[en] ²²⁶Ra data on eleven vertical profiles taken during the GEOSECS program from the Antarctic Ocean and its vicinity in both the Atlantic and the Pacific are presented. Replicate measurements were made on each sample using the Rn-emanation method. The precision (1 sigma) based on triplicate analyses averages about +-2.5%. Waters all around the Antarctic continent below 2 km depth appear to exhibit a uniform ²²⁶Ra concentration of 21.5+-1 dpm/100 kg, except perhaps locally such as the Ross Sea and the Drake Passage where small variations may be present. Higher in the water column, the ²²⁶Ra contents decrease toward the surface with gradients which vary as a function of the influence exerted by the Antarctic Convergence. Across this oceanic front, a north-to-south increase of ²²⁶Ra occurs (the increase being the largest near the surface: from 8 to 18 dpm/100 kg), reflecting the combining effect of deep-water upwelling and meridional water mixing. The core layer of the Antarctic Intermediate Water contains about 14 dpm/100 kg of ²²⁶Ra and that of the Circumpolar Intermediate Water (O₂ minimum and local T maximum) about 18 dpm/100 kg. To a first approximation, ²²⁶Ra covaries with Si in the circumpolar waters. (Auth.)

[en] A relativistic field-emission-limited diode employing a high-transparency mesh anode is investigated via a self-consistent approach. The field emission process is described quantum mechanically by the Fowler-Nordheim equation. The cathode plasma and surface properties are considered within the framework of the effective work function approximation. Space-charge effects are described by Poisson's equation including relativistic effects. Ionization effects at the high-transparency mesh anode are ignored. The numerical calculations are carried out on a time scale much shorter than the emergence of the gap closure. The quasistationary state of the diode exhibits a cutoff voltage. The electric field on the cathode surface is found to be saturated in the high-voltage regime and determined by the effective work function only

[en] Hydrogenation of TiO₂ is relevant to hydrogen storage and water splitting. We have carried out a detailed mechanistic study on TiO₂ hydrogenation through H and/or H₂ diffusion from the surface into subsurface layers of anatase TiO₂ (101) by periodic density functional theory calculations implementing on-site Coulomb interactions (DFT + U). Both H atoms and H₂ molecules can migrate from the crystal surface into TiO₂ near subsurface layer with 27.8 and 46.2 kcal/mol energy barriers, respectively. The controlling step for the former process is the dissociative adsorption of H₂ on the surface which requires 47.8 kcal/mol of energy barrier. Both hydrogen incorporation processes are expected to be equally favorable. The barrier energy for H₂ migration from the first layer of the subsurface O_sub1 to the 2nd layer of the subsurface oxygen O_sub2 requires only 6.6 kcal. The presence of H atoms on the surface and inside the subsurface layer tends to promote both H and H₂ penetration into the subsurface layer by reducing their energy barriers, as well as to prevent the escape of the H₂ from the cage by increasing its escaping barrier energy. The H₂ molecule inside a cage can readily dissociate and form 2HO-species exothermically (ΔH =−31.0 kcal/mol) with only 26.2 kcal/mol barrier. The 2HO-species within the cage may further transform into H₂O with a 22.0 kcal/mol barrier and 19.3 kcal/mol exothermicity relative to the caged H₂ molecule. H₂O formation following the breaking of Ti–O bonds within the cage may result in the formation of O-vacancies and surface disordering as observed experimentally under a high pressure and moderately high temperature condition. According to density of states analysis, the projected density of states of the interstitial H, H₂, and H₂O appear prominently within the TiO₂ band gap; in addition, the former induces a shift of the band gap position notably towards the conduction band. The thermochemistry for formation of the most stable sub-surface species (2HO and H₂O) has been predicted. These results satisfactorily account for the photo-catalytic activity enhancement observed experimentally by hydrogenation at high temperatures and high pressures.

[en] The Taiwan Light Source (TLS) is the first dedicated synchrotron light source facility that used a superconducting RF cavity to upgrade its existing RF system. More then one year has passed since the superconducting RF cavity was successfully integrated with the existing RF subsystem for routine operation. The operating status shows that the superconducting RF (SRF) system has effectively suppressed the instabilities of the beam that are caused by the interaction of the electron beam with the higher-order-modes of the cavity. The original goal of doubling the photon intensity has been reached by applying top-up mode injection at the beam current of 300 mA. However, compared to the original RF system, the beam loading is much higher, and the frequency of the superconducting cavity is also more easily perturbed by the mechanical vibrations. Those make the operation of the SRF system facing the challenge on the machine reliability, especially for a dedicated synchrotron light source facility. Here we report the operating status of the SRF system, analyze the trip sources, and present our efforts to improve the machine reliability and performance since the SRF system began routine operation

