arXiv subjects

Markus H. Thoma

Publications and source records attributed to Markus H. Thoma.

At least 19 recordsLinked to original sources

arXiv2024

Surface Modifications of PCB-Based Plasma Sources Induced by Atmospheric Plasma: A Comparative Study of Dielectric and Electrode Materials

The study investigated the effects of atmospheric plasma on various dielectric materials (FR-4, PTFE, Al$_2$O$_3$) and electrode materials (copper, silver, gold-plated copper) used in surface micro-discharge plasma sources. XPS and laser microscopy were used to analyze changes in surface properties and chemical composition after 10 hours of plasma exposure. Al$_2$O$_3$ showed the highest resistance, with no significant changes in Al/O ratio or oxidation state. PTFE underwent oxidation, with fluorine substituted by oxygen, forming carbonyl, hydroxyl and aldehyde groups. FR-4 showed the most substantial changes, with etching of the epoxy matrix exposing glass fibers. This was in line with the XPS results, which show a higher O/C-ratio, less epoxy groups and higher nitrogen signals. Silver electrodes were most resilient, with sintered particles redistributed across the Al$_2$O$_3$ surface. Copper and gold-plated copper electrodes were more susceptible to oxidation and degradation, especially in areas exposed to plasma filaments. The results highlight the importance of carefully selecting materials based on resistance to plasma exposure and compatibility with the application.

physics.plasm-ph↗

arXiv2022

A Novel Approach on Dielectric Barrier Discharge using Printed Circuit Boards

Drug resistant bacteria, prions and nosocomial infections underline the need of more effective sterilizing technologies. The cold plasma technology is expected to bring a benefit in this context. Six different plasma sources, based on printed circuit boards, were evaluated fourfold. This include measurements of the power consumption, the ignition behavior by an ICCD-camera and ozone formation by absorption spectroscopy at 254 nm. To evaluate the biocidal effect, four bacterial test series were performed with Escherichia coli. The entirety of the tests analyze the plasma inactivation process from the input parameters to the desired biocidal effect. The discharge current and time resolved ignition behaviors indicated a simultaneous formation of filaments at the beginning of the negative half-cycle. The dynamics of the ozone production showed a saturated exponential growth upon a maximum value of 435 ppm. Additionally, the microbiological test series unveiled differences between the plasma source concepts. A total reduction rate of Log-4 within a minute was achievable. An air flow through slits within the plasma sources destabilized the plasma. Minor changes of the electrode geometry changed all measured parameters. Hence, to develop a pathogen inactivating plasma source, these results recommend a comb-shaped electrode design, which is laminated on a dielectric.

physics.plasm-ph↗

arXiv2022

Influence of Temporal Variations in Plasma Conditions on the Electric Potential Near Self-Organized Dust Chains

The self-organization of dust grains into stable filamentary dust structures (or "chains") largely depends on dynamic interactions between the individual charged dust grains and the complex electric potential arising from the distribution of charges within the local plasma environment. Recent studies have shown that the positive column of the gas discharge plasma in the Plasmakristall-4 (PK-4) experiment onboard the International Space Station (ISS) supports the presence of fast-moving ionization waves, which lead to variations of plasma parameters by up to an order of magnitude from the average background values. The highly-variable environment resulting from ionization waves may have interesting implications for the dynamics and self-organization of dust particles, particularly concerning the formation and stability of dust chains. Here we investigate the electric potential surrounding dust chains in the PK-4 by employing a molecular dynamics model of the dust and ions with boundary conditions supplied by a Particle-in-Cell with Monte Carlo collisions (PIC-MCC) simulation of the ionization waves. The model is used to examine the effects of the plasma conditions within different regions of the ionization wave and compare the resulting dust structure to that obtained by employing the time-averaged plasma conditions. Comparison between simulated dust chains and experimental data from the PK-4 shows that the time-averaged plasma conditions do not accurately reproduce observed results for dust behavior, indicating that more careful treatment of plasma conditions in the presence of ionization waves is required. It is further shown that commonly used analytic forms of the electric potential do not accurately describe the electric potential near charged dust grains under these plasma conditions.

