Microcalorimeter measurement of x-ray spectra from a high-temperature magnetically confined plasma

A NASA-built x-ray microcalorimeter spectrometer has been installed on the MST facility at the Wisconsin Plasma Physics Laboratory and has recorded x-ray photons emitted by impurity ions of aluminum in a majority deuterium plasma. Much of the x-ray microcalorimeter development has been driven by the needs of astrophysics missions, where imaging arrays with few-eV spectral resolution are required. The goal of our project is to adapt these single-photon-counting microcalorimeters for magnetic fusion energy research and demonstrate the value of such measurements for fusion science. Microcalorimeter spectrometers combine the best characteristics of the x-ray instrumentation currently available on fusion devices: high spectral resolution similar to an x-ray crystal spectrometer and the broadband coverage of an x-ray pulse height analysis system. Fusion experiments are increasingly employing high-Z plasma-facing components and require measurement of the concentration of all impurity ion species in the plasma. This diagnostic has the capability to satisfy this need for multi-species impurity ion data and will also contribute to measurements of impurity ion temperature and flow velocity, Z_eff, and electron density. Here, we introduce x-ray microcalorimeter detectors and discuss the diagnostic capability for magnetic fusion energy experiments. We describe our experimental setup and spectrometer operation approach at MST, and we present the results from an initial measurement campaign.

Recent enhancements in the performance of the Orion high-resolution x-ray spectrometers

During the past few years, the Orion high-resolution x-ray spectrometers have been successful tools for measuring x-ray spectra from plasmas generated in the Orion laser facility. Duplicate spectrometers also operate successfully at the Livermore EBIT-I and SuperEBIT electron beam ion traps for measuring x-ray polarization. We have recently implemented very high-quality, optically bonded, spherically bent quartz crystals to remove the structure in the x-ray image that had been observed in earlier measurements. The structure had been caused by focusing defects and limited the accuracy of our measurements. We present before and after images that show a drastic improvement. We, furthermore, have implemented a spherically bent potassium acid phthalate (KAP) crystal on one of our spectrometers. The KAP crystal was prepared in a similar fashion, and we present measurements of the N Ly-β and Ne Lyβ lines taken in first- and second-order reflections at 600 and 1200 eV, respectively. These measurements confirm that KAP crystals can be produced at a quality suitable for extending the spectral coverage to wavelengths longer than those accessible by different quartz crystals, especially those that cover the astrophysically important lines of iron.

Absolute throughput calibration of multiple spherical crystals for the Orion High-REsolution X-ray spectrometer (OHREX)

We present absolute throughput analysis of several crystals for the Orion High-REsolution X-ray (OHREX) imaging crystal spectrometer using ray tracing and experimental measurements. The OHREX spectrometer is a high-resolution x-ray spectrometer designed to measure spectral line shapes at the Orion laser facility. The spectrometer is fielded with up to two spherical crystals simultaneously covering two independent spectral ranges. Each crystal has a nominal radius of curvature of R = 67.2 cm and is fielded at a nominal Bragg angle of 51.3°. To cover different bands of interest, several different crystals are available, including Ge (111), KAP, and several cuts of quartz, whose resolving power λ/Δλ exceeds 10 000. The calibrated response of the available crystals has previously been reported from measurements at the EBIT-I electron beam ion trap at Lawrence Livermore National Laboratory. Here, we model the absolute throughput of each crystal using ray tracing and verify the results using experimental data for the quartz (101¯1) crystal.

The Warm Electron Beam Ion Trap (WEBIT): An instrument for ground calibration of space-borne x-ray spectrometers

The warm electron beam ion trap (WEBIT) at Lawrence Livermore National Laboratory is being developed as a pre-launch, ground calibration source for space-borne, high-throughput, high-resolution x-ray spectrometers, such as the x-ray imaging and spectroscopy mission Resolve quantum calorimeter. Historically, calibration sources for calorimeter spectrometers have relied on characteristic line emission from x-ray tubes, fluorescing metals, and radioactive sources. The WEBIT, by contrast, relies on emission from x-ray transitions in highly charged ions, for example, hydrogen-like and helium-like ions, whose energies are well known and whose line shapes are relatively simple. The WEBIT can create astrophysically relevant ions whose x-ray emission falls in the 0.3-12 keV science bandpass of Resolve and has a portable design advantageous for a calibration source. The WEBIT will be used to help calibrate Resolve's instrumental line shape and gain scale as a function of various operational parameters during both detector subsystem level testing and instrumental level testing.

Experimental comparison of spherically bent HAPG and Ge crystals

The Orion high-resolution X-ray (OHREX) imaging spherically bent crystal spectrometer, operated with both image plates and CCD cameras, provides time-averaged plasma diagnostics through high-resolution spectroscopy with good signal-to-noise at the Orion laser facility. In order to provide time-resolved spectra, the OHREX will be outfitted with a streak camera, and in this case, even higher signal to noise will be desired. Using the OHREX's sister instrument, the EBIT High-resolution X-ray (EBHiX) spectrometer, at the LLNL electron beam ion trap EBIT-I, we therefore compare the efficiency of a high-quality Ge (111) crystal (2d = 6.532 Å) with that of a higher integrated reflectivity, but lower-resolution highly annealed pyrolytic graphite (HAPG) crystal (2d = 6.708 Å) in the energy range 2408-2452 eV. We find that the HAPG provides overall more signal across the entire image; however, because of the much better focusing properties of the Ge crystal, the latter provides more signal within the central 100 μm of the spatial profile in the cross-dispersion direction and is thus more suitable for the narrow entrance window of the Livermore-built streak camera.

