Vietnam Academy of Science and Technology: Journals Online
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Amplification of Ultrashort Titan-sapphire Laser Pulses Using Chirped-pulse Amplification Technique
Get PDFWe report on a Chirped Pulse Amplification (CPA)-based Titanium:Sapphire (Ti:Al2O3 or Ti:Sapphire) amplifier that uses a single grating stretcher and compressor configuration. The 8-pass amplifier configuration is able to yield 70 μJ pulse energy at 10 Hz repetition rate, which corresponds to a gain of 7000 times. The amplified pulses are expected to be transform-limited with pulse duration of 85 fs. This amplifier will expand the application of our Ti:Sapphire laser at the Vietnam Academy of Sciences (VAST)
New record of the genus Brulleia Szépligeti, 1904 ( Hymenoptera: Braconidae: Helconinae ), with description of three new species from VietNam
Get PDFThe genus Brulleia Szépligeti, 1904 was recorded for the first time in Vietnam, three new species, namely Brulleia flavosoma Long, sp. n.; B. mellicrus Long, sp. n. and B. nigrisoma Long, sp. n., were descripbed and illustrated. A key to Brulleia species from Vietnam is provided. The checklist of the already known species of the genus Brulleia is also given
Inverse dynamic analysis of milling machining robot, application in calibration of cutting force
Get PDFThis article presents analysis of inverse dynamics of serial manipulators in milling process. Cutting forces and complicated motion involve to difficulties in solving dynamics problems of robots. In general, cutting forces are determined by using empirical formulas that lead to errors of cutting force values. Moreover, the cutting forces are changing and causing vibration during machining process. Errors of cutting force values affect to the accuracy of the dynamic model. This paper proposes an algorithm to compute the cutting forces based on the feedback values of the robot's motion.
New records and morphological assessments of long-nosed fruit bats (chiroptera: pteropodidae: Macroglossus spp.) from Vietnam
Get PDFLong-nosed fruit bat is a common name of the genus Macroglossus which comprises two species: Dagger-toothed long-nosed fruit bat (M. minimus) and Greater long-nosed fruit bat (M. sobrinus). These two species were rarely recorded from Vietnam or neighboring countries. Within Vietnam, M. minimus has been recorded only from two localities in southern Vietnam while M. sobrinus was known from all northern, central and southern regions of the country. Morphological features of these species in Vietnam were poorly documented in previous publications. With results from a rapid examination of all specimens and recently captured individuals, we here confirm that M. sobrinus is distinctively larger than M. minimus in all external and craniodental measurements. Two species are also distinguishable by their nostril shapes and mandible symphyses. This paper provides new distributional records of both M. sobrinus and M. minimus from Vietnam with remarks on their ecology and habitats.
Multichannel Photon Counting Lidar Measurements Using USB-based Digital Storage Oscilloscope
Get PDFWe present a simple method of making multichannel photon counting measurements of weak lidar signal from large ranges, using commonly available USB-based digital storage oscilloscopes. The single photon pulses from compact photomultiplier tubes are amplified and stretched so that the pulses are large and broad enough to be sampled efficiently by the USB oscilloscopes. A software interface written in Labview is then used to count the number of photon pulses in each of the prescribed time bins to form the histogram of LIDAR signal. This method presents a flexible alternative to the modular multichannel scalers and facilitate the development of sensitive lidar systems
Real-Space Approach for the Electronic Calculation of Twisted Bilayer Graphene Using the Orthogonal Polynomial Technique
Get PDFWe discuss technical issues involving the implementation of a computational method for the electronic structure of material systems of arbitrary atomic arrangement. The method is based on the analysis of time evolution of electron states in the real lattice space. The Chebyshev polynomials of the first kind are used to approximate the time evolution operator. We demonstrate that the developed method is powerful and efficient since the computational scaling law is linear. We invoked the method to study the electronic properties of special twisted bilayer graphene whose atomic structure is quasi-crystalline. We show the density of states of an electron in this graphene system as well as the variation of the associated time auto-correlation function. We find the fluctuation of electron density on the lattice nodes forming a typical pattern closely related to the typical atomic pattern of the quasi-crystalline bilayer graphene configuration