physics.plasm-ph↗

arXiv2021

Cold Atmospheric Plasma Sterilization of FFP3 Face Masks and Long-Term Material Effects

The use of cold atmospheric plasmas (CAP) to sterilize sensitive surfaces is an interesting new field of applied plasma physics. Motivated by the shortages of face masks and safety clothing at the beginning of the corona pandemic, we conducted studies on the sterilization of FF3 face masks with CAP and the resulting material effects. Therefore, the bactericidal and sporicidal efficacy of CAP afterglow sterilization of FFP3 mask material was investigated by inoculating fabric samples with test germs Escherichia coli (E. coli) and Bacillus atrophaeus (B. atrophaeus) and subsequent CAP afterglow treatment in a surface-micro-discharge (SMD) plasma device. In addition, a detailed analysis of the changes in long-term plasma treated (15h) mask material and its individual components - ethylene vinyl acetate (EVA) and polypropylene (PP) - was carried out using surface analysis methods such as laser microscopy, contact angle measurements, X-ray photoelectron spectroscopy (XPS) as well as fabric permeability and resistance measurements. The experiments showed that E. coli and B. atrophaeus could both be effectively inactivated by plasma treatment in nitrogen mode (12 kVpp, 5 kHz). For B. atrophaeus inactivation of more than 4-log was achieved after 30 minutes. E. coli population could be reduced by 5-log within one minute of CAP treatment and after five minutes a complete inactivation (> 6-log) was achieved. Material analysis showed that long-term (> 5 h) plasma treatment affects the electrostatic properties of the fabric. From this it can be deduced that the plasma treatment of FFP3 face masks with the CAP afterglow of an SMD device effectively inactivates microorganisms on the fabric. FFP3 masks can be plasma decontaminated and reused multiple times but only to a limited extent, as otherwise the permeability levels no longer meet the DIN EN 149 specifications.

physics.plasm-ph↗

arXiv2020

Comparative Study of the Surface Potential of Magnetic and Non-magnetic Spherical Objects in a Magnetized RF Discharge Plasma

We report measurements of the time-averaged surface floating potential of magnetic and non-magnetic spherical probes (or large dust particles) immersed in a magnetized capacitively coupled discharge. In this study, the size of the spherical probes is taken greater than the Debye length. The surface potential of a spherical probe first increases, i.e. becomes more negative at low magnetic field (B $<$ 0.05 T), attains a maximum value and decreases with further increase of the magnetic field strength (B $>$ 0.05 T). The rate of change of the surface potential in the presence of a B-field mainly depends on the background plasma and types of material of the objects. The results show that the surface potential of the magnetic sphere is higher (more negative) compared to the non-magnetic spherical probe. Hence, the smaller magnetic sphere collects more negative charges on its surface than a bigger non-magnetic sphere in a magnetized plasma. The different sized spherical probes have nearly the same surface potential above a threshold magnetic field (B $>$ 0.03 T), implicating a smaller role of size dependence on the surface potential of spherical objects. The variation of the surface potential of the spherical probes is understood on the basis of a modification of the collection currents to their surface due to charge confinement and cross-field diffusion in the presence of an external magnetic field.

