REVIEW OF SCIENTIFIC INSTRUMENTS 85, 103114 (2014)

A doubly curved elliptical crystal spectrometer for the study of localized x-ray absorption in hot plasmas Adam D. Cahill,a) Cad L. Hoyt, Sergei A. Pikuz, Tania Shelkovenko, and David A. Hammer Cornell University, Electrical and Computer Engineering, Ithaca, NY 14853, USA

(Received 18 June 2014; accepted 6 October 2014; published online 23 October 2014) X-ray absorption spectroscopy is a powerful tool for the diagnosis of plasmas over a wide range of both temperature and density. However, such a measurement is often limited to probing plasmas with temperatures well below that of the x-ray source in order to avoid object plasma emission lines from obscuring important features of the absorption spectrum. This has excluded many plasmas from being investigated by this technique. We have developed an x-ray spectrometer that provides the ability to record absorption spectra from higher temperature plasmas than the usual approach allows without the risk of data contamination by line radiation emitted by the plasma under study. This is accomplished using a doubly curved mica crystal which is bent both elliptically and cylindrically. We present here the foundational work in the design and development of this spectrometer along with initial results obtained with an aluminum x-pinch as the object plasma. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4898339] I. INTRODUCTION

The value of x-ray absorption spectroscopy for the diagnosis of plasmas over a large range of temperatures and densities has long been appreciated.1–3 This is in contrast to x-ray emission spectroscopy which generally requires higher temperatures before signals sufficiently above the system noise floor can be recorded. The primary cause of this requirement is that emission spectra are visible only when adequate populations of excited ions are present. This requires sufficiently high temperatures to excite the upper state of the emission lines and sufficient densities to have a high signal to noise ratio. Absorption spectroscopy, on the other hand, carries with it only the requirement that a population of ground state ions exists. This makes absorption spectra an ideal diagnostic for colder and lower density plasmas. As plasma temperatures and densities rise, object plasma emission lines begin to appear in the spectra from any spectrometer with a line of sight to the object plasma, as would be the case in point projection absorption spectroscopy.4 This complicates, and in some cases makes impossible, the interpretation of absorption spectra as both emission and absorption lines transition between the same excited and ground states. Since emission spectra enjoy an increasing signal-tonoise ratio with rising temperature and density, it would seem that absorption techniques have little utility in the study of high temperature plasmas. Furthermore, intensely radiating plasmas are often surrounded by regions of cooler and lower density material which can have a significant impact on a plasma’s evolution.5 Spectra from such edge plasmas, whether they be emissive or absorptive, are often overpowered by emission spectra generated in nearby regions of high temperature and/or density. In order to investigate conditions in such a mixed system, we have developed an x-ray spectrometer that is able to record a) [email protected]

0034-6748/2014/85(10)/103114/8/$30.00

spatially resolved absorption spectra of a plasma under study in the presence of bright emission from that plasma. The object plasma for this study is a two wire x-pinch, which presents a hot emissive plasma in close proximity to a cool non-radiative plasma, the plasma jet between the two wires. A high temperature plasma is generated at the location where the two wires cross; along the central axis of the pinch, both above and below the crossing point, plasma jets form that are cooler and of lower density. Such a plasma is impossible to probe using point projection absorption techniques due to the emission from the crossing point and the lack of uniformity along the axis. Emission spectroscopy is able to characterize the radiation emitted from the crossing point, but is blind to the cooler jets along the axis just millimeters away from the higher temperature emissive plasma. The x-ray spectrometer presented here collects spatially resolved absorption spectra along the object plasma’s axis while preventing emission lines from the object plasma from contaminating the absorption spectrum. The spectrometer is realized by using a combination of elliptical and cylindrical geometries in orthogonal planes to define the x-ray crystal’s surface, together with a novel arrangement of experimental components. Contrary to a point projection absorption experiment, in which source radiation first interacts with an object plasma before dispersion by an x-ray optic, this design disperses probe radiation before interaction with the object. It is this feature that generates a resolved absorption spectrum while leaving the object plasma emission spectrum unresolved. The remainder of this paper is laid out in six sections. The details of the spectrometer’s geometry will be addressed in Sec. II, including the arrangement of the four main components of the system: x-ray source, object plasma, x-ray crystal, and detector. Additionally, equations defining the system’s geometry are presented. Section III describes an analysis of the system using a simple x-ray ray tracing scheme. Section IV compares the elliptical spectrometer to a point projection

