[en] Infrared spectroscopy has been employed to study the reduction of carbon dioxide on supported catalyst films. The investigation included isotopic labeling using D₂ as the reduction gas. Isotopic exchange was observed for both CO₂/D₂ and CH₄/D₂ mixtures. The mechanism of this isotopic exchange involves migration of hydrogen from the support to the Rh sites, an ''inverse spillover effect''. A key intermediate in the dissociation of CO₂ on the supported Rh films was a carbonyl hydride species. 21 refs., 2 figs

[en] The ITER ECH heating and current drive system delivers 24 MW at 170 GHz, which can be directed to either the equatorial (EL) or upper (UL) port launching antennas (launchers) depending on the desired physics application. The UL reference design uses a front steering (FS) mirror that sweeps the beam in a poloidal plane providing co-ECCD over the outer third of the plasma cross section. A novel frictionless, backlash-free steering mechanism has been developed for an increased reliability. The design avoids components such as bearings and push-pull rods, which tend to grip in conventional launching systems in use on present ECH systems. Flexure pivots replace of bearings and a pneumatic seal-less actuator using pressurised helium integrated into the rotating mirror assembly offers a fast and precise response avoiding push-pull rods, linkages representing sliding bearings and remote actuators. The result is a complete self-contained frictionless kinematic assembly rotating the steering mirror up to ±7 deg (±14 deg. for RF beam). The launcher has a single dedicated purpose of stabilising the neoclassical tearing modes (NTM), with the launcher steering range accessing the region in which the q=3/2 or 2 flux surfaces are expected to be found for scenarios susceptible to NTM. The performance of the FS launcher far exceeds (by a factor of 1.5 to 3) that required by the physics to stabilize the NTM. The two mirror (focusing and steering) system of the FS launcher essentially decouples the steering and focusing functions of the FS launcher, which offers the flexibility to increase the access range beyond that required by the NTM stabilization such that the launcher can access further inward for sawteeth control. Extending the range of the UL can relax the EL steering range, and optimize the launcher for enhanced physics performance with an optimised central deposition and potential for counter ECCD. (author)

[en] Full text: TCV's high power ECRH system with its flexible launchers is used to tailor the current profile and to generate hollow current profiles with off-axis co-electron cyclotron current drive (ECCD). In this manner, steady-state electron internal transport barriers (eITB) have been obtained where the confinement is strongly enhanced over L-mode scaling. eITBs may be parametrised by the strength of the barrier, i.e. the depth of the hollow current profile, and by its volume, i.e. the region enclosed within the barrier. The formation of the eITB has been found to occur on an even faster time scale than the energy confinement time (∼ 2ms). The barrier is formed off-axis and causes the confinement to improve throughout the core of the plasma. The fast increase in confinement occurs without any change in external parameters and without any additional heating deposited in the core. A simplified model of the current density profile evolution from a peaked to hollow profile that correlates the barrier formation with the inversion of current profile, along with the evidence that the barrier is formed in a very localized zone (both temporal and spatial), will be discussed in detail. After the barrier is formed, its strength can be controlled by adjusting the depth of the central current trough. For example, small amounts of ohmic power (∼3 kW) are used to drive up to ∼50% counter current (relative to the total current), resulting in further confinement enhancement of up to 50%. Conversely, ohmic co-current in the center has been shown to weaken and even destroy the barrier. These experiments demonstrate that the experimentally observed confinement enhancement associated with the eITB strongly depends on the depth and width of the current trough in current density profile. (author)

[en] The positive ion yield resulting from the interaction of a pulsed 266-nm laser with yttria-stabilized cubic zirconia crystals is investigated. Although the photon energy (4.66 eV) is well below the nominal band-gap energy for ZrO₂ (5.0-5.5 eV), photon stimulated ion desorption (PSD) of Y+, Zr+, YO+ and ZrO+ begins at approx. 2.5 MW/cm². We interpret this as the onset of laser ablation. Cation mass spectra collected using higher laser fluences resemble those obtained via secondary-ion mass spectrometry (SIMS). The similarity between the laser ablation and SIMS data demonstrates the importance of surface electronic structure effects in photon induced degradation of this material

[en] The additional power absorbed by the plasma can be determined from the time derivative of the total plasma energy, which can be estimated from the diamagnetic flux of the plasma using a Diamagnetic Loop (DML). The main difficulty in using diamagnetic measurements to estimate the kinetic energy is the compensation of the flux measurement sensitivity to poloidal magnetic fields, which is not always easy to adjust. A method based on the temporal variations of the diamagnetic flux of the plasma during Modulated Electron Cyclotron Heating (MECH) has been developed. Using MECH has the advantage that these poloidal fields are not significantly modulated and a good compensation of these fields is not necessary. However, a good compensation of the vessel poloidal image current is crucial to ensure a sufficiently large bandwidth. The application of this diagnostic to studies of the extraordinary mode (X-mode) absorption at the third electron cyclotron harmonic frequency (X3) has been performed on the TCV Tokamak in plasmas pre-heated by X-mode at the second harmonic (X2). A MECH frequency scan has allowed the determination of an optimum modulation frequency, situated at about 200- 250 Hz. Based on this diagnostic, full single-pass absorption of the injected X3 power was measured with the X2 pre-heating in co-current drive. This high absorption is more than a factor of 2 higher than the one predicted by the linear ray tracing code TORAY. Experimental evidence indicates that a large fraction of the X3 power is absorbed by electrons in an energetic tail created by the X2 pre-heating. (author)

