New application of hyperspectral imaging to steady-state plasma observations.

A new application of hyperspectral imaging (HSI) to steady-state plasma emission observations is proposed because of its prominent feature: an HSI camera records a two-dimensional image, and each spatial pixel contains spectral data typically with more than a hundred bands, while conventional digital cameras have only three bands. The characterization of an HSI camera (Specim IQ) has been performed during steady-state plasma-material interaction experiments using the linear plasma device PISCES-A. By easily subtracting the background/continuum emission in contrast to conventional filter cameras, two-dimensional images of multiple emission lines at different wavelengths are simultaneously obtained during a single measurement, demonstrating the advantage in plasma emission observations.

Development of a LIBS system for in situ surface measurements during plasma exposure in PISCES-A.

A laser-induced breakdown spectroscopy (LIBS) system has been developed with a Q-switched Nd:YAG laser (wavelength = 1064 nm and pulse width ∼5 ns) to conduct in situ surface measurements during plasma exposure in the PISCES-A linear divertor plasma simulator. The LIBS signal enhancement is obtained with both the magnetic field normal to the surface of a target and steady-state background plasma. Migration of sputtered Ta impurities onto the neighboring Cr surface is identified during He plasma exposure, only when cone structures are formed on the Cr surface. D retention in W during D plasma exposure is observed to decrease with increasing the sample temperature. The temporal evolution of D outgassing from W is measured in a time range of ∼10-420 s right after D plasma exposure. A power law fit, t -α , yields α ∼ 0.34 ± 0.09, which is nearly consistent with calculated and measured values.

Morphologies of tungsten nanotendrils grown under helium exposure.

Nanotendril "fuzz" will grow under He bombardment under tokamak-relevant conditions on tungsten plasma-facing materials in a magnetic fusion energy device. We have grown tungsten nanotendrils at low (50 eV) and high (12 keV) He bombardment energy, in the range 900-1000 °C, and characterized them using electron microscopy. Low energy tendrils are finer (~22 nm diameter) than high-energy tendrils (~176 nm diameter), and low-energy tendrils have a smoother surface than high-energy tendrils. Cavities were omnipresent and typically ~5-10 nm in size. Oxygen was present at tendril surfaces, but tendrils were all BCC tungsten metal. Electron diffraction measured tendril growth axes and grain boundary angle/axis pairs; no preferential growth axes or angle/axis pairs were observed, and low-energy fuzz grain boundaries tended to be high angle; high energy tendril grain boundaries were not observed. We speculate that the strong tendency to high-angle grain boundaries in the low-energy tendrils implies that as the tendrils twist or bend, strain must accumulate until nucleation of a grain boundary is favorable compared to further lattice rotation. The high-energy tendrils consisted of very large (>100 nm) grains compared to the tendril size, so the nature of the high energy irradiation must enable faster growth with less lattice rotation.

Experimental evidence of intermittent convection in the edge of magnetic confinement devices.

Probe measurements in the PISCES linear device indicate the presence of plasma radially far from where it is produced. We show that this is mainly caused by large-scale structures of plasma with high radial velocity. Data from the Tore Supra tokamak show striking similarities in the shape of these intermittent events as well as the fluctuation density probability distribution and frequency spectrum. The fact that intermittent, large-scale events are so similar in linear devices and tokamaks indicates the universality of convective transport in magnetically confined plasmas.