One-dimensional space resolving flat-field holographic grating soft x-ray framing camera spectrograph for laser plasma diagnostics

A 1D space resolving x-ray spectrum diagnostic system has been developed to study the radiation opacity of hot plasma on SG-II laser facility. The diagnostic system consists of a 2400 lines/mm flat-field holographic grating and a gated microchannel plate coupled with an optical CCD and covers the wavelength range of 5-50 A?. The holographic grating was compared with a ruled one by measuring the emission spectra from a laser-produced molybdenum plasma. The results indicate that the holographic grating possesses better sensitivity than the ruled grating having nearly similar spectral resolution. The spectrograph has been used in radiative opacity measurement of Fe plasma. Simultaneous measurements of the backlight source and the transmission spectrum in appointed time range in one shot have been accomplished successfully with the holographic grating spectrometer. The 2p-3d transition absorption of Fe plasma near 15.5 A? in has been observed clearly.     

A new correction method for high-resolution energy-dispersive x-ray analyses in the environmental scanning electron microscope

Spurious x-ray signals, which previously prevented high-resolution energy-dispersive x-ray analysis (EDS) in the environmental scanning electron microscope (ESEM), can be corrected using a simple method presented here. As the primary electron beam travels through the gas in the ESEM chamber, a significant fraction of the primary electrons is scattered during collisions with gas molecules. These scattered electrons form a broad skirt that surrounds the primary electron beam as it impacts the sample. The correction method assumes that changes in the width of the electron skirt with pressure are less important than changes in the skirt intensity; this method works as follows: The influence of the gas on the overall x-ray data is determined by acquiring EDS spectra at two pressures. Subtracting the two spectra provides us with a difference spectrum which is then used to correct the original data, using extrapolation, back to the x-ray spectrum expected under high-vacuum conditions. Low-noise data are required to resolve small spectral peaks; however, the principle should apply equally to x-ray maps and even to low-magnification images.     

High resolution far-infrared Fourier transform spectroscopy of radicals at the AILES beamline of SOLEIL synchrotron facility

Experimental far-infrared (FIR) spectroscopy of transient species (unstable molecules, free radicals, and ions) has been limited so far in both emission and absorption (mainly by the low probability of spontaneous emission in that spectral range and the low brightness of continuum sources used for absorption measurements, respectively). Nevertheless, the FIR spectral range recently became of high astrophysical relevance thanks to several new observational platforms (HERSCHEL, ALMA...) dedicated to the study of this region suitable for the detection of the emission from cold objects of the interstellar medium. In order to complete the experimental dataset concerning transient species, three discharge experiments dedicated to the recording of high resolution FIR spectra of radicals have been developed at the Advanced Infrared Line Exploited for Spectroscopy (AILES) which extracts the bright FIR synchrotron continuum of the synchrotron facility SOLEIL. These experiments make use of a high resolution (R = 0.001 cm(-1)) Bruker IFS125 Fourier transform (FT) spectrometer. An emission setup (allowing to record spectra of radicals excited at high rotational and vibrational temperatures) and two absorption setups (exploiting the bright synchrotron source at the highest resolution available on the FT) are alternatively connected to the FT. The advantages and limitations of these techniques are discussed on the basis of the recent results obtained on OH and CH radicals. These results constitute the first FIR spectra of radicals using synchrotron radiation, and the first FIR spectrum of a C-bearing radical using FT-spectroscopy.     

High resolution absorption spectroscopy of exploding wire plasmas using an x-pinch x-ray source and spherically bent crystal

We present here the use of absorption spectroscopy of the continuum radiation from x-pinch-produced point x-ray sources as a diagnostic to investigate the properties of aluminum plasmas created by pulsed power machines. This technique is being developed to determine the charge state, temperature, and density as a function of time and space under conditions that are inaccessible to x-ray emission spectroscopic diagnostics. The apparatus and its characterization are described, and the spectrometer dispersion, magnification, and resolution are calculated and compared with experimental results. Spectral resolution of about 5000 and spatial resolution of about 20 μm are demonstrated. This spectral resolution is the highest available to date in an absorption experiment. The beneficial properties of the x-pinch x-ray source as the backlighter for this diagnostic are the small source size (<5 μm), smooth continuum radiation, and short pulse duration (<0.1 ns). Results from a closely spaced (1 mm) exploding wire pair are shown and the general features are discussed.     

