X-pinch Plasma Research Articles

Modeling of the L-shell copper X-pinch plasma produced by the compact generator of Ecole polytechnique using pattern recognition

Principal component analysis is applied and compared with the line ratios of special Ne-like transitions for investigating the electron beam effects on the L-shell Cu synthetic spectra. The database for the principal component extraction is created over a non Local Thermodynamic Equilibrium (non-LTE) collisional radiative L-shell Copper model. The extracted principal components are used as a database for Artificial Neural Network in order to estimate the plasma electron temperature, density, and beam fractions from a representative time-integrated spatially resolved L-shell Cu X-pinch plasma spectrum. The spectrum is produced by the explosion of 25-μm Cu wires on a compact LC (40 kV, 200 kA, and 200 ns) generator. The modeled plasma electron temperatures are about Te ∼ 150 eV and Ne = 5 × 1019 cm−3 in the presence of the fraction of the beams with f ∼ 0.05 and a centered energy of ∼10 keV.

Polarization analysis of CuXX-lines emitted from X-pinch

Soft x-ray emission from CuXX L-shell lines emitted by a dense X-pinch plasma have been investigated with high-resolution curved Bragg crystals at different angles of orientation. Single shot time integrated spectra show clear evidences of polarization for the Ne-like spectral lines 2s22p6 1S0 → 2s22p53s 1P1 (λ = 12.570 Å), 2s22p6 1S0 → 2s22p53s 3P1 (λ = 12.8277 Å). The variation of the intensity ratio of these two well-separated L-shell lines is discussed in view of its application for suprathermal electron characterization under real experimental conditions of pinch plasmas. We demonstrated that the simultaneous use of two different polarization spectrometers (means 4 Bragg crystals) permitted a high level of confidence for the analysis of the variation of the line ratios due to polarization.

Principal component analysis of electron beams generated in K-shell aluminum X-pinch plasma produced by a compact LC-generator

Principal component analysis (PCA) method is applied and compared with the line ratios of H-like and He-like transitions, in order to investigate the effects of electron beam on the K-shell Aluminum synthetic spectra. It is also used as a diagnostics to estimate the plasma parameters of K-shell Al X-pinch plasma spectrum. This spectrum is produced by the explosion of two 25-μm Al wires on a compact LC (40 kV, 200 kA) generator. The database for the principal component extraction is created over a previously developed, non-LTE, collisional radiative K-shell Aluminum model. As a result, PCA shows an agreement with the line ratios which are sensitive to plasma electron temperatures, densities and beam fractions. Principal component analysis also illustrates that the addition to the non-LTE model of a fraction f of electrons in an energetic beam, generates the clusters in a three dimensional vector space which are translations of each other and follows reverse v-shaped cascade trajectories, except for the f = 0.0 case. Modeling of a typical shot by PCA gives the plasma electron temperature of Te = 100 eV, density of Ne = 1 × 1020 cm−3 and hot electron fraction of f = 0.2 (with a beam energy centered at 10 keV).

Visualization of the magnetic field and current path in Z-pinch and X-pinch plasmas

Laser diagnostics at the wavelength of 266 nm allow investigation of wire array Z-pinches and X-pinches at the 1 MA pulse power generator. Faraday rotation diagnostics at 266 nm is applied to study MG magnetic fields in Z-pinch plasma. Faraday diagnostics can qualitatively visualize magnetic fields in dense plasma and give additional information about the current flow even if the plasma density cannot be reconstructed with interferometry. A comparison of images from the three-channel polarimeter shows strong localized magnetic fields, revealing the path for the electric current inside the plasma. Faraday images present current switched to the trailing plasma.

Spectroscopic analysis of X-pinch plasma produced on the compact LC-generator of Ecole Polytechnique using artificial neural networks

A back-propagation artificial neural network algorithm is applied to a Mo X-pinch to estimate plasma parameters from typical L-shell spectra in the keV region. The spectrum was generated by a very compact LC-generator (40 kV, 200 kA) for driving 25-μm Mo two-wire X-pinches with a current rise-time of 200 ns. The neural network was trained over a set of synthetic spectra generated using a previously developed L-shell non-LTE collisionally radiative model. As a result, electron temperature and density of the Mo plasma were estimated as Te = 1000 eV and ne = 9 × 1020 cm−3. Furthermore, effects of electron beams on plasma parameters have been investigated through the inclusion of hot electrons in the kinetic model. The small fraction of hot electrons resulted in a better fitting of spectrum and decreased parameters Te = 650 eV and ne = 3 × 1020 cm−3.

