Synthesis of nanodimensional TiO2 thin films using energetic ion beam

Nanophases of TiO₂ are achieved by irradiating polycrystalline thin films of TiO₂ by 100 MeV Au ion beam at varying fluence. The surface morphology of pristine and irradiated films is studied by atomic force microscopy (AFM). Phase of the film before and after irradiation is identified by glancing angle X-ray diffraction (GAXRD). The blue shift observed in UV-vis absorption edge of the irradiated films indicates nanostructure formation. Electron spin resonance (ESR) studies are carried out to identify defects created by the irradiation. The nanocrystallisation induced by SHI irradiation in polycrystalline thin films is studied.     

Large aperture electrostatic dust detector

Diagnosis and management of dust inventories generated in next-step magnetic fusion devices is necessary for their safe operation. A novel electrostatic dust detector, based on a fine grid of interlocking circuit traces biased to 30 or 50 V has been developed for the detection of dust particles on remote surfaces in air and vacuum environments. Impinging dust particles create a temporary short circuit and the resulting current pulse is recorded by counting electronics. Up to 90% of the particles are ejected from the grid or vaporized suggesting the device may be useful for controlling dust inventories. We report measurements of the sensitivity of a large area (5 × 5 cm) detector to microgram quantities of dust particles.     

Existence states of deuterium irradiated into LiAlO2

The existence states of deuterium in LiAlO₂ were analyzed by in situ IR absorption spectroscopy during irradiation with 3 keV at room temperature. Multiple IR absorption peaks that were related to O-D stretching vibrations were observed, mainly at 2650 cm⁻¹ (O-D_α), 2600 cm⁻¹ (O-D_β), and 2500 cm⁻¹ (O-D_γ). The O-D_α was assigned to the surface O-D. The O-D_β and O-D_γ were interpreted as two distinct O-D states for three candidates: O-D of substitutional D⁺ for Li⁺; O-D of substitutional D⁺ for Al³⁺; and O-D of interstitial D⁺. O-D_β was the dominant O-D state for deuterium irradiated into LiAlO₂, and had higher stability than O-D_γ. Heating after ion irradiation led to the desorption of D₂ and an increase in the intensity of O-D_β, which implies that some of the deuterium irradiated into LiAlO₂ exists in non-O-D states, such as D⁻ captured by F centers.     

Analysis of fuel retention in plasma-facing components from controlled fusion devices

First wall components in controlled fusion devices undergo severe modification by various physical and chemical processes arising from plasma-wall interactions: material erosion, its transport in the plasma and re-deposition. The intention of this work is to give a concise overview of key issues in the characterization of plasma-facing materials and components in tokamaks. The importance of surface analysis in studies of fuel inventory and material migration is presented. Experimental procedures and analysis methods are briefly reviewed with emphasis on ion beam techniques which play a prominent role in studies of wall components exposed to hot plasmas. Practical aspects in the analytical approach are addressed and special instrumentation used in these studies is described.     

Deposition of gold nano-particles and nano-layers on polyethylene modified by plasma discharge and chemical treatment

Polyethylene (PE) was treated in Ar plasma discharge and then grafted from methanol solution of 1,2-ethanedithiol to enhance adhesion of gold nano-particles or sputtered gold layers. The modified PE samples were either immersed into freshly prepared colloid solution of Au nano-particles or covered by sputtered, 50 nm thick gold nano-layer. Properties of the plasma modified, dithiol grafted and gold coated PE were studied using XPS, UV-VIS, AFM, EPR, RBS methods and nanoindentation. It was shown that the plasma treatment results in degradation of polymer chain, creation of excessive free radicals and conjugated double bonds. After grafting with 1,2-ethanedithiol the concentration of free radicals declined but the concentration of double bonds remained unchanged. Plasma treatment changes PE surface morphology and increases surface roughness too. Another significant change in the surface morphology and roughness was observed after deposition of Au nano-particles. The presence of Au on the sample surface after the coating with Au nano-particles was proved by XPS and RBS methods. Nanoindentation measurements shown that the grafting of plasma activated PE surface with dithiol increases significantly adhesion of sputtered Au nano-layer.     

