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Collective Thomson scattering measurements of fast-ion transport due to sawtooth crashes in ASDEX Upgrade
No full textSawtooth instabilities can modify heating and current-drive profiles and potentially increase fast-ion losses. Understanding how sawteeth redistribute fast ions as a function of sawtooth parameters and of fast-ion energy and pitch is hence a subject of particular interest for future fusion devices. Here we present the first collective Thomson scattering (CTS) measurements of sawtooth-induced redistribution of fast ions at ASDEX Upgrade. These also represent the first localized fast-ion measurements on the high-field side of this device. The results indicate fast-ion losses in the phase-space measurement volume of about 50% across sawtooth crashes, in good agreement with values predicted with the Kadomtsev sawtooth model implemented in TRANSP and with the sawtooth model in the EBdyna_go code. In contrast to the case of sawteeth, we observe no fast-ion redistribution in the presence of fishbone modes. We highlight how CTS measurements can discriminate between different sawtooth models, in particular when aided by multi-diagnostic velocity-space tomography, and briefly discuss our results in light of existing measurements from other fast-ion diagnostics.</p
Chemical sputtering by H2+ and H3+ ions during silicon deposition
No full textWe investigated chemical sputtering of silicon films by Hy + ions (with y being 2 and 3) in an asymmetric VHF Plasma Enhanced Chemical Vapor Deposition (PECVD) discharge in detail. In experiments with discharges created with pure H2 inlet flows, we observed that more Si was etched from the powered than from the grounded electrode, and this resulted in a net deposition on the grounded electrode. With experimental input data from a power density series of discharges with pure H2 inlet flows, we were able to model this process with a chemical sputtering mechanism. The obtained chemical sputtering yields were (0.3–0.4) ± 0.1 Si atom per bombarding Hy + ion at the grounded electrode and at the powered electrode the yield ranged from (0.4 to 0.65) ± 0.1. Subsequently, we investigated the role of chemical sputtering during PECVD deposition with a series of silane fractions SF (SF(%) = [SiH4]/[H2]*100) ranging from SF = 0% to 20%. We experimentally observed that the SiHy + flux is not proportional to SF but decreasing from SF = 3.4% to 20%. This counterintuitive SiHy + flux trend was partly explained by an increasing chemical sputtering rate with decreasing SF and partly by the reaction between H3 + and SiH4 that forms SiH3 +
Experimental Simulation of the Behaviour of Diagnostic First Mirrors Fabricated of Different Metals for ITER Conditions
No full textIn the experimental fusion reactor ITER, the plasma-facing component of each optical and/or laser diagnostic needs to be based on reflective optics with at least one mirror (first mirror) facing the thermonuclear plasma. The different kinds of radiation emanating from the burning plasma (neutrons, neutral atoms, electromagnetic radiation) create hostile operating conditions for the first mirrors. Therefore, a special program has been set up under the ITER framework aimed at solving the first mirror problem. This paper will review the main results in this field that have been obtained in the Institute of Plasma Physics, National Science Center “Kharkov Institute of Physics and Technology” (in many cases in cooperation with groups of other countries, as indicated in corresponding parts of the manuscript) during long-term investigations directed to find a solution of this problem, i.e., to find a
material and accompanying precautions in order to satisfy the requirements for first mirrors. The main efforts were devoted to finding solutions to overcome the impact of the most severe deteriorating factors resulting in degradation of the optical properties of mirrors: sputtering by charge exchange atoms and deposition of contaminants. The obtained results are focused on: the effects of long term sputtering on mirror specimens fabricated from different metals with different structures (polycrystals, single crystals, metal film on metal substrates, amorphous), the effects of contaminating film and the possible protection to avoid of its appearance, the role of chemical processes for some metal mirrors, and the choice of material of laser mirrors
Rotation profile flattening and toroidal flow shear reversal due to the coupling of magnetic islands in tokamaks
No full textNote: Paper PI3 1, Bull. Am. Phys. Soc. 60, 261 (2015).</p
Experimental investigation of geodesic acoustic modes on JET using Doppler backscattering
No full textGeodesic acoustic modes (GAMs) have been investigated in JET ohmic discharges using mainly Doppler backscattering. Characteristics and scaling properties of the GAM are studied. Time and spatial resolved measurements of the perpendicular velocity indicate that GAMs are located in a narrow layer at the edge density gradient region with amplitude corresponding to about 50% of the mean local perpendicular velocity. GAMs on JET appear to be regulated by the turbulence drive rather than by their damping rate. It is also shown that the GAM amplitude is 20% larger in deuterium than in hydrogen plasmas
Dislocation-mediated trapping of deuterium in tungsten under high-flux high-temperature exposures
No full textThe effect of severe plastic deformation on the deuterium retention in tungsten exposed to high-flux low-energy plasma (flux ∼1024 m−2 s−1, energy ∼50 eV and fluence up to 5 × 1025 D/m2) was studied experimentally in a wide temperature range (460–1000 K) relevant for application in ITER. The desorption spectra in both reference and plastically-deformed samples were deconvoluted into three contributions associated with the detrapping from dislocations, deuterium-vacancy clusters and pores. As the exposure temperature increases, the positions of the release peaks in the plastically-deformed material remain in the same temperature range but the peak amplitudes are altered as compared to the reference material. The desorption peak attributed to the release from pores (i.e. cavities and bubbles) was suppressed in the plastically deformed samples for the low-temperature exposures, but became dominant for exposures above 700 K. The observed strong modulation of the deuterium storage in “shallow” and “deep” traps, as well as the reduction of the integral retention above 700 K, suggest that the dislocation network changes its role from “trapping sites” to “diffusion channels” above a certain temperature. The major experimental observations of the present work are in line with recent computational assessment based on atomistic and mean field theory calculations available in literature
Full vectorial mapping of the complex electric near-fields of THz resonators
No full textUsing micro-structured photo-conducting probes, we demonstrate full vectorial mapping of the complex electric fields in the near-field region of a resonant structure at THz frequencies. The investigated structure represents the simplest case of a resonator: a metallic rod. We show field amplitude as well as phase maps for the three field components at the half wavelength (λ/2) resonance of the rod. The field as well as the phase distributions are in excellent agreement with our physical understanding of local electric-field distributions in the vicinity of λ/2 resonant structures and are validated by numerical simulations. These measurements can be a platform for performance optimization of the emerging field of THz photonic and plasmonic devices with complex sub-wavelength structures