Measurements of the effective electron density in an electron beam ion trap using extreme ultraviolet spectra and optical imaging

In an electron beam ion trap (EBIT), the ions are not confined to the electron beam, but rather oscillate in and out of the beam. As a result, the ions do not continuously experience the full density of the electron beam. To determine the effective electron density, n _e,eff, experienced by the ions, the electron beam size, the nominal electron density n _e, and the ion distribution around the beam, i.e., the so-called ion cloud, must be measured. We use imaging techniques in the extreme ultraviolet (EUV) and optical to determine these. The electron beam width is measured using 3d → 3p emission from Fe xii and xiii between 185 and 205 Å. These transitions are fast and the EUV emission occurs only within the electron beam. The measured spatial emission profile and variable electron current yield a nominal electron density range of n _e ∼ 10¹¹-10¹³ cm^-3. We determine the size of the ion cloud using optical emission from metastable levels of ions with radiative lifetimes longer than the ion orbital periods. The resulting emission maps out the spatial distribution of the ion cloud. We find a typical electron beam radius of ∼60 μm and an ion cloud radius of ∼300 μm. These yield a spatially averaged effective electron density, n _e,eff, experienced by the ions in EBIT spanning ∼ 5 × 10⁹-5 × 10¹¹ cm^-3.

High resolution, high signal-to-noise crystal spectrometer for measurements of line shifts in high-density plasmas

The Orion high-resolution x-ray (OHREX) spectrometer has been a successful tool for measuring the shapes of density-broadened spectral lines produced in short-pulse heated plasmas at the Orion laser facility. We have recently outfitted the instrument with a charge-couple device (CCD) camera, which greatly increased the accuracy with which we can perform line-shift measurements. Because OHREX is located on the outside of the Orion target chamber, no provisions for the shielding of electromagnetic pulses are required. With the CCD, we obtained a higher signal-to-noise ratio than we previously obtained with an image-plate detector. This allowed us to observe structure in the image produced by the diffraction from the two OHREX crystals, which was highly reproducible from shot to shot. This structure will ultimately limit the accuracy of our spectroscopic measurements.

Imaging crystal spectrometer for high-resolution x-ray measurements on electron beam ion traps and tokamaks

We describe a crystal spectrometer implemented on the Livermore electron beam ion traps that employ two spherically bent quartz crystals and a cryogenically cooled back-illuminated charge-coupled device detector to measure x rays with a nominal resolving power of λ/Δλ ≥ 10 000. Its focusing properties allow us to record x rays either with the plane of dispersion perpendicular or parallel to the electron beam and, thus, to preferentially select one of the two linear x-ray polarization components. Moreover, by choice of dispersion plane and focussing conditions, we use the instrument either to image the distribution of the ions within the 2 cm long trap region, or to concentrate x rays of a given energy to a point on the detector, which optimizes the signal-to-noise ratio. We demonstrate the operation and utility of the new instrument by presenting spectra of Mo³⁴⁺, which prepares the instrument for use as a core impurity diagnostic on the NSTX-U spherical torus and other magnetic fusion devices that employ molybdenum as plasma facing components.

Experimentally determining the relative efficiency of spherically bent germanium and quartz crystals

We have used the EBIT-I electron beam ion trap at the Lawrence Livermore National Laboratory and a duplicate Orion High Resolution X-ray Spectrometer (OHREX) to measure the relative efficiency of a spherically bent quartz (101̄1) crystal (2d = 6.687 Å) and a spherically bent germanium (111) crystal (2d = 6.532 Å). L-shell X-ray photons from highly charged molybdenum ions generated in EBIT-I were simultaneously focussed and Bragg reflected by each crystal, both housed in a single spectrometer, onto a single CCD X-ray detector. The flux from each crystal was then directly compared. Our results show that the germanium crystal has a reflection efficiency significantly better than the quartz crystal, however, the energy resolution is significantly worse. Moreover, we find that the spatial focussing properties of the germanium crystal are worse than those of the quartz crystal. Details of the experiment are presented, and we discuss the advantages of using either crystal on a streak-camera equipped OHREX spectrometer.

Characterization of an atomic hydrogen source for charge exchange experiments

We characterized the dissociation fraction of a thermal dissociation atomic hydrogen source by injecting the mixed atomic and molecular output of the source into an electron beam ion trap containing highly charged ions and recording the x-ray spectrum generated by charge exchange using a high-resolution x-ray calorimeter spectrometer. We exploit the fact that the charge exchange state-selective capture cross sections are very different for atomic and molecular hydrogen incident on the same ions, enabling a clear spectroscopic diagnostic of the neutral species.

