Deconvolution of Stark broadened spectra for multi-point density measurements in a flow Z-pinch

Stark broadened emission spectra, once separated from other broadening effects, provide a convenient non-perturbing means of making plasma density measurements. A deconvolution technique has been developed to measure plasma densities in the ZaP flow Z-pinch experiment. The ZaP experiment uses sheared flow to mitigate MHD instabilities. The pinches exhibit Stark broadened emission spectra, which are captured at 20 locations using a multi-chord spectroscopic system. Spectra that are time- and chord-integrated are well approximated by a Voigt function. The proposed method simultaneously resolves plasma electron density and ion temperature by deconvolving the spectral Voigt profile into constituent functions: a Gaussian function associated with instrument effects and Doppler broadening by temperature; and a Lorentzian function associated with Stark broadening by electron density. The method uses analytic Fourier transforms of the constituent functions to fit the Voigt profile in the Fourier domain. The method is discussed and compared to a basic least-squares fit. The Fourier transform fitting routine requires fewer fitting parameters and shows promise in being less susceptible to instrumental noise and to contamination from neighboring spectral lines. The method is evaluated and tested using simulated lines and is applied to experimental data for the 229.69 nm C III line from multiple chords to determine plasma density and temperature across the diameter of the pinch. These measurements are used to gain a better understanding of Z-pinch equilibria.     

Measurement of the D alpha spectrum produced by fast ions in DIII-D

Fast ions are produced by neutral beam injection and ion cyclotron heating in toroidal magnetic fusion devices. As deuterium fast ions orbit around the device and pass through a neutral beam, some deuterons neutralize and emit D(alpha) light. For a favorable viewing geometry, the emission is Doppler shifted away from other bright interfering signals. In the 2005 campaign, we built a two channel charge-coupled device based diagnostic to measure the fast-ion velocity distribution and spatial profile under a wide variety of operating conditions. Fast-ion data are acquired with a time resolution of approximately 1 ms, spatial resolution of approximately 5 cm, and energy resolution of approximately 10 keV. Background subtraction and fitting techniques eliminate various contaminants in the spectrum. Neutral particle and neutron diagnostics corroborate the D(alpha) measurement. Examples of fast-ion slowing down and pitch angle scattering in quiescent plasma and fast-ion acceleration by high harmonic ion cyclotron heating are presented.     

A spectral system with the spatial resolution for plasma motion detection in the GOL-3 multi-mirror trap

A spectral system with the spatial resolution intended for diagnosing rotation of the plasma in the GOL-3 facility is described. The system is capable of measuring the radial dependences of the Doppler shift and broadening of spectral lines emitted by the plasma from various points in it. A DFS-24 double focusing spectrometer with a reciprocal linear dispersion of 0.45 nm/mm is the basis for the spectral system. If a radiation source is axially symmetric and extended, it is possible to obtain the intensity of spectral lines on the surface of the photosensitive area of a CCD array with the spatial resolution across the plasma by forming an image of the plasma in the sagittal focal plane of the entrance collimation lens, though the spectrometer astigmatism is strong. As a result, a spatial resolution of 0.8 mm has been attained in the developed system at a 55-mm field of view.     

Fast-ion Dα measurements of the fast-ion distribution (invited)

The fast-ion Dα (FIDA) diagnostic is an application of charge-exchange recombination spectroscopy. Fast ions that neutralize in an injected neutral beam emit Balmer-α light with a large Doppler shift. The spectral shift is exploited to distinguish the FIDA emission from other bright sources of Dα light. Background subtraction is the main technical challenge. A spectroscopic diagnostic typically achieves temporal, energy, and transverse spatial resolution of ～1?ms, ～10?keV, and ～2?cm, respectively. Installations that use narrow-band filters achieve high spatial and temporal resolution at the expense of spectral information. For high temporal resolution, the bandpass-filtered light goes directly to a photomultiplier, allowing detection of ～50?kHz oscillations in FIDA signal. For two-dimensional spatial profiles, the bandpass-filtered light goes to a charge-coupled device camera; detailed images of fast-ion redistribution at instabilities are obtained. Qualitative and quantitative models relate the measured FIDA signals to the fast-ion distribution function. The first quantitative comparisons between theory and experiment found excellent agreement in beam-heated magnetohydrodynamics (MHD)-quiescent plasmas. FIDA diagnostics are now in operation at magnetic-fusion facilities worldwide. They are used to study fast-ion acceleration by ion cyclotron heating, to detect fast-ion transport by MHD modes and microturbulence, and to study fast-ion driven instabilities.     

