J. C. Xu

Effect of hydrogenation on the microwave absorption properties of BaTiO3 nanoparticles

Microwave absorbing materials (MAMs) have numerous important applications in electronic communications, signal protection, radar dodging, etc. Although it has been proposed as a promising MAM, BaTiO3 has a high reflection coefficient at the interface with air, causing a large reflection. Thus, its efficiency of microwave absorption is not satisfactory. Here, we report that hydrogenation has largely improved the microwave absorption of BaTiO3 nanoparticles. Hydrogenation is performed on BaTiO3 nanoparticles by treating pristine BaTiO3 nanoparticles at 700 °C for 4 hours in a pure H2 environment. The enhanced microwave absorption efficiency with a reflection loss value (−36.9 dB) is attributed to the increased resonance of polar rotations with the incident electromagnetic field which is amplified by the increased interfacial polarization caused by the built-in electrical field along the boundaries between different grains created within these nanoparticles.

Efficient acceleration of monoenergetic proton beam by sharp front laser pulse

Stable acceleration of relativistic ions by the radiation pressure of a superintense, circularly polarized laser pulse with sharp front is investigated by analytical modeling and particle-in-cell simulation. For foils with given density and thickness, the suitable steepness of the laser front is found to suppress instabilities and efficiently drive a stable monoenergetic ion beam. With a laser pulse of peak amplitude a{sub 0}=200, a proton beam of energy about 10 GeV can be generated. The dynamics of the laser-compressed electron layer and the ions in the hole-boring stage are investigated. In the case studied, the ions initially in the middle of the target are found to be accelerated to the back surface of the target ahead of the other ions.

The origin of the strong microwave absorption in black TiO2

In this study, the mechanism of the strong microwave absorption in black TiO2 nanoparticles has been investigated both experimentally and theoretically. In experiment, the amorphous TiO2 nanoparticles/paraffin wax composites show the reflection loss (RL) of −4.0 dB, which is much smaller compared with the RL of −49.0 dB in those core/shell structure ones. Theoretically, the calculation illustrates that the accumulated charge of 1013 cm−3 at the core/shell interface results in the plasmon resonance with the incident microwave frequency at 9.3 GHz and 27.0 GHz. The microwave absorption enhancement of the black TiO2 nanoparticles is proposed to originate from the synergy mechanism between their crystalline-cores and amorphous-shells, rather than the defects and impurities in amorphous phase.

Fast electron flux driven by lower hybrid wave in the scrape-off layer

The fast electron flux driven by Lower Hybrid Wave (LHW) in the scrape-off layer (SOL) in EAST is analyzed both theoretically and experimentally. The five bright belts flowing along the magnetic field lines in the SOL and hot spots at LHW guard limiters observed by charge coupled device and infrared cameras are attributed to the fast electron flux, which is directly measured by retarding field analyzers (RFA). The current carried by the fast electron flux, ranging from 400 to 6000 A/m2 and in the direction opposite to the plasma current, is scanned along the radial direction from the limiter surface to the position about 25 mm beyond the limiter. The measured fast electron flux is attributed to the high parallel wave refractive index n|| components of LHW. According to the antenna structure and the LHW power absorbed by plasma, a broad parallel electric field spectrum of incident wave from the antennas is estimated. The radial distribution of LHW-driven current density is analyzed in SOL based on Landau d...

A Novel Green TiO2 Photocatalyst with a Surface Charge-Transfer Complex of Ti and Hydrazine Groups

The optical property of TiO2 plays an important role in its various and promising photocatalytic applications. Previous efforts in improving its optical properties include doping with various metal and/or non-metal elements, coupling with other colorful semiconductors or molecules, and hydrogenating to crystalline/disordered core/shell nanostructures. Here, we report a beautiful green TiO2 achieved by forming the charge-transfer complex of colorless hydrazine groups and surface Ti4+ , which extends the optical absorption into the near infrared region (≈1100 nm, 1.05 eV). It shows an enhanced photocatalytic performance in hydrogen generation under simulated sunlight, and degradation of organic pollution under visible light due to an impurity state (about 0.28 eV) resulting in fast electron-hole separation and injection of electrons from the ligand to the conduction band of TiO2 . This study demonstrates an alternative approach to tune the optical, impurity state and photocatalytic properties of TiO2 nanoparticles and we believe this will spur a wide interest in related materials and applications.

