1,721,003 research outputs found
Alternatives to current dark energy models
No full textThe envisioned characteristics of a multi-component, inhomogeneous ``cosmological plasma" model are considered whose heavier visible component consists of ``loaded galaxy clusters". These include the optically observed clusters of galaxies, the characteristic electromagnetic radiation (X-rays) emitting plasmas associated with them, whose mass is larger or comparable to that of the galaxies, and the containing dark matter.
The other visible component consists of the lighter but far more numerous ``field galaxies". A third component is the ``pervading dark matter" in which the visible components are immersed.
On this basis, an energy transport equation, involving a ``dark wind", is proposed for the accelerating expansion energy of the multi-component plasma, rather than a homogeneous, ``dark energy" equation of state. This is in analogy to transport equations based on processes relevant to laboratory and space plasmas that are not in thermal equilibrium and for which equations of state cannot be formulated
Active black holes: Relevant plasma structures, regimes and processes involving all phase space
Full text linkThe presented theory is motivated by the growing body of experimental information on the characteristics, connected with relevant spectral, time, and space resolutions, of the radiation emission from objects considered as rotating black holes. In the immediate surroundings of these objects, three plasma regions are identified: an innermost Buffer Region, an intermediate Three-regime Region, and a Structured Peripheral Region. In the last region, a Composite Disk Structure made of a sequence of plasma rings corresponding to the formation of closed magnetic surfaces is considered to be present and to allow intermittent accretion flows along the relevant separatrices. The nonlinear “Master Equation” describing composite disk structures is derived and solved in appropriate asymptotic limits. A ring configuration, depending on the state of the plasma at the microscopic level: (i) can be excluded from forming given the strongly nonthermal nature of the electron distribution (in momentum space) within the Three-regime Region allowing the onset of a spiral structure; the observed High Frequency Quasi Periodic Oscillations are associated with these tridimensional structures; (ii) may be allowed to propagate to the outer edge of the Buffer Region where successive rings carrying currents in opposite directions are ejected vertically (in opposite directions) and originate the observed jets; or (iii) penetrates in the Three-regime Region and is dissipated before reaching the outer edge of the Buffer Region. The absence of a coherent composite disk structure guiding accretion in the presence of a significant magnetic field background is suggested to characterize quiescent black holes.United States. Dept. of Energ
MgB₂ Coil Options for Fusion Poloidal Magnets
No full textA hybrid solution is proposed for the IGNITOR research fusion machine by using of superconducting coils for some poloidal magnets, in association with high field copper magnets for the central solenoid and for the toroidal field coils. The choice to be made among the various superconductors is based on the materials performances in high magnetic field, on the mechanical strength of the conductors and on the cost of manufacturing large coils. In this study we analyze a coil based on MgB₂, a "medium temperature" superconducting material that we expect will avoid, in association with others high temperature superconductors, the use of liquid He in future machines. The external poloidal magnet, 5 m in diameter and subjected to a magnetic field of 5 T, represents a real test bench of the technical issues which should be addressed in the exploitation of the future fusion reactors. To fulfil the technical characteristics of the selected magnet we must optimize the fill factor of the superconducting MgB₂ wires, increasing the presently obtained 30% value. Accordingly, the effective current density in the superconducting wire should be of about 1500 A/mm[superscript 2] (@10 K, 5 T), a value which is compatible with the present best MgB₂ laboratory short wires, doped by C or SiC
High energy plasmas, general relativity and collective modes in the surroundings of black holes
No full textThe theoretical finding of composite disk structures around compact objects (e.g. black holes) and recent experimental observations indicate that highly coherent and dynamically important magnetic field configurations exist in the core of these plasma structures. Coherent configurations involving closed magnetic surfaces provide a means to resolve the 'accretion paradox' for a magnetized disk while the formation of jets emitted in the close vicinity of the compact object is related to these configurations. The absence of vigorous accretion activity in the presence of black holes in old galaxies can be attributed to the loss of the surrounding coherent magnetic configurations during their history. The relevant dynamics include axisymmetric (ballooning) modes as well as tridimensional spirals which can be excited from disks with a 'seed' magnetic field, under the effects of differential rotation and of the vertical plasma pressure gradient. The properties of these spirals are strongly dependent on their vertical structure. Axisymmetric modes can produce vertical flows of thermal energy and particles in opposing directions that can be connected to the winds emanating from disks in active galactic nuclei (AGNs). A similarity to the effects of temperature gradient driven modes in magnetically confined laboratory plasmas is pointed out. Spiral modes that are oscillatory in time and in the radial direction can produce transport of angular momentum toward the outer region of the disk structure, a necessary process for the occurrence of accretion. The excitation of radially localized density spirals co-rotating with the plasma, at a distance related to the Schwartzchild radius RS = 2GM*/c2, is proposed as the explanation for high frequency quasiperiodic oscillations (HFQPOs) of non-thermal x-ray emission from compact objects.U.S. Department of Energ
Ignitor siting in the frame of the IGNIR collaboration
No full textThe IGNIR collaboration between Italy and Russia is centred on the construction of the core of the Ignitor machine in Italy and its installation and operation within the TRINITI site (Troitsk, Russia). A Preliminary Safety Report is under preparation for the siting of the Ignitor experiment in Troitsk. A specific site has been selected, with characteristics determined from informations regarding TRINITI. This site has many positive characteristics, such as the presence of electrical facilities because of nearby distribution nodes, and the full availability of the required room for the arrangement of all Ignitor buildings. The results of the analyses, both regarding doses to population and personnel, and other environmental impacts, show that the TRINITI site in Troitsk is a consistent choice for Ignitor localisation, according to the safety and the environmental impact point of view
