Hongbin Ding

System-dependent melting behavior of icosahedral anti-Mackay nanoalloys

Anti-Mackay icosahedral clusters of composition Ag32M13, where M is either Cu, Ni, or Co, have been recently shown to possess special structural stability both by calculations and experiments. These nanoalloys assume a core–shell arrangement, with an icosahedral core of 13 M atoms surrounded by an Ag shell of anti-Mackay structure. In this paper we study the melting of these three nanoalloys, showing that, despite the close similarity of the structures, melting takes place through quite different mechanisms. In particular, we find that Ag32Co13 and Ag32Ni13 present a premelting phenomenon which involves only the shell of the cluster while the core melts at higher temperatures, in agreement with previous calculations. On the contrary, in Ag32Cu13, melting occurs through stages that involve the shell and the core at the same time. These findings are rationalized in terms of the different features of the energy landscape of these nanoalloys. Our simulations, in which special care has been devoted to avoid non-ergodicity problems, show also that the particles keep their core–shell structures even in the liquid phase, indicating an incomplete miscibility of Ag with Ni, Co or Cu at the nanoscale up to quite high temperatures.

High spatial resolution mapping of deposition layers on plasma facing materials by laser ablation microprobe time-of-flight mass spectroscopy

A laser ablation microprobe time-of-flight mass spectroscopy (LAM-TOF-MS) system with high spatial resolution, ~20 nm in depth and ~500 μm or better on the surface, is developed to analyze the composition distributions of deposition layers on the first wall materials or first mirrors in tokamak. The LAM-TOF-MS system consists of a laser ablation microprobe combined with a TOF-MS and a data acquisition system based on a LabVIEW program software package. Laser induced ablation combined with TOF-MS is an attractive method to analyze the depth profile of deposited layer with successive laser shots, therefore, it can provide information for composition reconstruction of the plasma wall interaction process. In this work, we demonstrate that the LAM-TOF-MS system is capable of characterizing the depth profile as well as mapping 2D composition of deposited film on the molybdenum first mirror retrieved from HL-2A tokamak, with particular emphasis on some of the species produced during the ablation process. The presented LAM-TOF-MS system provides not only the 3D characterization of deposition but also the removal efficiency of species of concern.

Laser-based diagnostics applications for plasma-surface interaction studies

Several laser based diagnostics are implemented on to the linear plasma generator Magnum-PSI, wherein ITER divertor relevant plasma-wall conditions are realized. Laser Induced Desorption Quadrupole Mass Spectroscopy (LID-QMS) and Laser Induced Breakdown Spectroscopy (LIBS) are installed to measure deuterium retention in plasma facing components. Combined with Thermal Desorption Spectroscopy, LID-QMS can be used to measure lateral retention profiles. LIBS is used to measure the surface composition qualitatively, after plasma exposure. An advanced Thomson Scattering (TS) system measures electron density, neutral density and electron temperature profiles (spatial resolution < 2 mm) across the maximum 100 mm plasma diameter. Very low electron density (9 × 1018 m−3) can be measured within seconds with accuracies better than 6%. The minimum measurable electron density and temperature are ~ 1 × 1017 m−3 and ~ 0.07 eV, respectively. By virtue of the high system sensitivity, single pulse TS can be performed on high density pulsed plasmas (used for replicating ELMs). For measuring the ion temperature and flow velocity of the plasma a Collective TS system (CTS) is being built: the small Debye length of the Magnum-PSI plasma enables application of this method at relatively short laser wavelength. In a feasibility study it was shown that forward CTS with a seeded Nd:YAG laser operating at 1064 nm, can be applied at Magnum-PSI to measure ion temperature and axial velocity with an accuracy of < 8% and < 15%, respectively. Two high spectral resolution ( ~ 0.005 nm) detection schemes are applied simultaneously: an Echelle grating spectrometer (enabling profile measurements) and a system based on a Fabry-Perot etalon that enables wavelength scanning over its free spectral range, by tilting the device. The status and performance of the various laser based plasma and surface diagnostics will be reported along with experimental results.

