Ivan N. Tilikin

Hybrid X-pinch with conical electrodes

A hybrid X-pinch configuration consisting of solid conical electrodes connected by a wire has been tested on a 45 ns risetime, 500 kA peak current pulsed power generator. Wires of different materials were loaded through holes in the cones; wire lengths were varied from 0.6 to 2 mm. Most of these hybrid X-pinches generated an intense single burst of soft x-rays and developed a single hot spot that was of micron-scale size. Hybrid X-pinches generate less hard x-ray intensity than standard X-pinches.

X-pinch source of subnanosecond soft X-ray pulses based on small-sized low-inductance current generator

For the first time, the regime of a micrometer-size hot spot formation is impemented for an X-pinch in a plasma, which is fed from a current generator based on low-inductance capacitors and rapid current switches. The configurations of X-pinches, which can be used effectively as point sources of soft X-rays with this type of current generator, are determined. A prototype of a small-size radiation source for high-resolution point projection X-ray radiography has been constructed. The main parameters of X-pinch as a radiation source are analyzed and compared with X-pinch parameters in high-voltage setups with shaping lines. An analysis of the data on the operation of X-pinches in generators with different parameters has led to simple relations that can be used to select optimal initial X-pinch parameters.

Hybrid X-pinches

Results from experimental studies of a hybrid X-pinch with an initial configuration in the form of a high-current diode with conical tungsten electrodes spaced by 1–2 mm and connected to one another with 20- to 100-μm-diameter wires are presented. The experiments were carried out at four facilities with a current amplitude from 200 to 1000 kA and front duration from 45 to 200 ns. It is shown that, in spite of their simpler configuration, hybrid X-pinches with a short rise time of the current pulse (50–100 ns) are highly competitive with standard X-pinches in the generated soft X-ray power and the formation of a single hot spot in them is much more stable, while hard X-ray emission is almost absent. The possibility of using hybrid X-pinches as soft X-ray sources for point projection X-ray imaging of plasma objects is considered.

A source of hard X-ray radiation based on hybrid X pinches

X pinches are well known to produce very small, dense plasma pinches (“hot spots”) that emit sub-nanosecond bursts of 1–8 keV radiation. Hard X-ray radiation in the range from 8 to 300 keV or more is also emitted, and only a small portion of which is associated with the X-pinch hot spot. In hybrid X-pinches (HXP), the  10 ns hard X-ray pulse is terminated by fast closure of the gap between the two conical electrodes of the HXP by rapidly expanding electrode plasmas. The temporal, spectral, and spatial properties of this higher energy radiation have been studied. This radiation was used for point-projection imaging with magnification between 1.5 and 6, and spatial resolution of 20–100 μm was demonstrated.

Study of the hybrid X-pinch with an external axial magnetic field

Summary form only given. The hot spot that develops in the gap between the electrodes of a hybrid X-pinch (HXP) [1] is a unique source of X-rays. The hot spot location is within 100-1000 μm of the center of symmetry of the X-pinch, and uncertainty depends on the gap size. Investigation of the X-pinch neck cascade process [2] showed that the hot spot tended to develop on one side of the minidiode that constitutes the z-pinch electrode structure in an X-pinch. The addition of an axial magnetic field component as a means to shift the hot spot in a specific direction and to stabilize its location has been tested in HXP experiments on 250 kA-1.2 MA pulsers [3]. It was shown that a configuration with twisted return current rods can generate too strong an axial magnetic, seriously disturbing the neck cascade process and hot spot development. In this work we have been continuing to study magnetized HXBs using a large set of available diagnostics: laser probing, X-ray, XUV and optical imaging, tiny magnetic probes and X-ray spectroscopy. The magnetic field profile around an HXP with twisted return current rods was measured on BIN (270 kA current, 100 ns risetime) and COBRA (1.2 MA current, 100 ns risetime) pulsers and optimal for x-ray production axial/azimuthal magnetic field ratio was determined.

Study of explosive electron emission from a pin cathode using high resolution point-projection x-ray radiography

Summary form only given. Explosive electron emission (EE) is well-known phenomenon widely used in high-current electronics [1]. Most theories of EE are based on mechanism of cathode flares development after explosion of metal micropins on the cathode surface [2]. It was shown that the EE have a complicated inhomogeneous spatial and temporal structure, but experimental methods at that time were incapable of revealing the internal structure of the flame and could not adequately explain the processes occurring during pin explosion. The physical state of the pin material, the spatial structure of the flame and its origin are still a matter of conjecture. In this work we used high-resolution point projection x-ray radiography [3] with Hybrid X-pinch (HXP) as the source of radiation [4] to directly observe pin explosion in the high-current diode. Pins made from thin (10-25) Cu or Mo wires were soldered to the cathode of the diode placed in return current circuit of HXP in BIN pulser (250 kA current, 100 ns risetime). Radiograph images with x7.4 magnification were recorded on the Kodak DR film with 12.5 μm Ti filter. Pin length and pin-anode gap were varied in wide range. The diode current and voltage were measured. In experiments with small pin-anode gap (0.1-1 mm) development of the expanded dense core of the pin wire was observed except pin tip with length 100-200 μm. In experiments with bigger gap there was no visible wire core expansion within spatial resolution of the experimental technique (2 μm).

