Speaker
Description
Injecting impurity gases into the divertor region to enhance volumetric recombination and form detached plasmas is a promising approach to mitigate intense divertor heat loads. Understanding the underlying physics of divertor detachment is important for controlling heat and particle fluxes in the divertor while maintaining core plasma performance.
Molecular activated recombination (MAR) has relatively high reaction rates even at moderately high electron temperatures (Te). In contrast, electron–ion recombination (EIR) becomes dominant at Te ≪ 1 eV, resulting in a shift of the dominant recombination process along with the decrease in Te [1, 2].
So far, we have utilized the divertor simulation experimental module (D-module) of a tandem-mirror, GAMMA 10/PDX, and observed two phases of emission patterns during the formation of a detached plasma by injecting molecular hydrogen into the end-loss plasma flowing into the V-shaped tungsten target plates of the D-module [3].
As the neutral gas pressure (pn) in the D-module increased, Te decreased; a strong Hα emission associated with MAR was observed [4], and the emission region moved upstream as the electron density (ne) decreased. A further increase in pn (> ~ 5 Pa) led to the second phase, which is characterized by a drastic rise in emissions from hydrogen atoms in higher excited states, indicating the occurrence of EIR. At that point, Te was calculated to have dropped to ~ 0.1 eV using the Boltzmann plot method [3]. Notably, during this second phase, despite the low Te, a sharp increase in ne up to ~ 5 x 10^18 m^-3 was measured by the microwave interferometer, which then gradually decreased until the end of the plasma discharge.
In the presentation, the mechanism of the shift in recombination processes from MAR to EIR and the re-increase in ne will be discussed by comparing the results of molecular and atomic emissions obtained by a spectrometer with multiple lines of sight, complemented by newly implemented Mach-probe measurements between the V-shaped target plates.
This work was partly supported by JST SPRING Grant Number JPMJSP2124, JSPS KAKENHI Grant Numbers 22H01198, 23K22469, and NIFS Collaboration Research program (NIFS23KUGM174, NIFS23KUGM186, NIFS25KFFT001).
[1] K. Verhaegh et al., Nucl. Fusion 63 (2023) 016014.
[2] J. Shi et al., Physica Scripta 98 (2023) 115605.
[3] S. Takahashi et al., Nuclear Materials and Energy 43 (2025) 101945.
[4] M. Sakamoto et al., Nuclear Materials and Energy 12 (2017) 1004–1009.