Abstract
Two new vertical neutron cameras characterized by high detection efficiency were developed on the Large Helical Device in order to observe poloidal structures of helically trapped beam ions created by the perpendicularly injected positive-ion based neutral beam (P-NB) and are newly operated since 2018. In this work, the neutron fields at the vertical neutron cameras are investigated using the Monte Carlo N-particle transport code to evaluate the performance of its collimators. The results indicate that neutrons are attenuated by the heavy concrete and are well collimated through the collimator to detectors. Neutron spectra at the detector position show over 99% of uncollided 2.45 MeV neutrons. Time evolution of neutron emission profiles during the short pulse of P-NB injection is measured by the vertical neutron cameras. Peaks on the neutron emission profiles corresponding to the helically trapped beam ion are successfully obtained, as designed. The decrease in line integrated neutron flux at the peak positions after the P-NB stops is consistent with the behavior of the total neutron emission rate measured by the neutron flux monitor.
Highlights
Understanding of energetic particle transport and loss due to energetic-particle-driven magnetohydrodynamic (MHD) mode is one of the key issues in order to obtain better plasma performance in fusion devices.[1]
The horizontal and vertical neutron cameras were installed at the Joint European Torus (JET).[4,5]
The VNC2 and VNC3 are installed parallel at LHD lower port to view different plasma cross-sections in order to observe the poloidal structure of helically-trapped beam ions created by the positive-ion based neutral beam (P-NB).[24]
Summary
Understanding of energetic particle transport and loss due to energetic-particle-driven magnetohydrodynamic (MHD) mode is one of the key issues in order to obtain better plasma performance in fusion devices.[1]. The understanding of energetic particle physics obtained by the neutron camera will provide essential information for the achievement of burning plasma in ITER. In stellarators and helical devices, in particular, the behavior of helically-trapped beam ion which is created by the perpendicularly neutral beam injections is crucial in the core plasma physics regarding the loss of the energetic ion or the limiting of the ion temperature because of a lack of symmetry of the system. In order to sustain the high-temperature state, the study of helically-trapped beam ion behavior is important and will provide the crucial information for the deep understanding of energetic ions confinement property. To study the energetic particle transport through the time-resolved and the spatially-resolved measurement, the neutron cameras have been developed and used in LHD. This paper is devoted to discussing the performance of VNC2 and VNC3 using numerical simulation and shows experimental results
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