Energetic ion confinement studies using comprehensive neutron diagnostics in the Large Helical Device

Abstract

Understanding of energetic particle (EP) confinement is one of the critical issues in realizing fusion reactor. In stellarator/helical devices, the research on EP confinement is one of the key topics to obtain better confinement by utilizing the flexibility of three-dimensional magnetic field. Study of EP transport in the Large Helical Device (LHD) has been performed by means of escaping EP diagnostics in hydrogen plasma operation. By starting deuterium operation of the LHD, confinement study of EPs has progressed remarkably by using newly developed comprehensive neutron diagnostics providing the information of EPs confined in the core region. The total neutron emission rate (Sn) increases due to the relatively low deviation of beam ion orbit from the flux surface with the inward shift of the magnetic axis. Sn has the peak around the electron density of 2×10 19 m -3 to 3×10 19 m -3 , as predicted. It is found that the fraction of beam-beam components in Sn is evaluated to be approximately 20% by the Fokker-Planck models TASK/FP in the plasma with both co-and counter-neutral beam injections. The equivalent fusion gain in DT plasma achieved 0.11 in a negative-ion-based neutral beam heated 2 plasma. Time evolution of Sn following the short pulse neutral beam injection into the electroncyclotron-heated low-beta plasma is reproduced by drift kinetic simulation, indicating that transport of beam ion injected by short pulse neutral beam can be described with neoclassical models in magnetohydrodynamic quiescent low-beta plasmas. The vertical neutron camera works successfully, demonstrating that in the co-neutral beam injected plasma, neutron emission profile shifts according to magnetic axis position. The shift of the neutron emission profile is reproduced by orbit-following models. The triton burnup study is performed for the first time in stellarator/heliotron so as to understand the alpha particle confinement. It is found that the triton burnup ratio, which largely increases at inward shifted configurations due to the better triton orbit and better plasma performance in inward shifted configuration is similar to that measured in tokamak having a similar minor radius with the LHD. We study the confinement capability of EPs toward a helical reactor in magnetohydrodynamic -quiescent region and expansion of the energetic-ion physics study in toroidal fusion plasmas.

Overview of first Wendelstein 7-X high-performance operation

“…Variation between the configurations for the DT fusion rates are smaller than for the DD fusion rates, and the triton burn-up ratio was approximately 0.2 % in all of the configurations. Measurements with a SciFi detector at the LHD heliotron have yielded triton burn-up ratios in the same order of magnitude, between 0.05 % to 0.45 % [25]. The constant burnup ratio implies that the DT fusion rate is mainly determined by the triton slowing-down distribution and consequently effected by the triton confinement properties.…”

Advanced Helical Plasma Research towards a Steady-State Fusion Reactor by Deuterium Experiments in Large Helical Device

“…Confinement improvement that has been identified in LHD deuterium experiments should widen the scope of the operation. Quantitative assessment for start-up scenarios reaching such an identified Neutrons produced in deuterium plasmas in conjunction with a well-prepared set of neutron diagnostics [15] (e.g., neutron emission rate and triton burn-up ratio) have provided the capacity for the quantitative assessment of the energetic particles' confinement property [16,17] and their interaction with MHD modes [18,19] in LHD plasmas.…”

Estimation of 14 MeV neutron rate from triton burn‐up in future W7‐X deuterium plasma campaigns

“…Studying the energetic neutrons from the deuteron-triton (DT) reaction, called triton burn-up, allows to infer the confinement and slowing-down properties of the tritons, which was done in, for example, the ASDEX Upgrade, JET, PDX, PLT, and TFTR tokamaks as well as recently in the LHD heliotron. [5][6][7][8][9][10] Since the -particles and tritons are both emitted isotropically and have similar gyroradii at their production energies, predictions of the -particle confinement can be assessed, which is of major interest for future fusion reactors. [5] These fast ions are meant to be the main heating source in burning DT fusion plasmas.…”

“…[11] Recently, similar detectors were operated at LHD, yielding reasonable statistics for 14 MeV neutron rates above 10 12 s −1 . [10,[15][16][17][18][19] In order to study the capability of SciFi at W7-X, a one-dimensional diffusionless estimation has been developed that does not take losses into account explicitly.…”