Ross Filter Research Articles

Characterization of bright betatron radiation generated by direct laser acceleration of electrons in plasma of near critical density

Directed x-rays produced in the interaction of sub-picosecond laser pulses of moderate relativistic intensity with plasma of near-critical density are investigated. Synchrotron-like (betatron) radiation occurs in the process of direct laser acceleration (DLA) of electrons in a relativistic laser channel when the electrons undergo transverse betatron oscillations in self-generated quasi-static electric and magnetic fields. In an experiment at the PHELIX laser system, high-current directed beams of DLA electrons with a mean energy ten times higher than the ponderomotive potential and maximum energy up to 100 MeV were measured at 1019 W/cm2 laser intensity. The spectrum of directed x-rays in the range of 5–60 keV was evaluated using two sets of Ross filters placed at 0° and 10° to the laser pulse propagation axis. The differential x-ray absorption method allowed for absolute measurements of the angular-dependent photon fluence. We report 1013 photons/sr with energies >5 keV measured at 0° to the laser axis and a brilliance of 1021 photons s−1 mm−2 mrad−2 (0.1%BW)−1. The angular distribution of the emission has an FWHM of 14°–16°. Thanks to the ultra-high photon fluence, point-like radiation source, and ultra-short emission time, DLA-based keV backlighters are promising for various applications in high-energy-density research with kilojoule petawatt-class laser facilities.

Elemental Contrast X-ray Tomography Using Ross Filter Pairs with a Polychromatic Laboratory Source

The majority of current laboratory based X-ray sources are polychromatic and are not tuneable. This lack of monochromaticity limits the range of applications for these sources and in particular it reduces the elemental specificity of laboratory based X-ray imaging experiments. Here we present a solution to this problem based on the use of Ross filter pairs. Although such Ross filter arrangements have been applied in proof-of-principle spectroscopy experiments, to date there have been no reports of this approach used for full-field X-ray imaging. Here we report on the experimental demonstration of Ross filter pairs being used for quasi-monochromatic, full-field imaging. This arrangement has several important benefits for laboratory based X-ray imaging including, as we demonstrate, elemental contrast enhancement. The method is demonstrated both for two-dimensional radiography and for three-dimensional X-ray tomography.

罗斯滤片法测量激光打靶X射线辐射通量

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Measurement of intense X-Ray spectra: Methods based on CXR and Ross filter

The interaction between X-ray radiation and the material of secondary targets is simulated using GEANT4 for two methods of measurements of intense X-ray spectra. In the first case, characteristic X-ray radiation (CXR) from the secondary target to the back hemisphere is detected; in the second case, primary radiation passing through pairs of foils (a Ross filter) is registered. Using Ross filters and the CXR technique, the primary spectra of X-ray radiation are recovered. Two methods for recovery primary radiation spectra are compared.

Imaging of high-energy x-ray emission from cryogenic thermonuclear fuel implosions on the NIF

Accurately assessing and optimizing the implosion performance of inertial confinement fusion capsules is a crucial step to achieving ignition on the NIF. We have applied differential filtering (matched Ross filter pairs) to provide broadband time-integrated absolute x-ray self-emission images of the imploded core of cryogenic layered implosions. This diagnostic measures the temperature- and density-sensitive bremsstrahlung emission and provides estimates of hot spot mass, mix mass, and pressure.

Development of a short duration backlit pinhole for radiography on the National Ignition Facility

Experiments on the National Ignition Facility (NIF) will require bright, short duration, near-monochromatic x-ray backlighters for radiographic diagnosis of many high-energy density systems. This paper details a vanadium pinhole backlighter producing (1.8±0.5)×10(15) x-ray photons into 4π sr near the vanadium He-like characteristic x-ray energy of 5.18 keV. The x-ray yield was quantified from a set of Ross filters imaged to a calibrated image plate, with the Dante diagnostic used to confirm the quasimonochromatic nature of the spectrum produced. Additionally, an x-ray film image shows a source-limited image resolution of 26 μm from a 20 μm diameter pinhole.