Beam emission spectroscopy turbulence imaging system for the MAST spherical tokamak

A new beam emission spectroscopy turbulence imaging system has recently been installed onto the MAST spherical tokamak. The system utilises a high-throughput, direct coupled imaging optics, and a single large interference filter for collection of the Doppler shifted D(α) emission from the ~2 MW heating beam of ~70 keV injection energy. The collected light is imaged onto a 2D array detector with 8 × 4 avalanche photodiode sensors which is incorporated into a custom camera unit to perform simultaneous 14-bit digitization at 2 MHz of all 32 channels. The array is imaged at the beam to achieve a spatial resolution of ~2 cm in the radial (horizontal) and poloidal (vertical) directions, which is sufficient for detection of the ion-scale plasma turbulence. At the typical photon fluxes of ~10(11) s(-1) the achieved signal-to-noise ratio of ~300 at the 0.5 MHz analogue bandwidth is sufficient for detection of relative density fluctuations at the level of a few 0.1%. The system is to be utilised for the study of the characteristics of the broadband, ion-scale turbulence, in particular its interaction with flow shear, as well as coherent fluctuations due to various types of MHD activity.     

Microscopic spectral imaging using a video camera

A technique is described which permits the simultaneous acquisition of multiple fluorescent emission and/or absorption spectra from discrete regions of a specimen under microscopic observation. The instrument consists of a modified inverted microscope, an optical diffraction grating, a silicon intensified target (SIT) camera, and a digital video image processor. Observation of the zero diffraction order of the grating with the SIT camera permits an optical slice of the specimen to be selected by positioning the region of interest over the entrance slit of the grating housing. To obtain the spectral characteristics of this optical slice, the grating is rotated to impinge the first order diffraction on the camera. The video image of this first order diffraction maintains spatial integrity along the slit's long axis and provides spectral dispersion on the perpendicular axis. Thus, each of the horizontal video lines along the long axis of the slit represents a spectral analysis of the corresponding spatial location within the specimen. The spectral resolution (0.2 nm/channel) of each video line is determined by the resolution of the camera system in conjunction with the resolution of the grating. The image processing system acquires and processes all 500 spectra in 33 ms and permits the accurate localization of the source of each spectrum in the slice. This type of topological spectral analysis permits the determination of both spatial and spectral characteristics of intrinsic or extrinsic chromophores within the specimen. In addition, this technique permits the detection of and the possible correction for photobleaching, light scattering and image plane effects. The application of this technique to the study of single cells is discussed and an example of the technique in determining the fluorescence spectra of acridine orange within the nucleus of an intact mammalian blastocyst is described.     

Layout and results from the initial operation of the high-resolution x-ray imaging crystal spectrometer on the Large Helical Device

First results of ion and electron temperature profile measurements from the x-ray imaging crystal spectrometer (XICS) diagnostic on the Large Helical Device (LHD) are presented. This diagnostic system has been operational since the beginning of the 2011 LHD experimental campaign and is the first application of the XICS diagnostic technique to helical plasma geometry. The XICS diagnostic provides measurements of ion and electron temperature profiles in LHD with a spatial resolution of 2 cm and a maximum time resolution of 5 ms (typically 20 ms). Ion temperature profiles from the XICS diagnostic are possible under conditions where charge exchange recombination spectroscopy (CXRS) is not possible (high density) or is perturbative to the plasma (low density or radio frequency heated plasmas). Measurements are made by using a spherically bent crystal to provide a spectrally resolved 1D image of the plasma from line integrated emission of helium-like Ar(16 +). The final hardware design and configuration are detailed along with the calibration procedures. Line-integrated ion and electron temperature measurements are presented, and the measurement accuracy is discussed. Finally central temperature measurements from the XICS system are compared to measurements from the Thomson scattering and CXRS systems, showing excellent agreement.     