[en] A copper prototype of a 1.5-GHz cavity was manufactured to simulate a superconducting radio-frequency cavity for technique development. Frequency tuning with longitudinal compression of this prototype and cryogenic cooling with liquid nitrogen were performed to examine the numerical results from finite-element models, mainly the corresponding shifts of the fundamental resonant frequency. An appropriate element option improved the accuracy of the resonant frequency and the distribution of the magnetic field. Effects of geometry distortion of an uneven length on the frequency shift of this shell-type cavity as loaded on longitudinal compression are also examined and discussed. (paper)

[en] A simple method for the fabrication of highly photoactive nanocrystalline two-layer TiO₂ electrodes for solar cell applications is presented. Diluted titanium acetylacetonate has been used as a precursor for covering SnO₂:F (FTO) films with dense packed TiO₂ nanocrystallites. The nanoporous thick TiO₂ film follows the dense packed thin TiO₂ film as a second layer. For the latter, amorphous TiO₂ nanoparticles have been successfully synthesized by a sol-gel technique in an acidic environment with pH<1 and hydrothermal growth at a temperature of 200 deg. C. The acidic nanoparticle gel was neutralized by basic ammonia and a TiO₂ gel of pH 5 was obtained; this pH value is higher than the recently reported value of 3.1 (Park et al 2005 Adv. Mater. 17 2349-53). Highly interconnected, nanoporous, transparent and active TiO₂ films have been fabricated from the pH 5 gel. SEM, AFM and XRD analyses have been carried out for investigation of the crystal structure and the size of nanoparticles as well as the surface morphology of the films. Investigation of the photocurrent-voltage characteristics has shown improvement in cell performance along with the modification of the surface morphology, depending on pH of the TiO₂ gel. Increasing the pH of the gel from 2.1 to 5 enhanced the overall conversion efficiency of the dye-sensitized solar cells by approximately 30%. An energy conversion efficiency of 8.83% has been achieved for the cell (AM1.5, 100 mWcm^-2 simulated sunlight) compared to 6.61% efficiency in the absence of ammonia in the TiO₂ gel

[en] A superconducting 500-MHz cavity module has been installed into the electron storage ring of NSRRC. This SRF module is tested on both the RF and cryogenic performances, before and after installation into the electron ring. Calibrations and measurements on its cryogenic load at different operating helium bath pressures are described and concluded. The test results of unloaded quality factor are reported. Meanwhile the excellent regulation on helium bath pressure is so advantageous to all these measurements. During normal operation with RF power, fluctuations of the helium bath pressure and liquid helium level are +/- 1.38 mbar (0.02 psi) and +/-0.2%, respectively

[en] When particles are injected according to the Fowler-Nordheim (FN) field emission equation, the transmitted current density will transition to the space charge limited (SCL) current density, with increasing applied diode voltage. The actual transmitted current density is the so-called SCL-FN current density. In this work, Barbour's analytic solution for the SCL-FN current density is modified with consideration of injection velocity and also geometric effects, by solving the advanced FN equation with the effective field enhancement factor, the energy conservation equation with an initial velocity term, and Poisson's equation simultaneously. The solution is also extended to the relativistic regime where similar transition process is found. This solution has been verified using particle-in-cell simulation with varying diode voltage, electron injection velocity, and field enhancement factor

[en] The use of an addressable, faceted cathode has been proposed as a method of modulating current injection in a magnetron to improve performance and control phase. To implement the controllable electron emission, five-sided and ten-sided faceted planar cathodes employing gated field emitters are considered as these emitters could be fabricated on flat substrates. For demonstration, the conformal finite-difference time-domain particle-in-cell simulation, as implemented in VORPAL, has been used to model a ten-cavity, rising sun magnetron using the modulated current sources and benchmarked against a typical continuous current source. For the modulated, ten-sided faceted cathode case, the electrons are injected from three emitter elements on each of the ten facets. Each emitter is turned ON and OFF in sequence at the oscillating frequency with five emitters ON at one time to drive the five electron spokes of the π-mode. The emitter duty cycle is then 1/6th the Radio-Frequency (RF) period. Simulations show a fast start-up time as low as 35 ns for the modulated case compared to 100 ns for the continuous current cases. Analysis of the RF phase using the electron spoke locations and the RF magnetic field components shows that the phase is controlled for the modulated case while it is random, as typical, for the continuous current case. Active phase control during oscillation was demonstrated by shifting the phase of the electron injection 180° after oscillations started. The 180° phase shift time was approximately 25 RF cycles.

[en] Solid-state for high power RF application is an attracting and interesting technology which is now become popular in accelerator field. To adopt and master such technique, a 500MHz, 80kW solid-state amplifier is thus developed in NSRRC. The amplifier is consisted of 100 900W amplifier modules which are driving by identical modules. Each module contains input and output directional couplers and status monitoring circuits. To have longer life time and better performance, the RF power transistors are integrated with water cooled heat sink directly. In such a way, the transistors have higher output power and better efficiency. The RF power of each module is combined through coaxial combiner while its DC power is provided by parallel connected DC power supplies which can provide better redundancy and reliability. The home-made solid-state amplifier is demonstrated to have quite high quality RF power and reliability with acceptable power combination efficiency. (paper)