Plasma Density Gradient Research Articles

On the profile of intense high-altitude auroral electric fields at magnetospheric boundaries

Abstract. The profile of intense high-altitude electric fields on auroral field lines has been studied using Cluster data. A total of 41 events with mapped electric field magnitudes in the range between 0.5–1 V/m were examined, 27 of which were co-located with a plasma boundary, defined by gradients in particle flux, plasma density and plasma temperature. Monopolar electric field profiles were observed in 11 and bipolar electric field profiles in 16 of these boundary-associated electric field events. Of the monopolar fields, all but one occurred at the polar cap boundary in the late evening and midnight sectors, and the electric fields were typically directed equatorward, whereas the bipolar fields all occurred at plasma boundaries clearly within the plasma sheet. These results support the prediction by Marklund et al. (2004), that the electric field profile depends on whether plasma populations, able to support intense field-aligned currents and closure by Pedersen currents, exist on both sides, or one side only, of the boundary.

Intense turbulence observed above a mesospheric temperature inversion at equatorial latitude

Results from a sounding rocket experiment launched on September 19, 2004 from Kwajalein Atoll, Marshall Islands are reported. A large modulation of the temperature profile in the upper mesosphere was observed with a local maximum at 92 km, 40 K warmer than 2 km below. The temperature gradient between 92 and 102 km was near‐adiabatic, suggesting strong mixing. Turbulence was observed in the lower part of the mixed layer, as evidenced by neutral and plasma density fluctuations on both the upleg and downleg portions of the flight. The plasma density gradient was less steep in the mixed region. The turbulent energy dissipation rate was found to be 170 mW/kg. The thermal structure can be described as an upper mesospheric inversion layer, possibly caused by enhanced wave breaking or turbulent heat transport.

Single-Shot Characterization of Plasmas Generated by 17 TW 37 fs Laser Pulses

Single-shot measurement of plasma parameters is very important in laser wakefield acceleration research aiming to generate compact high-quality femtosecond electron beams. To achieve the efficient and stable monoenergetic acceleration of plasma electrons, effects of laser prepulses, plasma density gradients, wave-breaking processes, laser pulse instability, and energy spectra of accelerated electrons have to be measured simultaneously, granting reproducible laser pulse parameters. A strong correlation between the laser prepulse forming the plasma cavity and electron acceleration has been experimentally confirmed.

On collisionless energy absorption in plasmas: Theory and experiment in spherical geometry

An investigation of the rf impedance characteristics of a small spherical probe immersed in a laboratory plasma is ongoing in the large Space Physics Simulation Chamber [D. N. Walker et al., Rev. Sci. Instrum. 65, 661 (1994)] at the Naval Research Laboratory. The data taken are from network analyzer measurements of the reflection coefficient obtained when applying a low level rf signal to the probe which is either near floating potential or negatively dc biased in a low pressure plasma. As is well known, sheaths form around objects placed inside plasmas. The electron density is smaller inside the sheath, and the reduction in density alters the plasma impedance. Surprisingly, the impedance becomes “resistive,” even though the plasma is effectively collisionless, at frequencies below the bulk plasma frequency, thus leading to collisionless energy absorption. This behavior comes directly from Maxwell's equations along with the cold fluid equations. The solutions obtained indicate that this form of plasma resistance is inversely proportional to the plasma density gradient evaluated at the location where the plasma frequency is equal to the applied frequency. The sphere for this work is typically near plasma potential or biased more negatively into the ion collection regime. Applying a supplemental, negative dc bias to the sphere thickens the sheath and tends to raise its resistance as the density gradient is softened. Much of the earlier work in the area of collisionless resistance concentrated primarily on planar probes as opposed to the present work which is concerned with spheres. Interpreting the results is simpler for a sphere and the results obtained agree well with theory as described. For comparison to the theory we use only the S11 parameter outputs (or reflection coefficients) of the network analyzer in the experimental series. Significant energy absorption is observed at frequencies generally near one-half the plasma frequency. One result of this work is that the most efficient transfer of power to the plasma occurs not unexpectedly when there is impedance matching between input impedance and output (collisionless) impedance. This paper is an exposition of these ideas along with data results and a comparison to theory for the spherical probe which has not been published in this form.

