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Aerospace aircraft has significantly improved the life quality of human beings and extended the capability of space explosion since its appearance in 1903, in which liquid propellants or fuels provide the key power source. For jet fuels, its property of energy density plays an important role in determining the flight range, load, and performance of the aircraft. Therefore, the design and fabrication of high-energy-density (HED) fuels attract more and more attention from researchers all over the world. Herein, we briefly introduce the development of liquid jet fuels and HED fuels and demonstrate the future development of HED fuels. To further improve the energy density of fuel, the approaches of design and construction of multi-cyclic and stained molecule structures are proposed. To break through the density limit of hydrocarbon fuels, the addition of energetic nanoparticles in HED fuels to produce nanofluid or gelled fuels may provide a facile and effective method to significantly increase the energy density. This work provides the perspective for the development of HED fuels for advanced aircrafts.

Collapse of the edge flow shear as the line-averaged density approaches the Greenwald density limit has been observed as a precursor to the enhanced edge particle flux characteristic of proximity to the density limit regime. Here, we report the use of a biased electrode to sustain the edge shear layer in high density discharges, in which the shear layer would otherwise collapse. A stable increase in line-averaged density is observed along with a strong increase in edge density. These experiments were carried out on the J-TEXT tokamak. The Reynolds stress at the edge is enhanced, and the zonal flow sustained, while density perturbation levels, the flux of turbulence internal energy (i.e., turbulence spreading), and particle and heat flux all decrease significantly. Electron adiabaticity increases, and bias voltage modulation experiments show that an increase in the edge shear leads the increase in adiabaticity. These results suggest that external edge E × B flow shear drive may be of interest for sustaining edge plasma states at high density, and support the hypothesis that collapse of the edge shear layer triggers the onset of the strong transport and turbulence characteristic of the density limit regime.

The overall performance of a tokamak strongly depends on phenomena that take place in a thin region between the main plasma and the vessel wall, which is denoted as tokamak boundary. In fact, the formation of transport barriers in this region can significantly improve plasma confinement and, therefore, the tokamak fusion performance. In addition, the tokamak boundary determines the peak heat flux to the wall, an essential quantity for the design and the operation of fusion power plants, as well as the level of impurities in the core, the removal of fusion ash and the dynamics of neutral particles. The dynamics in the plasma boundary is strongly nonlinear and characterized by a wide range of length and time scales as well as by a complex magnetic field geometry that may feature one or more nulls of the poloidal magnetic field. Large-scale, three-dimensional turbulence simulations are therefore often required to disentangle the complex physical mechanisms that govern this region. The thesis is focused on the analysis of the different turbulent transport regimes present in the plasma boundary as they appear from three-dimensional, flux-driven, global, two-fluid turbulence simulations carried out by using the GBS code, which is significantly extended here to allow the self-consistent simulation of the plasma dynamics coupled to a kinetic single-species neutral model in arbitrarily complex magnetic geometries. Considering single-null magnetic configurations, three turbulent transport regimes are identified: (i) a regime of suppressed turbulent transport at low values of collisionality and large values of heat source, (ii) a regime of developed turbulent transport at intermediate values of collisionality and heat source, and (iii) a regime of very large turbulent transport at high value of collisionality and density, which can be associated to the crossing of the density limit. By leveraging the results of GBS simulations, theory-based scaling laws of the pressure and density decay lengths in the near and far scrape-off layer are derived in the developed transport regime from a balance among heat source, turbulent transport across the separatrix and parallel losses at the vessel wall. The theoretical scaling of the pressure decay length in the near scrape-off layer is successfully validated against a multi-machine database of SOL width measurements at the outer target. By carefully analysing the transition to the regime of large turbulent transport, we show that the density limit can be explained by an enhancement of turbulent transport at the tokamak boundary when the density increases. This analysis leads to a theory-based scaling law of the maximum edge density achievable in tokamaks, which is in better agreement with a multi-machine database than the widely used Greenwald empirical scaling, thus significantly improving our understanding and predictive capability of the density limit, with important implications for the design and the operation of future fusion power plants. The thesis concludes by presenting the first turbulent simulations carried out in various snowflake magnetic configurations, which are used to investigate the effect of turbulence and equilibrium flow on the heat flux distribution among the different strike points.

