Sun Shin

Dynamics of mode competition in a gigawatt-class magnetically insulated line oscillator

An axial mode competition is observed in a 1GHz magnetically insulated line oscillator operating at gigawatt power level with a pulse duration of 130ns. A fast-growing axial mode adjacent to desired π-mode starts up first and hops to the slow-growing and stable π mode. The dynamics of the mode competition is found to be strongly dependent on the time-varying axial velocity of the magnetically insulated electron beam. The experimental observation is verified by the particle-in-cell simulation using a time-frequency analysis.

A tree-like nanoporous WO3 photoanode with enhanced charge transport efficiency for photoelectrochemical water oxidation

We report a tree-like nanoporous tungsten trioxide (WO3) photoanode that largely improves the photoelectrochemical water oxidation performance. These novel WO3 photoanodes were prepared using a pulsed laser deposition method and their porosity was controlled by adjusting the oxygen partial pressure during the deposition process. The tree-like nanoporous WO3 photoanode has a nanoporous structure with a partially preferred alignment of the individual WO3 nanocrystals, which greatly improves the charge transport efficiency. Under simulated solar light illumination, the aforementioned features resulted in ∼9 times higher photocurrent density (1.8 mA cm−2 at 1.23 V vs. RHE) than a dense WO3 photoanode. An incident photon-to-current conversion efficiency of over 70% was also obtained at wavelengths of 350–400 nm.

Direct printing synthesis of self-organized copper oxide hollow spheres on a substrate using copper(II) complex ink: gas sensing and photoelectrochemical properties.

The direct printing synthesis of metal oxide hollow spheres in the form of film on a substrate is reported for the first time. This method offers facile, scalable, high-throughput production and device fabrication processes. The printing was carried out via a doctor-blade method using Cu(II) complex ink with controllable high viscosity based on formate-amine coupling. Following only thermal heating in air, well-defined polycrystalline copper oxide hollow spheres with a submicrometer diameter (≤1 μm) were formed spontaneously while being assembled in the form of a film with good adhesion on the substrate. This spontaneous hollowing mechanism was found to result from the Kirkendall effect during oxidation at elevated temperature. The CuO films with hollow spheres, prepared via direct printing synthesis at 500 °C, led to the creation of a superior p-type gas sensor and photocathode for photoelectrochemical water splitting with completely hollow cores, a rough/porous shell structure, a single phase, high crystallinity, and no organic/polymer residue. As a result, the CuO hollow-sphere films showed high gas responses and permissible response speeds to reducing gases and high photocurrent density compared to conventional CuO powder films and the values previously reported. These results exemplify the successful realization of a high-throughput printing fabrication method for the creation of superior nanostructured devices.

Ta-substituted SnNb2−xTaxO6 photocatalysts for hydrogen evolution under visible light irradiation

Ta-substituted SnNb2−xTaxO6 was successfully prepared via a solid-state reaction to study the effect of Ta insertion in Nb sites on the crystal structure, photophysical properties, and photocatalytic activities for hydrogen evolution. Analyses of X-ray diffraction patterns and Raman spectra revealed that the substitution of Ta caused not only a more tightly packed atomic structure with greater crystal structural distortion, but also a shorter M–O bond length in MO6 octahedra. Additionally, we observed a gradual increase in the band gap, changing the photoabsorption property and conduction band electronic structure. The SnNb1.4Ta0.6O6 photocatalyst showed enhanced hydrogen evolution compared to pristine SnNb2O6. This result was mainly attributed to better transport ability of the photo-generated charge carriers.

Boosting the solar water oxidation performance of a BiVO4 photoanode by crystallographic orientation control

Materials with low crystal symmetry often exhibit anisotropic properties, allowing the tuning of their physical and chemical properties via crystallographic orientation and exposed facet control. Herein, for the first time, we have demonstrated that pristine BiVO4 with a preferred [001] growth orientation and exposed (001) facets exhibits excellent intrinsic charge transport properties and surface reactivity. Using preferentially [001]-oriented BiVO4 (p-BVO) as a photoanode for photoelectrochemical water splitting, an impressive photocurrent density at 1.23 V vs. the reversible hydrogen electrode (RHE) is achieved, which is approximately 16 times higher than that exhibited by a photoanode based on randomly oriented BiVO4. Importantly, when the surface of p-BVO is further roughened and decorated with an oxygen evolution electrocatalyst, photocurrent densities of ∼3.5 and ∼6.1 mA cm−2 are achieved at 0.6 and 1.23 VRHE, respectively; the latter value corresponds to ∼82% of the theoretically achievable photocurrent density for BiVO4 under 1 sun illumination. Our results demonstrate the effectiveness of crystal orientation and exposed facet control in optimizing materials for solar water-splitting applications.

Pulse shortening by RF breakdown in relativistic backward wave oscillator

Relativistic backward wave oscillator (RBWO) is the suitable source for generating high power electromagnetic wave (HPEM). The RBWO has a high efficiency by means of the interaction region between a backward wave and an electron beam. However, the pulse shortening problem occurs frequently in experiments of RBWO because of the RF breakdown on rippled slow wave structure. Experimental study is done 500 MW RBWO in progress.

Effect of anode plasma on pulse shortening in magnetically insulated transmission line oscillator

A competition between two axial mode of TM01 is observed in the experiment of a L-band magnetically insulated transmission line oscillator (MILO) (D. H. Kim, et al., 2007). This mode competition can be affected by anode plasma formation during the operation. The transient time between two modes becomes shorter with additional anode plasma in MAGIC2D simulation.

Experimental Study on Dynamics of Mode Competition in Magnetically Insulated Transmission Line Oscillator (MILO)

A mode competition phenomenon of gigawatts-nanonoseconds magnetically insulated transmission line oscillator (MILO) is experimentally investigated. A pulsed power system of 500 kV-35 kA and 130 ns pulse length is used for generating the relativistic electron beam. The microwave with peak power of 2.8 GW, pulse duration of 70 ns, and frequency of 1.14 GHz is observed. A spatiotemporal measurement using RF B-dotprobe is in a reasonable agreement with a numerical simulation using a particle-in-cell (PIC) code confirming mode competition between axial modes.

Experimental Study on X-band Relativistic Backward Wave Oscillator (RBWO)

A Relativistic backward wave oscillator (RBWO) is a reliable source of a high power electromagnetic wave (HPEM). The RBWO is capable of high efficiency by a relatively long interaction region between a backward wave and an electron beam. But the pulse shortening problem occurs frequently in experiments of RBWO because of the mode competition, the RF breakdown and so on. After this mode competition between axial modes of TM01, the RF power is vanished. There also occurs a mode competition between TM01 and TM02 when the radii of the hollow electron beam are smaller than a critical value. Mode competition which is one of the main causes of pulse shortening is found to be very sensitive to the position of the electron beam in this RBWO device. Experimental study is done on 100MW RBWO and in progress on X-band hundreds MWRBWO.

Axial Mode Competition in a Magnetically Insulated Line Oscillator (MILO)

Summary form only given. Experimental research on L-band magnetically insulated transmission line oscillator (MILO) is carried out for the generation of the microwave of ~GW power. This MILO is composed of 6-uniform cavities and single coupling cavity and two choke cavities. The measured output power of 2.8 GW shows a reasonable agreement with the simulated one of 2.75 GW. In this experiment, 500 kV-36 kA pulsed electron beam with the pulse duration of 130nsec is used. The pulsewidth of the microwave is limited to 70ns mainly due to a mode competition between axial modes. The mode change during the pulse is measured by RF B-dot probes at each cavity. Dynamic of mode competition is studied with time-frequency analysis.