Plasma cleaning of ITER first mirrors in magnetic field

To avoid reflectivity losses in ITER optical diagnostic systems, plasma sputtering of metallic First Mirrors is foreseen in order to remove deposits coming from the main wall (mainly beryllium and tungsten). Therefore plasma cleaning has to work on large mirrors (up to a size of 200*300 mm) and under the influence of strong magnetic fields (several Tesla). This work presents the results of plasma cleaning of aluminium and aluminium oxide (used as beryllium proxy) deposited on molybdenum mirrors. Using radio frequency (13.56 MHz) argon plasma, the removal of a 260 nm mixed aluminium/aluminium oxide film deposited by magnetron sputtering on a mirror (98 mm diameter) was demonstrated. 50 nm of pure aluminium oxide were removed from test mirrors (25 mm diameter) in a magnetic field of 0.35 T for various angles between the field lines and the mirrors surfaces. The cleaning efficiency was evaluated by performing reflectivity measurements, Scanning Electron Microscopy and X-ray Photoelectron Spectroscopy.Comment: 5 pages, 6 figures and 1 table. Results presented on the 21st Plasma Surface Interaction conference held in Kanazawa Japan, May 201

Decomposition studies of NH₃ and ND₃ in presence of H₂ and D₂ with Pt/Al₂O₃ and Ru/Al₂O₃ catalysts

In the fusion reactor ITER, ammonia will be produced as a result of the interaction between the hydrogen isotopes used as fuel and nitrogen used to spread the power loads of a larger area. As part of the fuel management in ITER, NQ3 (NQ3, Q = H, D, T) will have to be decomposed using a palladium membrane reactor. The decomposition of pure NH3 and ND3 was studied in this work using commercial platinum (Pt) and ruthenium (Ru) catalysts on alumina (0.5 wt% loading), in a conventional reactor configuration (i.e., without a palladium membrane). With Pt/Al2O3, decomposition fractions larger than 90% were achieved with NH3 above 800 K using the lowest flow-to-mass ratio (/g-cat) of 0.015 sccm g−1. However, with the increase of /g-cat to 0.220 sccm g−1, similar decompositions were achieved only at  K. In contrast, with Ru/Al2O3 decomposition fractions above 90% were attained already below 700 K, regardless of /g-cat. With both catalysts the decomposition of NH3 was found to be more efficient than that of ND3 at a wide range of temperatures, thus evidencing the existence of isotopic effect. A strong inhibition of both NH3 and ND3 in presence of, respectively, H2 and D2 with Pt/Al2O3 was observed. This effect was stronger at lower temperatures and larger hydrogen partial pressures. The inhibition effect with Ru/Al2O3 was less pronounced and it was suppressed at 629 K. Isotopic exchange reactions with equimolar mixtures of NH3-D2 and ND3-H2 revealed that the most and least abundant isotopologue are, respectively, NH2D and ND3. At the relevant temperature window in which the PMR will be operated (673–823 K), the Ru-based catalyst exhibits superior performances in terms of decomposition rates, negligible isotopic and inhibition effects. A slight reduction of the performances with this catalyst was observed with 0.200 sccm g−1. This work suggests that 0.5 wt% Ru/Al2O3 is the most suitable catalyst to be used during ITER operation

Reducing the hydrogen content in liquid helium

Helium has the lowest boiling point of any element in nature at normal atmospheric pressure. Therefore, any unwanted substance like impurities present in liquid helium will be frozen and will be in solid form. Even if these solid impurities can be easily eliminated by filtering, liquid helium may contain a non negligible quantity of molecular hydrogen. These traces of molecular hydrogen are the causes of a known problem worldwide: the blocking of fine capillary tubes used as flow resistors in helium evaporation cryostats to achieve temperatures below 4.2 K. This problem seriously affects a wide range of cryogenic equipment used in low temperature physics research and leads to a dramatic loss of time and costs due to the high price of helium. Here, we present first the measurement of molecular hydrogen content in helium gas. Three measures to decrease this molecular hydrogen are afterward proposed; (i)improving the helium quality, (ii) release of helium gas in the atmosphere during purge time for the regeneration cycle of the helium liquefierâEurotms internal purifier, and (iii) installation of two catalytic converters in a closed helium circuit. These actions have eliminated all blockages of capillaries at low temperatures now for more than two years

Ion flux-energy distributions across grounded grids in an RF plasma source with DC-grounded electrodes

