X-ray Charge-coupled Device Camera Research Articles

Soft x-ray laser beamline for surface processing and damage studies.

We have developed a soft x-ray laser (SXRL) beamline equipped with an intensity monitor dedicated to ablation study such as surface processing and damage formation. The SXRL beam having a wavelength of 13.9 nm, pulse width of 7 ps, and pulse energy of around 200 nJ is generated from Ag plasma mediums using an oscillator-amplifier configuration. The SXRL beam is focused onto the sample surface by the Mo/Si multilayer coated spherical mirror. To get the correct irradiation energy/fluence, an intensity monitor composed of a Mo/Si multilayer beam splitter and an x-ray charge-coupled device camera has been installed in the beamline. The Mo/Si multilayer beam splitter has a large polarization dependence in the reflectivity around the incident angle of 45°. However, by evaluating the relationship between reflectivity and transmittance of the beam splitter appropriately, the irradiation energy onto the sample surface can be derived from the energy acquired by the intensity monitor. This SXRL beamline is available to not only the ablation phenomena but also the performance evaluation of soft x-ray optics and resists.

Quantitative Analysis of Ultralow-Density Materials Using Laboratory-Based Quasi-Monochromatic Radiography

The measurement of the density of materials, especially ultralow-density foams, is difficult in that the measurement must be precise and localizable. The density of the material is often governed by its cellular (i.e., porous) structure, and many techniques exist to create that structure. Often, the cellular structure can vary from one location within the material to another, and when at low densities (i.e., densities lower than ~500 mg/cm3), it can vary due to shrinkage during syneresis, collapse under the weight of gravity, or gas/water vapor uptake. Quantifying this variation is important for a variety of applications, especially when used in plasma physics targets. Knowing the density and its variation across the sample is critical for experimental results to be accurately predicted by physics calculations and for modeling the results of the physics targets. The use of quasi-monochromatic radiography provides a means to image the two-dimensional (2-D) distribution of density variation within silica aerogel materials and to quantitatively measure that variation from sample to sample and lot to lot. For this study, two batches of silica aerogels with targeted densities of ~20 mg/cm3 were created, one batch at Lawrence Livermore National Laboratory, and the other batch at Los Alamos National Laboratory. Outlined here is a quasi-monochromatic radiography system using various X-ray sources coupled to a doubly curved crystal optic and X-ray charge-coupled device camera to image and characterize these materials. It was found that measuring the density both gravimetrically and using quasi-monochromatic radiography were statistically identical, although the two batches were found to be slightly higher than their targeted density due to shrinkage. The radiography system also provided 2-D information as to the aerogel quality, i.e., presence of voids, chipped material, or inclusions.

Design of a prototype split-and-delay unit for XFEL pulses, and their evaluation by synchrotron radiation X-rays.

A prototype split-and-delay unit (SDU) for X-ray free-electron laser (XFEL) pulses is proposed based on the Graeff-Bonse four-Bragg-reflection interferometer by installing 12.5° slopes. The SDU can continuously provide a delay time from approximately -20 to 40 ps with a resolution of less than 26 fs. Because the SDU was constructed from a monolithic silicon crystal, alignment is straightforward. The obtained thoroughputs of the SDU reached 0.7% at 7.46 keV and 0.02% at 14.92 keV. The tunability of the delay time using the proposed SDU was demonstrated by finding the interference effects of the split X-rays, and the time resolution of the proposed SDU was evaluated using the width of the interference pattern recorded on the X-ray charge-coupled device camera by changing the energy, i.e. longitudinal coherence length, of the incident X-rays. It is expected that the proposed SDU will be applicable to XFEL experiments using delay times from tens of femtoseconds to tens of picoseconds, e.g. intensity correlation measurements.

VUV/XUV measurements of impurity emission in plasmas with liquid lithium surfaces on LTX

The VUV/XUV spectrum has been measured on the Lithium Tokamak eXperiment (LTX) using a transmission grating imaging spectrometer (TGIS) coupled to a direct-detection x-ray charge-coupled device camera. TGIS data show significant changes in the ratios between the lithium and oxygen impurity line emission during discharges with varying lithium wall conditions. Lithium coatings that have been passivated by lengthy exposure to significant levels of impurities contribute to a large O/Li ratio measured during LTX plasma discharges. Furthermore, previous results have indicated that a passivated lithium film on the plasma facing components will function as a stronger impurity source when in the form of a hot liquid layer compared to a solid lithium layer. However, recent TGIS measurements of plasma discharges in LTX with hot stainless steel boundary shells and a fresh liquid lithium coating show lower O/Li impurity line ratios when compared to discharges with a solid lithium film on cool shells. These new measurements help elucidate the somewhat contradictory results of the effects of solid and liquid lithium on plasma confinement observed in previous experiments.

