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We present experimental results on the investigation of hotspot formation in PFP-I, a small 3.8 kJ plasma focus device operating in hydrogen-argon mixtures, at pressures from below 0.2 torr upward. A combination of multipinhole and slit-wire X-ray photography is used to measure the characteristic size and temperature of the hotspots, over a range of pressure and gas mixing ratios. Filtered p-i-n diodes and a beam-target detector are used to investigate the time evolution of the hotspots. Typical size for the hottest emitting region, at temperatures between 200 and 400 eV, is found to be around 150 /spl mu/m, with a typical duration of the high temperature phase of the order of 10 ns. In general, the temperature in the final phase of the time evolution of the hotspots reaches values which are nearly twice those of the plasma column where they are formed. Characteristic size of the hotspots is about half of that of the initial plasma column.

The behavior of a medium energy (/spl sim/1 kJ) pulse-power-driven vacuum spark is shown to depend on the electrode material and form of the anode in otherwise similar conditions of operation. The dynamical evolution of the discharge is followed from its soft X-ray emission. The electrode materials compared are titanium and aluminum with a form of anode that is tubular or conical. The use of a tubular anode favors a more uniform sheath and a better formation of a dense Z-pinch and the ensuing hot spots are found to be at least twice as bright. The position of the brightest spots differs according to the material and the electrode shape, and is found to coincide with the shape of the sheath. The energy output is measured and the density of the plasma in the hot spots may be calculated.

Experimental observations of the plasma dynamics in a vacuum spark are presented which permit measurements of the electron density and temperature during a large part of the compression phase of the pinch. The vacuum spark is generated by a low-impedance pulse forming line with a maximum current in excess of 100 kA. The discharge is operated in the hybrid mode in a titanium plasma. A laser focused onto the cathode provides the preionizing source. Soft x-ray emission from the current sheath is observed with an x-ray framing camera well before maximum compression. These observations are compared with holographic interferograms, both showing the formation of an axisymmetric rhombic boundary to the plasma sheath. The temperature and density of both the sheath and the internal plasma are observed until the formation of hot spots at the time of maximum compression. The temperature evolution of the hot spots is presented, showing a repeatable behavior that depends on the axial position.

Discharge formation in the A–K space of transient hollow cathode discharge (THCD) is causally linked with the emission of high energy electron beams originating in the hollow cathode region (HCR). Ionization in the A–K gap proceeds through the formation of a moving virtual anode, whose time evolution is strongly correlated with different periods in the electron beam activity. Here, we report on time and space resolved observations of different ionization events inside the HCR, which are time correlated with ionization processes inside the A–K gap. The experiments have been performed in Hydrogen, at pressures between 50 and 400 mTorr. A statistical study of the characteristic times associated with the different ionization events, based on von Laue plots, shows that the time distribution of events is well described by a single Gaussian distribution.

Experimental studies on a fast pulsed capillary discharge are presented. The discharge operates in a 0.8 mm inner diameter capillary, at 12 kV applied voltage. On axis discharge initiation is achieved by means of the hollow cathode effect. A short, less than 10 ns, XUV pulse is produced. Preliminary time resolved spectroscopic studies indicate that a hot, fast evolving short duration capillary plasma is produced.

We report on experimental observations in PFP-I, a small 3.8 kJ plasma focus, which is operated in Hydrogen-Argon mixtures to investigate the effect of parameter modifications on the overall performance of the device. An extensive array of diagnostics is been used, which includes voltage and current probes in the external circuit, a novel small magnetic probes array located along the cathode rods, filtered PIN diodes located side and end on, and filtered multi-pinhole and slit-wire X-ray camera. Extended operating range from below 0.2 Torr upwards has been achieved. Hot spot formation has been investigated as a function of H2-Ar mixing ratio. Hot spot sizes around 150 μm in the soft X-ray region, have been inferred from the slit-wire measurements. Pin-hole time integrated X-ray pictures and time resolved PIN diode measurements have been used to determine characteristic hot-spot temperatures in the 350 to 450 eV range.

Summary form only given. The Vacuum Spark is a small scale device which allows the achievement of plasma conditions leading to radiative collapse on a nanosecond timescale. We report on the plasma formations in a Vacuum Spark operated in the hybrid mode when driven from a 1.5 Ohm coaxial pulse forming line with a maximum current of 140 kA in a 120 ns pulse. A comparatively low power Nd:YAG laser pulse incident onto the cathode initiates the discharge. The evolution of the titanium plasma electron density and temperature is followed both in the visible and the soft X-ray part of the spectrum. The plasma electron density dynamics are followed using a novel micro holographic interferometric technique, based on the Mach-Zehnder interferometer, permitting a spatial resolution better than 20 /spl mu/m. This technique is compared with simultaneous results from image plane interferometric holograms. The probing beam is a 1 ns frequency doubled Nd:YAG laser.