[en] Torsatron and stellarator plasma confinement devices rely on magnetic surface mapping to determine the critical vacuum magnetic field structure. A recently developed method employing an emissive filament offers some advantages over the traditional technique of mapping with a directed electron beam. On the Auburn Torsatron a comparative study between the emissive filament and directed electron beam techniques has been conducted. The parameters varied in the comparative study are: filament geometry, emission current, bias voltage, background gas pressure, and magnetic field strength. This comparative study indicates that the emissive filament technique is reliable over a broad and easily accessible range of parameters. We have also measured the spatial distribution of electrons on a given magnetic surface. As an application of the emissive filament technique, the optimization of the magnetic surfaces on the Auburn Torsatron is shown

[en] A small component of ECCD exists when heating off-axis with no toroidal injection due to the field helicity. The sign of the ECCD will change depending on the absorption location in a poloidal plane, the beam injection relative to ∇B_vac, and the direction of the toroidal magnetic field. The importance of the ECCD component can be confirmed by the behavior of the sawteeth when the ECH deposition crosses the q=1 surface. The sawtooth period is very sensitive to the local shear at q=1 and it is shown that difference in sawtooth period when heating above and below the midplane are consistent with the presence of a small ECCD component. Experiments have also shown that the sawtooth period is strongly determined by the absorbed ECH power near the q=1 surface. In particular significant changes in the sawtooth period are observed as the absorption location is varied on a scale smaller than the beam spot size. A narrow region of rapidly changing confinement is shown to exist near the q=1 surface to be located outside the q=1 surface by a factor of 1.3 in radii. The region can be used as a method for measuring the beam deposition width in the plasma

[en] 'High-level radioactive waste storage tanks within DOE sites contain significant amounts of organic components (solid and liquid phases) in the form of solvents, extractants, complexing agents, process chemicals, cleaning agents and a variety of miscellaneous compounds. These organics pose several safety and pretreatment concerns, particularly for the Hanford tank waste. Remediation technologies are needed that significantly reduce the amounts of problem organics without resulting in toxic or flammable gas emissions, and without requiring thermal treatments. These restrictions pose serious technological barriers for current organic destruction methods which utilize oxidation achieved by thermal or chemical activation. This project focuses on using ionizing radiation (a,b,g) to catalytically destroy organics over oxide materials through reduction/oxidation (redox) chemistry resulting from electron-hole (e^-/h⁺) pair generation. Conceptually this process is an extension of visible and near-UV photocatalytic processes known to occur at the interfaces of narrow bandgap semiconductors in both solution and gas phases. In these processes, an electron is excited across the energy gap between the filled and empty states in the semiconductor. The excited electron does reductive chemistry and the hole (where the electron was excited from) does oxidative chemistry. The energy separation between the hole and the excited electron reflects the redox capability of the e^-/h⁺ pair, and is dictated by the energy of the absorbed photon and the bandgap of the material. The use of ionizing radiation overcomes optical transparency limitations associated with visible and near-UV illumination (g-rays penetrate much farther into a solution than UV/Vis light), and permits the use of wider bandgap materials (such as ZrO₂) which possess potentially greater redox capabilities than those with narrow bandgap materials. Experiments have been aimed at understanding the mechanism(s) of g-radiocatalysis and extending the body of knowledge about e^-/h⁺ pair chemistry of semiconducting metal oxide (MO) materials by examining the influence of surface structure, defects and dopants on the photocatalytic activity of narrow bandgap materials (TiO₂). An outcome of this proposed work will be a more thorough evaluation of the use of ionizing radiation in the catalytic remediation of organics (and other problem species) in high-level mixed waste.'

[en] Torsatron and stellarator plasma confinement devices rely on magnetic surface mapping to determine the critical vacuum magnetic field structure. A recently developed method employing an emissive filament offers some advantages over the traditional technique of mapping with a directed electron beam. On the Auburn torsatron a comparative study between the emissive filament and directed electron beam techniques has been conducted. The parameters varied in the comparative study are filament geometry, emission current, bias voltage, background gas pressure, and magnetic field strength. This comparative study indicates that the emissive filament technique is reliable over a broad and easily accessible range of parameters. We have also measured the spatial distribution of electrons on a given magnetic surface. As an application of the emissive filament technique, the optimization of the magnetic surfaces on the Auburn torsatron is shown

[en] Magnetic surface optimization studies have been conducted on the Auburn Torsatron. Winding errors in the original helical field coil resulted in a displacement and an up-down asymmetry in the magnetic surfaces. The displacement was initially corrected by unbalancing the outer Helmholtz coils. Computer modelling showed that the observed displacement and asymmetry are expected for a helical field coil winding error having a cos (θ) modulation. A helical correction coil with the appropriate modulation was designed and installed. Magnetic surface mappings with and without the helical correction coil were conducted. With the correction coil, there was a significant improvement in surface symmetry. The experimental results are in good agreement with the predictions of the computer modelling. (author). 8 refs, 6 figs, 1 tab