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Two additional angles, α and β, are of importance. Angle α is defined as the angle between the tangent at point B and the major axis. The second of the two angles, β, is the angle between a ray leaving the first focal point A and the major axis. An expression for α is found by determining the slope of the tangent line at point B from Eq. (1) and equating this to tan(α):   −bx . (4) α = tan−1 √ a a2 − x 2 FIG. 1. The geometry of an ellipse with a = 3 13 and b = 2 23 ( = 0.6). Points A and D lie at the two foci of the ellipse. The central angle BCF (φ) is used to parameterize x-ray reflections at point B.

system. Section V is dedicated to the details of the experimental set-up, while Sec. VI presents an example absorption spectrum obtained with the spectrometer. Section VII presents a summary of the work. II. GEOMETRY A. Elliptical curvature

The spectrometer layout is based on the geometry of an ellipse, as shown in Fig. 1, similar to the TREX spectrometers on Sandia’s Z machine.6 Points A and D are the focal points of the ellipse and lie along the major axis. Point C is the center of the ellipse and points E, B, and F lie on the elliptical surface. A defining feature of this geometry is that any ray emitted from one of the focal points in the plane of the page is reflected from the elliptical surface toward the second focal point. For example, a ray emitted along path AB is reflected along path BD. The curve in Fig. 1 is described by Eq. (1) and depends on the two parameters a and b, the lengths CF and CE, respectively. Lengths AC and CD are both equal to the focal length f. The eccentricity of the ellipse is computed using the ratio of a and b in Eq. (2) and is also equal to the ratio of f to a: y2 x2 + = 1, a2 b2  =

1−

(1)

 2 b f = . a a

The angle β is equal to β = tan−1



 r sin(φ) . f + r cos(φ)

(5)

The reflection of x-rays at point B follows Bragg’s Law: nλ = 2d sin(θ ).

(6)

Here n is the reflection order of the crystal and d is the spacing between the crystal’s reflecting planes. The angle θ is between an incoming x-ray and the reflecting planes. In the present analysis these planes are taken to follow the elliptical surface. It is seen that the angle θ in Eq. (6) is equal to the sum of α and β and allows the reflected wavelength to be found as a function of the central angle φ: 2d sin(α(φ) + β(φ)). (7) n Equation (7) allows an arc of the elliptical surface to be selected for x-ray reflection given the elliptical curve, crystal parameters, and a desired band of probing x-rays. For the plot in Fig. 2, an x-ray bandwidth of 8.2Åto 9.5Åis selected. The ellipse of Fig. 1 with  = 0.6 is used, and a mica crystal (2d = 19.94Å) is assumed to be reflecting the x-rays in the 2nd order (n = 2). Second order reflections are used because they are required for a physically realizable spectrometer design. Mica cannot reflect the desired x-ray bandwidth above the 2nd order, and 1st order reflections require a very high eccentricity that would place excessive stress on a mica crystal. The range of φ is found to be [0.36, 1.09] radians for this case. Equation (6) enforces a limit on the maximum wavelength that a crystal may reflect. The minimum in Fig. 2 as λ(φ) =

(2)

To simplify the analysis of this geometry, the angle φ is defined to be the angle between a ray from the center of the ellipse to a point of interest on the ellipse and the major axis. This angle serves to parametrize the point of interest, B. The length CB is a function of φ: r(φ) = 

ab (b cos(φ))2 + (a sin(φ))2

.

(3)

The coordinates of B are then x = r(φ) cos(φ), y = r(φ) sin(φ).

FIG. 2. The elliptical parameters of Fig. 1 are used with a mica crystal reflecting x-rays in the 2nd order to determine the reflected x-ray wavelength as a function of the central angle, φ. Limits on φ are determined by selecting a band of probe x-rays.

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φ approaches π2 is due to the nature of the elliptical geometry. A minimum eccentricity is required for the reflection of a given band of x-rays. Expressions for calculating this eccentricity are found by considering the case in which an x-ray is reflected from point E in Fig. 1. In this case, φ = π2 and Eqs. (2) and (3) are simplified and combined with Eq. (5). If λmin is the shortest desired wavelength, then the corresponding θ , θ min , and minimum required eccentricity,  min , are given by Eqs. (8) and (9), respectively,   nλmin , (8) θmin = sin−1 2d  min =

1 1 + tan2 (θmin )

= cos(θmin ).