Cytocompatibility of Ar plasma treated and Au nanoparticle-grafted PE

Polyethylene (PE) was irradiated with inert Ar plasma, and the chemically active PE surface was grafted with Au nanoparticles. The composition and the structure of the modified PE surface were studied using X-ray photoelectron spectroscopy (XPS) and Rutherford backscattering spectroscopy (RBS). Changes in the surface wettability were determined from the contact angle measured in a reflection goniometer. The changes in the surface roughness and morphology were followed by atomic force microscopy (AFM). The modified PE samples were seeded with rat vascular smooth muscle cells (VSMC) or mouse NIH 3T3 fibroblasts, and their adhesion and proliferation were studied. We found that plasma discharge and Au grafting lead to dramatic changes in the surface morphology and roughness of PE. The Au nanoparticles were found not only on the sample surface, but also in the sample interior up to the depth of about 100 nm. In addition, plasma modification of the PE surface, followed with grafting Au-nanoparticles, significantly increased the attractiveness of the PE surface for the adhesion and growth of VSMC, and particularly for mouse embryonic 3T3 fibroblasts.     

Hydrogen reflection in low-energy collisions with amorphous carbon

Reflection and sticking of hydrogen atoms at amorphous carbon surfaces is studied at impact energies ranging from 0.1 to 50 eV using molecular dynamics simulations. We show that the reflection coefficient at the lowest energies is large and is very sensitive to both the many-body potential used in the simulations and the degree of hydrogen enrichment of the surfaces.     

Heat load behaviors of plasma sprayed tungsten coatings on copper alloys with different compliant layers

Plasma sprayed tungsten (PS-W) coatings with the compliant layers of titanium (Ti), nickel-chromium-aluminum (NiCrAl) alloys and W/Cu mixtures were fabricated on copper alloys, and their properties of the porosity, oxygen content, thermal conductivity and bonding strength were measured. High heat flux tests of actively cooled W coatings were performed by means of an electron beam facility. The results indicated that APS-W coating showed a poorer heat transfer capability and thermo-mechanical properties than VPS-W coating, and the compliant layers improved W coating performance under the heat flux load. Among three compliant layers, W/Cu was the preferable because of its better effects on heat removal and stress alleviating. The optimization of W/Cu compliant layer found that 0.1 mm and 25 vol.%W was optimum compliant layer structure for 1 mm W coating, which induced a 23% reduction of the maximum stress compared to the sharp interface, and the plastic strain was reduced to 0.01% from 1.55%.     

On the carbon and tungsten sputtering rate in a magnetron discharge

Magnetron discharge as sputtering source can serve as an alternative tool for the study of the plasma-wall interaction, with applications for ITER divertor. The present work reports on the influence of the target power density and the nature of the projectile on the erosion of C and W targets. The experimental results concern the sputtering rate of carbon and tungsten targets of a d.c. magnetron discharge in argon and helium atmosphere, at different gas pressures in the range of 10-100 mTorr and discharge power densities up to 40 W cm⁻² while the discharge current intensity was used as control parameter. In this investigation, carbon and tungsten sputtering rates were measured using two conventional methods based on gravimetric mass loss and profilometry. Target erosion profiles were compared with the profiles of the ion energy flux bombarding the target, calculated from a 2D fluid model.     

Effect of crystallinity on the memory effect of Ge nanocrystals synthesized by atom beam sputtering

The effect of crystallinity of Ge nanocrystals on the charge storage properties of the metal oxide semiconductor (MOS) structure has been investigated. MOS structure with Ge nanocrystals embedded in the oxide has been fabricated by using atom beam sputtering technique. After annealing at 600 °C in Ar + H₂ atmosphere, capacitance-voltage (C-V) measurements show flat band voltage shift of ∼0.9 V. It which is a clear indication of the memory effect of Ge nanocrystals, while unannealed structure doesnot show any hysteresis in the C-V curve. Micro Raman spectroscopy and X-ray diffraction (XRD) analyses show that crystalline content of Ge nanoparticles in the MOS structure has increased after annealing.     