Cascaded target normal sheath acceleration

A cascaded target normal sheath acceleration (TNSA) scheme is proposed to simultaneously increase energy and improve energy spread of a laser-produced mono-energetic proton beam. An optimum condition that uses the maximum sheath field to accelerate the center of the proton beam is theoretically found and verified by two-dimensional particle-in-cell simulations. An initial 10 MeV proton beam is accelerated to 21 MeV with energy spread decreased from 5% to 2% under the optimum condition during the process of the cascaded TNSA. The scheme opens a way to scale proton energy lineally with laser energy.

Upgrade of Langmuir probe diagnostic in ITER-like tungsten mono-block divertor on experimental advanced superconducting tokamak

In order to withstand rapid increase in particle and power impact onto the divertor and demonstrate the feasibility of the ITER design under long pulse operation, the upper divertor of the EAST tokamak has been upgraded to actively water-cooled, ITER-like tungsten mono-block structure since the 2014 campaign, which is the first attempt for ITER on the tokamak devices. Therefore, a new divertor Langmuir probe diagnostic system (DivLP) was designed and successfully upgraded on the tungsten divertor to obtain the plasma parameters in the divertor region such as electron temperature, electron density, particle and heat fluxes. More specifically, two identical triple probe arrays have been installed at two ports of different toroidal positions (112.5-deg separated toroidally), which can provide fundamental data to study the toroidal asymmetry of divertor power deposition and related 3-dimension (3D) physics, as induced by resonant magnetic perturbations, lower hybrid wave, and so on. The shape of graphite tip and fixed structure of the probe are designed according to the structure of the upper tungsten divertor. The ceramic support, small graphite tip, and proper connector installed make it possible to be successfully installed in the very narrow interval between the cassette body and tungsten mono-block, i.e., 13.5 mm. It was demonstrated during the 2014 and 2015 commissioning campaigns that the newly upgraded divertor Langmuir probe diagnostic system is successful. Representative experimental data are given and discussed for the DivLP measurements, then proving its availability and reliability.

Cascaded proton acceleration by collisionless electrostatic shock

A new scheme for proton acceleration by cascaded collisionless electrostatic shock (CES) is proposed. By irradiating a foil target with a moderate high-intensity laser beam, a stable CES field can be induced, which is employed as the accelerating field for the booster stage of proton acceleration. The mechanism is studied through simulations and theoretical analysis, showing that a 55 MeV seed proton beam can be further accelerated to 265 MeV while keeping a good energy spread. This scheme offers a feasible approach to produce proton beams with energy of hundreds of MeV by existing available high-intensity laser facilities.

Broad range energy absorption enabled by hydrogenated TiO2 nanosheets: from optical to infrared and microwave

Efficient energy harvesting is critical in developing various future clean energy sources and technologies from our ultimate clean energy source – the Sun, which covers a broad range of photon energies ranging from ultraviolet, to visible, infrared, and microwave regions. Absorption is the first key step in the uptake of solar energy for various energy conversions and utilizations. Materials with broad-range electromagnetic interaction are therefore highly desirable. Here, we demonstrate that such broad-range energy absorption from visible light to microwave regions can be achieved with hydrogenated TiO2 nanosheets. A large near-infrared and visible-light absorption (>60%), a broad mid-IR absorption, and a highly efficient absorption in the microwave region have been obtained with hydrogenated TiO2 nanosheets. In contrast, barely any absorption is observed for pristine TiO2 nanosheets in these regions. Therefore, this study shows that with such high absorption across such a broad energy range, hydrogenated TiO2 nanosheets obviously have a large capability of absorbing solar energy across a broad energy region, which can be potentially useful for various photo, photoelectric, photochemical applications, such as semiconductor devices, photocatalysis, photovoltaics, infrared detection, microwave communication, etc.

Angular distribution of emitted electrons due to intense p-polarized laser foil interaction

The angular distribution of electrons emitting from a foil surface illuminated by p -polarized laser pulses is studied using particle-in-cell simulation for incident angles of θ 1 = 22.5 ° , 45 ° , 67.5 ° and laser amplitudes of a = 0.5 , 1, 2. Theoretical prediction of the emission direction, based on canonical momentum conservation along the target surface, is verified. Surface ablation, the Alfven current limit, as well as self-generated electromagnetic fields on the surface are numerically investigated and found to play important roles in the modulation of the angular distribution of the emitted electrons. The emitted electrons of higher energy are found to be directly accelerated to near the polarization direction of the incident laser light. The simulation results agree very well with the recent experimental results from Al targets irradiated by a 60 fs, 180 mJ laser pulse.