physics.plasm-ph↗

arXiv2020

Disalignment rate coefficient of argon $\mathrm{2p_8}$ due to nitrogen collision

Tunable diode laser induced fluorescence (TDLIF) measurements are discussed and quantitatively evaluated for nitrogen admixtures in argon plasma under the influence of a strong magnetic field. TDLIF measurements were used to evaluate light-transport properties in a strongly magnetized optically thick argon/nitrogen plasma under different pressure conditions. Therefore, a coupled system of rate balance equations was constructed to describe laser pumping of individual magnetic sub-levels of $\mathrm{2p_8}$ state through frequency-separated sub-transitions originating from $\mathrm{1s_4}$ magnetic sub-levels. The density distribution (alignment) of $\mathrm{2p_8}$ multiplet was described by balancing laser pumping with losses including radiative decay, transfer of excitation between the neighboring sub-levels in the $\mathrm{2p_8}$ multiplet driven by neutral collisions (argon and nitrogen) and quenching due to electron and neutral collisions. Resulting $\mathrm{2p_8}$ magnetic sub-level densities were then used to model polarization dependent fluorescence, considering self-absorption, which could be directly compared with polarization-resolved TDLIF measurements. This enables to estimate the disalignment rate constant for the $\mathrm{2p_8}$ state due to collisions by molecular nitrogen. A comparison to molecular theory description is given providing satisfactory agreement. The presented measurement method and model can help to describe optical emission of argon and argon-nitrogen admixtures in magnetized conditions and provides a basis for further description of optical emission spectra in magnetized plasmas.

physics.plasm-ph↗

arXiv2020

Rotational properties of annulus dusty plasma in a strong magnetic field

The collective dynamics of annulus dusty plasma formed between a co-centric conducting (non-conducting) disk and ring configuration is studied in a strongly magnetized radio-frequency (rf) discharge. A superconducting electromagnet is used to introduce a homogeneous magnetic field to the dusty plasma medium. In absence of the magnetic field, dust grains exhibit thermal motion around their equilibrium position. The dust grains start to rotate in anticlockwise direction with increasing magnetic field (B $>$ 0.02 T), and the constant value of the angular frequency at various strengths of magnetic field confirms the rigid body rotation. The angular frequency of dust grains linearly increases up to a threshold magnetic field (B $>$ 0.6 T) and after that its value remains nearly constant in a certain range of magnetic field. Further increase in magnetic field (B $>$ 1 T) lowers the angular frequency. Low value of angular frequency is expected by reducing the width of annulus dusty plasma or the input rf power. The azimuthal ion drag force due to the magnetic field is assumed to be the energy source which drives the rotational motion. The resultant radial electric field in the presence of magnetic field determines the direction of rotation. The variation of floating (plasma) potential across the annular region at given magnetic field explains the rotational properties of the annulus dusty plasma in the presence of magnetic field.

physics.plasm-ph↗

arXiv2020

3-Dimensional Dusty Plasma in a Strong Magnetic Field: Observation of Rotating Dust Tori

The paper reports on the dynamics of a 3-dimensional dusty plasma in a strong magnetic field. An electrostatic potential well created by a conducting or non-conducting ring in the rf discharge confines the charged dust particles. In the absence of the magnetic field, dust grains exhibit a thermal motion about their equilibrium position. As the magnetic field crosses a threshold value (B $>$ 0.02 T), the edge particles start to rotate and form a vortex in the vertical plane. At the same time, the central region particles either exhibit thermal motion or $\vec{E} \times \vec{B}$ motion in the horizontal plane. At B $>$ 0.15 T, the central region dust grains start to rotate in the opposite direction resulting in a pair of counter-rotating vortices in the vertical plane. The characteristics of the vortex pair change with increasing the strength of the magnetic field (B $\sim$ 0.8 T). At B $>$ 0.8 T, dust grains exhibit very complex motion in the rotating torus. The angular frequency variation of rotating particles indicates a differential or sheared dust rotation in a vortex. The angular frequency increases with increasing the magnetic field from 0.05 T to 0.8 T. The ion drag force and dust charge gradient along with the E-field are considered as possible energy sources for driving the edge vortex flow and central region vortex motion, respectively. The directions of rotation also confirm the different energy sources responsible for the vortex motion.

physics.plasm-ph↗

arXiv2020

Quantitative evaluation of laser-induced fluorescence in magnetized plasma accounting for disalignment effect