Spectral properties and dynamics of gold nanorods revealed by EMCCD‐based spectral phasor method

Gold nanorods (NRs) with tunable plasmon‐resonant absorption in the near‐infrared region have considerable advantages over organic fluorophores as imaging agents due to their brightness and lack of photobleaching. However, the luminescence spectral properties of NRs have not been fully characterized at the single particle level due to lack of proper analytic tools. Here, we present a spectral phasor analysis method that allows investigations of NRs' spectra at single particle level showing the spectral variance and providing spatial information during imaging. The broad phasor distribution obtained by the spectral phasor analysis indicates that spectra of NRs are different from particle to particle. NRs with different spectra can be identified in images with high spectral resolution. The spectral behaviors of NRs under different imaging conditions, for example, different excitation powers and wavelengths, were revealed by our laser‐scanning multiphoton microscope using a high‐resolution spectrograph with imaging capability. Our results prove that the spectral phasor method is an easy and efficient tool in hyper‐spectral imaging analysis to unravel subtle changes of the emission spectrum. We applied this method to study the spectral dynamics of NRs during direct optical trapping and by optothermal trapping. Interestingly, different spectral shifts were observed in both trapping phenomena. Microsc. Res. Tech. 78:283–293, 2015. © 2015 Wiley Periodicals, Inc.     

A multi-crystal wavelength dispersive x-ray spectrometer

A multi-crystal wavelength dispersive hard x-ray spectrometer with high-energy resolution and large solid angle collection is described. The instrument is specifically designed for time-resolved applications of x-ray emission spectroscopy (XES) and x-ray Raman scattering (XRS) at X-ray Free Electron Lasers (XFEL) and synchrotron radiation facilities. It also simplifies resonant inelastic x-ray scattering (RIXS) studies of the whole 2d RIXS plane. The spectrometer is based on the Von Hamos geometry. This dispersive setup enables an XES or XRS spectrum to be measured in a single-shot mode, overcoming the scanning needs of the Rowland circle spectrometers. In conjunction with the XFEL temporal profile and high-flux, it is a powerful tool for studying the dynamics of time-dependent systems. Photo-induced processes and fast catalytic reaction kinetics, ranging from femtoseconds to milliseconds, will be resolvable in a wide array of systems circumventing radiation damage.     

Note: study of extreme ultraviolet and soft x-ray emission of metal targets produced by laser-plasma-interaction

Different metal targets were investigated as possible source material for tailored laser-produced plasma-sources. In the wavelength range from 1 to 20 nm, x-ray spectra were collected with a calibrated spectrometer with a resolution of λ/Δλ = 150 at 1 nm up to λ/Δλ = 1100 at 15 nm. Intense line emission features of highly ionized species as well as continuum-like spectra from unresolved transitions are presented. With this knowledge, the optimal target material can be identified for the envisioned application of the source in x-ray spectrometry on the high energy side of the spectra at about 1 keV. This energy is aimed for because 1 keV-radiation is ideally suited for L-shell x-ray spectroscopy with nm-depth resolution.     

Wave-dispersive x-ray spectrometer for simultaneous acquisition of several characteristic lines based on strongly and accurately shaped Ge crystal

Si and Ge are widely used as analyzing crystals for x-rays. Drastic and accurate shaping of Si or Ge gives significant advance in the x-ray field, although covalently bonded Si or Ge crystals have long been believed to be not deformable to various shapes. Recently, we developed a deformation technique for obtaining strongly and accurately shaped Si or Ge wafers of high crystal quality, and the use of the deformed wafer made it possible to produce fine-focused x-rays. In the present study, we prepared a cylindrical Ge wafer with a radius of curvature of 50 mm, and acquired fluorescent x-rays simultaneously from four elements by combining the cylindrical Ge wafer with a position-sensitive detector. The energy resolution of the x-ray fluorescence spectrum was as good as that obtained using a flat single crystal, and its gain was over 100. The demonstration of the simultaneous acquisition of high-resolution x-ray fluorescence spectra indicated various possibilities of x-ray spectrometry, such as one-shot x-ray spectroscopy and highly efficient wave-dispersive x-ray spectrometers.     

