In the Large Helical Device (LHD) operated with a metallic (stainless steel) first wall, it is found that the iron density, nFe, at the plasma core is fairly low (nFe ⩽ 108 cm−3) in general neutral beam (NB)-heated discharges, while the iron quickly increases with the appearance of impurity accumulation when a multi-hydrogen ice pellet is injected or the NB input power is largely reduced. Although the highest iron density (nFe ⩽ 1010 cm−3) at the plasma centre in the LHD is observed from such discharges, it suggests a still low iron concentration (nFe/ne < 10−3). Therefore, the edge iron transport in the ergodic layer, which determines the iron influx to the core plasma, is studied to clarify why the iron density in the core plasma is low. A line ratio of Fe XV located in the vicinity of the last closed flux surface to Fe VIII (or Fe IX) located in the ergodic layer decreases with density. The two-dimensional (2D) edge iron emission of Fe XVI and Fe IX is enhanced in the vicinity of the X-point with a larger number of magnetic field lines directly connected to divertor plates, which suggests that iron ions from the first wall move downstream. The density of edge Fe15+ ions giving the iron influx to the core plasma is analysed with the 2D distribution. The analysis also shows that the iron influx to the core plasma decreases with density. These results clearly indicate that the screening effect developed in the ergodic layer works well for iron ions coming from the first wall. A three-dimensional edge transport simulation with EMC3-EIRENE can also predict an effective impurity screening for heavy impurities compared to light impurities.

The positions of emission of berylliumlike oxygen ions in the core region as well as the hydrogen atoms in the boundary region of the limiter shadow have been measured by means of the difference of the Zeeman patterns in the spectral shape in the poloidal section of the TRIAM-1M super conducting tokamak [H. Zushi et al., Nucl. Fusion 43, 1600 (2003)]. For determining magnetic field strength, the σ components of the Zeeman spectra are resolved by a linear polarizer. In addition to the emission region, the local hydrogen neutral temperature and the recycling flow velocity are measured in the boundary region, and the bulk ion temperature is measured in the core region.

Atomic and molecular processes in volumetric recombination phenomena relevant to divertor detachment are investigated in a divertor simulator MAP (Material and Plasma) -II. In the recombining plasmas, quantitative measurements of parameters, especially using a Langmuir probe, are diffi cult, so that the development of alternative diagnostics is important. Recombination can be induced in He plasma by puffi ng of He or H 2 . In the He puffi ng case, the Rydberg spectra show an electron temperature of lower than 0.1 eV, while in the H 2 puffi ng case, the Rydberg spectra disappear even though the reduction of the ion fl ux is apparent, showing that another type of recombination occurs. Negative hydrogen ions are observed in the peripheral region of the plasma column.

A diagnostic technique for the local measurement of emissions from the molecular hydrogen has been developed. In the presence of an external magnetic field, the emission position can be deduced from the Zeeman patterns in the spectral line shape, and with the aid of the coronal model the rovibronic emission intensity provides the local rovibrational temperatures. In order to evaluate the Zeeman effect on the hydrogen molecule, a quantum-mechanical calculation is performed. The experimentally observed Fulcher-α rovibronic band spectra under a magnetic field strength of about 7T are well represented by the calculation. The temporal evolution of the emission position as well as the rovibrational temperatures are measured in the 8.2GHz lower hybrid current drive discharges of the TRIAM-1M tokamak [H. Zushi et al., Nucl. Fusion 45, S142 (2005)]. The measured emission is found to originate from the region where the plasma comes in contact with the limiter, and the observed increase in the rotational temperature suggests a rise in the limiter surface temperature.

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Neutral particle dynamics is measured in the edge plasma of the TRIAM-1M tokamak by means of the Zeeman patterns in the spectral shape. This measurement can provide the emission position with high spatial resolution, as well as the local drift velocity and temperature. In the TRIAM-1M, the inward neutral flow velocity driven by the radial neutral pressure gradient is observed. The temporal evolution of the flow velocity suggests that the distance between the last closed flux surface and the first wall changes the recycling flux. Thereby, the neutral pressure gradient and the neutral flow velocity are varied. This technique can be an effective way for the measurement of the local neutral behaviour in the edge region.

An electric probe exposed to laser irradiation during negative ion density measurement by applying the laser photodetachment technique may suffer ablation even though the laser power is not significantly high. In this paper, the ablation effects on laser photodetachment signals were examined in a linear plasma device using plasmas of both helium and a mixture of helium and hydrogen. It was found that there was a specific threshold laser power necessary to cause the ablation, and that it depended on the electron density. The ablation signals were also detected using a negatively biased probe. A method for monitoring the ablation phenomena is then presented. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)