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Laser induced fluorescence in nanosecond repetitively pulsed discharges for CO2 conversion
No full textA CO2 nanosecond repetitively pulsed discharge (NRP) is a harsh
environment for laser induced fluorescence (LIF) diagnostics. The
difficulties arise from it being a strongly collisional system in which
the gas composition, pressure and temperature, have quick and strong
variations. The relevant diagnostic problems are described and
illustrated through the application of LIF to the measurement of the OH
radical in three different discharge configurations, with gas mixtures
containing CO2 + H2O. These range from a dielectric barrier NRP with He
buffer gas, a less hostile case in which absolute OH density measurement
is possible, to an NRP in CO2+H2O, where the full set of drawbacks is at
work. In the last case, the OH density measurement is not possible with
laser pulses and detector time resolution in the ns time scale.
Nevertheless, it is shown that with a proper knowledge of the
collisional rate constants involved in the LIF process, a collisional
energy transfer-LIF methodology is still applicable to deduce the gas
composition from the analysis of LIF spectra
Turbulent transport stabilization by ICRH minority fast ions in low rotating JET ILW L-mode plasmas
Get PDFThe first experimental demonstration that fast ion induced stabilization of thermal turbulent transport takes place also at low values of plasma toroidal rotation has been obtained in JET ILW (ITER-like wall) L-mode plasmas with high ( 3 He)-D ICRH (ion cyclotron resonance heating) power. A reduction of the gyro-Bohm normalized ion heat flux and higher values of the normalized ion temperature gradient have been observed at high ICRH power and low NBI (neutral beam injection) power and plasma rotation. Gyrokinetic simulations indicate that ITG (ion temperature gradient) turbulence stabilization induced by the presence of high-energetic 3 He ions is the key mechanism in order to explain the experimental observations. Two main mechanisms have been identified to be responsible for the turbulence stabilization: a linear electrostatic wave-fast particle resonance mechanism and a nonlinear electromagnetic mechanism. The dependence of the stabilization on the 3 He distribution function has also been studied.</p
Non-Maxwellian fast particle effects in gyrokinetic GENE simulations
No full textFast ions have recently been found to significantly impact and partially suppress plasma turbulence both in experimental and numerical studies in a number of scenarios. Understanding the underlying physics and identifying the range of their beneficial effect is an essential task for future fusion reactors, where highly energetic ions are generated through fusion reactions and external heating schemes. However, in many of the gyrokinetic codes fast ions are, for simplicity, treated as equivalent-Maxwellian-distributed particle species, although it is well known that to rigorously model highly non-thermalised particles, a non-Maxwellian background distribution function is needed. To study the impact of this assumption, the gyrokinetic code GENE has recently been extended to support arbitrary background distribution functions which might be either analytical, e.g., slowing down and bi-Maxwellian, or obtained from numerical fast ion models. A particular JET plasma with strong fast-ion related turbulence suppression is revised with these new code capabilities both with linear and nonlinear gyrokinetic simulations. It appears that the fast ion stabilization tends to be less strong but still substantial with more realistic distributions, and this improves the quantitative power balance agreement with experiments.</p
Confining light to the atomic scale
No full textA graphene sheet near a metal nanoantenna squeezes infrared photons into a subnanometric gap, pushing the limits of nanophotonics. © 2018 The Author(s)
First principle integrated modeling of multi-channel transport including Tungsten in JET
Get PDFFor the first time, over five confinement times, the self-consistent flux driven time evolution of heat, momentum transport and particle fluxes of electrons and multiple ions including Tungsten (W) is modeled within the integrated modeling platform JETTO [Romanelli M et al PFR 2014], using first principle-based codes: namely, QuaLiKiz [Bourdelle C. et al. PPCF 2016] for turbulent transport and NEO [Belli E A and Candy J PPCF 2008] for neoclassical transport. For a JET-ILW pulse, the evolution of measured temperatures, rotation and density profiles are successfully predicted and the observed W central core accumulation is obtained. The poloidal asymmetries of the W density modfying its neoclassical and turbulent transport are accounted for. Actuators of the W core accumulation are studied: removing the central particle source annihilates the central W accumulation whereas the suppression of the torque reduces significantly the W central accumulation. Finally, the presence of W slightly reduces main ion heat turbulent transport through complex nonlinear interplays involving radiation, effective charge impact on ITG and collisionality.</p
Power handling limit of liquid lithium divertor targets
Get PDFA model is formulated to make a first estimate of the maximum tolerable power of liquid lithium divertor targets, and to gain insight into their behavior in terms of lithium loss rate and surface temperature. The model,formulated as a simple analytical expression, states that the incoming power is balanced by heat conduction through the target and by the lithium which is dissipating energy via evaporation, radiation and ion-neutral friction. A target is considered to fail when the net lithium loss flux from the surface exceeds the available supply. The model is evaluated over a range of input parameters: lithium supply rate, surface layer thickness, redeposition coefficient, and dissipated energy per Li particle lost to the plasma. Based on the results, first, surface temperature locking is expected above a deposited power of 10 MW/m2. Second, lithium targets are expected to be extremely robust against power deposited during short transient events. A surface layer thickness of 50 micron is sufficient to withstand 60 MJ/m2 vertical displacement events or 20 MJ/m2 disruptions.</p
Broadband optical response of graphene measured by terahertz time-domain spectroscopy and FTIR spectroscopy
No full textThis work describes the broadband optical response of graphene grown by chemical vapor deposition by combining terahertz time-domain spectroscopy (THz-TDS) at 0.25–1.7 THz (8–60 cm−1) with Fourier Transform Infrared Spectroscopy (FTIR) at 11–210 THz (370–7000 cm−1). It is shown that the measured response is well described by a combination of the Drude model, expressing free-carrier absorption/reflection, and an expression for interband absorption. The THz-TDS and FTIR transmittance curves are fitted consistently using this model. This indicates that these techniques are complementary to each other. Both can thus be employed together or individually to characterize the electronic properties of graphene (e.g., carrier density and mobility). To exemplify this, we demonstrate that this equivalency can be used to study the effect of substrate-dependent doping on the optical response of graphene. Furthermore, the amount of reflection and absorption corresponding to the measured transmittance curve is calculated. The toolbox for the non-invasive broadband characterization of graphene is thus extended by this work
Interaction and coherence of a plasmon-exciton polariton condensate
No full textPolaritons are quasiparticles arising from the strong coupling of electromagnetic waves in cavities and dipolar oscillations in a material medium. In this framework, localized surface plasmon in metallic nanoparticles defining optical nanocavities have attracted increasing interests in the past decade. This interest results from their sub-diffraction mode volume, which offers access to extremely high photonic densities by exploiting strong scattering cross sections. However, high absorption losses in metals have hindered the observation of collective coherent phenomena, such as condensation. In this work, we demonstrate the formation of a nonequilibrium room temperature plasmon–exciton–polariton condensate with a long-range spatial coherence, extending a hundred of microns, well over the excitation area, by coupling Frenkel excitons in organic molecules to a multipolar mode in a lattice of plasmonic nanoparticles. Time-resolved experiments evidence the picosecond dynamics of the condensate and a sizable blueshift, thus measuring for the first time the effect of polariton interactions in plasmonic cavities. Our results pave the way to the observation of room temperature superfluidity and novel nonlinear phenomena in plasmonic systems, challenging the common belief that absorption losses in metals prevent the realization of macroscopic quantum states