Enhanced particle transport events are discovered and analyzed as the density limit of the J-TEXT tokamak is approached. Edge shear layer collapse is observed and the ratio of Reynolds power to turbulence production decreases. Simultaneously, the divergence of turbulence internal energy flux (i.e. turbulence spreading) increases, indicating that shear layer collapse triggers an outward spreading event. Studies of correlations show that the enhanced particle transport events are quasi-coherent, and manifested primarily in density fluctuations which exhibit positive skewness. Electron adiabaticity emerges as the critical parameter which signals transport event onset. For α < 0.35 as density approaches the Greenwald density, both turbulence spreading and density fluctuations rise rapidly. Taken together, these results elucidate the connections between edge shear layer, density fluctuations, particle transport events, turbulence spreading and plasma edge cooling as the density limit is approached.

The optimized superconducting stellarator device Wendelstein 7-X (with major radius , minor radius , and plasma volume) restarted operation after the assembly of a graphite heat shield and 10 inertially cooled island divertor modules. This paper reports on the results from the first high-performance plasma operation. Glow discharge conditioning and ECRH conditioning discharges in helium turned out to be important for density and edge radiation control. Plasma densities of with central electron temperatures were routinely achieved with hydrogen gas fueling, frequently terminated by a radiative collapse. In a first stage, plasma densities up to were reached with hydrogen pellet injection and helium gas fueling. Here, the ions are indirectly heated, and at a central density of a temperature of with was transiently accomplished, which corresponds to with a peak diamagnetic energy of and volume-averaged normalized plasma pressure . The routine access to high plasma densities was opened with boronization of the first wall. After boronization, the oxygen impurity content was reduced by a factor of 10, the carbon impurity content by a factor of 5. The reduced (edge) plasma radiation level gives routinely access to higher densities without radiation collapse, e.g. well above line integrated density and central temperatures at moderate ECRH power. Both X2 and O2 mode ECRH schemes were successfully applied. Core turbulence was measured with a phase contrast imaging diagnostic and suppression of turbulence during pellet injection was observed.

The EUROfusion JET-ILW pedestal database is described, with emphasis on three main issues. First, the technical aspects are introduced, including a description of the data selection, the datasets, the diagnostics used, the experimental and theoretical methods implemented and the main definitions. Second, the JET-ILW pedestal structure and stability are described. In particular, the work describes the links between the engineering parameters (power, gas and divertor configuration) and the disagreement with the peeling-ballooning (PB) model implemented with ideal magnetohydrodynamics equations. Specifically, the work clarifies why the JET-ILW pedestal tends to be far from the PB boundary at high gas and high power, showing that a universal threshold in power and gas cannot be found but that the relative shift (the distance between the position of the pedestal density and of the pedestal temperature) plays a key role. These links are then used to achieve an empirical explanation of the behavior of the JET-ILW pedestal pressure with gas, power and divertor configuration. Third, the pedestal database is used to revise the scaling law of the pedestal stored energy. The work shows a reasonable agreement with the earlier Cordey scaling in terms of plasma current and triangularity dependence, but highlights some differences in terms of power and isotope mass dependence.