X-ray absorption spectroscopy of X-pinch plasmas

Summary form only given. X-ray absorption spectroscopy is a powerful diagnostic technique for determining the charge state, temperature and density of plasmas under a wide range of conditions and situations. It has been employed recently to study the core-corona system generated in electrically exploded wires and wire array Z-pinches. Using an X-pinch as the source of probing radiation and a spherically bent quartz crystal as the spectral analyzer as in those experiments required the object to be uniformly elongated in one direction. The important central part of an X-pinch is localized in the limited space and so, to probe it using x-ray radiation from an auxiliary source, a complicated focusing x-ray optical scheme must be used and is under development now. In the work reported here, we observed “self-absorption” spectra of hybrid X-pinches (HXP) when the continuum radiation from the X-pinch hot spot is absorbed in the surrounding hot spot plasmas or in the trailing material of the X-pinch neck. To extend the choice of materials that can be used in HXPs we developed hollow tube loads that are filled by powder or fine grains of salt. We have tested twisted Al wires wetted with the melt sulfur and single Ti and Mo wires covered by mixtures of epoxy or super-glue with CaCl2 salt grains. Using ELICS and Johann spectrographs we recorded intense K-shell spectra containing both emission and absorption spectral lines. Plasma parameters were estimated using Prizm program.

Investigation of initial stage of hybrid X pinches

Summary form only given. The initial stage a hybrid X pinch (HXP) plasma formation has been studied using laser probing x-ray radiography and electrical measurements. This stage is especially interesting in HXPs in comparison with standard X-pinches because of the strong influence of the electrode plasmas on the process of neck development. Electrode material evaporated by strong UV radiation from the exploding wire can shorten the electrode gap before hot spot formation. The interaction of electrodes plasmas and the exploding wire plasma determines the discharge parameters required for proper HXP operation. The experiments in this work were performed on different pulsers with output current from 4 kA to 1.2 MA and current rise time from 50 ns to 340 ns. The low inductance super small pulse generator Micro based on ceramic capacitors and a flashover vacuum optically triggered switch was specially designed to study the initial phase of the HXP. This pulser with a peak current of 5 kA and current rise rate 100 A/ns reproduced very well the processes in HXPs on more powerful devices in first tenths nanoseconds of the discharge and enables us to study the details of plasma formation without powering big machines. The results obtained on the Micro pulser were compared with the results obtained in experiments on our older pulsers GVP (10 kA, 350 ns) and BIN (250 kA, 100 ns) at the Lebedev Institute, and XP (400 kA, 100 ns) and COBRA (1 MA, 100 ns) at Cornell University. It was shown that in HXPs from materials with low melting temperature and high core expansion rate (Al, Cu, Ag, Au) the wire core expands and fills inter-electrode gap and prevents fast diode shortening by the electrode plasma. That makes possible using relatively long pulse drivers for HXP.

Dynamics of hybrid X pinches

Summary form only given. The hybrid X pinch (HXP) configuration, consisting of solid conical electrodes connected by a wire, has been studied on pulsers with current from 250 kA to 1.2 MA and current rise time from 45 ns to 170 ns1. The experiments have shown that for each generator, it is possible to find a wire material, diameter and length for which the HXP generates an intense single burst of soft x-rays and develops a single hot spot with micron size. However, the physics of the process of single hot spot formation has not been studied in detail. In order to understand the physical processes that occur in the HXPs we are carrying out an in-depth investigation of those processes and comparing them with the physics of standard X pinches (SXP). The early stages of HXP and SXP hot spot formation have been studied on the BIN (250 kA, 100 ns) and XP (400 kA, 100 ns) pulsers. Point-projection x-ray radiography with HXPs as the source of the x-ray radiation was used in most experiments2. Hybrid and standard X-pinches were placed in the return current circuit of the pulsers in place of one of the two rods. 70 kA and 120 kA current through the X pinches were measured on the BIN and XP pulsers, respectively. The experiments have shown that the early stages of standard and hybrid X-pinch hot spot formation are very different. The final stage of hot spot formation has been studied in the configuration with two HXPs placed in parallel in the full current central electrode gap. The final pinching process in the HXP is similar to the same stage of SXP. The influence of the inter electrode plasma on HXP dynamics also has been studied, and it does have a substantial effect.

Study of hybrid X-pinches with current up to 1.2 MA

A hybrid X-pinch configuration consisting of solid conical electrodes connected by a wire was first tested on the 45 ns rise time, 500 kA peak current XP pulsed power generator1. In more recent work the hybrid X-pinch was studied on generators with peak currents from 250 kA to 1.2 MA and current rise time from 45 ns to 170 ns. In all experiments the hybrid pinches have the same 60° conical electrodes made of tungsten (W) with 5% copper (Cu). The wires were loaded trough 1 mm holes in the cones. The wire diameter and the gap between electrodes were varied to fit the pulser peak current and rise time. Mg, Al, Ti, Ni, NiCr, Cu, Mo, Pd, Ag, W and Au wires having lengths ranging from 0.6 to 2.5 mm were tested in the experiments. The wire diameters were varied from 12 to 200 µm for different experimental conditions. The experiments have shown that for each generator, it is possible to find a wire material, diameter and length for which the X-pinches generate an intense single burst of soft x-rays and develop a single hot spot with micron size. Also they generated less hard x-ray intensity than that measured from comparable standard X-pinches. Absence of x-rays with photon energies > 20 keV associated with long-lived electron beams2 is explained by fast closure of the gap between conical electrodes by expanding dense plasma from them. At the same time short-lived electron beam radiation produces bright small-size x-ray source in 6–15 keV spectral band usable for point-projection radiography. Hybrid X-pinches have been used as a source of continuum radiation with a flat spectrum for absorption x-ray spectroscopy of relatively cold plasma of exploded Al wires and wire arrays3. In place of the ∼ 100 µm diameter wires between the conical electrodes we have tested hollow tubes from Al, Ni and polyethylene. Both empty and filled with various materials tubes were tested as a load on COBRA pulser (1.2 MA, 100 ns rise time). This possibility with the hybrid X-pinch configuration greatly expands the range of materials that can be studied under extreme conditions of an X-pinch.