Structural studies of Ge nanocrystals embedded in SiO2 matrix

Germanium nanoparticles embedded in SiO₂ matrix were prepared by atom beam sputtering on a p-type Si substrate. The as-deposited films were annealed at temperatures of 973 and 1073 K under Ar + H₂ atmosphere. The as-deposited and annealed films were characterized by Raman, X-ray diffraction and Fourier transform infrared spectroscopy (FTIR). Rutherford backscattering spectrometry was used to quantify the concentration of Ge in the SiO₂ matrix of the composite thin films. The formation of Ge nanoparticles were observed from the enhanced intensity of the Ge mode in the Raman spectra as a function of annealing, the appearance of Ge(3 1 1) peaks in the X-ray diffraction data and the Ge vibrational mode in the FTIR spectra. We have irradiated the films using 100 MeV Au⁸⁺ ions with a fluence of 1 × 10¹³ ions/cm² and subsequently studied them by Raman and FTIR. The results are compared with the ones obtained by annealing.     

Saturation in deuterium retention of CFC graphite targets exposed to beryllium-seeded plasmas on PISCES-B

ITER strike-plates are foreseen to be of carbon-fiber-composite (CFC). In this study the CFC bulk deuterium retention in ITER-relevant conditions is investigated. DMS 701 (Dunlop) CFC targets were exposed to plasma in PISCES-B divertor plasma simulator. Samples were exposed to both pure deuterium plasma and beryllium-seeded plasma at high fluences (up to ) and high surface temperature (1070 K). The deuterium contents of the exposed samples have been measured using both thermal-desorption-spectrometry (TDS) during baking at 1400 K and ion beam nuclear reaction analysis (NRA). The total deuterium inventory has been obtained from TDS while NRA measured the deuterium depth distribution. In the analysed fluence range at target temperature of 1070 K, no fluence dependence was observed. The measured released deuterium is . In the case of target exposure with beryllium-seeded plasma no change in the released amount of deuterium was found. The deuterium concentration inside the samples is almost constant until the probed depth of ?m, except in the first 1 μm surface layer, where it is 5 times higher than in the bulk. No C erosion/redeposition was observed in the Be-seeded plasma cases. The measured retention, applied to 50 m² of ITER CFC surface, would imply a tritium saturated value of 0.3 gT, much lower than the ITER safety limit of 350 g.     

Assessment of DNA damage of Lewis lung carcinoma cells irradiated by carbon ions and X-rays using alkaline comet assay

DNA damage and cell reproductive death determined by alkaline comet and clonogenic survival assays were examined in Lewis lung carcinoma cells after exposure to 89.63 MeV/u carbon ion and 6 MV X-ray irradiations, respectively. Based on the survival data, Lewis lung carcinoma cells were verified to be more radiosensitive to the carbon ion beam than to the X-ray irradiation. The relative biological effectiveness (RBE) value, which was up to 1.77 at 10% survival level, showed that the DNA damage induced by the high-LET carbon ion beam was more remarkable than that induced by the low-LET X-ray irradiation. The dose response curves of “Tail DNA (%)” (TD) and “Olive tail moment” (OTM) for the carbon ion irradiation showed saturation beyond about 8 Gy. This behavior was not found in the X-ray curves. Additionally, the carbon ion beam produced a lower survival fraction at 2 Gy (SF2) value and a higher initial Olive tail moment 2 Gy (OTM2) than those for the X-ray irradiation. These results suggest that carbon ion beams having high-LET values produced more severe cell reproductive death and DNA damage in Lewis lung carcinoma cells in comparison with X-rays and comet assay might be an effective predictive test even combining with clonogenic assay to assess cellular radiosensitivity.     

