Ionization Layer Research Articles

Pulsational instability of supergiant protostars: do they grow supermassive by accretion?

Supermassive stars (SMSs; M>10^5 Msun) and their remnant black holes are promising progenitors for supermassive black holes (SMBHs) observed in the early universe at z>7. It has been postulated that SMSs forms through very rapid mass accretion onto a protostar at a high rate exceeding 0.01 Msun/yr. According to recent studies, such rapidly accreting protostars evolve into "supergiant protostars", i.e. protostars consisting of a bloated envelope and a contracting core, similar to giant star. However, like massive stars as well as giant stars, both of which are known to be pulsationally unstable, supergiant protostars may also be also unstable to launch strong pulsation-driven outflows. If this is the case, the stellar growth via accretion will be hindered by the mass loss. We here study the pulsational stability of the supergiant protostars in the mass range M<10^3 Msun through the method of the linear perturbation analysis. We find that the supergiant protostars with M>600 Msun and very high accretion rate Mdot>1.0 Msun/yr are unstable due to the kappa mechanism. The pulsation is excited in the He^+ ionization layer in the envelope. Even under a conservative assumption that all the pulsation energy is converted into the kinetic energy of the outflows, the mass-loss rate is ~10^-3 Msun/yr, which is lower than the accretion rate by more than two orders of magnitude. We thus conclude that the supergiant protostars should grow stably via rapid accretion at least in the mass range we studied. As long as the rapid accretion is maintained in the later stage, protostars will become SMSs, which eventually produce seeds for the high-z SMBHs.

Ion gyro‐harmonic structuring in the stimulated radiation spectrum and optical emissions during electron gyro‐harmonic heating

Stimulated electromagnetic emissions (SEEs) are secondary radiation produced during active space experiments in which the ionosphere is actively heated with high power high frequency (HF) ground‐based radio transmitters. Recently, there has been significant interest in ion gyro‐harmonic structuring the SEE spectrum due to the potential for new diagnostic information available such as electron acceleration and creation of artificial ionization layers. These relatively recently discovered gyro‐harmonic spectral features have almost exclusively been studied when the transmitting frequency is near the second electron gyro‐harmonic frequency. The first extensive systematic experimental investigations of the possibility of these spectral features for third electron gyro‐harmonic heating are provided here. Discrete spectral features shifted from the transmit frequency ordered by harmonics of the ion gyro‐frequency were observed for third electron gyro‐harmonic heating for the first time at a recent campaign at the High Frequency Active Auroral Research Program (HAARP) facility. These features were also closely correlated with a broader band feature at a larger frequency shift from the transmit frequency known as the downshifted peak (DP). The power threshold of these spectral features was measured, as well as their behavior with heater beam angle, and proximity of the transmit frequency to the third electron gyro‐harmonic frequency. Comparisons were also made with similar spectral features observed during second electron gyro‐harmonic heating during the same campaign. A theoretical model is provided that interprets these spectral features as resulting from parametric decay instabilities in which the pump field ultimately decays into high frequency upper hybrid/electron Bernstein and low frequency neutralized ion Bernstein IB and/or obliquely propagating ion acoustic waves at the upper hybrid interaction altitude. Coordinated optical and SEE observations were carried out in order to provide a better understanding of electron acceleration and precipitation processes. Optical emissions were observed associated with SEE gyro‐harmonic features for pump heating near the second electron gyro‐harmonic during the campaign. The observations affirm strong correlation between the gyro‐structures and the pump‐induced optical emissions.

Kinetic Numerical Simulation of the Cathode Attachment Zone of Constricted High-Current Vacuum Arcs

This paper is devoted to numerical simulation of the cathode attachment zone of constricted high-current vacuum arcs driven by a transverse magnetic field. With help of particle-in-cell and direct simulation Monte Carlo methods, a 1-D model of the attachment zone was developed, and the stationary-cathode-spot mode was investigated. The states of the charged particles and atoms in the Knudsen layer, the ionization layer, and the plasma sheath are determined in detail. The results support the adaptation of simplified cathode attachment models to be able to reduce the computational effort needed for self-consistent numerical simulations of magnetically driven constricted high-current vacuum arcs.

