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

Multi-view Imaging of Energetic Electron during High-Guide Field Magnetic
Reconnection in Tokamak Merging Experiment
Shinjiro Takeda1, Yukehi Okunishi1, Yunhan Cai1, Hiroshi Tanabe1, Yasushi Ono1
1
The University of Tokyoe-mail (speaker): stakeda@ts.t.u-tokyo.ac.jp
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Reconnection electric is found to accelerate electrons tothe downstream for the first time during high guidemagnetic fields, forming a quadrupole electrostatic
potential profile for the following ion acceleration andheating. In this important process, the production ofenergetic electrons has been detected in both PIC
simulations [1] and laboratory plasmas [2,3]. The spatial
profile of the energetic electrons is often measured byobserving the bremsstrahlung emitted from the electrons.
In this study, we measured the high-guide-fieldreconnection of two merging tokamaks using a stereoview soft X-ray cameras [4], which enables us tosimultaneously measure two-dimensional images of theBremsstrahlung through two different filters. A compact
vacuum vessels with a built-in microchannel plate andfiltered pinholes are installed in the TS-6 sphericaltokamak merging device. The two filters are a 1-μm
Mylar film for high-energy electrons and a 2.5-μm
aluminum film for low-energy electrons. Twodimensional soft X-ray emission profiles werereconstructed from the measured line-integrated stereoimages.
The soft X-ray emission contours were measured asshown in Fig. (a). The left column shows the highenergy electron image measured through a Mylar filter,
and the middle column shows the low-energy electron
image through an aluminum filter. The time evolution of
the electrostatic potential shown in the right column
agrees qualitatively with the scenario of accelerated
energetic electrons forming a quadrupole potential. It canbe seen that the emission peaks are localized around theX-point at t = 470 μs in the high-energy electron image
and at 468 μs in the low-energy electron image. The
downstream emission peak at t = 475 μs was observed inboth the high-energy and low-energy images. When theexperiment was performed with different intensities of
the reconnecting and guide magnetic fields, the magnetic
field dependence of the emission intensity at the X point
was confirmed as shown in Fig. (b): the intensity of thehigh energy emission peak increases with the guidemagnetic field amplitude, while the intensity of the low
energy emission peak decreases. This fact suggests thatthe electron acceleration near the X point increases withthe length of the magnetic field lines in the X point
region where no poloidal magnetic field exists. On theother hand, the downstream emission peaks increased
with reconnecting field strength for both high and low
energy electrons (Fig. (c)). This emission is considered
to come from the electrons accelerated to thedownstream region by an electric field parallel to themagnetic field. It may also include high-energy electrons
due to Fermi acceleration [5]. In order to clarify themechanism for the electron acceleration, we haveincreased the number of X-ray images through different
filters from two to six and are obtaining two-dimensional
Bremsstrahlung spectral information, and are currently
conducting experiments with the system extended to four
viewpoints.

Time evolution of soft X-ray emission in twoenergy bands and floating potential (a) andmagnetic field dependence of emission intensity
in two energy bands at the X-point (top) and in
the downstream region (bottom) (b).

References[1] P. Pritchett, et al.: J. Geophys. Res., Vol.109, A01220(2004)[2] T. Yamada, et al.: Nucl. Fusion, Vol.56, 106019(2016)[3] M. Inomoto, et al.: Nucl. Fusion, Vol.59, 086040(2019)[4] J. Xiang, et al.: Rev. Sci. Instrum., Vol.92, 083504(2021)[5] J. T. Dahlin: Phys. Plasmas, Vol. 27, 100601 (2020)