The signatures of magnetic reconnection according to the microwave drifting bursts observations

A series of drifting microwave bursts during the 30 March 2001 flare are analyzed using the Siberian Solar radiotelescope (SSRT) images at 5.7 GHz and dynamic spectra obtained simultaneously by the spectropolarimeters of National Astronomic Observatories in China (NAOC) in the range 5.2–7.6 GHz. While observing the event with the SSRT, the burst sources were simultaneously recorded at two frequencies, which allowed their relative spatial shifts to be measured and source velocity along the flare loop (observed in soft X-ray and ultraviolet emission) to be evaluated. Estimates were made of the plasma density gradient along the source movement direction, the plasma emission being assumed to be generated at the second harmonic. Drifting burst series occur during transient hard X-ray brightenings. Burst drift rates ranged from 10 to 20 GHz/s, with a mean value of about 6 GHz/s. The shape of the drift rate distribution around the mean value is nearly symmetric. It is suggested that the mean value distribution may be related to increased plasma density in the source of subsecond pulses. In particular, the corresponding density variations may be associated with magnetic reconnection processes.

Whistler propagation in inhomogeneous plasma

We present a numerical study of the propagation of VLF whistler waves in the magnetospheric plasma. In this study the plasma is considered to be homogeneous in the direction along the ambient magnetic field and strongly inhomogeneous across it. The goal of this investigation is to understand whistler propagation in magnetic‐field‐aligned channels (also called ducts) with either enhanced or depleted plasma density. In particular, the paper is focused on situations where the transverse scale size of the duct is comparable to or smaller than the perpendicular wavelength of the whistler. In this case, classical analysis of the whistler dynamics based on the geometrical optic approximation becomes invalid, and numerical solutions of the full wave equations should be performed. Our simulations extend the earlier analysis based on the ray‐tracing technique and analytical studies of the very low frequency wave equations. We show that high‐density ducts are inherently leaky and this leakage depends on the perpendicular wavelength of the wave inside the duct. We also show that whistler trapping occurs not only at density maxima and minima but also at critical points along a density gradient. This effect can explain whistler guiding along strong transverse plasma density gradients at the plasmapause.

Improvement of electron beam quality in optical injection schemes using negative plasma density gradients

Enhanced electron trapping using plasma density down-ramps as a method for improving the performance of laser injection schemes is proposed and analyzed. A decrease in density implies an increase in plasma wavelength, which can shift a relativistic electron from the defocusing to the focusing region of the accelerating wakefield, and a decrease in wake phase velocity, which lowers the trapping threshold. The specific method of two-pulse colliding pulse injector is examined in detail using a three-dimensional test particle tracking code. A density down-ramp with a change of density on the order of tens of percent over distances greater than the plasma wavelength leads to an enhancement of charge by two orders in magnitude or more, up to the limits imposed by beam loading. The accelerated bunches are ultrashort (fraction of the plasma wavelength--e.g., approximately 5 fs), high charge ( > 20 pC at modest injection laser intensity approximately 10(17) W/cm(2)), with a relative energy spread of a few percent at a mean energy of approximately 25 MeV, and a normalized root-mean-square emittance of the order of 0.5 mm mrad.

Convective Ionospheric Storms: A Major Space Weather Problem

Convective Ionospheric Storms: A Major Space Weather Problem

Dynamics and characteristics of electric-field structures in the auroral return current region observed by Cluster

The temporal evolution and other characteristics of intense quasi-static electric fields in the return current region are discussed using Cluster observations. A narrow-scale, divergent electric field, the high-altitude signature of a positive U-shaped potential structure, was observed at the poleward edge of the central plasma sheet, close to magnetic midnight at a geocentric distance of about 4.2 Earth radii. Its acceleration potential increased from less than 1 to 3 kV on a 100 s timescale, similar to the formation time for ionospheric plasma density holes, and consistent with previous results for this kind of structure. In the adjacent upward current region, an energy decrease in inverted-V ions was observed some minutes prior to this. The inverted-V potential decrease was roughly equal to the subsequent perpendicular potential increase in the return current region, suggesting that a potential redistribution took place between the two adjacent current branches. Other characteristics of this and three other return current structures are summarized, to illustrate both common and different features of these. The structures are characterized by (all values have been mapped to the ionospheric level) peak electric-field magnitudes of ≈1 V m−1, bipolar or unipolar profiles, occurrence at plasma boundaries associated with plasma density gradients, perpendicular scale sizes of ≈10 km, downward field-aligned currents of ≈10 μA m−2, and upward electron beams with characteristic energies of a few hundred to a few thousand eV. The bipolar and unipolar electric-field profiles are proposed to reflect whether plasma populations, dense enough to support upward field-aligned currents (by which the return current can close) exist on both sides, or on one side only of the boundary.

