Non-thermal Ion Distributions Research Articles

Analysis of radiative gravitational modes in magnetized dusty plasma with non-thermal ion distribution

The characteristics of radiative gravitational modes in non-thermal magnetized dusty plasma with charge fluctuation and the polarization force of dust particles are investigated. The electron species are assumed to be radiative and the ions are assumed to be non-thermal. A normal mode analysis is applied to derive the general dispersion relation for radiative magnetized self-gravitating dusty plasma. Modified radiative condensation and self-gravitational instability criteria for both perpendicular and parallel modes of propagation are obtained. Analytically, it is found that the presence of a non-thermal ion population significantly modified the usual radiative condensation instability criterion, and damped the growth rate of the radiative mode. The obtained analytical results are further discussed in the numerical result section. We investigate the fact that the non-thermal ion population acts as a stabilizing source for radiative condensation instability in magnetized self-gravitating dusty plasma fluid, including the dust polarization force and charge fluctuation. The thermal conductivity with non-thermal ion distribution quickly moves the system towards stability. The implications of the results in the region of the interstellar medium (ISM) are discussed.

First evidence of non-Gaussian solar flare EUV spectral line profiles and accelerated non-thermal ion motion

The properties of solar flare plasma can be determined from the observation of optically thin lines. The emitting ion distribution determines the shape of the line, with an isothermal Maxwellian ion distribution producing a Gaussian profile. Non-Gaussian lines may indicate more complex ion distributions. We investigate the possibility of determining flare-accelerated non-thermal ion or plasma velocity distributions. We study EUV spectral lines during a flare SOL2013-05-15T01:45 using the Hinode EUV Imaging Spectrometer (EIS). The flare is located close to the eastern solar limb with an extended loop structure, allowing the different flare features: ribbons, hard X-ray (HXR) footpoints and the loop-top source to be clearly observed in UV, EUV and X-rays. EUV line spectroscopy is performed in seven different regions. We study the line profiles of isolated and unblended Fe XVI lines (262.976 A) formed at temperatures of 2-4 MK. Fe XVI line profiles at one time close to the peak soft X-ray emission and free of directed mass motions are examined using: 1. a higher moments analysis, 2. Gaussian fitting, and 3. by fitting a kappa distribution profile convolved with a Gaussian to account for the EIS instrumental profile. Fe XVI line profiles in the loop-top, HXR footpoint and ribbon regions can be confidently fitted with a kappa line profile, giving low, non-thermal kappa values between 2-3.3. A higher moments analysis also finds that many of the line kurtosis values are higher than the Gaussian value of 3, even with the presence of a broad Gaussian instrumental profile. A flare-accelerated non-thermal ion population could account for both the observed non-Gaussian line profiles, and for the Fe XVI excess broadening found from Gaussian fitting, if the emitting ions are interacting with a thermalised 4 MK electron population, and the instrumental profile is well-approximated by a Gaussian.

Ion firehose instability in a dusty plasma considering product-bi-kappa distributions for the plasma particles

We study the dispersion relation for low frequency waves in the whistler mode propagating along the ambient magnetic field, considering ions and electrons with product-bi-kappa (PBK) velocity distributions and taking into account the presence of a population of dust particles. The results obtained by numerical analysis of the dispersion relation show that the decrease in the κ indexes in the ion PBK distribution contributes to the increase in magnitude of the growth rates of the ion firehose instability and the size of the region in wave number space where the instability occurs. It is also shown that the decrease in the κ indexes in the electron PBK distribution contribute to decrease in the growth rates of instability, despite the fact that the instability occurs due to the anisotropy in the ion distribution function. For most of the interval of κ values which has been investigated, the ability of the non-thermal ions to increase the instability overcomes the tendency of decrease due to the non-thermal electron distribution, but for very small values of the kappa indexes the deleterious effect of the non-thermal electrons tends to overcome the effect due to the non-thermal ion distribution.

Ignition threshold for non-Maxwellian plasmas

An optically thin p-11B plasma loses more energy to bremsstrahlung than it gains from fusion reactions, unless the ion temperature can be elevated above the electron temperature. In thermal plasmas, the temperature differences required are possible in small Coulomb logarithm regimes, characterized by high density and low temperature. Ignition could be reached more easily if the fusion reactivity can be improved with nonthermal ion distributions. To establish an upper bound for the potential utility of a nonthermal distribution, we consider a monoenergetic beam with particle energy selected to maximize the beam-thermal reactivity. Comparing deuterium-tritium (DT) and p-11B, the minimum Lawson criteria and minimum ρR required for inertial confinement fusion (ICF) volume ignition are calculated with and without the nonthermal feature. It turns out that channeling fusion alpha energy to maintain such a beam facilitates ignition at lower densities and ρR, improves reactivity at constant pressure, and could be used to remove helium ash. On the other hand, the reactivity gains that could be realized in DT plasmas are significant, the excess electron density in p-11B plasmas increases the recirculated power cost to maintain a nonthermal feature and thereby constrains its utility to ash removal.

