[en] Erosion and redeposition processes have been investigated using a linear plasma device and a Monte-Carlo simulation code. Molybdenum substrate is exposed to argon plasmas for oblique incidence of magnetic field under well-defined plasma conditions. The effect of magnetic field directions on the erosion distribution is clearly observed. We have a qualitative explanation for the erosion profiles with the effect of prompt redeposition of high Z material in comparison with numerical code predictions. However, we have quantitative differences for their erosion rates. (author)

[en] The current-voltage characteristics, the structure of electric potential around an ion sensitive probe and the particle flux on the ion collector have been simulated by the two dimensional particle-in-cell code (Berkeley Code). Concerning the separate mechanism of ions and electrons on the probe, the importance of electric potential profile around the electrode was pointed out. It was found that the E x B drift motion of electrons moving along the equipotential surface plays an essential role in the ISP measurement. (orig.)

[en] A new divertor simulator called NAGDIS-II has been constructed in order to investigate edge plasma physics in fusion devices. Improved TP-D type plasma source, which consists of LaB₆ cathode with a Mo hollow shield and external heating system, water-cooled intermediate electrode and anode was employed to make a high density plasma in the NAGDIS-II. The performance and reliability of the discharge system was confirmed by quantitatively measuring neutral pressure, heating efficiency and plasma parameters. (author)

[en] Measurements of ion and electron temperatures have been performed in detaching helium--hydrogen plasmas in a linear divertor simulator experiment using spectroscopy, a Langmuir probe, and an omegatron mass spectrometer. Detachment in these plasmas is characterized by a significant (∼20 x) reduction in the central plasma flux at the target plate as the target region neutral pressure is increased from 2 to 12 mTorr. The data indicate that partially detached gas-target plasmas consist of a hot (T_e∼5 eV) core region along the axis of the plasma column, surrounded by a cold (T_e∼0.1 eV) halo region of recombining plasma. At T_e=5 eV, plasma recombination is negligible compared with ionization; these experiments therefore provide evidence that detachment is primarily caused by radial transport and by a gradual drop in the ionization source as the temperature of the core region drops below 5 eV

[en] Heat load reduction to plasma-facing components is one of the most crucial issues for the next generation fusion reactors such as ITER, which will realize a long pulse or a steady state operation. For the reason mentioned above, it is quite important to investigate fundamental property and structure of detached plasmas associated with plasma recombinations, which can decrease heat and particle flux to the divertor plate. We have done basic experiments on the plasma detachment in the divertor plasma simulator, NAGDIS-II device. We have observed both electron-ion recombination (EIR), which includes three-body and radiative recombinations, and molecular activated recombination (MAR) in NAGDIS-II. (orig.)

[en] The first reliable measurement of ion temperature in the divertor plasma of the Large Helical Device has been done by using an ion sensitive probe. The satisfactory current-voltage characteristics of the ion collector for evaluating the ion temperature were obtained at the outer part of the divertor leg. Furthermore, simultaneous ion and electron temperature measurements were successfully done in this part. The results show that the ion temperature is higher than the electron temperature in the part. There is a possibility that the profiles of the evaluated ion temperature which shows relatively higher than the electron temperature at the outside of divertor leg are qualitatively explained by particle's orbits around the edge and divertor region

[en] A simulated gas target divertor experiment has been performed to investigate fundamental physics of plasma detachment in the linear plasma device, TPD-I, which has a high heat flux and high density plasma in steady and pulse state. The existence of density threshold for plasma detachment was observed in our experiment. It is found that the electron-ion temperature relaxation process is a key to determine the density dependence of the plasma detachment. Furthermore, the preliminary experimental results for dynamic behavior of the detached plasma for heat pulse plasma are shown. (author)

[en] Full text: In Plasma Research Center, University of Tsukuba, divertor simulation experiments have been conducted at the end region of GAMMA 10/PDX. The high temperature end-loss plasmas of GAMMA10/PDX are a functional tool for simulating edge and divertor plasmas and contribute to developing a deeper understanding of the physics involved in plasma detachment. Our aim is to study detachment phenomena under equivalent conditions for ITER SOL and divertor plasma under high temperature and strong magnetic field. So far, we have performed characterization of plasma detachment from high temperature plasma (ion temperature has achieved a few hundred eV) produced by a large tandem mirror device for various radiator gases. For ideal detached plasma operation, the amount of impurities is expected to be as low as possible. In this study, we have investigated the synergistic effect of a combination of various impurity gases and hydrogen gas on plasma detachment of high temperature plasma, equivalent to SOL plasma of tokamaks in the GAMMA10/PDX end region, utilizing an open magnetic field configuration. A small puff of an impurity gas (N₂, Ne, Ar, Kr, Xe) in combination with a puff of H₂ gas is examined to evaluate their synergistic effect on the formation of detached plasma; the following results are obtained: i) A combination of N₂ and H₂ puffs showed clear decrease of electron density and ion flux; ii) N₂ and H₂ puffs form a strong density gradient along the axial direction; and iii) other noble impurity gases showed insufficient synergistic effect. The new results indicate the possibility of achieving a reliable divertor operation scheme and the importance of investigating molecular processes in further detail. We can contribute to the optimization of detached plasma formation through a deeper understanding of the H₂ and N₂ assisted recombination process. (author)

[en] Highlights: • Transition of a detached to attached plasma by ECH in GAMMA 10/PDX tandem mirror. • Heat and ion fluxes increased significantly during ECH and with increasing impurity. • The impurity ion emission increases remarkably during ECH. • Transition characteristics in detached plasmas produced by various radiator gases. • N2 seeding has a significant tolerance of sustaining detached plasma during ECH. -- Abstract: The transition of a detached to attached plasma experiment has been performed in GAMMA 10/PDX by applying an additional plasma heating pulse of electron cyclotron heating (ECH). In a plasma detachment experiment, a short pulse (25 ms) of ECH was applied at the east plug-cell to examine the effects of electron heating on the plasma parameters in the D-module. It was found that the ion flux increased significantly during ECH injection. In the ECH injection period, the ion flux increased with the increasing impurity injection, which indicates the impurity ion components are enhanced by ionization in the D-module due to the application of ECH. During ECH injection period, the increase of the ion flux near the corner of the target plate shows a clear dependence on the gas species. The 2D visible emission has been captured by the high-speed camera and a bright emission near the corner of the target has been observed. The spectroscopic measurement in the D-module shows that the impurity ion emission increases remarkably during ECH injection. These results indicate the detached plasma change to the attached state.

[en] In order to find the causes of the strong anomaly of current-voltage characteristics of Langmuir probe observed in detached recombining plasmas in a linear divertor plasma simulator, NAGDIS-II, we have investigated plasma resistance along a magnetic field and potential fluctuations in the detached recombining plasmas. Simple calculation on the ratio between the plasma length, at which plasma resistance and resistance of ion sheath formed around a probe tip become equal, and an electron collection length indicates that the evaluation of electron temperature T_e becomes inaccurate at T_e of less than 0.6 eV when plasma density and neutral pressure are 1.0 x 10¹⁸ m^-3 and 10 mtorr, respectively. The potential fluctuation in detached recombining plasmas was found to be so large compared to T_e/e, which can also modify the probe characteristics. (orig.)