DIFFER: Publications
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MHD instabilities in astrophysical plasmas: very different from MHD instabilities in tokamaks!
Get PDFThe extensive studies of MHD instabilities in thermonuclear magnetic confinement experiments, in particular of the tokamak as the most promising candidate for a future energy producing machine, have led to an \u27intuitive\u27 description based on the energy principle that is very misleading for most astrophysical plasmas. The \u27intuitive\u27 picture almost directly singles out the dominant stabilizing field line bending energy of the Alfvén waves and, consequently, concentrates on expansion schemes that minimize that contribution. This happens when the wave vector k 0 of the perturbations, on average, is perpendicular to the magnetic field B. Hence, all macroscopic instabilities of tokamaks (kinks, interchanges, ballooning modes, ELMs, neoclassical tearing modes, etc) are characterized by satisfying the condition k 0 -|- B, or nearly so. In contrast, some of the major macroscopic instabilities of astrophysical plasmas (the Parker instability and the magneto-rotational instability) occur when precisely the opposite condition is satisfied: k 0 | | B. How do those instabilities escape from the dominance of the stabilizing Alfvén wave? The answer to that question involves, foremost, the recognition that MHD spectral theory of waves and instabilities of laboratory plasmas could be developed to such great depth since those plasmas are assumed to be in static equilibrium. This assumption is invalid for astrophysical plasmas where rotational and gravitational accelerations produce equilibria that are at best stationary, and the associated spectral theory is widely, and incorrectly, believed to be non-self adjoint. These complications are addressed, and cured, in the theory of the Spectral Web, recently developed by the author. Using this method, an extensive survey of instabilities of astrophysical plasmas demonstrates how the Alfvén wave is pushed into insignificance under these conditions to give rise to a host of instabilities that do not occur in laboratory plasmas.</p
Dispersion Anisotropy of Plasmon–Exciton–Polaritons in Lattices of Metallic Nanoparticles
No full textWhen the electromagnetic modes supported by plasmonic-based cavities interact strongly with molecules located within the cavity, new hybrid states known as plasmon–exciton–polaritons (PEPs) are formed. The properties of PEPs, such as group velocity, effective mass, and lifetime, depend on the dispersive and spectral characteristics of the optical modes underlying the strong coupling. In this work, we focus on lattice modes supported by rectangular arrays of plasmonic nanoparticles known as surface lattice resonances (SLRs). We show that SLRs arising from different in-plane diffraction orders in the lattice can couple with the molecular excitons, leading to PEPs with distinct dispersions and thus different group velocities. These results illustrate the possibility of tailoring the transport of PEPs through the design of lattices of plasmonic particles
Conversion of CO2 in a packed-bed dielectric barrier discharge reactor
No full textThe conversion of CO2 into CO and O2 in a quartz cylindrical packed-bed dielectric reactor has been studied using CO2 and Ar gas mixtures at atmospheric pressure and near ambient temperature with quartz wool, γ-Al2O3, and TiO2 packing. The highest energy efficiencies and conversion rates were observed with TiO2 packing in 20% CO2 in Ar: 30% conversion with 2.9% energy efficiency, and 17.5% conversion with 5.0% energy efficiency. Both γ-Al2O3 and quartz wool also showed an enhancement in conversion over an unpacked reactor. The percentage of CO2 to Ar in the range of 20%–60% is shown to have only a minor effect on reactor performance. Conversion as a function of power input and flow rate is studied in detail for γ-Al2O3 and TiO2 packing with similar particle sizes. In both cases, simple chemical kinetic models show that the CO production rate is nearly equal for both materials, while reverse reaction rates to CO2 are doubled for γ-Al2O3 compared to TiO2. From detailed charge–voltage (Q-V) analysis of all four reactor configurations, it is revealed that the electric field at which discharging occurs is higher for both γ-Al2O3 and TiO2 as compared to the empty or quartz wool filled reactors. Comparing kinetic model results with the electrical Q-V analysis, it appears likely that the higher and similar magnitude electric fields occurring with γ-Al2O3 and TiO2 are directly responsible for the increased CO production rates via increased electron energies in the discharge. The higher reverse reaction rates for γ-Al2O3, and its subsequent poorer performance compared to TiO2, can be attributed to a significantly higher effective surface area, which increases undesirable surface reactions between CO and oxygen species
Plasmonic Nanoantenna Arrays as Efficient Etendue Reducers for Optical Detection
No full textOptical detectors require the efficient collection of incident light onto a photodetector. Refractive or reflective optics are commonly used to increase the collected power. However, in the absence of losses, such optics conserve etendue and therefore pose a limit on the field of view and the active area of the detector. A promising method to overcome this limitation is to use an intermediate layer of fluorescent material that omnidirectionally absorbs the incident light and preferentially emits toward the photodetector. We demonstrate here that plasmonic nanoantenna phased arrays are a promising platform to improve the emission efficiency of thin luminescent layers and provide an efficient method to reduce optical etendue. In particular, we show an almost constant optical absorption of the luminescent layer on top of the array with the angle of incidence and a strong beamed emission in small solid angles in the forward direction. These results pave the way for novel optical communication detectors incorporating nanofabricated plasmonic materials as optical etendue reducers
Plasma radiation studies in Magnum-PSI using resistive bolometry
