Plasma Radiation Source Research Articles

Neutron Emission Characteristics of NX-3 Plasma Focus Device: Speed Factor as the Guiding Rule for Yield Optimization

This paper reports the results of characterization and optimization experiments carried out on a newly developed NX-3 dense plasma focus device (20 kJ at 20 kV, a quarter time period of ~ 3 μs, and 10 kJ/600 kA at 14 kV) at the Plasma Radiation Source Laboratory, NIE, Nanyang Technological University, Singapore. Initial experiments were conducted with an electrode assembly configuration having anode radius and length of 20 and 160 mm, respectively, for detailed neutron emission characterization of NX-3 device followed by further optimization of neutron yield using various other electrode configurations designed using the Lee Code. At ≥10-kJ operation, the average neutron yield on the order of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> neutrons/pulse in 4πsr was obtained for the deuterium filling gas pressure range of 6-8 mbar. The neutron yield of ~ 4.6 ×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> neutrons/pulse at 10 kJ/6 mbar is the highest ever reported for a device with the same stored energy. The neutron anisotropy measurements point to the beam-target mechanism as the dominant neutron production mechanism for NX-3 plasma focus device. Further optimization of neutron yield in NX-3 was achieved with the peak average neutron yield being enhanced from ~ (2.38 ±0.31) ×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> neutrons/shot for the initial electrode configuration to about ~ (3.40 ±0.43) ×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> neutrons/shot for the electrode configuration with anode radius and length of 26 and 140 mm, respectively. The analysis of neutron yield results for various electrode assembly configurations demonstrates the speed factor as a key optimization tool for maximization of neutron yield.

Determination of arsenic and selenium by hydride generation and headspace solid phase microextraction coupled with optical emission spectrometry

A hydride generation headspace solid phase microextraction technique has been developed in combination with optical emission spectrometry for determination of total arsenic and selenium. Hydrides were generated in a 10 mL volume septum-sealed vial and subsequently collected onto a polydimethylsiloxane/Carboxen solid phase microextraction fiber from the headspace of sample solution. After completion of the sorption, the fiber was transferred into a thermal desorption unit and the analytes were vaporized and directly introduced into argon inductively coupled plasma or helium microwave induced plasma radiation source. Experimental conditions of hydride formation reaction as well as sorption and desorption of analytes have been optimized showing the significant effect of the type of the solid phase microextraction fiber coating, the sorption time and hydrochloric acid concentration of the sample solution on analytical characteristics of the method developed. The limits of detection of arsenic and selenium were 0.1 and 0.8 ng mL − 1 , respectively. The limit of detection of selenium could be improved further using biosorption with baker's yeast Saccharomyces cerevisiae for analyte preconcentration. The technique was applied for the determination of total As and Se in real samples.

The Electromagnetic Instability in Electron Flow with Ion Background

Electromagnetic (EM) behavior and instability resulting from the interaction between EM wave and plasma wave are analyzed based on linear perturbation theory. It is shown that the instability is caused by the the coupling between high frequency electromagnetic field and electron transverse oscillation derived from the deflection of electron longitudinal oscillation due to self-produced magnetic field. The influences of the self-produced magnetic field and plasma density on the instability are studied. In addition, characteristics of EM wave propagation at different angles are investigated. The present results are of significance to new type plasma radiation source, ion accelerator and plasma diagnostic techniques.

Xenon Discharge-Produced Plasma Radiation Source for EUV Lithography

Extreme ultraviolet (EUV) radiation with wavelengths of 11-14 nm is seen as the most promising candidate for a new lithographic technology. Compared with synchrotron radiation sources and laser-produced plasmas, gas discharge-produced plasma sources for EUV radiation are expected to offer lower cost of ownership. Using xenon, a broadband emission in the investigated wavelength range from 10 to 17 nm is observed. Very short current pulses, having a fast rise time of 85- or 140-ns duration and 23-kA amplitude, were applied across the xenon-filled Z-pinch capillary (3-mm diameter and 5-mm length) to produce EUV radiation. An EUV radiation from the Z-pinch plasma was characterized, which is based on the temporal behavior of EUV intensity and the pinhole images. Two maximum EUV radiations occur, which are sensitive to the xenon flow rate and the discharge current. The first radiation is relatively of short duration, while the second radiation lasts as long as the first period of the current flows. The EUV source size due to the first radiation is approximately 300 μm, i.e., half of the one due to the second radiation.

