arXiv subjects

A. Bock

Publications and source records attributed to A. Bock.

10 recordsLinked to original sources

arXiv2026

Flux pumping and bifurcated relaxations of helical core in 3D magnetohydrodynamic modelling of ASDEX Upgrade plasmas

Flux pumping was achieved in recent hybrid scenario experiments in the ASDEX Upgrade (AUG) tokamak, which is characterized by a sawtooth-free helical quiescent state and the anomalous radial redistribution of toroidal current density and poloidal magnetic flux. In this article, the self-regulation mechanism of the AUG core plasma during flux pumping is investigated at realistic parameters using the JOREK code based on the two-temperature, nonlinear, full magnetohydrodynamic (MHD) model. A key milestone in AUG flux pumping modelling is achieved by quantitatively reproducing the clamped current density and safety factor profiles in the plasma core, demonstrating the effectiveness of the dynamo effect in sustaining the flux pumping state. The dynamo term, that is of particular interest, is primarily generated by the pressure-gradient driven m/n = 1/1 quasi-interchange-like MHD instability. The work systematically extrapolates the parameter regimes of flux pumping from the above AUG base case by scanning dissipation coefficients and plasma beta. The simulation results reveal bifurcated plasma behaviours at different Hartmann numbers, including distinct states such as flux pumping (helical core with a flat current density), sawteeth (periodic kink-cycling), single crash (without subsequent cycle), and quasi-stationary magnetic island (peaked current density). Transitions from marginal flux pumping state to sawteeth are observed in long-term simulations. The relationships between system dissipation, plasma beta, and different plasma states are carefully analyzed. For practical purposes, the potential operational window for flux pumping, as determined by plasma density and temperature, is estimated. The modelling efforts advance the understanding of flux pumping and facilitate the development of a fast surrogate model for efficient evaluation of flux pumping.

physics.plasm-ph↗

arXiv2024

Integrated Data Analysis and Validation

A major challenge in nuclear fusion research is the coherent combination of data from heterogeneous diagnostics and modelling codes for machine control and safety as well as physics studies. Measured data from different diagnostics often provide information about the same subset of physical parameters. Additionally, information provided by some diagnostics might be needed for the analysis of other diagnostics. A joint analysis of complementary and redundant data allows, e.g., to improve the reliability of parameter estimation, to increase the spatial and temporal resolution of profiles, to obtain synergistic effects, to consider diagnostics interdependencies and to find and resolve data inconsistencies. Physics-based modelling and parameter relationships provide additional information improving the treatment of ill-posed inversion problems. A coherent combination of all kind of available information within a probabilistic framework allows for improved data analysis results. The concept of Integrated Data Analysis (IDA) in the framework of Bayesian probability theory is outlined and contrasted with conventional data analysis. Components of the probabilistic approach are summarized and specific ingredients beneficial for data analysis at fusion devices are discussed.

physics.plasm-ph↗

arXiv2021

An NLP approach to quantify dynamic salience of predefined topics in a text corpus

The proliferation of news media available online simultaneously presents a valuable resource and significant challenge to analysts aiming to profile and understand social and cultural trends in a geographic location of interest. While an abundance of news reports documenting significant events, trends, and responses provides a more democratized picture of the social characteristics of a location, making sense of an entire corpus to extract significant trends is a steep challenge for any one analyst or team. Here, we present an approach using natural language processing techniques that seeks to quantify how a set of pre-defined topics of interest change over time across a large corpus of text. We found that, given a predefined topic, we can identify and rank sets of terms, or n-grams, that map to those topics and have usage patterns that deviate from a normal baseline. Emergence, disappearance, or significant variations in n-gram usage present a ground-up picture of a topic's dynamic salience within a corpus of interest.

cs.CL↗

arXiv2020

Kinetic modelling of runaway electron generation in argon-induced disruptions in ASDEX Upgrade

Massive material injection has been proposed as a way to mitigate the formation of a beam of relativistic runaway electrons that may result from a disruption in tokamak plasmas. In this paper we analyse runaway generation observed in eleven ASDEX Upgrade discharges where disruption was triggered using massive gas injection. We present numerical simulations in scenarios characteristic of on-axis plasma conditions, constrained by experimental observations, using a description of the runaway dynamics with self-consistent electric field and temperature evolution in two-dimensional momentum space and zero-dimensional real space. We describe the evolution of the electron distribution function during the disruption, and show that the runaway seed generation is dominated by hot-tail in all of the simulated discharges. We reproduce the observed dependence of the current dissipation rate on the amount of injected argon during the runaway plateau phase. Our simulations also indicate that above a threshold amount of injected argon, the current density after the current quench depends strongly on the argon densities. This trend is not observed in the experiments, which suggests that effects not captured by 0D kinetic modeling -- such as runaway seed transport -- are also important.

physics.plasm-ph↗

arXiv2015

Shocks in unmagnetized plasma with a shear flow: Stability and magnetic field generation

A pair of curved shocks in a collisionless plasma is examined with a two-dimensional particle-in-cell (PIC) simulation. The shocks are created by the collision of two electron-ion clouds at a speed that exceeds everywhere the threshold speed for shock formation. A variation of the collision speed along the initially planar collision boundary, which is comparable to the ion acoustic speed, yields a curvature of the shock that increases with time. The spatially varying Mach number of the shocks results in a variation of the downstream density in the direction along the shock boundary. This variation is eventually equilibrated by the thermal diffusion of ions. The pair of shocks is stable for tens of inverse ion plasma frequencies. The angle between the mean flow velocity vector of the inflowing upstream plasma and the shock's electrostatic field increases steadily during this time. The disalignment of both vectors gives rise to a rotational electron flow, which yields the growth of magnetic field patches that are coherent over tens of electron skin depths.