Rarefactive and compressive soliton waves in unmagnetized dusty plasma with non-thermal electron and ion distribution

Sagdeev's pseudo potential method is employed to study dust acoustic solitary waves in an unmagnetized plasma containing negatively charged dusts with non-thermal electron and ion. The range of parameters for the existence of solitary waves using the analytical expression of the Sagdeev potential has been found. It is observed that, depending on the values of the plasma parameters like ion to electron temperature ratio σ, non-thermal parameters β and γ, electron to ion density ratio μ, and the value of the Mach number M, both rarefactive and compressive solitary waves may exist.

Dust acoustic solitons in a charge varying dusty plasma in the presence of ion nonthermality and background nonextensivity

Dust acoustic (DA) solitons are addressed in a charge varying dusty plasma in the presence of ion nonthermality and background nonextensivity. A physically meaningful nonthermal nonextensive ion distribution is outlined. The correct non-Maxwellian ion charging current is derived based on the orbit-limited motion theory. Under grain-current balance, the variable dust charge is expressed in terms of the Lambert function. It is found that nonthermality and its nonextensive nature may act concurrently and influence the restoring force and hence the soliton profile. Due to the flexibility provided by the nonextensive parameter, we think that our model should provide a better fit of the space observations.

Multi-point, high-speed passive ion velocity distribution diagnostic on the Pegasus Toroidal Experiment

A passive ion temperature polychromator has been deployed on Pegasus to study power balance and non-thermal ion distributions that arise during point source helicity injection. Spectra are recorded from a 1 m F/8.6 Czerny-Turner polychromator whose output is recorded by an intensified high-speed camera. The use of high orders allows for a dispersion of 0.02 Å/mm in 4th order and a bandpass of 0.14 Å (~13 km/s) at 3131 Å in 4th order with 100 μm entrance slit. The instrument temperature of the spectrometer is 15 eV. Light from the output of an image intensifier in the spectrometer focal plane is coupled to a high-speed CMOS camera. The system can accommodate up to 20 spatial points recorded at 0.5 ms time resolution. During helicity injection, stochastic magnetic fields keep T(e) low (100 eV) and thus low ionization impurities penetrate to the core. Under these conditions, high core ion temperatures are measured (T(i) ≈ 1.2 keV, T(e) ≈ 0.1 keV) using spectral lines from carbon III, nitrogen III, and boron IV.

Mediation of the solar wind termination shock by non-thermal ions

Broad regions on both sides of the solar wind termination shock are populated by high intensities of non-thermal ions and electrons. The pre-shock particles in the solar wind have been measured by the spacecraft Voyager 1 (refs 1-5) and Voyager 2 (refs 3, 6). The post-shock particles in the heliosheath have also been measured by Voyager 1 (refs 3-5). It was not clear, however, what effect these particles might have on the physics of the shock transition until Voyager 2 crossed the shock on 31 August-1 September 2007 (refs 7-9). Unlike Voyager 1, Voyager 2 is making plasma measurements. Data from the plasma and magnetic field instruments on Voyager 2 indicate that non-thermal ion distributions probably have key roles in mediating dynamical processes at the termination shock and in the heliosheath. Here we report that intensities of low-energy ions measured by Voyager 2 produce non-thermal partial ion pressures in the heliosheath that are comparable to (or exceed) both the thermal plasma pressures and the scalar magnetic field pressures. We conclude that these ions are the >0.028 MeV portion of the non-thermal ion distribution that determines the termination shock structure and the acceleration of which extracts a large fraction of bulk-flow kinetic energy from the incident solar wind.

The Effect of Higher-Order Corrections on the Propagation of Nonlinear Dust-Acoustic Solitary Waves in a Dusty Plasma with Nonthermal Ions Distribution

Propagation of nonlinear dust-acoustic (DA) waves in a unmagnetized collisionless mesospheric dusty plasma containing positively and negatively charged dust grains and nonthermal ion distributions are investigated. For nonlinear DA waves, a reductive perturbation method is employed to obtain a Korteweg-de Vries (KdV) equation for the first-order potential. As it is well-known, KdV equations contain the lowest-order nonlinearity and dispersion, and consequently can be adopted for only small amplitudes. As the wave amplitude increases, the width and velocity of a soliton can not be described within the framework of KdV equations. So, we extend our analysis and take higher-order nonlinear and dispersion terms into account to clarify the essential effects of higher-order corrections. Moreover, in order to study the effects of higher-order nonlinearity and dispersion on the output solution, we address an appropriate technique, namely the renormalization method.

