In the Large Helical Device (LHD) experiment, complete detachment takes place when the plasma column shrinks inside the last-closed-flux-surface (LCFS). The density at the LCFS that results in this shrinking corresponds to the maximum LCFS density achievable under the attached condition. The critical LCFS density increases with the square root of the heating power, as is predicted by the conventional density limit scaling for helical plasmas, called the Sudo scaling. High line-averaged electron densities reaching 3 × 1020 m−3, which correspond to ∼3 times as high as the Sudo scaling, have been achieved in the plasmas with strongly peaked density profiles generated by hydrogen ice pellet injection. Even in the pellet-fuelled plasmas, however, the LCFS densities are similar to those in gas-fuelled plasmas with flat density profiles and well reproduced by Sudo scaling with a factor 0.8. According to these observations, Sudo scaling has been reinterpreted as the ‘edge’ density limit scaling. The square root type power dependence in the Sudo scaling has been reconsidered. Instead of a simple power balance between the heating power and the radiation loss, it is deduced by combining the critical LCFS temperature for complete detachment and the electron temperature dependence on the heating power and the electron density. Higher edge density than in the attached plasmas can be sustained in the completely detached plasmas, where the plasma edge shrinks inside the LCFS. The edge density limit scaling is extended for completely detached plasmas by taking into account the shrinking plasma edge.

The L–H transition in a helical-axis heliotron, Heliotron J, is investigated. For electron cyclotron heating (ECH), neutral beam injection (NBI) heating and ECH + NBI combination heating plasmas, the confinement quality of the H-mode is examined with an emphasis on its magnetic configuration dependence. The vacuum edge rotational transform, ι(a)/2π, is chosen as a label for the magnetic configuration where ι/2π is the rotational transform and a is the average plasma minor radius in metres. The experimental ι(a)/2π dependence of the enhancement factor over the L-mode confinement reveals that specific configurations exist where high-quality H-modes (1.3 < HISS95 < 1.8) are attained. is the experimental global energy confinement time and is the confinement time scaling from the international stellarator database given as . R is the plasma major radius in metres, is the line-averaged plasma density in 1019 m−3, PL is the power loss in megawatts that accounts for the time derivative of the total plasma energy content and Bt is the toroidal magnetic field strength in tesla (Stroth U. et al 1996 Nucl. Fusion 36 1063). The ι (a)/2π ranges for these configurations are near values that are slightly less than those of the major natural resonances of Heliotron J, i.e. n/m = 4/8, 4/7 and 12/22. To better understand this configuration dependence, the geometrical poloidal viscous damping rate coefficient, Cp, is calculated for different values of ι(a)/2π and compared with the experimental results. The threshold line-averaged density of the H-mode, which depends on the configuration, is in the region of 0.7–2.0 × 1019 m−3 in ECH (0.29 MW) + NBI (0.57 MW) operation. As for the edge plasma characteristics, Langmuir probe measurements have shown a reduced fluctuation-induced transport in the region that begins inside the last closed flux surface (LCFS) and extends into the scrape-off layer. In addition, a negative radial electric field Er (or Er-shear) is simultaneously formed near the LCFS at the transition.

Non-inductive currents of electron cyclotron heated plasmas have been examined in the helical-axis heliotron device, Heliotron J. The bootstrap and EC currents were separated by comparing experiments with positive and negative magnetic field. The estimated bootstrap current was found to be affected by the magnetic field configuration. It increases with an increase in the bumpy component of the magnetic field spectrum, which agrees well with a neoclassical prediction calculated using the SPBSC code. The EC current driven by oblique launch with respect to the magnetic field strongly depends on the field configuration and the location of the EC power deposition. The EC current is enhanced when the EC power is deposited on the magnetic axis. The maximum EC current is I EC = −4.6 kA and the current drive efficiency is η = n e RI p/P EC = 8.4 × 1016 A W−1 m−2. The flow direction of the EC current depends on the magnetic field ripple structure where the EC power is deposited.

Self-sustained detachment has been obtained in the Large Helical Device (LHD). Strong hydrogen gas puffing of ∼200 Pa m3 s−1 after a density feedback phase detaches the plasma from the divertor plates with high reproducibility. High electron density of over 1 × 1020 m−3 is sustained without gas puffing until the heating beam stops and a high-density flat top for 2 s has been demonstrated. Throughout the self-sustained detachment phase, the minor radius of the hot plasma column shrinks to ∼90% of the last-closed-flux-surface, which corresponds to the rational surface. This new state has been named the ‘Serpens mode’, for self-regulated plasma edge 'neath the last-closed-flux-surface. Global energy confinement of the Serpens mode is compared with the international stellarator scaling 1995 (ISS95) and the recently established scaling for high-density LHD plasmas (HD scaling), where shrinking confinement volume and shallow penetration of the heating beams are taken into account. Although the energy confinement of the Serpens mode seems deteriorated compared with ISS95, as in the case of high-density attached plasmas, it is consistent with the HD scaling. This suggests that the energy confinement properties of detached plasmas in LHD are similar to those in high-density attached plasmas.

Neptunium I Solubility / Hydrolysis /Migration /Quartz /Liquid chromatography SummaryTo examine the chemical forms of neptunium (V) in aqueous solutions, the solubility was measured by an ultrafiltration method. The existence of three species of NpOj, NpOjOH and NpOj (OH) 2 were expected from the resulting pH-dependent solubility curve. Their hydrolytic data were obtained as follows; log0, =5.7, log0 3 = 8.6 and log K sp = -10.7 at ionic strength of 0.01 M.A high performance liquid chromatography was applied to study the migration behaviour of neptunium (V) in a quartzpacked column. The presence of two components of neptunium (V) which were different from each other in the migration velocity was demonstrated successfully. The one component was the unretarded component moving with the eluent, and the other the retarded component. The distribution coefficient of the retarded component decreased with the increasing ionic strength of the eluent and increased with the increasing pH. Mechanisms of the migration behaviour are discussed for each component.