Current Research into Applications of Tomography for Fusion Diagnostics (original) (raw)
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Review of Scientific Instruments, 2015
A method of tomographic reconstruction of the neutron emissivity in the poloidal cross section of the Joint European Torus (JET, Culham, UK) tokamak was developed. Due to very limited data set (two projection angles, 19 lines of sight only) provided by the neutron emission profile monitor (KN3 neutron camera), the reconstruction is an ill-posed inverse problem. The aim of this work consists in making a contribution to the development of reliable plasma tomography reconstruction methods that could be routinely used at JET tokamak. The proposed method is based on Phillips-Tikhonov regularization and incorporates a priori knowledge of the shape of normalized neutron emissivity profile. For the purpose of the optimal selection of the regularization parameters, the shape of normalized neutron emissivity profile is approximated by the shape of normalized electron density profile measured by LIDAR or high resolution Thomson scattering JET diagnostics. In contrast with some previously developed methods of ill-posed plasma tomography reconstruction problem, the developed algorithms do not include any post-processing of the obtained solution and the physical constrains on the solution are imposed during the regularization process. The accuracy of the method is at first evaluated by several tests with synthetic data based on various plasma neutron emissivity models (phantoms). Then, the method is applied to the neutron emissivity reconstruction for JET D plasma discharge #85100. It is demonstrated that this method shows good performance and reliability and it can be routinely used for plasma neutron emissivity reconstruction on JET. [
Modified Phillips–Tikhonov regularization for plasma tomography
Current Applied Physics, 2010
The tomography has been used as a powerful diagnostic method for visualizing cross-sectional images in plasma research. Without any biasing data, the Phillips-Tikhonov (P-T) regularization method was attempted in this work to reconstruct cross-sectional phantom images of the plasma by minimizing the gradient between adjacent pixel data. A comparison of the tomographic reconstructions of the cross-sectional images similar to the tokamak plasmas by the P-T method and the maximum entropy (ME) method showed that the P-T method produced more accurate results. In addition, the P-T method was modified by adding an iteration procedure with a second-order correction to improve the accuracy of the reconstruction for noisy line-integrated data. Tomographic reconstructions in the presence of Gaussian-distributed random noise demonstrated that the modified P-T method with only several iterations significantly reduced the mean reconstruction error by (30-50)% of the original root-mean-square error caused by the random noise.
Fast tomographic methods for the tokamak ISTTOK
AIP Conference Proceedings, 2008
The achievement of long duration, alternating current discharges on the tokamak IST-TOK requires a real-time plasma position control system. The plasma position determination based on magnetic probes system has been found to be inadequate during the current inversion due to the reduced plasma current. A tomography diagnostic has been therefore installed to supply the required feedback to the control system. Several tomographic methods are available for soft X-ray or bolometric tomography, among which the Cormack and Neural networks methods stand out due to their inherent speed of up to 1000 reconstructions per second, with currently available technology. This paper discusses the application of these algorithms on fusion devices while comparing performance and reliability of the results. It has been found that although the Cormack based inversion proved to be faster, the neural networks reconstruction has fewer artifacts and is more accurate. FIGURE 1. The tomography camera system of the tokamak ISTTOK: 3 cameras with 8 diodes each. Sight lines in the poloidal installation are drawn.
Into the Fast Tomographic Postprocessing in Tokamaks
Informatyka, Automatyka, Pomiary w Gospodarce i Ochronie Środowiska, 2017
The collaboration of authors led to implementing advanced and fast systems for diagnostics of plasma content in tokamaks. During the development of systems it is planned to add new functionalities, in particular, the algorithms of tomographic reconstruction to obtain information on three dimensional distribution of plasma impurities. In the article the idea of tomographic reconstruction is introduced and issues of performance and adequate hardware selection are presented.
Singular Value Decomposition Combined With Analytical Method for Tokamak Plasma Tomography
2010
Several arrays of soft X-ray detectors are used with the Fourier expansion for angular direction and Zernicke polynomial on radial direction during an analytical method tomography. A truncation of two expansions is necessary. We used singular value decomposition in order to solve the set of equations. This technique reconstructs better images of tomography.
