Plasma Relaxation Research Articles

Electron beam relaxation in inhomogeneous plasmas

Abstract. In this work, we studied the effects of background plasma density fluctuations on the relaxation of electron beams. For the study, we assumed that the level of fluctuations was so high that the majority of Langmuir waves generated as a result of beam-plasma instability were trapped inside density depletions. The system can be considered as a good model for describing beam-plasma interactions in the solar wind. Here we show that due to the effect of wave trapping, beam relaxation slows significantly. As a result, the length of relaxation for the electron beam in such an inhomogeneous plasma is much longer than in a homogeneous plasma. Additionally, for sufficiently narrow beams, the process of relaxation is accompanied by transformation of significant part of the beam kinetic energy to energy of accelerated particles. They form the tail of the distribution and can carry up to 50% of the initial beam energy flux.

Reduced coupled-mode approach to electron-ion energy relaxation

We present a reduced model for the energy transfer via coupled collective modes in two-temperature plasmas based on quantum statistical theory. The model is compared with exact numerical evaluations of the coupled-mode (CM) energy transfer rate and with alternative reduced approaches over a range of conditions in the warm dense matter (WDM) and inertial confinement fusion (ICF) regimes. Our approach shows excellent agreement with an exact treatment of the CM rate and supports the importance of the coupled-mode effect for the temperature and energy relaxation in WDM and ICF plasmas. We find that electronic damping of collective ion density fluctuations is crucial for correctly describing the mode spectrum and, thus, the energy exchange. The reduced CM approach is studied over a wide parameter space, enabling us to establish its limits of applicability.

Comment on “Woltjer-Taylor State without Taylor’s Conjecture: Plasma Relaxation at All Wavelengths”

We find that the proof in the recent paper$\textsuperscript{\cite{14}}$ can not justify the authors' conclusion. We provide a real proof that any state will eventually evolves to the Woltjer-Taylor state exponentially. However, this kind of evolution is is mainly due to Joule heat, which also makes the magnetic field vanishes exponentially. Zero Woltjer-Taylor states are not physically attractive. Instead of examine $\Delta$, we introduce the quantity $\theta_{\nabla \times \vec{B},\vec{B}}$ and $R$ to examine if the plasma reaches to the strong (general) Woltjer-Taylor state, and then derive the condition for the evolution to the strong/general Woltjer-Taylor state.

Effective potential theory for transport coefficients across coupling regimes.

A plasma transport theory that spans weak to strong coupling is developed from a binary collision picture, but where the interaction potential is taken to be an effective potential that includes correlation effects and screening self-consistently. This physically motivated approach provides a practical model for evaluating transport coefficients across coupling regimes. The theory is shown to compare well with classical molecular dynamics simulations of temperature relaxation in electron-ion plasmas as well as simulations and experiments of self-diffusion in one-component plasmas. The approach is versatile and can be applied to other transport coefficients as well.

Dynamics of the formation of intense magnetic flux tubes on the sun by a converging plasma flow

We study the dynamics of the formation of an intense magnetic flux tube (IMFT) from the background magnetic field by a converging plasma flow with nonlinear magnetic diffusion due to the Cowling conductivity. We confirm the formation of a stationary profile of the magnetic field that was obtained earlier and estimate its onset time. We also consider relaxation of the background field inhomogeneities in the course of their advection into the magnetic flux tube. In the case of nonzero projection of the electrical current on the magnetic field, we found a new mode of the field inhomogeneity relaxation in plasmas with a highly anisotropic resistivity tensor. In this mode, unlike the case of absence of currents along the magnetic flux tube axis, which was studied earlier, an impulsive component of the magnetic energy release is observed, and the magnetic field outside the impulsive component is nearly force-free. A qualitative description of this phenomenon is proposed, analytical estimates are given, and the results of numerical calculations are presented.

Effects of magnetic field on anisotropic temperature relaxation

In a strongly magnetized plasma, where the particles' thermal gyro-radii are smaller than the Debye length, the magnetic field greatly affects the plasma's relaxation processes. The expressions for the time rates of change of the electron and ion parallel and perpendicular temperatures are obtained and calculated analytically for small anisotropies through considering binary collisions between charged particles in the presence of a uniform magnetic field by using perturbation theory. Based on these expressions, the effects of the magnetic field on the relaxation of anisotropic electron and ion temperatures due to electron-electron collisions, ion-ion collisions, and electron-ion collisions are investigated. Consequently, the relaxation times of anisotropic electron and ion temperatures to isotropy are calculated. It is shown that electron-ion collisions can affect the relaxation of an anisotropic ion distribution in the strong magnetic field.

