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Deuterium blistering in tungsten and tungsten vanadium alloys
No full textIn order to evaluate D blistering behavior in W based plasma facing materials, rolled W and different grades of W-V targets were exposed to high flux of 1.2 × 1024 m−2 s−1, low energy (38 eV) D plasma at two different surface temperatures. The blistering behavior was investigated by means of scanning electron microscopy, accompanied by electron back-scattering diffraction. Highest numbers of blisters were observed on the surface of rolled tungsten. The addition of V precursor to W suppressed D blister formation. In the case of W-V alloys, comparatively submicron V-containing materials have shown high tendency but small size blisters formation than micron V-containing samples. A high density of blisters was observed near the (111) plane on the surface of both V-containing alloys. Nano-sized blisters were also observed on V enriched surface.</p
Exploring the electron density in plasma induced by EUV radiation: I. Experimental study in hydrogen
No full textPlasmas induced by EUV radiation are unique since they are created without the need of any discharge. Moreover, it is essential to characterize these plasmas to understand and predict their long term impact on highly delicate optics in EUV lithography tools. In this paper we study plasmas induced by 13.5 nm EUV radiation in hydrogen gas. The electron density is measured temporally resolved using a non-invasive technique known as microwave cavity resonance spectroscopy. The influence of the EUV pulse energy and gas pressure on the temporal evolution of the electron density has been explored over a parameter range relevant for industry. Our experimental results show that the maximum electron density is in the order of 10 14 m −3 and depends linearly on the EUV pulse energy. Furthermore, the maximum electron density depends quadratically on the pressure; the linear term is caused by photoionization and the quadratic term by subsequent electron impact ionization. The decay of the plasma is governed by ambipolar diffusion and, hence, becomes slower at elevated pressures. Similarities and differences of the same processes in argon are highlighted in this paper.</p
Effect of gas properties on the dynamics of the electrical slope asymmetry effect in capacitive plasmas: comparison of Ar, H2 and CF4
No full textTailored voltage excitation waveforms provide an efficient control of the ion energy (through the electrical asymmetry effect) in capacitive plasmas by varying the ‘amplitude’ asymmetry of the waveform. In this work, the effect of a ‘slope’ asymmetry of the waveform is investigated by using sawtooth-like waveforms, through which the sheath dynamic can be manipulated. A remarkably different discharge dynamic is found for Ar, H 2 , and CF 4 gases, which is explained by the different dominant electron heating mechanisms and plasma chemistries. In comparison to Argon we find that the electrical asymmetry can even be reversed by using an electronegative gas such as CF 4 . Phase resolved optical emission spectroscopy measurements, probing the spatiotemporal distribution of the excitation rate show excellent agreement with the results of particle-in-cell simulations, confirming the high degree of correlation between the excitation rates with the dominant heating mechanisms in the various gases. It is shown that, depending on the gas used, sawtooth-like voltage waveforms may cause a strong asymmetry.</p
Real-time control of ELM and sawtooth frequencies: similarities and differences
No full textELMs and Sawteeth, located in different parts of the plasma, are similar from a control engineering point of view. Both manifest themselves through quiescent periods interrupted by periodic collapses. For both, large collapses, following long quiescent periods, have detrimental effects while short periods are associated with decreased confinement. Following the installation of the all metal ‘ITER like wall’ on JET, sawteeth and ELMs also play an important role by expelling tungsten from the core and edge of the plasma respectively. Control of tungsten has therefore been added to divertor heat load reduction, NTM avoidance and helium ash removal as reasons for requiring ELM and sawtooth control. It is therefore of interest to implement control systems to maintain the sawtooth and ELM frequencies in the desired ranges. On JET, ELM frequency control uses radial field ‘kicks’ and pellet and gas injection as actuators, while sawtooth control uses ion cyclotron resonance heating (ICRH). JET experiments have, for the first time, established feedback control of the ELM frequency, via real time variation of the injected gas flow [1]. Using this controller in conjunction with pellet injection allows the ELM frequency to be kept as required despite variations in pellet ELM triggering efficiency. JET Sawtooth control experiments have, for the first time, demonstrated that low field side ICRH, as foreseen for ITER, can shorten sawteeth lengthened by central fast ions [2]. The development of ELM and sawtooth control could be key to achieve stable high performance JET discharges with minimal tungsten content. Integrating such schemes into an overall control strategy will be required in future tokamaks and gaining experience on current tokamaks is essential.</p
