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Our multi-view soft X-ray measurement system detected for the first time high-energy electrons localized at the X-point of two merging tokamak plasmas. We found their energies increase with the guide toroidal field. These electrons are considered to be accelerated by the reconnection electric field along the guide magnetic field.
Under a high guide field, the reconnection electric field is observed to accelerate electrons to the downstream, forming quadra-pole type electrostatic potential profiles for the following ion acceleration and heating. In this key process, the production of energetic electrons has been studied both in PIC simulations [1], and in laboratory plasmas [2,3]. The spatial profile of energetic electrons is often measured by observing Bremsstrahlung emitted from them. In this study, we measured soft X-ray images of high-guide field reconnection formed by two merging tokamaks using a multi-view soft X-ray camera, which can simultaneously measure four images of Bremsstrahlung emission through four different filters.
On the midplane of TS-6 spherical merging tokamak, we installed a multi-view soft X-ray camera composed of four built-in microchannel plates, four pinholes, and four filters: 1 µm and 2 µm mylar films for >100eV and >200eV electrons and 1 µm and 2.5 µm aluminum films for >20eV and >50eV electrons. We simultaneously measured the line-integrated images of soft X-ray emissions whose R-Z images were reconstructed using Tikhonov-Philips regularization and the minimum GCV criterion [4].
The emission profiles from high energy electrons: >200eV and >100eV peaked at the X-point. It is noted that the emission from >200eV electrons increased with the guide magnetic field strength. Therefore, the emission is thought to be from energetic electrons accelerated by the reconnection electric field along the guide magnetic field in the vicinity of the X point.
[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] N. Iwama, et al.: Appl. Phys. Lett., Vol.54, 502 (1989)