A study of a dataset of JET H-mode plasma with the Be/W ITER-like wall (JET-ILW) shows that reaching the edge MHD ballooning limit leads to confinement degradation. However, unlike JET plasmas with a carbon wall (JET-C), the JET-ILW plasmas stay in a marginal dithering phase for a relatively long period, associated with a higher (≈20%) H-mode density limit (HDL) than JET-C equivalents. This suggests that ITER could be operated in H-mode with higher density than the scaling based on carbon wall devices, but likely with a dithering phase plasma with lower confinement. A new, reliable estimator for JET E r, min has been derived by combining HRTS measurements of pedestal gradient and edge-SOL decay lengths. JET radial E r ETB wells are observed in the range of −15 to −60 kV m−1 in high performance H-modes, consistent with previous CXRS results in ASDEX Upgrade. The results imply that a higher positive E × B shear in the near SOL plays a role in sustaining a marginal phase in JET-ILW which leads to a higher HDL than that in JET-C. The results of the JET-ILW dataset show agreement with the Goldston finite collisionality HD model for SOL broadening at high collisionality. A hypothesis for the dithering H-mode phase is proposed: as n e,SOL increases, ν ∗,SOL increases, SOL broadens, E r shear decreases, triggers L-mode; n e drops, ν ∗,SOL decreases, SOL becomes narrower, and E r shear increases, triggering H-mode, resulting in a cycle of H–L–H- oscillations. For burning plasma devices, such as ITER, operating just below the MHD limit for the dithering phase could be a promising regime for maximising core density, and fusion performance while minimising plasma-material interaction. The oscillatory signal during the dithering phase could be used as a precursor of undesirable plasma performance for control purposes.

openaire: EC/H2020/633053/EU//EUROfusion Experiments in JET with the Be/W ITER-like wall show that in pure hydrogen low-confinement mode (L-mode) plasmas the density limit is approximately 20% higher than their corresponding deuterium plasmas. The maximum achievable density in L-mode plasmas is limited by the magnetohydrodynamic stability of the m/n = 2/1 tearing mode. Studies with the edge fluid-neutral Monte-Carlo code package EDGE2D-EIRENE show that the density of hydrogen atoms inside the separatrix is two times lower than for deuterium in plasma conditions preceding the density limit. The difference between the isotopes is caused by the non-linear process at density limit onset which leads to more efficient dissociation and ionization of hydrogen molecules and atoms in hydrogen than in deuterium plasmas at the high-field X-point region at electron temperatures lower than 2 eV. The m/n = 2/1 island size is estimated to be 30% smaller islands for hydrogen than for deuterium cases for equal fuelling conditions and radial transport assumptions.

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In FTU, the density profile in the presence of large MARFE becomes more and more peaked with increasing density, forming strong density gradients close to the radial region affected by the MARFE. The temperature at the edge drops to few eV, driving a drop of the whole profile. The estimated particle source cannot justify the change of the density gradient, which instead is well-explained by a change of the pinch. A thermo-diffusion term well-describes the pinch evolution and the experimental behavior of the density at those radii where temperature measurements are reliable.

Identification of the mechanisms for the H-mode density limit in machines with fully metallic walls, and their scaling to future devices is essential to find for these machines the optimal operational boundaries with the highest attainable density and confinement. Systematic investigations of H-mode density limit plasmas in experiments with deuterium external gas fuelling have been performed on machines with fully metallic walls, JET and AUG and results have been compared with one another. Basically, the operation phases are identical for both tokamaks: the stable H-mode phase, degrading H-mode phase, breakdown of the H-mode with energy confinement deterioration usually accompanied by a dithering cycling phase, followed by the l -mode phase. The observed H-mode density limit on both machines is found close to the Greenwald limit (n/n GW = 0.8-1.1 in the observed magnetic configurations). The similar behavior of the radiation on both tokamaks demonstrates that the density limit (DL) is neither related to additional energy losses from the confined region by radiation, nor to an inward collapse of the hot discharge core induced by overcooling of the plasma periphery by radiation. It was observed on both machines that detachment, as well as the X-point MARFE itself, does not trigger a transition in the confinement regime and thus does not present a limit on the plasma density. It is the plasma confinement, most likely determined by edge parameters, which is ultimately responsible for the transition from H-to L-mode. The measured Greenwald fractions are found to be consistent with the predictions from different theoretical models [16,30] based on MHD instability theory in the near-SOL. For complete list of authors see http://dx.doi.org/10.1016/j.nme.2017.01.005