IMPACT OF A REALISTIC DENSITY STRATIFICATION ON A SIMPLE SOLAR DYNAMO CALCULATION

In our Sun, the magnetic cycle is driven by the dynamo action occurring inside the convection zone, beneath the surface. Rotation couples with plasma turbulent motions to produce organized magnetic fields that erupt at the surface and undergo relatively regular cycles of polarity reversal. Among others, the axisymmetric dynamo models have been proved to be a quite useful tool to understand the dynamical processes responsible for the evolution of the solar magnetic cycle and the formation of the sunspots. Here, we discuss the role played by the radial density stratification on the critical layers of the Sun on the solar dynamo. The current view is that a polytropic description of the density stratification from beneath the tachocline region up to the Sun's surface is sufficient for the current precision of axisymmetric dynamo models. In this work, by using an up-to-date density profile obtained from a standard solar model, which is itself consistent with helioseismic data, we show that the detailed peculiarities of the density in critical regions of the Sun's interior, such as the tachocline, the base of the convection zone, the layers of partial ionization of hydrogen and helium, and the super-adiabatic layer, play a non-negligible role on the evolution of the solar magnetic cycle. Furthermore, we found that the chemical composition of the solar model plays a minor role in the formation and evolution of the solar magnetic cycle.

Pulsations in the atmosphere of the roAp star HD 24712 - II. Theoretical models

We discuss pulsations of the rapidly oscillating Ap (roAp) star HD 24712 (HR 1217) based on nonadiabatic analyses taking into account the effect of dipole magnetic fields. We have found that all the pulsation modes appropriate for HD 24712 are damped; i.e., the kappa-mechanism excitation in the hydrogen ionization layers is not strong enough to excite high-order p-modes with periods consistent with observed ones, all of which are found to be above the acoustic cut-off frequencies of our models. The main (2.721 mHz) and the highest (2.806 mHz) frequencies are matched with modified $l=2$ and $l=3$ modes, respectively. The large frequency separation ($\approx 68 \mu$Hz) is reproduced by models which lay within the error box of HD 24712 on the HR diagram. The nearly equally spaced frequencies of HD 24712 indicate the small frequency separation to be as small as $\approx 0.5\mu$Hz. However, the small separation derived from theoretical $l=1$ and 2 modes are found to be larger than $\sim 3\mu$Hz. The problem of equal spacings could be resolved by assuming that the spacings correspond to pairs of $l=2$ and $l=0$ modes. The amplitude distribution on the stellar surface is strongly affected by the magnetic field resulting in the predominant concentration at the polar regions. Amplitudes and phases of radial-velocity variations for various spectral lines are converted to relations of amplitude/phase versus optical depth in the atmosphere. Oscillation phase delays gradually outward in the outermost layers indicating the presence of waves propagating outward. The phase changes steeply around $\log\tau\sim-3.5$, which supports a $T-\tau$ relation having a small temperature inversion there.

Sporadic E layer observations over Arecibo using coherent and incoherent scatter radar: Assessing dynamic stability in the lower thermosphere

A patchy sporadic E ionization layer was observed using the 430 MHz incoherent scatter radar at the Arecibo Radio Observatory and a new, 30 MHz coherent scatter radar imager located on St. Croix. The former measured state parameters of the layer (plasma density, temperature, and composition) while the latter observed small‐scale plasma waves and the associated quasiperiodic (QP) echoes. Regularization was used to estimate horizontal vector ion drifts from Arecibo line‐of‐sight data. Assuming the ion temperatures and drifts to be representative of the neutral atmospheric temperatures and drifts in the mesosphere lower‐thermosphere (MLT) region, we evaluated the Richardson number criterion for neutral dynamic instability in regions occupied by the layers. We find that the condition for instability was generally satisfied and thus that dynamic instability is consistent with the layer breakup. The electrodynamics of the resulting patchy layers could then give rise to the QP echoes.

