Spectroscopic observation of super-Alfvénic field-reversed configuration merging process by mixing of tracer ions

• Published in: Review of scientific instruments 2022-10, Vol.93 (10), p.103526-103526
• Main Authors: Kobayashi, D., Seki, T., Asai, T., Takahashi, Ts, Morelli, J., Inomoto, M., Takahashi, T., Dettrick, S., Gota, H.
• Format: Article
• Language: English
• Subjects: Bandpass filters; Configurations; Framing cameras; Magnetic structure; Magnetohydrodynamics; Plasmas (physics); Time
• ISSN: 0034-6748; 1089-7623
• DOI: 10.1063/5.0101756

Visualization of the collisional merging formation process of field-reversed configuration (FRC) has been attempted. In the collisional merging formation process, two initial FRC-like plasmoids are accelerated toward each other by a magnetic pressure gradient. The relative speed of the collision reaches several times the typical ion sonic speed and Alfvénic speed. The magnetic structure of the initial-FRCs is disrupted in the collision process, but the FRC-like magnetic structure is reformed in ∼30 µs after the collision. Magnetic reconnection should occur in this process; however, general theoretical models in magnetohydrodynamics approximation cannot be applied to this process because of the high-beta nature of FRC and super-Alfvénic/sonic relative speed. In this work, the spectroscopic observation of the collisional merging FRC formation was conducted to evaluate the timescale and geometry of merging. A slight amount of tracer element (e.g., helium) was mixed into one of two initial-FRCs. Mixing of the tracer did not cause serious adverse effects on the performance of the initial-FRC in the collision and merging processes. The collision and merging processes were visualized successfully and observed using a fast-framing camera with a bandpass filter. The timescale of merging and the outflow speed in the collisional merging process of FRCs were optically evaluated for the first time.