Emission Tomography for JET Plasma Diagnosis
At the JET Tokamak a major goal is the production and measurement of high levels of neutron emission from d-d and d-t fusion reactions. A variety of neutron diagnostics are used independently at JET to measure both fast and thermal ion behavior. The available neutron diagnostics include a neutron profile monitor consisting of a vertical and horizontal camera. The JET neutron cameras, a unique instrument among similar diagnostics available at large fusion research facilities, consist of two concrete shields of which each includes a fan-shaped array of collimators. These collimators define a total of 19 lines of sight, grouped in two cameras (horizontal and vertical).
Schematic view of the JET neutron emission profile monitor showing the lines of sight. |
The plasma coverage determined by the 19 line of sight can be used for neutron or g-ray tomography. It ensures a 2D arrangement for measurements and distribution determination. The 2D slice is located in the plane defined by the major torus radius (R) and the major torus axis (Z). The thickness of the plasma slice along the toroidal direction, determined by the collimation system, is approximately 75 mm. The reconstructions are useful for the study the thermal and beam-induced sources of neutron emission and to analyze the evolution of fast ion populations. The existence of only two views and the coarse sampling in each projection leads to a highly limited data set tomographic problem. For example, in the case of a reconstruction grid with 20 x 35 pixels (pixel size of 90 x 90 mm) an image with 700 pixel values must be retrieved from 19 experimental data. In consequence special algorithms which are suitable and specific to the machine and its constrains, allowing effective tomography from the available limited data must were developed.
A reconstruction method based on the maximum likelihood (ML) principle was developed for solving the reconstruction problem [1]. A smoothing operator, defined as median filtering which uses a sliding window moving along magnetic contour lines, is used to compensate for the lack of experimental information. The method was tested on numerically simulated phantoms with shapes characteristic for this kind of tomography. The ML method was used during in JET campaingns C20-26.
Temporal evolution of the neutron emissivity for shot 61141 (from 60.87 s to 61.32 s) |
Besides ML, three other methods have been developed: a method based on the statistical principle of maximum entropy (ME), a Tikhonov regularization (TR) technique and a Monte Carlo back-projection algorithm (MCBP). Extensive work was dedicated to the assessment of all the developed methods and a comprehensive comparative study was reported [2]. The evaluation reveals that the ML method is the only one able to encompass the reconstruction, with a good quality, of all structures of the emissive distribution. The ML method provides the finest results in terms of shapes reconstruction and resolution and produces artefact free images.
The test phantoms and their reconstruction. Each row corresponds to a phantom. The first column corresponds to the phantom image, the other columns correspond to reconstructions obtained using the different methods used in this paper – from left to right: ML, ME, TR, MCBP. These numerically simulated emissive distributions are characteristic for JET neutron and gamma tomography. They cover most of the range of possible distributions for this kind of tomography. Simple but frequent shapes are considered together with the retrieval of sophisticated structure in the emissive distribution which proved to be essential for a complete image of the quality and reliability of the methods. |
- T. Craciunescu, G. Bonheure, V. Kiptily, A. Murari, S. Soare, I. Tiseanu, V. Zoita. The Maximum Likelihood Reconstruction Method for JET Neutron Tomography. Nuclear Inst. and Methods in Physics Research, A, 595, p. 623–630, 2008. http://dx.doi.org/10.1016/j.nima.2008.07.145
- T. Craciunescu, G. Bonheure, V. Kiptily, A. Murari, I. Tiseanu, V. Zoita. A comparison of four reconstruction methods for JET neutron and gamma tomography , 2009. http://dx.doi.org/10.1016/j.nima.2009.03.224