Get PDFBoth the physics of divertor detachment and vapour shielding are characterized by a relatively large amount of radiation produced in the divertor. The linear plasma generator Magnum-PSI is well-suited to study such processes due to its ITER-divertor relevant plasma conditions, simplied geometry and diagnostic accessibility. The need the quantify the plasma radiated power close to the target surface motivated the development of a 4-channel resistive bolometer for Magnum-PSI, and marks the first deployment of such a diagnostic on a linear device. An axially resolved measurement of plasma emission at arbitrary distances from the target surface is now possible. The radial position of the detector can be varied, hereby viewing the full diameter of the plasma column or down to a central region. The overall system design is discussed alongside a comparison of the spectral absorbance of carbon-coated versus non-coated Au/Al bolometer sensors. Despite low electron temperatures of the plasma (1-5 eV), the observed power densities were found to be 10-37 times the sensor noise floor of ∼0.1 W m-2. A synthetic diagnostic based on collisional radiative model calculations from ADAS could well match observed values from H and Ne plasmas while the measured values for Ar and He were more difficult to reproduce. The obtained findings allow for approximate power balance calculations in Magnum-PSI indicating that maximally ∼47 % and ∼14 % of the total power is lost by radiation in the cases of Ar and Ne/He respectively. The results demonstrate the feasibility of resistive bolometry in low temperature high density plasma regions and on long timescales (>450 s) which is of relevance to ITER. Due to long-term temperature drifts which were observed, a planned upgrade involves the installation of a shutter and FPGA-based electronics for increased accuracy.</p
A forward model for the helium plume effect and the interpretation of helium charge exchange measurements at ASDEX Upgrade
No full textThe analysis of the charge exchange measurements of helium is hindered by an additional emission contributing to the spectra, the helium \u27plume\u27 emission (Fonck et al 1984 Phys. Rev. A 29 3288), which complicates the interpretation of the measurements. The plume emission is indistinguishable from the active charge exchange signal when standard analysis of the spectra is applied and its intensity is of comparable magnitude for ASDEX Upgrade conditions, leading to a significant overestimation of the He2+ densities if not properly treated. Furthermore, the spectral line shape of the plume emission is non-Gaussian and leads to wrong ion temperature and flow measurements when not taken into account. A kinetic model for the helium plume emission has been developed for ASDEX Upgrade. The model is benchmarked against experimental measurements and is shown to capture the underlying physics mechanisms of the plume effect, as it can reproduce the experimental spectra and provides consistent values for the ion temperature, plasma rotation, and He2+ density
Stabilizing Lead-Free All-Inorganic Tin Halide Perovskites by Ion Exchange
No full textBecause of its thermal stability, lead-free composition, and nearly ideal optical and electronic properties, the orthorhombic CsSnI3 perovskite is considered promising as a light absorber for lead-free all-inorganic perovskite solar cells. However, the susceptibility of this three-dimensional perovskite toward oxidation in air has limited the development of solar cells based on this material. Here, we report the findings of a computational study which identifies promising RbyCs1-ySn(BrxI1-x,)(3) perovskites for solar cell applications, prepared by substituting cations (Rb for Cs) and anions (Br for I) in CsSnI3. We show the evolution of the material electronic structure as well as its thermal and structural stabilities upon gradual substitution. Importantly, we demonstrate how the unwanted yellow phase can be suppressed by substituting Br for I in CsSn(BrxI1-x)(3) with x >= 1/3. We predict that substitution of Rb for Cs results in a highly homogeneous solid solution and therefore an improved film quality and applicability in solar cell devices.</p
Performance mapping of cation exchange membranes for hydrogen-bromine flow batteries for energy storage
No full textNon-oxidative methane coupling to C2 hydrocarbons in a microwave plasma reactor
Get PDFNon‐oxidative methane activation is carried out in a microwave plasma reactor for coupling to higher hydrocarbons. Fourier transform infrared spectroscopy (FTIR) was used to measure absolute concentrations of the major hydrocarbon species. Hydrogen concentration was also independently inferred from pressure‐based change in molar flow measurements. By closing both the carbon and hydrogen balance, from stoichiometry of the reactions, the amount of deposits was obtained as well. Additionally, core gas temperatures up to 2500 K were measured with Raman scattering when nitrogen acted as probing molecule in sample mixture discharges. At low gas temperatures, ethane and ethylene were significant products based on plasma chemistry, with ethane selectivities reaching up to 60%. At higher gas temperatures, thermal effects become stronger shifting the selectivity toward acetylene and deposits, resembling more with equilibrium calculations. The energy efficiency of the methane conversion reached up to 15% from which 10% represented coupling efficiency to higher hydrocarbons. It is concluded that there is an interplay between plasma and thermal chemistry where plasma generates radicals and final distribution is set by thermodynamics. CH4 ->plasma-> CH3 ->coupling-> C2H6 ->dehydrogenation-> C2H5 ->….</p
A 2.5 T, 1.25 m Free Bore Superconducting Magnet for the Magnum-PSI Linear Plasma Generator
Get PDFDIFFER\u27s main experiment Magnum-PSI is the only laboratory setup in the world capable of exposing materials to plasma conditions similar to those of future fusion reactors. The success of the Magnum-PSI experiment depends on the generation of a 2.5 T magnetic field without restricting the diagnostic access and operational aspects of the experiment. This has been achieved with a magnet consisting of five superconducting solenoids wound on a 2.5 m long stainless steel coil former positioned in a cryostat offering a 1.25 m warm bore. A copper stabilized multifilamentary NbTi conductor with a 3.48 mm 2 cross section has been used, thus the magnet exhibits a total inductance of 500 H and a stored energy of 16 MJ. This presents quite a challenge for the protection scheme that has been implemented using a mix of back-to-back cold diodes and external dump resistors. The coils generate a plateau shaped magnetic field adjustable up to 2.5 T while the distance between the coils allows for 16 room temperature view-ports. The coils are cooled with liquid helium using a re-condensing system operated with cryocoolers, while the magnet system cycles between zero and full field up to once per day.</p