Dry etching characteristics of TaN absorber for extreme ultraviolet mask with Ru buffer layer

The extreme ultraviolet (EUV) mask using reflective optics has a structure totally different from that of the conventional mask. This study investigated the dry etching characteristics of the EUV mask layer in which TaN and Ru were used as the absorber and buffer layer, respectively. The TaN absorber etching requires high etch selectivity of TaN over Ru to achieve the high EUV reflectivity. In this work, the TaN etching rate was investigated with various halogen gases in the capacitively coupled plasma type etching system. The etching rates with the fluorinated gases were higher than that with the chlorinated gas. Ru could not be etched without oxygen in all of the considered etch gases. The etching rate of TaN and its etch selectivity to Ru were increased with the increase of RF power, pressure and SF6 gas flow ratio. When the real EUV mask specimens were 100% over etched under the highest selectivity etching condition, the Ru buffer layer still remained, which was confirmed by x-ray photoelectron spectroscopy analysis. The surface roughness of etched reflective mirror was very small, and the EUV reflectivity was equivalent to that of the reference mirror (EUV reflectivity is over 60%) which was confirmed by measuring the intensity and the diffracted pattern of 13.5 nm EUV obtained using x-ray charge coupled device camera. The Ru buffer layer could be removed by the O2 plasma used in the posterior photo resist ashing process without affecting the EUV reflectivity.

Three-dimensional observation of biological specimens using a laser plasma soft x-ray microscope

Laser plasma soft x-ray microscope using Wolter mirror optics was applied to three-dimensional observation of a biological specimen. To obtain a high-resolution image with an x-ray charge-coupled device camera, a high-magnification imaging system with a relay mirror was used. To obtain the three-dimensional image of a biological specimen, imaging tomography was used. The three-dimensional image of a diatom was obtained with a spatial resolution of 250nm.

Development of X-ray CCD camera system with high readout rate using ASIC

We report on the development of an X-ray charge-coupled device (CCD) camera system with high readout rate using application-specific integrated circuit (ASIC) and Camera Link standard. The distinctive Δ Σ type analog-to-digital converter is introduced into the chip to achieve effective noise shaping and to obtain a high resolution with relatively simple circuits. The unit test proved moderately low equivalent input noise of 70 μ V with a high readout pixel rate of 625 kHz, while the entire chip consumes only 100 mW. The Camera Link standard was applied for the connectivity between the camera system and frame grabbers. In the initial test of the whole system, we adopted a P-channel CCD with a thick depletion layer developed for X-ray CCD camera onboard the next Japanese X-ray astronomical satellite. The characteristic X-rays from 109Cd were successfully read out resulting in the energy resolution of 379 ( ± 7 ) eV (FWHM) @22.1 keV, that is, Δ E / E = 1.7 % with a readout rate of 44 kHz.

Development of a compact x-ray particle image velocimetry for measuring opaque flows

A compact x-ray particle image velocimetry (PIV) system employing a medical x-ray tube as a light source was developed to measure quantitative velocity field information of opaque flows. The x-ray PIV system consists of a medical x-ray tube, an x-ray charge coupled device camera, a programmable shutter for a pulse-type x ray, and a synchronization device. Through performance tests, the feasibility of the developed x-ray PIV system as a flow measuring device was verified. To check the feasibility of the developed system, we tested a tube flow at two different mean velocities of 1 and 2 mm/s. The x-ray absorption of tracer particles must be quite different from that of working fluid to have a good contrast in x-ray images. All experiments were performed under atmospheric pressure condition. This system is unique and useful for investigating various opaque flows or flows inside opaque conduits.

Flat field anomalies in an x-ray charge coupled device camera measured using a Manson x-ray source

The static x-ray imager (SXI) is a diagnostic used at the National Ignition Facility (NIF) to measure the position of the x rays produced by lasers hitting a gold foil target. The intensity distribution taken by the SXI camera during a NIF shot is used to determine how accurately NIF can aim laser beams. This is critical to proper NIF operation. Imagers are located at the top and the bottom of the NIF target chamber. The charge coupled device (CCD) chip is an x-ray sensitive silicon sensor, with a large format array (2k x 2k), 24 microm square pixels, and 15 microm thick. A multianode Manson x-ray source, operating up to 10 kV and 10 W, was used to characterize and calibrate the imagers. The output beam is heavily filtered to narrow the spectral beam width, giving a typical resolution E/DeltaE approximately = 10. The x-ray beam intensity was measured using an x-ray photodiode that has an accuracy better than 1% up to the Si K edge and better than 5% at higher energies. The x-ray beam provides full CCD illumination and is flat, within +/-1% maximum to minimum. The spectral efficiency was measured at ten energy bands ranging from 930 to 8470 eV. We observed an energy dependent pixel sensitivity variation that showed continuous change over a large portion of the CCD. The maximum sensitivity variation occurred at 8470 eV. The geometric pattern did not change at lower energies, but the maximum contrast decreased and was not observable below 4 keV. We were also able to observe debris, damage, and surface defects on the CCD chip. The Manson source is a powerful tool for characterizing the imaging errors of an x-ray CCD imager. These errors are quite different from those found in a visible CCD imager.