Estimations of Mo X-pinch plasma parameters on QiangGuang-1 facility by L-shell spectral analyses

Plasma parameters of molybdenum (Mo) X-pinches on the 1-MA QiangGuang-1 facility were estimated by L-shell spectral analysis. X-ray radiation from X-pinches had a pulsed width of 1 ns, and its spectra in 2–3 keV were measured with a time-integrated X-ray spectrometer. Relative intensities of spectral features were derived by correcting for the spectral sensitivity of the spectrometer. With an open source, atomic code FAC (flexible atomic code), ion structures, and various atomic radiative-collisional rates for O-, F-, Ne-, Na-, Mg-, and Al-like ionization stages were calculated, and synthetic spectra were constructed at given plasma parameters. By fitting the measured spectra with the modeled, Mo X-pinch plasmas on the QiangGuang-1 facility had an electron density of about 1021 cm−3 and the electron temperature of about 1.2 keV.

Determining the resistance of X-pinch plasma

The current and the voltage of an X-pinch were measured. The inductance of the X-pinch was assumed to be a constant and estimated by the calculation of the magnetic field based on the well-known Biot—Savart's Law. The voltage of the inductance was calculated with L · di/dt and subtracted from the measured voltage of the X-pinch. Then, the resistance of the X-pinch was determined and the following results were obtained. At the start of the current flow the resistance of the exploding wires is several tens of Ohms, one order of magnitude, higher than the metallic resistance of the wires at room temperature, and then it falls quickly to about 1 Ω, which reflects the physical processes occurring in the electrically exploding wires, i.e., a current transition from the highly resistive wire core to the highly conductive plasma. It was shown that the inductive contribution to the voltage of the X-pinch is less than the resistive contribution. For the wires we used, the wires' material and diameter have no strong influence on the resistance of the X-pinch, which may be explained by the fact that the current flows through the plasma rather than through the metallic wire itself. As a result, the current is almost equally divided between two parallel X-pinches even though the diameter and material of the wires used for these two X-pinches are significantly different.

Role of nonideality of multiply ionized plasma in phenomena of X-ray burst and X-pinch hot spot explosion

We present the results of resistive magnetohydrodynamic calculations for the final stage of X-pinch plasma compression, where we use our model of nonideality in multiply ionized dense plasma proposed before. Introducing this model into the code made provides the possibility to find the earlier inaccessible range of plasma parameters and obtain a pattern of X-ray burst and hot spot explosion qualitatively adequate to available observations. These data show the crucial significance of the necessity of taking into account the effects of nonideality in electron transfer processes for the description of X-pinch dynamics at the final stage of compression.

Theoretical and experimental studies of material radiative properties and their applications to laser and heavy ion inertial fusion

AbstractTheoretical and experimental studies of radiative properties of substances heated by pulsed current devises or lasers and used as X-ray sources have been carried out depending on plasma conditions, and specific spectra of X-ray absorption and radiation for different materials have been calculated. Important features of the theoretical model, known as the ion model of plasma, are discussed. This model can be applied for calculations of the radiative properties of complex materials over a wide range of plasma parameters. For purposes of indirect-driven inertial fusion based on the hohlraum concept, an optimization method is used for the selection of an effective complex hohlraum wall material, which provides high radiation efficiency at laser interaction with the wall. The radiation efficiency of the resulting material is compared with the efficiency of other composite materials that have previously been evaluated theoretically. A similar theoretical study is performed for the optically thin X-pinch plasma produced by exploding wires. Theoretical estimations of radiative efficiency are compared with experimental data that have been obtained from measurements of X-pinch radiation energy yield using two exploding wire materials, NiCr and Alloy 188. It is shown that the theoretical results agree well with the experimental data. A symmetric multilayer X-pinch, where W and Mo wires are used, is as well considered. The theoretical explanation of experimental phenomena is discussed based on the W and Mo radiative spectra. The ion model was as well applied for interpretation of experimental results on opacities of CHO-plasma obtained via indirect heating of low density polymer layers by means of soft X-rays. The new diagnostics method based on the deformation of the of the Carbon absorption K-edge when foam layer is heated to plasma is discussed. The spectral coefficients for X-ray absorption in CHO-plasma are calculated in the photon energy region around the Carbon K-edge for different plasma temperatures and mean foam density. In this case, the Carbon K-edge position on the energy scale can be used for plasma temperature diagnostic.

Dynamics of vapor emissions at wire explosion threshold

X-pinch plasmas have been actively studied in the recent years. Numerical simulation of the ramp-up of metallic vapor emissions from wire specimens shows that under impulsive Ohmic heating the wire core invariably reaches a supercritical state before explosion. The heating rate depends sensitively on the local wire resistance, leading to highly variable vapor emission flux along the wire. To examine the vapor emission process, we have visualized nickel wire explosions by means of shock formation in air. In a single explosion as captured by shadowgraphy, there usually appear several shocks with spherical or cylindrical wave front originating from different parts of the wire. Growth of various shock fronts in time is well characterized by a power-law scaling in one form or another. Continuum emission spectra are obtained and calibrated to measure temperature near the explosion threshold. Shock front structures and vapor plume temperature are examined.

