4 frames X-ray camera, time resolved X-ray crystal spectrometer, filtered PIN diodes and pinhole camera. Current and voltage probes located at the load area and in the transfer line were used to monitor the electrical parameters. A Nd-YAG laser doubled at 532 nm with a pulse length of 400 ps was used as a light source for schlieren photography and interferometry. Registration of laser probing images with high spatial resolution was done by CCD cameras. To obtain schlieren photographs of plasma evolution in the single discharge the laser pulse was split into four pulses separated by 20 ns.

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.

A series of experiments carried out in a gas embedded compressional Z-pinch are presented. A de micro discharge of 150 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-2 and D-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 similar to 200 kA with a dI/dt > 10(12) AVs. The use of H-2 and D-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.