Neutron-Induced Radioactivity in a Compact Ignition Experiment
No full textA study on the activation of the Ignitor machine is performed. The machine has been modelled with the neutron transport code ANISN, in orderto calculate the neutron fluxes. These have been input in the inventory code ANITA, and the neutron-induced radioactivity in the structures of the machine is obtained. It becomes clear that the activation of certain parts of the machine (vessel, magnets) requires the adoption of remote handling equipment for their maintenance. The substitution of INCONEL 625 (vessel material for Ignitor) by a material with lower long-term activation is discusse
Endogenous and asymmetric magnetic reconnection with associated processes of relevance to fusion burning plasmas
No full textAn endogenous magnetic reconnection process is characterized by a driving factor that lays within the layer where a drastic change of magnetic field topology occurs. This kind of process is shown to take place in the presence of an electron temperature gradient in a well-confined plasma where, referring to quasi-collisionless regimes, the resulting electron temperature fluctuations can be anisotropic. Then a class of (radially) localized reconnecting modes is identified. These involve a transverse generated field (B) over tilde (x) of odd parity (as a function of the radial variable), and have finite (phase) velocities of propagation contrary to commonly considered reconnecting modes. The widths of the relevant reconnection layers remain significant even when large macroscopic distances are considered. Given that there are plasmas in the Universe with considerable electron thermal energy contents, these features can be relied upon in order to produce magnetic field generation, or conversion of magnetic energy into particle energy when the coupling of the localized odd modes to extended even modes can be significant. In any case, the resulting magnetic islands are not symmetric. With their excitation these modes can extract momentum from the main body of the plasma column which should recoil in the opposite direction. The excitation of antisymmetric endogenous modes is shown to be relevant to the electron temperature heating due to the reaction products in a fusion burning plasma as, in this case, the longitudinal thermal conductivity on selected rational magnetic surfaces can be decreased, relative to its collisional value, by the effects of reconnection. This kind of steepening is proposed to have a role in enhancing the growth rate of the instability involved in disruption events of the plasma column. The best agreement between theory and experiments concerning the onset of magnetic reconnection is (probably) represented by the theory of the internal kink mode. The observed accelerated reconnection rate following the onset is suggested as being explained by the formation of a relatively large magnetic island with a local steepening of the electron temperature gradient. A new kind of odd 'thermonuclear heating' mode associated with symmetric reconnection is identified
Ignitor-like Toroidal Devices for Neutron Production
No full textCompact fusion toroidal machines operating in DT have the potential to become efficient sources of neutrons for material testing. An Ignitor-like device could be envisaged for this purpose, making full use of the intense neutron flux that it can generate without reaching ignition. Preliminary radiation damage estimates for some fusion-relevant materials have shown that few full-power months of operation would provide adequate dpa levels. The main features and technological issues of a High Field Neutron Source Facility based on the Columbus concept, with about 50% more volume than Ignitor, are illustrated and discussed. Optimization of the plasma temperature and density relative to the reference ignition scenario (with the assistance of auxiliary heating power) can achieve considerable reductions of duty cycle requirements. The constraints imposed by flux availability, magnet heating and wall loading will inevitably impose a complete redesign of the machine, with the adoption of novel materials (such as MgB2 superconductor already adopted for Ignitor), and new modes of operation will need be investigate
Design basis accident analysis for the Ignitor experiment
No full textA safety analysis study has been applied to the Ignitor machine. The main initiating events have been identified, and accident sequences have been studied. A deterministic assessment of the main accidental sequences has been performed. The consequences of the radioactive environmental releases have been assessed by means of a population dose code. This paper analyzes the deterministic consequences of two accidental sequences, serving as the "design basis accidents" because of the extent of radioactive release involved, either outside or inside the building. The two sequences with higher releases have been considered. The deterministic analysis has achieved the following results: the Ignitor machine, both during routine operation and accidental sequences, presents a negligible environmental impact and radiological risk
Interpretation of the I-Regime and transport associated with relevant heavy particle modes
Full text linkThe excitation of a novel kind of heavy particle [1, 2] mode at the edge of the plasma column is
considered as the signature of the I-con nement Regime [3{7]. The outward transport of impurities
produced by this mode is in fact consistent with the observed expulsion of them from the main
body of the plasma column (a high degree of plasma purity is a necessary feature for fusion burning
plasmas capable of approaching ignition). Moreover, the theoretically predicted mode phase velocity,
in the direction of the electron diamagnetic velocity, has been con rmed by relevant experimental
analyses [8] of the excited
uctuations (around 200 kHz). The plasma \spontaneous rotation" in the
direction of the ion diamagnetic velocity is also consistent, according to the Accretion Theory [9] of
this phenomenon, with the direction of the mode phase velocity. Another feature of the mode that
predicted by the theory is that the I-Regime exhibits a knee of the ion temperature at the edge of
the plasma column but not one of the particle density as the mode excitation factor is the relative
main ion temperature gradient exceeding the local relative density gradient. The net plasma current
density appearing in the saturation stage of the relevant instability, where the induced particle and
energy
uxes are drastically reduced, is associated with the signi cant amplitudes of the poloidal
magnetic eld
uctuations [6, 7] observed to accompany the density
uctuations. The theoretical
implications of the signi cant electron temperature
uctuations [10] observed are discussed.United States. Dept. of Energ
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