X-pinch Plasma Research Articles

Effect of wire number on x-pinch discharges

An investigation has been carried out into the x-ray emission and plasma dynamics of a “tabletop” x-pinch plasma generated by passing a 40ns, 35kA current pulse through two or four 5μm thick tungsten wires. It was found that a four-wire x pinch consistently resulted in an order of magnitude higher energy hard x-ray yield (>3.5keV) than a two-wire pinch. The total x-ray emission from a four-wire pinch also had a shorter pulse length such that the total x-ray power was increased by more than a factor of 2. Observations of x-pinch discharges driven by a higher energy 160kA, 80ns generator are also presented. In this case x pinches with two, four, and six wires were made using thicker aluminum and molybdenum wires and the number of wires is shown to play a significant role in the plasma dynamics. Increasing the number of wires enhanced the reproducibility of the discharge and the x-ray pulse.

Radiation properties and implosion dynamics of planar and cylindrical wire arrays, asymmetric and symmetric, uniform and combined X-pinches on the UNR 1-MA zebra generator

In the following experiments, we studied implosions of different wire arrays and X-pinches produced on the 1-MA Zebra generator at the University of Nevada, Reno. Diagnostics included both spatially-resolved and time-gated X-ray imaging and spectroscopy, and laser probing. In particular, we compared planar wire arrays, to which little energy could be coupled via the conventional magnetic-to-kinetic conversion mechanism, to cylindrical wire arrays of comparable dimensions and mass. The planar wire arrays were shown to radiate much higher peak power and more energy in subkiloelectronvolt and kiloelectronvolt spectral ranges than cylindrical wire arrays. We tested the theoretical conjecture that enhanced resistivity due to the small-scale inhomogeneity of wire-array plasmas has a major effect on dynamics, energy coupling and radiation performance of wire-array Z-pinches. The study of Al, Alumel, and W cylindrical wire arrays shows a wide variety of characteristic behaviors in plasma implosions discussed hereinafter. Additional experimental results for symmetric and asymmetric, uniform stainless steel, Cu, Mo, combined Al/Mo, Mo/Al, Al/W, W/Al, and Mo/W X-pinches are also presented. New data for the total radiation yield are obtained. The planar structures of X-pinch plasma and the corresponding electron beam was observed for most of X-pinches. The generation of hot spots along original wires positions-cooler than those from the cross-wire region-and arc structures with hot spots between wires were found for X-pinches composed from Al, Cu, and W wires.

Technique to measure sub-microsecond magnetic field pulses using magnetic (CoPt) thin films

We introduce a technique to measure the maximum magnetic field of a submicrosecond duration pulse using magnetic CoPt thin films. In the present experiment, this technique yields a lower limit for the field intensity and reveals the sense of that peak field. The time-varying magnetic field was generated by an exploding wire array plasma called an X pinch. Using a Quantum Design Superconducting Quantum Interference Device magnetometer, two thin films were initialized with remnant magnetization along a specific direction. The two films were then placed near an X-pinch plasma column with magnetizations in opposite directions. The current driven through the X pinch induced a change in magnetization in the films, which implied a lower bound of 17 kOe for the magnitude of the maximum magnetic field to which the film was exposed.

Determination of the size and structure of an X-pinch x-ray source from the diffraction pattern produced by microfabricated slits

X-pinch plasma emits subnanosecond bursts of x rays in the 3-10-keV energy range from a small source. As such, it has been used for high-resolution point-projection imaging of small, dense, rapidly changing plasmas as well as for submillimeter-thick biological samples. In addition to the effect of source size on geometric resolution, a small source size can also provide high spatial coherence of x rays, enabling the rays to be used for imaging weakly absorbing objects with excellent spatial resolution by a method called phase-contrast imaging. To determine the source size, we microfabricated gold slits and imaged them in a point-projection radiography configuration. The shape of the shadow image pattern depends on the source size and energy band of the x rays, the shape and material used for the slits, and the geometry of the experiment. Experimental results have been compared with wave-optics calculations of the expected image pattern as a function of all the parameters listed above. For example, assuming a Gaussiansource distribution, an effective source size in 2.5-4.1 A radiation (1 A = 0.1 nm) of 1.2 +/- 0.5 microm (full width at half-maximum) was determined for a 20-microm Mo wire X pinch. Characterization of the size and structure of the x-ray bursts from X pinches by the use of different wire materials and different slit structures is made.