Dense transient plasmas of high energy density, can be easily produced in a plasma focus device, on a cost effective basis. High energy ion beam emission is one, among several others, of the very interesting transient plasma phenomena observed in plasma focus discharges. Although some of the properties of these ion beams have been investigated in the past, the physical mechanisms involved in their generation are still not clear, and further investigations are required on order to establish the exact correlations of the ion beam emission with other plasma focus phenomena. We present a preliminary series of measurements on ion beams in a plasma focus operating with hydrogen/gas mixtures, using an XRD probe array, which allows the characteristic ion energy to be determined from the time of flight across the probe array, and the time of the ion beam emission to be correlated with plasma emission events associated with the soft X-ray pulses detected by the probes.

In this work we present experimental observations of the XUV and electron beam emission properties of an ultrafast pulsed capillary discharge, which combines the features of a transient hollow cathode discharge with the inherent characteristics of the capillary discharge.

The recording of the spatial structure of a high temperature plasma in the hard X-ray region is limited by the problem of shine-through associated with a thin aperture material, when fine resolution is required, as well as the low-emission flux from the plasma in the high energy spectral region. In this paper, the design and performance of a hard X-ray camera is presented to monitor plasma radiation structure emitting in a spectral region above 12 keV, using the slit-wire camera design. In a series of experiments on GIT-4, the camera was used successfully to identify the spatial structure of the hard X-ray emission from the plasma and demonstrated the existence of a sub-100 /spl mu/m diameter, cm long plasma column emitting in the 18-30 keV region.

Experiments on the GIT-4 pulsed power generator have been performed in order to find out more about the composite pinch formation, stability and performances [1,2]. Different fibre materials and different external shell masses were tried out, in order to study their influence on the pinch behavior [3]. We present here results obtained from space resolved multi-pinhole, multi-filter X-ray camera under a variety of load conditions. The analysis was performed using a filter analysis [4] code with a full simulated spectra from the high Z materials. The temperature distribution along the composite pinch in those different experimental situations has been estimated, in different spectral regions. The influence of shell mass loading on the pinch uniformity will be discussed.

Neutron emission of a low energy 3.3 kJ (15 kV) plasma focus operated in pure deuterium and in deuterium-argon admixtures is studied. Both the time resolved and time integrated techniques are employed simultaneously. The time resolved measurements are made using six plastic scintillator-photomultiplier detectors placed at different angular positions. Two sets of Indium foil activation counters are used for the time integrated measurements. In general, two periods of neutron emission are observed with the second period emission being more energetic. Thus at least two different mechanisms may be involved in the neutron production.

For a pinch under equilibrium, the Pease-Braginskii current limit sets the ceiling when the ohmic heating rate is balanced by the bremsstrahlung loss rate. Under a dynamical condition. Shearer showed that, when the pinch current exceeds the Pease-Braginskii limit, the difference in ohmic heating and bremsstrahlung loss rate could lead to the phenomenon of radiative contraction, producing a plasma structure of ultra high energy density. The work was extended by Haines and Robson independently to describe the conditions for radiative collapse. These analytical results show that under certain circumstances, a high temperature pinch can be self-compressed to an extremely high density, limited only by photon self-absorption or electron degeneracy. In this paper, we present an analysis on the radiative collapse formalism including the effect of an external circuit. It is shown that the degree of contraction is governed by the intrinsic rate of energy delivery to the system. Two expressions are derived which define the ultimate pinch dimension with the plasma parameters and that of the electrical circuit.

Experimental studies on a fast pulsed capillary discharge are presented. The discharge operates in a 0.8 mm inner diameter alumina capillary, at 10 to 30 kV applied voltage. On axis discharge initiation is achieved by means of the hollow cathode effect. A short, less than 10 ns, XUV pulse is produced. Preliminary time resolved spectroscopic studies indicate that a hot, fast evolving, short duration capillary plasma is produced.