Quantitative evaluation of tunable diode laser induced fluorescence (TDLIF) measurements in magnetized plasma take into account Zeeman splitting of energetic levels and intra-multiplet mixing defining the density distribution (alignment) of excited $\mathrm{2p_8}$ multiplet is discussed in this paper. TDLIF measurements were used to evaluate light-transport properties in a strongly magnetized optically thick argon plasma under different pressure conditions. Therefore, a coupled system of rate balance equations were constructed to describe laser pumping of individual magnetic sub-levels of $\mathrm{2p_8}$ state through frequency separated sub-transitions originating from $\mathrm{1s_4}$ magnetic sub-levels. The density distribution of $\mathrm{2p_8}$ multiplet was described by balancing laser pumping with losses including radiative decay, transfer of excitation between the neighboring multiplets driven by neutral collisions and quenching due to electron and neutral collisions. Resulting $\mathrm{2p_8}$ magnetic sub-level densities were then used to model polarization dependent fluorescence, consider self-absorption, which could be directly compared with measured polarization resolved TDLIF measurements. This enables to obtain unique solutions for the $\mathrm{1s_4}$ and $\mathrm{1s_5}$ magnetic sub-level densities which were in good agreement with the densities obtained by laser absorption measurements. It is shown that LIF measurements in magnetized plasma conditions have strong pressure dependence that should be corrected consider effective disalignment rate. The presented measurement method and model can help further understanding and improve description of optical emission of argon in magnetized conditions.

physics.plasm-ph↗

arXiv2019

Influence of External Magnetic Field on Dust$-$Acoustic Waves in a Capacitive RF Discharge

This paper reports experiments on self$-$excited dust acoustic waves (DAWs) and its propagation characteristics in a magnetized rf discharge plasma. The DAWs are spontaneously excited in dusty plasma after adding more particles in the confining potential well and found to propagate in the direction of streaming ions. The spontaneous excitation of such low-frequency modes is possible due to the instabilities associated with streaming ions through the dust grain medium. The background E-field and neutral pressure determine the stability of excited DAWs. The characteristics of DAWs strongly depend on the strength of external magnetic field. The magnetic field of strength B $<$ 0.05 T only modifies the characteristics of propagating waves in dusty plasma at moderate power and pressure, P = 3.5 W and p = 27 Pa respectively. It is found that DAWs start to be damped with increasing the magnetic field beyond B $>$ 0.05 T and get completely damped at higher magnetic field B $\sim$ 0.13 T. After lowering the power and pressure to 3 W and 23 Pa respectively, the excited DAWs in the absence of B are slightly unstable. In this case, the magnetic field only stabilizes and modifies the propagation characteristics of DAWs while the strength of B is increased up to 0.1 T or even higher. The modification of the sheath electric field where particles are confined in the presence of the external magnetic field is the main cause of the modification and damping of the DAWs in a magnetized rf discharge plasma.

physics.plasm-ph↗

arXiv2012

Screening Masses in Gluonic Plasma

Both electric and magnetic screening masses in a nonperturbative gluonic background are investigated using operator product expansion. The magnetic screening mass is found to agree with lattice results whereas the electric screening mass is somewhat smaller than the one found on the lattice.

hep-ph↗

arXiv2011

Conserved Density Fluctuation and Temporal Correlation Function in HTL Perturbation Theory

Considering recently developed Hard Thermal Loop perturbation theory that takes into account the effect of the variation of the external field through the fluctuations of a conserved quantity we calculate the temporal component of the Euclidian correlation function in the vector channel. The results are found to be in good agreement with the very recent results obtained within the quenched approximation of QCD and small values of the quark mass ($\sim 0.1T$) on improved lattices of size $128^3\times N_τ$ at ($N_τ=40, \ T=1.2T_C$), ($N_τ=48, \ T=1.45T_C$), and ($N_τ=16, \ T=2.98T_C$), where $N_τ$ is the temporal extent of the lattice. This suggests that the results from lattice QCD and Hard Thermal Loop perturbation theory are in close proximity for a quantity associated with the conserved density fluctuation.