Evolution of ionization front and its effects on laser pulse propagation in gaseous targets

When an intense laser pulse propagates in a gaseous target, an ionization front is produced at the leading edge of the laser pulse, which moves at the speed of light. By use of a one-dimensional particle-in-cell code with the field ionization included, we simulate the interaction between the laser pulse and its ionization front. It is found that, because of the discontinunities in atomic ionization potentials for different ionization orders, a flat-top region of the electron density appears inside the ionization front at the beginning. The ionization induces blue shif in the laser frequency, which modifies the laser pulse into a step-like profile. This new pulse profile in turn shortens the flat-top region in the ionization front. Generally the ionization front evolves into several parts, corresponding to different ionization orders, with different plasma density gradients. The effects of different gas elements and initial gas densities on the produced ionization fronts have been investigated. The conditions ith regard to the laser intensity and gas species to produce steep density gradients in ionization fronts have been studied for applications in frequency up-conversion by use of counter-propagating light pulses. Finally, the spectrum evolution of the ionizing pulse is investigated and the resulting fine spectrum structure located between the fundamental and the second harmonic is analyzed.

Spontaneous edge E × B sheared flow development studies in the TJ-II stellarator

The influence of plasma density and edge gradients in the development of perpendicular sheared flow has been investigated in the plasma edge region of the TJ-II stellarator. It has been experimentally observed that the generation of spontaneous perpendicular sheared flow (i.e. the naturally occurring shear layer) requires a minimum plasma density or gradient. It has been found that there is a coupling between the onset of sheared flow development and an increase in the level of plasma edge turbulence; once sheared flow is fully developed the level of fluctuations and turbulent transport slightly decreases whereas edge gradients and plasma density increase. The resulting shearing rate is close to the one required to trigger a transition to improved confinement regimes with reduction of edge turbulence, showing that spontaneous sheared flows and fluctuations keep themselves near marginal stability.

Is there a plasma density gradient role on the generation of short-scale Farley-Buneman waves?

Abstract. The physics of the unstable E-region plasma is based on the modified two stream, or Farley-Buneman, and the gradient drift instabilities. The theory combines both mechanisms into a single dispersion relation which applies for the directly generated short-scale plasma waves, known as type 1 irregularities. In the absence of a plasma gradient it is only the two stream mechanism acting which favors wave excitation if E×B electron drifts relative to the ions exceed a threshold slightly above the ion acoustic speed. On the other hand, the theory also predicts that a destabilizing (stabilizing) electron density gradient acts to decrease (increase) the ion acoustic threshold, and hence the wave phase velocities at threshold, depending on the gradient strength and the wavelength. Given a destabilizing plasma gradient, the threshold reduction is larger at longer than shorter wavelengths and thus the best way to test the gradient role is by simultaneous observations of type 1 waves at two or more radio backscatter frequencies. The present paper relies on dual frequency backscatter observations of 1.1 m and 3.1 m type 1 irregularities made simultaneously at 144 MHz and 50 MHz, respectively, in mid-latitude sporadic E-layers. Using as typical plasma gradient scale lengths for destabilized sporadic E-layers those that are obtained from rocket electron density profiles, the radar observations are compared with the predictions of kinetic theory. The results suggest that the plasma density gradient effect on meter scale Farley-Buneman waves is not important. This is reinforced further by the analysis of backscatter from destabilized meteor trail plasma when very steep gradients are expected in electron density. The present findings, and more from past studies, question the electron density gradient role in the generation of short-scale plasma waves as predicted by the linear instability theory. This deserves attention and more study.