L-shell spectroscopic diagnostics of radiation from krypton HED plasma sources

X-ray spectroscopy is a useful tool for diagnosing plasma sources due to its non-invasive nature. One such source is the dense plasma focus (DPF). Recent interest has developed to demonstrate its potential application as a soft x-ray source. We present the first spectroscopic studies of krypton high energy density plasmas produced on a 3 kJ DPF device in Singapore. In order to diagnose spectral features, and to obtain a more comprehensive understanding of plasma parameters, a new non-local thermodynamic equilibrium L-shell kinetic model for krypton was developed. It has the capability of incorporating hot electrons, with different electron distribution functions, in order to examine the effects that they have on emission spectra. To further substantiate the validity of this model, it is also benchmarked with data gathered from experiments on the electron beam ion trap (EBIT) at Lawrence Livermore National Laboratory, where data were collected using the high resolution EBIT calorimeter spectrometer.

Responsivity calibration of the LoWEUS spectrometer

We performed an in situ calibration of the relative responsivity function of the Long-Wavelength Extreme Ultraviolet Spectrometer (LoWEUS), while operating on the Lithium Tokamak Experiment (LTX) at Princeton Plasma Physics Laboratory. The calibration was accomplished by measuring oxygen lines, which are typically present in LTX plasmas. The measured spectral line intensities of each oxygen charge state were then compared to the calculated emission strengths given in the CHIANTI atomic database. Normalizing the strongest line in each charge state to the CHIANTI predictions, we obtained the differences between the measured and predicted values for the relative strengths of the other lines of a given charge state. We find that a 3rd degree polynomial function provides a good fit to the data points. Our measurements show that the responsivity between about 120 and 300 Å varies by factor of ∼30.

Three new extreme ultraviolet spectrometers on NSTX-U for impurity monitoring

Three extreme ultraviolet (EUV) spectrometers have been mounted on the National Spherical Torus Experiment-Upgrade (NSTX-U). All three are flat-field grazing-incidence spectrometers and are dubbed X-ray and Extreme Ultraviolet Spectrometer (XEUS, 8-70 Å), Long-Wavelength Extreme Ultraviolet Spectrometer (LoWEUS, 190-440 Å), and Metal Monitor and Lithium Spectrometer Assembly (MonaLisa, 50-220 Å). XEUS and LoWEUS were previously implemented on NSTX to monitor impurities from low- to high-Z sources and to study impurity transport while MonaLisa is new and provides the system increased spectral coverage. The spectrometers will also be a critical diagnostic on the planned laser blow-off system for NSTX-U, which will be used for impurity edge and core ion transport studies, edge-transport code development, and benchmarking atomic physics codes.

Calibration of the OHREX high-resolution imaging crystal spectrometer at the Livermore electron beam ion traps

We report the calibration of the Orion High-Resolution X-ray (OHREX) imaging crystal spectrometer at the EBIT-I electron beam ion trap at Livermore. Two such instruments, dubbed OHREX-1 and OHREX-2, are fielded for plasma diagnostics at the Orion laser facility in the United Kingdom. The OHREX spectrometer can simultaneously house two spherically bent crystals with a radius of curvature of r = 67.2 cm. The focusing properties of the spectrometer allow both for larger distance to the source due to the increase in collected light and for observation of extended sources. OHREX is designed to cover a 2.5°-3° spectral range at Bragg angles around 51.3°. The typically high resolving powers at these large Bragg angles are ideally suited for line shape diagnostics. For instance, the nominal resolving power of the instrument (>10 000) is much higher than the effective resolving power associated with the Doppler broadening due to the temperature of the trapped ions in EBIT-I. The effective resolving power is only around 3000 at typical EBIT-I conditions, which nevertheless is sufficient to set up and test the instrument's spectral characteristics. We have calibrated the spectral range for a number of crystals using well known reference lines in the first and second order and derived the ion temperatures from these lines. We have also made use of the 50 μm size of the EBIT-I source width to characterize the spatial focusing of the spectrometer.

Development of a high resolution x-ray spectrometer for the National Ignition Facility (NIF)

A high resolution (E/ΔE = 1200-1800) Bragg crystal x-ray spectrometer is being developed to measure plasma parameters in National Ignition Facility experiments. The instrument will be a diagnostic instrument manipulator positioned cassette designed mainly to infer electron density in compressed capsules from Stark broadening of the helium-β (1s²-1s3p) lines of krypton and electron temperature from the relative intensities of dielectronic satellites. Two conically shaped crystals will diffract and focus (1) the Kr Heβ complex and (2) the Heα (1s²-1s2p) and Lyα (1s-2p) complexes onto a streak camera photocathode for time resolved measurement, and a third cylindrical or conical crystal will focus the full Heα to Heβ spectral range onto an image plate to provide a time integrated calibration spectrum. Calculations of source x-ray intensity, spectrometer throughput, and spectral resolution are presented. Details of the conical-crystal focusing properties as well as the status of the instrumental design are also presented.