Theory of High-Energy-Photon K-Shell Recombination Continuum Radiation From $Z$-Pinch Plasmas

This paper presents explicit analytical formulas relating the X-ray yields in various energy bins of the free-bound recombination continuum to the mass, temperature, dimensions, and confinement time of the plasma participating in the K-shell emission. Estimates based on these formulas support the idea that K-shell recombination continuum radiation of argon, titanium, and iron can be a viable alternative to the K-shell line radiation of higher atomic number ions for advancing the operation of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Z</i> -pinch plasma radiation sources into the warm X-ray photon energy range of 10-20 keV. The potential of the refurbished Z accelerator for generating multikilojoule radiation yields in this energy range via recombination continuum is assessed.

Measurement of Recombination Coefficient of Atomic Oxygen

We developed the equipment that measured the recombination coefficient of the atomic oxygen from heat balance of the temperature sensor referring to the method of measuring the recombination coefficient of the atomic hydrogen1). We added the following two improvements to the method for calculating the recombination coefficient of hydrogen1): Firstly, to suppress the influence of thermal radiation from a plasma radiation source as much as possible, we set the sensor to the position that the sensor was not exposed to the radiation from plasma directly. that the sensor Secondly, we made it possible for sensors to measure higher temperatures in some areas. The source of the oxygen radicals was microwave plasma operating at 900 W for O2 pressures of 126 Pa. The measured atomic oxygen recombination coefficients on quartz and chemically polished aluminum alloy (ALpika®) surfaces were 1.94×10−4 and 3.75×10−3, respectively.

Multi-dimensional radiation transport for modeling axisymmetric Z pinches: Ray tracing compared to Monte Carlo solutions for a two-level atom

Radiation plays a critical role in the dynamical evolution and energetics of Z pinches and a dominant role for those loads designed as radiation sources in the keV photon energy region. Therefore, in modeling such plasmas it is essential to employ an accurate and realistic calculation of radiation generation and transport. This study presents a straightforward method to carry out three-dimensional radiation transport calculations in an azimuthally symmetric cylindrical geometry for use in magnetohydrodynamic simulations of high energy Z-pinch plasma radiation sources. The present approach is based on the integral solution of the radiative transfer equation using long characteristics with equal weight angular quadratures, and uses either a multifrequency grid or probability-of-escape-based zone-to-zone coupling. Using previously published Monte Carlo calculations of a two-level atom in cylindrical geometry as a benchmark, the accuracy of the approach is assessed as a function of the frequency grid, number of rays employed, and spatial grid. An analytic expression for the frequency averaged escape probability of an emission line is found to be as accurate for the population kinetics as the multifrequency method. While the escape probability description sacrifices information on the line profile, it is far more computationally efficient yet sufficiently accurate for line dominated plasmas such as Z-pinch radiation sources.

Interaction between electromagnetic waves and plasma waves in motional plasma

The electromagnetic wave (EM wave) behavior and the electromagnetic instability caused by the interaction between an EM wave and a plasma wave in motional plasma are studied. The dispersion relation of EM waves and the dielectric tensor of motional plasma are derived by magnetohydrodynamics, and the wave phenomenon in motional plasma is displayed. As a result, the electromagnetic instability, which is excited by the interaction between the EM waves and the plasma waves, is revealed. The mechanism of the instability is the coupling between high frequency electromagnetic field and the transverse electron oscillation derived from the deflection of longitudinal electron oscillation due to self-magnetic field. The present research is useful with regard to the new type of plasma radiation source, ion-focusing accelerator, and plasma diagnostic technique.

