Review of LHD-ECRH activities and beyond

Masaki Nishiura; Hiroe Igami; Hiromi Takahashi; Naoki Kenmochi; Ryoma Yanai; Kenji Ueda; Yoshinori Mizuno; Toshiki Takeuchi; Yasuo Yoshimura; Takashi Shimozuma; Shin Kubo; Toru Ii Tsujimura; Sakuji Kobayashi; Tsuyoshi Kariya

arXiv · 2609.32265

Review of LHD-ECRH activities and beyond

Masaki Nishiura·Hiroe Igami·Hiromi Takahashi·Naoki Kenmochi·Ryoma Yanai·Kenji Ueda·Yoshinori Mizuno·Toshiki Takeuchi·Yasuo Yoshimura·Takashi Shimozuma·Shin Kubo·Toru Ii Tsujimura·Sakuji Kobayashi·Tsuyoshi Kariya

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Abstract

The electron cyclotron resonance heating (ECRH) system on the Large Helical Device (LHD) played a key role in the progress of helical fusion research from the first plasma in 1998 to the final discharge on December 25, 2025. This paper presents a historical review of ECRH activities on LHD, highlighting the stepwise advances in hardware, including gyrotrons, transmission lines, and launcher systems, together with the exploration of advanced plasma regimes such as overdense heating and long-pulse steady-state operation. Beyond plasma heating, the ECRH system was also used as a high-power probe for advanced diagnostics, including collective Thomson scattering (CTS) as well as correlation electron cyclotron emission (CECE), and as a flexible actuator for transport studies and real-time control experiments. Finally, we discuss how the technological assets and operational expertise accumulated over the nearly three decades of the LHD program are being further developed and applied to next-generation helical devices, particularly CHD and CHD-U, as well as to broader gyrotron and millimeter-wave applications.

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Masaki Nishiura, Hiroe Igami, Hiromi Takahashi, Naoki Kenmochi, Ryoma Yanai, Kenji Ueda, Yoshinori Mizuno, Toshiki Takeuchi, Yasuo Yoshimura, Takashi Shimozuma, Shin Kubo, Toru Ii Tsujimura, Sakuji Kobayashi, Tsuyoshi Kariya. 2026-09-26. Review of LHD-ECRH activities and beyond. https://arxiv.org/abs/2609.32265

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