hep-ph↗

arXiv2010

Low Mass Dilepton Rate from the Deconfined Phase

We discuss low mass dilepton rates ($\le 1$ GeV) from the deconfined phase of QCD using both perturbative and non-perturbative models and compare with those from lattice gauge theory and in-medium hadron gas. Our analysis suggests that the rate at very low invariant mass ($ M\le 200$ MeV) using the nonperturbative gluon condensate in a semiempirical way within the Green function dominates over the Born-rate and independent of any uncertainty associated with the choice of the strong coupling in perturbation theory. On the other hand the rate from $ρ-q$ interaction in the deconfined phase is important between 200 MeV $\le M \le 1 GeV as it is almost of same order of the Born-rate as well as in-medium hadron gas rate. Also the higher order perturbative rate, leaving aside its various uncertainties, from HTL approximation becomes reliable at $M\ge 200$ MeV and also becomes comparable with the Born-rate and the lattice-rate for $M\ge 500$ MeV, constraining on the broad resonance structures in the dilepton rate at large invariant mass. We also discuss the lattice constraints on the low mass dilepton rate. Furthermore, we discuss a more realistic way to advocate the quark-hadron duality hypothesis based on the dilepton rates from QGP and hadron gas than it is done in the literature.

hep-ph↗

arXiv2010

Neutrino decay into fermionic quasiparticles in leptogenesis

We calculate the decay rate of the lightest heavy Majorana neutrino in a thermal bath using finite temperature cutting rules and effective Green's functions according to the hard thermal loop resummation technique. Compared to the usual approach where thermal masses are inserted into the kinematics of final states, we find that deviations arise through two different leptonic dispersion relations. The decay rate differs from the usual approach by more than one order of magnitude in the temperature range which is interesting for the weak washout regime. This work summarizes the results of Ref. [1], to which we refer the interested reader.

hep-ph↗

arXiv2010

Spontaneous disordering and symmetry breaking in complex plasmas

Spontaneous symmetry breaking is an essential feature of modern science. We demonstrate that it also plays an important role in the physics of complex plasmas. Complex plasmas can serve as a powerful tool for observing and studying discrete types of symmetry and disordering at the kinetic level that numerous many-body systems exhibit.

physics.plasm-ph↗

arXiv2010

Decay of a Yukawa fermion at finite temperature and applications to leptogenesis

We calculate the decay rate of a Yukawa fermion in a thermal bath using finite temperature cutting rules and effective Green's functions according to the hard thermal loop resummation technique. We apply this result to the decay of a heavy Majorana neutrino in leptogenesis. Compared to the usual approach where thermal masses are inserted into the kinematics of final states, we find that deviations arise through two different leptonic dispersion relations. The decay rate differs from the usual approach by more than one order of magnitude in the temperature range which is interesting for the weak washout regime. We discuss how to arrive at consistent finite temperature treatments of leptogenesis.

hep-ph↗

arXiv2009

Field Theoretic Description of Ultrarelativistic Electron-Positron Plasmas

Ultrarelativistic electron-positron plasmas can be produced in high-intensity laser fields and play a role in various astrophysical situations. Their properties can be calculated using QED at finite temperature. Here we will use perturbative QED at finite temperature for calculating various important properties, such as the equation of state, dispersion relations of collective plasma modes of photons and electrons, Debye screening, damping rates, mean free paths, collision times, transport coefficients, and particle production rates, of ultrarelativistic electron-positron plasmas. In particular, we will focus on electron-positron plasmas produced with ultra-strong lasers.

physics.plasm-ph↗

arXiv2008

Ultrarelativistic Electron-Positron Plasma

hep-ph↗