Structural studies of silicon oxynitride layers formed by low energy ion implantation

Silicon oxynitride (Si_xO_yN_z) layers were synthesized by implanting ¹⁶O₂⁺ and ¹⁴N₂⁺ 30 keV ions in 1:1 ratio with fluences ranging from 5 × 10¹⁶ to 1 × 10¹⁸ ions cm⁻² into single crystal silicon at room temperature. Rapid thermal annealing (RTA) of the samples was carried out at different temperatures in nitrogen ambient for 5 min. The FTIR studies show that the structures of ion-beam synthesized oxynitride layers are strongly dependent on total ion-fluence and annealing temperature. It is found that the structures formed at lower ion fluences (∼1 × 10¹⁷ ions cm⁻²) are homogenous oxygen-rich silicon oxynitride. However, at higher fluence levels (∼1 × 10¹⁸ ions cm⁻²) formation of homogenous nitrogen rich silicon oxynitride is observed due to ion-beam induced surface sputtering effects. The Micro-Raman studies on 1173 K annealed samples show formation of partially amorphous oxygen and nitrogen rich silicon oxynitride structures with crystalline silicon beneath it for lower and higher ion fluences, respectively. The Ellipsometry studies on 1173 K annealed samples show an increase in the thickness of silicon oxynitride layer with increasing ion fluence. The refractive index of the ion-beam synthesized layers is found to be in the range 1.54-1.96.     

Photoluminescence study of Ge nanocrystals irradiated by reactor neutron flux

Samples of Ge nanocrystals (Ge-ncs) embedded in amorphous SiO₂ film were prepared by Ge ion implantation and subsequent primary thermal annealing. These samples were irradiated by neutron flux in a nuclear reactor followed by the second annealing. Irradiation with thermal neutrons leads to doping of nanocrystals with Ga, As and Se impurities due to nuclear transmutation of isotope ⁷⁰Ge into ⁷¹Ga, isotope ⁷⁴Ge into ⁷⁵As, isotope ⁷⁶Ge into ⁷⁷Se, respectively (neutron transmutation doping, NTD). Irradiation with fast neutrons leads to appearance of radiation damages, which are expected to be removed after the second annealing. Photoluminescence (PL) measurements show that PL is quenched after neutron irradiation, and restored after annealing higher than 500 °C. The PL spectra of doped Ge-ncs samples show more intense exciton radiative luminescence than of undoped Ge-ncs sample, which is related to that the donor and acceptor impurities recombine the nonradiative centers in the interface of Ge-ncs and SiO₂ matrix, and enhance the probability of exciton recombination.     

Radiosensitivity of hepatoma cell lines and human normal liver cell lines exposed in vitro to carbon ions and argon ions at the HIRFL

Human hepatoma (SMMC-7721) and normal liver (L02) cells were irradiated with γ-rays, ¹²C⁶⁺ and ³⁶Ar¹⁸⁺ ion beams at the Heavy Ion Research Facility in Lanzhou (HIRFL). By using the Calyculin-A induced premature chromosome condensation technique, chromatid-type breaks and isochromatid-type breaks were scored separately. Tumor cells irradiated with heavy ions produced a majority of isochromatid break, while chromatid breaks were dominant when cells were exposed to γ-rays. The relative biological effectiveness (RBE) for irradiation-induced chromatid breaks were 3.6 for L02 and 3.5 for SMMC-7721 cell lines at the LET peak of 96 keVμm⁻¹¹²C⁶⁺ ions, and 2.9 for both of the two cell lines of 512 keVμm⁻¹³⁶Ar¹⁸⁺ ions. It suggested that the RBE of isochromatid-type breaks was pretty high when high-LET radiations were induced. Thus we concluded that the high production of isochromatid-type breaks, induced by the densely ionizing track structure, could be regarded as a signature of high-LET radiation exposure.     

