We have newly developed a large-scale equilibrium database for real-time magnetic coordinate mapping system in the Large Helical Device. Thousands of free-boundary equilibria for each vacuum configuration have been calculated under wide ranges of central beta, pressure peaking factor, toroidal current and current peaking factor. We have also prepared a line of sight database which tabulates pre-calculated mapping results for all equilibria along several selected lines of sight to accelerate the mapping procedure. A user library has been developed to retrieve results of the inverse mapping as well as additional equilibrium parameters from the databases. A mapping program iteratively searches for the best-fitted equilibrium so as to minimize the discrepancy between inboard and outboard side of an electron temperature profile measured by the Thomson scattering diagnostic. Real-time mapping of all the time slices of the Thomson data enables us to provide time evolutions of equilibrium parameters and electron temperature/density profiles as functions of effective minor radius, which can be applied specifically for subsequent analyses of transport phenomena based on experiments.
We report the experimental finding of n=1 helical cores (HCs) accompanied by saturated m/n=2/1 tearing modes (TMs) with low mode frequencies in JT-60U. The HCs accompanied by TMs were observed after an increase in the mode amplitude and a decrease in the mode frequency of m/n=2/1 precursors with tearing parity. The decreased mode frequency is typically lower than 20 Hz. With various diagnostics, the coupling of n=1 HCs and m/n=2/ 1 TMs has been clearly observed. Because the coherent oscillations in the ion temperature are observed in both the core region and the edge region, the flux surfaces including the m/n=2/1 magnetic island appear to have m=1 helical deformation. It has also been suggested that the m/n=2/1 TM and the HC rotate in the electron diamagnetic direction keeping f m/n=1/1(HC) =2f m/n=2/1(TM) in several plasmas. Here, f m/n=1/1(HC) is the mode frequency of HCs and f m/n=2/1(TM) is the mode frequency of TMs. In addition, the core seems to be shifted to the high-field side when the O-points of the m/n=2/1 magnetic island line up in the midplane, which is confirmed by reconstructions of magnetohydrodynamic equilibria with motional Stark effect measurement and the MEUDAS code. Our observation of m/n=2/1 TMs having HCs contributes to the understanding of the excitation mechanism of HCs in tokamak plasmas.
A high-performance data acquisition (DAQ) system has been developed for steady-state fusion experiments at the Large Helical Device (LHD). Its significant characteristics are 110 MB s−1 continuous DAQ capability and the performance scalability using an unlimited number of DAQ units. Incoming data streams are first transferred temporarily onto the shared random access memory, and then cut into definite time chunks to be stored. They are also thinned out to 1/N to be served for the real-time monitoring clients. In LHD steady-state experiment, the DAQ cluster has established the world record for acquiring 90 GB/shot. The established technology of this steady-state acquisition and store can contribute to the ITER experiments whose data amount is estimated in the range 100 or 1000 GB/shot. This system also acquires experimental data from multiple remote sites through the fusion-dedicated virtual private network in Japan. The speed lowering problem in long-distance TCP/IP data transfer has been improved by the packet pacing optimization. The demonstrated collaboration scheme will be analogous to that of ITER and the supporting machines.
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