Local Emissivity Profiles Research Articles

Development of a multi-channel visible bremsstrahlung measurement system for the effective charge in the versatile experiment spherical torus (VEST).

A diagnostic system for measuring the effective charge in the versatile experiment spherical torus (VEST) has been developed. The system utilizes a toroidal array to observe the plasma radius on the low magnetic field side, providing a spatially resolved Zeff. The target wavelength of visible bremsstrahlung (VB) was carefully selected to avoid contamination by line emissions. The detector signal was calibrated using a halogen light source and an integrating sphere to obtain an absolute value of the radiative power from each chord. The local emissivity profile was reconstructed from the line-integrated VB emission using the Abel inversion method. Reconstruction tests were performed on various shapes of phantom profiles to effectively reconstruct the local emissivity from the measurements. We found that the initial measurements of the multi-channel VB system were consistent with the results of other independent measurements, supporting the validity of the new measurements. Finally, we obtained the initial result of Zeff in the VEST.

Inversion techniques to obtain local rotation velocity and ion temperature profiles for the x-ray crystal spectrometer on EAST

Inversion techniques are conducted based on the tangential x-ray crystal spectroscopy (TXCS) geometry on EAST to obtain the local profiles of ion temperature (T i) and toroidal rotation velocity (v t). Firstly, local emissivity profiles of the impurity argon are obtained using the asymmetrical Abel inversion. Then, the local v t and T i profiles are calculated by considering the local emissivity profiles and the TXCS detailed geometry. In addition, how the changes in the v t profiles affect the accuracy in the T i profiles is discussed in detail. It is also found that the line-integrated T i profiles are becoming less accurate with the increase in the radial gradient in the local v t profiles. Nonetheless, accurate T i radial profiles are reconstructed after considering the effects of the emissivity and velocity, which are verified by comparing the inverted v t and T i profiles with those local profile measurements from the Charge eXchange Recombination Spectroscopy (CXRS) on EAST.

WEST tokamak hard x-ray tomography inversion

Long-pulse steady-state discharges with lower hybrid current drive were successfully obtained in the limiter configuration machine, Tore Supra. The hard x-ray (HXR) diagnostic system in Tore Supra associated with tomographic inversion provided a good approximation of the lower hybrid power deposition profile. The accurate plasma current control through this tomographic reconstruction leads to an improvement of plasma confinement. Tore Supra was upgraded to WEST, Tungsten (W) Environment in Steady-state Tokamak. The plasma geometry was changed to the D-shaped geometry. Therefore, a novel inversion code for WEST has been developed. The method used in the code is based on the Bessel function of the first kind of order zero. The code validation was performed using phantom emissivity profiles and WEST experimental data. It has been found that the line radiation of the tungsten divertor in WEST and noise significantly affect the inversion. The local HXR emissivity profile with acceptable accuracy can be achieved by applying a singular value decomposition filter and suppressing the lines of sight that intersect the divertors.

Multi-energy reconstructions, central electron temperature measurements, and early detection of the birth and growth of runaway electrons using a versatile soft x-ray pinhole camera at MST.

A multi-energy soft x-ray pinhole camera has been designed, built, and deployed at the Madison Symmetric Torus to aid the study of particle and thermal transport, as well as MHD stability physics. This novel imaging diagnostic technique employs a pixelated x-ray detector in which the lower energy threshold for photon detection can be adjusted independently on each pixel. The detector of choice is a PILATUS3 100K with a 450 μm thick silicon sensor and nearly 100 000 pixels sensitive to photon energies between 1.6 and 30 keV. An ensemble of cubic spline smoothing functions has been applied to the line-integrated data for each time-frame and energy-range, obtaining a reduced standard-deviation when compared to that dominated by photon-noise. The multi-energy local emissivity profiles are obtained from a 1D matrix-based Abel-inversion procedure. Central values of Te can be obtained by modeling the slope of the continuum radiation from ratios of the inverted radial emissivity profiles over multiple energy ranges with no a priori assumptions of plasma profiles, magnetic field reconstruction constraints, high-density limitations, or need of shot-to-shot reproducibility. In tokamak plasmas, a novel application has recently been tested for early detection, 1D imaging, and study of the birth, exponential growth, and saturation of runaway electrons at energies comparable to 100 × Te,0; thus, early results are also presented.