Experimental study of X-pinch in a submicrosecond regime

X-pinch plasmas are known as point-like X-ray sources with the potential application for backlighting diagnostics. X-pinches are commonly driven by large pulsed power generators delivering high-voltage pulses with short rise-times, typically under 100 ns. When operating with slower capacitor banks, X-pinches become less reliable because of the arising X-ray pulse jitter and because of the appearance of two or more X-ray bursts coming from multiple hot spots. In this paper, a very compact inductance-capacitance (LC) generator with the current rise-time of 200 ns was used to drive molybdenum and tungsten wire X-pinches. A single peak, small shot-to-shot jitter emission of X-rays was obtained. Time-integrated penumbral imaging recorded the X-ray source dimension of less than 15 /spl mu/m in the spectral region above 2.4 keV. The total yield of more than 80 mJ was registered with the radiation pulse duration as short as 1.5 ns. The appearance of single- or multiple-source core structures is discussed in correlation with used wire material and X-pinch torsion angle. The results confirm the possibility of using an X-pinch driven by a fast compact capacitor bank for backlighting applications.

Electron-beam-generated x rays from X pinches

X pinches are well known to produce very small, dense plasma pinches (“micropinches”) that emit short bursts of 1.5–8keV radiation [Shelkovenko et al., Phys. Plasmas 9, 2165 (2002)]. X-ray radiation in the 8–100keV range is also emitted, only a small portion of which is associated with the micropinches. Beginning immediately after the soft x-ray burst, higher energy x-ray emission is observed that is attributed to energetic electrons accelerated in the gaps that appear in the X-pinch plasma structure. The temporal, spectral, and spatial properties of this higher energy radiation (8–100keV) have been studied using two ∼0.1μs pulsed power generators, one operating at up to 450kA peak current and the other up to 270kA. This radiation was also used for imaging in a low magnification configuration, and spatial resolution of a few tens of micrometers was demonstrated.

Investigation of the radiation properties of L- and M-shell X-pinch plasma x-ray sources using a transmission grating spectrometer

A transmission grating spectrometer (TGS) with one dimensional spatial resolution was used to investigate the radiative properties of X-pinch plasmas produced using wires made of NiCr and CoNiCr alloys, and Mo. Experiments were carried out on the Cornell XP machine. The TGS was used to estimate of the energy yields of L-shell Cr and Ni, and M-shell Mo radiation. The absolute energy yields in the wavelength range 10<λ<15 Å were 1–5% of the total energy delivered to the load (10–40 J for L-shell NiCr, and 20–50 J for M-shell Mo). An analysis of the spatial structure of X-pinch emission regions in different wavelength ranges shows that the source structure is changing with wavelength and the size varies from less than 0.1 up to 1 mm. For example, in one Mo X-pinch test shown, three separate soft x-ray sources radiating in the wavelength range λ<42 Å can be seen, but only one of those sources radiates in the wavelength range λ>42 Å. The results are compared with data obtained previously with the TGS on a 1 MA pulse powered machine Zebra.

Spectroscopic analysis of x-ray bursts from nichrome and conichrome X-pinch plasmas

Radiative properties of plasmas from X pinches with nichrome and conichrome wires have been studied using the presented diagnostic arrangement. The experimental results have been produced at the Cornell XP facility with a peak current of 450 kA and a full width at half maximum pulse duration of 100 ns. The spatially resolved, time-integrated x-ray line spectra from the region of the X-pinch cross point have been recorded using different crystal spectrometers. In particular, K-shell and L-shell x-ray spectra of Ni and Cr have been recorded through different filters in the same pulses for X pinches from two different wire alloys. A nonlocal thermodynamic equilibrium collisional–radiative atomic kinetic model of Ni has been developed to identify the useful diagnostic spectroscopic features and to model experimental spectra. The results of the modeling and radiative properties of different materials from nichrome and conichrome alloys are compared and discussed.

X-ray imaging of an X-pinch plasma with a bubble compound refractive lens

We present diagnostic images taken of an X-pinch plasma x-ray source driven by the XP pulser (100 ns, 500 kA) at Cornell University using an x-ray bubble compound refractive lens. The lens consists of a 200 μm inside diameter glass capillary that contains about 100 biconcave microlenses formed by a string of bubbles in epoxy. A precise system for lens alignment with of 3–5 arcmin accuracy is described. X-ray images of four-wire X pinches were obtained with a spatial resolution of approximately 2 μm.

Table-top X-pinch for x-ray radiography

Measurements of the optical and x-ray emission from a small 40-kA, 30-ns (10%–90%) rise-time X-pinch plasma discharge are reported. This device is truly “table-top,” having a laboratory footprint of less than 1 m2. With tungsten wires, the total energy (hν>800 eV) is 60 mJ in a pulse of less than 10 ns. The size of the emitting region is less than 10 μm. The emission is reproducible with a jitter relative to the current pulse less than 5 ns. Initial experiments demonstrating the use of this X-pinch for applications in x-ray radiography are presented.