Observation of spontaneously excited waves in the ion cyclotron frequency range on JT-60U

In this paper, experimental observations of spontaneously excited waves in the ion cyclotron range of frequency (ICRF) on JT-60U are described. The fluctuations in ICRF are driven by the presence of non-thermal ion distribution in magnetically confined plasmas. Two types of magnetic fluctuations are detected: one is due to high-energy D ions from neutral beam (NB) injections and the other is due to fusion products (FPs) of 3He and T ions. These fluctuations have been reported as ion cyclotron emissions (ICEs) in the burning plasma experiments on large tokamaks. This paper describes the first measurement of the spatial structures of the excited modes in the poloidal and toroidal directions. It is confirmed by using ICRF antennas as pickup loops that all modes excited spontaneously have magnetic components. The modes due to D ions have zero or a small toroidal wave number kz. On the other hand, the measurement of finite kz in the modes due to FP ions supports the excitation of the Alfvén waves, which is the possible origin of FP-ICEs. It is also observed that the excited modes due to FP ions (3He and T ions) have different characteristics: driven by different NBs and having different parameter dependences. ICE due to T ions has no harmonics and the value of ω/Ωci is smaller than that due to 3He. Both beam-driven ICEs and FP-ICEs are clearly observed and their spatial structures are obtained on JT-60U.

Effect of non-thermal ion distributions on the Jeans instability in dusty plasmas

Non-thermal ion distributions are a characteristic feature of space plasmas, and in the current work the influence of such non-thermal ion distributions on the Jeans instability is examined. Following previous general models, the electrons are taken to be Boltzmann on the timescales involved, whilst the negative dust component is modelled by the continuity and momentum equations. The ion dynamics are described via a non-thermal Cairns distribution. The linear dispersion relation is derived and analysed for varying degrees of deviation from ordinary thermal behaviour, determined by the characteristic parameter of the Cairns distribution.

Incoherent scatter measurements of ring‐ion beam distributions produced by space shuttle exhaust injections into the ionosphere

When the space shuttle Orbiting Maneuver Subsystem (OMS) engines burn in the ionosphere, two types of effects are produced. First, charge exchange between the exhaust molecules and the ambient O+‐ions yields beams of high‐speed molecular ions that can excite plasma turbulence. Second, the molecular ions eventually recombine with electrons to yield a plasma hole. The ion‐beam interactions and the formation of artificial plasma holes in the ionosphere have been studied with ground‐based, incoherent‐scatter radars (ISRs) during the Shuttle Ionospheric Modification with Pulsed Localized Exhaust (SIMPLEX) series of experiments. The SIMPLEX II experiment took place in late July 1999 during the STS‐93 flight of the Space Shuttle Columbia. The Orbital Maneuver Subsystem (OMS) engines provided controlled ion injections over the incoherent scatter radar (ISR) facilities located at Arecibo, Puerto Rico to excite unusual radar signatures. After charge exchange between the exhaust and the ambient plasma, pickup ions were produced with velocities near 10 km/s using a ram‐burn orientation of the OMS engines relative to the vehicle orbit vector. During the SIMPLEX II experiment, the ISR spectra of the exhaust‐modified plasma were obtained for the first time. The formation of ring‐ion beam distributions was determined from curve fitting to the radar spectra. These spectra show the presence of the nonthermal ion distributions and enhanced scatter from electrons for thermal ion distributions with elevated ion temperatures. Analysis of the ion distributions in the modified ionosphere indicates that they were unstable and may have quickly generated plasma waves that along with ion‐neutral collisions changed the ion‐velocity distributions. The observations show that the perpendicular ion speed was rapidly reduced from 10 km/s to about 1 km/s. These observations open up the possibility of conducting a new series of experiments studying ring‐ion beam instabilities that occur naturally in the auroral‐region ionosphere and artificially in the space shuttle exhaust where there is large relative motion between the ion and neutral species.

Electrostatic Solitary and Shock Structures in Dusty Plasmas

We present a rigorous theoretical investigation of electrostatic solitary and shock structures that may exist in dusty plasmas, by employing the reductive perturbation technique as well as the pseudo-potential approach. We have focused on dust acoustic solitary waves (DASWs) in a plasma whose constituents are negatively charged cold dust fluid and Boltzmann electrons and ions. We have also investigated the effects of adiabatic dust fluid temperature, non-isothermal (trapped and nonthermal) ion distributions, non-planer (spherical and cylindrical) geometries, strong dust correlation, etc. It is shown that such effects do not only significantly modify the dust acoustic solitary structures, but also introduce some important new features to the DASWs. Furthermore, we have considered the nonlinear propagation of dust acoustic and dust ion-acoustic waves in a dissipative dusty plasma and have shown that they can propagate as shock waves. The implications of our results to space and laboratory dusty plasmas are briefly mentioned.