Modular bolometric/soft x-ray diagnostic in Sino-UNIted Spherical Tokamak.

A bolometer/soft x-ray diagnostic based on modular absolute extreme ultraviolet photodiodes has been built to obtain the local emission profile of a poloidal cross section in Sino-UNIted Spherical Tokamak (SUNIST). The system consists of seven photodiode arrays of 112 channels located around the low field side of a poloidal cross section. All arrays are mounted into 81 × 36 × 33mm3 modules, which could be installed flexibly on the wall of the vacuum vessel. Tungsten shields with adjustable pinholes restrict the radiation flux. Compact flexible printed circuits with multilayer copper polygon poured on both sides are designed to transmit signals with electronic noise, which is reduced to less than 1/10 before. A two-stage transimpedance amplifier array of 700 kHz bandwidth is fixed on the atmosphere side of a vacuum electrical feedthrough and is powered by battery packs. A synchronized switch shuts down the circuit to reduce the heat production and increase the battery life. A line-integrated radiation emission profile with 2cm spatial resolution and temporal resolution up to 2 µs is obtained, and the local emission profile of SUNIST is reconstructed with the help of a newly developed tomography scheme for movable detector modules.

Space-resolved extreme ultraviolet spectrometer for impurity diagnostics in HL-2A

Abstract A space-resolved extreme ultraviolet (EUV) spectrometer system has been developed in the HL-2A tokamak for impurity monitoring and transport studies. A charge-coupled-device (CCD) detector is mounted on a two-stage wavelength scanning mechanism for measuring the spatial profile of impurity line emissions in the wavelength range of 30−600 A. A good spatial profile of impurity line emissions is observed throughout the lower half of the HL-2A plasma along the vertical direction by installing a space-resolved slit in front of an entrance slit. The vertical observation range is ∼43 cm covering a wide plasma area from the plasma central region to the vicinity of divertor separatrix X-point. In order to make a critical analysis of the impurity transport, in particular, in the plasma edge, an accurate position calibration in the observed vertical impurity line intensity profile is necessary. For the purpose a thin metal bar is installed at the diagnositc port. Then, the vertical position of the impurity profile can be accurately determined throughout the entire wavelength range. Based on the present method, the local emissivity profile of impurity line emissions is well reconstructed with good accuracy through Abel inversion technique. Resultant CIV and CVI local emissivity profiles show a clear difference in the carbon transport between ohmic and NBI heating phases.

Density evaluation of tungsten W24+, W25+, and W26+ ions using unresolved transition array at 27–34 Å in Large Helical Device

The extreme ultraviolet (EUV) spectra of a tungsten unresolved transition array (UTA) at 15–70 Å have been studied in Large Helical Device (LHD) by injecting a tungsten pellet. Vertical profiles of the UTA line are measured with a space-resolved EUV spectrometer. In our previous study, it has been found that the UTA line at wavelength intervals of 32.16–33.32, 30.69–31.71, and 29.47–30.47 Å is composed of a single ionization stage of W24+, W25+, and W26+, respectively. In this report, therefore, the densities of W24+, W25+, and W26+ ions are evaluated from the radial profile measured at the above-mentioned wavelength intervals. To evaluate ion density, the photon emission coefficients of W24+, W25+, and W26+ ions are calculated using a collisional-radiative (CR) model. The chord-integrated radial profile of UTA lines is converted to a local emissivity profile using the Abel inversion technique. The density profiles of W24+, W25+, and W26+ ions are thus obtained from the local emissivity profile and the photon emission coefficient in addition to the temperature and density profiles. The obtained density profile of the W24+ ion is analyzed in detail by investigating the dependences of the electron density and the number of tungsten particles injected by the tungsten pellet. The total tungsten ion density nW near ρ = 0.7 where the W24+ ion locates is also estimated from the W24+ ion density with fractional abundance in ionization equilibrium calculated with the Atomic Data and Analysis Structure (ADAS) code. The nW evaluated from the present CR model seems to be larger than that estimated from the number of tungsten particles injected by the pellet. Discussions are made with the nW evaluated from the photon emission coefficient in the CL version of the ADAS code.