(9)

(10)

B. Cylindrical curvature

The application of cylindrically bent crystals to x-ray spectroscopy has been covered extensively in the literature.7, 8 The defining feature of a cylindrical geometry is that light emitted from a point source along the crystal’s axis of revolution is focused to a line along that axis of revolution. The present work makes use of this cylindrical shape to extend the elliptical arc of Sec. II A out of the plane and define a two-dimensional surface. This surface describes the desired geometry of the final crystal used in the system. The remaining degree of freedom in the design of the crystal surface is the axis around which the elliptical arc is to be revolved. A standard off-axis ellipsoid optic takes the major axis as the axis of revolution. With this choice, light from point A of Fig. 1 is focused back to a point at D. This is undesirable as an object plasma at point D would be probed only at a single point lying in the plane of the ellipse. Instead, an axis of revolution is chosen to focus the probing radiation into a line perpendicular to the plane of the ellipse at the second focal point D. The chosen axis intersects the first focal point of the ellipse, point A, and makes an angle of 45◦ with the major axis. This axis is shown in Fig. 3. The coordinates of the crystal’s surface are obtained by applying the transformation of Eq. (11) to the elliptical arc defined in Sec. II A. This extends the geometry into and out of the page. The transform proceeds by first translating the geometry so that the axis of revolution passes through the origin. This is accomplished by adding the focal length f to x (x → x + f ) so that the focal point at position A becomes the new origin of the system. The rotation is then applied and the translation is removed: ⎡ ⎤ ⎛⎡ ⎤ ⎡ ⎤⎞ ⎡ ⎤ x(φ, γ ) r(φ)cos(φ) f f ⎣ y(φ, γ ) ⎦ = R(γ ) ⎝⎣ r(φ)sin(φ) ⎦ + ⎣ 0 ⎦⎠ − ⎣ 0 ⎦. z(φ, γ ) 0 0 0

FIG. 3. The elliptical arc is revolved around an axis containing point A and making a 45◦ angle with the major axis. Two focal lines are generated.

fined by constructing a vector through that point which is also perpendicular to the axis of revolution. This vector is shown for an arbitrary point, G, on the arc in Fig. 3. The angle between this vector and the elliptical plane is γ , as shown in Fig. 4. The factors of √12 in the rotation matrix arise from sine and cosine terms involving the angle between the axis of revolution and the major axis of the ellipse, which has been chosen to be 45◦ : ⎡ cos(γ )+1 cos(γ )−1 −sin(γ ) ⎤ 2

⎢ cos(γ )−1 R(γ ) = ⎢ ⎣ 2 sin(γ √ ) 2

2 cos(γ )+1 2 sin(γ √ ) 2



2 sin(γ √ ) 2

⎥ ⎥. ⎦

(12)

cos(γ )

By requiring the axis of revolution to include the focal point A and not point D, the cylindrical bend of the crystal generates an astigmatic focusing of the x-rays. A vertical focal line perpendicular to the plane of the ellipse appears at the object plasma location. A horizontal focal line is also generated along the axis of revolution. This astigmatism is introduced into the geometry to aid the separation of the emission and absorption spectra as shown in Fig. 4. Object plasma radiation is entirely unfocused unlike the source spectrum reflected and focused by the crystal. This presents an opportunity to place a

(11) The rotation matrix, R(γ ), is presented in Eq. (12). The rotation angle, γ , for a given point on the elliptical arc is de-

FIG. 4. The probe x-rays (red) are focused by the crystal through the sample and then the aperture. Emission x-rays from the sample (blue) only reach the detector through the aperture. Point D is the same as is found in Fig. 3.

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slit aperture on the axis of revolution and limit the detector’s exposure to object plasma x-rays. The selection of a 45◦ angle for the revolution axis is not tightly constrained. Smaller angles cause a smaller portion of the object’s axis to be probed and require the aperture to be placed closer to the sample. Larger angles cause spectral dispersion at the detector to become larger. The choice made here probes a reasonable amount of plasma, places the aperture in a convenient location, and does not excessively disperse the absorption spectrum. III. RAY TRACING

