Ion Tiseanu

Evaluation of reconstruction methods for time-resolved spectroscopy of short-pulsed neutron sources

A comparison of five methods for the reconstruction of the time-resolved neutron energy spectrum of short-pulsed neutron sources from time-of-flight measurements is reported. The first method is an analog Monte Carlo reconstruction technique (AMCRT), expressly designed for the optimization of such measurements. It was proved that the studied problem can be treated as a tomographic one with a limited data set. A Fourier convolution and backprojection method and three other tomographic methods, which have been shown to work with a limited data set, are used: the maximum entropy method, the algebraic reconstruction technique, and a Monte Carlo implementation of the backprojection (MCBP) technique. Through numerical tests, the quality of reconstructions in different image geometries at various noise levels has been studied. Besides the AMCRT method, which produces the best results, good reconstructions are also obtained using MCBP and maximum entropy. If computing time must be minimized, the maximum entropy algorithm is most convenient. This algorithm could be used routinely in time-resolved spectroscopy measurements.

Advanced x-ray imaging of metal-coated/impregnated plasma-facing composite materials

A combination of x-ray imaging techniques is employed in the characterization of the coated/impregnated carbon-based composite materials to be used as plasma-facing components (PFC) in fusion devices. X-ray micro-tomography (μXCT) is applied in the visualization of carbon fiber composites (CFC)–Cu joined samples and in the characterization of CFC material porosity networks. Quantitative determination of the thickness of tungsten coating on carbon materials is performed using a combined absorption/fluorescence x-ray technique. The method was applied in the analysis of W-coated fine-grain graphite and CFC tiles. The x-ray imaging techniques allow rapid analysis with high spatial resolution.

Evaluation of reconstruction errors and identification of artefacts for JET gamma and neutron tomography

The Joint European Torus (JET) neutron profile monitor ensures 2D coverage of the gamma and neutron emissive region that enables tomographic reconstruction. Due to the availability of only two projection angles and to the coarse sampling, tomographic inversion is a limited data set problem. Several techniques have been developed for tomographic reconstruction of the 2-D gamma and neutron emissivity on JET, but the problem of evaluating the errors associated with the reconstructed emissivity profile is still open. The reconstruction technique based on the maximum likelihood principle, that proved already to be a powerful tool for JET tomography, has been used to develop a method for the numerical evaluation of the statistical properties of the uncertainties in gamma and neutron emissivity reconstructions. The image covariance calculation takes into account the additional techniques introduced in the reconstruction process for tackling with the limited data set (projection resampling, smoothness regularization depending on magnetic field). The method has been validated by numerically simulations and applied to JET data. Different sources of artefacts that may significantly influence the quality of reconstructions and the accuracy of variance calculation have been identified.

Investigation on the erosion/deposition processes in the ITER-like wall divertor at JET using glow discharge optical emission spectrometry technique

As a complementary method to Rutherford back scattering (RBS), glow discharge optical emission spectrometry (GDOES) was used to investigate the depth profiles of W, Mo, Be, O and C concentrations into marker coatings (CFC/Mo/W/Mo/W) and the substrate of divertor tiles up to a depth of about 100 μm. A number of 10 samples cored from particular areas of the divertor tiles were analyzed. The results presented in this paper are valid only for those areas and they cannot be extrapolated to the entire tile. Significant deposition of Be was measured on Tile 3 (near to the top), Tile 6 (at about 40 mm from the innermost edge) and especially on Tile 0 (HFGC). Preliminary experiments seem to indicate a penetration of Be through the pores and imperfections of CFC material up to a depth of 100 μm in some cases. No erosion and a thin layer of Be (<1 μm) was detected on Tiles 4, 7 and 8. On Tile 1 no erosion was found at about 1/3 from bottom.

Overview of image processing tools to extract physical information from JET videos

In magnetic confinement nuclear fusion devices such as JET, the last few years have witnessed a significant increase in the use of digital imagery, not only for the surveying and control of experiments, but also for the physical interpretation of results. More than 25 cameras are routinely used for imaging on JET in the infrared (IR) and visible spectral regions. These cameras can produce up to tens of Gbytes per shot and their information content can be very different, depending on the experimental conditions. However, the relevant information about the underlying physical processes is generally of much reduced dimensionality compared to the recorded data. The extraction of this information, which allows full exploitation of these diagnostics, is a challenging task. The image analysis consists, in most cases, of inverse problems which are typically ill-posed mathematically. The typology of objects to be analysed is very wide, and usually the images are affected by noise, low levels of contrast, low grey-level in-depth resolution, reshaping of moving objects, etc. Moreover, the plasma events have time constants of ms or tens of ms, which imposes tough conditions for real-time applications. On JET, in the last few years new tools and methods have been developed for physical information retrieval. The methodology of optical flow has allowed, under certain assumptions, the derivation of information about the dynamics of video objects associated with different physical phenomena, such as instabilities, pellets and filaments. The approach has been extended in order to approximate the optical flow within the MPEG compressed domain, allowing the manipulation of the large JET video databases and, in specific cases, even real-time data processing. The fast visible camera may provide new information that is potentially useful for disruption prediction. A set of methods, based on the extraction of structural information from the visual scene, have been developed for the automatic detection of MARFE (multifaceted asymmetric radiation from the edge) occurrences, which precede disruptions in density limit discharges. An original spot detection method has been developed for large surveys of videos in JET, and for the assessment of the long term trends in their evolution. The analysis of JET IR videos, recorded during JET operation with the ITER-like wall, allows the retrieval of data and hence correlation of the evolution of spots properties with macroscopic events, in particular series of intentional disruptions.