A Real Time Bolometer Tomographic Reconstruction Algorithm in Nuclear Fusion Reactors
2021
In tokamak nuclear fusion reactors, one of the main issues is to know the total emission of radiation, which is mandatory to understand the plasma physics and is very useful to monitor and control the plasma evolution. This radiation can be measured by means of a bolometer system that consists in a certain number of elements sensitive to the integral of the radiation along straight lines crossing the plasma. By placing the sensors in such a way to have families of crossing lines, sophisticated tomographic inversion algorithms allow to reconstruct the radiation tomography in the 2D poloidal cross-section of the plasma. In tokamaks, the number of projection cameras is often quite limited resulting in an inversion mathematic problem very ill conditioned so that, usually, it is solved by means of a grid-based, iterative constrained optimization procedure, whose convergence time is not suitable for the real time requirements. In this paper, to illustrate the method, an assumption not val...
Vector and scalar tomography on fusion plasmas using Hamiltonian and variational methods
Plasma Physics and Controlled Fusion, 1998
A method is presented to obtain tomographic inversions of line-integrated measurements in magnetized plasmas, both for the cases of scalar and vector fields. For the Hamiltonian flow of the magnetic field in tokamaks the vector tomographic problem can be reduced to a scalar one. In many fusion experiments the amount of measured data is insufficient to allow an inversion without further assumptions. We present a variational method, where such assumptions can easily be implemented by the use of constraints. We outline the procedures using the example of obtaining the function on tokamaks from Faraday rotation measurements.
Deep neural networks for plasma tomography with applications to JET and COMPASS
Journal of Instrumentation, 2019
Convolutional neural networks (CNNs) have found applications in many image processing tasks, such as feature extraction, image classification, and object recognition. It has also been shown that the inverse of CNNs, so-called deconvolutional neural networks, can be used for inverse problems such as plasma tomography. In essence, plasma tomography consists in reconstructing the 2D plasma profile on a poloidal cross-section of a fusion device, based on line-integrated measurements from multiple radiation detectors. Since the reconstruction process is computationally intensive, a deconvolutional neural network trained to produce the same results will yield a significant computational speedup, at the expense of a small error which can be assessed using different metrics. In this work, we discuss the design principles behind such networks, including the use of multiple layers, how they can be stacked, and how their dimensions can be tuned according to the number of detectors and the desired tomographic resolution for a given fusion device. We describe the application of such networks at JET and COMPASS, where at JET we use the bolometer system, and at COMPASS we use the soft X-ray diagnostic based on photodiode arrays.
Design of multi-range tomographic system for transport studies in tokamak plasmas
Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment, 2010
The integrated spectroscopic system for visible plasma radiation, soft X-ray, and bolometric measurements has been designed for the COMPASS tokamak. This diagnostic allows tomographic reconstruction at a few microseconds time base and features an improved spatial resolution about 1 cm in the pedestal region, where the highest gradient of plasma pressure drives turbulent structures and their propagation towards the wall. In combination with other developed diagnostics on COMPASS, it represents an ideal tool for a measurement and characterization of these radiating plasma structures, which are responsible for an extremely high particle/energy transport across magnetic field. The design of the new integrated multi-channel spectroscopic diagnostics is reviewed and corresponding technical challenges are described. Additionally, the methods, which will be used for data processing, are briefly mentioned and target physics is discussed.
Journal of Computational Physics, 2012
The problem of equilibrium of a plasma in a Tokamak is a free boundary problem described by the Grad-Shafranov equation in axisymmetric configurations. The right hand side of this equation is a non linear source, which represents the toroidal component of the plasma current density. This paper deals with the real time identification of this non linear source from experimental measurements. The proposed method is based on a fixed point algorithm, a finite element resolution, a reduced basis method and a least-square optimization formulation.