First simultaneous lidar observations of sodium layers and VHF radar observations of E-region field-aligned irregularities at the low-latitude station Gadanki

Abstract. Simultaneous observations of atmospheric sodium (Na) made by a resonance lidar and E-region field-aligned-irregularities (FAI) made by the Indian MST radar, both located at Gadanki (13.5° N, 79.2° E) and horizontal winds acquired by a SKiYMET meteor radar at Trivandrum (8.5° N, 77° E) are used to investigate the relationship among sodium layer, FAI and neutral winds in the mesosphere and lower thermosphere region. The altitudes and descent rates of higher altitude (~95 km) Na layer and FAI agree quite well. The descending structures of the higher altitude Na layer and FAI are found to be closely related to the diurnal tidal phase structure in zonal winds observed over Trivandrum. At lower altitudes, the descent rate of FAI is larger than that of Na layer and zonal tidal phase. These observations support the hypothesis that the metallic ion layers are formed by the zonal wind shear associated with tidal winds and subsequently get neutralized to manifest in the form of descending Na layers. The descending FAI echoing layers are manifestation of the instabilities setting in on the ionization layer. In the present observations, the altitudes of occurrence of Na layer and FAI echoes being low, we surmise that it is quite possible that the FAI echoes are due to the descent of already formed irregularities at higher altitudes.

Ion diagnostics of a discharge in crossed electric and magnetic fields for electric propulsion

The velocity distribution function (VDF) of metastable Xe+ ions was measured along the channel centerline of the high-power PPS®X000 Hall effect thruster by means of laser induced fluorescence (LIF) spectroscopy at 834.72 nm for various discharge voltages (300–700 V) and propellant mass flow rates (6–15 mg s−1). The development of the on-axis profile of the velocity dispersion reveals the interrelation between ionization and acceleration layers. The ion velocity profiles are in accordance with outcomes of a hybrid numerical model in which the electron mobility is assessed from particle-in-cell simulations. The axial distribution of the effective electric field is inferred from the mean ion velocity profile, despite the parasitic effect due to ions created in the acceleration region. Most of the acceleration process takes place outside the thruster channel. The electric field augments and it moves upstream when the applied voltage is ramped up. The impact of the xenon mass flow rates is found to depend upon the voltage. A novel approach based on the moments of the experimental VDFs in combination with the Boltzmann's equation is introduced in order to determine the real electric field distribution. The method also provides the ionization frequency profile. The LIF diagnostics reveals the existence at the end of the acceleration region of fast ions of which the kinetic energy is above the supplied energy. The fraction of these supra-sped up ions grows when the voltage increases. The ion VDFs were also recorded in the plasma plume far field by way of a retarding potential analyzer (RPA). The shape of the RPA traces as well as their evolution with operating conditions are in agreement with trends observed by means of LIF spectroscopy. Finally, physical mechanisms at the origin of supra-sped up ions are discussed in light of numerical simulation outcomes and a set of new experimental results.

Artificial E-region field-aligned plasma irregularities generated at pump frequencies near the second electron gyroharmonic

Abstract. E region ionospheric modification experiments have been performed at HAARP using pump frequencies about 50 kHz above and below the second electron gyroharmonic frequency. Artificial E region field-aligned plasma density irregularities (FAIs) were created and observed using the imaging coherent scatter radar near Homer, Alaska. Echoes from FAIs generated with pump frequencies above and below 2Ωe did not appear to differ significantly in experiments conducted on summer afternoons in 2008, and the resonance instability seemed to be at work in either case. We argue that upper hybrid wave trapping and resonance instability at pump frequencies below the second electron gyroharmonic frequency are permitted theoretically when the effects of finite parallel wavenumbers are considered. Echoes from a sporadic E layer were observed to be somewhat weaker when the pump frequency was 50 kHz below the second electron gyroharmonic frequency. This may indicate that finite parallel wavenumbers are inconsistent with wave trapping in thin sporadic E ionization layers.