Component investigation of ionization stages on tungsten unresolved transition array spectra for plasma diagnostics based on space-resolved extreme-ultra violet spectroscopy in large helical device

A series of experiments on tungsten spectroscopy has been carried out in a Large Helical Device with tungsten pellet injection to study the tungsten spectra. In particular, understanding of an unresolved transition array (UTA) is still insufficient, whereas the UTA is very important for the diagnostics and transport study of tungsten ions in edge plasmas of International Thermonuclear Experimental Reactor. For the purpose, extreme-ultra violet (EUV) spectra of UTA observed in the wavelength range of 15 Å–70 Å are analyzed at two different wavelength intervals of 15–45 Å and 45–70 Å, which mainly consist of Δn = 1, 2 and Δn = 0 transitions for n = 4 partially ionized tungsten ions, respectively. At first, the UTA line intensity is analyzed against central electron temperature at the temperature recovery phase after the pellet injection to examine the presence of blended lines. Next, vertical profiles measured with two space-resolved EUV spectrometers are analyzed against electron temperature profiles for further precise investigation of the UTA spectra. For the analysis, local emissivity profiles are obtained from the measured vertical intensity profiles with the Abel inversion method based on magnetic surface structures calculated by the VMEC code. It is then possible to investigate the ionization stage of tungsten ions comprising the UTA. As a result, it is found that the wavelength intervals of 49.24 Å ≤ λ ≤ 49.46 Å, 48.81 Å ≤ λ ≤ 49.03 Å, and 47.94 Å ≤ λ ≤ 48.15 Å, which are identified as W27+, W26+, and W24+, respectively, are applicable to the tungsten diagnostics. The result of the line component analysis on the tungsten UTA is summarized in tables.

Optimized tomography methods for plasma emissivity reconstruction at the ASDEX Upgrade tokamak.

The soft X-ray (SXR) emission provides valuable insight into processes happening inside of high-temperature plasmas. A standard method for deriving the local emissivity profiles of the plasma from the line-of-sight integrals measured by pinhole cameras is the tomographic inversion. Such an inversion is challenging due to its ill-conditioned nature and because the reconstructed profiles depend not only on the quality of the measurements but also on the inversion algorithm used. This paper provides a detailed description of several tomography algorithms, which solve the inversion problem of Tikhonov regularization with linear computational complexity in the number of basis functions. The feasibility of combining these methods with the minimum Fisher information regularization is demonstrated, and various statistical methods for the optimal choice of the regularization parameter are investigated with emphasis on their reliability and robustness. Finally, the accuracy and the capability of the methods are demonstrated by reconstructions of experimental SXR profiles, featuring poloidal asymmetric impurity distributions as measured at the ASDEX Upgrade tokamak.

Electron density profile measurements from hydrogen line intensity ratio method in Versatile Experiment Spherical Torus.

Electron density profiles of versatile experiment spherical torus plasmas are measured by using a hydrogen line intensity ratio method. A fast-frame visible camera with appropriate bandpass filters is used to detect images of Balmer line intensities. The unique optical system makes it possible to take images of Hα and Hβ radiation simultaneously, with only one camera. The frame rate is 1000 fps and the spatial resolution of the system is about 0.5 cm. One-dimensional local emissivity profiles have been obtained from the toroidal line of sight with viewing dumps. An initial result for the electron density profile is presented and is in reasonable agreement with values measured by a triple Langmuir probe.

Up-down asymmetry measurement of tungsten distribution in large helical device using two extreme ultraviolet (EUV) spectrometers.

Two space-resolved extreme ultraviolet spectrometers working in wavelength ranges of 10-130 Å and 30-500 Å have been utilized to observe the full vertical profile of tungsten line emissions by simultaneously measuring upper- and lower-half plasmas of LHD, respectively. The radial profile of local emissivity is reconstructed from the measured vertical profile in the overlapped wavelength range of 30-130 Å and the up-down asymmetry is examined against the local emissivity profiles of WXXVIII in the unresolved transition array spectrum. The result shows a nearly symmetric profile, suggesting a good availability in the present diagnostic method for the impurity asymmetry study.