Due to the unique geometry of the crystal surface, it is desirable to validate the geometry of the spectrometer system before beginning construction. To accomplish this task, a simplified x-ray ray tracing program was used to compute the path taken by x-rays leaving one of the two elliptical foci. The geometry is that of Sec. II with elliptical parameters a and b equal to 8.47 cm and 6.77 cm, respectively. The aim of the program is to compute the distribution of x-rays as they intersect three key planes in the system. The first two are the meridional and sagittal planes. The meridional plane contains points A and D and is perpendicular to the elliptical plane. The sagittal plane includes the axis of revolution from Fig. 3 and is also perpendicular to the ellipse. The final plane investigated is that of the detector which sits behind and parallel to the sagittal plane. The placement of the detector behind the sagittal focus allows the absorption spectrum to defocus after passing through the focal line and to recover the spatial information obtained from the object plasma in the meridional plane. The software works by emitting a fixed number of x-rays from an optically thin volume source at point A in Fig. 1. The spectrum of these x-rays is taken to be of a constant intensity, but may also be taken from the output of an atomic modelling code. The rays are directed toward the crystal surface where they are reflected. The software does not treat the Bragg reflections of x-rays in the detailed manner of some codes,9 but only considers them as specular reflections. As a consequence, modifications to the reflected spectrum such as broadening mechanisms due to the atomic structure of the crystal are not investigated here. Finally, the rays’ intersections with the specified plane are computed and reported. The first of the planes investigated is the meridional plane. This plane contains the axis of the object plasma which is assumed to be azimuthally symmetric and the major axis of the ellipse. The results are shown in Fig. 5 where all wavelengths of x-rays are seen to be tightly grouped around a focal line. The finite width of the focus is due to two factors. Near the center of the plot, the width is due to the finite size of the source emitting the x-rays, which is set to a radius of 10 μm. Near the ends of the distribution, the observed defocusing is due to the cylindrical curvature of the crystal. As the angle between the axis of revolution and the major axis is decreased, both the defocusing and height of this distribution decrease. The horizontal axis of Fig. 5 is expanded to accentuate the spectral spread of the x-rays as they pass through this plane. In an experimental setting, the deviations from a perfect focal

FIG. 5. The x-rays are distributed along a curved focal line in the meridional focal plane. This provides spatial resolution at the object location to the spectrometer. Notice the difference in horizontal and vertical scales.

line are much smaller than the object plasma and are of no consequence. The focus in the sagittal plane is shown in Fig. 6. X-rays are spectrally dispersed while passing through a horizontal focus. The finite height of the focus is due to the 10 μm radius of the source. Fig. 7 presents the x-ray distribution at the detector plane. In contrast to Figs. 5 and 6, Fig. 7 provides two dimensions of resolution. Each point on the detector plane contains information about the absorption of a given wavelength at a specific height along the object plasma’s axis. A demagnification of 67% is calculated by comparing the distribution heights in Figs. 5 and 7. The magnification can be adjusted by varying the distance between the sagittal and detector planes. IV. COMPARISON TO POINT PROJECTION

A point projection x-ray absorption spectroscopy experiment is depicted at the top of Fig. 8. X-rays are generated

FIG. 6. The x-rays are distributed along a horizontal focal line in the sagittal focal plane.

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FIG. 7. The absorption spectrum seen at the detector plane has both spatial (vertical) and spectral (horizontal) resolutions.

from a pinch and immediately interact with an object plasma. A spherical crystal then disperses the probe x-rays and reflects them to a focal line. A detector is placed behind this focus to allow the spectrum to defocus and recover spatial resolution. The detector image contains both spectral and spatial information. This scheme is described in detail in the literature.3 A key requirement for such an experiment is that uniformity exist along the sample’s axis. This must be enforced to ensure that each wavelength of radiation interacts with plasma at the same temperature and density. Were this uniformity to not exist, long wavelength photons would interact with regions of plasma under different conditions than those probed by shorter wavelengths. Because analysis of spectral data relies on features such as line ratios and continuum slope that span the probing bandwidth, such non-uniformity would make interpretation of the data impossible. Another requirement for point projection absorption spectroscopy is that the object plasma is hot enough to produce ground state ions for absorption but not so hot that those ions transition into an excited state. If they do become excited, they will, upon falling back to the ground state, emit a photon whose energy exactly matches that of an absorption line.

