| 61. |
- Ivonin, Igor A.
(author)
-
Global Turbulent EquiPartition (TEP) in Tokamak Plasma
- 2001
-
Doctoral thesis (other academic/artistic)abstract
- We studied analytically and numerically the establishment of global turbulent equipartition (TEP) of magnetised electrons in a tokamak plasma. This allows us to investigate self-consistently the inhomogeneous distributions of the plasma density, the electron and the ion temperatures, the generation of radial electric field and the poloidal and the toroidal rotation in the turbulent tokamak plasma. We have obtained the level and the inhomogeneous distribution of the turbulent energy, which allows us to estimate the rate of turbulent mixing at any point in a tokamak. We have used the invariant kinetic description for the electrons and semi hydrodynamics approach for the ions (separately for the trapped and passing ions) to describe the TEP state of the plasma in tokamak. As a result, we found two qualitatively different TEP distributions, that am similar to the L and H-modes in tokamak. We have studied these modes in details. We have got the scaling laws of the density and the temperature in both cases. We have found that, in both cases, the turbulence level is sufficiently large to provide the quick rate of the turbulent mixing in the central tokamak region, which is required for the validity of the global TEP approach. We have found that, in the second case of H-mode, the turbulent suppression zone, the poloidal rotation and the negative radial electric field is developed at the plasma boundary. The turbulence level in the turbulence suppression zone, which appears in the H-mode, is rather- small to provide quick turbulent mixing in this region. It means, that two different TEPS, one for the hot central plasma and another one for the cold plasma at the separatrix could appear outside the turbulence suppression zone. We have studied this coexistence by using different models.
|
|
| 62. |
- Sandberg, Ingmar, 1974-
(author)
-
Drift and Mirror Modes in Magnetized Plasmas
- 2002
-
Doctoral thesis (other academic/artistic)abstract
- Low frequency plasma instabilities driven by plasma inhomogeneity and velocity anisotropies are major candidates for the explanation of various phenomena observed in fusion and space plasmas. For low-β fusion plasma, the influence of plasma rotation and finite ion temperature on the structure and the type of toroidal drift eigenmodes in tokamaks is investigated rigorously, including the effects induced by the toroidal geometry such as the coupling mode, the radial variation of plasma rotation velocity and the magnetic shear. The conditions at which global or propagating drift modes are formed and the analytical dispersion relations are obtained for various cases. For high-β space plasma, a unified theory of the mirror instability based on a quasi-hydrodynamic approach and a unified fully kinetic theory of the drift mirror instabilities have been developed. The obtained results are applicable for arbitrary distribution functions and can be used to determine whether mirror and drift mirror modes are stable or unstable in a multi-component plasma, including electron temperature effects and electron pressure anisotropies. A major outcome of the theory of drift mirror instabilities, is the prediction of a hydrodynamic drift mirror instability with threshold lower than the classic ion drift mirror instability.
|
|
| 63. |
- Kobyakov, Dmitry, 1985-
(author)
-
Hydrodynamics of Binary Bose-Einstein Condensates and Hydro-elasticity of the Inner Crust of Neutron Stars
- 2014
-
Doctoral thesis (other academic/artistic)abstract
- In the present thesis, “Hydrodynamics of Binary Bose-Einstein Condensates and Hydro-elasticity of the Inner Crust of Neutron Stars”, the hydrodynamic effects, instabilities and superfluid turbulence in binary immiscible ultracold gases, and hydro-elastic macroscopic coupled modes and microscopic structure of the inner layers of the crust of neutron stars, are studied. The ultracold gas dynamics can be realized in the laboratory. The excitation modes of the inner crust determine a number of observable properties such as elasticity, thermal properties and mass transport properties. Here we focus on expanding the details, rather than repeating the results presented in the published articles.In the part of the thesis related to atomic ultracold gases, we utilize the physical parameters in the experimentally realizable parameter region. We numerically simulate the coupled non-linear Schrödinger equations, and calculate observable quantities, such as phase and modulus of the order parameter, conditions needed for observation of the Rayleigh-Taylor instability and for turbulence generation. The numerical calculations are accompanied by analytical description of the processes. The dispersion relation for capillary-gravitational waves at the interface between two ultracold gases, is derived straightforwardly from the superfluid Lagrangian. The equations of motion for centre-of-mass of the superfluids are derived, and then used in our model of the quantum swapping of immiscible superfluids pressed by a strong external force. By numerical simulation, we find that the Kelvin-Helmholtz instability which occurs at the non-linear stage of the Rayleigh-Taylor instability, can generate quantum turbulence with peculiar properties. We find that two-dimensional superfluid