Pinch Radiation Research Articles

Effect of Atomic Number on Plasma Pinch Properties and Radiative Emissions

The aim of the research is to examine the dependence of plasma pinch properties and radiation emissions on the atomic number of the operating gas within the dense plasma focus device (NX2) when using hydrogen and argon gases. Simulation was performed with Lee’s code on an NX2 dense plasma focus at a constant gas pressure value ( P 0 = 0.5 torr ). The results showed that the minimum radius of the plasma focus in the case of the hydrogen plasma pinch was 0.30 cm and in the case of the argon plasma pinch 0.17 cm, and this affected the value of the radiation emission as it was 7.8 × 10 − 6 J and 11 J for the hydrogen and argon pinch, respectively. The energy of the ion beam released by the breakdown of the plasma pinch was found as E n = 23.8 J in the state of hydrogen and E n = 105 J in the state of argon.

Coarse spectral characterization of warm x-rays at the Z facility using a filtered thermoluminescent dosimeter array

A new collimated filtered thermoluminescent dosimeter (TLD) array has been developed at the Z facility to characterize warm x-rays (hν > 10 keV) produced by Z pinch radiation sources. This array includes a Kapton debris shield assembly to protect the TLDs from the source debris, a collimator array to limit the field of view of the TLDs to the source region, a filter wheel containing filters of aluminum, copper and tungsten up to 3 mm thick to independently filter each TLD, and a hermetically sealed cassette containing the TLDs as well as tungsten shielding on the sides and back of the array to minimize scattered radiation reaching the TLDs. Experimental results from a krypton gas puff and silver wire array shot are analyzed using two different functional forms of the energy spectrum to demonstrate the ability of this diagnostic to consistently extend the upper end of the x-ray spectrum characterization from ∼50 keV to >1 MeV.

Study of the termination phase of plasma production and the formation of magnetic flux breakthroughs during wire array implosion

The phenomenon of magnetic flux breakthrough into a wire array during its implosion was studied experimentally at the Angara-5-1 facility. It is shown that breakthroughs develop in the final stage of plasma production from the wire material and occur near the initial wire position. The spatial distributions of the azimuthal magnetic field within tungsten, molybdenum, copper, and aluminum wire arrays were studied using magnetic probes. The distributions of the azimuthal magnetic field B φ(z, t) along the array height in different stages of implosion were measured, and the characteristic dimensions of regions with a nonuniform magnetic field that appear during magnetic flux breakthroughs at the outer boundary of the wire array plasma were determined. The dimensions of these regions are compared with those of the regions with depressed plasma radiation observed in frame and time-integrated X-ray images. The dynamics of the distribution B φ(z, t) in regions with a nonuniform magnetic field during breakthroughs of the azimuthal magnetic flux is compared with that of the spatial distribution of pinch radiation in the frame X-ray images in different stages of implosion. The experimental data on the characteristics of spatially nonuniform breakthroughs of the magnetic flux into the wire array are analyzed using the plasma rainstorm model proposed by V.V. Aleksandrov et al. (JETP 97, 745 (2003)). The plasma density in the region of magnetic flux breakthrough is estimated.

Study of the radiation spectra of fast Z-pinches formed during the implosion of wire arrays in the Angara-5-1 facility

Results are presented from measurements of the radiation spectra of the Z-pinch tungsten plasma produced during the implosion of cylindrical wire arrays with a linear mass of 200–400 μg/cm and an initial diameter of 12–20 mm at a current of ∼3 MA in the experiments performed at the Angara-5-1 facility. The radiation spectra in the photon energy range of 50–900 eV were recorded on a UF-4 X-ray film by using a spectrograph with a transmission grating. The radiation spectrum in the photon energy range of 1–3 keV was recorded using a crystalline panoramic spectrograph. A curtain shutter was used to protect the transmission grating from fast microparticles produced due to the erosion of high-voltage electrodes. The total radiation yield was measured with a thermocouple calorimeter. It is shown that most of the tungsten plasma radiation energy is emitted in the photon energy range of 80–300 eV. Measurements of the spectral intensity of pinch radiation with a spatial resolution along the pinch radius showed that the effective transverse diameter of the pinch did not exceed 2 mm, which agrees with independent current measurements of the pinch size. The results of measurements of the spectral intensity of pinch radiation were compared with calculations per-formed under the assumption of a stationary homogeneous plasma.

Nested multilayered X pinches for generators with mega-ampere current level

A symmetric X pinch configuration that is conducive to using large numbers of wires on ≥1 MA pulsed power generators has been tested at 1 MA. Using an initial configuration of wires before their twisting, similar to nested cylindrical wire arrays, enables a geometrically simple, compact, multilayer wire configuration at the X pinch crossing region. Multilayer X pinches with the same or different materials in the inner and outer wire layers were tested. Optimization resulted in X pinch radiation sources with peak power comparable to the most successful single layer X pinch, but with a compact, single bright X radiation source more reliably obtained using the nested configuration.

