Speaker
Description
In detached divertor experiments, impurity gas injection used to achieve detachment has been reported to cause core plasma cooling near the X-point and, in some cases, radiative collapse. These adverse effects have been attributed to the approach of the recombination front toward the X-point, highlighting the need to establish a technique for controlling the recombination front position.
Measurement of spatial profile along a magnetic field line under limited observation ports is an issue in the recombination front study. We have developed an estimation method of spatial density and temperature profiles using single observation port. Applicability of the method is demonstrated with electrostatic probe diagnostics in a linear plasma device NUMBER.
NUMBER consists of a 2 m vacuum chamber. Helium plasma is generated using electron cyclotron resonance (ECR) by injecting microwaves (2.45 GHz, 6 kW) through a quartz window at z = 0 m. Plasma flowing along the magnetic field lines (z-axis) undergoes volumetric recombination when additional neutral gas is supplied from behind the endplate installed at z = 1.98 m. Double probes are installed at z = 1.38, 1.53, and 1.68 m, enabling measurement of the axial plasma distribution with 150 mm spacing.
In order to estimate spatial profile, a forked probe is installed at z = 1.68 m, which is capable of measuring the axial gradients of electron temperature and density with a high spatial resolution of 40 mm. We derived a new method for estimating the z-profile using forked probe. This technique can reconstruct the z-profile by utilizing the pressure dependance of electron temperature and density together with the pressure dependance of their local axial gradients.
The measurements revealed that, in the central region of the plasma, the electron temperature decreases monotonically with increasing z, with typical values of a few eV. In contrast, the electron density exhibits a “rollover” structure: it increases with z, reaches a peak value of 2.0 × 10¹⁸ m⁻³, and then decreases. This structure was confirmed using both the double probe and the forked probe, supporting applicability of the estimation method. The rollover position provides a useful indicator for identifying the recombination front location.
Furthermore, by adjusting the balance between neutral gas injection and pumping, we conducted experiments under different neutral pressure distributions and observed the resulting movement of the recombination front. This finding suggests that controlling the neutral pressure distribution can be effectively applied to control the recombination front position.