astro-ph.HE↗

arXiv2001

Superconductivity-induced changes of the phonon resonances in RBa_2Cu_3O_7 (R=rare earth)

We observe a characteristic energy $\hbarω_{c}\approx 2.1$ eV which separates regions of different behavior of the phonon intensities in the Raman spectra of the $R\rm Ba_2Cu_3O_7$ system. A superconductivity-induced drop of phonon intensities is found for the oxygen modes O(4) and O(2)-O(3) only for excitation energies below $\hbarω_c$. This intensity drop indicates an order parameter which affects energies in the vicinity of $\hbarω_c$.

cond-mat.supr-con↗

arXiv1999

Electronic Raman Scattering and Phonon Self-Energy Effects in the R-123 System: Signatures of Gap and Pseudogap

Using Raman spectroscopy and transport measurements we investigate thin epitaxial films of Y{1-x}(Pr,Ca)xBa2Cu3O{6+y}. We explore the electronic Raman responses obtained after subtraction of phononic excitations, and the 2Delta peaks that form out of the electronic background below Tc. We find that the energy of the B1g 2Delta peak increases monotonically with decreasing doping until the peaks become unresolvable. In contrast, the peaks in A1g symmetry follow Tc being resolvable in the Pr-doped films. The B2g responses are weak and a 2Delta peak is only detected at the highest doping level. As a consequence of strong electron-phonon coupling, the B1g phonon at 340 cm^-1 exhibits a pronounced Fano-type line shape. We use a phenomenological model to describe the line shape that takes into account real and imaginary part of the electronic response. It allows us to obtain the self-energy corrections and the mass- enhancement factor lambda as a measure of the coupling. In the normal state we find lambda=0.015 around optimal doping and decreasing values with decreasing doping. The electron-phonon coupling increases strongly below T_c in overdoped samples in which the B1g 2Delta peaks appear in the vicinity of the phonon. Self-energy effects observed in the superconducting state can only partly be assigned to the redistributing electronic response. Anomalies with respect to frequency, linewidth, and intensity remain. They appear at increasing tempera- tures with decreasing doping and we provide evidence that they are connected to the presence of the pseudogap. We supplement our study by a comparison with single crystal data and investigate the influence of site-substitutional disorder on the electronic response and the electron-phonon interaction.

cond-mat.supr-con↗

arXiv1999

Electron-Phonon Coupling Deduced from Phonon Line Shapes

We investigate the Fano-type line shape of the Ba mode of Y_{1-x}Ca_xBa_2Cu_3O_{6+y} films observed in Raman spectra with A_1g symmetry. The line shape is described with an extended Fano formula that allows us to obtain the bare phonon parameters and the self-energy effects based on a phenomenological description of the real and imaginary part of the low-energy electronic response. It turns out that the phonon intensity originates almost entirely from a coupling to this electronic response with negligible contributions from interband (high-energy) electronic excitations. In the normal state we obtain a measure of the electron-phonon coupling via the mass-enhancement factor lambda. We find lambda=6.8 +/- 0.5 % around optimum doping and only weak changes of the self-energy in the superconducting state. With increased disorder at the Ba site we find a decreased intensity of the Ba mode which we can relate to a decreased electron-phonon coupling.

cond-mat.supr-con↗

arXiv1999

Anomalous Self-Energy Effects of the B_1g Phonon in Y_{1-x}(Pr,Ca)_xBa_2Cu_3O_7 Films

In Raman spectra of cuprate superconductors the gap shows up both directly, via a redistribution of the electronic background, the so-called "2Delta peaks", and indirectly, e.g. via the renormalization of phononic excitations. We use a model that allows us to study the redistribution and the related phonon self-energy effects simultaneously. We apply this model to the B_1g phonon of Y_{1-x}(Pr,Ca)_xBa_2Cu_3O_7 films, where Pr or Ca substitution enables us to investigate under- and overdoped samples. While various self-energy effects can be explained by the strength and energy of the 2Δpeaks, anomalies remain. We discuss possible origins of these anomalies.

cond-mat.supr-con↗

arXiv1999

Resonant Raman Study of Superconducting Gap and Electron-Phonon Coupling in YbBa2Cu3O7-delta

We investigate the electronic background as well as the O2-O3 mode at 330 cm^-1 of highly doped YbBa2Cu3O7-delta in B1g symmetry. Above the critical temperature Tc the spectra consist of an almost constant electronic background and superimposed phononic excitations. Below Tc the superconducting gap opens and the electronic background redistributes exhibiting a 2Delta peak at 320 cm^-1. We use a model that allows us to separate the background from the phonon. In this model the phonon intensity is assigned to the coupling of the phonon to inter- and intraband electronic excitations. For excitation energies between 1.96 eV and 2.71 eV the electronic background exhibits hardly any resonance. Accordingly, the intraband contribution to the phonon intensity is not affected. In contrast, the interband contribution vanishes below Tc at 1.96 eV while it remains almost unaffected at 2.71 eV.

cond-mat.supr-con↗