In vitro and in vivo evaluation of superparamagnetic iron oxide nanoparticles coated by bisphosphonates: The effects of electrical charge and molecule length

Physicochemical coating properties are often considered to be determining factors for in vivo characteristics of superparamagnetic iron oxide nanoparticles, used as contrast agent in Magnetic Resonance Imaging (MRI). To investigate the electrical charge (modified by zero, one or two ammonium groups) and the molecule length (3, 5 or 7 methylene chains) effects of bisphosphonate-type coatings, we assessed the complement activation, in vivo plasma and tissue relaxation time alterations of intravenously injected small iron oxide nanoparticles (<25nm) on male healthy Wistar rats. The presence of ammonium groups induces a weak activation of the complement whatever the size and the concentration of particles, whereas hydroxyethylenebisphosphonate (HEBP)-coated particles are poor complement activators only at the lowest concentration. In vivo, HEBP-coated nanoparticles have the greatest prolonged relaxation time effects, despite their higher negative electrical charge, contrary to two ammonium bearing coatings. No significant differences were observed between mono-ammonium molecular coatings.

Electron-ion temperature relaxation in hydrogen plasmas

We present an ad hoc method to correct the Coulomb logarithm of known models in order to match electron and ion temperature relaxation based on molecular dynamic simulations. A thermodynamic analysis is done. Numerical results are presented and discussed.

Effect of Martensitic Transformation on the Optical Spectra of Cu-Mn-Al Alloy

The effect of martensitic transformation on the transport and optical properties of Cu-4.9Mn-23.4Al (at.%) alloy is reported. The resistivity measurements revealed the transformation temperatures Ms =284K and Af=303K. The optical properties of Cu-4.9Mn-23.4Al (at.%) alloy have been investigated at several temperatures near the region of austenite-martensite transformation. It was shown that intraband absorption is dominates in alloy for ħω &lt; 2.5 eV energy range. Plasma Ωp and relaxation γ frequencies of free charge carriers for Cu-4.9Mn-23.4Al (at.%) alloy were evaluated at several temperatures on the basis of Drude analysis of the alloy optical properties in the intraband absorption region. The growth of electrical resistance induced by martensitic transformation upon alloy cooling obtained directly from the resistivity measurements correlates nicely with the results derived from the optical measurements and can be explained by the reduction of the plasma frequency Ωp as well as increase in relaxation frequency γ of free charge carriers.

Woltjer-Taylor State without Taylor’s Conjecture: Plasma Relaxation at all Wavelengths

In the process of dissipative relaxation, there is strong astrophysical and laboratory evidence that plasmas tend to evolve towards the well-known Woltjer-Taylor state, specified by [nabla]×B=αB for constant α. To explain how such a state is reached, Taylor developed his famous theory based on the conjecture that relaxation is dominated by short wavelength fluctuations. However, there is no conclusive experimental or numerical evidence in support of Taylor's conjecture. A new theory is developed, which predicts that the system will evolve towards the Woltjer-Taylor state for an arbitrary fluctuation spectrum.

Electron acceleration during three-dimensional relaxation of an electron beam-return current plasma system in a magnetic field

We investigate the effects of acceleration during non-linear electron-beam relaxation in magnetized plasma in the case of electron transport in solar flares. The evolution of electron distribution functions is computed using a three-dimensional particle-in-cell electromagnetic code. Analytical estimations under simplified assumptions are made to provide comparisons. We show that, during the non-linear evolution of the beam-plasma system, the accelerated electron population appears. We found that, although the electron beam loses its energy efficiently to the thermal plasma, a noticeable part of the electron population is accelerated. For model cases with initially monoenergetic beams in uniform plasma, we found that the amount of energy in the accelerated electrons above the injected beam-electron energy varies depending the plasma conditions and could be around 10-30% of the initial beam energy. This type of acceleration could be important for the interpretation of non-thermal electron populations in solar flares. Its neglect could lead to the over-estimation of accelerated electron numbers. The results emphasize that collective plasma effects should not be treated simply as an additional energy-loss mechanism, when hard X-ray emission in solar flares is interpreted, notably in the case of RHESSI data.