systems with weak inter-component repulsion are different from previously studied strongly repulsive binary superfluids, because the quantum Kelvin-Helmholtz instability in weakly repulsive superfluids rolls up the whole interface forming a vortex bundle, similarly to dynamics of the shear fluid layers in the classical hydrodynamics. Production of vortex bundles favours the Kolmogorov spectrum of turbulence, and we find that the Kolmogorov scaling indeed is present in a freely decaying turbulence.In the part of the thesis related to neutron stars, we study the inner crust of neutron stars, where the fully ionized atomic nuclei coexist with a superfluid of neutrons. The interaction between superfluid neutrons and the crystallized Coulomb plasma is due to the interaction between density perturbations (interaction of the scalar type), and between the current - the non-dissipative entrainment effect (interaction of the vector type). We calculate velocities of the collective modes of the crystal coupled to superfluid neutrons. As an input we use the results of microscopic nuclear calculations in the framework of the compressible liquid drop model (the Lattimer and Swesty equation of state), and more recent effective Thomas-Fermi calculations with shell corrections (N. Chamel, and the Brussels theoretical nuclear physics group). Knowledge of velocities as functions of the matter density in the inner crust is important for calculation of a number of dynamic and transport properties. The heat transport properties of the inner crust are directly observable in accreting binary systems (low-mass x-ray binaries). The mass transport properties of the inner crust are directly linked to the rotational evolution, being a key physical ingredient of the pulsar glitch phenomenon. The elastic properties are related to the vibrational modes of the star, and to the breaking stress of the crust.In the second part of our work on neutron stars we investigate the microscopic structure of the inner crust treating the structure as an anisotropic crystal coupled to s-wave superfluid neutron liquid. As we analyse dynamics of the elementary excitations at higher wavenumbers (smaller scales), we reach the edge of the first Brillouin zone. The Lattimer-Swesty data is applicable for wavenumbers much smaller than the edge of the first Brillouin zone. We extrapolate the data through the whole first Brillouin zone to calculate the fastest growth rate of the unstable modes. The crucial step is to calculate the mode velocities in anisotropic crystal incorporating both the induced neutron-proton interactions, and the electron screening properties. We find that the combined influence of these two effects leads to softening of the longitudinal phonon of the lattice above about the Thomas-Fermi screening wavenumber of the electrons. The critical wavenumber when the frequency becomes purely imaginary is about 1/5 - 2/3 of the reciprocal lattice vector, thus validating our assumption. The imaginary mode frequency implies instability at finite wavenumbers. Our calculations suggest that the mode at the first Brillouin zone edge is the most unstable, and thus the structure experiences a displacive phase transition when the central ion of a unit cell of the body-cubic-centred lattice, is displaced to the cube face. Thus, the electronic structure of matter at densities above the neutron drip [1], is richer than previously appreciated, and new microscopic calculations of nuclear structure are necessary which take into account the high-wavenumber physics. Such calculations will provide crucial input to models interpreting the quasi-periodic oscillations in Soft Gamma Repeaters as magnetar x-ray flares, and to the theory of glitches of neutron stars.[1] The neutron drip density is ~3×1011 g cm-3.
|
|
| 64. |
- Akrami, Yashar, 1980-
(author)
-
Supersymmetry vis-à-vis Observation : Dark Matter Constraints, Global Fits and Statistical Issues
- 2011
-
Doctoral thesis (other academic/artistic)abstract
- Weak-scale supersymmetry is one of the most favoured theories beyond the Standard Model of particle physics that elegantly solves various theoretical and observational problems in both particle physics and cosmology. In this thesis, I describe the theoretical foundations of supersymmetry, issues that it can address and concrete supersymmetric models that are widely used in phenomenological studies. I discuss how the predictions of supersymmetric models may be compared with observational data from both colliders and cosmology. I show why constraints on supersymmetric parameters by direct and indirect searches of particle dark matter are of particular interest in this respect. Gamma-ray observations of astrophysical sources, in particular dwarf spheroidal galaxies, by the Fermi satellite, and recording nuclear recoil events and energies by future ton-scale direct detection experiments are shown to provide powerful tools in searches for supersymmetric dark matter and estimating supersymmetric parameters. I discuss some major statistical issues in supersymmetric global fits to experimental data. In particular, I further demonstrate that existing advanced scanning techniques may fail in correctly mapping the statistical properties of the parameter spaces even for the simplest supersymmetric models. Complementary scanning methods based on Genetic Algorithms are proposed.
|
|
| 65. |
- Hassan, S. Fawad, et al.