Laser mass-spectrometry for online diagnosis of reactive plasmas with many species

The purpose of this study is to design a diagnostic system for reactive plasma environment by combining molecular-beam time-of-flight (TOF) mass spectroscopy with laser spectroscopy technique. The combination of TOF mass spectrometers and pulsed lasers is favorable in the diagnosis of intermediate species distribution since they allow the simultaneous but separate recording of the spectra of different species. In the plasma system, the intermediate species in electronic ground state or low lying excited state is pumped to higher energy level with resonant laser excitation, and then, the ionization with a second laser system is possible which can readily be detected by the TOF analyzer. The ionization itself is only used as a detection mechanism for the observation of the excitation of these states. In this manner, the population distribution of intermediate species can be determined with state-selective and mass-selective feature. Also, in this article, a flexible data acquisition and automatic control system based on LABVIEW was designed to integrate all the stand-alone measurement instruments including a TOF spectrometer, a laser system, a high performance oscilloscope, and a digital delay generator into a single personal computer-based control unit. Moreover, a virtual Boxcar integrator with hundreds of channels has been developed to enhance the signal while filtering out the random noises. Finally, the many potentials of this technique in the application of plasma diagnosis will be discussed.

High sensitive and high temporal and spatial resolved image of reactive species in atmospheric pressure surface discharge reactor by laser induced fluorescence

The current paucity of spatial and temporal characterization of reactive oxygen and nitrogen species (RONS) concentration has been a major hurdle to the advancement and clinical translation of low temperature atmospheric plasmas. In this study, an advanced laser induced fluorescence (LIF) system has been developed to be an effective antibacterial surface discharge reactor for the diagnosis of RONS, where the highest spatial and temporal resolution of the LIF system has been achieved to ∼100 μm scale and ∼20 ns scale, respectively. Measurements on an oxidative OH radical have been carried out as typical RONS for the benchmark of the whole LIF system, where absolute number density calibration has been performed on the basis of the laser Rayleigh scattering method. Requirements for pixel resolved spatial distribution and outer plasma region detection become challenging tasks due to the low RONS concentration (∼ppb level) and strong interference, especially the discharge induced emission and pulsed laser induced stray light. In order to design the highly sensitive LIF system, a self-developed fluorescence telescope, the optimization of high precision synchronization among a tunable pulsed laser, a surface discharge generator, intensified Charge Coupled Device (iCCD) camera, and an oscilloscope have been performed. Moreover, an image BOXCAR approach has been developed to remarkably improve the sensitivity of the whole LIF system by optimizing spatial and temporal gating functions via both hardware and software, which has been integrated into our automatic control and data acquisition system on the LabVIEW platform. In addition, a reciprocation averaging measurement has been applied to verify the accuracy of the whole LIF detecting system, indicating the relative standard deviation of ∼3%.

Evolution of metastable state molecules N2(A3 Σu+) in a nanosecond pulsed discharge: A particle-in-cell/Monte Carlo collisions simulation

A particle-in-cell plus Monte Carlo collisions method has been employed to investigate the nitrogen discharge driven by a nanosecond pulse power source. To assess whether the production of the metastable state N2(A3 Σu+) can be efficiently enhanced in a nanosecond pulsed discharge, the evolutions of metastable state N2(A3 Σu+) density and electron energy distribution function have been examined in detail. The simulation results indicate that the ultra short pulse can modulate the electron energy effectively: during the early pulse-on time, high energy electrons give rise to quick electron avalanche and rapid growth of the metastable state N2(A3 Σu+) density. It is estimated that for a single pulse with amplitude of −9 kV and pulse width 30 ns, the metastable state N2(A3 Σu+) density can achieve a value in the order of 109 cm−3. The N2(A3 Σu+) density at such a value could be easily detected by laser-based experimental methods.

