Abstract
In future burning plasma devices measuring the plasma position and its shape with great spatial and temporal resolution is a crucial task. Nowadays these information are obtained by means of magnetic coils installed inside the vacuum vessel that in the future devices (like ITER and DEMO), due to the harsh environment caused by the high plasma temperature, will experience degradation over the time. Furthermore, the long plasma discharges will result in large integration drift causing a lack of accuracy in the measured signal. In order to assist the magnetic diagnostics and at the same time provide a novel tool to benchmark them, here the measurement of the plasma magnetic axis position by means of a collimated neutron flux monitor is proposed. Three different methods are here described and applied on JET by means of the neutron camera: a weighted average, the asymmetry method and a neural network. The methods are calibrated on a large database of plasma discharges including NBI and ICRH heated ones, and then compared with the magnetic axis position reconstructed by EFIT. The neural network outperforms the two other methods. In particular, the asymmetry method results to be sensitive to MHD activity, NBI power variation and to neutron emissivity profiles presenting a strong asymmetry, such as in case of peripheral NBI deposition due to high density plasmas or ICRH resonance layer. A possible application to vertical displacement events and disruptions is discussed and envisaged for future applications on DEMO. Finally, the performances of the neural network and of the asymmetry methods are studied in the case of one or two missing channels in the neutron flux monitor, showing how in general the reconstruction of the radial magnetic axis in both methods is more sensitive to the lack of channels than the vertical one. The methods here proposed can be applied and benchmarked on DTT and ITER neutron cameras as part of a future real-time control system.
Highlights
Plasma position control is a key element for the safe and reliable operation of present-day and future fusion reactors
The performances of the neural network and of the asymmetry methods are studied in the case of one or two missing channels in the neutron flux monitor, showing how in general the reconstruction of the radial magnetic axis in both methods is more sensitive to the lack of channels than the vertical one
In DEMO reflectometry has been proposed as a possible alternative to the magnetic diagnostics to measure the plasma shape [6], even though it still needs to be calibrated against the magnetic signals
Summary
Plasma position control is a key element for the safe and reliable operation of present-day and future fusion reactors. The magnetic axis position must be known a priori in order to calibrate the asymmetry and the weighted average and to train the neural network In this respect, JET offers an ideal test for this approach for several reasons: i) its extensive set of magnetic diagnostics coupled to EFIT are used to determine the magnetic axis position in real-time; ii) the neutron camera consists of both a vertical and a horizontal set of LoS that can be used to estimate radial and ver tical plasma displacement and iii) a large set of plasma discharges in several different plasma scenarios and additional heating configurations including VDEs and disruptions which allow to test the approach pro posed here in a wide range of conditions.
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