(author)
-
Bimetric gravity from ghost-free massive gravity
- 2012
-
In: Journal of High Energy Physics (JHEP). - 1126-6708 .- 1029-8479. ; 1202
-
Journal article (peer-reviewed)abstract
- Generically, non-linear bimetric theories of gravity suffer from the same Boulware-Deser ghost instability as non-linear theories of massive gravity. However, recently proposed theories of massive gravity have been shown to be ghost-free. These theories are formulated with respect to a flat, non-dynamical reference metric. In this work we show that it is possible to give dynamics to the reference metric in such a way that the consistency of the theory is maintained. The result is a non-linear bimetric theory of a massless spin-2 field interacting with a massive spin-2 field that is free of the Boulware-Deser ghost. To our knowledge, this is the first construction of such a ghost-free bimetric theory.
|
|
| 66. |
- Aalbers, J., et al.
(author)
-
A next-generation liquid xenon observatory for dark matter and neutrino physics
- 2023
-
In: Journal of Physics G: Nuclear and Particle Physics. - : IOP Publishing. - 0954-3899 .- 1361-6471. ; 50:1
-
Research review (peer-reviewed)abstract
- The nature of dark matter and properties of neutrinos are among the most pressing issues in contemporary particle physics. The dual-phase xenon time-projection chamber is the leading technology to cover the available parameter space for weakly interacting massive particles, while featuring extensive sensitivity to many alternative dark matter candidates. These detectors can also study neutrinos through neutrinoless double-beta decay and through a variety of astrophysical sources. A next-generation xenon-based detector will therefore be a true multi-purpose observatory to significantly advance particle physics, nuclear physics, astrophysics, solar physics, and cosmology. This review article presents the science cases for such a detector.
|
|
| 67. |
- Abbasi, R., et al.
(author)
-
A convolutional neural network based cascade reconstruction for the IceCube Neutrino Observatory
- 2021
-
In: Journal of Instrumentation. - : Institute of Physics Publishing (IOPP). - 1748-0221. ; 16:7
-
Journal article (peer-reviewed)abstract
- Continued improvements on existing reconstruction methods are vital to the success of high-energy physics experiments, such as the IceCube Neutrino Observatory. In IceCube, further challenges arise as the detector is situated at the geographic South Pole where computational resources are limited. However, to perform real-time analyses and to issue alerts to telescopes around the world, powerful and fast reconstruction methods are desired. Deep neural networks can be extremely powerful, and their usage is computationally inexpensive once the networks are trained. These characteristics make a deep learning-based approach an excellent candidate for the application in IceCube. A reconstruction method based on convolutional architectures and hexagonally shaped kernels is presented. The presented method is robust towards systematic uncertainties in the simulation and has been tested on experimental data. In comparison to standard reconstruction methods in IceCube, it can improve upon the reconstruction accuracy, while reducing the time necessary to run the reconstruction by two to three orders of magnitude.
|
|
| 68. |
- Hobiger, Thomas, 1978, et al.
(author)
-
Combining VLBI and GPS for inter-continental frequency transfer
- 2015
-
In: 22nd European VLBI for Geodesy and Astrometry (EVGA) Working Meeting.
-
Conference paper (other academic/artistic)abstract
- For decades the Global Positioning System (GPS) has been the only space geodetic technique routinely used for inter-continental frequency transfer applications. In the past VLB) has also been considered for this purpose and the method's capabilities were studied several times. However, compared to GPS current VLBI technology only provides few observations per hour, thus limiting its potential to improve frequency comparisons. We therefore investigate the effect of combining VLBI and GPS on the observation level in order to draw the maximum benefit from the strength of each individual technique. As a test-bed for our study we use the CONT11 campaign observed in 2011. First we review the frequency transfer performance that can be achieved with independent technique-specific analyses. With this analysis approach both techniques, GPS and VLBI, show similar frequency link instabilities at the level of 1e-14 to 1e-15 (MDEV) on inter-continental baselines for averaging times of one day. We also perform a combined analysis of VLBI and GPS data on the observation level and demonstrate that our combination approach leads to small but consistent improvements for frequency transfer of up to 10%, in particular for averaging periods longer than 3000 s. We discuss the implications of these findings and present our ideas about how VLBI can contribute to international frequency transfer tasks.
|
|
| 69. |
|
|
| 70. |
- Kobyakov, Dmitry, et al.
(author)
-
Towards a metallurgy of neutron star crusts
- 2014
-
In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 112:11, s. 112504-
-
Journal article (peer-reviewed)abstract
- In the standard picture of the crust of a neutron star, matter there is simple: a body-centered-cubic (bcc) lattice of nuclei immersed in an essentially uniform electron gas. We show that at densities above that for neutron drip (~4×1011 g cm-3 or roughly one thousandth of nuclear matter density), the interstitial neutrons give rise to an attractive interaction between nuclei that renders the lattice unstable. We argue that the likely equilibrium structure is similar to that in displacive ferroelectric materials such as BaTiO3. As a consequence, properties of matter in the inner crust are expected to be much richer than previously appreciated and we mention possible consequences for observable neutron star properties.
|
|