Phase Congruency Image Classification for MARFE Detection on JET with a Carbon Wall

Multifaceted asymmetric radiation from the edge (MARFE) instabilities may reduce confinement leading to harmful disruptions. They cause a significant increase in impurity radiation, and therefore, they leave a clear signature in the video data. This information can be exploited for automatic identification and tracking. A MARFE classifier, based on the phase congruency theory, has been developed and adjusted to extract the structural information in the images of Joint European Torus (JET) cameras. This approach has the advantage of using a dimensionless quantity and providing information that is invariant to image illumination, contrast, and magnification. The method was tested on JET experimental data and has proved to provide a good prediction rate.

Upgrade of the JET Gamma‐Ray Cameras

The JET gamma‐ray camera diagnostics have already provided valuable information on the gamma‐ray imaging of fast ion in JET plasmas /1,2/. The applicability of gamma‐ray imaging to high performance deuterium and deuterium‐tritium JET discharges is strongly dependent on the fulfilment of rather strict requirements for the characterisation of the neutron and gamma‐ray radiation fields. These requirements have to be satisfied within very stringent boundary conditions for the design, such as the requirement of minimum impact on the co‐existing neutron camera diagnostics. The JET Gamma‐Ray Cameras (GRC) upgrade project deals with these issues with particular emphasis on the design of appropriate neutron/gamma‐ray filters (“neutron attenuators”). Several design versions have been developed and evaluated for the JET GRC neutron attenuators at the conceptual design level. The main design parameter was the neutron attenuation factor. The two design solutions, that have been finally chosen and developed at the leve...

X-ray micro-laminography for the ex situ analysis of W-CFC samples retrieved from JET ITER-like wall

x-ray micro-laminography was qualified and implemented as a complementary solution for the 3D microstructural analysis of tungsten coated carbon-fibre reinforced carbon (W/CFC) samples retrieved fr ...

EFD-P(13)39 Supervised Image Processing Learning for Wall MARFE Detection Prior to Disruption in JET with a Carbon Wall

In the last years, several diagnostic systems have been installed on Joint European Torus (JET) providing new information that may be potentially useful for disruption prediction. The fast visible camera can deliver information about the occurrence of multifaceted asymmetric radiation from the edge (MARFE) instabilities that precede disruptions in density limit discharges. Two image processing methods - the sparse image representation using overcomplete dictionaries and the Histogram of oriented gradients (HOGs) - have been used for developing MARFE classifiers with supervised learning. The methods have been tested with JET experimental data and a good prediction rate has been obtained. The HOG method is able to provide predictions useful for online disruption prediction.

X-ray Micro-Tomography as a New and Powerful Tool for Characterization of MgB2 Superconductor

Applied superconductivity is virtually important for all human activities solving different problems in the fields such as power and energy, electronics, computing and communications, medical equipment and sensors, fast transportation and so on. However, the progress in this field is relatively slow when compared to other young industries. The reasons are diverse, and among them are the prohibitive prices vs. performance of the superconducting technologies. In this regard, superconducting-based materials or products with improved working characteristics at constant or lower prices are always of interest. One superconductor of practical interest is MgB2. This superconductor has several advantages as follows: 1. Critical temperature Tc = 39 K of MgB2 [1] is the highest among simple superconducting compounds. This is a unexpectedly high Tc and, although MgB2 can be considered a swave classical Bardeen-Cooper-Schriffer (BCS) superconductor, it shows elements of unconventional superconductivity: two-gap superconductivity was observed with gap values of Δσ = 7.4 meV and Δπ = 2.1 meV at 4.2 K corresponding to critical temperatures of 15 K and 45 K, with σ and π being the bands of the boron electrons [2]. In fact, there are two hole-type quasi-two-dimensional σ bands (σ1 and σ2), an electron-type (π 1) and a hole-type (π 2) three-dimensional π bands [3,4]. Such situation generates new physical effects with markedly different behaviors in many properties some of them of practical meaning when compared with single band superconductors. 2. It is a simple compound composed of only two elements. 3. It is a cheap compound composed of relatively cheap and available elements. 4. It is not toxic and it is considered stable in the air. 5. It is a light compound with low theoretical density of 2.63 g/cm3. Crystal structure is relatively simple of layered hexagonal type. 6. It has anisotropy, but it is not as high as in high temperature superconductors.