Lineshape spectroscopy with a very high resolution, very high signal-to-noise crystal spectrometer

We have developed a high-resolution x-ray spectrometer for measuring the shapes of spectral lines produced from laser-irradiated targets on the Orion laser facility. The instrument utilizes a spherically bent crystal geometry to spatially focus and spectrally analyze photons from foil or microdot targets. The high photon collection efficiency resulting from its imaging properties allows the instrument to be mounted outside the Orion chamber, where it is far less sensitive to particles, hard x-rays, or electromagnetic pulses than instruments housed close to the target chamber center in ten-inch manipulators. Moreover, Bragg angles above 50° are possible, which provide greatly improved spectral resolution compared to radially viewing, near grazing-incidence crystal spectrometers. These properties make the new instrument an ideal lineshape diagnostic for determining plasma temperature and density. We describe its calibration on the Livermore electron beam ion trap facility and present spectral data of the K-shell emission from highly charged sulfur produced by long-pulse as well as short-pulse beams on the Orion laser in the United Kingdom.

High-resolution grazing-incidence grating spectrometer for temperature measurements of low-Z ions emitting in the 100-300 Å spectral band

We have constructed a high-resolution grazing-incidence spectrometer designed for measuring the ion temperature of low-Z elements, such as Li(+) or Li(2 +), which radiate near 199 Å and 135 Å, respectively. Based on measurements at the Livermore Electron Beam Ion Trap we have shown that the instrumental resolution is better than 48 mÅ at the 200 Å setting and better than 40 mÅ for the 135-Å range. Such a high spectral resolution corresponds to an instrumental limit for line-width based temperature measurements of about 45 eV for the 199 Å Li(+) and 65 eV for the 135 Å Li(2 +) lines. Recently obtained survey spectra from the Lithium Tokamak Experiment at the Princeton Plasma Physics Laboratory show the presence of these lithium emission lines and the expected core ion temperature of approximately 70 eV is sufficiently high to demonstrate the feasibility of utilizing our high-resolution spectrometer as an ion-temperature diagnostic.

A high-resolution imaging x-ray crystal spectrometer for high energy density plasmas

Adapting a concept developed for magnetic confinement fusion experiments, an imaging crystal spectrometer has been designed and tested for HED plasmas. The instrument uses a spherically bent quartz [211] crystal with radius of curvature of 490.8 mm. The instrument was tested at the Titan laser at Lawrence Livermore National Laboratory by irradiating titanium slabs with laser intensities of 10(19)-10(20) W/cm(2). He-like and Li-like Ti lines were recorded, from which the spectrometer performance was evaluated. This spectrometer provides very high spectral resolving power (E/dE > 7000) while acquiring a one-dimensional image of the source.

X-ray tests of a two-dimensional stigmatic imaging scheme with variable magnifications

A two-dimensional stigmatic x-ray imaging scheme, consisting of two spherically bent crystals, one concave and one convex, was recently proposed [M. Bitter et al., Rev. Sci. Instrum. 83, 10E527 (2012)]. The Bragg angles and the radii of curvature of the two crystals of this imaging scheme are matched to eliminate the astigmatism and to satisfy the Bragg condition across both crystal surfaces for a given x-ray energy. In this paper, we consider more general configurations of this imaging scheme, which allow us to vary the magnification for a given pair of crystals and x-ray energy. The stigmatic imaging scheme has been validated for the first time by imaging x-rays generated by a micro-focus x-ray source with source size of 8.4 μm validated by knife-edge measurements. Results are presented from imaging the tungsten Lα1 emission at 8.3976 keV, using a convex Si-422 crystal and a concave Si-533 crystal with 2d-spacings of 2.21707 Å and 1.65635 Å and radii of curvature of 500 ± 1 mm and 823 ± 1 mm, respectively, showing a spatial resolution of 54.9 μm. This imaging scheme is expected to be of interest for the two-dimensional imaging of laser produced plasmas.

A new spectrometer design for the x-ray spectroscopy of laser-produced plasmas with high (sub-ns) time resolution

This paper describes a new type of x-ray crystal spectrometer, which can be used in combination with gated x-ray detectors to obtain spectra from laser-produced plasmas with a high (sub-ns) time resolution. The spectrometer consists of a convex, spherically bent crystal, which images individual spectral lines as perfectly straight lines across multiple, sequentially gated, strip detectors. Since the Bragg-reflected rays are divergent, the distance between detector and crystal is arbitrary, so that this distance can be appropriately chosen to optimize the experimental arrangement with respect to the detector parameters. The spectrometer concept was verified in proof-of-principle experiments by imaging the Lβ1- and Lβ2-lines of tungsten, at 9.6735 and 9.96150 keV, from a micro-focus x-ray tube with a tungsten target onto a two-dimensional pixilated Pilatus detector, using a convex, spherically bent Si-422 crystal with a radius of curvature of 500 mm.

Development of a ten inch manipulators-based, flexible, broadband two-crystal spectrometer

We have developed and implemented a broadband X-ray spectrometer with a variable energy range for use at the Atomic Weapons Establishment's Orion Laser. The spectrometer covers an energy bandwidth of ∼1-2 keV using two independently mounted, movable Bragg diffraction crystals. Using combinations of cesium hydrogen pthlate, ammonium dihydrogen phosphate, and pentaerythritol crystals, spectra covering the 1.4-2.5, 1.85-3.15, or 3.55-5.1 keV energy bands have been measured. Image plate is used for detection owing to its high dynamic range. Background signals caused by high energy X-rays and particles commonly produced in high energy laser experiments are reduced by a series of tantalum baffles and filters installed between the source and crystal and also between the crystals and detector.

