Development of quantitative atomic modeling for tungsten transport study using LHD plasma with tungsten pellet injection

Quantitative tungsten study with reliable atomic modeling is important for successful achievement of ITER and fusion reactors. We have developed tungsten atomic modeling for understanding the tungsten behavior in fusion plasmas. The modeling is applied to the analysis of tungsten spectra observed from plasmas of the large helical device (LHD) with tungsten pellet injection. We found that extreme ultraviolet (EUV) emission of W24+ to W33+ ions at 1.5–3.5 nm are sensitive to electron temperature and useful to examine the tungsten behavior in edge plasmas. We can reproduce measured EUV spectra at 1.5–3.5 nm by calculated spectra with the tungsten atomic model and obtain charge state distributions of tungsten ions in LHD plasmas at different temperatures around 1 keV. Our model is applied to calculate the unresolved transition array (UTA) seen at 4.5–7 nm tungsten spectra. We analyze the effect of configuration interaction on population kinetics related to the UTA structure in detail and find the importance of two-electron-one-photon transitions between 4p54dn+1– 4p64dn−14f. Radiation power rate of tungsten due to line emissions is also estimated with the model and is consistent with other models within factor 2

Indirect energy transfer channel between fast ions via nuclear elastic scattering observed on the large helical device

An energy transfer phenomenon between energetic ions, which cannot be explained only considering the Coulomb scattering process, was observed on a large helical device (LHD). This phenomenon often occurs in fusion reactivity enhancement and fast-ion slowing-down process that can be observed as a delay in the decay time of the D(d,n)3He neutron generation rate. The transferred energy required to induce such a reactivity enhancement or delay in the fast-ion slowing-down time (neutron decay time) was examined based on the Boltzmann−Fokker−Planck analysis in which a discrete energy transfer process, called nuclear elastic scattering (NES), is included. It was shown that even though the cross section of the NES is smaller than that of the Coulomb scattering, enough knock-on population appears in the energetic region in ion distribution function to induce the observable NES effects; thus, enough energy is transferred from beam ions to fast component of bulk ion distribution function indirectly and the transferred energy per unit time via NES is comparable to the Coulomb scattering rate. This study analytically demonstrates that the observed phenomena on LHD can be explained smoothly by considering the alternative indirect energy transfer channel between energetic ions, which can be comparable with the one via Coulomb scattering

Extreme ultraviolet spectroscopy and atomic models of highly charged heavy ions in the Large Helical Device

We report recent results of extreme ultraviolet (EUV) spectroscopy of highly charged heavy ions in plasmas produced in the Large Helical Device (LHD). The LHD is an ideal source of experimental databases of EUV spectra because of high brightness and low opacity, combined with the availability of pellet injection systems and reliable diagnostic tools. The measured heavy elements include tungsten, tin, lanthanides and bismuth, which are motivated by ITER as well as a variety of plasma applications such as EUV lithography and biological microscopy. The observed spectral features drastically change between quasicontinuum and discrete depending on the plasma temperature, which leads to some new experimental identifications of spectral lines. We have developed collisional-radiative models for some of these ions based on the measurements. The atomic number dependence of the spectral feature is also discussed

A comprehensive study on impurity behavior in LHD long pulse discharges

Impurity behavior is studied in a variety of LHD (Large Helical Device) long pulse discharges, i.e. standard hydrogen plasmas, super dense core plasmas, helium plasmas with ICH (Ion Cyclotron Frequency Heating), multi-species plasmas mixed with H and He. Density scan experiments show a specific density range of impurity accumulation for only hydrogen discharges. Strong suppression of impurity accumulative behavior is observed in high temperature plasmas with high power heating. The main contributions to impurity transport are extracted by a comprehensive study on impurity behavior, i.e. investigating the critical conditions for impurity accumulation and the parameter dependences. It is found that the impurity behavior is determined by three dominant contributions, i.e. neoclassical transport mainly depending on radial electric field, turbulent transport increasing with heating power and impurity screening at high edge collisionality in the ergodic layer. The mapping of impurity behavior on n-T (electron density and temperature) space at the plasma edge shows a clear indication of the domain without impurity accumulation and provides operation scenarios to build up fusion-relevant plasmas

Density Regimes of Complete Detachment and Serpens Mode in LHD

In the Large Helical Device (LHD), the hot plasma column shrinks at the high-density regime and complete detachment takes place. Hydrogen volume recombination is observed at complete detachment. This phase isself-sustained under specific experimental conditions and called the Serpens mode (self-regulated plasma edge ‘neath the last-closed-flux-surface). The Serpens mode is achieved after either rapid or slow density ramp up, and either by hydrogen or helium gas puffing. The threshold conditions for complete detachment and the Serpens mode are experimentally documented in the parameter space of heating power and density. The threshold density for the Serpens mode transition increases with ? 0.4 power of the heating power. The total radiation is shown to be not adequate to describe the threshold conditions, since it mainly includes the information of very edge region outside the hot plasma column. The operational density limit in LHD, which is sustainable in steady state, has been extended to 1.7 times as high as the Sudo density limit, by applying pellet injection to the Serpens plasmas

The isotope effect on impurities and bulk ion particle transport in the Large Helical Device

The isotope effect on impurities and bulk ion particle transport is investigated by using the deuterium, hydrogen, and isotope mixture plasma in the Large Helical Device (LHD). A clear isotope effect is observed in the impurity transport but not the bulk ion transport. The isotope effects on impurity transport and ion heat transport are observed as a primary and a secondary effect, respectively, in the plasma with an internal transport barrier (ITB). In the LHD, an ion ITB is always transient because the impurity hole triggered by the increase of ion temperature gradient causes the enhancement of ion heat transport and gradually terminates the ion ITB. The formation of an impurity hole becomes slower in the deuterium (D) plasma than the hydrogen (H) plasma. This primary isotope effect on impurity transport contributes the longer sustainment of the ion ITB state because the low ion thermal diffusivity can be sustained as long as the normalized carbon impurity gradient R/Ln,c, where , is above the critical value (~−5). Therefore, the longer sustainment of the ITB state in the deuterium plasma is considered to be a secondary isotope effect due to the mitigation of the impurity hole. The radial profile of H and D ion density is measured using bulk charge exchange spectroscopy inside the isotope mixture plasma. The decay time of H ion density after the H-pellet injection and the decay time of D ion density after D-pellet injection are almost identical, which demonstrates that there is no significant isotope effect on ion particle transport

Recent Results from LHD Experiment with Emphasis on Relation to Theory from Experimentalist’s View

he Large Helical Device (LHD) has been extending an operational regime of net-current free plasmas towardsthe fusion relevant condition with taking advantage of a net current-free heliotron concept and employing a superconducting coil system. Heating capability has exceeded 10 MW and the central ion and electron temperatureshave reached 7 and 10 keV, respectively. The maximum value of β and pulse length have been extended to 3.2% and 150 s, respectively. Many encouraging physical findings have been obtained. Topics from recent experiments, which should be emphasized from the aspect of theoretical approaches, are reviewed. Those are (1) Prominent features in the inward shifted configuration, i.e., mitigation of an ideal interchange mode in the configuration with magnetic hill, and confinement improvement due to suppression of both anomalous and neoclassical transport, (2) Demonstration ofbifurcation of radial electric field and associated formation of an internal transport barrier, and (3) Dynamics of magnetic islands and clarification of the role of separatrix