A tabletop femtosecond time-resolved soft x-ray transient absorption spectrometer

A laser-based, tabletop instrument is constructed to perform femtosecond soft x-ray transient absorption spectroscopy. Ultrashort soft x-ray pulses produced via high-order harmonic generation of the amplified output of a femtosecond Ti:sapphire laser system are used to probe atomic core-level transient absorptions in atoms and molecules. The results provide chemically specific, time-resolved dynamics with sub-50-fs time resolution. In this setup, high-order harmonics generated in a Ne-filled capillary waveguide are refocused by a gold-coated toroidal mirror into the sample gas cell, where the soft x-ray light intersects with an optical pump pulse. The transmitted high-order harmonics are spectrally dispersed with a homebuilt soft x-ray spectrometer, which consists of a gold-coated toroidal mirror, a uniform-line spaced plane grating, and a soft x-ray charge coupled device camera. The optical layout of the instrument, design of the soft x-ray spectrometer, and spatial and temporal characterizations of the high-order harmonics are described. Examples of static and time-resolved photoabsorption spectra collected on this apparatus are presented.

Operation of a single-photon–counting x-ray charge-coupled device camera spectrometer in a petawatt environment

The use of a single-photon–counting x-ray charge-coupled device (CCD) camera as an x-ray spectrometer is a well-established technique in ultrashort-pulse laser experiments. In single-photon–counting mode, the pixel value of each readout pixel is proportional to the energy deposited from the incident x-ray photon. For photons below 100 keV, a significant fraction of the events deposits all the energy in a single pixel. A histogram of the pixel readout values gives a good approximation of the x-ray spectrum. This technique requires almost no alignment, but it is very sensitive to signal-to-background issues, especially in a high-energy petawatt environment. Shielding the direct line of sight to the target was not sufficient to obtain a high-quality spectrum, for the experiments reported here the CCD camera had to be shielded from all sides with up to 10 cm of lead.

Energy and spatial resolved measurement of soft x-ray emission with photon counting x-ray charge coupled device camera in compact helical system

Two-dimensional profiles (32×16 spatial channels) of energy spectra of x-ray emission are measured with an energy resolution of 0.16 keV using the photon counting soft x-ray charge coupled device (CCD) camera by optimizing the intensity of x ray with attenuation Be filter in the compact helical system. The energy spectra measured with the CCD camera agrees with that estimated from electron temperature and density measured with YAG Thomson scattering.

In situ calibration of a flat-field grazing incidence spectrograph with an x-ray charge-coupled device camera using a transmission grating and a laser-produced plasma for x-ray laser research

An in situ calibration experiment of a soft x-ray spectrograph for x-ray laser research has been performed. The calibrated spectrograph was a flat-field grazing incidence spectrometer with an x-ray charge-coupled device camera. The absolute sensitivity of the spectrograph was obtained for the wavelength range from 2.6 to 4.0 nm using a transmission grating spectrograph as a reference spectrograph set along the axial direction of a line-focused laser-produced plasma. The absolute sensitivity determined in this work shows nearly the same value as that deduced from the calibration experiment using synchrotron radiation.

Near-field spatial imaging of a Ni-like Ag 140-Å x-ray laser

We measure the two-dimensional, near-field spatial distribution of a 140-\AA{} nickel-like silver x-ray laser at the output aperture with high magnification using a curved multilayer x-ray mirror to image the output onto an x-ray charge-coupled device camera. Lasing is created by illuminating silver slab targets with a pair of 75 ps laser pulses separated by 2.2 nsec from the Vulcan laser. The two-dimensional, high-resolution, spatial image shows the x-ray laser source size and its position relative to the target surface. A dramatic change in both the position and source size are observed for the refraction compensating curved target as compared with the flat targets.

Laboratory characterization of Wölter x-ray optics

We have conducted characterization measurements of a Wölter grazing incidence x-ray microscope. The measurements were carried out on 5% sectors of a Wölter x-ray optic in a laboratory utilizing a new, very versatile, high brightness x-ray source. Absolute reflectance measurements as a function of x-ray energy were made with Si(Li) detectors to acquire continuum spectra prior to and after reflecting off the Wölter optic. Spatial resolution characteristics of the Wölter were mapped out using backilluminated pinholes or grids imaged onto film or an x-ray charge-coupled device camera. The implications of the asymmetric point spread function are considered.

High precision Wölter optic calibration facility

We have developed an off-line facility for very precise characterization of the reflectance and spatial resolution of the grazing incidence Wölter type I x-ray optics used at Nova. The primary component of the facility is a new, very versatile, high brightness x-ray source consisting of a focused DC electron beam incident onto a precision manipulated target-pinhole array. The data are recorded with a selection of detectors. For imaging measurements we use direct exposure x-ray film modules or an x-ray charge-coupled device camera. For energy-resolved reflectance measurements, we use lithium drifted silicon detectors and a proportional counter. An in situ laser alignment system allows precise location and rapid periodic alignment verification of the x-ray point source, the statically mounted Wölter optic, and the chosen detector.