arXiv subjects

Andres Cathey

Publications and source records attributed to Andres Cathey.

4 recordsLinked to original sources

arXiv2023

Emission dynamics and spectrum of a nanoshell-based plasmonic nanolaser spaser

We study theoretically the emission and lasing properties of a single nanoshell spaser nanoparticle, or plasmonic nanolaser, made of an active core (gain material) and a plasmonic metal shell. Based on an analytical framework coupling together time-dependent equations for the gain and the metal, we calculate the lasing threshold with the help of an instability analysis. We characterize the regime under the threshold, where the nanoshell behaves as an optical amplifier when excited by an incident probe field. We then investigate in depth the non-linear lasing regime above the threshold, under autonomous conditions (free lasing without external drive), by computing the system's dynamics both in the transient state and in the final steady state. We show that at threshold, the lasing starts at one frequency only, usually one of the plasmon resonances of the nanoshell; then as the gain is further raised, the emission widens to other frequencies. This differs significantly from previous findings in the literature, which found only one emission wavelength above threshold. We proceed to calculate the complete (maximal) emission spectrum of the nanolaser as well as its emission linewidth, both of which are evidenced to be affected by unusually strong frequency shifts (pull-out) effects. We find that the nanolaser emission is highly asymmetrical spectrally and only occurs on one side (high-frequency) of the plasmon resonance. Finally, we show that the spectral position of the emission line can be tuned across the whole visible range, by changing the geometrical aspect ratio of the nanoshell.

physics.optics↗

arXiv2023

MHD simulations of formation, sustainment and loss of Quiescent H-mode in the all-tungsten ASDEX Upgrade

Periodic edge localized modes (ELMs) are the non-linear consequences of pressure-gradient-driven ballooning modes and current-driven peeling modes becoming unstable in the pedestal region of high confinement fusion plasmas. In future tokamaks like ITER, large ELMs are foreseen to severely affect the lifetime of wall components as they transiently deposit large amounts of heat onto a narrow region at the divertor targets. Several strategies exist for avoidance, suppression, or mitigation of these instabilities, such as the naturally ELM-free quiescent H-mode (QH-mode). In the present article, an ASDEX Upgrade equilibrium that features a QH-mode is investigated through non-linear extended MHD simulations covering the dynamics over tens of milliseconds. The equilibrium is close to the ideal peeling limit and non-linearly develops saturated modes at the edge of the plasma. A dominant toroidal mode number of $n=1$ is found, for which the characteristic features of the edge harmonic oscillation are recovered. The saturated modes contribute to heat and particle transport preventing pedestal build-up to the ELM triggering threshold. The non-linear dynamics of the mode, in particular its interaction with the evolution of the edge safety factor is studied, which suggest a possible new saturation mechanism for the QH-mode. The simulations show good qualitative and quantitative agreement to experiments in AUG. In particular, the processes leading to the termination of QH-mode above a density threshold is studied, which results in the transition into an ELM regime. In the vicinity of this threshold, limit cycle oscillations are observed.

physics.plasm-ph↗

arXiv2022

Filamentary plasma eruptions and the heating and acceleration of electrons

We present test-particle simulations of electrons during a nonlinear MHD simulation of a type-I edge localized mode (ELM) to explore the effect of an eruptive plasma filament on the kinetic level. The electrons are moderately heated and accelerated during the filamentary eruption on a fast time scale of the order of 0.5 ms. A clearly non-thermal tail is formed in the distribution of the kinetic energy that is of power-law shape and reaches 90 keV for some particles. The acceleration is exclusively observed in the direction parallel to the magnetic field, i.e. with a clear preference in counter-current direction, and we show that the parallel electric field is the cause of the observed acceleration. Most particles that escape from the system leave at one distinct strike-line in the outer divertor leg at some time during their energization. The escaping high energy electrons in the tail of the energy distribution are not affected by collisions, they thus show characteristics of runaway electrons. The mean square displacement indicates that transport in energy space clearly is super-diffusive, and interpreting the acceleration process as a random walk, we find that the distributions of energy-increments exhibit exponential tails, and transport in energy space is equally important of convective (systematic) and diffusive (stochastic) nature. By analyzing the MHD simulations per se, it turns out that the histograms of the parallel electric field in the edge region exhibit power-law shapes, and this clearly non-Gaussian statistics is ultimately one of the reasons for the moderately anomalous phenomena of particle transport that we find in energy space.

physics.plasm-ph↗

arXiv2020

Non-linear extended MHD simulations of type-I edge localised mode cycles in ASDEX Upgrade and their underlying triggering mechanism

A triggering mechanism responsible for the explosive onset of edge localised modes (ELMs) in fusion plasmas is identified by performing, for the first time, non-linear magnetohydrodynamic simulations of repetitive type-I ELMs. Briefly prior to the ELM crash, destabilising and stabilising terms are affected at different timescales by an increasingly ergodic magnetic field caused by non-linear interactions between the axisymmetric background plasma and growing non-axisymmetric perturbations. The separation of timescales prompts the explosive, i.e. faster than exponential, growth of an ELM crash which lasts ${\sim}$ 500 $μ$s. The duration and size of the simulated ELM crashes compare qualitatively well with type-I ELMs in ASDEX Upgrade. As expected for type-I ELMs, a direct proportionality between the heating power in the simulations and the ELM repetition frequency is obtained. The simulations presented here are a major step forward towards predictive modelling of ELMs and of the assessment of mitigation techniques in ITER and other future tokamaks.

physics.plasm-ph↗