Pseudomonoenergetic x‐ray diffraction measurements using balanced filters for coherent‐scatter computed tomography

Coherent-scatter computed tomography (CSCT) is a method of "composition" imaging based on measurements of diffraction patterns from tissues. Use of an x-ray tube degrades scatter pattern angular resolution due to the x-ray spectral width, making it difficult to uniquely identify some materials. The use of two transmission filters with similar atomic numbers (balanced "Ross filters") to generate pseudomonoenergetic scatter patterns is described as it applies to CSCT. An analysis of angular-blur mechanisms reveals that focal spot size and beam width are the most important factors determining Bragg-peak width when Er-Tm filters are used. A relative RMS spectral width of 1% can be achieved in the difference spectrum and a Bragg-peak RMS angular width of approximately 0.14 degree (relative width of 3% at 5 degrees scatter angle) can be achieved with an effective energy of 58 keV.

Correlations between Argon Ion Beams and X-Rays in a Plasma Focus

We investigated the characteristics of X-rays and ions generated in a argon plasma focus discharge, and analyzed the correlation between X-ray emissions and ion emissions using the time of flight method composed with a Faraday cup, a capacitive current monitor, and p–i–n diodes covered with Ross filters. We obtained the energy spectra of argon ion beams, and observed X-rays of Ar Kα line and Cu Kα line. The optimum pressure range for ion emissions agreed with that for X-ray emissions, and ion energy distributions were shifted toward higher energies when X-ray emissions were good, which means the enhancement of the ion acceleration due to the mechanism of X-ray emission. The observed time correlations can not be explained by conventional ion acceleration models, but by a hybrid model describing the electric field induced by the moving current distribution, an anomalous resistivity, and the Hall effect.

Mixed gas Z pinch experiments using a shell-on-shell nozzle on Double-EAGLE

A series of Z pinch experiments using argon and krypton was conducted on the Double-EAGLE pulse power driver at 3.5 to 4.0 MA peak current and 170-190 ns implosion time. A shell-on-shell nozzle provided the opportunity to separate the two gases and to control which was driven more strongly (by virtue of being in the inner plenum). With argon in the inner plenum, 12 to 16/spl plusmn/3 kJ krypton L-shell and 8 to 10/spl plusmn/3 kJ of argon K-shell radiation was produced. With krypton in the inner plenum, 23.6 /spl plusmn/2.5 kJ of krypton L-shell and 2.5/spl plusmn/2.3 kJ of argon K-shell radiation were produced. Since the optimum implosion times for the two gases were different, changing the mass of the z pinch varied the ratio of the yields. Using a streak spectrograph and PCDs with a Ross filter pair, the time history of the krypton and the argon radiation could be distinguished and seen to be simultaneous. Spectroscopic measurements of a chlorine dopant in the inner shell gas demonstrated that the implosions with krypton achieved lower density and electron temperature than with argon whether the krypton was in the inner or outer plenum.

X-ray Ross filter method for impurity transport studies on DIII-D (abstract)