Two-dimensional self-consistent modelling of the arc/cathode interaction

A theory based on the literature works is developed to define a self-consistent model of the arc–cathode interaction in thermal plasma domain. The proposed model, applied in this paper in a two-dimensional coordinate system includes a physical description of the arc–cathode interaction, the cathode bulk, the sheath, the pre-sheath and the local thermodynamic equilibrium plasma column.This model allows assuming the electric current conservation through a two-temperature treatment of the sheath and pre-sheath regions. This model has been applied in a free burning arc configuration in an argon gas with a cylindrical tungsten cathode and current intensity equal to 200 A. The orders of magnitude of the current density and of the heat flux are, respectively, 106 A m−2 and 107 W m−2.Three parametric studies are performed: on the ionization layer length, on the value of the secondary emission coefficient and on the formulation of the electrical conductivity in the pre-sheath. The size of the ionization layer and the chosen value of the secondary emission coefficient have a significant influence on the properties of the discharge.

A time-resolved laser induced fluorescence study on the ion velocity distribution function in a Hall thruster after a fast current disruption

The temporal characteristics of the Xe+ ion axial velocity distribution function (VDF) were recorded in the course of low-frequency discharge current oscillations (∼14 kHz) of the 5 kW class PPS®X000 Hall thruster. The evolution in time of the ion axial velocity component is monitored by means of a laser induced fluorescence diagnostic tool with a time resolution of 100 ns. As the number of fluorescence photons is very low during such a short time period, a homemade pulse-counting lock-in system was used to perform real-time discrimination between background photons and fluorescence photons. The evolution in time of the ion VDF was observed at three locations along the thruster channel axis after a fast shutdown of the thruster power. The anode discharge current is switched off at 2 kHz during 5 μs without any synchronization with the current oscillation cycle. This approach allows to examine the temporal behavior of the ion VDF during decay and ignition of the discharge as well as during forced and natural plasma oscillations. Measurements show that the distribution function of the axial component of the Xe+ ion does change periodically in time with a frequency close to the current oscillation frequency in both forced and natural cases. The ion density and the mean velocity are found to oscillate, whereas the velocity dispersion stays constant, which indicates that ionization and acceleration layers have identical dynamics. Finally, variations over time in the electric field are for the first time experimentally evidenced in a crossed-field discharge.

PHOTOIONIZED MIXING LAYER MODELS OF THE DIFFUSE IONIZED GAS

It is generally believed that O stars, confined near the galactic midplane, are somehow able to photoionize a significant fraction of what is termed the "diffuse ionized gas" (DIG) of spiral galaxies, which can extend up to 1-2 kpc above the galactic midplane. The heating of the DIG remains poorly understood, however, as simple photoionization models do not reproduce the observed line ratio correlations well or the DIG temperature. We present turbulent mixing layer models in which warm photoionized condensations are immersed in a hot supersonic wind. Turbulent dissipation and mixing generate an intermediate region where the gas is accelerated, heated and mixed. The emission spectrum of such layers are compared with observations of Rand (ApJ 462, 712) of the DIG in the edge-on spiral NGC2363. We generate two sequence of models that fit the line ratio correlations between [SII]/H-alpha, [OI]/H-alpha, [NII]/[SII] and [OIII]/H-beta reasonably well. In one sequence of models the hot wind velocity increases while in the other the ionization parameter and layer opacity increases. Despite the success of the mixing layer models, the overall efficiency in reprocessing the stellar UV is much too low, much less than 1%, which compels us to reject the TML model in its present form.

An asymptotic model for steady wire-to-wire corona discharges

The purpose of this paper is to propose a new model based on an asymptotic analysis for the modelling of steady wire-to-wire corona discharges. The concept consists in dividing the discharges into regions of two kinds: two thin ionization layers at the vicinity of the electrodes and a much larger ion-drift region. Introducing the length of the ionization layer ε and using an asymptotic analysis to define a truncated system, a simplified kinetic can be considered for each region which allows finding quasi-analytical solutions. The method is validated by comparison with a classical scheme and the results show good agreement with experiments. Finally, an example of parametric study of the discharge is presented. Indeed, the ability of the model to provide solutions at low computational cost allows quickly performing such a study and could be useful to experimenters in order to suggest new geometric settings or experimental setups.