Investigation of a high spatial resolution method based on polar coordinate maximum entropy method for analyzing electron density fluctuation data measured by laser phase contrast

Laser phase contrast is a powerful diagnostic method to determine the spatial distribution of electron density fluctuations in magnetically confined plasmas, although its applicability depends on magnetic field configurations. The spatial resolution of fluctuations is linked with the resolution of the propagation direction that is derived from the two-dimensional spectral analysis of the wavenumber for the fluctuations. The method was applied to fluctuation measurements in a compact helical system. In order to improve the resolution of the propagation direction with a relatively small number of data points, the maximum entropy method with polar coordinates was employed. A spatial resolution of the order of 1 cm was obtained, which is satisfactory in a plasma with a 20 cm minor radius.     

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.     

Poloidal beam emission spectroscopy system for the measurement of density fluctuations in Large Helical Device

A system of beam emission spectroscopy (BES) for density fluctuation measurements having the sightlines passing through the plasma in the poloidal direction was developed in the Large Helical Device (LHD). Even though the angle between the beam and the sightline is slightly larger than a right angle, Doppler-shifted beam emission can be distinguished from background emission because of the high energy (120-170 keV) of the neutral beam for heating with negative ion sources. Spatial resolution is about 0.1-0.2 in the normalized radius. Compared with the prototype BES system with toroidal sightlines, the BES system with poloidal sightlines showed improved spatial resolution.     

Calculation of spatial response of 2D beam emission spectroscopy diagnostic on MAST

The beam emission spectroscopy (BES) turbulence diagnostic on MAST is to be upgraded in June 2010 from a one-dimensional trial system to a two-dimensional imaging system (8?radial×4?poloidal channels) based on a newly developed avalanche photodiode array camera. The spatial resolution of the new system is calculated in terms of the point spread function to account for the effects of field-line curvature, observation geometry, the finite lifetime of the excited state of the beam atoms, and beam attenuation and divergence. It is found that the radial spatial resolution is ～2-3?cm and the poloidal spatial resolution ～1-5?cm depending on the radial viewing location. The absolute number of detected photons is also calculated, hence the photon noise level can be determined.     

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.     

基于连续光抽运的布里渊光时域分析仪新技术

介绍了一种基于连续的抽运—探测光相关的布里渊分布式光纤传感器新技术,该技术采用频率调制连续的抽运光和探测光,通过检测沿光纤的后向布里渊散射光的频移实现温度和应变的分布式测量,其空间分辨力可达1cm,比常规基于脉冲抽运光的布里渊光时域分析仪(BOTDA)技术的分辨力高近两个数量级。还结合常规BOTDA讨论了该新技术高分辨力的优势和测量范围受限制的不足。     

Optical emission spectroscopy of the Linac4 and superconducting proton Linac plasma generators

CERN's superconducting proton Linac (SPL) study investigates a 50 Hz high-energy, high-power Linac for H(-) ions. The SPL plasma generator is an evolution of the DESY ion source plasma generator currently operated at CERN's Linac4 test stand. The plasma generator is a step towards a particle source for the SPL, it is designed to handle 100 kW peak RF-power at a 6% duty factor. While the acquisition of an integrated hydrogen plasma optical spectrum is straightforward, the measurement of a time-resolved spectrum requires dedicated amplification schemes. The experimental setup for visible light based on photomultipliers and narrow bandwidth filters and the UV spectrometer setup are described. The H(α), H(β), and H(γ) Balmer line intensities, the Lyman band and alpha transition were measured. A parametric study of the optical emission from the Linac4 ion source and the SPL plasma generator as a function of RF-power and gas pressure is presented. The potential of optical emission spectrometry coupled to RF-power coupling measurements for on-line monitoring of short RF heated hydrogen plasma pulses is discussed.     

