DIFFER: Publications
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Parameter dependence of ELM loss reduction by magnetic perturbations at low pedestal density and collisionality in ASDEX upgrade
No full textELM mitigation by magnetic perturbations is studied at low pedestal collisionalities down to ITER-like values (V∗ e PED = 0.1) in ASDEX Upgrade. A comprehensive database of ELM energy losses for varying plasma density, heating power, edge safety factor and magnetic perturbation structure has been assembled to investigate parameter dependencies of ELM mitigation. It is found that magnetic perturbations with a toroidal mode number n = 2 can reduce the ELM energy loss normalized to the energy stored in the plasma pedestal from about 30% to less than 5%, i.e. by a factor of six, below an electron pedestal collisionality of V∗ e PED = 0.4. At this level of ELM mitigation a significant reduction of the pedestal pressure and, therefore, global plasma confinement occurs. This pedestal pressure reduction is mostly due to a reduction of plasma density, the so-called pump-out effect. Refueling by neutral beams and in particular by pellet injection is possible and can re-establish confinement, however, the ELM energy loss increases as well with increasing density. © 2017 Published by Elsevier B.V.</p
Atmospheric pressure roll-to-roll plasma enhanced CVD of high quality silica-like bilayer encapsulation films
Get PDFA glow like atmospheric pressure dielectric barrier discharge in a roll-to-roll setup was used to synthesize 90 nm silica-like bilayer encapsulation films composed of a 30 nm dense “barrier layer” and a comparatively less dense 60 nm “buffer layer” onto a polyethylene 2,6 naphthalate substrate by means of plasma enhanced chemical vapor deposition. Tetraethyl orthosilicate was used as the precursor gas, together with a mixture of nitrogen, oxygen, and argon. The microstructure, chemical composition, morphology, and permeation properties of the films were studied as a function of the specific energy delivered per precursor molecule, and oxygen concentration in the gas mixture, during the deposition of the barrier layer. The presence of the buffer layer within the bilayer architecture critically enhanced the encapsulation performance of the bilayer films, and this in conjunction with increasing the specific energy delivered per precursor molecule during the barrier layer deposition to a value of 20 keV, enabled an effective water vapor transmission rate as low as 6.9 × 10−4 g m−2 d−1 (at 40 °C, 90% relative humidity (RH)) to be achieved. Furthermore, the bilayer film structure has given rise to a remarkable 50% reduction in deposition energy consumption per barrier area with respect to single layer silica-like films of equivalent encapsulation performance and thickness.</p
Plasma pressure and particle loss studies in the Pilot-PSI high flux linear plasma generator
No full textPlasma detachment in tokamak divertors reduces the particle and power fluxes to the plasma facing components and is essential for successful operation of ITER. The linear plasma generator Pilot-PSI can produce a high density (∼1021 m−3), low temperature (∼1 eV) plasma which is similar to that expected at the ITER divertor strike-points during the partially detached regime [1]. Given the simple geometry of the device, Pilot-PSI allows to diagnose the plasma beam at multiple axial positions. In this study, the incoherent Thomson scattering (TS) diagnostic [2] is exploited to measure the radial plasma Te and ne profiles at two locations of the hydrogen plasma beam: near the plasma source (upstream) and 2 cm from the target plate. At the target, the TS measurements are supported by an embedded Langmuir probe. These measurements prove the existence of parallel plasma pressure loss as well as particle loss, confirming that physical processes believed to cause detachment in tokamak divertors also hold in Pilot-PSI (ion-neutral friction, volume recombination). It is found that the fractional reduction of the plasma pressure varies between ∼4 and ∼5000, depending strongly on the pressure of the background neutrals. The importance of individual loss channels is discussed.</p
Time evolution of vibrational temperatures in a CO2 glow discharge measured with infrared absorption spectroscopy
No full textVibrational temperatures of CO2 are studied in a pulsed glow discharge by means of time-resolved in situ Fourier transform infrared spectroscopy, with a 10 μs temporal resolution. A method to analyze the infrared transmittance through vibrationally excited CO2 is presented and validated on a previously published CO2 spectrum, showing good agreement between fit and data. The discharge under study is pulsed with a typical duty cycle of 5–10 ms on–off, at 50 mA and 6.7 mbar. A rapid increase of the temperature of the asymmetric stretch vibration (T 3) is observed at the start of the pulse, reaching 1050 K, which is an elevation of 550 K above the rotational temperature (T rot) of 500 K. After the plasma pulse, the characteristic relaxation time of T 3 to T rot strongly depends on the rotational temperature. By adjusting the duty cycle, the rotational temperature directly after the discharge is varied from 530 to 860 K, resulting in relaxation times between 0.4 and 0.1 ms. Equivalently, as the gas heats up during the plasma pulse, the elevation of T 3 above T rot decreases strongly
Experimental studies of the snowflake divertor in TCV