Double planar wire array as a compact plasma radiation source

Magnetically compressed plasmas initiated by a double planar wire array (DPWA) are efficient radiation sources. The two rows in a DPWA implode independently and then merge together at stagnation producing soft x-ray yields and powers of up to 11.5kJ∕cm and more than 0.4TW∕cm, higher than other planar arrays or low wire-number cylindrical arrays on the 1MA Zebra generator. DPWA, where precursors form in two stages, produce a shaped radiation pulse and radiate more energy in the main burst than estimates of implosion kinetic energy. High radiation efficiency, compact size (as small as 3–5mm wide), and pulse shaping show that the DPWA is a potential candidate for ICF and radiation physics research.

Z-pinch plasma neutron sources

A deuterium gas-puff load imploded by a multi-MA current driver from a large initial diameter could be a powerful source of fusion neutrons, a plasma neutron source (PNS). Unlike the beam-target neutrons produced in Z-pinch plasmas in the 1950s and deuterium-fiber experiments in the 1980s, the neutrons generated in deuterium gas-puffs with current levels achieved in recent experiments on the Z facility at Sandia National Laboratories could contain a substantial fraction of thermonuclear origin. For recent deuterium gas-puff shots on Z, our analytic estimates and one- and two-dimensional simulations predict thermal neutron yields ∼3×1013, in fair agreement with the yields recently measured on Z [C. A. Coverdale et al., Phys. Plasmas (to be published)]. It is demonstrated that the hypothesis of a beam-target origin of the observed fusion neutrons implies a very high Z-pinch-driver-to-fast-ions energy transfer efficiency, 5 to 10%, which would make a multi-MA deuterium Z-pinch the most efficient light-ion accelerator. No matter what mechanism is eventually determined to be responsible for generating fusion neutrons in deuterium gas-puff shots on Z, the deuterium neutron yield is shown to scale as Yn∼Im4, where Im is the peak current of the pinch. Theoretical estimates and numerical modeling of deuterium gas-puff implosions demonstrate that the yields of thermonuclear fusion neutrons that can be produced on ZR and the next-generation machines are sufficiently high to make PNS the most powerful, cost- and energy-efficient laboratory sources of 2.5−14MeV fusion neutrons, just like plasma radiation sources are the most powerful sources of soft and keV x rays. In particular, the predicted deuterium-tritium thermal neutron-producing capability of PNS driven by the next-generation ZR and ZX accelerators is ∼5×1016 and ∼1018, respectively.

Characterization of the measurement uncertainty of a laboratory EUV reflectometer for large optics

Quality assurance for the production of optical components for extreme ultraviolet lithography (EUVL) strongly requires at-wavelength metrology. Initially, the at-wavelength characterization of mirrors and masks was developed using synchrotron radiation of electron storage rings, e.g. BESSY II. For the production process of EUV optics, however, the immediate access to metrology tools is necessary and the availability of laboratory devices is mandatory. Recently, an EUV reflectometer (EUVR) for large samples has been put into operation. It consists of a laser-produced plasma (LPP) radiation source, a monochromator and a large goniometer system. It can handle samples with diameters of up to 500 mm, thicknesses of up to 200 mm and weights of up to 30 kg. The wavelength can be varied from 10 nm to 16 nm. The spot size on the sample surface is about 2 mm in both directions. The angle of incidence can be varied from 3° to grazing incidence. In this paper, we describe the measurement uncertainties achieved in the EUVR with reference to measurements by the Physikalisch-Technische Bundesanstalt at its soft x-ray radiometry beamline at BESSY II. Using appropriate reference points for the wavelength calibration of the monochromator, the absolute wavelength uncertainty is below 4 pm in the spectral range from 11 nm to 15 nm. The measured peak reflectance of multilayer mirrors shows a constant offset of about 1% absolute and is reproduced within 0.2%.