Simulation of the atomic and ionic densities in the ionization layer of a plasma arc with a binary cathode

A physical model was developed to study the behaviour of the cathode material evaporated from a thoriated tungsten cathode of an atmospheric-pressure argon plasma arc. The densities of tungsten and thorium atoms and ions in the ionization layer were obtained, and the influence of the different physical processes on the evaporated cathode material was established. It was found that almost all of the neutral atoms evaporated from the cathode are ionized near the beginning of the ionization layer, i.e. near the boundary between the sheath and the ionization layer. Thorium ions are concentrated in a 4 µm region near the beginning of this layer, while tungsten ions are found in a region of 9 µm. The contribution of the electric force to the velocity of ions is the dominant contribution only near the beginning of the ionization layer. At a distance from the interface between the sheath and the ionization layer greater than 3.8 µm in the case of thorium ions, and greater than 5 µm in the case of tungsten ions, the contributions of the density gradient forces and the frictional forces are more important than the electric force contribution.

Multiple phase screen calculation of wide bandwidth propagation

This paper presents results from a two‐dimensional multiple phase screen (MPS) calculation that computes numerical realizations or sample functions of the electric field after propagation through the ionosphere. The MPS code solves the parabolic wave equation and allows for direct computation of realizations of the received signal. The ionization is represented by a series of random phase screens that characterize the severity and spectrum of the electron density fluctuations. For wide bandwidth signals, the MPS code is exercised for many frequencies over the bandwidth of the propagating signal; Fourier transform techniques are used to obtain the propagating signal in the time domain. The MPS simulation is quite general and may be applied to problems involving numerous, separated, layers of ionization characterized by spatially varying electron density power spectra. MPS techniques can handle all levels of ionospheric disturbances from the least severe, where only small phase fluctuations occur, to the most severe case of frequency selective scintillation. Several examples of propagating signals are presented that simultaneously show the development of the ionospheric transfer function and the impulse response function. The calculations are repeated, and the realizations are averaged to obtain the two‐frequency, two‐position mutual coherence function (MCF), Γ(x, f) and its Fourier transform S(q,τ), the generalized power spectrum. The wide bandwidth MPS results closely match results obtained from strong scatter theory as utilized in the phase screen diffraction method.

Breakdown Delay Times for Subnanosecond Gas Discharges at Pressures Below One Atmosphere

With a RADAN 303-A pulser (a rise time of 150 ps and a maximum voltage of 150 kV into matched load), fast breakdown in argon and air is investigated. An oil-filled coaxial transmission line is coupled with a lens to a biconical section and a radial millimeter-size gap operated at subatmospheric pressure. Diagnostics include capacitive voltage dividers which allow the determination of voltage across and current through the gap with a temporal resolution defined by the digitizer (20 Gs/s, 6 GHz) used. A scintillator-photomultiplier combination with different metal absorber foils and a temporal resolution of 2 ns is used as X-ray detector to obtain a rough energy spectrum of the X-rays and electrons in the range of 10-150 keV. Discharges are characterized by runaway electrons over much of the pressure range, with a strong excitation and ionization layer at the cathode surface, and ldquofree-fallrdquo conditions with negligible gaseous ionization for the rest of the gap. High-energy electrons (> 60 keV) are observed up to atmospheric pressure. Time-to-breakdown curves versus pressure have been measured for different applied voltage rise times. They resemble Paschen curves with a steep increase toward low pressure and a slow increase toward high pressure. The major experimental findings and particularly the time-to-breakdown curves are confirmed using simple force-equation modeling. Monte Carlo calculations simulating collisional ionizations and developing electron avalanches in three dimensions have been used to verify and explain the experimental results.