Rare-earth neutral metal injection into an electron beam ion trap plasma

We have designed and implemented a neutral metal vapor injector on the SuperEBIT high-energy electron beam ion trap at the Lawrence Livermore National Laboratory. A horizontally directed vapor of a europium metal is created using a thermal evaporation technique. The metal vapor is then spatially collimated prior to injection into the trap. The source's form and quantity constraints are significantly reduced making plasmas out of metal with vapor pressures ≤10(-7) Torr at ≥1000 °C more obtainable. A long pulsed or constant feed metal vapor injection method adds new flexibility by varying the timing of injection and rate of material being introduced into the trap.

Extended-range grazing-incidence spectrometer for high-resolution extreme ultraviolet measurements on an electron beam ion trap

A high-resolution grazing-incidence grating spectrometer has been implemented on the Livermore electron beam ion traps for performing very high-resolution measurements in the soft x-ray and extreme ultraviolet region spanning from below 10 Å to above 300 Å. The instrument operates without an entrance slit and focuses the light emitted by highly charged ions located in the roughly 50 μm wide electron beam onto a cryogenically cooled back-illuminated charge-coupled device detector. The measured line widths are below 0.025 Å above 100 Å, and the resolving power appears to be limited by the source size and Doppler broadening of the trapped ions. Comparisons with spectra obtained with existing grating spectrometers show an order of magnitude improvement in spectral resolution.

Characterization of spatially resolved high resolution x-ray spectrometers for high energy density physics and light source experiments

A high resolution 1D imaging x-ray spectrometer concept comprising a spherically bent crystal and a 2D pixelated detector is being optimized for diagnostics of small sources such as high energy density physics (HEDP) and synchrotron radiation or x-ray free electron laser experiments. This instrument is used on tokamak experiments for Doppler measurements of ion temperature and plasma flow velocity profiles. Laboratory measurements demonstrate a resolving power, E/ΔE of order 10,000 and spatial resolution better than 10 μm. Initial tests of the high resolution instrument on HEDP plasmas are being performed.

Hyperfine splitting of the 2s1/2 and 2p1/2 levels in Li- and be-like ions of (59)(141) Pr

High-resolution spectroscopy of the 2s(1/2)-2p(1/2) transition in the extreme ultraviolet region is shown to resolve the level splitting induced by the nuclear magnetic field of both the 2s(1/2) and the 2p(1/2) levels in lithiumlike (141)Pr(56+) and of the 2s(1/2)2p(1/2) (3)P(1) level in berylliumlike (141)Pr(55+). The (141)Pr ions are an ideal test of this measurement approach because their energy levels are known well from first principles and are unaffected by small energy contributions from QED and nuclear magnetization effects. The accuracy attained in the measured 196.5 ± 1.2 meV 2s(1/2) splitting is more than an order of magnitude better than that achieved before using crystal spectroscopy of the 2s(1/2)-2p(3/2) x-ray transition and at the level needed to implement a proposed scheme for disentangling the contributions from QED and nuclear magnetization effects in higher-Z ions, such as (209)Bi.

Observations of the effect of ionization-potential depression in hot dense plasma

The newly commissioned Orion laser system has been used to study dense plasmas created by a combination of short pulse laser heating and compression by laser driven shocks. Thus the plasma density was systematically varied between 1 and 10  g/cc by using aluminum samples buried in plastic foils or diamond sheets. The aluminum was heated to electron temperatures between 500 and 700 eV allowing the plasma conditions to be diagnosed by K-shell emission spectroscopy. The K-shell spectra show the effect of the ionization potential depression as a function of density. The data are compared to simulated spectra which account for the change in the ionization potential by the commonly used Stewart and Pyatt prescription and an alternative due to Ecker and Kröll suggested by recent x-ray free-electron laser experiments. The experimental data are in closer agreement with simulations using the model of Stewart and Pyatt.

High-resolution time-resolved extreme ultraviolet spectroscopy on NSTX

We report on upgrades to the flat-field grazing-incidence grating spectrometers X-ray and Extreme Ultraviolet Spectrometer (XEUS) and Long-Wavelength Extreme Ultraviolet Spectrometer (LoWEUS), at the National Spherical Torus Experiment (NSTX) at the Princeton Plasma Physics Laboratory. XEUS employs a variable space grating with an average spacing of 2400 lines/mm and covers the 9-64 Å wavelength band, while LoWEUS has an average spacing of 1200 lines/mm and is positioned to monitor the 90-270 Å wavelength band. Both spectrometers have been upgraded with new cameras that achieve 12.5 ms time resolution. We demonstrate the new time resolution capability by showing the time evolution of iron in the NSTX plasma.

