Transition in velocity and grouping of arc spot on different nanostructured tungsten electrodes

AbstractBehavior of arc spots was investigated in detail using a nanostructured tungsten specimen with different thicknesses of the nanostructured layer. From the observation using a fast framing camera, it was found that the velocity of the arc spots significantly altered as passing the boundary of the two layers. The changes in spot velocity and spot width were discussed theoretically using the ecton model. The fractal dimension of the arc trail evaluated by using a box-counting method was significantly changed. Also, the width of arc trail was increased with the nanostructured layer thickness. From the SEM analysis of the specimen, the amount of tungsten eroded by arcing for two different thickness cases was estimated, and the erosion rates were discussed

Influence of expanding and contracting magnetic field configurations on detached plasma formation in a linear plasma device

We investigated the effects of magnetic field structure on detached plasma formation by simulating magnetically expanding and contracting plasma in a linear plasma device. The present study helps to characterize the geometries of a conventional poloidal divertor and advanced divertors, e.g., super-X divertor. The total ion particle flux measured with a large-diameter target plate dramatically changed under the detached plasma condition compared to that in attached plasma. Under the detached plasma condition, the magnetically expanding plasma clearly exhibited a significant influence on the degradation of detached plasma formation. Further, the magnetically contracting plasma slightly enhanced the electron-ion recombination (EIR) processes. By changing the magnetic field structure from contraction to expansion, the electron density (ne) decreased and the electron temperature (Te) increased upstream from the recombination front, leading to the degradation of the EIR processes. The effect of the decrease in parallel flow velocity under the magnetically contracting plasma on the plasma detachment was not observed because the driven flow due to pressure gradient compensated the effect

Effect of temperature and incident ion energy on nanostructure formation on silicon exposed to helium plasma

Helium plasma can be used to deliver low‐energy (<100 eV) helium ions to stimulate the growth of nanostructures on silicon surfaces. This can produce a wide range of surface features including nanoscale roughening, nanowires and porous structures. In this study, nanostructure sizes varied from ∼10 to over 100 nm in diameter. The effect of these structures on surface reflectivity for photovoltaic and photocatalytic applications is also investigated. Broadband suppression of photoreflectivity is achieved across the 300-1,200 nm wavelength range studied for silicon exposed to helium plasma at 600°C, with an average reflectivity of 3.2% and 2.9% for incident helium ion energies of 42 and 62 eV, respectively.Japan Society for the Promotion ofScience, Grant/Award Numbers:17KK0132, 19H01874; AustralianResearch Council, Grant/Award Number:DP20010283

Blob/Hole Generation in the Divertor Leg of the Large Helical Device

We have analyzed ion saturation current fluctuation measured by a fast scanning Langmuir probe (FSP) in edge region of the Large Helical Device (LHD). Positive and negative spikes of the ion saturation current were observed in the private region and on the divertor leg, respectively. It was found that the boundary position between these regions corresponds to the low-field side (LFS) edge of the divertor leg where the gradient of the ion saturation current profile was the maximum. Such a positional relationship resembles that near the separatrix in the LFS in tokamaks, where blobs and holes are generated. Statistical analysis indicates similar fluctuation characteristics among different magnetic devices

Molecular Dynamics Simulation of Chemical Vapor Deposition of Amorphous Carbon: Dependence on H/C Ratio of Source Gas

By molecular dynamics simulation, the chemical vapor deposition of amorphous carbon onto graphite and diamond surfaces was studied. In particular, we investigated the effect of source H/C ratio, which is the ratio of the number of hydrogen atoms to the number of carbon atoms in a source gas, on the deposition process. In the present simulation, the following two source gas conditions were tested: one was that the source gas was injected as isolated carbon and hydrogen atoms, and the other was that the source gas was injected as hydrocarbon molecules. Under the former condition, we found that as the source H/C ratio increases, the deposition rate of carbon atoms decreases exponentially. This exponential decrease in the deposition rate with increasing source H/C ratio agrees with experimental data. However, under the latter molecular source condition, the deposition rate did not decrease exponentially because of a chemical reaction peculiar to the type of hydrocarbon in the source gas.Comment: accepted by Jpn. J. Appl. Phys. (2008

Double‐probe measurement in recombining plasma using NAGDIS‐II

We have studied the validity of the double-probe method in recombining plasmas. Electron temperature (Te) measured with a double probe was quantitatively evaluated by taking into account the influences of plasma potential fluctuation, plasma resistivity, and electron density fluctuation on the current–voltage characteristics. Differential potential fluctuation and plasma resistivity between two electrodes have a minor effect on Te especially when the inter-distance is small (typically 1 mm). Scattering of measured Te due to the density fluctuation was sufficiently suppressed by making the data acquisition time long (typically 4 s) and taking the average. There is a good agreement between Te measured with the optimized double-probe method and that with laser Thomson scattering diagnostics

Plasma detachment study of high density helium plasmas in the Pilot-PSI device

We have investigated plasma detachment phenomena of high-density helium plasmas in the linear plasma device Pilot-PSI, which can realize a relevant ITER SOL/Divertor plasma condition. The experiment clearly indicated plasma detachment features such as drops in the plasma pressure and particle flux along the magnetic field lines that were observed under the condition of high neutral pressure; a feature of flux drop was parameterized using the degree of detachment (DOD) index. Fundamental plasma parameters such as electron temperature (Te) and electron density in the detached recombining plasmas were measured by different methods: reciprocating electrostatic probes, Thomson scattering (TS), and optical emission spectroscopy (OES). The Te measured using single and double probes corresponded to the TS measurement. No anomalies in the single probe I–V characteristics, observed in other linear plasma devices [16, 17, 36], appeared under the present condition in the Pilot-PSI device. A possible reason for this difference is discussed by comparing the different linear devices. The OES results are also compared with the simulation results of a collisional radiative (CR) model. Further, we demonstrated more than 90% of parallel particle and heat fluxes were dissipated in a short length of 0.5 m under the high neutral pressure condition in Pilot-PSI

Imaging of radiation during impurity gas puffing in LHD

In LHD, several methods of detachment have been attempted, including impurity gaspuffing [1], and the application of an m/n=1/1 magnetic perturbation [2]. LHD is equipped with an imaging bolometer (IRVB) [3] that views the plasma from an upper port. Two scenarios are shown and compared, Ne puffing and N2 puffing. In the case of Ne puffing, radiation becomes more intense near the helical divertor X-point as the radiation increases. In the case of N2 puffing, a double stripe pattern evolves around the upper helical divertor X-point, which appears to be localized near the gas puff inlet. In addition, probe data also indicates that the drop in divertor flux with N2 is localized, while uniform with Ne

Experimental observations and modelling of radiation asymmetries during N2 seeding in LHD

N2 gas has been seeded in the Large Helical Device (LHD) to reduce the divertor heat load through enhanced radiation. Radiation is observed by two imaging bolometers, viewing the same poloidal cross-section from top and bottom ports, at a location which is 36° toroidally removed from the N2 gas puff nozzle located at the bottom of the machine. During N2 seeding, these measurements both confirm that additional radiation from the outboard side is coming exclusively from the top of the cross-section, indicating up/down asymmetry, which is also reproduced by modelling with EMC3-EIRENE using a half torus model. In addition, a toroidally localized, magnetic field direction-dependent radiation enhancement is observed with N2 seeding, but is not reproducible by the model