A novel technique for plasma density measurement using surface-wave transmission spectra

A technique for the measurement of the absolute electron density in low-pressure plasmas using microwaves is described. It is based on observing the propagation of electromagnetic surface waves (SW) at a plasma-sheath boundary, guided by a dielectric cylinder immersed in the plasma. The transmission spectrum is measured between two antennas situated at either end of the dielectric cylinder and connected to a network analyser. Analytical theory based on the Trivelpiece–Gould work (Trivelpiece and Gould 1959 J. Appl. Phys. 30 1784, Trivelpiece 1967 Slow-Wave Propagation in Plasma Waveguides) indicates that the lowest frequency at which the SW can propagate is equal to of the plasma frequency, which is directly related to the electron number density at the plasma-sheath boundary. We call this probe the plasma transmission probe (PTP) in contrast to the plasma absorption probe proposed by Sugai and co-workers (Kokura et al 1999 Japan. J. Appl. Phys. 38 5262). The PTP is promising for the measurement of low densities (⩾109 cm−3) at relatively high gas pressure (⩽1 Torr). An axi-symmetric finite element model of the probe is presented and used to calculate transmission spectra. Experimental spectra measured in a radio-frequency capacitively coupled discharge in argon at various plasma densities and pressures (40–750 mTorr) are presented and compared with the calculated ones. Plasma densities derived from the transmission spectra were compared with those obtained with a Langmuir probe. The PTP was also compared with a microwave 1/4-wave resonator (‘hairpin probe’) at low pressure (5–45 mTorr) in an ICP discharge in argon. The densities determined by the PTP were found to be lower by a factor of 0.5–0.7 compared with those obtained with a Langmuir and a hairpin probe. We believe this can be attributed to the pre-sheath plasma density gradient, as the PTP determines the sheath edge electron density, not the bulk value.

Ionospheric irregularities in the low‐latitude valley region observed with the Equatorial Atmosphere Radar

The geomagnetically low‐latitude valley region between the upper E region and the lower F1 region is studied with the Equatorial Atmosphere Radar (EAR) in Indonesia. Three‐meter‐scale field‐aligned irregularity echoes have been frequently observed in the valley region in association with the equatorial spread F (ESF) in the period from sunset to midnight. The valley region echoes usually appear at above 150 km and propagate downward with time. Rapid beam scanning of the EAR revealed that spatial structure, temporal variation, and drift velocity of the valley region echoes resemble those of ESF, which indicates that the dynamoelectric field in the equatorial F region controls the low‐latitude valley region irregularities. Perturbed electric fields associated with ESF map down to the low‐latitude valley region and can produce the perturbed plasma density structures as “images” of ESF structures. Image structure is effectively formed at altitudes below 200 km and is a source of the valley region irregularities observed with the EAR. It is suggested that intermediate layers should supply a plasma density gradient for excitation of 3‐m‐scale irregularities in the valley region through gradient drift instability.

Beam plasma electromagnetic instabilities in a smooth density gradient: Application to the fast ignition scenario

The integrated growth rate of various relativistic beam∕plasma instabilities in a weakly varying plasma density gradient is calculated using a WKB-like approximation. It is proven that such an assumption can be made in fast ignition scenario conditions. The formalism is applied to the two-stream, the filamentation, and the two-stream∕filamentation instabilities, the latter instability being a mixture of the former two, and is the fastest growing one. The results are restricted to collisionless plasmas and nonrelativistic beam and plasma temperatures. Filamentation instability is reduced by the density gradient and eventually does not develop in the core, whereas two-stream and two-stream∕filamentation instabilities should saturate even before they feel the gradient. Various effects connected to the density gradient are discussed. It is found they should be negligible as long as these later instabilities remain in their respective linear regime.

Observation of resistive drift Alfvén waves in a helicon plasma

A low-frequency, transverse electromagnetic wave is observed in a magnetized helicon plasma with 1⪢β>νe∕Ωe. The wave is localized to the vicinity of the largest plasma density gradient and appears only at a low neutral pressure. Based on the scaling of the wave frequency and amplitude with magnetic field strength, the wave is identified as the resistive drift Alfvén wave.