X-pinch plasma conditions from time resolved x-ray spectroscopy

X-pinch plasmas, produced by exploding two or more 10–50 μm metal wires that cross and touch in the middle in the form of an X, have been successfully developed as point sources of soft x rays for radiography. However, the parameters of the x-ray source as a function of time, space and wavelength band are not well understood. To determine the X-pinch plasma characteristics, time-resolved x-ray spectra produced by two- and multiwire X pinches made from different wire materials were recorded using an x-ray streak camera. A spherically bent mica crystal spectrograph was used with a ∼10 ps time resolution x-ray streak camera in a slitless configuration. Some spectra were dominated by continuum and others by line radiation, and observed spectral features varied on time scales ranging from 10 to 1000 ps, depending on the wire material. Electron temperatures and densities in Ti X pinches were estimated from line ratios based upon collisional-radiative calculations, and from the time delay between the appearance of He-like and H-like Ti lines, respectively, indicating peak values of ∼2.5 keV and ∼2×1023/cm3. Results are presented that demonstrate the necessity for 10–100 ps time-resolution spectral data to obtain plasma conditions from micropinch x rays.

Applications of advanced theoretical x-ray L-shell spectroscopy to various plasma and collision experiments

The University of Nevada, Reno has developed a collisional-radiative atomic kinetics model that has successfully described the spectra produced by widely different multicharged ion experiments. The application of the model to experiments that highlight the importance of particular kinetics effects, including time-dependence and non-Maxwellian electron distribution functions, will be presented. These experiments include ion beam collision and high-temperature femtosecond laser and X-pinch plasma experiments. Adequate descriptions of ion beam collision spectra and x-ray yield require time-dependent treatment of radiative cascades from high-n capture states. Spectra from plasmas with significant fractions of hot electrons, such as those produced by fs lasers and X-pinches, require calculation of hot electron effects on collisional rates. The kinetics model presented here extracts as much information as possible from advanced experimental diagnostics such as time- and space-resolved spectral measurements.

High-Z 0.9–1.0 MA X-pinch as a possible backlighter in 50–100 keV and sub-keV-10 keV spectral regions and a powerful soft x-ray source for surface modification research

The presence of collimated 1.5–2.0 MeV electron beams in 0.9–1.0 MA high-Z X-pinch plasmas leads to the generation of an anisotropic hard x-ray radiation (more than 50–100 keV) in the crossing point of the wires, with a 1–2 mm size source and a time duration of 100–150 ns. The side-on measured electron temperature is higher than the end-on. Introducing of a synchrotron radiation in a side-on direction is used for the explanation of the phenomenon. The new results of time-resolved 0.16–10 keV x-ray imaging and spectroscopic study of the development of jets and hot spots in X-pinches are presented. The plasma parameters varied significantly for consecutive x-ray bursts (hot spots) during the same X-pinch discharge. A minimum duration of x-ray pulses was 1.1 ns. The x-ray energy output was up to 10–11 kJ, and a maximum power can reach up to 3×1011 W. The X-pinch coupled with the x-ray glass-capillary converter for collecting and focusing of x rays can generate soft x-ray energy density up to (1–10) J/cm2, and the flux density up to (1–5)×109 W/cm2. The 1 MA X-pinch can be used as a possible hard and soft x-ray backlighter and a soft x-ray source for material science and microelectronics research.

Time-resolved spectroscopic measurements of ∼1 keV, dense, subnanosecond X-pinch plasma bright spots

Bright, ∼1 μm x-ray sources (micropinches) produced within exploding wire X pinches are found to be near solid density and ∼1 keV electron temperature. For example, with a Ti X pinch, a 90 ps lifetime, 1.5–1.8 keV electron temperature, ∼1023/cm3 electron density plasma was observed. These plasma characteristics were determined using time-resolved x-ray spectra produced by 2- and 4-wire X pinches and collected by an x-ray streak camera with <10 ps time resolution. Together with a spherically bent mica crystal spectrograph, the streak camera recorded the 1–10 keV radiation emitted from X pinches made from different wire materials. Some spectra were dominated by continuum and others by line radiation. Spectral features varied on time scales ranging from 10 to 300 ps, depending on the wire material. Results are presented that demonstrate the necessity of time-resolved data for determining plasma conditions from micropinch x-ray spectra.

