7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Visible-Light Imaging of Merging Formed FRCsby Wide-View Tomography Camera
R.Hayata, K.Hirama, T.Seki, D.Kobayashi, T.Takahashi, T.Asai
College of Science and Technology, Nihon Universitye-mail (speaker): csre23023@g.nihon-u.ac.jp
The Field-Reversed Configuration (FRC) plasma is
an extremely high beta compact torus confined solely bypoloidal flux. At Nihon University, collisional merging
experiments of FRC have been conducted on FAT-CMdevice. Two initial FRC-like plasmoids are formed within
two conical theta-pinch formation sections coaxiallyconnected to the both ends of confinement vessel. They
are accelerated by magnetic pressure gradient to collide
and merge at relative velocity exceeding the Alfvén
velocity to form a single FRC. While previous
experiments have validated a significant increment inplasma volume/magnetic flux and extended configuration
lifetime, the time evolution of the internal structure during
the collisional merging process, presumed to beaccompanied by dynamic structural transitions, has yet tobe observed [1, 2].
The tomography camera (T-cam) has beendeveloped to measure the translation and collisionalmerging process of FRCs via visible light imaging [3, 4].
The developed T-cam consists of a multi-anodephotomultiplier tube (PMT) as a photo detector for fast
response in microseconds, an interference filter (opticalbandpass filter: BPF) for observing specific wavelength
range, a slit to regulate incident lights intensity and aplanoconcave cylindrical lens for expansion of the field ofview.
The previous model of T-cam had successfullyreconstructed the internal structure of FRC and estimatedthe center of gravity of plasmoids in the translation
process. The view angle of T-cam was designed to observe
the FRC with a typical plasma radius of 0.25 m at thecollision. However, the field of view of the T-cam before
update was narrower than previously assumed to observe
the edge of plasma or outside. Consequently, it wasimpossible for T-cam to reconstruct the internal structureof FRC. In order to extend the field of view of the T-cam
to perform tomographic imaging of the collision/merging
processes and after merged FRCs, a new incident optics
system was designed.
To avoid the view being obstructed by the inner wallof the port, a viewport extended into the vacuum side wasdesigned and adopted. The incident optics is placed inside(Fig. 1). In addition, using the developed ray tracing
program, the configuration of the incident optical system
was designed by ray tracing including the lens system andthe shape of the window. This program loads the radius ofcurvature, height, and position for each lens and uses
Snell's law at the boundary of them for processing
refraction. The result of ray tracing shows that two planoconcave cylindrical lenses with different focal lengths canbe used to obtain a wider field of view. The view angle isexpanded from 60° to 105°. This corresponds to an expand

Figure 1. The incident optics is placed inside of thewindow (cross-section view).
in the measurement range from a radius of 250 mm to 300
mm. The observable range covers most of the area inside
the confinement chamber (Fig. 2). The viewing angle wasmeasured using an LED light source with a narrowed solid
angle by a pinhole. This verification confirmed that thefield of view was almost actualized result of ray tracing.
In this work, we report the findings derived fromobservations utilizing the wide-view visible-lighttomography camera.

Figure 2. Comparison of the field of view, newly built
T-cam and before upgrade.
This work was partially supported by the JSPS
KAKENHI under Grant No. JP20H00143 and Nihon
University, College of Science and Technology, Grant for
Project Research, Grant for the Promotion of Leading
Research and the research grant of Nihon University.
References[1] T. Asai et al., Nucl. Fusion 59, 056024 (2019).
[2] T. Asai et al., Nucl. Fusion 61, 096032 (2021).
[3] H. Tomuro et al., Rev. Sci. Instrum. 81, 10E525(2010).
[4] T. Seki et al., Rev. Sci. Instrum. 93, 103520 (2022).