A radiative collapse predictor has been developed using a machine-learning model based on high-density plasma experiments in the Large Helical Device (LHD). Concurrently, the physical background of radiative collapse was discussed based on the distinct features extracted by a sparse modeling, which is one of the frameworks of data-driven science. Electron density, CIV and OV line emissions, and electron temperature at the plasma edge have been extracted as the key parameters of radiative collapse. Those parameters are relevant to the physical knowledge that the major cause of radiative collapse is the enhancement of radiative loss by light impurities in the plasma-edge region. Using these four parameters, the likelihood of occurrence of radiative collapse has been estimated. The behavior of plasma at the edge—in particular, the carbon impurities outside the last closed flux surface—has been evaluated using EMC3-EIRENE code for the phase with increasing likelihood, that is, the plasma is getting close to the collapse. It is shown that the radiation caused by the C3+ ion, which corresponds to the CIV emission, is enhanced in the region where electron temperature is around 10 eV.

Efficient lower hybrid current drive (LHCD) is demonstrated at densities up to n[over ¯]_{e}≈1.5×10^{20} m^{-3} in diverted plasmas on the Alcator C-Mod tokamak by operating at increased plasma current-and therefore reduced Greenwald density fraction. This density exceeds the nominal "LH density limit" at n[over ¯]_{e}≈1.0×10^{20} m^{-3} reported previously, above which an anomalous loss of current drive efficiency was observed. The recovery of current drive efficiency to a level consistent with engineering scalings is correlated with a reduction in density shoulders and turbulence levels in the far scrape-off layer. Concurrently, rf wave interaction with the edge and/or scrape-off-layer plasma is reduced, as indicated by a minimal broadening of the wave frequency spectrum measured at the plasma edge. These results have important implications for sustaining steady-state tokamak operation and indicate a pathway forward for implementing efficient LHCD in a reactor.

The problem of the thermal equilibrium and density limit in tokamak-reactor are analyzed. The empirical Greenwald criterion determining the critical plasma density may be exceeded in tokamaks with auxiliary heating. One may expect that the fusion power may increase the critical plasma density. The thermal balance in tokamak-reactors is discussed. The critical density is defined by the equality of heating (auxiliary plus fusion) power and radiation losses at the plasma edge. The influence of fusion power input as well as auxiliary heating on the critical density is studied. The analytic model is based on some simplifying assumptions. The auxiliary heating as well as the thermonuclear one is assumed to be localized at the center of the plasma column. The slab geometry is assumed for simplicity. It is shown that the critical density rises up increasing the fusion output drastically using the feedback control.

High-density discharges on JET with ITER-like Wall (ILW) have been analysed with the aim of establishing a mechanism for the H-mode density limit (DL) and compared with experiments in the JET carbon material configuration. The density limit is up to 20% higher in the JET-ILW than in the JET-C machine. The observed H-mode density limit is found close to the Greenwald limit. It is sensitive to the main plasma shape and is almost independent of the heating power. It has been observed that the transition from H-mode to L-mode is not always an abrupt event but may exhibit a series of H–L–H transitions, the so-called “dithering H-mode”. It was observed that detachment, as well as the X-point MARFE itself, does not trigger the H–L transition and thus does not present a limit on the plasma density and that it is the plasma confinement which is ultimately responsible for the H-mode DL.

We analyzed broad-band X-ray and radio data of the magnetar SGR J1935+2154 taken in the aftermath of its 2014, 2015, and 2016 outbursts. The source soft X-ray spectrum, Comment: 17 pages, 7 figures, 5 tables, submitted to ApJ, comments welcome

Edge shear flow and its effect on regulating turbulent transport have long been suspected to play an important role in plasmas operating near the Greenwald density limit $ n_G $. In this study, equilibrium profiles as well as the turbulent particle flux and Reynolds stress across the separatrix in the HL-2A tokamak are examined as $ n_G $ is approached in ohmic L-mode discharges. As the normalized line-averaged density $ \bar{n}_e/n_G $ is raised, the shearing rate of the mean poloidal flow $ \omega_{\rm sh} $ drops, and the turbulent drive for the low-frequency zonal flow (the Reynolds power $ \mathcal{P}_{Re} $) collapses. Correspondingly, the turbulent particle transport increases drastically with increasing collision rates. The geodesic acoustic modes (GAMs) gain more energy from the ambient turbulence at higher densities, but have smaller shearing rate than low-frequency zonal flows. The increased density also introduces decreased adiabaticity which not only enhances the particle transport but is also related to a reduction in the eddy-tilting and the Reynolds power. Both effects may lead to the cooling of edge plasmas and therefore the onset of MHD instabilities that limit the plasma density.

