Volume recombination of C⁴⁺in detached divertor plasmas of JT-60U

Abstract

Volume recombination of C4+ and e− into C3+ is observed for the first time in detached divertor plasmas with an X-point MARFE. The recombination zone is located around the X-point, and the electron temperature and density are evaluated to be 6.3 eV and 7.8 × 1020 m−3, respectively. In this zone, the volume recombination flux is larger by two orders of magnitude than the ionization flux of C3+. However, the radiation power due to the recombination process is only 2% of the total radiation power, measured by a bolometer. In contrast, the radiation power due to the excitation process from the ground state of C3+ by electron collision dominates the total radiation power.

Radiation process of carbon ions in JT-60U detached divertor plasmas

“…If the C 3þ flux from the divertor region is higher than that from the main plasma, it is expected that auxiliary impurity seeded from the divertor, for instance, CD 4 , ionizes into C 3þ effectively and yields radiation efficiently, leading to high controllability of radiation loss. In contrast, if the C 3þ flux from the main plasma is higher than that from the divertor region, the controllability of the radiation loss by auxiliary impurity seeding may be low because of difficulties in controlling the transport via the main plasma.…”

“…In contrast, if the C 3þ flux from the main plasma is higher than that from the divertor region, the controllability of the radiation loss by auxiliary impurity seeding may be low because of difficulties in controlling the transport via the main plasma. Recently, it is reported that at the X-point in the detached plasma with MARFE, the volume recombination of C 4þ is one of the processes to produce C 3þ and that the electron temperature is too low to ionize C 3þ to C 4þ [4]. Hence it is expected that C 4þ at the X-point is transported from the upstream via the main plasma, and is not originated from the lowly ionized carbon ions transported from the downstream of the divertor.…”

“…From the comparison of these fluxes to the ionization flux of C 3þ and the recombination flux of C 4þ determined in Ref. [4], the C 3þ generation and loss processes are discussed. Further the radiation power from C 2þ is evaluated to compare it with that from C 3þ determined in Ref.…”

Impact of Differing Electron and Ion Temperatures on Emission Lines of Impurities in Tokamak Divertors

“…For such a purpose, spectroscopy is often a method of choice and spectroscopic measurements of impurity line radiation are routinely used for plasma diagnostics. In tokamaks with carbon tiles, such as JT-60U, carbon line radiation is one of the dominant energy loss channels [1][2][3]. In this device, impurity radiation is dominated by C III and C IV line emission.…”

“…In this device, impurity radiation is dominated by C III and C IV line emission. Intensities of lines emitted by these two species are used for the electron temperature determination and its spatial distribution in the divertor [1][2][3]. Under detachment conditions of divertor plasmas, high-members of the Balmer series of hydrogen or its isotope deuterium are observed.…”