Study of keV radiation properties of Mo and Ti X-pinch plasma sources using a pinhole transmission grating spectrometer

The properties of keV x-ray radiations from Mo and Ti X-pinch plasma sources at the current of 800 kA were investigated by a pinhole transmission grating spectrometer. The spectrometer was characterized by a high linear dispersion rate (2.9 Å/mm), and from its time-integrated diffraction images, rich information about the X-pinch sources (e.g., source number, source size, and absolute spectra) could be obtained. Multiple hot spots were produced in all the Mo tests with loads made of two or four 25 μm wires with or without a shunt wire, and obvious increases both in the radiation intensity and in the source size around the spectral region of 2.6 keV were observed. In Ti X-pinch tests, a single keV x-ray burst with a source size of ∼200 μm and a time duration of ∼200 ps in full width at half maximum was obtained using a load made of two 50 μm wires plus a shunt wire. The intensity of x-rays decreased sharply from ∼1011 photon eV−1 sr−1 at 1 keV to ∼108 photon eV−1 sr−1 at 4 keV. The energy-dependent source size in the band of 1–4 keV is less than 100 μm and seemed to shrink quickly as x-ray energy increases.

Absolute Spectral Radiation Measurements From 200-ns 200-kA $X$-Pinch in 10-eV–10-keV Range With 1-ns Resolution

A series of seven X-ray diodes and four Si p-i-n detectors with K- and L-filters was employed to measure the absolute time-resolved spectra of 200-ns 200-kA molybdenum and copper X-pinch plasmas. We observe a 10-mum-size 0.4-0.7-ns X-ray source at a total power yield level of 1.5 GW with about 35% in the range above 1 keV. The extreme ultraviolet part of the spectrum can be fitted by a Planckian function with a temperature of 65-75 eV. In the region above 800 eV, the spectrum can be fitted by an exponential distribution with an effective temperature of ~1 keV for Mo X-pinch and ~500 eV for Cu plasma. The X-ray source yields 200-550 mJ in this spectral range. The total XUV and X-ray yield varies in the range 10-30 J.

Dynamics of Multiple-Wire X-Pinches

X-pinches are one of the most localized sources of X-rays and are potentially useful for a number of applications. An investigation was carried out into the plasma dynamics of a "tabletop" X-pinch plasma generated by passing a 40-ns 35-kA current pulse through two or four 5-mum-thick tungsten wires. It was found that the number of wires significantly changes the dynamics of the discharge, as evidenced by images of the emission in the visible spectral region. It was also found that increasing the number of wires enhanced the reproducibility of the discharge and the X-ray pulse.

MHD processes during the cascade development of the neck and hot spot in an X-pinch

Results are presented from two-dimensional MHD simulations of X-pinch implosion. The simulations were performed in the (r, z) and (x, y) geometries for homogeneous (dense plasma) and heterogeneous (core-corona) loads. The formation of a minidiode, the development of a neck and an X-radiating hot spot, and the influence of the plasma corona on the implosion dynamics of the dense X-pinch plasma were investigated. For through simulations, the conical neck model was used, whereas a detailed analysis of the X-ray burst was performed in the parabolic neck model. The MHD processes occurring during the implosion of oblique shock waves and the onset of instability of the plasma column were examined. It is found that, due to the quasi-periodic character of these processes, the neck compression proceeds in a cascade fashion. The plasma state in a hot spot just before the break of the neck is analyzed, and the possibility of generating fast particle beams is considered.

The importance of EBIT data for Z-pinch plasma diagnostics

The results from the last six years of X-ray spectroscopy and spectropolarimetry of high-energy density Z-pinch plasmas complemented by experiments with the electron beam ion trap (EBIT) at the Lawrence Livermore National Laboratory (LLNL) are presented. The two topics discussed are the development of M-shell X-ray W spectroscopic diagnostics and K-shell Ti spectropolarimetry of Z-pinch plasmas. The main focus is on radiation from a specific load configuration called an “X-pinch”. In this work the study of X-pinches with tungsten wires combined with wires from other, lower Z materials is reported. Utilizing data produced with the LLNL EBIT at different energies of the electron beam the theoretical prediction of line positions and intensity of M-shell W spectra were tested and calibrated. Polarization-sensitive X-pinch experiments at the University of Nevada, Reno (UNR) provide experimental evidence for the existence of strong electron beams in Ti and Mo X-pinch plasmas and motivate the development of X-ray spectropolarimetry of Z-pinch plasmas. This diagnostic is based on the measurement of spectra recorded simultaneously by two spectrometers with different sensitivity to the linear polarization of the observed lines and compared with theoretical models of polarization-dependent spectra. Polarization-dependent K-shell spectra from Ti X-pinches are presented and compared with model calculations and with spectra generated by a quasi-Maxwellian electron beam at the LLNL EBIT-II electron beam ion trap.PACS Nos.: 32.30.Rj, 52.58.Lq, 52.70.La