Development of a time-resolved soft x-ray spectrometer for laser produced plasma experiments

A 2400 lines/mm variable-spaced grating spectrometer has been used to measure soft x-ray emission (8-22 A?) from laser-produced plasma experiments at Lawrence Livermore National Laboratory's Compact Multipulse Terrawatt (COMET) Laser Facility. The spectrometer was coupled to a Kentech x-ray streak camera to study the temporal evolution of soft x rays emitted from the back of the Mylar and the copper foils irradiated at 10(15)?W/cm(2). The instrument demonstrated a resolving power of ～120 at 19 A? with a time resolution of 31 ps. The time-resolved copper emission spectrum was consistent with a photodiode monitoring the laser temporal pulse shape and indicated that the soft x-ray emission follows the laser heating of the target. The time and spectral resolutions of this diagnostic make it useful for studies of high temperature plasmas.     

A dual-channel, focusing x-ray spectrograph with uniform dispersion for Z pinch plasmas measurement

A dual-channel, focusing x-ray spectrograph with uniform dispersion (i.e., the linear dispersion of this spectrograph is a constant) is described for measuring the x-ray spectra emission from the hot, dense Al Z pinch plasmas. The spectrograph uses double uniform-dispersed crystals (e.g., a Quartz 1010 crystal and a Mica 002 crystal) as dispersion elements and a double-film box as detector to achieve the simultaneous recording of the time integrated spectrum covering a wide spectral range of ~5-9 A?. Since this spectrograph disperse the x-rays on the detector plane with uniform spacing for every wavelength, it needs not the calibration of the wavelength with spatial coordinate, thereby own the advantages of easiness and veracity for spectra identification. The design of this spectrograph and the example of experiment on the "Yang" accelerator are presented.     

Application of beam emission spectroscopy to NBI plasmas of Heliotron J

This paper describes the application of the beam emission spectroscopy (BES) to Heliotron J, having the nonsymmetrical helical-magnetic-axis configuration. The spectral and spatial profile of the beam emission has been estimated by the numerical calculation taking the collisional excitation processes between plasmas (electrons/ions) and beam atoms. Two sets of the sightlines with good spatial resolution are presented. One is the optimized viewing chords which have 20 sightlines and observe the whole plasma region with the spatial resolution Δρ less than ±0.055 using the newly designed viewing port. The other is 15 sightlines from the present viewing port of Heliotron J for the preliminary measurement to discuss the feasibility of the density fluctuation measurement by BES. The beam emission has been measured by a monochromator with a CCD camera. A good consistency has been obtained between the spectral profiles of the beam emission measured by the monochromator and the beam emission spectrum deduced by the model calculation. An avalanche photodiode with an interference filter system was also used to evaluate the signal-to-noise (S/N) ratio of the beam emission in the present experimental setup. The modification of the optical system is being planned to improve the S/N ratio, which will enable us to estimate the density fluctuation in Heliotron J.     

Detection of x-ray emission in a nanosecond discharge in air at atmospheric pressure

Measurement of x-ray emission is an important parameter to investigate runaway behavior of fast electrons produced in nanosecond-pulse gas discharge. An online detection system of x rays is described in this paper, and the system consists of an x-ray detector with NaI (Tl) scintillator and photomultiplier tube, and an integrated multichannel analyzer. The system is responsible for detecting x-ray emission signal, processing the detected signals, and scaling the energy distribution. The calibration results show that every channel of the detection system represents a given x-ray energy and various x rays can be divided into different energy ranges between 10 and 130 keV. For a repetitive nanosecond-pulse breakdown between highly nonuniform gaps in open air, an energy distribution is obtained using the online detection system. It shows that the x-ray emission is a continuous spectrum and the x rays of above 60 keV dominate in the detected energy distribution.     