How to Maximize the Radius of a Wire-Array$Z$-Pinch for the 100-ns to 1-$muhboxs$Implosion Time Scales

The Centre d'Etudes de Gramat is now studying long implosion time (about 1 mus) plasma radiation sources. Z-pinches are routinely used by other laboratories in the 100-ns regime with the well-known experimental result, in which an about 2-cm initial radius should not be exceeded to preserve the quality of the implosion for a single array. In a many hundreds of nanoseconds implosion time scale, the question is raised if a bigger radius could be shot. This paper discusses a theoretical approach, which shows that a 7-cm radius is acceptable in the 1-mus regime. It is based on an analysis of magneto-Rayleigh-Taylor instabilities of the accelerating sheath in the r-z plane. It will be demonstrated that all the spatial features of the implosion are determined by the magnetic skin depth, and so essentially depend on the square root of the implosion time. Experiments were carried out with SPHINX generator at the 4-MA level with 800-ns implosion time and confirmed these results

Enhancement of x-ray yield from the Z-Beamlet laser for monochromatic backlighting by using a prepulse

The Z-Beamlet Laser (ZBL) is capable of providing on-target energies of up to 1.5kJ at 527nm in up to four separate 0.3–1.5ns pulses during a 20ns window. ZBL is routinely used as a source of x rays for backlighting experiments on the Sandia Z facility, a 20MA, 100ns rise-time, pulsed-power driver for z-pinch plasma radiation sources. Most backlighting experiments use monochromatic crystal imaging diagnostics at 1865 or 6151eV. We present calibration data demonstrating that the use of a 0.3–0.6ns, ∼200J pulse, followed 2ns later by a 1.0ns, ∼1kJ pulse results in more than twice the x-ray yield at 6151eV (a He-like Mn emission line) compared to a single 1.0ns, ∼1kJ pulse. The first pulse creates a plasma (and few x rays) that expands and approaches the critical density for the laser when the second pulse arrives, creating a more efficient coupling of laser light to the plasma. A similar improvement was also noted for He-like Ni emission lines, suggesting that this simple technique scales to higher photon energies.

A Trend in Pulse-Powered Plasma Radiation Source

Recent progress of semiconductor power devices has shifted our interests on pulsed-power energy driver from a huge energetic system to a high-average-power compact generator. Intense extreme ultraviolet radiation sources have been developed using those devices. It is considered that they can be the light sources for microscopy and the next generation microlithography. A trend and a prospect for the pulse powered radiation sources are briefly described.

Formation of tight plasma pinches and generation of high-power soft x-ray radiation pulses in fast Z-pinch implosions

Argon K-shell plasma radiation source experiments were carried out on the GIT-12 generator [Bugaev, S.P. et al., 1997, Russian Phys. Journal, 40, 38] for a long (300 ns) implosion regime. The performance of a shell-on-solid-fill double gas puff was characterized in the experiments with and without an external axial magnetic field. The maximum Ar K-shell radiation yield registered in the experiments without an axial magnetic field was at the level of 1100 J/cm. This yield is consistent with the theoretically predicted yield for a short (100 ns) implosion regime. The experiments showed that the initial magnetic field which provides stabilization of the shell-on-solid-fill double gas puff was lower than that required for stabilization of a single annular gas puff. Satisfactory stabilization of the double gas puff was observed at an initial axial magnetic field of 1.4 kG. The maximum Ar K-shell radiation yield registered in the experiments with the axial magnetic field did not exceed 400 J/cm. A sharp reduction of the K-shell yield cannot be explained only by taking into account the energy losses associated with the compression of the axial magnetic field.

Fast discharge energy storage development for advanced X-ray simulators

Design studies have been completed to investigate the impact of improvements in fast energy storage systems on the designs of: larger future simulators (such as a 15-MA plasma radiation source (PRS) simulator), simulator upgrades of operational machines (such as Double-EAGLE), and for very compact, smaller simulators. The fast energy storage system that has been investigated and is presently under development is a fast Marx generator (FMG) with inductance capacitance (LC) 1/2=200 ns and LC 1/2=300 ns, depending on the capacitance per stage. This new fast Marx energy storage system uses newly developed, low-inductance rail switches and low-inductance capacitors. These components are configured in a low-inductance FMG stage and then stacked in series to form a unit for the voltage required and a number of units in parallel for the required system inductance and stored energy. A four-stage fast Marx prototype has been demonstrated with a total of 60-kJ energy stored and an output voltage of 680 kV. This new FMG technology will provide the capability to build X-ray machines in a significantly more compact configuration. The new FMG technology minimizes or eliminates the need for storing the energy in a large water transfer capacitor. A design sketch of a 15-MA PRS machine driven by a fast Marx will be presented. This generator would consist of 48 eight-stage FMG units and would drive the PRS directly without further pulse compression. We will also present the concept of a high voltage (2-3 MV), compact X-ray machine that uses a nine-stage fast Marx module to directly charge a vacuum inductive store. A plasma opening switch (POS) is used to switch the inductive store and deliver the electron beam to the load.