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Should the temperature become high enough, these photons will fill in the absorption features on the detector and emission lines will appear. This contamination by object plasma emission can lead to an erroneous interpretation of the absorption spectrum. Thus this technique is effective only over a limited range of temperatures. In contrast, the elliptical spectrometer disperses probing x-rays before they interact with the sample, as illustrated in Fig. 8. This provides the opportunity to create an astigmatic focusing scheme that is not possible with the point projection setup. The crystal’s geometry first focuses the x-rays onto the object’s axis, shown with a vertical orientation in Fig. 8. At a given position along the object’s axis, all wavelengths are focused through the same localized plasma. Next, a horizontal focal line is generated behind the object in which all spatial information is compressed. As the x-rays pass this focal line and begin to defocus, spatial information is recovered and collected by a detector. In contrast to the point projection technique, there is no uniformity requirement for the elliptical spectrometer. This is because of the focusing that occurs at the sample’s location. With all wavelengths sampling each location in the object plasma, the problem of different wavelengths viewing different plasma conditions no longer occurs. This allows objects with variation along their axis to be investigated using absorption spectroscopy. Because a focal line exists between the object plasma and the detector, a slit aperture can be placed here without affecting the absorption spectrum. This blocks a majority of the radiation emitted by the object as shown in Fig. 4. The emission that does pass through the slit has not interacted with the crystal and so appears as a uniform background signal and not as resolved spectral lines. This can be subtracted from the data if the source is sufficiently bright. Any emission that does interact with the crystal is directed not toward the detector, but in the direction of the source where it is of no consequence. Thus, the placement of the crystal before the object plasma also eases the restricted temperature range of the point projection technique. V. EXPERIMENTAL SET-UP A. Current driver

The spectrometer has been demonstrated on the XP pulsed power generator at Cornell University. This device delivers a peak current of up to 500 kA in 100 ns pulses to an experimental load. The full machine current is used to drive a hybrid x-pinch10 that is located at point A of Fig. 1 to provide a source of probing x-rays. A portion of the full drive current in the return current circuit is directed to pass through a two wire x-pinch to form an object plasma at the second focal point D. The current passing through this x-pinch is estimated to be 20% of the full current. B. X-ray source plasma FIG. 8. Point projection spectroscopy (top) disperses the probing radiation after interaction with the object plasma. The elliptical geometry (bottom) disperses the radiation before interaction with the object.

Probing x-rays are generated by a hybrid x-pinch10 using a 50 μm diameter silver wire. Silver is chosen as the source

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material not only due to its generation of continuum radiation in the x-ray band of interest but also because it produces emission lines in this band which can be used for calibration. With calibrations completed, a gold source can be substituted if desired to produce a clean continuum spectrum without emission lines. The results presented in Sec. VI B were acquired using a silver source plasma. The hotspot of a hybrid x-pinch radiates on subnanosecond time scales.10 This is the only time during which an appreciable number of probing x-rays are generated. This short lifetime of the hotspot sets the time over which data is integrated on the detector. By using a short duration probe, time resolved data are collected on a time integrating detector.

C. Crystal

Mica is selected as the crystal of choice for this work. The crystal’s proximity to both pinches raises the concern of damage to the crystal due to pinch debris. This drives the selection away from higher cost crystals such as quartz. Thin sheets of mica are also relatively flexible, an important property for successfully conforming to the spectrometer’s geometry. The desired crystal geometry, as described in Sec. II with a = 8.47 cm and b = 6.77 cm, is cut into an Al substrate. The crystal is then pressed into the Al and held by a flexible nitrile rubber membrane using an air pressure driven press. A pressure of approximately 100 psia is maintained until an optical adhesive cures. The primary disadvantage of mica is its ability to reflect x-rays in multiple orders.8 The desired x-ray bandwidth requires the elliptical design to rely on x-ray reflections in the second order. However, mica reflects x-rays in the 1st, 3rd, and 5th orders with efficiencies comparable to that of 2nd order reflections. To combat this problem, x-ray filters, to be discussed shortly, are selected to minimize detector exposure to x-rays reflected by orders other than the 2nd.

D. Object plasma

The object plasma is chosen to be a two wire x-pinch. This is done because of the open geometry of the configuration. Other than a single crossing point, there are no obstructions to the propagation of the probing x-rays. Additionally, past work has documented the appearance of plasma jets above and below the hotspot of the pinch. Thus, this geometry provides a good example of a hot radiating plasma, the hotspot at the crossing point, surrounded by cooler structures, the jets, that are roughly cylindrically symmetric but vary along their axis. The material chosen for the object plasma is Al alloy 5056. The primary impurity in this alloy is magnesium at a concentration of 5%. These Mg ions serve as the absorbers in the sample, producing absorption features in the continuum spectrum of the silver source. These features can then be used to determine the temperature and density of the object plasma as a function of height along its axis. The x-pinch wires are separated by about 1 mm at the crossing point to reduce the intensity of the hotspot.