High brightness extreme ultraviolet (at 13.5 nm) emission from time-of-flight controlled discharges with coaxial fuel injection

Extreme ultraviolet (EUV) emission from discharge produced plasma with the coaxial injection of fuel vapor (tin and lithium) produced by laser ablation is experimentally studied. Multiple plasma pinches preceding a strong and long recombination radiation of EUV are observed in the first half cycle of a sinusoidal discharge current. Due to the time-of-flight control type of the discharge, the shape of pinch radiation pulses is almost identical. With the coaxial injection of time-of-flight controlled discharges, the highest brightness of EUV emission (maximum extracted energy of 244.3 mJ/2π sr per pulse with the emitter size of ∼1×0.3 mm2 in full width at half maximum) is provided with efficiency exceeding 2% of deposited energy into the plasma (or 1% of dissipated energy in the discharge) due to a much better matching with the optimal plasma parameters in the recombination regime and a decrease in the off-duty factor. Stability of emitting plasma of the repetitive pinches is essentially improved with use of a second laser pulse.

Multicolor Soft X-Ray Imaging of $Z$-Pinch Radiation Sources on the Z-Machine Pulsed-Power Driver

Time-resolved X-ray pinhole imaging is employed for diagnosing fast Z-pinch implosions on the Z accelerator. Reflection from a multilayer mirror produces monochromatic 277-eV images of the Z-pinch plasma in the final stages of implosion and stagnation. Simultaneous imaging with a filtered pinhole camera or using a second mirror can yield multicolor X-ray images which contain spectral as well as spatial information, allowing for the study of radiation physics in dense Z-pinches.

Microsecond Z-pinch as ultrasoft X-ray radiation source

The spectral characteristics of plasma radiation of a gas Z-pinch with a rise time of 1 μs and a current amplitude of 250 kA have been measured. The measurements were performed in the range of X-ray energy quanta from 30eV to 600 eV. A spectrograph with a transmission grating and a polychromator on the basis of multilayer X-ray mirrors served as the diagnostic devices. Absolutely calibrated semiconducting detectors AXUV-5 were used as radiation detectors in the polychromator. The full pinch radiation loss was measured with a thermocouple calorimeter. The radiation of He-, Li- and Be- like ions has been shown to dominate at the stage of maximum compression of the nitrogen Z-pinch. The results of the spectral measurements were employed for the absolute calibration of various X-ray detectors.

X-ray radiation of Z-pinch source

The spectral characteristics of the plasma radiation of a gas Z-pinch with a rise time of 1μs and a current amplitude of 250 kA have been measured. The measurements were performed in the range of X-ray energy quanta from 30 eV to 600 eV. A spectrograph with a transmission grating and a polychromator on the basis of multilayer X-ray mirrors served as the diagnostic device. Absolutely calibrated semi-conducting detectors AXUV-5 were used as radiation detectors in the polychromator. The full pinch radiation loss was measured with a thermocouple calorimeter. The radiation of He-, Li- and Be-like ions has been shown to dominate at the stage of maximum compression of the nitrogen Z-pinch. The results of the spectral measurements were employed for the absolute calibration of various X-ray detectors. Some of these calibration results are presented in this paper.

Soft x-ray (0.2<E<2.0 keV) imager for z-pinch plasma radiation sources

Z -pinches can produce intense fluxes of argon K-shell (3 keV) radiation, but typically only a fraction of the load mass near the axis of the pinch radiates in this spectral range. The majority of the mass does not get hot or dense enough to radiate efficiently in the K-shell. We have designed, built, and tested an instrument to image pinch emission, specifically the radial emission profile, at energies below the K-shell in order to track the location of the cooler mass. A gold mirror provides a high-energy cut-off at 2 keV while a transmission grating disperses the incoming radiation and provides a low-energy cutoff at 0.1 keV. A vertical slit images the pinch radiation in the radial direction and the emission profile is recorded with either an extreme ultraviolet-sensitive charge-coupled device camera (time-integrated) or a linear photodiode array (∼1 ns time resolution). We present results for the mirror, grating, and system characterization obtained at the Advanced Light Source synchrotron located at Lawrence Berkeley National Laboratory (Berkeley, CA).