激光诱导铜产生等离子体羽辉的颜色特性

为了研究等离子体羽的辐射特性,建立了激光诱导辐射模型。通过时空分辨光谱测量技术,测定了激光烧蚀铜诱导产生等离子体羽辉在氧气环境下的发射光谱及其发射强度随时间与空间的分布;利用快速照相法,拍摄了等离子体羽的照片。结果表明:等离子体羽光谱主要由Cu的原子谱线、一价离子谱线组成;其不同区域的颜色不同,且随环境气压而变化。认为等离子体羽的不同区域诱导发光机理不同,中心区域以连续辐射为主;中间区域以原子谱线和一价离子谱线为主;外围区域以原子谱线为主。连续辐射主要来自近靶处高能电子运动而产生的韧致辐射以及电子和一价离子的复合;一价离子谱线主要来自近靶处高能电子与一价离子碰撞传能;原子谱线主要来自电子与原子的碰撞传能以及近靶处电子与一价离子的复合。     

Water mapping in hydrated soft materials

We present a method based on spatially resolved electron energy-loss spectroscopy in the cryo-STEM to map the spatial distribution of water in frozen-hydrated polymers. The spatial resolution is limited by the dose constraints imposed by radiation damage, and to stay within these constraints, the use of fine electron-probe sizes comes at the cost of reduced counts in the energy-loss spectra. Thus, at the resolution limit, the detection of isolated water-rich pixels or the identification of minor variations in water content across the specimen is complicated because one must distinguish significant fluctuations from noise. Here we develop a criterion with which to guide such a distinction. We characterize the intrinsic noise associated with spectral measurements under given illumination and acquisition conditions. We then use that noise in combination with scatter diagrams to threshold spectrum images and objectively identify statistically significant compositional fluctuations. We illustrate these ideas using a simulated spectrum dataset for a hypothetical blend of hydrophilic and hydrophobic homopolymers. We show that while a direct inspection of the water map may not allow any meaningful conclusions to be drawn, after applying the thresholding approach we can clearly identify the regions of the specimen that are rich in water. We also experimentally study a model blend system comprised of hydrophilic poly(vinyl pyrrolidone) (PVP) dispersed in a hydrophobic matrix of poly(styrene) (PS). By MLS fitting using damaged and undamaged PVP reference spectra, we determine that the critical dose characteristic of dry PVP is approximately 8000 e/nm2 using 200 keV incident electrons. Irradiating frozen-hydrated PVP gives rise to noticeable hydrogen evolution at doses of approximately 1500 e/nm2. To stay within this constraint we use doses of 400 e/nm2 and a pixel spacing in the spectrum imaging of 100 nm. In order to quantitatively map the water, PVP, and PS compositions, we measure their total inelastic scattering cross-sections. Direct inspection of the composition maps reveals the presence of large water-rich domains of the order of approximately 1 microm and the scatter-diagram thresholding approach identifies small water-rich domains one pixel in size.     

Development of a diagnostic technique based on Cherenkov effect for measurements of fast electrons in fusion devices

A diagnostic technique based on the Cherenkov effect is proposed for detection and characterization of fast (super-thermal and runaway) electrons in fusion devices. The detectors of Cherenkov radiation have been specially designed for measurements in the ISTTOK tokamak. Properties of several materials have been studied to determine the most appropriate one to be used as a radiator of Cherenkov emission in the detector. This technique has enabled the detection of energetic electrons (70 keV and higher) and the determination of their spatial and temporal variations in the ISTTOK discharges. Measurement of hard x-ray emission has also been carried out in experiments for validation of the measuring capabilities of the Cherenkov-type detector and a high correlation was found between the data of both diagnostics. A reasonable agreement was found between experimental data and the results of numerical modeling of the runaway electron generation in ISTTOK.     