Radiative properties of implosions of combined X-pinches and planar wire arrays composed from different wire materials on the UNR 1 MA Z-pinch generator

The radiation from implosions of X-pinches and planar wire arrays, which consist of alloyed Al and Mo wires, on the UNR 1MA ZEBRA generator was studied. Combined, planar-loop X-pinches are good surrogate sources for studying radiative properties of Z-pinch plasmas that can be of small size, high density and high temperature as are standard X-pinches. Further, the X-pinches consist of the wires from different materials and can also exhibit distinct radiation properties. In particular, combined X-pinches studied in this work used an Al 5056 (5% Mg) wire in the anode (cathode) loop and a Mo wire in the cathode (anode) loop. Combined planar wire arrays had from 10 to 15 wires arranged in one plane and consisted of the same two wire materials as used in the X-pinches. We find that planar wire arrays radiate significant peak power. Spatially-resolved and spatially integrated X-ray spectral data as well as time integrated pinhole X-ray images were analyzed. Non-LTE kinetic models were applied to study the axial gradients of electron temperatures and densities in these X-pinch and Z-pinch plasmas. Specifically, X-ray spectra of K-shell Al and Mg, and L-shell Mo were modeled. As a result, we discuss the dependence of the plasma spatial structure, temperature, density, and opacity. The importance of using different materials or alloys for Z-pinch plasma diagnostics is illustrated.

A compact pulse generator for radiographic source supply

Radiography of short-living objects using X-pinch radiation is a new perspective direction in diagnostics developed at the P. N. Lebedev Institute of Physics. High spatial (up to a few tenth of a micrometer) and temporal resolutions (up to 0.1 ns) achieved with the use of the X pinch is extremely interesting to the researches of transient processes and substances in extreme states. The X pinch is produced by two or more crossed wires (with wire diameters up to 30 µm) exploded under the influence of current running through them. Scanning radiation is generated in a dense high-temperature plasma (up to several kiloelectron volts) arising in the region of wire crossing. Among the main requirements on the generator used to create the X pinch are current amplitudes of 150–300 kA and rates of current increase of 1–2 kA/ns. Up to now, these current pulse parameters have been provided only by bulky stationary generators weighing from 300 kg to several tons. In the present paper, a small-size (weighing 70 kg) high-current pulse generator (with current amplitude exceeding 300 kA and pulse rise time of 200 ns) of soft x-ray radiation from high-temperature X-pinch plasma is described.

Investigation of Magnetic Fields in 1-MA Wire Arrays and$X$-Pinches

A Faraday rotation diagnostic was applied for the investigation of magnetic fields in plasma of 1-MA wire arrays and X-pinches. Laser-probing diagnostics at the Zebra generator include a four-channel polarointerferometer and a four-frame shadowgraphy. The Faraday rotation diagnostic consists of shadow and Faraday channels, shearing air-wedge interferometer, and an additional schlieren channel. The implosion dynamics of the wire arrays were studied. A current in the plasma column of Al low-wire number arrays was found by the Faraday rotation diagnostic. Optical diagnostics showed a turbulent plasma and bubblelike objects in the plasma column of Al wire arrays. The Faraday rotation diagnostic demonstrated a complicated structure of magnetic fields in X-pinch plasma

Multiwire$X$-Pinches at 1-MA Current on the COBRA Pulsed-Power Generator

The rebuilt COBRA pulsed-power generator, which has a variable current-pulse waveform and amplitude (95-150-ns rise time and ~1-MA peak current), has extended the range of the current-pulse parameters that can be used to study the X-pinches. X-pinches with 4-12 wires of several different wire materials (from Al to W) with diameters from 25 to 75 mum have been studied. The influence of the rate of the rise of current and the X-pinch wire mass on the X-pinch plasma formation and pinch implosion dynamics has been studied using a set of diagnostics with spatial and/or temporal resolution. Multiwire X-pinches were placed in the diode center, and/or two four-wire X-pinches were placed in one of the four parallel return-current circuits of the diode. The experiments showed that the most important factor determining the first X-ray burst timing is the linear mass density of the X-pinch wires, and an optimal value for obtaining a single high-energy X-ray burst was found. Radiographic images of the different test objects, wires in wire-array Z-pinches, and the X-pinches, themselves, were obtained using a micrometer-scale spatial resolution

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.