High-resolution spectroscopy for Doppler-broadening ion temperature measurements of implosions at the National Ignition Facility

Future implosion experiments at the national ignition facility (NIF) will endeavor to simultaneously measure electron and ion temperatures with temporal and spatial resolution in order to explore non-equilibrium temperature distributions and their relaxation toward equilibrium. In anticipation of these experiments, and with understanding of the constraints of the NIF facility environment, we have explored the use of Doppler broadening of mid-Z dopant emission lines, such as krypton He-α at 13 keV, as a diagnostic of time- and potentially space-resolved ion temperature. We have investigated a number of options analytically and with numerical raytracing, and we have identified several promising candidate spectrometer designs that meet the expected requirements of spectral and temporal resolution and data signal-to-noise ratio for gas-filled exploding pusher implosions, while providing maximum flexibility for use on a variety of experiments that potentially include burning plasma.

Extreme ultraviolet spectroscopy and modeling of Cu on the SSPX Spheromak and laser plasma "Sparky"

Impurities play a critical role in magnetic fusion research. In large quantities, impurities can cool and dilute plasma creating problems for achieving ignition and burn; however in smaller amounts the impurities could provide valuable information about several plasma parameters through the use of spectroscopy. Many impurity ions radiate within the extreme ultraviolet (EUV) range. Here, we report on spectra from the silver flat field spectrometer, which was implemented at the Sustained Spheromak Physics experiment (SSPX) to monitor ion impurity emissions. The chamber within the SSPX was made of Cu, which makes M-shell Cu a prominent impurity signature. The Spect3D spectral analysis code was utilized to identify spectral features in the range of 115-315 Å and to more fully understand the plasma conditions. A second set of experiments was carried out on the compact laser-plasma x-ray∕EUV facility "Sparky" at UNR, with Cu flat targets used. The EUV spectra were recorded between 40-300 Å and compared with results from SSPX.

Molybdenum emission from impurity-induced m = 1 snake-modes on the Alcator C-Mod tokamak

A suite of novel high-resolution spectroscopic imaging diagnostics has facilitated the identification and localization of molybdenum impurities as the main species during the formation and lifetime of m = 1 impurity-induced snake-modes on Alcator C-Mod. Such measurements made it possible to infer, for the first time, the perturbed radiated power density profiles from which the impurity density can be deduced.

A new scheme for stigmatic x-ray imaging with large magnification

This paper describes a new x-ray scheme for stigmatic imaging. The scheme consists of one convex spherically bent crystal and one concave spherically bent crystal. The radii of curvature and Bragg reflecting lattice planes of the two crystals are properly matched to eliminate the astigmatism, so that the conditions for stigmatic imaging are met for a particular wavelength. The magnification is adjustable and solely a function of the two Bragg angles or angles of incidence. Although the choice of Bragg angles is constrained by the availability of crystals, this is not a severe limitation for the imaging of plasmas, since a particular wavelength can be selected from the bremsstrahlung continuum. The working principle of this imaging scheme has been verified with visible light. Further tests with x rays are planned for the near future.

Rest-wavelength fiducials for the ITER core imaging x-ray spectrometer

Absolute wavelength references are needed to derive the plasma velocities from the Doppler shift of a given line emitted by a moving plasma. We show that such reference standards exist for the strongest x-ray line in neonlike W(64+), which has become the line of choice for the ITER (Latin "the way") core imaging x-ray spectrometer. Close-by standards are the Hf Lβ(3) line and the Ir Lα(2) line, which bracket the W(64+) line by ±30 eV; other standards are given by the Ir Lα(1) and Lα(2) lines and the Hf Lβ(1) and Lβ(2) lines, which bracket the W(64+) line by ±40 and ±160 eV, respectively. The reference standards can be produced by an x-ray tube built into the ITER spectrometer. We present spectra of the reference lines obtained with an x-ray microcalorimeter and compare them to spectra of the W(64+) line obtained both with an x-ray microcalorimeter and a crystal spectrometer.

Optimization of the configuration of pixilated detectors based on the Shannon-Nyquist theory

The configurations of pixilated detectors for x-ray imaging crystal spectrometers and x-ray pinhole cameras can be optimized based on the Shannon-Nyquist sampling and interpolation theory, since the observed spectra are typically oversampled. The degree of oversampling is quantified and alternative uses for redundant pixels are proposed.

Application of spatially resolved high resolution crystal spectrometry to inertial confinement fusion plasmas

High resolution (λ∕Δλ ∼ 10 000) 1D imaging x-ray spectroscopy using a spherically bent crystal and a 2D hybrid pixel array detector is used world wide for Doppler measurements of ion-temperature and plasma flow-velocity profiles in magnetic confinement fusion plasmas. Meter sized plasmas are diagnosed with cm spatial resolution and 10 ms time resolution. This concept can also be used as a diagnostic of small sources, such as inertial confinement fusion plasmas and targets on x-ray light source beam lines, with spatial resolution of micrometers, as demonstrated by laboratory experiments using a 250-μm (55)Fe source, and by ray-tracing calculations. Throughput calculations agree with measurements, and predict detector counts in the range 10(-8)-10(-6) times source x-rays, depending on crystal reflectivity and spectrometer geometry. Results of the lab demonstrations, application of the technique to the National Ignition Facility (NIF), and predictions of performance on NIF will be presented.

