66 research outputs found
A new quasilinear model for turbulent momentum transport in tokamaks with flow shear and plasma shaping
In tokamak experiments, sufficiently strong flow shear reduces
turbulent transport, thereby improving the prospects for fusion power plants.
It is therefore of great importance to efficiently explore parameter space to
find where strong plasma flow can be achieved. To this end, we propose a new,
physically motivated quasi-linear model for estimating momentum transport from
turbulence in the presence of toroidal flow shear and plasma shaping. The
method gives good estimates of momentum transport for up-down asymmetric
geometries as well as low magnetic shear and tight aspect ratio. The results
are benchmarked with high-fidelity nonlinear GENE simulations, demonstrating
that it provides a fast and accurate estimate of momentum transport.Comment: 34 pages, 17 figure
Physical Regimes of Electrostatic Wave-Wave nonlinear interactions generated by an Electron Beam Propagation in Background Plasma
Electron-beam plasma interaction has long been a topic of great interest. The
validities of Quasi-Linear (QL) theory and Weak Turbulence (WT) theory are
limited by the requirement of sufficiently dense mode spectrum and small wave
amplitude. In this paper, by performing a large number of high resolution
two-dimensional (2D) particle-in-cell (PIC) simulations and using analytical
theories, we extensively studied the collective processes of a mono-energetic
electron beam emitted from a thermionic cathode propagating through a cold
plasma. We show that initial two-stream instability between the beam and
background cold electrons is saturated by wave trapping rather than QL theory.
Further evolution occurs due to strong wave-wave nonlinear processes. We show
that the beam-plasma interaction can be classified into four different physical
regimes in the parameter space for the plasma and beam parameters. The
differences between the different regimes are analyzed in detail. For the first
time, we identified a new regime in strong Langmuir turbulence featured by what
we call Electron Modulational Instability (EMI) that creates a local Langmuir
wave packet faster than ion frequency ({\omega}_pi) and ions initially do not
respond to EMI in the initial growing stage. On a longer timescale, the action
of the ponderomotive force produces very strong ion density perturbations so
that the beam-plasma wave interaction stops being resonant. Consequently, in
this EMI regime beam-plasma interaction is a periodic burst (intermittent)
process. The beams are strongly scattered, and the Langmuir wave spectrum is
significantly broadened, which gives rise to the strong heating of bulk
electrons. Some interesting phenomena in the strong turbulent regime are also
discussedComment: 65 pages, 19 figure
Electron Modulation Instability in the Strong Turbulent Regime for Electron Beam Propagation in Background Plasma
We study collective processes for an electron beam propagating through a
background plasma using simulations and analytical theory. A new regime where
the instability of a Langmuir wave packet can grow locally much faster than ion
frequency ({\omega}_pi) is clearly identified. The key feature of this new
regime is an Electron Modulational Instability that rapidly creates a local
Langmuir wave packet, which in its turn produces local charge separation and
strong ion density perturbations because of the action of the ponderomotive
force, such that the beam-plasma wave interaction stops being resonant. Three
evolution stages of the process and observed periodic burst features are
discussed. Different physical regimes in the plasma and beam parameter space
are clearly demonstrated for the first time.Comment: 19 pages, 3 figure
Fluidisable mesoporous silica composites for thermochemical energy storage
Salt hydrate based thermochemical energy storage has been widely recognised as a promising long-duration storage technology to decarbonize heating/cooling in buildings.However, currently there are few salt hydrate-based energy storage materials capable to fulfil the requirements for energy density, efficiency, scalability and stability due to inappropriate particle size of the material. In this study, a commercially available mesoporous silica with large pore volume and good fluidisability was used as the porous matrix to prepare salt composites containing different salts (CaBr2, MgBr2, MgSO4, CaCl2, and Al(NH4)(SO4)2) via a facile incipient wetness impregnation method. A variety of techniques including scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray powder diffraction (XRD), nitrogen physisorption measurements and thermogravimetric analysis (TGA) were used to characterize the physicochemical properties and water hydration/dehydration performance of mesoporous silica-based salt composites. The results showed that both salt loading level and salt type play critical roles in determining the water adsorption performance of salt composites. Tested under hydration conditions of 30°C and vapour pressure of 25mbar, the CaCl2 based salt composites exhibited the highest water adsorption capacity, which reached 109 wt% at the CaCl2 loading level of 50wt%, while the MgBr2 based salt composites had faster water adsorption rates than other salt composites. Most of the salt composites were capable of desorbing 70–80% of the adsorbed water at temperatures below 90°C, highlighting their great potential to store low-grade heat such as industrial waste heat or solar thermal energy. Advanced characterization demonstrated that the large pore volume and pore size improved the salt molecules' accessibility and water diffusivity inside the pores, leading to high water adsorption capacity and fast hydration/dehydration rate. In the aspects of particle size for future upscaling, this work presents an all new scalable and fluidisable salt composite material that opens up the potential to develop low-temperature fluidised bed based thermal energy storage systems for the first time
Direct Implicit and Explicit Energy-Conserving Particle-in-Cell Methods for Modeling of Capacitively-Coupled Plasma Devices
Achieving entire large scale kinetic modelling is a crucial task for the
development and optimization of modern plasma devices. With the trend of
decreasing pressure in applications such as plasma etching, kinetic simulations
are necessary to self-consistently capture the particle dynamics. The standard,
explicit, electrostatic, momentum-conserving Particle-In-Cell method suffers
from tight stability constraints to resolve the electron plasma length (i.e.
