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Molecular activated recombination (MAR) leading to plasma detachment has been occurred by additional hydrogen gas injection to the divertor simulation plasma in the tandem mirror GAMMA 10/PDX. Recently, the divertor simulation experimental module (D-module) has been installed in the end region of the mirror device to make best use of a linear plasma device with plasma confinement, meaning that the ion temperature of the plasma exposed to the D-module is high (i.e. a few hundreds eV), which is comparable to the SOL ion temperature in ITER. Noted that ion energy of the plasma is distributed, meaning that condition of PSI is equivalent to that of the torus plasma from a viewpoint of hydrogen recycling.
The D-module consists of a rectangular box (0.5 m square and 0.7 m in length) with an inlet aperture (0.2 m in diameter) at the front panel and a V-shaped tungsten target inside the box. The target size is 0.3 m in width and 0.35 m in length. The open-angle of the V-shaped target can remotely be changed from 15 deg. to 80 deg. A normal setting is 45 deg. Thirteen Langmuir probes have been installed on the upper target plate and two probes have been installed between the V-shaped target and the inlet. The plasma is exposed to the target through the inlet aperture.
In this study, additional hydrogen gas was injected from the vicinity of the inlet to the target with a different feed rate for each plasma to control the neutral pressure inside the D-module, which was measured with an ASDEX gauge. The electron temperature decreased in a whole region of the plasma with increase in the pressure. The temperature near the corner of the V-shaped target decreased from ~ 20 eV to ~ 1 eV. At that time, a clear density roll-over was observed. A position of the density maximum moved to upstream of the plasma with increase in the pressure and the density near the corner of the target decreased to detach the plasma from the target. The detachment must be caused by MAR, since the ratio of H alpha intensity to H beta intensity, which is a good monitor of MAR occurrence, increased. Two dimensional measurement of the ratio using a fast camera with an interference filter showed that the dominant region of MAR moved to upstream of the plasma with increase in the pressure.
In order to estimate an effect of the angle of the target on detachment properties, pressure dependence of the plasma density was measured at the target angle of 30 deg., 45 deg. and 60 deg. The plasma density in front of the target increased once and then decreased with the pressure. It is found that this density roll-over occurred at a smaller amount of gas injection for a smaller target angle, suggesting that a local pressure near the corner of the target became higher at a smaller target angle due to the recycling effect of the V-shape.
Molecular activated recombination in divertor simulation plasma on GAMMA 10/PDX