$\hbox{Xe}^{+}$ Ion Transport in the Crossed-Field Discharge of a 5-kW-Class Hall Effect Thruster

The velocity-distribution function (VDF) of metastable Xe <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ions was measured along the channel axis of the 5-kW-class PPSX000 Hall effect thruster by means of laser-induced fluorescence spectroscopy at 834.72 nm for various voltages, magnetic fields, and mass flow rates. Axial-velocity and dispersion profiles are compared to on-axis profiles obtained with the 1.5-kW-class PPS100 thruster. Outcomes of the comparison are threefold: 1) The broadening of the VDF across the region of strong magnetic field is a general feature for Hall thrusters. It originates in the overlap between ionization and acceleration layers. The kinetic-energy dispersion increases with the discharge voltage; it reaches up to 200 eV at 700 V. 2) Most of the acceleration potential is localized outside the thruster channel whatever the thruster size and operating conditions. The electric field moves upstream when the applied voltage is ramped up, i.e., the fraction of potential inside the channel increases with the voltage. On the contrary, the electric field is shifted downstream when the gas flow rate increases. The magnetic field has a little impact on the potential distribution. 3) A nonnegligible amount of very fast (kinetic energy higher than the applied potential) and very slow Xe <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ions are always observed. Such ions may find their origin in space and temporal oscillations of the electric field as suggested by numerical simulations carried out with both kinetic and hybrid models.

Influence of electrical properties of treated surface on RF-excited plasma needle at atmospheric pressure

We present a comparison of a finite element analysis of the atmospheric pressure RF-excited plasma needle interacting with different surfaces with corresponding experimental observations of light emission spatial profiles. The gas used is helium with 1 ppm nitrogen as an impurity. The needle has a point-to-plane geometry with a radius of 30 µm at the tip and an inter-electrode gap of 1 mm. We employ a fluid model in two-dimensional axisymmetric coordinates. Our simulation results indicate that the plasma structure strongly depends on the electrical properties of the treated surface as well as the discharge mode. In the lower power corona mode with a dielectric surface, the plasma is confined near the needle tip. As a result, particle fluxes to the dielectric surface are relatively low and follow a Gaussian-like radial profile. In the higher power glow mode with a dielectric surface, the particle fluxes to the surface are orders of magnitude higher and the spatial distribution of the particle fluxes becomes radially more uniform due to a uniform ionization layer just above the treated surface. When a conductive plate replaces the dielectric surface in the glow mode, a quite intense ionization spot appears near the surface closest to the needle tip. Consequently, the particle fluxes to the surface peak near the symmetry axis under these conditions. These simulation results are validated by experimental observation of light emission spatial profiles.

Influence of magnetic field and discharge voltage on the acceleration layer features in a Hall effect thruster

The axial velocity of singly charged xenon ion is determined by means of laser induced fluorescence spectroscopy at the exhaust of a PPS100-LM Hall effect thruster by analyzing the Doppler shifted spectral profile of the 834.72 nm Xe+ ion line. Measurements are carried out both inside and outside the thruster channel. Ion velocity distribution functions (VDF) are used to compute the ion accelerating potential. It is shown that such a potential can in fact be defined in several ways due to the overlap between the ionization and acceleration layers, which translates into broad VDF. The PPS100-LM thruster is operated under an applied voltage ranging from 100 to 300 V and a magnetic field produced by coils whose current is varied from 2.5 to 5.5 A. The broad range of working conditions allows one to determine the influence of these two parameters on the ion dynamics. Finally, the experimental results are used to confirm the outcomes of a 2D kinetic model.

Plasma lens and plume divergence in the Hall thruster

The effect of magnetic field curvature on the plume divergence in the Hall thruster is analyzed. The two-dimensional plasma flow and electric field are determined self-consistently within the paraxial approximation in this plasma lens, a nearly axial electric field perpendicular to the curved magnetic field lines. The ion radial velocity along the thruster is described analytically. The authors suggest positioning the ionization layer near the zero of the magnetic field in a reversing-direction field configuration for a minimal beam divergence. They also show that an additional emitting electrode can reduce plume divergence.