Gold charge state distributions in highly ionized, low-density beam plasmas

We present a systematic study of Au charge state distributions (CSDs) from low density, nonlocal thermodynamic equilibrium plasmas created in the Livermore electron beam ion traps (EBIT-I and EBIT-II). X-ray emission from Ni-like to Kr-like Au ions has been recorded from monoenergetic electron beam plasmas having E(beam)=2.66, 2.92, 3.53, and 4.54 keV, and the CSDs of the beam plasmas have been inferred by fitting the collisionally excited line transitions and radiative recombination emission features with synthetic spectra. We have modeled the beam plasmas using a collisional-radiative code with various treatments of the atomic structure for the complex M- and N-shell ions and find that only models with extensive doubly excited states can properly account for the dielectronic recombination (DR) channels that control the CSDs. This finding would be unremarkable for plasmas with thermal electron distributions, where many such states are sampled, and the importance of DR is well established. But in an EBIT source, the beam is resonant with only a subset of such states having spectator electrons in orbitals with high principal quantum number n (8≤n≤20). The inclusion of such states in the model was also necessary to obtain agreement with observed stabilizing transitions in the x-ray spectra.

Measurements of electron transport in foils irradiated with a picosecond time scale laser pulse

The heating of solid foils by a picosecond time scale laser pulse has been studied by using x-ray emission spectroscopy. The target material was plastic foil with a buried layer of a spectroscopic tracer material. The laser pulse length was either 0.5 or 2 ps, which resulted in a laser irradiance that varied over the range 10(16)-10(19) W/cm(2). Time-resolved measurements of the buried layer emission spectra using an ultrafast x-ray streak camera were used to infer the density and temperature conditions as a function of laser parameters and depth of the buried layer. Comparison of the data to different models of electron transport showed that they are consistent with a model of electron transport that predicts the bulk of the target heating is due to return currents.

Viscoelastic suppression of gravity-driven counterflow instability

Attempts to achieve "top kill" of flowing oil wells by pumping dense drilling "muds," i.e., slurries of dense minerals, from above will fail if the Kelvin-Helmholtz instability in the gravity-driven counterflow produces turbulence that breaks up the denser fluid into small droplets. Here we estimate the droplet size to be submillimeter for fast flows and suggest the addition of a shear-thickening or viscoelastic polymer to suppress turbulence. We find in laboratory experiments a variety of new physical effects for a viscoelastic shear-thickening liquid in a gravity-driven counterstreaming flow. There is a progression from droplet formation to complete turbulence suppression at the relevant high velocities. Thick descending columns show a viscoelastic analogue of the viscous buckling instability. Thinner streams form structures resembling globules on a looping filament.

X-ray spectroscopy of Cu impurities on NSTX and comparison with Z-pinch plasmas

X-ray spectroscopy of mid-Z metal impurities is important in the study of tokamak plasmas and may reveal potential problems if their contribution to the radiated power becomes substantial. The analysis of the data from a high-resolution x-ray and extreme ultraviolet grating spectrometer, XEUS, installed on NSTX, was performed focused on a detailed study of x-ray spectra in the range 7-18 Å. These spectra include not only commonly seen iron spectra but also copper spectra not yet employed as an NSTX plasma impurity diagnostic. In particular, the L-shell Cu spectra were modeled and predictions were made for identifying contributions from various Cu ions in different spectral bands. Also, similar spectra, but from much denser Cu plasmas produced on the UNR Z-pinch facility and collected using the convex-crystal spectrometer, were analyzed and compared with NSTX results.

Experimental setup for tungsten transport studies at the NSTX tokamak

Tungsten particles have been introduced into the National Spherical Torus Experiment (NSTX) in Princeton with the purpose to investigate the effects of tungsten injection on subsequent plasma discharges. An experimental setup for the study of tungsten particle transport is described where the particles are introduced into the tokamak using a modified particle dropper, otherwise used for lithium-powder injection. An initial test employing a grazing-incidence extreme ultraviolet spectrometer demonstrates that the tungsten-transport setup could serve to infer particle transport from the edge to the hot central plasmas of NSTX.

Calculation of the Johann error for spherically bent x-ray imaging crystal spectrometers

New x-ray imaging crystal spectrometers, currently operating on Alcator C-Mod, NSTX, EAST, and KSTAR, record spectral lines of highly charged ions, such as Ar(16+), from multiple sightlines to obtain profiles of ion temperature and of toroidal plasma rotation velocity from Doppler measurements. In the present work, we describe a new data analysis routine, which accounts for the specific geometry of the sightlines of a curved-crystal spectrometer and includes corrections for the Johann error to facilitate the tomographic inversion. Such corrections are important to distinguish velocity induced Doppler shifts from instrumental line shifts caused by the Johann error. The importance of this correction is demonstrated using data from Alcator C-Mod.