Plasma conditions for improved energy coupling into the gain region of the Ni-like Pd transient collisional x-ray laser

We have directly probed the conditions in which the Ni-like Pd transient collisional x-ray laser is generated and propagates by measuring the near-field image and by utilizing picosecond resolution soft x-ray laser interferometry of the preformed Pd plasma gain medium. The electron density and gain region of the plasma have been determined experimentally and are found to be in good agreement with simulations. We observe a strong dependence of the laser pump-gain medium coupling on the laser pump parameters. The most efficient coupling occurs with the formation of lower density gradients in the preformed plasma and when the duration of the main heating pulse is comparable to the gain lifetime (approximately 10 ps for mid- Z Ni-like schemes). This increases the output intensity by more than an order of magnitude relative to the commonly utilized case where the same pumping energy is delivered within a shorter heating pulse duration (<3 ps) . In contrast, the higher intensity heating pulses are observed to be absorbed at higher electron densities and in regions where steep density gradients limit the effective length of the gain medium.

Threshold for sheared flow and turbulence development in the TJ-II stellarator**This paper was first presented at the EPS04 conference.

The influence of plasma density and edge gradients on the development of perpendicular sheared flow has been investigated in the plasma edge region of the TJ-II stellarator. The development of the naturally occurring velocity shear layer requires a minimum plasma density. Experimental findings have shown that there is a coupling between the onset of sheared flow development and an increase in the level of plasma edge turbulence; once sheared flow is fully developed the level of fluctuations and turbulent transport slightly decreases whereas edge gradients and plasma density increases. Electron density profiles show a broadening evolution as density increases above the critical value where sheared flow is developed, while the temperature profile remains similar, reflecting the strong impact of plasma density in the global confinement scaling. Furthermore, the shearing rate of the spontaneous sheared flow turns out to be close to the one needed to trigger a transition to improved confinement regimes. Density ramp experiments show, within the experimental uncertainty, no evidence of hysteresis during the spontaneous shear development. Power modulation, in the proximity of the critical plasma density, allows the characterization of plasma potential and electric field relaxation during the transition. The present results have a direct impact on the understanding of the physics mechanisms underlying the generation of critical sheared flow, pointing to the important role of turbulent driven flow.

Gyrokinetic simulations of ion and impurity transport

A systematic study of turbulent particle and energy transport in both pure and multicomponent plasmas is presented. In this study, gyrokinetic results from the GYRO code [J. Candy and R. E. Waltz, J. Comput. Phys. 186, 545 (2003)] are supplemented with those from the GLF23 [R. E. Waltz, G. M. Staebler, W. Dorland et al., Phys. Plasmas 4, 2482 (1997)] transport model, as well as from quasilinear theory. Various results are obtained. The production of a particle pinch driven by temperature gradients (a thermal pinch) is demonstrated, and further shown to be weakened by finite electron collisionality. Helium transport and the effects of helium density gradient and concentration in a deuterium plasma are examined. Interestingly, it is found that the simple D-v (diffusion versus convective velocity) model of impurity flow is consistent with results obtained from nonlinear gyrokinetic simulations. Also studied is the transport in a 50-50 deuterium-tritium plasma, where a symmetry breaking is observed indicating the potential for fuel separation in a burning plasma. Quasilinear theory together with linear simulations shows that the symmetry breaking which enhances the tritium confinement arises largely from finite-Larmor-radius effects. To justify the numerical methods used in the paper, a variety of linear benchmarks and nonlinear grid refinement studies are detailed.

Ion Acceleration in a Dipole Vortex in a Laser Plasma Corona

Particle-in-cell simulations show that the inhomogeneity scale of the plasma produced in the interaction of high-power laser radiation with gas targets is of fundamental importance for ion acceleration. In a plasma slab with sharp boundaries, the quasistatic magnetic field and the associated electron vortex structure produced by fast electron beams both expand along the slab boundary in a direction perpendicular to the plasma density gradient, forming an extended region with a quasistatic electric field, in which the ions are accelerated. In a plasma with a smooth density distribution, the dipole magnetic field can propagate toward the lower plasma density in the propagation direction of the laser pulse. In this case, the electron density in an electric current filament at the axis of the magnetic dipole decreases to values at which the charge quasineutrality condition fails to hold. In electric fields generated by this process, the ions are accelerated to energies substantially higher than those characteristic of plasma configurations with sharp boundaries.