A novel imaging x-ray microscope based on a spherical crystal

A novel, compact, large field, and spectrally tunable imaging x-ray microscope is presented. It is based on the use of an isotropic point x-ray source and a spherically curved crystal. The x-ray beam intensity is modulated by the object attenuation, then monochromatized and enlarged using a spherical crystal and, finally, imaged using a detector downstream from the crystal. We demonstrate by ray tracing simulations that this system allows microscopy studies with high spatial resolution, high magnification ratios, and large field of view. Microscopes using this model can be easily built using different x-ray sources, like conventional x-ray tube generators, x rays emitted by laser-generated plasmas or x-pinch plasmas, and also synchrotron radiation when used in combination with other condenser optics. Preliminary experiments are presented to demonstrate the feasibility of the proposed setup. High resolution (∼4 μm) monochromatic (δλ/λ∼10−5–10−3) images over a large field of view (few mm2) were recorded in the spectral range 8–14 Å using a laser-generated plasma source and a spherical mica crystal. Compared to x-ray crystal imagers used in relation with plasma sources, the new configuration produces high quality stigmatic images working at many different Bragg angles, thus improving the spectral tunability and allowing a more flexible design.

Radiographic and spectroscopic studies of X-pinch plasma implosion dynamics and x-ray burst emission characteristics

Experimental results are presented from studies of the dynamics of X-pinch plasmas, formed using two fine wires that cross and touch at a single point (in the form of an X) as the load of a high current pulser. High-resolution (sub-ns in time and 2–3 μm in space) x-ray radiographs of X pinches driven by current pulses that rise to 200–250 kA peak current in 40 ns show that ⩽300 μm long Z pinches form in the region of the original wire cross-point a few ns prior to the first sub-ns intense x-ray bursts that are characteristic of an X pinch. The Z pinches implode to ⩽10 μm diam and then appear to develop gaps in the radiographic images in one or two places, coincident in time with the x-ray burst(s). The emission spectra of the intense x-ray bursts from different wire materials indicate electron temperatures of 500–1300 eV and densities in excess of 1022/cm3.

Effect of an electron beam generated in an X-pinch plasma on the structure of the K spectra of multiply charged ions

The first experimental studies of an electron beam generated in an X pinch on the XP machine (Cornell University, USA) and the BIN machine (P. N. Lebedev Physical Institute, Russian Academy of Sciences) are reported. It is shown that it is possible in an X pinch to isolate the effect of a plasma-generated electron beam on the multiply charged ion radiation. The intensities of the satellite lines corresponding to Li-, Be-, B-, and C-like ions are calculated for the Al spectrum on the basis of a collisional-radiative model with a non-Maxwellian electron distribution in the plasma. The effect of an electron beam on the multiply charged light ion radiation in an X-pinch plasma is demonstrated. Comparing our calculations with the experimental spectra, we conclude that the present model can be used to estimate the electron beam intensity.

Nitrogen laser system for diagnosing z-pinch and x-pinch plasmas

A nitrogen laser system built to probe dense z-pinch and x-pinch plasmas is described. This low-cost system consists of an atmospheric pressure oscillator and a low pressure amplifier. It generates a 1-cm-diam 3371 Å laser beam with approximately 0.33 mJ in a ≲1 ns pulse, and with a coherence length of at least 5 cm. This laser has been used in several modes of operation, such as for simultaneous schlieren and interferometry imaging of dense z-pinch and x-pinch plasmas. Example results are presented.

Ray-tracing for a monochromatic x-ray backlighting scheme based on spherically bent crystal

The properties of a monochromatic x-ray backlighting system based on a spherically bent crystal were investigated. Simulations of spatial resolution for spherically bent crystals with different radius of curvature and for several wavelengths of incidence radiation were done using the ray-tracing method. Test experiments were carried out to demonstrate the high spatial resolution using the incoherent soft x-ray radiation produced by laser- and x-pinch plasmas. Both experimental and theoretical results show that a spatial resolution of at least 4 μm can be obtained in a field of view ranging from a few mm to cm. The narrow spectral band of the diffracted beam (Δλ/λ ∼ 3 × 10−4), the high efficiency of the presented scheme and the possibility of significantly reducing the self radiation of the studied object make it one of the most promising backlighting systems. This scheme permits the essential reduction of requirements of the source of radiation and could be a less costly alternative to using x-ray lasers for such purposes.

Precision measurements of the wavelengths of the Al XII 1snp1P1—1s2 1S0 (n = 6–12) lines in the emission spectra of laser and X-pinch plasmas

The emission spectra of a high-temperature recombining laser plasma and of an X-pinch plasma were used in high-precision (± 0.03–0.05 pm) measurements of the wavelengths of the Al XII lines resulting from the 1snp1P1—1s2 1S0 (n = 6–12) transitions. The lines due to transitions from the 1s11p and 1s12p levels of the He-like Al XII ion were observed for the first time. The experimental results were compared with calculations carried out by several theoretical methods.