X-ray spectral diagnostics of neon photoionization experiments on the Z-machine

We report on an initial spectroscopic study of low-density, x-ray photoionized neon with x-ray spectroscopy. These experiments, carried out on the Z-machine at Sandia, are optimized to produce a gradient-free, collisionless plasma, and to explore issues related to the rapid x-ray photoionization of relatively cold, low-density plasmas. The initial experiments used time-integrated absorption spectroscopy, backlit by the pinch radiation, to determine the ionization balance in the gas cell. Future experiments will use time-resolved spectroscopy in both absorption and emission. The emission spectra are expected to be similar to those seen from photoionized astrophysical sources, such as x-ray binaries. Indeed, in addition to addressing basic plasma and atomic physics issues, these experiments are designed to help the astrophysical community better understand the new, high-resolution spectra being produced by the Chandra and XMM-Newton telescopes, and to benchmark spectral synthesis codes.

X-ray absorption spectroscopy measurements of thin foil heating by Z-pinch radiation.

Absorption spectroscopy measurements of the time-dependent heating of thin foils exposed to intense z-pinch radiation sources are presented. These measurements and their analysis provide valuable benchmarks for, and insights into, the radiative heating of matter by x-ray sources. Z-pinch radiation sources with peak powers of up to 160 TW radiatively heated thin plastic-tamped aluminum foils to temperatures approximately 60 eV. The foils were located in open slots at the boundary of z-pinch hohlraums surrounding the pinch. Time-resolved Kalpha satellite absorption spectroscopy was used to measure the evolution of the Al ionization distribution, using a geometry in which the pinch served as the backlighter. The time-dependent pinch radius and x-ray power were monitored using framing camera, x-ray diode array, and bolometer measurements. A three-dimensional view factor code, within which one-dimensional (1D) radiation-hydrodynamics calculations were performed for each surface element in the view factor grid, was used to compute the incident and reemitted radiation flux distribution throughout the hohlraum and across the foil surface. Simulated absorption spectra were then generated by postprocessing radiation-hydrodynamics results for the foil heating using a 1D collisional-radiative code. Our simulated results were found to be in good general agreement with experimental x-ray spectra, indicating that the spectral measurements are consistent with independent measurements of the pinch power. We also discuss the sensitivity of our results to the spectrum of the radiation field incident on the foil, and the role of nonlocal thermodynamic equilibrium atomic kinetics in affecting the spectra.

Time-resolved spectroscopy of Al, Ti, and Mo X pinch radiation using an X-ray streak camera

Time-resolved spectroscopic measurements of the radiation emitted from Al, Ti, and Mo X pinches have been made with <10 ps time resolution. The radiation is emitted from micropinch plasmas with sizes of order 1 μm in times in the 10– 100 ps range. Spectra implied that dense, ∼1 keV plasmas were produced, such as a 90 ps lifetime, 1.5– 1.8 keV electron temperature and near solid-density Ti plasma. The experimental systems and analysis methods are described in detail, including line ratio calculations for μm-scale Ti and Al plasmas with ion densities of 10 19–10 24 cm −3 and ∼1 keV electron temperatures.

Radiation science using Z-pinch x rays

Present-day Z-pinch experiments generate 200 TW peak power, 5–10 ns duration x-ray bursts that provide new possibilities to advance radiation science. The experiments support both the underlying atomic and plasma physics, as well as inertial confinement fusion and astrophysics applications. A typical configuration consists of a sample located 1–10 cm away from the pinch, where it is heated to 10–100 eV temperatures by the pinch radiation. The spectrally-resolved sample-plasma absorption is measured by aiming x-ray spectrographs through the sample at the pinch. The pinch plasma thus both heats the sample and serves as a backlighter. Opacity measurements with this source are promising because of the large sample size, the relatively long radiation duration, and the possibility to measure opacities at temperatures above 100 eV. Initial opacity experiments are under way with CH-tamped NaBr foil samples. The Na serves as a thermometer and absorption spectra are recorded to determine the opacity of Br with a partially-filled M-shell. The large sample size and brightness of the Z pinch as a backlighter are also exploited in a novel method measuring re-emission from radiation-heated gold plasmas. The method uses a CH-tamped layered foil with Al+MgF2 facing the radiation source. A gold backing layer that covers a portion of the foil absorbs radiation from the source and provides re-emission that further heats the Al+MgF2. The Al and Mg heating is measured using space-resolved Kα absorption spectroscopy and the difference between the two regions enables a determination of the gold re-emission. Measurements are also performed at lower densities where photoionization is expected to dominate over collisions. Absorption spectra have been obtained for both Ne-like Fe and He-like Ne, confirming production of the relevant charge states needed to benchmark atomic kinetics models. Refinement of the methods described here is in progress to address multiple issues for radiation science.