The commonly adopted scaling for the maximum achievable plasma density in tokamak fusion devices, the so-called 'Greenwald limit', refers to the line-averaged density along a central chord and depends only on the average plasma current density. However, the Greenwald limit has been exceeded in tokamak experiments in the case of peaked density profiles, indicating that the edge density is the real parameter responsible for the density limit. Furthermore, the Greenwald limit has been obtained for fixed density profiles, so that the scaling can be very different when introducing density profile dependencies on plasma parameters. Dedicated density limit experiments were performed in recent years on the Frascati Tokamak Upgrade, exploring the high density domain in a wide range of values of plasma current, toroidal magnetic field and edge safety factor. New data were collected in the latest experimental campaigns, extending the study of the density limit towards lower values of toroidal magnetic field and plasma current. These experiments confirmed the edge nature of the density limit, as a Greenwald-like scaling was obtained for the maximum achievable line-averaged density along a peripheral chord, while a clear scaling of the maximum achievable line-averaged density along a central chord with the toroidal magnetic field only was found and successfully interpreted as due to interplay between the peripheral Greenwald limit and the specific density profile behavior when approaching the density limit. In particular, an analytical relation between the peripheral and the central density limit was derived for the first time, with the introduction of a generalized parabolic density profile with the peaking factor dependent on the plasma parameters. © 2017 IOP Publishing Ltd.

The thermal pressure coefficients in liquid n-Pentadecane (C15), n-Heptadecane (C17), n-octadecane (C18) and n-nonadecane (C19) was measured using pVT data. The measurements were carried out at pressures up to 150 MPa in the temperature range from 293 to 383 K. The experimental results have been used to evaluate various thermophysical properties such as thermal pressure coefficients up to 150 MPa with the use of density and temperature data at various pressures. New parameters of the linear isotherm regularity, the so-called LIR equation of state, are used to calculate of thermal pressure coefficients of n-Pentadecane (C15), n-Heptadecane (C17), n-octadecane (C18) and n-nonadecane (C19) dense fluids. In this paper, temperature dependency of linear isotherm regularity parameters in the form of a first order has been developed to second and third order and their temperature derivatives of new parameters are used to calculate thermal pressure coefficients. The resulting model predicts accurately thermal pressure coefficients from the lower density limit at the Boyle density at the from triple temperature up to about double the Boyle temperature. The upper density limit appears to be reached at 1.4 times the Boyle density. These problems have led us to try to establish a function for the accurate calculation of the thermal pressure coefficients based on the linear isotherm regularity theory for different fluids.

Discrete element method (DEM) was used to study the effect of mill speed and lifter face angle on the characteristics of charge group. Based on the bin algorithm, a position density limit is proposed to study the motion of the charge. The result shows that the angle of charge group is mainly affected by mill speed and the measured result proves that the predicted equation is reasonable.

This study demonstrates that any temperature dependency introduced in the term describing the inter-molecular repulsive forces of analytical EOS models leads to prediction of the infinite value for the isochoric heat capacity at the density limit conditions. The temperature-dependent hard sphere diameters and co-volumes necessarily result in the non-physical negative infinite values of this property at the infinite pressure, which in turn leads to the non-physical predictions of other properties, such as the sound velocities. In addition, this study demonstrates that the inconsistency existing at the imaginary state of infinite pressure may have a very strong negative impact on predicting the data of real fluids at the experimentally available pressures. Therefore making the repulsion compressibility factor temperature-dependent seems highly non-recommended.