arXiv subjects

Gregor Morfill

Publications and source records attributed to Gregor Morfill.

5 recordsLinked to original sources

arXiv2013

Effect of strong wakes on waves in two-dimensional plasma crystals

We study effects of the particle-wake interactions on the dispersion and polarization of dust lattice wave modes in two-dimensional plasma crystals. Most notably, the wake-induced coupling between the modes causes the branches to "attract" each other, and their polarizations become elliptical. Upon the mode hybridization the major axes of the ellipses (remaining mutually orthogonal) rotate by $45^\circ$. To demonstrate importance of the obtained results for experiments, we plot spectral densities of the longitudinal and transverse waves revealing distinct fingerprints of the mixed polarization. Furthermore, we show that at strong coupling the hybrid mode is significantly shifted towards smaller wave numbers, away from the border of the first Brillouin zone (where the hybrid mode is localized for a weak coupling).

physics.plasm-ph↗

arXiv2013

High-voltage nanosecond pulses in a low-pressure radiofrequency discharge

An influence of a high-voltage (3-17 kV) 20 ns pulse on a weakly-ionized low-pressure (0.1-10 Pa) capacitively-coupled radiofrequency (RF) argon plasma is studied experimentally. The plasma evolution after pulse exhibits two characteristic regimes: a bright flash, occurring within 100 ns after the pulse (when the discharge emission increases by 2-3 orders of magnitude over the steady-state level), and a dark phase, lasting a few hundreds μs (when the intensity of the discharge emission drops significantly below the steady-state level). The electron density increases during the flash and remains very large at the dark phase. 1D3V particle-in-cell simulations qualitatively reproduce both regimes and allow for detailed analysis of the underlying mechanisms. It is found that the high-voltage nanosecond pulse is capable of removing a significant fraction of plasma electrons out of the discharge gap, and that the flash is the result of the excitation of gas atoms, triggered by residual electrons accelerated in the electric field of immobile bulk ions. The secondary emission from the electrodes due to vacuum UV radiation plays an important role at this stage. High-density plasma generated during the flash provides efficient screening of the RF field (which sustains the steady-state plasma). This leads to the electron cooling and, hence, onset of the dark phase.

physics.plasm-ph↗

arXiv2012

On the heterogeneous character of the heartbeat instability in complex (dusty) plasmas

A hypothesis on the physical mechanism generating the heartbeat instability in complex (dusty) plasmas is presented. It is suggested that the instability occurs due to the periodically repeated critical transformation on the boundary between the microparticle-free area (void) and the complex plasma. The critical transformation is supposed to be analogous to the formation of the sheath in the vicinity of an electrode. The origin of the transformation is the loss of the electrons and ions on microparticles surrounding the void. We have shown that this hypothesis is consistent with the experimentally measured stability parameter range, with the evolution of the plasma glow intensity and microparticle dynamics during the instability, as well as with the observed excitation of the heartbeat instability by an intensity-modulated laser beam, tuned to the atomic transition of a working gas.

physics.plasm-ph↗

arXiv2010

Agglomeration of microparticles in complex plasmas

Agglomeration of highly charged microparticles was observed and studied in complex plasma experiments carried out in a capacitively coupled rf discharge. The agglomeration was caused by strong dust density waves triggered in a particle cloud by decreasing neutral gas pressure. Using a high-speed camera during this unstable regime, it was possible to resolve the motion of individual microparticles and to show that the relative velocities of some particles were sufficiently high to overcome the mutual Coulomb repulsion and hence to result in agglomeration. After stabilising the cloud again through the increase of the pressure, we were able to observe the aggregates directly with a long-distance microscope. We show that the agglomeration rate deduced from our experiments is in good agreement with theoretical estimates. In addition, we briefly discuss the mechanisms that can provide binding of highly charged microparticles in a plasma.

physics.plasm-ph↗

arXiv2004

Detecting non-linearities in data sets. Characterization of Fourier phase maps using the Weighted Scaling Indices

We present a methodology for detecting non-linearities in data sets based on the characterization of the structural features of the Fourier phase maps. A Fourier phase map is a 2D set of points $M= \{(ϕ_{\vec{k}}, ϕ_{\vec{k} + \vecΔ})\}$, where $ ϕ_{\vec{k}}$ is the phase of the $k$-mode of the Fourier transform of the data set and $\vecΔ$ a phase shift. The information thus rendered on this space is analyzed using the spectrum of weighted scaling indices to detect phase coupling at any scale $\vecΔ$. We propose a statistical test of significance based on the comparison of the properties of phase maps created from both the original data and surrogate realizations. We have applied our method to the Lorenz system and the logarithmic stock returns of the Dow Jones index. Applications to higher dimensional data are straightforward. The results indicate that both the Lorenz system and the Dow Jones time series exhibit significant signatures of non-linear behavior.

physics.data-an↗