Validation of a geometrical model for the calculation of the integrated effective emissivity of conical cavities with a lid

In this paper, a geometrical model for the numerical calculation of the integrated effective emissivity of conical isothermal cavities with a lid, is described in detail. We make use of the Montecarlo method and the ‘back ray tracing’ technique, assuming diffuse reflection and a detector situated at an arbitrary distance from the cavity aperture. First, the geometry of the problem is discussed and the local hemispherical emissivity profiles along the cone generatrix are calculated for different configurations. Then we proceed to the validation of the model, by calculating the distribution of the reflected photons in the interior of the cavity, such as it is provided by the numerical algorithm. The calculated distribution is compared with theoretical values, obtained from the expressions of the view factors for the conical geometry.The calculated values for the local and integrated effective emissivity are compared with results published by other authors, highlighting the differences between them, and the internal consistency of our model is demonstrated. Special attention to the calculation of the view factors in conical cavities affected by vignetting due to the obstruction produced by the lid, is paid. The view factors of points at the penumbral region are numerically calculated, applying the Montecarlo method, including a complete analysis of the uncertainty.

Impact of spatial resolution on air quality simulation: A case study in a highly industrialized area in Shanghai, China

The air pollution contribution from highly industrialized areas has been a prominent issue in regional air quality control. Particular emphasis on local industrial emissions is necessary to understand the complexity of air pollution over highly industrialized areas. Baoshan District, one of the most important industrialized areas in China and the most competitive steel and iron production base worldwide, was selected as the study area in this work. The WRF/CMAQ modeling system with local emission profile was applied to study the impact of spatial resolution on air quality modeling. The simulation results for SO2, NO, NO2, CO and PM10 at both 3–km and 1–km resolutions were verified by ground level observations. The results showed that the allocation of the emission inventory is improved by using finer resolution grids, which allow the consideration of detailed emission features. The influence of model resolution was more significant for air quality than for meteorology simulation. The relative errors using the finer resolution method ranged from –25% to 59%, an obvious improvement over the error value of 26%–245% obtained using the coarse resolution method. The changing tendencies of air pollutants in urban and rural areas were generally better modeled at finer than coarser resolution. However, the detailed variation in the most heavily polluted areas was still difficult to capture, and the model performance was not evidently improved by the use of a fine resolution. To improve the model performance over highly industrialized areas for future studies, combining the dynamic emission profile with detailed industrial activities and accurate local meteorological fields is suggested.

Enhancement of edge impurity transport with ECRH in the HL-2A tokamak

Edge impurity transport is studied in electron cyclotron resonance heating (ECRH) L-mode plasmas of the HL-2A tokamak based on space-resolved vacuum ultraviolet spectroscopy with which radial profiles of impurity line emissions are measured from the core region inside the last closed flux surface (LCFS) and the edge region in the scrape-off layer, simultaneously. The radial profile of carbon emissions of C V (2271 Å: 1s2s 3S–1s2p 3P) reconstructed into the local emissivity profile is analysed with a one-dimensional impurity transport code, and the diffusion coefficient and convective velocity of impurity ions are determined in the core region of the HL-2A tokamak. The impurity source is also determined with the measured absolute emissivity profiles of C IV (1548 Å: 1s22s 2S–1s22p 2P) located at the LCFS. The ratio of C V to C IV can therefore be used as an index to characterize the core impurity transport between the LCFS and the radial region of the C V emission at a normalized radius of about ρ = 0.6. The ratio measured from ohmic discharges shows a gradual decrease with electron density. However, the ratio suddenly decreases by a factor of three when the ECRH focused in the plasma centre is switched on, suggesting a strong enhancement of the impurity transport. The analysis with the transport code indicates a change in the convective term. The convective velocity of C4+ ions changes from inward to outward direction during the ECRH phase, while an inward velocity usually exists in the ohmic phase. Possible mechanisms for the reversal of the convective velocity are discussed.

