Recent experiments in a gas embedded compressional Z-pinch are presented. The experiments have been carried out in H 2 and D 2 , using a pulse power generator capable of delivering a dI/dt ≥ 10 12 A/s. The pinch is initiated by a focused laser pulse, which is coaxial with a cylindrical DC microdischarge. This configuration results in double column pinch at early times, which at current rise evolves into a gas embedded compressional Z-pinch. Diagnostics used are Rogowskii coil, single frame holographic interferometry and holographic shadowgraphy, visible streak camera images from which, current, density, line density, pinch radius and plasma motion are obtained. The pinch is characterised by a maximum on axis density which is much higher than the expected value from filling pressure, with a Bennett temperature of 75 eV at 180 kA.

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.

The authors study, experimentally, high power density matter (HPDM) of hot microplasmas collected in the interelectrodes space in a discharge at "commercial" vacuum, and the hard X-ray yields related. In particular, a Marx generator (under just 1 J stored energy) triggers a discharge through a 50 /spl Omega/ transmission cable in a hollow cathode geometry. The appropriate choice of anode and cathode configurations and their efficient combination with some gas fulfilment may provide the consequent manipulation both with cold solid density grains of anode material, and hot microplasmas originating from them after breakdown. Dynamic structures of hard X-ray from plasmas "dust" were registered using a sensitive imaging device with 5 ns exposure time. The numbers of X-ray spots registered beyond of electrodes were up to few hundreds per shot. The results for single grain (cluster) overheating and explosion accompanied by X-rays seem similar to recent data on high-intensity laser (10/sup 15-18/ W/cm/sup 2/) focusing on clusters. However, hard X-ray emission efficiency registered and well reproduced in these discharges (0.1-0.3%) seems essentially higher.

The present work deals with a comparative study in a compressional gas embedded Z-pinch in H-2 and in D-2 are presented. The use of H-2 and D-2 allows discharges with the same electrical properties, but different dynamics. Pressures of 1/3, 1/6 and 3/70 atm were used to carry out the experiments. The pinch is initiated by a focused laser pulse, which is coaxial with a cylindrical DC microdischarge. This configuration results in a double column pinch at early times, which as current rises, coalesces into a single column becoming a gas embedded compressional Z-pinch. The maximum electron density achieved on axis is greater than mice the expected value from the filling pressure, in contrast with a traditional gas embedded pinch. The expansion rate is reduced to a third of the observed value for the single channel laser initiated gas embedded pinch. This observation is consistent with a central current channel in the composite pinch. The experimental results, electron density at the centre and lower expansion rate, confirm the high degree of compression achievable with the composite preionization scheme.

A hollow cathode discharge using a tungsten pointed anode is fed by a low power Marx generator (1 J, 70 kV, 50 ns). The x-ray emission duration varies from 20 to 10 ns in the pressure range mbar, in air. The spatial distribution of the emitting sites has been determined, using a sensitive imaging device with a 5 ns exposure time, in the same range of pressures. This emission is strongly influenced by the occurrence of the `pseudo-spark' regime. X-rays are emitted by different mechanisms: (a) electron bremsstrahlung and by Auger cascade associated with particle emission from the Teflon insulator and also tungsten ( and ) characteristic lines; (b) pinching and collapse of the hot plasma surrounding the anode and the insulator. X-ray spots are observed. Their number is about 200 per shot, their radius is less than 40 m and their emission intensity is compatible with that of a dense plasma: with a temperature in the range 100 eV to 1 keV. Some of them are seen to follow linear trajectories, suggesting a 1 ns period wave propagation at 300 km which triggers the collapse.