Spectroscopic comparison between 1200 grooves/mm ruled and holographic gratings of a flat-field spectrometer and its absolute sensitivity calibration using bremsstrahlung continuum

A flat-field extreme ultraviolet (EUV) spectrometer with a varied line spacing groove grating (1200 grooves/mm at grating center) has been developed to study the emission spectra from highly ionized medium Z impurities in large helical device (LHD). It covers a wavelength range of 50-500 A using a mechanically ruled grating, which was later replaced by a newly developed laminar-type holographic grating for comparative studies. Differences in spectral resolution, intensities of higher order spectra, and sensitivities of the spectrometer were studied between the two gratings by observing the emission spectra of LHD plasmas. Although the achieved resolution was alike between them, i.e., deltalambda approximately 0.24 A at 200 A, the holographic grating was much superior in suppressing the higher order light than the ruled grating. The relative sensitivity between the two gratings was evaluated using continuum radiation from LHD plasmas. As a result, it was found that the holographic grating has a flat response in the full wavelength range, but the sensitivity of the ruled grating drops sharply below 200 A. A new technique for the absolute calibration of the EUV holographic grating spectrometer was tried by combining the continuum radiation with a branching ratio of C IV lines (3p-3s: 5800 A/3p-2s: 312 A), and an accurate absolute sensitivity has been successfully obtained.     

2.2 microm axial resolution optical coherence tomography based on a 400 nm-bandwidth superluminescent diode

We demonstrate 2.2 microm axial resolution optical coherence tomography (OCT) in 1.1-1.7 microm wavelength regime by using a nonidentical multiple-quantum-well (MQW) superluminescent diode (SLD) with record-bandwidth emission. The compact, low-cost, and reliable light source with extreme broadband emission demonstrates significant potentials for spectroscopic and commercial OCT applications requiring ultrahigh spatial resolution.     

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.     

A glow-discharge-pumped broadband short-wavelength lamp

A broadband ultraviolet-vacuum-ultraviolet lamp, which emits at wavelengths of 150–300 nm, and its output characteristics are described. The lamp is excited by a longitudinal glow discharge, which is initiated in a quartz tube with an inner diameter of 1.4 cm and an interelectrode distance of 10 cm. The tube is filled with a He/Br₂ working mixture at a total pressure of 0.2–1.0 kPa. The lamp emission spectrum consists of a 163.3-nm spectral line of bromine atoms and continuum in the spectral region 165–300 nm. A continuum is formed as a result of an overlap of broad emission bands of bromine molecules. The total emission power of the lamp reaches 4–5 W at an electric power of up to 60 W. The lamp’s efficiency is 5–10%, and its service life in the gas-static mode is 300–400 h.     

Emission spectroscopy of laser ablation plasma with time gating by acousto-optic modulator

The capability of acousto-optic modulator (AOM) to perform time-gated measurements for laser ablation plasma spectroscopy has been examined. Especially, we focused on the capability of the "AOM gating" to exclude the continuum and extremely broadened spectra usually observed immediately after the laser ablation. Final goal of the use of the AOM is to achieve considerable downsizing of the system for in situ and on-site analyses. In the present paper, it is shown that narrow and clear spectral lines can be obtained with the AOM gating even if the target is submerged in water. Also, application of this technique to the targets in air is demonstrated. It has been revealed that the AOM gating is fast enough to exclude the continuum and broadened lines, while effectively acquiring sufficiently narrow atomic lines lasting slightly longer than the continuum.     

Low-hazard metallography of moisture-sensitive electrochemical cells

A better understanding of paper properties requires a detailed knowledge about the spatial arrangement of its constituent materials in its structure. This paper presents a novel approach for the analysis of the three-dimensional paper structure at the fibre level. A technique combining a rotary microtome and an optical microscopy was developed allowing serial sectioning of hundreds of cuts. The microscope is fixed on a moveable stage and mounted in front of a microtome. Repeatedly, thin slices are cut off an embedded paper sample and the cut block surface is scanned in a fully automated process. The prototype built is able to digitize paper samples with a size of more than 1 cm(2) at a possible three-dimensional resolution below 1 μm. Advanced computer vision methods are applied to extract relevant information from the digitized samples. Currently, the most important applications are the analysis of pigment coating layers on the paper surfaces and the analysis of fibre transverse morphology. Besides the analysis of paper structures, this technique is also suited for the spatial analysis of other materials, if the structural features are accessible with light optical microscopy.