arXiv subjects

P. Ivanov

Publications and source records attributed to P. Ivanov.

6 recordsLinked to original sources

arXiv2024

On the importance of parallel magnetic-field fluctuations for electromagnetic instabilities in STEP

[ABRIDGED] This paper discusses the importance of parallel perturbations of the magnetic-field in gyrokinetic simulations of electromagnetic instabilities and turbulence at mid-radius in the burning plasma phase of the conceptual high-$\beta$, reactor-scale, tight-aspect-ratio tokamak STEP. Previous studies have revealed the presence of unstable hybrid kinetic ballooning modes (hKBMs) at binormal scales approaching the ion Larmor radius. In this STEP plasma it was found that the hKBM requires the inclusion of parallel magnetic-field perturbations to be linearly unstable. Here, the extent to which the inclusion of fluctuations in the parallel magnetic-field can be relaxed is explored through gyrokinetic simulations. In particular, the frequently used MHD approximation (dropping $\delta \! B_{\parallel}$ and setting the $\nabla B$ drift frequency equal to the curvature drift frequency) is discussed and simulations explore whether this approximation is useful for modelling STEP plasmas. It is shown that the MHD approximation can reproduce some of the linear properties of the full STEP gyrokinetic system, but is too stable at low $k_y$ and nonlinear simulations using the MHD approximation result in very different transport states. It is demonstrated that the MHD approximation is challenged by the high $\beta^{\prime}$ values in STEP, and that the approximation improves considerably at lower $\beta^{\prime}$. Furthermore, it is shown that the sensitivity of STEP to $\delta \! B_{\parallel}$ fluctuations is primarily because the plasma sits close to marginality and it is shown that in slightly more strongly driven conditions the hKBM is unstable without $\delta \! B_{\parallel}.$ Crucially, it is demonstrated that the state of large transport typically predicted by local electromagnetic gyrokinetic simulations of STEP plasmas is not solely due to $\delta \! B_{\parallel}$ physics.

physics.plasm-ph↗

arXiv2022

A new avenue in the search for CP violation: Moessbauer spectroscopy of 227-Ac

This work proposes a new avenue in the search for CP-violating odd-electric and even-magnetic nuclear moments. A promising candidate to find such moments in the ground state is the quadrupole-deformed and octupole-correlated nucleus 227-actinium. In this nucleus, the 27.4-keV $E1$~transition that connects the $3/2^+$ parity-doublet partner and the $3/2^-$ ground state is perfectly suited to apply the sensitive technique of recoil-free selfabsorption, commonly known as M\"ossbauer spectroscopy. In this experimental approach, the lifetime of the $3/2^+$ upper parity-doublet partner allows an estimate of the lower limit of $\Delta E = 2\cdot \Gamma_{\gamma}$= $23.7(1) \times 10^{-9}$ eV for the achievable energy resolution to be made. This resolution must be exceeded by the interaction of a CP-violating moment and the corresponding multipole moment of the field distribution in the lattice. This work presents the first ideas for patterns caused by CP-violating moments on the expected quadrupole splitting and nuclear Zeeman effect.

nucl-ex↗

arXiv2015

A stochastic quasi-classical wavefunction of the Universe from the third quantization procedure

(abbreviated) We study quantized solutions of WdW equation describing a closed FRW universe with a $Λ$ term and a set of massless scalar fields. We show that when $Λ\ll 1$ in the natural units and the standard $in$-vacuum state is considered, either wavefunction of the universe, $Ψ$, or its derivative with respect to the scale factor, $a$, behave as random quasi-classical fields at sufficiently large values of $a$, when $1 \ll a \ll e^{2\over 3Λ}$ or $a \gg e^{2\over 3Λ}$, respectively. Statistical r.m.s value of the wavefunction is proportional to the Hartle-Hawking wavefunction for a closed universe with a $Λ$ term. Alternatively, the behaviour of our system at large values of $a$ can be described in terms of a density matrix corresponding to a mixed state, which is directly determined by statistical properties of $Ψ$. It gives a non-trivial probability distribution over field velocities. We suppose that a similar behaviour of $Ψ$ can be found in all models exhibiting copious production of excitations with respect to $out$-vacuum state associated with classical trajectories at large values of $a$. Thus, the third quantization procedure may provide a 'boundary condition' for classical solutions of WdW equation.

gr-qc↗

arXiv2011

A fully relativistic twisted disk around a slowly rotating Kerr black hole: derivation of dynamical equations and the shape of stationary configurations

(abbreviated) In this paper we derive equations describing dynamics and stationary configurations of a twisted fully relativistic thin accretion disc around a slowly rotating black hole. We find that the disc dynamics and stationary shapes are determined by a pair of equations for two complex variables describing orientation of the disc rings and velocity perturbations in the disc. We analyse shapes of stationary twisted configurations. It is shown that the stationary configurations depend on two parameters - the $α$parameter and $\tilde δ= δ_{*}/\sqrt a$, where $δ_{*}\sim h/r$ is the disc opening angle (h is the disc halfthickness) and $a$ is the black hole rotational parameter. When $a > 0$ and $\tilde δ\ll 1$ the shapes depend drastically on value of $α$. When $α$ is small the disc inclination angle oscillates with radius with amplitude and radial frequency of the oscillations dramatically increasing towards the last stable orbit. For moderate values of $α$ the oscillations do not take place but the disc does not align with the equatorial plane at small radii. Its inclination angle is either increasing towards $R_{ms}$ or exhibits a non-monotonic dependence on the radial coordinate. Finally, when $α$ is sufficiently large the disc aligns with the equatorial plane at small radii. When $a < 0$ the disc aligns with the equatorial plane for all values of $α$. The results reported here may have implications for determining structure and variability of accretion discs close to $R_{ms}$ as well as for modelling of emission spectra coming from different sources, which are supposed to contain black holes.

astro-ph.HE↗

arXiv2004

Gamma background and neutron flux measurements at Basic Ecological Observatory Moussala

The Basic Ecological Observatory (BEO) Moussala is located at 2925m above sea level and one of the main activities is connected with gamma background and neutron dose rate monitoring. The neutron and gamma measurements are carried out with help of site radiation monitor Harwell Type 3208-1. The dose rate of ambient gamma background is measured with SBN-90, improved with the embedded microprocessor system using automatic PC based system. A cross check and inter calibration with existing TLD (thermo luminescent dosimeters) at BEO Moussala which is a part of the monitoring network of INRNE is presented. A polyethylene sphere with a layer of lead serving as a neutron breeder, extending the energy range from about 40 MeV to several hundred MeV is presented as a project in development. Another project in development is the construction of active detector based on SNM-15 detectors for a dose rate measurement of cosmic neutrons is described. Preliminary study of the performances of the detectors are carried out. A further project for muonic telescope similar to the yet developed is discussed.

physics.ins-det↗

arXiv1997

Non-linear metric perturbations and production of primordial black holes

We consider the simple inflationary model with peculiarity in the form of "plateau" in the inflaton potential. We use the formalism of coarse-grained field to describe the production of metric perturbations $h$ of an arbitrary amplitude and obtain non-Gaussian probability function for such metric perturbations. We associate the spatial regions having large perturbations $h\sim 1$ with the regions going to primordial black holes after inflation. We show that in our model the non-linear effects can lead to overproduction of the primordial black holes.

astro-ph↗