Study of temporal pulse shape effects on W using simulations and laser heating
Get PDFTransient heat pulses with triangular, square, and ELM-like temporal shapes are investigated in order to further understand how transient plasma instabilities will affect plasma facing components in tokamaks. A solution to the 1D heat equation for triangular pulses allows the peak surface temperature to be written analytically for arbitrary rise times. The solution as well as ANSYS simulations reveal that a positive ramp (maximum rise time) triangular pulse has a higher peak surface temperature by a factor of sqrt{2} compared to that from a negative ramp (rise time = 0) pulse shape with equal energy density, peak power, and pulse width. Translating the results to ITER, an ELM or disruption pulse with the shortest rise time is the most benign compared to other pulse shapes with the same peak heat flux and same energy density.</p
JET experiments with tritium and deuterium–tritium mixtures
No full textExtensive preparations are now underway for an experiment in the Joint European Torus (JET) using tritium and deuterium–tritium mixtures. The goals of this experiment are described as well as the progress that has been made in developing plasma operational scenarios and physics reference pulses for use in deuterium–tritium and full tritium plasmas. At present, the high performance plasmas to be tested with tritium are based on either a conventional ELMy H-mode at high plasma current and magnetic field (operation at up to 4 MA and 4 T is being prepared) or the so-called improved H-mode or hybrid regime of operation in which high normalised plasma pressure at somewhat reduced plasma current results in enhanced energy confinement. Both of these regimes are being re-developed in conjunction with JET\u27s ITER-like Wall (ILW) of beryllium and tungsten. The influence of the ILW on plasma operation and performance has been substantial. Considerable progress has been made on optimising performance with the all-metal wall. Indeed, operation at the (normalised) ITER reference confinement and pressure has been re-established in JET albeit not yet at high current. In parallel with the physics development, extensive technical preparations are being made to operate JET with tritium. The state and scope of these preparations is reviewed, including the work being done on the safety case for DT operation and on upgrading machine infrastructure and diagnostics. A specific example of the latter is the planned calibration at 14 MeV of JET neutron diagnostics
Molecular Dynamics Study of Superelastic Performance of a β-Si3N4 Nanohelical Coil
No full textSuperealsticity behavior has been discovered for the β-Si3N4 nanohelical coil. By a series of cyclic loading and unloading simulation tests, the nanohelical coil could elastically recover its original shape after a relatively long axial tension far beyond the elastic limit, accompanying little residual plastic deformation. The spring constant is first a steady-state value of 0.72 N m–1 in the linear stage and then increases dramatically for up to 1.5 N m–1 in the nonlinear region. The fascinating performance of the nanocoil makes it stand out from its bulk counterparts in the development of future electronic devices such as nanoscale elastic energy storage
Orientation Sensitivity of Oxygen Evolution Reaction on Hematite
Get PDFThe sensitivity of the surface orientation on photoelectrochemical water oxidation has recently been reported by experimental studies. However, a detailed theoretical understanding is still missing. Density functional theory + Hubbard U (DFT + U) calculations are therefore carried out in order to investigate the oxygen evolution reaction (OER) on hematite (Fe2O3) surfaces for five surface orientations, namely (100), (210), (101), (021), and (211). The free energies of four proton-coupled electron transfer steps and the OER overpotential were calculated, and the trend in activity was analyzed. For the (100) orientation, two adsorbate–adsorbate distances were studied. Interestingly, a very low overpotential of 0.52 V was found for the (100) surface with a bridge site (adsorbate on a bridge of two Fe atoms) configuration that benefited from adsorbate–adsorbate interactions.</p
Directional sideward emission from luminescent plasmonic nanostructures
No full textPeriodic arrays of metallic nanoparticles can be used to enhance the emission of light in certain directions. We fabricated hexagonal arrays of aluminium nanoparticles combined with thin layers of luminescent material and optimized period (275 nm) and thickness (1500 nm) to obtain sideward directional emission into glass for a wavelength band around 620 nm. The key physics is that the luminescent layer acts as a waveguide, from which light is emitted at preferential angles using diffractive effects. This phenomenon has applications in the field of solid-state lighting, where there is a desire for small, bright and directional sources
