Acknowledgements

The authors are grateful for the fruitful discussions with Prof. S. Masuzaki, Prof. S. Morita, Dr Y. Narushima, Prof. N. Ohno, Prof. B.J. Peterson, Prof. R. Sakamoto, Dr R. Seki, Dr H. Tanaka, Dr T. Tokuzawa, Prof. K.Y. Watanabe, Prof. H. Yamada, Dr I. Yamada, Prof. M. Yokoyama, Dr. J.W. Ahn, Dr Y. Feng, Prof. K. Ida, Prof. K. Itoh, Prof. S. Kajita, Dr G. Kawamura, Prof. T. Morisaki, Dr K. Mukai, Dr K. Nagaoka, Prof. M. Osakabe, Dr H. Takahashi, and Prof. K. Tanaka. The authors also thank the LHD experimental group for the excellent operation of LHD. The work has been financially supported by JSPS KAKENHI with grant numbers JP16H04622 and JP19H01878 and NIFS budget code ULPP026.