The possibility of attaining high temperature density plasma by passing a high current through a thin wire or fibre has long been recognized. In the paper experiment, another concept is prosed to produce the composite pinch configuration. Rather than the external injection of an additional plasma cylinder, an additional surface coating on the thin fibre is used. By the application of a controlled preheat current, this surface coating is then vaporised to form the additional plasma jacket over the fibre. Different coating methodologies were explored and the resulting plasma structure studied. (AIP)

A micro‐channel using corona effect at atmospheric pressure, either in hydrogen or in air, is created prior to the triggering of the main discharge. The channel diameter has been measured to have about 10 μm diameter, with a cw intensity of the order of 0.1 mA. The main discharge is powered by a Marx generator delivering a current pulse of 240 kA during 200 ns at the end of a water line having 1 Ω impedance. Four pictures of the pinch are taken at each shot delayed in time, each from the other, from 10 ns to 260 ns.The discharge is seen to remain stable against usual instabilities during more than 500 ns. When a m = 1 instability appears sometime, its relative amplitude seem to remain constant as a function of time, at a very low level.The expansion of the channel takes place at very high velocities. In air, the expansion is slowered at a time corresponding to the maximum intensity.The influence of the micro‐channel characteristics has been clearly checked: when it is not well‐established, the discharge becomes unstable very early, even with the use of the pointed electrodes.

A vacuum spark having a tungsten pointed anode and a hollow cathode is feeded by a small Marx generator (11). It produces energetic beams of tungsten ions (E= 0.5 to 70 keV) and electrons beams in the first ignition phase. The electrons produce. an X-ray emission through Bremsstrahlung in the Tenon insulator and by Auger cascade in relation with particle emission. In the second phase. when the applied electric field is low and the current intensity is high, the electrons give rise to W (Lα and Lβ) X-ray emission. The mechanism of this last part is ascribed to interactions either with the massive anode or a plasma surrounding the anode. In the case of dense plasma interaction. the emission is of the same type as observed in laser-plasma interaction. Spots of X-rays are also observed which may originate from pinching or interaction with drops or dust particles in the interelectrode spacing. Experimental results obtained using X-ray imaging are discussed.

Applicability of pulsed X-ray sources depends greatly of criteria like spot size, repetition rate, lifetime, simplicity of operation, wavelength adjustability. We present results on such points for two miniature sources operating in the nanosecond regime. The low energy may limit applications with respect to high sensitivity detectors required

Intense point-like X-ray sources are of interest for plasma diagnostics, especially density measurement at high values in Z-pinches. Thee device presented here is designed to deliver pulsed power above some GW during 50ns. It is based on a 8 stages Marx generator and a water dielectric pulse forming line. The load is a low inductance vacuum diode with a small emitting zone delivering X-ray bursts on the fundamental lines of the metal material.

In this paper we report observations on a gas‐embedded pinch in which the initial preionized channel is a stable, very narrow 10 micron diameter channel. Observations show no instability develops and that the electron density is 1021/cm3 at a temperature of 5 eV. We describe the preionization system and the experimental set−up including the plasma diagnostics used. (AIP)

Two distinct phases of X-ray emission in a small vacuum spark with a plasma trigger have been identified. The first phase of emission is associated with the trigger plasma and originates from beam-target X-rays issued from the pointed anode. The source of electrons at this period is from the trigger plasma. The second phase of emission is associated with the breakdown of the main gap. The source of X-rays is a combination of beam-target and beam-plasma X-rays. It is observed that the angular distribution is constant. The X-ray energies vary regularly with the applied voltage, and the triggered vacuum spark as a high brightness.< >

A small source, based on a hollow cathode vacuum discharge, produces nanosecond X-ray pulses, with a stored electrical energy less than 1 Joule. Previous measurements have shown that the emitting region is of small size (< 50 μm) and very reproducible in position (δ1 < 50 μm). This type of discharge creates a powerful beam of electrons coming from the hollow cathode plasma. Two different and successive emission mechanisms exist: a beam-target one from trigger plasma and a beam-plasma interaction during the breakdown phase. The spatial distribution is imaged using a highly sensitive camera.

A miniature X-ray source, based on a small vacuum discharge, is studied. It was designed to be of small size and low cost. The electrical characteristics are given. Evidence of pinches is shown even with kiloampere pulses. The size and the reproducibility of the source are estimated by radiography: the source diameter and its excursion are certainly smaller than 50 μm. The total energy scales as V...

We study the oscillations superimposed on an intense discharge feeding a dense Z pinch. The periods, measured in different conditions, allow the measurement of the plasma inductance. We show that the variation of this inductance is related to the current penetration in the plasma, the anomalous current distribution, and the superconductive behavior of a dense turbulent plasma.