Calibration of a high resolution grating soft x-ray spectrometer

The calibration of the soft x-ray spectral response of a large radius of curvature, high resolution grating spectrometer (HRGS) with a back-illuminated charge-coupled device detector is reported. The instrument is cross-calibrated for the 10-50 Å waveband at the Lawrence Livermore National Laboratory electron beam ion trap (EBIT) x-ray source with the EBIT calorimeter spectrometer. The HRGS instrument is designed for laser-produced plasma experiments and is important for making high dynamic range measurements of line intensities, line shapes, and x-ray sources.

Development of a spatially resolving x-ray crystal spectrometer for measurement of ion-temperature (T(i)) and rotation-velocity (v) profiles in ITER

Imaging x-ray crystal spectrometer (XCS) arrays are being developed as a US-ITER activity for Doppler measurement of T(i) and v profiles of impurities (W, Kr, and Fe) with ∼7 cm (a/30) and 10-100 ms resolution in ITER. The imaging XCS, modeled after a prototype instrument on Alcator C-Mod, uses a spherically bent crystal and 2D x-ray detectors to achieve high spectral resolving power (E/dE>6000) horizontally and spatial imaging vertically. Two arrays will measure T(i) and both poloidal and toroidal rotation velocity profiles. The measurement of many spatial chords permits tomographic inversion for the inference of local parameters. The instrument design, predictions of performance, and results from C-Mod are presented.

The ITER core imaging x-ray spectrometer: x-ray calorimeter performance

We describe the anticipated performance of an x-ray microcalorimeter instrument on ITER. As part of the core imaging x-ray spectrometer, the instrument will augment the imaging crystal spectrometers by providing a survey of the concentration of heavy ion plasma impurities in the core and possibly ion temperature values from the emission lines of different elemental ions located at various radial positions.

Calibration of a flat field soft x-ray grating spectrometer for laser produced plasmas

We have calibrated the x-ray response of a variable line spaced grating spectrometer, known as the VSG, at the Fusion and Astrophysics Data and Diagnostic Calibration Facility at the Lawrence Livermore National Laboratory (LLNL). The VSG has been developed to diagnose laser produced plasmas, such as those created at the Jupiter Laser Facility and the National Ignition Facility at LLNL and at both the Omega and Omega EP lasers at the University of Rochester's Laboratory for Laser Energetics. The bandwidth of the VSG spans the range of ∼6-60 Å. The calibration results presented here include the VSG's dispersion and quantum efficiency. The dispersion is determined by measuring the x rays emitted from the hydrogenlike and heliumlike ions of carbon, nitrogen, oxygen, neon, and aluminum. The quantum efficiency is calibrated to an accuracy of 30% or better by normalizing the x-ray intensities recorded by the VSG to those simultaneously recorded by an x-ray microcalorimeter spectrometer.

Vacuum ultraviolet impurity spectroscopy on the Alcator C-Mod tokamak

Vacuum ultraviolet spectroscopy is used on the Alcator C-Mod tokamak to study the physics of impurity transport and provide feedback on impurity levels to assist experimental operations. Sputtering from C-Mod's all metal (Mo+W) plasma facing components and ion cyclotron range of frequency antenna and vessel structures (sources for Ti, Fe, Cu, and Ni), the use of boronization for plasma surface conditioning and Ar, Ne, or N(2) gas seeding combine to provide a wealth of spectroscopic data from low-Z to high-Z. Recently, a laser blow-off impurity injector has been added, employing CaF(2) to study core and edge impurity transport. One of the primary tools used to monitor the impurities is a 2.2 m Rowland circle spectrometer utilizing a Reticon array fiber coupled to a microchannel plate. With a 600 lines/mm grating the 80<λ<1050 Å range can be scanned, although only 40-100 Å can be observed for a single discharge. Recently, a flat-field grating spectrometer was installed which utilizes a varied line spacing grating to image the spectrum to a soft x-ray sensitive Princeton Instruments charge-coupled device camera. Using a 2400 lines/mm grating, the 10<λ<70 Å range can be scanned with 5-6 nm observed for a single discharge. A variety of results from recent experiments are shown that highlight the capability to track a wide range of impurities.

Measurement of anomalously strong emission from the 1s-9p transition in the spectrum of H-like phosphorus following charge exchange with molecular hydrogen

We have measured K-shell x-ray spectra of highly ionized argon and phosphorus following charge exchange with molecular hydrogen at low collision energy in an electron beam ion trap using an x-ray calorimeter array with ∼6  eV resolution. We find that the emission at the high end of the Lyman series is greater by a factor of 2 for phosphorus than for argon, even though the measurement was performed concurrently and the atomic numbers are similar. This does not agree with current theoretical models and deviates from the trend observed in previous measurements.

Relativistic quasimonoenergetic positron jets from intense laser-solid interactions

Detailed angle and energy resolved measurements of positrons ejected from the back of a gold target that was irradiated with an intense picosecond duration laser pulse reveal that the positrons are ejected in a collimated relativistic jet. The laser-positron energy conversion efficiency is ∼2×10{-4}. The jets have ∼20 degree angular divergence and the energy distributions are quasimonoenergetic with energy of 4 to 20 MeV and a beam temperature of ∼1  MeV. The sheath electric field on the surface of the target is shown to determine the positron energy. The positron angular and energy distribution is controlled by varying the sheath field, through the laser conditions and target geometry.