Diagnostics on Z (invited)

The 100 ns, 20 MA pinch-driver Z is surrounded by an extensive set of diagnostics. There are nine radial lines of sight set at 12° above horizontal and each of these may be equipped with up to five diagnostic ports. Instruments routinely fielded viewing the pinch from the side with these ports include x-ray diode arrays, photoconducting detector arrays, bolometers, transmission grating spectrometers, time-resolved x-ray pinhole cameras, x-ray crystal spectrometers, calorimeters, silicon photodiodes, and neutron detectors. A diagnostic package fielded on axis for viewing internal pinch radiation consists of nine lines of sight. This package accommodates virtually the same diagnostics as the radial ports. Other diagnostics not fielded on the axial or radial ports include current B-dot monitors, filtered x-ray scintillators coupled by fiber optics to streak cameras, streaked visible spectroscopy, velocity interferometric system for any reflector, bremsstrahlung cameras, and active shock breakout measurement of hohlraum temperature. The data acquisition system is capable of recording up to 500 channels and the data from each shot is available on the Internet. A major new diagnostic presently under construction is the BEAMLET backlighter. We will briefly describe each of these diagnostics and present some of the highest-quality data from them.

Simulating the radiation environment in Z experiments using a 3D view factor code with 1D radiation-hydrodynamics emission modeling

We present an approach for simulating the multidimensional radiation environment in Z-pinch hohlraums at the Sandia Z facility. In this approach we use a three-dimensional (3D) view factor code to compute the incident flux distribution onto all surface elements of the view factor grid. At each surface element, the frequency-dependent re-emission is computed by performing a one-dimensional (1D) radiation-hydrodynamics simulation for the element. The pinch radiation source is treated as a uniformly emitting cylinder, with a size and emission flux given by the experimentally measured time-dependent radius and radiation power. Here, we describe our modeling procedure, and present results from simulations of Z experiments in which a plastic-tamped aluminum foil was located in an open slot in the Z-pinch hohlraum (i.e., the return current structure). We also discuss the sensitivity of the Al plasma conditions to the incident spectrum onto the sample and the re-emission characteristics of the hohlraum walls.

Streaked x-ray diagnostic for Double EAGLE z-pinch radiation sources

Gated-microchannel-plate cameras have primarily been used for time-resolved x-ray measurements on high current z pinches because they are compact and unaffected by magnetic fields and hard x rays. X-ray streak cameras are relatively large, and sensitive to magnetic fields, but they provide continuous time records with subnanosecond resolutions. We have demonstrated that an x-ray streak camera can be fielded successfully at the multimegampere Double EAGLE facility. We describe initial spectroscopy and imaging measurements and the vacuum housing and rf shielding developed to field the camera.

XUV emission of nitrogen-puff Z-pinch

In order to increase X-ray output from the K shells of a highly ionized plasma in a small gas-puff Z-pinch, work with nitrogen or carbon instead of neon or argon is suggested. Basic measurements of the radiation of such a plasma, made by vacuum X-ray diodes and thin filters, are presented. Two working regimes with nitrogen at pinch currents 160 and 200 kA, with different contributions of line and continuous spectra in the K shell output, are discussed. It is shown that at a typical energy input of small Z-pinches (several kilo Joules) the suggested lighter elements can be ionized to He-like and H-like states not only in the hot spots, as has been observed with neon and argon, but also in the bulk. The yield of nitrogen K-shell radiation (about 10 J per shot) is therefore substantially higher than that of neon. Besides the pinch physics and radiation dynamics, the K-shell emission of the lighter elements may also be interesting in X-ray sources in the spectral region of a 'water window'.

Observation of directed bremsstrahlung from a hollow cathode plasma

Directed bremsstrahlung emitted from a pinched hydrogen plasma source ( T e = 9 keV) is observed in pulsed power driven pseudo spark discharges. The source is located on the discharge axis at the exit of the hollow cathode; the volume of the source is below 1 mm 3. The emission half angles are 2°, 4° and 14°, depending on gas pressure and preionization before the discharge; the central axis of the emission cone is identical to the discharge axis. The 9 keV X-ray source is embedded in a pinch having a temperature of 1.5 keV. The onset of the pinch radiation follows the Bennett relation. The X-ray emissions obey the Kramer law of a radiating fully ionized plasma.

Evaluation of the gas puff z pinch as an x-ray lithography and microscopy source

Soft x rays (100–10 000 eV), due to their short wavelength (0.1–10 nm) can play an important role in high resolution microscopy and lithography. The gas puff Z pinch is an intense source of soft x rays. Calorimeter and x ray diode measurements showed that 10% of the stored electrical energy was converted to radiation in the range of 1–10 nm. Commercial photoresist polymethyl methacrylate (PMMA) and some new resists—CR 39, nitrocellulose, were exposed to the pinch radiation. The developed images on the resists have been studied with a scanning electron microscope. The resolution was found to be source limited, but a simple modification can improve the resolution by more than an order of magnitude.