The injection of Ar into the region of the DIII-D divertor is a promising technique for energy dissipation (through radiation and collisions) and consequently for reduction of the heat load on the plates. An important problem related to this technique, is the inherent poisoning of the core plasma by migrating Ar. The Ar core contamination seems also to improve the thermal transport in an advanced operating mode of the tokamak. It is therefore of great importance to measure the evolution of the impurity concentration profile within the core plasma. This goal could be achieved by using the Ross filter method in conjunction with the existing x-ray diagnostics on DIII-D. A basic Ross filter system consists of two identical detectors placed behind two different x-ray absorbing foils looking at the same plasma volume. The foils are made of different elements or compounds with adjacent or nearly adjacent atomic numbers. Their accurate thickness causes the x-ray transmission curves of the two foils to be effectively identical over the entire energy range except within the narrow region between their absorption edges. Since the transmission characteristics of the foils above and below their absorption edges are the same, any difference in the two detected signals is proportional to the total x-ray power of the emission spectrum between these two edge energies. An x-ray Ross filter with its energy pass band centered on the Ar XVII Kα line at 3.14 keV has been designed. This allows for the discrimination of the Ar Kα line only, regardless of Ar ionization state, against any background radiation with energies outside the energy pass band. The Ross filter was installed in front of two of the fan shaped poloidal x-ray arrays on DIII-D. The first measurements showed very good discrimination against Ne, another injected impurity. Emissivity profile evolution of the Kα lines and Ar enhanced continuum within the energy pass band of the Ross filter can be determined from the x-ray brightness signals by inverting techniques and by using the Te, ne, and Ar16+ profiles as measured by other diagnostics. The transport code MIST1 can be used to calculate both the emissivity profiles of the Kα of all the ions and their concentration profiles when the measured Te, and ne are used as input. The Ar16+ profiles as measured by charge exchange spectroscopy can be used as a constraint for the MIST code to accurately calculate the Ar18+ profile and thus unfold all the Ar ions Kα emissivity profiles. From these one can determine the Ar concentration profile evolution and the particle diffusion coefficient. In conclusion, using the Ross filter method with the existing x-ray imaging systems results in a powerful and cost-effective diagnostic for impurity transport studies.

Argon Kα measurement on DIII–D by Ross filters technique (abstract)

Techniques to reduce the heat flux to the divertor plates in tokamak power plants and the consequent erosion of, and subsequent damage to the divertor target plates include the injection of impurities such as argon, that can dissipate the energy (through radiative or collisional processes) before it reaches the target plates. An important issue with this type of scheme is poisoning of the plasma core by the impurities introduced in the divertor region. Subsequently, there is a desire to measure the profiles of the injected impurities in the core. X-ray Ross filters with an effective narrow band pass centered on the argon Kα line at 3.2 keV, have been installed on two of the existing x-ray arrays on DIII–D in order to help determine the argon concentration profiles. Emissivity profiles of the Kα lines and the emissivity profiles for the argon enhanced continuum can be inferred from the inverted filtered x-ray brightness signals if Te, ne, and Ar18+ profiles are known. The MISTReference 1 code is used to couple the filtered x-ray signals to the time dependent measurements of Te and ne. Further, the Ar16+ profiles measured by charge transfer spectroscopy, are used as a constraint on the MIST code runs to calculate Ar18+ profiles and unfold the argon emissivity profiles. A discussion of the Ross filters, the DIII–D argon data, and the data analysis scheme for inferring argon emissivity profiles will be discussed. Estimates of the total argon concentration in the core determined from this technique in DIII–D argon puff experiments will be presented.

Design of an optimal Ross filter system for x-ray spectra measurements in the range of 8.98–88 keV

This article describes wide-band Ross filter spectrometers for short-pulse plasma devices. The spectrometer consists of a number of channels, each representing a Ross filter pair in conjunction with x-ray detectors. Sensitivity bands of these channels cover the energy interval of interest without gaps. We consider as optimal the spectrometer whose channels have constant response within their sensitivity ranges and minimal response outside them. The main innovation is the addition of light material into one filter of the Ross pair, which reduces significantly (up to 5 times) the response outside the sensitivity range. Examples are presented of the optimal six-channel (8.98–18.99–29.2–38.92–50.24–67.42–88 keV) spectrometers with three types of detectors: Pb calorimeters, CsI scintillators, and Al2O3 thermoluminescent detectors and filters of readily available materials. The effect of uncertainty in x-ray cross sections and material thicknesses and the effect of filter-scattered photons hitting the detector on the accuracy of measurements are calculated.