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E. Detector

The detectors for the spectrometer system are BAS-TR image plates. Previous studies11 have indicated that these plates are well suited to the detection of soft x-rays due to their lack of a protective Mylar layer found on BAS-SR image plates. Images are developed using a Typhoon FLA 7000 scanner with a resolution of 25 μm.

F. Debris

The XP generator is being used to drive both the source and sample pinches, and so it is necessary to keep the pinches in close proximity to each other to keep the overall inductance of the load as low as possible. However, this requirement is countered by the need to keep the crystal at a safe distance from both pinches. As the distance between focal points is reduced, the distance between the crystal and the focal points must also shrink. The current design, with a focal length of 5.08 cm, represents a compromise between these two competing needs. The load inductance is experimentally found to be acceptable, reducing the driver current from 500 kA to 450 kA and slowing the rise time from 100 ns to 120 ns. The distance between the crystal and the object plasma x-pinch is also keep large enough to allow protective filters to be installed and prevent catastrophic damage by debris.

G. Filters

To protect the crystal from debris a 4 μm polypropylene filter is installed in front of the mica. This represents an effective thickness of 8 μm of filter that source x-rays must pass though. This film has been experimentally determined to be sufficient protection for the mica and also serves to attenuate 1st order reflections. An additional Al filter of 8 μm thickness is placed over the image plate detector. This filter serves a dual purpose. It protects the active layer of the detector which lacks the protective Mylar found in SR image plates. It also attenuates reflections from the mica in the 1st and 3rd orders. This alleviates the primary disadvantage of using mica as the dispersive element in the system. VI. RESULTS A. Pinhole imaging

A useful consequence of the spectrometer’s elliptical geometry is an unobstructed line of sight between the source and sample, as illustrated in Fig. 9. This allows time integrated pinhole images of the source and sample to be taken during the experiment. Results are shown on the right of Fig. 9. To capture these images, a 300 μm diameter unfiltered pinhole has been placed off of the ellipse’s major axis to record time integrated images of both the source and object plasmas onto a detector. The pinhole diameter is chosen to ensure that adequate radiation from the object plasma would reach the detector. However, it is comparable to the feature

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FIG. 9. The geometry of the spectrometer allows an offset pinhole to capture time integrated images of the source, 2, and sample, 1. Boxes 1 and 2 show time integrated pinhole images of emission from the sample and source, respectively. The magnifications of the images in boxes 1 and 2 are 0.68 and 0.36, respectively.

sizes of the source and object plasmas. This limits the resolution of the images in boxes 1 and 2 to 0.7 mm and 1.1 mm, respectively. Box 1 of Fig. 9 contains an emission image of the object plasma while box 2 holds an image of the source pinch. Box 1 reveals a vertical column of plasma along the axis of the object x-pinch. The crossing point of the x-pinch is near the bottom of the image. Variation in the intensity with height indicates changing plasma conditions and is consistent with changes in the absorption spectrum presented in Sec. VI B. This stands in contrast to a pinhole image filtered with 4 μm of Al (not shown) in which no emission is observed. This indicates that a majority of the sample’s emission lies below aluminum’s transmission window of 1 keV to 1.5 keV. B. Absorption spectrum

An experimental spectrum is shown in Fig. 10. The top image is a scan of an image plate detector. The source pinch emits a sub-nanosecond burst of x-rays from a hotspot. A small portion of these x-rays pass through the object plasma and are absorbed. This generates the spectrum seen in the middle of the image. The sample also contributes to the exposure through its emission, which appears as a background signal underlying the absorption spectrum. The absorption spectrum occupies the center of the image and contains continuum and line radiation from the

