G. Kawamura

EMC3-EIRENE modelling of edge impurity transport in the stochastic layer of the large helical device compared with extreme ultraviolet emission measurements

The transport properties and line emissions of carbon impurity in the stochastic layer of the Large Helical Device have been investigated with the 3D edge transport code EMC3-EIRENE. A parameter study has been performed to examine the sensitivity of the simulation results on each transport term in the impurity transport model and the impurity source characteristics, i.e. the source amount and the location. The modelling has revealed that in order to reproduce the experimental results of the emission distribution, the impurity perpendicular transport coefficient (D imp) and the first wall source play important roles, while changes to the ion thermal and the friction forces are rather irrelevant. The detailed study of flux tube tracing and magnetic field structure in the edge stochastic layer, in relation to impurity transport, has shown that the deeper penetration of impurity into the higher plasma density region due to the enhanced D imp and the first wall source is responsible for the change of emission pattern as well as the intensity. The analysis indicates that D imp might be larger than that of background plasma by a few factors and also that there probably exists a substantial amount of first wall impurity source.

Studies of dust transport in long pulse plasma discharges in the large helical device

Three-dimensional trajectories of incandescent dust particles in plasmas were observed with stereoscopic fast framing cameras in a large helical device. It proved that the dust is located in the peripheral plasma and most of the dust moves along the magnetic field lines with acceleration in the direction that corresponds to the plasma flow. ICRF heated long pulse plasma discharges were terminated with the release of large amounts of dust from a closed divertor region. After the experimental campaign, the traces of exfoliation of carbon rich mixed-material deposition layers were found in the divertor region. Transport of carbon dust is investigated using a modified dust transport simulation code, which can explain the observed dust trajectories. It also shows that controlling the radius of the dust particles to less than 1 mm is necessary to prevent the plasma termination by penetration of dust for the long pulse discharges. Dust transport simulation including heavy metal dust particles demonstrates that high heating power operation is effective for shielding the main plasma from dust penetration by an enhanced plasma flow effect and a high heat load onto the dust particles in the peripheral plasma. It shows a more powerful penetration characteristic of tungsten dust particles compared to that of carbon and iron dust particles.

Vertical profiles and two-dimensional distributions of carbon line emissions from C2+−C5+ ions in attached and RMP-assisted detached plasmas of large helical device

In Large Helical Device (LHD), the detached plasma is obtained without external impurity gas feed by supplying an m/n = 1/1 resonant magnetic perturbation (RMP) field to a plasma with an outwardly shifted plasma axis position of Rax = 3.90 m where the magnetic resonance exists in the stochastic magnetic field layer outside the last closed flux surface. The plasma detachment is triggered by the appearance of an m/n = 1/1 island when the density, increased using hydrogen gas feed, exceeds a threshold density. The behavior of intrinsically existing impurities, in particular, carbon originating in the graphite divertor plates, is one of the important key issues to clarify the characteristic features of the RMP-assisted plasma detachment although the particle flux still remains on some divertor plates even in the detachment phase of the discharge. For this purpose, vertical profiles and two-dimensional (2-D) distributions of edge carbon emissions of CIII to CVI have been measured at extreme ultraviolet wavelen...