We present an experimental investigation of the ion flux–energy distribution functions (IFEDFs) obtained across grounded grids in an asymmetric capacitively coupled RF source using a helium discharge. The powered electrode in the RF source is DC-grounded via a λ/4 filter, which lifts its DC potential to zero. Grids of different dimensions (hole width, thickness, and geometric transparency) were used to confine the plasma, while the IFEDF of the ion beam departing the grid and reaching the reactor walls was studied using a retarding field energy analyser. The IFEDF obtained was double-peaked, indicating the presence of fast ions arriving from the plasma source, and cold ions generated upon charge exchange collisions between the fast ions and neutrals. The flux, as well as the peak energies of the two ion groups, depended significantly on the process parameters: RF power, He pressure, the distance between grids and walls, and the dimensions of the grids. The results indicate that confining plasma with grids can reduce the ion flux at the walls by over 60%, significantly lowering the wall sputtering rate. This was confirmed with a dedicated long-exposure plasma discharge with a gridded plasma reactor, wherein less than 1 nm of Cu deposition was found on the DC-grounded powered electrode, and the surface reflectivity was preserved to pristine values. In contrast, a similar experiment in a gridless reactor resulted in Cu deposition of 35 nm with a drastic drop in surface reflectivity. These studies are of great importance for the application of similar RF plasma sources with in-situ cleaning of diagnostic mirrors in fusion devices, as well as in a variety of plasma processing applications

Anisotropic Etching of Graphite and Graphene in a Remote Hydrogen Plasma

We investigate the etching of a pure hydrogen plasma on graphite samples and graphene flakes on SiO$_2$ and hexagonal Boron-Nitride (hBN) substrates. The pressure and distance dependence of the graphite exposure experiments reveals the existence of two distinct plasma regimes: the direct and the remote plasma regime. Graphite surfaces exposed directly to the hydrogen plasma exhibit numerous etch pits of various size and depth, indicating continuous defect creation throughout the etching process. In contrast, anisotropic etching forming regular and symmetric hexagons starting only from preexisting defects and edges is seen in the remote plasma regime, where the sample is located downstream, outside of the glowing plasma. This regime is possible in a narrow window of parameters where essentially all ions have already recombined, yet a flux of H-radicals performing anisotropic etching is still present. At the required process pressures, the radicals can recombine only on surfaces, not in the gas itself. Thus, the tube material needs to exhibit a sufficiently low H radical recombination coefficient, such a found for quartz or pyrex. In the remote regime, we investigate the etching of single layer and bilayer graphene on SiO$_2$ and hBN substrates. We find isotropic etching for single layer graphene on SiO$_2$, whereas we observe highly anisotropic etching for graphene on a hBN substrate. For bilayer graphene, anisotropic etching is observed on both substrates. Finally, we demonstrate the use of artificial defects to create well defined graphene nanostructures with clean crystallographic edges.Comment: 7 pages, 4 color figure

Decorating Nanostructured Surfaces with Antimicrobial Peptides to Efficiently Fight Bacteria

With conventional antibiotic therapies being increasingly ineffective, bacterial infections with subsequent biofilm formation represent a global threat to human health. Here, an active and a passive strategy based on polymeric micelles were combined to fight bacterial growth. The passive strategy involved covalent immobilization of polymeric micelles through Michael addition between exposed maleimide and thiol functionalized surfaces. Compared to the bare surface, micelle-decorated surfaces showed reduced adherence and survival of bacteria. To extend this passive defense against bacteria with an active strategy, the immobilized micelles were equipped with the antimicrobial peptide KYE28 (KYEITTIHNLFRKLTHRLFRRNFGYTLR). The peptide interacted nonspecifically with the immobilized micelles where it retained its antimicrobial property. The successful surface decoration with KYE28 was demonstrated by a combination of X-ray photoelectron spectroscopy and quartz crystal microbalance with dissipation monitoring. The initial antimicrobial activity of the nanostructured surfaces against Escherichia coli was found to be increased by the presence of KYE28. The combination of the active and passive strategy represents a straightforward modular approach that can easily be adapted, for example, by exchanging the antimicrobial peptide to optimize potency against challenging bacterial strains, and/or to simultaneously achieve antimicrobial and anti-infection properties

Reimann Brake Ramp for planar flow casting processes and analysis of ribbon gluing

Planar flow casting is a rapid solidification process used to manufacture thin, metallic ribbons, and foil. Liquid metal is forced through a nozzle against a heat-sink wheel, and it rapidly solidifies into thin ribbons. A puddle of molten metal, held by surface tension, forms between the nozzle and wheel. This study examines a well-defined periodic surface defect called herringbone (HB), which is commonly produced when casting zirconium based alloys. The presence of this defect is related to processing conditions and puddle dynamics. Its formation has been correlated with the pinning of the liquid puddle at the nozzle edge. Here, the uniformity of thickness along a ribbon was successfully controlled (over a length of 50 m) using the Reimann Brake Ramp, which reduces the wheel speed at the start of the cast. For the alloy used in this study, the variation in the dimensionless thickness parameter, , with the Euler number () at assigned values of followed an allometric scaling, with an exponent value close to the theoretical value of 1/3. Furthermore, the nozzle inclination was related to the ribbon thickness, , and the ribbon quality. Moreover, a newly developed automatized melt spinner permitted monitoring and controlling of the process parameters, elucidating the gluing phenomenon of the ribbon observed during the starting phase. The ribbon gluing was influenced by the puddle geometry, the recirculation in the puddle, and later, to the ribbon cooling rate. Within these results, high-quality ribbons with control thickness over a considerable length are achieved

