ABSTRACT
Background: The Mekong River is the 10th largest river in the world. It is recognised as the most productive river in Southeast Asia and economically essential to the region, with an estimated 60-65 million people living in the lower Mekong Basin. The Mekong Delta within Vietnam is considered a highly vulnerable ecosystem under threat from increasing anthropogenic pressure, such as dam construction and, as a consequence, the Delta is sinking and altering the natural hydrological cycle. Dams also lead to eutrophication and pollution of downstream water from regulated water flux and water stagnation. Another threat is climate change coupled with the lower rainfall, which could lead to an increased risk of drought in the Mekong Delta Basin. Thus, these project data represent an important baseline reference. The ecological health of the Mekong Delta's environment, as indicated by the quality and availability of its water and biological resources, largely determines the economic and social development of the region, which produces about half of the agriculture and aquaculture products of Vietnam. New information: This paper reports quantitative data on the biodiversity of six groups of aquatic organisms: bottom and pelagic fish, macrozoobenthos, microorganisms, phyto- and zooplankton in the Mekong Delta within Vietnam, as well as data on the physicochemical parameters of water and bottom sediments. The data were collected during 2018-2022 as part of the Ecolan E-3.4 programme within the framework of the research plan of the Joint Russian-Vietnamese Tropical Research and Technological Center. All presented datasets are published for the first time.
ABSTRACT
We discovered a specific property of the eigenmodes of a cylindrical waveguide, due to which efficient mode excitation at ultrahigh (s=4n+1, n is integer) multiples of the gyrofrequency can be provided by a polyhelical weakly relativistic electron beam, standardly used for gyrotron operation. In the proof-of-principle experiment with a V-band gyrotron driven by a 25-keV, 2-A beam, about 100 mW radiation power at the fifth cyclotron harmonic (0.22 THz) has been detected in the cw regime. Based on the gyrotron theory, we demonstrate in simulations that, using parameters of existing gyrotrons, this method can be applied for producing cw radiation at a power level of up to several watts in the terahertz gap.
ABSTRACT
Many state-of-the-art fundamental and industrial projects need the use of terahertz radiation with high power and small linewidth. Gyrotrons as radiation sources provide the desired level of power in the sub-THz and THz frequency range, but have substantial free-running frequency fluctuations of the order of 10-4. Here, we demonstrate that the precise frequency stability of a high-power sub-THz gyrotron can be achieved by a phase-lock loop in the anode voltage control. The relative width of the frequency spectrum and the frequency stability obtained for a 0.263 THz/100 W gyrotron are 4 × 10-12 and 10-10, respectively, and these parameters are better than those demonstrated so far with high-power sources by almost three orders of magnitude. This approach confirms its potential for ultra-high precision spectroscopy, the development of sources with large-scale radiating apertures, and other new projects.