No full textTo address the risk that, in a fusion reactor, the conventional single-null divertor (SND) configuration may not be able to handle the power exhaust, alternative divertor configurations, such as the Snowflake divertor (SFD), are investigated in TCV. The expected benefits of the SFD-minus in terms of power load and peak heat flux are discussed and compared to experimental measurements. In addition, key results obtained during the last years are summarized
Control of the intrinsic microstructure in AP-PECVD synthesised amorphous silica thin films
No full textAmorphous single layered silica films deposited using industrially scalable roll-to-roll atmospheric pressure-plasma enhanced chemical vapor deposition were evaluated in terms of structure-performance relationships. Polarised attenuated total reflectance-Fourier transform infrared absorption spectroscopy and heavy water exposure to induce hydrogen-deuterium exchange revealed it was possible to control the film porosity simply by varying the precursor flux and plasma residence times. Denser silica network structures with fewer hydroxyl impurities, shorter Si-O bonds, decreased Si-O-Si bond angles and a greater magnitude of isolated pores were found in films deposited with decreased precursor flux and increased plasma residence times, and consequently exhibited significantly improved encapsulation performance
Direct visualization of hydrogen absorption dynamics in individual palladium nanoparticles
No full textDeuterium trapping and surface modification of polycrystalline tungsten exposed to a high-flux plasma at high fluences
No full textDeuterium (D) retention and surface modifications of hot-rolled polycrystalline tungsten (W) exposed to a low-energy ( 40 eV D −1 ), high-flux (2–5 × 10 23 D m −2 s −1 ) D plasma at temperatures of 380 K and 1140 K to fluences up to 1.2 × 10 28 D m −2 have been examined by using nuclear reaction analysis, thermal desorption spectroscopy, and scanning electron microscopy. The samples exposed at 380 K exhibited various types of surface modifications: dome-shaped blister-like structures, stepped flat-topped protrusions, and various types of nanostructures. It was observed that a large fraction of the surface was covered with blisters and protrusions, but their average size and the number density showed almost no fluence dependence. The D depth distributions and total D inventories also barely changed with increasing fluence at 380 K. A substantial amount of D was retained in the subsurface region, and thickness correlated with the depth where the cavities of blisters and protrusions were located. It is therefore suggested that defects appearing during creation of blisters and protrusions govern the D trapping in the investigated fluence range. In addition, a large number of small cracks was observed on the exposed surfaces, which can serve as fast D release channels towards the surface, resulting in a reduction of the effective D influx into the W bulk. On the samples exposed at 1140 K no blisters and protrusions were found. However, wave-like and faceted terrace-like structures were formed instead. The concentrations of trapped D were very low (<10 −5 at. fr.) after the exposure at 1140 K.</p
Nanostructuring of iron thin films by high flux low energy helium plasma
Get PDFHigh flux, low energy He plasma exposure is proven to nanostructure iron thin films over their entire thickness to a highly open structure with large surface area. From a large set of plasma exposure parameters, the ion flux, the surface temperature, and the plasma exposure time are found to be the most relevant parameters to process mechanically stable, nanostructured Fe thin films on brittle glass substrates. The nanostructure stays stable during oxidation. Different surface morphologies are found, depending on the location where the plasma plume interacts with the thin film. This method paves the way to a new direction in top down nanostructuring of thin films, which can be adopted for many functional materials in diverse applications that require a high ratio of active to projected surface area.</p
Infrared gas phase study on plasma-polymer interactions in high-current diffuse dielectric barrier discharge
Get PDFA roll-to-roll high-current diffuse dielectric barrier discharge at atmospheric pressure was operated in air and Ar/N2/O2 gas mixtures. The exhaust gas from the discharge was studied using a high-resolution Fourier-transform infrared spectrometer in the range from 3000 to 750?cm-1 to unravel the plasma-polymer interactions. The absorption features of HxNyOz, COx, and HCOOH (formic acid) were identified, and the relative densities were deduced by fitting the absorption bands of the detected molecules. Strong interactions between plasma and polymer (Polyethylene-2,6-naphthalate, or PEN) in precursor-free oxygen-containing gas mixtures were observed as evidenced by a high COx production. The presence of HCOOH in the gas effluent, formed through plasma-chemical synthesis of COx, turns out to be a sensitive indicator for etching. By adding tetraethylorthosilicate precursor in the plasma, dramatic changes in the COx production were measured, and two distinct deposition regimes were identified. At high precursor flows, a good agreement with the precursor combustion and the COx production was observed, whereas at low precursor flows an etching-deposition regime transpires, and the COx production is dominated by polymer etching.</p