Ultra Fast Shutter Driven by Pulsed High Current

Radiation simulation utilizing plasma radiation sources (PRS) generates a large number of undesirable debris, which may damage the expensive diagnosing detectors. An ultra fast shutter (UFS) driven by pulsed high current can erect a physical barrier to the slowly moving debris after allowing the passage of x-ray photons. The UFS consists of a pair of thin metal foils twisting the parallel axes in a Nylon cassette, compressed with an outer magnetic field, generated from a fast capacitor bank, discharging into a single turn loop. A typical capacitor bank is of 7.5 μF charging voltages varying from 30 kV to 45 kV, with corresponding currents of approximately 90 kA to 140 kA and discharging current periods of approximately 13.1 μs. A shutter closing time as fast as 38 microseconds has been obtained with an aluminium foil thickness of 100 micrometers and a cross-sectional area of 15 mm by 20 mm. The design, construction and the expressions of the valve-closing time of the UFS are presented along with the measured results of valve-closing velocities.

Heating of on-axis plasma heating for keV X-ray production with Z-pinches

We discuss a new opportunity of using Z-pinch plasma radiation sources for generating Ar K-shell radiation and harder keV quanta. Our approach to keV X-ray generation is based upon an analogy with laser fusion, where the imploding shell compressionally heats the low-density inner mass. The suggested design of a Z-pinch load consists then of one or two heavy outer shell(s) with a lower mass on-axis fill (i.e., central gas jet) producing most of the radiation. The outer shell is not supposed to radiate and thus does not need to have high specific energy characterized by the large /spl eta/ parameter (Whitney et al., 1990). Thus, the heavy outer shell does not need to have a very large initial diameter for its implosion to be matched to the long-pulse current driver. Rather, we want to have a large amount of energy from the driver coupled to this shell by the moment when the shell collides with the low-density fill and eventually converts much of this energy to the thermal energy of the on-axis plasma. This configuration is investigated numerically in the framework of a one-dimensional radiation-magneto-hydrodynamics model for the case of Ar K-shell radiators. It is demonstrated that the Ar fill is heated in two stages. The first stage corresponds to the shock heating and thermal conduction in an initially low-density fill, and it allows preheating the fill while avoiding significant losses in soft radiation. The fill radiator is then compressed quasi-adiabatically and is heated-up to the temperature optimum for K-shell quanta generation. Diffusion of the driving magnetic field is shown to always suppress the conductive heat losses from the hot on-axis plasma to the cold outer shell. Absorption of the K-lines emitted near the axis in the surrounding plasma could be avoided by filling the outer shell with a different gas (like N-on-Ar), which allows a substantial increase in the observed keV X-ray radiation yields.

High resolution x-ray absorption spectroscopy using a laser plasma radiation source

In this article we present a setup of a spectrometer for near edge x-ray absorption spectroscopy at the carbon K-edge (284 eV). The spectrometer works with a laser produced plasma as x-ray source and a single x-ray optical element in grazing incidence configuration called off-axis reflection zone plate. Using different sample foils we were able to demonstrate a spectral resolution of E/ΔE≈600. The positions of specific absorption peaks are in agreement with data measured at synchrotron sources. Since the off-axis zone plate is a focusing system, the x-ray flux in the detector plane is high enough to record an absorption spectrum on a charge-coupled device detector with a single shot. In the single shot modus a spectrum can be recorded before a sample degradation takes place. This will enable time-resolved experiments using pump-probe techniques.

Soft x-ray (0.2&amp;lt;E&amp;lt;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).