Enhancement of Laser-Induced Breakdown Spectroscopic Signals in a Liquid Jet with Glow Discharge:

In this work, emission signals of laser-induced breakdown (LIBS) plasma of a flowing liquid jet in the absence and presence of an air-supported glow discharge have been investigated. In combination with a needle-to-needle glow discharge, a Q-switched neodymium-doped yttrium aluminum garnet (Nd:YAG) laser (1064 nm, 5 ns) with power density ∼1010 W/cm2 was used to generate a plasma from a liquid jet. Emission lines of Mg, Ca, Al, Li, Na, and K all showed significant enhancements in the presence of the glow discharge. Lower continuum background was also observed. Mechanisms of the line emission enhancement and continuum radiation reduction were discussed.

Laser-induced breakdown spectroscopy for Wendelstein 7-X stellarator limiter tile analysis

Laser-induced breakdown spectroscopy (LIBS) is a well-established elemental composition analysis method as well as one of the most promising candidates for in situ first wall diagnosis of fusion devices. In this work, limiter graphite tiles, which were exposed in the initial operational phase (OP1.1) of the Wendelstein 7-X stellarator to He and H plasma, are analyzed ex situ by LIBS employing a picosecond pulsed laser for the first time and compared with post mortem analysis techniques. Depth profiles of each element and 2D profile of the ratio of H and C atoms on the surface are investigated. Both H content and retention depth on the deposition dominated zone are higher than on the erosion dominated zone due to the formation of C-H co-deposition layer. The results from LIBS are in agreement with those from the cross-sectional scanning electron microscopic image and electron dispersive x-ray spectroscopy.

Average OH density in alternating current dielectric barrier discharge by laser-induced fluorescence technique

The average OH density in atmospheric He–H2O(0.4%) needle-plate dielectric barrier discharge (DBD) was measured by the asynchronous laser-induced fluorescence (LIF) technique and the fluctuation of OH radical density was measured simultaneously to prove that the average OH density can be obtained by the asynchronous LIF technique. The evolution of the average OH density in four different discharge patterns, namely, negative barrier corona discharge, glow discharge, multi glow discharge, and streamer discharge, was studied, and it was found that the average OH density has an observable increase from corona discharge to streamer discharge. The main mechanism of OH production in the four different discharge patterns was analyzed. It was shown that the main mechanism of OH production in negative barrier corona discharge is electron direct collision dissociation, whereas in the other three discharge patterns the He metastable Penning ionization is the main process.

Remote in situ laser-induced breakdown spectroscopic approach for diagnosis of the plasma facing components on experimental advanced superconducting tokamak

The diagnosis of the fuel retention and impurity deposition on the plasma facing components (PFCs) is very important for monitoring plasma-wall interactions and improving the performance of long-pulse operation for tokamak devices. In this study, a remote in situ laser-induced breakdown spectroscopic (RIS-LIBS) system has been developed to be an effective and routine method for the diagnosis of the composition of the PFCs on Experimental Advanced Superconducting Tokamak (EAST). The RIS-LIBS system can be operated between EAST discharges via a remote network control system. This allows a flexible diagnosis for the PFCs at a specific EAST discharge operation or under planned plasma scenarios according to the experimental requirement. Measurements on the fuel retention and impurity deposition of the PFCs have been performed for the test of the RIS-LIBS system, and the depth resolution and the lateral resolution of the RIS-LIBS system have been achieved to be ∼100 nm and ∼3.0 mm, respectively. For the test of detectable elements, the fuel (deuterium) and impurities have been detected and identified clearly. In addition, the measurement of fuel abundance on the first wall as a function of the days of EAST deuterium plasma discharges has been carried out for the first time. These results well manifest a significant prospect of the RIS-LIBS for the diagnosis of the PFCs in the upcoming fusion devices like China Fusion Engineering Test Reactor (CFETR) and ITER.