7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Effect of ion temperature on detached plasma formation using a linear divertor
simulator TPDsheet-U
Naonori Okada1, Ryuichi Ohnuma,Akira Tonekgawa1 and Kohnosuke Sato2
1Tokai University,4-1-1 Kitakaname, Hiratsuka-shi, Kanagawa 259-1292, Japan
2
Research Institute for Applied Mechanics, Kyushu University, Hukuoka, Japane-mail (speaker):8BSP1107@gmail.com

The divertor is subject to damage by high heat flow
particles emitted from the core plasma. Therefore, it isconsidered to reduce the heat load on the divertor plates
by introducing neutral gas to generate a detached plasma.
In the divertor of future DEMO-class devices, further
reduction of the heat load is required, and simulation
experiments of advanced divertors employing a divergent
magnetic field configuration, such as the Super X divertor,
are in progress. Therefore, basic research on heat load
reduction in detached plasma is being actively conductedin small linear divertor simulators.
However, the ion temperature of the plasma producedby most linear divertors is as low as ～3 eV, which issignificantly different from the ion temperature of thedivertor plasma of large devices such as the International
Thermonuclear Experimental Reactor (ITER), which is～10-20 eV. Therefore, the effect of ion temperature on
detached plasma generation has been an open question inbasic research using linear divertor simulators. In addition,
the process of detached plasma generation in the divergent
field configuration used in advanced divertors has also
been mainly studied by simulation, and few basic
experiments have been conducted.
The objective of this study is to investigate thegeneration process of detached plasma with respect to ion
temperature by ion cyclotron resonance (ICR) heating of
high-density sheet plasma generated in a linear divertor
simulator (TPDsheet-U). The sheet plasma used here is aboundary plasma, and compared to ordinary cylindricalplasma, it is thought that radiofrequency waves from theoutside can penetrate the plasma more easily andefficiently heat the ions.
In this experiment, RF power in the ion cyclotron
frequency band (1.0-1.3 MHz) was applied to a densehydrogen sheet plasma (ne～1019 m-3, Te～10eV) and theenergy of the heated plasma was measured using amagnetic loop coil for the ICR heating effect. Detachedplasma formation such as electron density, electron
temperature, and Hγ/Hα were analyzed using Langmuirprobes and visible spectroscopy, respectively. As a result,it was observed for the first time that the electron
temperature of the detached plasma increased and theBalmer series emission intensity ratio decreased when theion temperature was increased by applying a resonancefrequency ωRF, which is about 1.2 times the ion cyclotron
frequency ωci of the hydrogen ions. This suggests that
increasing the ion temperature causes a transition fromdetached plasma to attached plasma.
Figure1 shows the energy relaxation time ratio(Te=10eV) versus ion temperature and plasma density for
Te > Ti, the temperature relaxation time τei is much smaller
than the particle confinement time τp, so Te decreases andthe electron energy loss increases. On the other hand,electron energy is transferred to ions, and ion energy islost through charge exchange. As a result, radiative
recombination and three-body recombination (EIR) occur.
Conversely, for Te≤Ti, as the ion temperature increases,
the temperature relaxation time τei is equal to or greaterthan the particle confinement time τp, and energy istransferred from the ion to the electron in the opposite
direction. Therefore, the electron temperature does notdecrease sufficiently, and the formation of detachedplasma is suppressed.
References[1] Okada, Naonori, et al. "Effects of ion temperature on
detached plasma formation using a linear divertor
simulator TPDsheet-U." Fusion Engineering and Design
192 (2023) 113596.

Figure 1 Dependence of temperature relaxation time on plasma density.