Surface chemistry of rare-earth oxide surfaces at ambient conditions: reactions with water and hydrocarbons

Rare-earth (RE) oxide surfaces are of significant importance for catalysis and were recently reported to possess intrinsic hydrophobicity. The surface chemistry of these oxides in the low temperature regime, however, remains to a large extent unexplored. The reactions occurring at RE surfaces at room temperature (RT) in real air environment, in particular, in presence of polycyclic aromatic hydrocarbons (PAHs), were not addressed until now. Discovering these reactions would shed light onto intermediate steps occurring in automotive exhaust catalysts before reaching the final high operational temperature and full conversion of organics. Here we first address physical properties of the RE oxide, nitride and fluoride surfaces modified by exposure to ambient air and then we report a room temperature reaction between PAH and RE oxide surfaces, exemplified by tetracene (C18H12) on a Gd2O3. Our study evidences a novel effect-oxidation of higher hydrocarbons at significantly lower temperatures (similar to 300 K) than previously reported (>500 K). The evolution of the surface chemical composition of RE compounds in ambient air is investigated and correlated with the surface wetting. Our surprising results reveal the complex behavior of RE surfaces and motivate follow-up studies of reactions between PAH and catalytic surfaces at the single molecule level

Study of Wall Re-Deposition on DC-grounded ITER-relevant Mirrors with RF Plasma in a First Mirror Unit

In ITER, several first mirrors (FMs) are expected to be DC-grounded with the water cooling lines being implemented as a quarter wavelength ($lambda$/4) RF-filter. DC-grounding of the FMs can significantly increase the plasma potential V p, which could trigger an increased wall sputtering and associated re-deposition on the FMs during plasma cleaning. To understand the scope of this impact, helium discharges were excited with DC-grounded FMs in an ITER-sized mock-up of a first mirror unit (FMU) using wall materials with different sputtering energy thresholds (E th). Additionally, a part of the FM was electrically isolated from the RF to study its impact on the erosion/re-deposition properties on the surface. The E th of the wall materials, as well as its native oxide layers, had a significant influence on the re-deposition observed on the FMs. With high E th where walls were unsputtered, both the DC-grounded and electrically isolated parts of the FM were free of deposits. However, with low E th where the walls were sputtered, there was a net wall re-deposition on the DC-grounded parts of the FM, while electrically isolated parts were still relatively clean. Further, to study the impact of floating wall components, Cu walls in the FMU were isolated from the ground. Here the walls developed a floating potential V f and the ion energy at the walls was lowered to e(V p - V f). The floating walls, in this case, were relatively unsputtered and the FMs experienced a net cleaning with total reflectivity of the mirror preserved at pristine mirror levels. This work shows that electrically isolating the FM as well as the wall surface minimizes wall re-deposition in presence of $lambda$/4 filter and therefore are promising techniques for effective FM cleaning in ITER

Plasma Cleaning of Steam Ingressed ITER First Mirrors

In ITER, the first mirrors (FMs) are vulnerable to an in-vessel coolant leak which could severely diminish their optical properties. To understand the scope of this potential impact, several FM samples were exposed to a steam and humidity test simulating the event in ITER. Both rhodium and molybdenum mirrors, observed a loss in specular reflectivity as a result (the loss being greater for the Mo mirror). Their surfaces were tarnished with the development a thin Rh oxide and a thick Mo oxide (120–170 nm). This study focusses on capacitively coupled radio frequency (CCRF) plasma cleaning of steam ingressed (SI) FM samples and follow their optical recovery. Plasma cleaning experiments were performed with 13.56 MHz CCRF plasma using argon and/or hydrogen as process gas (with 230 eV ion energy). Initial and final reflectivity measurements, chemical surface analysis using in vaccuo X-ray photoelectron spectroscopy, scanning electron microscopy, focused ion beam and roughness measurements, were carried out for each sample to evaluate the cleaning efficiency. Using the plasma cleaning technique, it was possible to remove the SI induced contamination from the mirror surfaces and recover their optical properties to the pristine levels. Several ‘voids/inclusions’ were seen to arise along the grain boundaries as a result of the SI procedure. The concentration of these ‘voids/inclusions’ was observed to increase till a certain point followed by a decrease with increasing cleaning time