Plasma relaxation and topological aspects in Hall magnetohydrodynamics

Parker’s formulation of isotopological plasma relaxation process in magnetohydrodynamics (MHD) is extended to Hall MHD. The torsion coefficient α in the Hall MHD Beltrami condition turns out now to be proportional to the potential vorticity. The Hall MHD Beltrami condition becomes equivalent to the potential vorticity conservation equation in two-dimensional (2D) hydrodynamics if the Hall MHD Lagrange multiplier β is taken to be proportional to the potential vorticity as well. The winding pattern of the magnetic field lines in Hall MHD then appears to evolve in the same way as potential vorticity lines in 2D hydrodynamics.

Characteristics of THz carrier dynamics in GaN thin film and ZnO nanowires by temperature dependent terahertz time domain spectroscopy measurement

We present a comprehensive study of the characteristics of carrier dynamics using temperature dependent terahertz time domain spectroscopy. By utilizing this technique in combination with numerical calculations, the complex refractive index, dielectric function, and conductivity of n-GaN, undoped ZnO NWs, and Al-doped ZnO NWs were obtained. The unique temperature dependent behaviors of major material parameters were studied at THz frequencies, including plasma frequency, relaxation time, carrier concentration and mobility. Frequency and temperature dependent carrier dynamics were subsequently analyzed in these materials through the use of the Drude and the Drude–Smith models.

Relaxation of a magnetized two ion species dusty plasma

A possibility of relaxation of an electronegative dusty plasma composed of two flowing ion species with stationary dust particles is explored. It is shown that magnetic field in such a system has the potential to self-organize giving rise to multiple Beltrami fields. When all the inertial and noninertial forces are taken into account, the relaxed state comes out to be a triple curl Beltrami field. On ignoring the inertia of lighter ions relative to heavy ones, double curl Beltrami field appears as a relaxed state which is equivalent to the superposition of two Beltrami fields. It shows the importance of inertia of plasma components in the formation of relaxed magnetic field structures. It is also shown numerically that scale separation allows the simultaneous existence of magnetic field and velocities varying at vastly different length scales. This gives rise to a dynamo mechanism at relaxed equilibrium. The study bears a strong relevance to the formation of multi scale structures in electronegative dusty plasmas, found in space and laboratory.

Mid- to long-wavelength infrared plasmonic-photonics using heavily doped n-Ge/Ge and n-GeSn/GeSn heterostructures

Heavily doped n-type Ge and GeSn are investigated as plasmonic conductors for integration with undoped dielectrics of Si, SiGe, Ge, and GeSn in order to create a foundry-based group IV plasmonics technology. N-type Ge1-xSnx with compositions of 0 ≤ x ≤ 0.115 are investigated utilizing effective-mass theory and Drude considerations. The plasma wavelengths, relaxation times, and complex permittivities are determined as functions of the free carrier concentration over the range of 10(10) to 10(21) cm-3. Basic plasmonic properties such as propagation loss and mode height are calculated and example numerical simulations are shown of a dielectric-conductor-dielectric ribbon waveguide structure are shown. Practical operation in the 2 to 20 μm wavelength range is predicted.

Infrared surface polaritons on antimony

The semimetal antimony, with a plasma frequency ~80 times less than that of gold, is potentially useful as a host for infrared surface polaritons (SPs). Relevant IR SP properties, including the frequency-dependent propagation length and penetration depths for fields into the media on either side of the interface, were determined from optical constants measured on optically-thick thermally-evaporated Sb films over the wavelength range 1 to 40 μm. Plasma and carrier relaxation frequencies were determined from Drude-model fits to these data. The real part of the permittivity is negative for wavelengths beyond 11 μm. Distinct resonant decreases in specular reflected intensity were observed for Sb lamellar gratings in the wavelength range of 6 to 11 μm, where the real part of the permittivity is positive. Both resonance angles and the angular reflectance spectral line shapes are in agreement with theory for excitation of bound surface electromagnetic waves (SPs). Finite element method (FEM) electrodynamic simulations indicate the existence of SP modes under conditions matching the experiments. FEM results also show that such waves depend on having a significant imaginary part of the permittivity, as has been noted earlier for the case of surface exciton polaritons.