Absolute intensity calibration of flat-field space-resolved extreme ultraviolet spectrometer using radial profiles of visible and extreme ultraviolet bremsstrahlung continuum emitted from high-density plasmas in Large Helical Device

A precise absolute intensity calibration of a flat-field space-resolved extreme ultraviolet (EUV) spectrometer working in wavelength range of 60-400 Å is carried out using a new calibration technique based on radial profile measurement of the bremsstrahlung continuum in Large Helical Device. A peaked vertical profile of the EUV bremsstrahlung continuum has been successfully observed in high-density plasmas (n(e) ≥ 10(14) cm(-3)) with hydrogen ice pellet injection. The absolute calibration can be done by comparing the EUV bremsstrahlung profile with the visible bremsstrahlung profile of which the absolute value has been already calibrated using a standard lamp. The line-integrated profile of measured visible bremsstrahlung continuum is firstly converted into the local emissivity profile by considering a magnetic surface distortion due to the plasma pressure, and the local emissivity profile of EUV bremsstrahlung is secondly calculated by taking into account the electron temperature profile and free-free gaunt factor. The line-integrated profile of the EUV bremsstrahlung continuum is finally calculated from the local emissivity profile in order to compare with measured EUV bremsstrahlung profile. The absolute intensity calibration can be done by comparing measured and calculated EUV bremsstrahlung profiles. The calibration factor is thus obtained as a function of wavelength with excellent accuracy. It is also found in the profile analysis that the grating reflectivity of EUV emissions is constant along the direction perpendicular to the wavelength dispersion. Uncertainties on the calibration factor determined with the present method are discussed including charge-coupled device operation modes.

Development of an automated method for in situ measurement of the geometrical properties of the ITER bolometer diagnostic

Abstract In order to derive the local emission profile of the plasma radiation in a fusion device using the line-integrated measurements of the bolometer diagnostic, tomographic reconstruction methods have to be applied to the measurements from many lines-of-sight. A successful reconstruction needs to take the finite sizes of detectors and apertures and the resulting non-ideal measurements into account. In ITER a method for in situ measurement of the geometrical properties of the various components of the bolometer diagnostic after installation is required as the viewing cones have to pass through narrow gaps between components. The method proposed to be used for ITER uses the beam of a laser with high intensity to illuminate the bolometer assembly from many different angles ξ and θ. A light-weight robot from Kuka Robotics is used to efficiently position the laser on many points covering the complete viewing cone of each line-of-sight and to direct the beam precisely into the entrance aperture of the bolometer. Measuring the response of the bolometer allows for the calculation of the transmission function t( ξ, θ), the angular etendue and finally the geometric function in reconstruction space, which is required for the tomography algorithms. Measuring the transmission function for a laboratory assembly demonstrates the viability of the proposed method. Results for a collimator-type camera from a prototype envisaged for ITER are presented. The implemented procedure is discussed in detail, in particular with respect to the automatisation applied which takes the achievable positioning and alignment accuracies of the robot into account. This discussion is extended towards the definition of requirements for a remote-handling tool for ITER.

Active and passive spectroscopic imaging in the DIII-D tokamak

Wide-angle, 2D imaging of Doppler-shifted, Balmer alpha (Dα) emission from high energy injected neutrals, charge exchange recombination (CER) emission from neutral beam interaction with thermal ions and fully stripped impurity ions and visible bremsstrahlung (VB) from the core of DIII-D plasmas has been carried out. Narrowband interference filters were used to isolate the specific wavelength ranges of visible radiation for detection by a tangentially viewing, fast-framing camera. Measurements of the Dα emission from fast neutrals injected into the plasma from the low field side reveal the vertical distribution of the beam, its divergence and the variation in its radial penetration with density. Modeling of this emission using both a full Monte Carlo collisional radiative code as well as a simple beam attenuation code coupled to Atomic Data and Analysis Structure emissivity lookup tables yields qualitative agreement, however the absolute magnitudes of the emissivities in the predicted distribution are larger than those measured. Active measurements of carbon CER brightness are in agreement with those made independently along the beam midplane using DIII-D's multichordal, CER spectrometer system, confirming the potential of this technique for obtaining 2D profiles of impurity density. Passive imaging of VB, which can be inverted to obtain local emissivity profiles, is compared with measurements from both a calibrated filter/photomultiplier array and the standard multichordal CER spectrometer system.