Numerical solution of the problems for plates on some complex partial internal supports
Get PDFIn the recent works, Dang and Truong proposed an iterative method for solving some problems of plates on one, two and three line partial internal supports (LPISs), and a cross internal support. In nature they are problems with strongly mixed boundary conditions for biharmonic equation. For this reason the method combines a domain decomposition technique with the reduction of the order of the equation from four to two. In this study, the method is developed for plates on internal supports of more complex configurations. Namely, we examine the cases of symmetric rectangular and H-shape supports, where the computational domain after reducing to the first quadrant of the plate is divided into three subdomains. Also, we consider the case of asymmetric rectangular support where the computational domain needs to be divided into 9 subdomains. The problems under consideration are reduced to sequences of weak mixed boundary value problems for the Poisson equation, which are solved by difference method. The performed numerical experiments show the effectiveness of the iterative method
Some scientific issues related to biological resource use and management toward sustainable development of marine economy in the costal provinces in South Central Vietnam
No full textBased on referring recent target scientific issues related to sustainable development in the world and considering practices of economic development in the provinces of South Central Vietnam, some issues were selected for analyses in terms of concepts, needs and orientations applicable to the region. They include, inter alia: Economic valuation of marine ecosystems; Important levels of habitats in coastal ecosystems; Rehabilitation and restoration of degraded resources; Natural hazards - prevention and resilience; Marine protected areas and fisheries enhancement; Deoxygenation in marine environment and economic losses in fisheries
Coral reef fishes in the banks and rocky islands in the offshore waters of Binh Thuan province
Get PDFBinh Thuan province is located in the South Central Coast of Vietnam with favorable conditions for living coral reefs. The coral reefs were mainly investigated in coastal water areas and Phu Quy island, meanwhile other areas such as the banks and rocky islands seem to lack information. This study was conducted to evaluate the species composition and distribution of coral reef fish communities in the Royal Bishop banks and around rocky islands in Julia shoal in the offshore waters of Binh Thuan province in April 2019. A total of 151 species belonging to 86 genera and 33 families of coral reef fishes were recorded. Among them, 4 species for the first time recorded in Vietnam are Helcogramma striata (Hansen, 1986) (Tripterygiidae), Pseudanthias squamipinnis (Peters, 1855) (Serranidae), Bodianus bilunulatus (Lacepède, 1801) and Coris aygula (Lacepède, 1801) (Labridae). The total average density of coral reef fish in the offshore waters of Binh Thuan is quite high at 1,104.9 ± 617 individuals/250 m2, most of them are small sized fish and ornamental fish group, the large sized fish and target fish group accounts for a relatively low rate, but their density here is much higher compared to that in the coastal waters of Binh Thuan province. The comparative analysis also shows that sites in coral reef around rocky islands have species richness and density of coral reef fish higher than the bank area
Ciguatoxin-like toxin in mackerel Acanthocybium soladria in Vietnam
Get PDFTwo processed mackerel products were sent to VAST keylab on “Food and Environmental Safety” in Institute of Oceanography for Ciguatoxin criteria as exported seafood requirement. The specimens were caught from offshore region of Vietnam South-Eastern sea in April 2019. By DNA sequencing of COI gene, both specimens were identified as Acanthocybium solandri, one of high commercial mackerel species which is well-known to be distributed in Central and South-Eastern sea of Vietnam. In mouse bioassay, both extracts from 2 samples exhibited acute toxicity, which killed tested mice within 6 and 15 hours after i.p. injection. In SIM LC/MS-MS analysis, the extracts showed the peak with retention time closed to that in CTX-1B standard in both cases of optimized and non-optimized solid phases. The result suggested that at least there was occurrence of CTX-like toxin in these mackerel specimens. The equivalent toxin levels were estimated as 0.51 ng/g and 0.84 ng/g; 5 and 8 times higher than regulation limit for CTXs in seafood product (0.1 ng CTX-1B/g). Further verification is in progress to confirm toxin in these mackerel samples in Vietnam. However, this data is the first alert on potential poisoning by consumption of mackerel, not only in Vietnam but also in Asia