The capillary discharge is of great interest as a high brightness, short pulse XUV and soft X-ray source. Intense light emission from the visible to the soft X-ray region can be produced with very high brightness due to the high aspect ratio. Both broadband continuum and narrow band line radiation can be produced. Lasing gain in long capillary discharges have been demonstrated in a number of experiments. To increase the plasma temperature the pressure has to be decreased. The difficulties at low pressure are the tendency for the discharge to be formed on the surface of the wall and long build-up time of the discharge. This introduces significant materials from the wall discharge which is not directly under external control. To overcome these problems we have developed a device for intense XUV production which combines the features of a transient hollow cathode discharge with the characteristics of the capillary discharge. In this paper, some of the pulsed power aspects of this pulsed hollow cathode capillary discharge are presented together with some of the results obtained. The potential underlying the understanding, as well as the control, of the operating regime of a high temperature capillary discharge open up several areas of innovative research. The implication of the successful realization of a table-top X-ray laser in both research and application could not be over emphasized. (3 pages)

An X-ray filter analysis code, XRAYFIL, has been developed to allow more accurate non-dispersive X-ray plasma diagnosis with absorption filters by using a series of trial spectra of the emitting species. The code calculates a set of emission spectra for a given plasma using a CRE level population model package, RATION[1], and convolves it with the transmission characteristics of the filter set used, as well as the response function of the detector chosen. Comparison of the ratio of the signal through the different filters from these calculated values to that recorded in the experiment allows us to obtain a measurement of the plasma temperature and density. The code incorporates a number of options for various emission scenario and detector choices.

Preliminary experimental results on the investigation of hotspot formation in a small 3.8 kJ Plasma Focus device operating at constant mass density in Hydrogen-Argon and Hydrogen-Nitrogen mixtures are presented, at pressures from below 0.2 Torr upward, with relative percentages of Hydrogen ranging from 80% to 100%. The diagnostics includes voltage and current measurements, multi pinhole and slit-wire X-ray photography, filtered PIN diode array, and an array of small magnetic probes located along the cathode rods to study the current sheath structure during the run down phase of the discharge. A measurement of the hot-spot characteristic size in different spectral regions is obtained from the slit-wire images, whereas the time resolved information from the PIN diodes is used to infer the temperature evolution of the plasma points. Typical results show more reproducible hotspot formation in H2-Ar then in H2-N2 mixtures, with characteristic temperature in the 300 to 570 eV range, and hotspot size between 130...

A series of collaborative experiments on complex plasma loads have been carried out on the large inductive energy storage (IES) generator, GIT-4, over the last three years. The aim of the experiments is to explore the different configurations for the formation of ultra-high energy density plasmas in high voltage pulsed power systems by direct electromagnetic energy coupling. In this paper, the authors present some of the underlying philosophy on these experiments and the results obtained. Particular emphasis is placed on the pulsed power aspects and the effect of source-load coupling.

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)

Hot spots are dense plasma formations emitting strongly in the medium to hard X‐ray regions. Their presence can be detected in a dense plasma focus when high Z impurities are added to the main gas of hydrogen or deuterium. Experiments have been carried out on DPF 78, a medium size, 28 kJ, 60 kV Mather type Plasma Focus operating with heavy gas admixtures to the hydrogen or deuterium fill gas. Using a range of time and space resolved diagnostics in the optical and X‐ray region, the dynamics and properties of hot spots were investigated. The complementary nature of these diagnostics working in conjunction allowed us to construct a physical description on the evolution of these dense plasma formations. The first appearance of hot spots was detected within 5 ns of first compression, at a time when the bulk of the plasma column is grossly stable. However this gross stability coexists with strong local density gradients as observed in Schlieren photographs which are correlated to the hot spots. The existence of a macroscopically stable plasma column at this time suggests that the large scale m=0 MHD instability, which is observed to take place at a later time at high z position and leads to the disruption of the integrity of the column, is not responsible for the hot spots formation in the initial period.

Summary form only given. The dynamics of the plasma configuration in the pinch and post-pinch phases and the properties of the X-ray and neutron emissions were studied on a medium-energy (28 kJ, 60 kV) plasma focus device in terms of the relative deuterium-to-noble gas concentration and effective atomic number of the gas filling. The investigation was carried out with various additions of noble gases (neon, argon, and krypton) to the main deuterium gas filling using a broad range of diagnostics methods. The increase in the effective ion charge due to the use of mixtures of deuterium with heavier gases leads to the reduction of both the average neutron yield and the neutron emission reproducibility, as well as the concentration of the soft X-ray emission to bright spots having dimensions of a few ten of /spl mu/m. An intense late soft X-ray emission was observed in noble gas seeded discharges, which could be due to a strongly radiating recombining plasma confined as a lower-density plasma configuration of large dimensions.