Speaker
Description
Excessive heat flux to the divertor plates is one of the most serious issues in magnetic fusion reactors. A detached divertor, in which neutral gas injection enhances radiation and volumetric recombination, is the most promising candidate for mitigating the divertor heat load. Thus integrated divertor simulation codes for future fusion reactors are actively developed. Linear plasma devices, which have high discharge reproducibility and allow detailed measurements in recombining plasmas, have been used for fundamental research on the divertor plasma physics. Additionally, they are recently playing an important role in benchmarking divertor simulation codes through detailed measurement in recombining plasmas. However, most measurements have focused only on electrons and excited atoms, and measurements of ions and comparisons with simulation results have rarely been performed. This has made it challenging to evaluate the accuracy of ion-related reaction and heat-transport models. In our previous work, we fabricated a Retarding Field Analyzer (RFA) and successfully obtained the radial profile of the ion temperature[1]. We are now developing a compact RFA that can translate along magnetic field lines. The axial RFA allows measurements of the axial distributions of both ion temperature and electron temperature in recombining plasmas.
The axial RFA is being introduced into the linear RF divertor simulator DT-ALPHA to measure detailed axial distributions of recombining plasma. DT-ALPHA can generate both helium ionization plasmas and steady-state recombining plasmas. Plasma is produced by $13.56\ \mathrm{MHz}$ radio-frequency discharge. Working gas is supplied from the upstream side of the device. In addition, helium secondary gas is supplied from the downstream side to enhance volumetric recombination. Typical parameters of the recombining plasma are approximately $T_\mathrm{e}\sim1.0\ \mathrm{eV}$, $n_\mathrm{e}\sim10^{18}\ \mathrm{m}^{-3}$.
The newly developed axial RFA consists of four grids and a collector. The distance between electrodes is $100\ \mu\mathrm{m}$, which allows analysis of the energy distributions of ions and electrons without being affected by space-charge limitations even in high density recombining plasmas. The outer and aperture diameters are $5\ \mathrm{mm}$ and $1\ \mathrm{mm}$, respectively, both of which are sufficiently smaller than the plasma diameter in DT-ALPHA. In the presentation, details of the axial RFA and experimental results for both helium ionization plasmas and recombining plasmas will be reported and discussed.
This work is supported by JSPS KAKENHI (Grant Nos. 24K00607 and 25KJ0528).
[1] S. Kagaya et al., Phys. Plasmas 32, 112103 (2025).