Debye length) and time scales (i.e. plasma period). This results in very high
computational cost, making this technique generally prohibitive for the large
volume entire device modeling (EDM). We explore the Direct Implicit algorithm
and the explicit Energy Conserving algorithm as alternatives to the standard
approach, which can reduce computational cost with minimal (or controllable)
impact on results. These algorithms are implemented into the well-tested
EDIPIC-2D and LTP-PIC codes, and their performance is evaluated by testing on a
2D capacitively coupled plasma discharge scenario. The investigation revels
that both approaches enable the utilization of cell sizes larger than the Debye
length, resulting in reduced runtime, while incurring only a minor compromise
in accuracy. The methods also allow for time steps larger than the electron
plasma period, however this can lead to numerical heating or cooling. The study
further demonstrates that by appropriately adjusting the ratio of cell size to
time step, it is possible to mitigate this effect to acceptable level
Dynamic Changes in Flavor Quality of Wuyi Rock Tea at Different Storage Times
Seven samples of Wuyi rock tea (cv. Shuixian) stored for different times (0, 5, 10, 15, 20, 25, and 30 years) were collected to investigate the effect of storage time on the flavor quality of Wuyi rock tea by the combined use of sensory evaluation, non-volatile and volatile composition analysis and chemometrics methods. The results showed that the transformation of the flavor quality of Wuyi rock tea during storage was divided into three stages: 0–5, 10–15, and 20–30 years. As the storage time increased, the taste gradually transformed from mellow to stale, and a sour taste appeared at the middle stage of storage and then faded away at the late stage. The aroma gradually changed from flowery and fruity to aged, woody and herbal. A significant decrease in the content of tea polyphenols, catechins-like and theaflavins and an increase in the content of soluble sugars were the major reasons for the taste changes. Changes in the contents of 15 characteristic volatiles, including indole, trans-nerolidol, dihydroactinidiolide, hotrienol, α-terpineol, methyl salicylate, β-ionone, and (Z)-hexanoic acid-3-hexenyl ester, were the key factors affecting the aroma changes. This study provides a scientific reference for the storage and consumption of Wuyi rock tea
Analysis of Flavor Characteristics and Characteristic Components of White Tea Made from Oolong Tea Cultivars
In order to investigate the differences in flavor quality between white tea made from Oolong tea cultivars and traditional white tea, white teas made from eight Oolong tea cultivars such as Zimeigui and Fuding Dahao white tea as a control were analyzed by sensory evaluation, biochemical assays and multivariate statistical analysis. The results showed that the appearance and infusion color of Oolong white tea were darker, while the taste and aroma were better than those of traditional white tea. The biochemical analysis revealed that the differences in conductivity, pH, and the contents of soluble sugars, free amino acids, gallocatechin gallate (GCG) and epigallocatechin gallate (EGCG) were important factors causing the differences in taste between traditional white tea and white tea made from Oolong tea cultivars. Volatile composition analysis showed that trans-2-nonenal, cis-3-nonen-1-ol, methyl palmitate, linalool, methyl linoleate, cedrol, geranyl formate, phenethyl alcohol, nerolidol, methyl salicylate, dibutyl phthalate and phytone were the key differential aroma components contributing to the difference in aroma between Oolong and traditional white tea. Findings from this study will provide a theoretical reference for flavor diversification of white tea
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