Effect of nonthermal ion distribution and dust temperature on nonlinear dust acoustic solitary waves

The effects of nonthermal ion distribution and finite dust temperature are incorporated in the investigation of nonlinear dust acoustic waves in an unmagnetized dusty plasma. Sagdeev pseudopotential method which takes into account the full nonlinearity of plasma equations, is used here to study solitary wave solutions. Possibility of co-existence of refractive and compressive solitons as a function of Mach number, dust temperature and concentration of nonthermal ions, is considered. For the fixed value of nonthermal ions, it is found that the effect of increase in dust temperature is to reduce the range of co-existence of compressive and refractive solitons. Particular concentration of nonthermal ions results in disappearance of refractive solitons while the decrease in dust temperature, at this concentration restores the lost refractive solitons.

Effects of vortex-like and non-thermal ion distributions on non-linear dust-acoustic waves

The effects of vortex-like and non-thermal ion distributions are incorporated in the study of nonlinear dust-acoustic waves in an unmagnetized dusty plasma. It is found that owing to the departure from the Boltzmann ion distribution to a vortex-like phase space distribution, the dynamics of small but finite amplitude dust-acoustic waves is governed by a modified Kortweg–de Vries equation. The latter admits a stationary dust-acoustic solitary wave solution, which has larger amplitude, smaller width, and higher propagation velocity than that involving adiabatic ions. On the other hand, consideration of a non-thermal ion distribution provides the possibility of coexistence of large amplitude rarefactive as well as compressive dust-acoustic solitary waves, whereas these structures appear independently when the wave amplitudes become infinitely small. The present investigation should help us to understand the salient features of the non-linear dust-acoustic waves that have been observed in a recent numerical simulation study.

Electron and ion temperature gradients and suprathermal tail strengths at Parker's solar wind sonic critical point

New constraints are placed on the electron and ion temperature gradients at Parker's sonic point, demonstrated here as the location for an ion where the force of gravity and the electric field parallel to the magnetic field are balanced. The equal‐temperature, one‐fluid versions of these results precisely recover Parker's necessary and sufficient conditions for the supersonic expansion. The two species constraints are largely within the Parker envelope. For all possible critical points, the electron partial pressure must exceed 0.2437 of the plasma pressure at this point. The location of the critical point relative to the isothermal critical point is determined by the partial pressure average of the electron and ion logarithmic derivatives of temperature at the critical point. The bulk acceleration at the critical point is controlled by (1) the ion (as distinct from the electron) temperature gradient at this point and (2) the ratio of the ion to electron pressure at the critical point. The sonic point is a velocity space deLaval nozzle rather than a traditional configuration space nozzle. Critical points are most frequently accompanied by positive radial ion and electron temperature gradients. The electron and ion temperature gradients required for a critical point are coupled and not arbitrary. Parker's one‐fluid discussion requires, by this analysis, both electron and ion temperature gradients to have positive radial exponents β = 3/4 near the critical point and a local effective gas polytrope behavior of γ = 10/13 < 1. For equal electron and ion temperatures, all possible critical points require positive ion temperature gradients at the critical point. Nonthermal electron and ion distributions are demonstrated to be generally required near the sonic critical point of a spherically symmetric solar wind. Effective polytrope exponents near the critical point range from γi(r*) < 1 and 1.5 > γe(r*) > 1/3. The allowed critical points with γ < 1 are used to infer a range of nonthermal κ distributions that permit this unusual γ < 1 behavior; ion nonthermal suprathermal tail indices ranging between 2 < κi < 5 are required, reflective of decidedly nonthermal distributions at the critical point and consistent with previous inferences of their size [Scudder, 1992b]. Even when unequal electron and ion temperatures are considered, the vast preponderance of the critical points delineated also is found with 1/3 < γe < 1, with the attendant range of electron nonthermal indices, κe, ranging from 2 to the Maxwellian limit of ∞. Critical points with maximal acceleration invariably occur for a positive electron temperature power law exponent of βe ≃ 1.12 that corresponds to κe ≃ 3.3, whether or not the ions and electrons have a common temperature at the critical point.

Upstream parameter dependence of anisotropies in the ion distributions downstream of quasi-perpendicular shocks

The reflection of part of the incident ions at quasi-perpendicular collisionless shocks results in highly structured and nonthermal ion distributions immediately downstream of such shocks. These distributions are unstable and lead to the generation of waves which in turn act to smooth out the distributions and thermalize the ions. Precisely which instabilities dominate the downstream turbulence depends sensitively on a number of parameters, such as the temperature anisotropy of the ion distributions and the velocity drift of the reflected ions relative to the directly transmitted/core ions. In this paper we conduct a series of one-dimensional hybrid simulations in order to estimate the values of these parameters immediately downstream of quasi-perpendicular shocks, and characterize their dependence on upstream conditions.