A simple multifoil spectrometer for routine carbon and oxygen measurements

A simple multifoil Ross filter spectrometer for monitoring principal (i.e., n=2−1) soft x-ray lines of H- and He-like carbon and oxygen has been installed on the Madison symmetric torus reversed-field pinch. It consists of four silicon surface barrier diodes, each covered by a different multilayer foil filter but viewing a common plasma volume. Each filter is composed of a combination of thin film absorbers with strategically located absorption edges, giving a peak transmission of around 10% at a wavelength of interest (C v 40.30 Å, C vi 33.74 Å, O vii 21.70 Å, O viii 18.90 Å) with good rejection of the others. A linear combination of these four lines is isolated by a given filter and a matrix inversion of the four simultaneous measurements produces the intensities of the individual lines. The time evolution of these lines shows evidence of radial shell structure and gives useful information of the dynamics of the electron temperature during sawtooth activity. This multifoil spectrometer provides a simple and inexpensive alternative to grazing incidence spectrometers as a monitor of C and O influx and radiated power in high temperature tokamaks, and may be especially applicable in a high neutron flux environment.

Pulsed high intensity monoenergetic low energy X-ray source and absolute X-ray monitor

An X-ray source of 1.5, 4.5, or 8.0 keV with 10 ns durations has been produced utilizing a pulsed electron source to excite the characteristic K-line radiation of elemental targets. A parallel plate ionization chamber was calibrated in the NBS standard monoenergetic K X-ray beams and used with Ross filters to measure the pulsed source K X-ray fluence. Typical measured peak X-ray flux intensities are 10 18−10 19 K X-rays per sr per s. The pulsed source contains four identical ports for simultaneous measurements of the K X-ray intensity by the ionization chamber and detectors being calibrated. The pulsed K X-ray source and the NBS standard steady state K X-ray source, which differ in intensity by 10 orders of magnitude, have been utilized to determine the responses of a photoelectric diode, a scintillator-vacuum photodiode combination, and a silicon PIN diode.

The GSFC EUV and X-ray spectroheliograph on OSO-7

The Goddard Space Flight Center instrument carried on the pointed section of the OSO-7 satellite is described. This instrument contains: An extreme ultraviolet spectroheliograph using glancing incidence optics of Wolter's Type II to focus the Sun's light on the entrance slit of a concave grating spectrometer; an auxiliary Hα system; two X-ray spectroheliographs using mechanical collimators for spatial resolution and Ross filters to isolate spectral bands of interest, and a flare polarimeter operating in the 15–40 keV X-ray region. These subsystems may be operated in a number of modes which, when combined with the spacecraft modes, give the instrument great flexibility for making solar observations. Representative results from each of the subsystems are presented.

Study of the x-ray production mechanism of a dense plasma focus

The x-ray emission from a 375-kJ plasma focus is reported as measured on a time-resolved basis with Ross filters and silicon diode detectors. Thick-target electron beam spectra are calculated with a Monte Carlo electron beam transport program and fit to the experimental time-integrated x-ray spectra. A good fit is obtained with a 2-MA E−2 power law electron beam, with cutoff energies at 8 and 200 keV, incident normal to a thick tungsten target. The source of the beam is judged to be electrons which are accelerated under the influence of strong electric fields. A 20-nsec Q-switched ruby laser pulse injected tungsten ions via laser ablation prior to the initial dense pinch, and the x-ray emission below 4 keV was enhanced and that above 10 keV was reduced. The enhanced low-energy x-ray emission is in agreement with calculated radiation for collisional M-shell excitation of highly stripped tungsten in a 1.5-keV thermal plasma. A 900- μsec ruby laser pulse injected sufficient tungsten ions to degrade greatly the initial dense plasma pinch and reduce the beam target x-ray emission. The time-resolved x-ray emission from this configuration appeared as a smooth Gaussian function in time with FWHM∼200 nsec.