Wide band focusing x-ray spectrograph with spatial resolution

A new, wide spectral bandwidth x-ray spectrograph, the wide-bandwidth focusing spectrograph with spatial resolution (WB-FSSR), based on spherically bent mica crystals, is described. The wide bandwidth is achieved by combining three crystals to form a large aperture dispersive element. Since the WB-FSSR covers a wide spectral band, it is very convenient for application as a routine diagnostic tool in experiments in which the desired spectral coverage is different from one test to the next. The WB-FSSR has been tested in imploding wire-array experiments on a 1 MA pulsed power machine, and x-ray spectra were recorded in the 1-20 A spectral band using different orders of mica crystal reflection. Using a two mirror-symmetrically placed WB-FSSR configuration, it was also possible to distinguish between a real spectral shift and a shift of recorded spectral lines caused by the spatial distribution of the radiating plasma. A spectral resolution of about 2000 was demonstrated and a spatial resolution of approximately 100 microm was achieved in the spectral band of 5-10 A in second order of mica reflection. A simple method of numerical analysis of spectrograph capability is proposed.     

Multichannel linear detector for x-ray spectroscopy

This paper considers a new linear multichannel x-ray detector designed on the basis of a BLPP-369M4 silicon photodiode array (2612 photodiodes, array pitch 12.5 µm, height 4 mm, and dynamic range 10⁴). The structure and characteristics of the multichannel detector are given, along with the Kα1,2, Kβ ₁, and Kβ ₅ x-ray emission lines and a K-edge absorption spectrum of metallic copper recorded on a universal URS-2I spectrometer using this detector. The resolution and the signal/noise ratio of these spectra are superior to those of spectra recorded by an SRPP-21 gas ionization counter at the same recording time. The detector has a spatial resolution of 20 µm and an x-ray detection limit to equal 1 quantum at λ = 1.54 Å. It is suitable for studying the fine structure of absorption spectra at 1–10 Å.     

Calibration of an imaging crystal spectrometer for low x-ray energies

An x-ray imaging crystal spectrometer was designed for the Hanbit magnetic mirror device to observe spectra of heliumlike neon at 13.4474 A. The spectrometer consists of a spherically bent mica crystal and an x-ray sensitive vacuum charge coupled device camera. This spectrometer can provide spatially resolved spectra, making it possible to obtain profiles of the ion charge state distribution from line ratios and profiles of the plasma rotation velocity from Doppler shift measurements. The paper describes measurements of spectral resolution of this instrument for low x-ray energies.     

Electron spectrometer in adjustable triode configuration for photo-induced field emission measurements

We have constructed a new ultrahigh vacuum apparatus with a triode configuration for the systematic investigation of photo-induced field emission (PFE) from metallic or semiconducting cathodes. These are exposed to electric fields up to 400 MV∕m and laser irradiation by means of hole or mesh gates. Cathodes and gates are in situ exchangeable and adjustable with high precision to ensure a homogeneous extraction of electrons which are partially transmitted to the fixed electron spectrometer. Its hemispherical sector analyzer provides an energy resolution limit of 8 meV. The commissioning of the measurement system has been performed with a tungsten needle. Its temperature showed up in the high-energy tail of the electron spectrum, while its work function was derived from the spectral low-energy part combined with the integral current-voltage curve. First PFE measurements on B-doped Si-tip arrays yielded a small field emission current increase under green laser illumination. A shift and splitting of the energy spectra was observed which revealed different emission regimes as well as the photosensitivity of the cathode due to carrier excitation into the conduction band. For the full exploitation of the PFE system, a tunable laser over a wide eV-range is required.     

Measurements of the fast electron bremsstrahlung emission during electron cyclotron resonance heating in the HL-2A tokamak

A fast electron bremsstrahlung (FEB) diagnostic technique based on cadmium telluride (CdTe) detector has been developed recently in the HL-2A tokamak for measurements of the temporal evolution of FEB emission in the energy range of 10-200 keV. With a perpendicular viewing into the plasma on the equatorial plane, the hard x-ray spectra with eight different energy channels are measured. The discrimination of the spectra is implemented by an accurate spectrometry. The system also makes use of fast digitization and software signal processing technology. An ambient environment of neutrons, gammas, and magnetic disturbance requires careful shielding. During electron cyclotron resonance heating, the generation of fast electrons and the oscillations of electron fishbone (e-fishbone) have been found. Using the FEB measurement system, it has been experimentally identified that the mode strongly correlates with the electron cyclotron resonance heating produced fast electrons with 30-70 keV.