Elsevier

Fusion Engineering and Design

Volumes 66–68, September 2003, Pages 905-909
Fusion Engineering and Design

Present and perspective roles of soft X-ray tomography in tokamak plasma position measurements

https://doi.org/10.1016/S0920-3796(03)00374-0Get rights and content

Abstract

This paper shows the importance and feasibility of real-time tomography in fusion experiments for the example of soft X-ray (SXR) position measurements. The requirement of non-magnetic real-time diagnostics in low frequencies for ITER is discussed. This is illustrated by recent results of rapid tomographic inversion of SXR measurements on tokamak TCV. Comparison with the magnetic reconstruction data has not only shown the valuable resolution capabilities of both techniques, but also revealed a slight dependence of magnetic measurements on toroidal magnetic field and an unnoticed drift of plasma position observer. A feasibility study using current hardware capacities for programmable real-time tomographic system with plasma position feedback output was carried out. A compact solution is found to be tractable opening wide perspectives for development.

Section snippets

Introduction, ITER relevance

The computerised tomography technique, which allows the reconstruction of the emissivity cross-section from its line integral measurements, has been applied in fusion plasma science since the early 1980s. A review of plasma tomography methods is presented in Ref. [1]. In the past two decades, plasma tomography applications have profited from the computer speed and capacity increase so that quite sophisticated algorithms, with a numerically rigorous introduction of constraints, have been

Rapid tomography and SXR position measurements on TCV

On TCV, minimum Fisher regularisation (MFR) is successfully applied in the tomographic reconstruction [8] of both SXR and bolometric data. The SXR diagnostics consist of ten pinhole cameras, see Fig. 1, each with linear arrays of 20 Centronix photodiodes and 47 μm beryllium filter foil [9]. In the MFR, the reconstructed images of emissivity cross-section are obtained on a grid of rectangular pixels. The default size of a pixel is 37 mm, full coverage of the TCV cross-section corresponds to

Technical feasibility of the real time tomography hardware

Real-time SXR tomography with position feedback on TCV translates to digitising 200 analogue signals fi, multiplying them with a pre-calculated inversion matrix Mji (typically 630×200 real numbers), finding the maximum of the resulting emissivity tomograms gj (630 numbers), extracting the core of emissivity gj, determining its center of gravity [rsxr, zsxr], possibly multiplying it by compensation factors and, if necessary, converting the calculated position to analogue signals. On TCV, the

Acknowledgements

The authors appreciate the professional support of Peter Milne from D-TACQ Solutions Ltd., http://www.d-tacq.com/. This work was partly supported by the Swiss National Science Foundation.

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