A review of relaxation and structure in some turbulent plasmas: magnetohydrodynamics and related models

A brief overview is provided as an introduction to hydrodynamic-like turbulence that characterizes the dynamics of plasmas in several parameter regimes. This includes magnetohydrodynamics (MHD), the electron fluid plasma, which is closely related to two-dimensional hydrodynamics, and the solar wind, which is usually viewed as a laboratory for three-dimensional MHD, with more involved plasma physics at the dissipative scales. An emphasis is placed on energy decay, spectra, relaxation processes, coherent structures, and higher statistics with selected applications in solar wind and laboratory plasmas.

Plasma dynamics and structural modifications induced by femtosecond laser pulses in quartz

We have investigated plasma formation and relaxation dynamics induced by single femtosecond laser pulses at the surface of crystalline SiO2 (quartz) along with the corresponding topography modifications. The use of fs-resolved pump-probe microscopy allows combining spatial and temporal resolution and simultaneous access to phenomena occurring in adjacent regions excited with different local fluences. The results show the formation of a transient free-electron plasma ring surrounding the location of the inner ablation crater. Optical microscopy measurements reveal a 30% reflectivity decrease in this region, consistent with local amorphization. The accompanying weak depression of ≈15nm in this region is explained by gentle material removal via Coulomb explosion. Finally, we discuss the timescales of the plasma dynamics and its role in the modifications produced, by comparing the results with previous studies obtained in amorphous SiO2 (fused silica). For this purpose, we have conceived a new representation concept of time-resolved microscopy image stacks in a single graph, which allows visualizing quickly suble differences of the overall similar dynamic response of both materials.

Effect of Nano-sized Carbon Black Particles on Lung and Circulatory System by Inhalation Exposure in Rats

ObjectivesWe sought to establish a novel method to generate nano-sized carbon black particles (nano-CBPs) with an average size smaller than 100 nm for examining the inhalation exposure risks of experimental rats. We also tested the effect of nano-CBPs on the pulmonary and circulatory systems.MethodsWe used chemical vapor deposition (CVD) without the addition of any additives to generate nano-CBPs with a particle size (electrical mobility diameter) of less than 100nm to examine the effects of inhalation exposure. Nano-CBPs were applied to a nose-only inhalation chamber system for studying the inhalation toxicity in rats. The effect on the lungs and circulatory system was determined according to the degree of inflammation as quantified by bronchoalveolar lavage fluid (BALF). The functional alteration of the hemostatic and vasomotor activities was measured by plasma coagulation, platelet activity, contraction and relaxation of blood vessels.ResultsNano-CBPs were generated in the range of 83.3-87.9 nm. Rats were exposed for 4 hour/day, 5 days/week for 4 weeks to 4.2 × 106, 6.2 × 105, and 1.3 × 105 particles/cm3. Exposure of nano-CBPs by inhalation resulted in minimal pulmonary inflammation and did not appear to damage the lung tissue. In addition, there was no significant effect on blood functions, such as plasma coagulation and platelet aggregation, or on vasomotor function.ConclusionWe successfully generated nano-CBPs in the range of 83.3-87.9 nm at a maximum concentration of 4.2 × 106 particles/cm3 in a nose-only inhalation chamber system. This reliable method can be useful to investigate the biological and toxicological effects of inhalation exposure to nano-CBPs on experimental rats.

Ionization transition in low-density plasma

Ionization equilibrium in low-density low-temperature plasma is considered. It is demonstrated using hydrogen and cesium as examples that the Saha equation predicts an almost jump-like change in the electron density on isochors in a narrow temperature range. Thus, in contrast to a smooth rise in the degree of ionization with increasing temperature at high plasma densities, an increase in the temperature in low-density plasma should result in a sharp transition from a neutral state to a fully ionized plasma. This transition is accompanied by a jump-like increase in the electric conductivity. The relation of these effects to the recombination transition in the model of the early Universe is discussed. The possibility of observing such a transition experimentally and the problems concerning the time of plasma relaxation into an equilibrium state at long free path lengths of plasma particles are considered.