Space-resolved extreme ultraviolet spectrometer for impurity emission profile measurement in Large Helical Device

A space-resolved extreme ultraviolet (EUV) spectrometer working in 60-400 A range has been developed to observe impurity emission profiles in core and edge plasmas of Large Helical Device (LHD). A flat focus is made for plane surface detector by using a varied line spacing holographic grating with an angle of incidence of 87 degrees. An excellent spectral resolution of 0.22 A at 200 A is then obtained after careful adjustment of the optical components. In the profile measurement of the LHD plasmas, the toroidal resolution is important as well as the radial resolution because the magnetic surfaces of LHD quickly change when the observation chord is tilted at a slightly different toroidal location. Horizontal dispersion is therefore selected for the present spectrometer. As a result, the toroidal resolution of 75 mm is achieved at the plasma position. An enough radial resolution of 10 mm is also obtained at spatial-resolution slit width of 0.2 mm. In order to measure the full radial profile of LHD plasmas the spectrometer is placed at a distance of approximately 9200 mm away from the plasma center and a backilluminated charge-coupled device with a size of 6.6x26.6 mm(2) is set vertical to the horizontal dispersion. Half of the LHD plasma, i.e., approximately 50 cm, can be measured along the vertical direction at horizontally elongated plasma cross section. A full vertical profile can be obtained by changing the vertical angle of the EUV spectrometer. As an example the full vertical profile of edge C IV (312.4 A) emission is presented. A wavelength interval of 35-65 A can be simultaneously observed, which varies according to the wavelength to be measured. A local emission profile of Fe XX (132.67 A) is also presented after Abel inversion as a typical example of the core EUV emission. Finally, the EUV spectrometer is absolutely calibrated using EUV bremsstrahlung continuum profile in comparison with absolute values of visible bremsstrahlung continuum profile because the bremsstrahlung continuum can be easily detected in high-density discharges of LHD.

Determination of Iron Density at the Plasma Center Using Radial Profile of FeKα Lines in LHD

Radial profiles of Kα X-ray spectra of metallic impurities have been measured using an assembly of soft X-ray pulse-height analyzers in Large Helical Device. The assembly is quipped with a radial scanning system, which gives us detailed profile information, especially in long pulse discharges. The local emissivity profiles of the Fe-Kα X-ray spectra have been quantitatively obtained by an Abel inversion technique. The density profile of the iron impurity in each charge state is analyzed using the energy shift profiles of the Fe-Kα spectra with the help of an impurity code analysis. As an example of the present method, the iron density of 1.2× 109 cm-3 in the plasma center is obtained at the line-averaged electron density of 2.6× 1013 cm-3.

Feedback control of the lower hybrid power deposition profile on Tore Supra

The Tore Supra facility is well suited to study ITER relevant topics such as the real-time control of plasma current and the sustaining of steady-state discharges. This work describes a tool that was recently developed and implemented on Tore Supra to control in real time, by means of the direct knowledge of the suprathermal electron local emission profile, the width of the lower hybrid power deposition profile. This quantity can be considered to some extent equivalent to the width of the plasma current density profile in case of fully non-inductive discharges. This system takes advantage of an accurate hard x-ray diagnostics, of an efficient lower hybrid additional heating and of a reliable real-time communication network. The successful experiments carried out to test the system employed, as actuators, the parallel refractive index n// and the total power PLH. The control of the suprathermal electron local emission profile through n// was also integrated with the feedback control of the total plasma current IP with PLH and of the loop voltage Vloop with the central solenoid flux. These results demonstrate that the system is robust, reliable and able to counterbalance destabilizing events. This tool can be effectively used in the future in fully non-inductive discharges to improve the MHD stability and to maintain internal transport barriers or lower hybrid enhanced performance modes. The real-time control of the lower hybrid power deposition profile could also be used in conjunction with the electron-cyclotron radiofrequency heating for synergy studies.