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source along with absorption features from the object, but not spectrally resolved emission from the object. The spectral resolution is along the horizontal axis and spatial resolution is in the vertical direction which corresponds to height along the axis of the object plasma. The lineouts, taken at three heights, demonstrate that the object plasma is not uniform along its axis. All three lineouts share two silver emission peaks that are visible around 1.29 keV (9.6 Å) and 1.38 keV (9.0 Å), respectively. These are used to calibrate the spectral axis. Experiments are planned to substitute gold for the silver source as it does not contain any emission features in this x-ray band. The spectrum will then contain only absorption lines. The other features of importance are the absorption features between 1.26 keV (9.8 Å) and 1.29 keV (9.6 Å). These absorption lines are due to the Mg ions present in the object plasma. These lines can be used to infer temperature and density of the object. The spectrum covers a spatial range that includes, and extends a few millimeters above, the crossing point of the object x-pinch. Lineout number 1 (red) is taken at this crossing point. It is seen that source emission is greatly attenuated here. This is due to the relatively large amounts of mass along the line of sight at this height. Lineouts 2 (green) and 3 (blue) are taken at 1.3 mm and 2.6 mm, respectively, above the crossing point where densities are expected to be much lower. It is here that absorption features due to helium-like Mg satellites are visible. An examination of these lines reveals changes in the relative intensities of the lines verses height that can be used for diagnostic purposes. Further examination of lineout number 1 (red), in which a majority of the continuum radiation has been absorbed, reveals a peak at 1.34 keV. The intensity of this line falls below the continuum level in the other two lineouts. This is a silver line at 3.35 keV (3.7 Å) that has been reflected in mica’s 5th, order indicating that the source temperature is sufficiently high to make non-negligible line contributions to the spectrum from higher energy bands. A detailed analysis of the absorption spectrum will require a careful study of the source spectrum in the 5th order to quantify the degree of contributions from higher order reflections of continuum radiation. The background noise seen around the spectrum is primarily due to object radiation that is passed by the 8 μm Al filter protecting the detector. This radiation is time integrated. Removal of this component of the image is done by masking the data and fitting a 2D 2nd order polynomial to the background around the edges of the image. This polynomial is then subtracted from the image before the lineouts are taken for Fig. 10. VII. SUMMARY

FIG. 10. Lineouts are taken from the absorption spectrum at three heights along the sample axis. Magnesium absorption features are highlighted between 1.26 keV (9.8 Å) and 1.29 keV (9.6 Å).

A spectrometer combining elliptical and cylindrical geometries has been described and shown to be capable of collecting localized and time resolved absorption spectra of a hot plasma sample. By focusing the probe radiation before interaction with the sample, the spectrometer records object plasma absorption spectra and emission lines from the source

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if they satisfy Bragg’s law. The majority of the object plasma emission radiation is rejected by an aperture and filter. The remainder that falls on the detector is not spectrally resolved. This results in spectra that contain only emission features from the source and absorption features from the object. By selecting a source devoid of emission features in the chosen x-ray band, it is possible to generate a spectrum containing only absorption features due to object conditions. This absence of object emission characteristics allows the technique of absorption spectroscopy to be utilized for the study of plasmas over a much greater range of object plasma temperatures than previously possible.

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MacFarlane et al., Phys. Rev. E 66, 046416 (2002). Audebert et al., Phys. Rev. Lett. 94, 025004 (2005). 3 P. F. Knapp et al., Rev. Sci. Instrum. 82, 063501 (2011). 4 P. F. Knapp et al., Phys. Plasmas 19, 056302 (2012). 5 P. A. Gourdain and C. E. Seyler, Phys. Rev. Lett. 110, 015002 (2013). 6 P. W. Lake et al., Rev. Sci. Instrum. 75, 3690 (2004). 7 A. P. Shevelko, Current Russian Research in Optics and Photonics: New Methods and Instruments for Space- and Earth-based Spectroscopy in XUV, UV, IR, and Millimeter Waves, Proc. SPIE, Vol. 3406, edited by I. I. Sobelman and V. A. Slemzin (SPIE, 1998), pp. 91–108. 8 G. Hölzer et al., Phys. Scr. 57, 301 (1998). 9 I. Uschmann et al., J. Appl. Crystallogr. 26, 405 (1993). 10 T. A. Shelkovenko et al., Phys. Plasmas 17, 112707 (2010). 11 A. L. Meadowcroft, C. D. Bentley, and E. N. Stott, Rev. Sci. Instrum. 79, 113102 (2008). 2 P.

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A doubly curved elliptical crystal spectrometer for the study of localized x-ray absorption in hot plasmas.

X-ray absorption spectroscopy is a powerful tool for the diagnosis of plasmas over a wide range of both temperature and density. However, such a measu...
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