An x-ray spectral measurement system for nanosecond plasmas

A 15-channel Ross filter/silicon diode detection system has been developed for measuring nanosecond pulsed x-ray spectra in the energy range from 4.5 to 116 keV. The theory and practical aspects of Ross filters and silicon diode detectors are discussed as applied to this system. The data read-out system consists of a current integrator array, a simultaneous multichannel digital voltmeter, and oscilloscopes. The system is designed for compatibility with a small computer to allow real time spectral measurements. The x-ray spectrum of a 24 kJ dense plasma focus has been obtained with this system and a theoretical thick target electron beam x-ray spectrum fit to the experimental data. The best fit is obtained with an E−2 power law electron beam spectrum extending from 5 to 125 keV.

Soft X-Ray Vacuum Ultraviolet Diagnostics of High Density, High Temperature Plasmas at the Air Force Weapons Laboratory

AbstractSoft X-ray, vacuum ultraviolet diagnostics of high temperature, high density plasmas are used to determine the temporal, spatial, spectral and total fluence of radiation emitted from plasmas. Some of the radiation diagnostics used to characterize these plasmas are: metallic photocathodes with thin anodes; Ross filter - PIN detector combination; time-resolved pinhole cameras; metallic calorimeter; bent crystal and grazing incidence spectrographs; and differentially pumped, windowless, Samson-type ionization chambers.

X-Ray Analysis for Electron Beam Enhancement in the Plasma Focus Device*

AbstractThe plasma focus device, a form of linear pinch discharge, produces an intense x-ray and neutron (D2) burst from a magnetically heated dense plasma. Rapidly changing magnetic fields at pinch time generate large axial electric fields which accelerate electrons and ions. In the experiments reported here the x-ray production during the plasma pinch of a 96 kilojoule (at 20 kV) plasma focus device was measured.The purpose of these experiments was to evaluate the energy in accelerated electrons in the plasma focus device and to learn how to enhance these electron hursts. Well focused, megampere electron beams at a few hundred kilovolts, lasting less than 10 nanoseconds have applications in fusionable pellet heating experiments. (1) X-rays were monitored to evaluate these electron bursts using a defocusing bent crystal spectrometer, doubly diffused silicon (PIN) detectors, with Ross filters, thermoluminescent dosimeters (TLDs) with filters, and x-ray pinhole photography.Thermoluminescent dosimeters indicated maximum x-ray yields of 140 joules above 3 keV at 57.3 kilojoules stored energy (16 kV) for a conversion efficiency to x-rays of 0.2%. 40 joules are above 60 keV and 15 joules above 80 keV. The hard x-ray pulse typically rises in 3 ns and frequently has a pulse width less than 10 ns. The low energy x-ray spectrum consists almost entirely of lines from the high Z anode insert, and the high energy spectrum is characteristic of a nonthermal power law distribution with an exponent of 2.2 ± 0.8. Peak hard x-ray production is obtained at 1 torr deuterium in contrast to peak neutron production (3 x 1010) at 5 torr. The addition of argon reduces total x-ray yield and increases the relative fraction of soft x-rays.These measurements suggest that the plasma focus produces 1200 joules of electrons with an average energy of 150 keV, in 10 nanoseconds with a stored energy of 57.3 kilojoules. This is a power of 1.2 × 1011 watts and power density of 1.5 × 1013 watts cm−2.

The measurement of X-ray intensities by means of Ross filters and photographic films

X-ray intensity measurements in a narrow range of the spectrum can be achieved by means of Ross filters. Nearly always they are used in connection with ionization detectors (especially pulse counters). With regard to certain applications, however, the employment of photographic film is of great interest, e.g. in microradiography or diffraction studies on amorphous materials. If the Ross-filter technique is combined with the photographic method instead of the counter method, the random error (arising from variations of the thickness or the speed of the emulsion, from nonuniform development, the graininess, etc.) becomes of paramount importance. Its dependence on filter thickness and X-ray voltage is discussed for Ni-Co and Zr-Sr filter pairs. Practical rules, which take into consideration the random error, as well as the error of monochromatism, are given.