Plasma and antenna coupling characterization in ICRF-wall conditioning experiments

Ion Cyclotron Wall Conditioning (ICWC) discharges, in pulsed-mode operation, were carried out in the divertor tokamaks ASDEX Upgrade (AUG) and JET to simulate the scenario of ITER wall conditioning at half-field (AUG) and full-field (JET). ICWC-plasma and antenna coupling characterization results obtained during the Ion Cyclotron Resonance Frequency (ICRF)-Wall Conditioning experiments performed in helium-hydrogen mixture in AUG and helium-deuterium mixtures in JET are presented here. Safe operational regimes for optimum ICWC in ITER could be explored for different magnetic fields. Satisfactory antenna coupling in the Mode Conversion scenario along with reproducible generation of ICRF plasmas and reliable wall conditioning were achieved by coupling RF power from one or two ICRF antennas at two (AUG, JET) different resonant frequencies. These results are in qualitative agreement with the predictions of 1-D TOMCAT code. Present study of ICWC indicates towards the beneficial effect of application of an additional (along with toroidal magnetic field) stationary vertical (B_V ? B_T) magnetic field on antenna coupling and plasma parameters. The results obtained from JET and AUG tokamaks, presented in this paper, emphasizes the proposed phenomenological schemes for further development of ICWC in superconducting tokamaks.     

Structural characterization of swift heavy ion irradiated polycarbonate

Makrofol-N polycarbonate thin films were irradiated with copper (50 MeV) and nickel (86 MeV) ions. The modified films were analyzed by UV-VIS, FTIR and XRD techniques. The experimental data was used to evaluate the formation of chromophore groups (conjugated system of bonds), degradation cross-section of the special functional groups, the alkyne formation and the amorphization cross-section. The investigation of UV-VIS spectra shows that the formation of chromophore groups is reduced at larger wavelength, however its value increases with the increase of ion fluence. Degradation cross-section for the different chemical groups present in the polycarbonate chains was evaluated from the FTIR data. It was found that there was an increase of degradation cross-section of chemical groups with the increase of electronic energy loss in polycarbonate. The alkyne and alkene groups were found to be induced due to swift heavy ion irradiation in polycarbonate. The radii of the alkyne production of about 2.74 and 2.90 nm were deduced for nickel (86 MeV) and copper (50 MeV) ions respectively. XRD analysis shows the decrease of the main XRD peak intensity. Progressive amorphization process of Makrofol-N with increasing fluence was traced by XRD measurements.     

A quartz-crystal-microbalance technique to investigate ion-induced erosion of fusion relevant surfaces

We describe a highly sensitive quartz-crystal-microbalance technique capable of determining erosion as well as implantation and retention rates for fusion relevant surfaces under ion bombardment. Total sputtering yields obtained with this technique for Ar ion impact on polycrystalline gold and tungsten surfaces are presented. The results compare well with existing experimental data as well as theoretical predictions and thus demonstrate the feasibility of the developed technique. Our setup is capable of detecting mass-changes as small as 10⁻⁵ μg/s, which corresponds to a removal of only 10⁻⁴ W monolayers/s.     

Study on C-W interactions by molecular dynamics simulations

By means of molecular dynamics simulations using bond-order potential (BOP), we have investigated the interactions between carbon (C) atoms and bcc tungsten (W). At finite temperature (T = 300 K) with incident energy of C atoms ranging from 0.5 to 100 eV at normal incidence, the projected range distribution as a function of incident energy and the average depth have been depicted. The properties of vacancy, vacancy migration, interstitial and substitutional C atoms in W have been determined. The most stable configuration for an interstitial C atom in W is in octahedral position and the lattice distortion around the C atom in octahedral interstitial configuration occurs along 〈1 0 0〉 and 〈1 1 0〉 directions. The mutual interaction between a vacancy and near interstitial C atom is also studied.