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LIBS detection of erosion/deposition and deuterium retention resulting from exposure to Pilot-PSI plasmas
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Tungsten dust remobilization under steady-state and transient plasma conditions
No full textRemobilization is one of the most prominent unresolved fusion dust-relevant issues, strongly related to the lifetime of dust in plasma-wetted regions, the survivability of dust on hot plasma-facing surfaces and the formation of dust accumulation sites. A systematic cross-machine study has been initiated to investigate the remobilization of tungsten micron-size dust from tungsten surfaces implementing a newly developed technique based on controlled pre-adhesion by gas dynamics methods. It has been utilized in a number of devices and has provided new insights on remobilization under steady-state and transient conditions. The experiments are interpreted with contact mechanics theory and heat conduction models.</p
Broadband characterization of congruent lithium niobate from mHz to optical frequencies
No full textLithium niobate (LiNbO3) is a well known uniaxial ferroelectric material. Using impedance measurement, quasi-optical free-space characterization, THz time domain spectroscopy (THz-TDS) and ellipsometry, its dielectric permittivity/refractive index was characterized depending on the crystal orientation over a broad frequency range: 1 mHz to 1 PHz (lambda = 300 nm). Three different frequency ranges, separated by well identified resonances, are observed: low frequency \u27free-piezoelectric\u27 response, intermediate frequency \u27clamped-ionic\u27 response and high frequency \u27electronic\u27 response. These features are discussed with an emphasis on the role of the crystallographic structure and piezoelectric response
Power deposition on misaligned castellated tungsten blocks in the Magnum-PSI and Pilot-PSI linear devices
No full textFor the final design of the ITER divertor it is important to determine whether shaping of each tungsten monoblock to eliminate leading edges is required or not. In order to aid this decision, two experiments were performed in DIFFER\u27s linear plasma devices to study heat loads on misaligned water cooled blocks at glancing incidence. First, a series of tungsten blocks were exposed to a high parallel heat flux (26 MW m -2) hydrogen plasma beam in the Magnum-PSI linear device. The blocks were exposed at an oblique angle between 4 degree and 7 degree with respect to the plasma beam, approaching the low angle of incidence expected in the ITER divertor strike-point regions. One block was vertically misaligned with respect to the others by a value between 0 and 1.2 mm, and the surface temperature evolution monitored. This was compared to a finite element thermo-mechanical model of the system, to investigate any discrepancies between the two. The importance of Larmor orbit smoothing effects could also be considered, as the Larmor radius was larger than half the misalignment height. Next, a row of identical tungsten blocks were exposed at an angle of 3 degree to ~500–700 μs pulsed hydrogen plasmas of peak power between 275–625 MW m -2 in the Pilot-PSI linear device, to simulate the effect of edge localized mode (ELM) transients. One block was misaligned with respect to the others by 1 mm. In all cases, the model and experiment were found to be in excellent agreement (to ~1%), demonstrating clearly that an optical approximation for plasma power loading on misaligned edges is appropriate. A simple model including Larmor smoothing was applied, the results of which give a similar prediction to the optical approximation for the temperature of the misaligned edges, within the error of the temperature measurements, though a small decrease in edge temperature is calculated. Therefore it can be concluded that the application of the optical approximation is generally appropriate as an input physics assumption for the study of shaping alternatives
An analytical force balance model for dust particles with size up to several Debye lengths
No full textIn this study, we developed a revised stationary force balance model for particles in the regime a/labda D<10. In contrast to other analytical models, the pressure and dipole force were included too, and for anisotropic plasmas, a novel contribution to the dipole moment was derived. Moreover, the Coulomb logarithm and collection cross-section were modified. The model was applied on a case study where carbon dust is formed near the plasma sheath in the linear plasma device Pilot-PSI. The pressure force and dipole force were found to be significant. By tracing the equilibrium position, the particle radius was determined at which the particle deposits. The obtained particle radius agrees well with the experimentally obtained size and suggests better agreement as compared to the unrevised model.</p
Enhancing light absorption in graphene with plasmonic lattices
No full textWe present a novel strategy based on metallic arrays of nanoparticles for enhancing the optical absorption in a monolayer of graphene. The collective resonances sustained by the metallic array and arising from the radiative coupling of localized surface plasmon resonances favour the interaction between the graphene layer and the light distributed in the structure. We measure the dispersion diagram of the allowed hybrid plasmonic-photonic modes and we calculate that one of these modes leads to an enhancement of the optical absorption of graphene by a factor 7. We propose to exploit this result for the rational design of graphene-based photodetectors
Accurate Characterization of Triple-Junction Polymer Solar Cells
No full textTriple-junction device architectures represent a promising strategy to highly efficient organic solar cells. Accurate characterization of such devices is challenging, especially with respect to determining the external quantum efficiency (EQE) of the individual subcells. The specific light bias conditions that are commonly used to determine the EQE of a subcell of interest cause an excess of charge generation in the two other subcells. This results in the build-up of an electric field over the subcell of interest, which enhances current generation and leads to an overestimation of the EQE. A new protocol, involving optical modeling, is developed to correctly measure the EQE of triple-junction organic solar cells. Apart from correcting for the build-up electric field, the effect of light intensity is considered with the help of representative single-junction cells. The short-circuit current density (JSC) determined from integration of the EQE with the AM1.5G solar spectrum differs by up to 10% between corrected and uncorrected protocols. The results are validated by comparing the EQE experimentally measured to the EQE calculated via optical-electronic modeling, obtaining an excellent agreement. © 2017 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei
Radiation transport modelling for the interpretation of oblique ECE measurements
No full textThe electron cyclotron emission (ECE) diagnostic provides routinely electron temperature (Te) measurements. At ASDEX Upgrade an electron cyclotron forward model, solving the radiation transport equation for given Te and electron density profile, is used in the framework of integrated data analysis. With this method Te profiles can be obtained from ECE measurements even for plasmas with low optical depth. However, due to the assumption of straight lines of sight and an absorption coefficient in the quasi-perpendicular approximation this forward model is not suitable for the interpretation of measurements by ECE diagnostics with an oblique line of sight.
Since radiation transport modelling is required for the interpretation of oblique ECE diagnostics we present in this paper an extended forward model that supports oblique lines of sight. To account for the refraction of the line of sight, ray tracing in the cold plasma approximation was added to the model. Furthermore, an absorption coefficient valid for arbitrary propagation was implemented. Using the revised model it is shown that for the oblique ECE Imaging diagnostic at ASDEX Upgrade there can be a significant difference between the cold resonance position and the point from which most of the observed radiation originates
Non-equilibrium Microwave Plasma for Efficient High Temperature Chemistry
No full textThis article describes a flowing microwave reactor that is used to drive efficient non-equilibrium chemistry for the application of conversion/activation of stable molecules such as CO2, N2 and CH4. The goal of the procedure described here is to measure the in situ gas temperature and gas conversion