Plasma Density Perturbations Research Articles

Application of a vacuum photodiode detector for X-ray recording on the T-10 tokamak

The results of vacuum photodiode detector (VPD) testing on the T-10 tokamak proposed for plasma tomography in the International Thermonuclear Experimental Reactor are described. It was shown that the VPD signal depends mainly on the plasma electron density and slightly depends on the plasma electron temperature; therefore it can be used for investigations into plasma density perturbation.

Plasma induced laser beam smoothing below the filamentation threshold

This paper deals with a statistical approach for description of the laser field interaction with underdense plasmas and modification of the laser beam temporal coherence during its propagation through a plasma at power well below the filamentation threshold. The main properties of the plasma density perturbations driven by a randomized laser beam are derived from a stochastic wave equation. The laser spectral and angular broadening is shown to occur on a distance that depends essentially on the ratio of the average power in a speckle to the critical power for filamentation. The coherence time of the transmitted light is reduced to the plasma acoustic time of response to the laser. It is typically a few picoseconds. Dedicated diagnostics have been developed for the interaction code PARAX in order to analyze the laser and plasma statistical properties. The effect of the plasma length on the transmitted light coherence is found to be in good agreement with theoretical predictions. Forward stimulated Brillouin scattering is shown to play a key role in the laser coherence loss in this low-intensity regime. The limitations of the analytical model are discussed in terms of the deviation of the electric field distribution from the Gaussian statistics and creation of density-electric field correlations. This regime of laser induced incoherence is especially interesting in that the associated angular broadening is not as deleterious as observed for higher intensities. Moreover, beam smoothing can be achieved in low-density plasmas where energy losses due to absorption and backscattering are not too important.

The effect of the rotating helical fields on the plasma edge in the HYBTOK-II Tokamak

An experiment with rotating helical magnetic fields (RHMF) in the HYBTOK-II tokamak is reported. The effect of the RHMF rotation with a frequency of 5 kHz on the poloidal rotation of plasma density perturbation was studied by a movable multi-pin Langmuir probe. The wavelet technique has been used to study the poloidal rotation of the bursts. During the RHMF operation the poloidal direction of the plasma density perturbation rotation coincides with the direction of the RHMF rotation. The effect of the RHMF is found to be essential up to minor radius r = 6 cm.

Dynamics of nonlinearly coupled magnetic-field-aligned electromagnetic electron-cyclotron waves near the zero-group-dispersion point in magnetized plasmas

The nonlinear coupling between two magnetic-field-aligned electromagnetic electron-cyclotron (EMEC) waves in plasmas is considered. Evaluating the ponderomotive coupling between the EMEC waves and quasistationary plasma density perturbations, a pair of coupled nonlinear Schrödinger equations (CNLSEs) is obtained. The CNLSEs are then used to investigate the occurrence of modulational instability in magnetized plasmas. Waves in the vicinity of the zero-group-dispersion point are considered, so that the group dispersion terms may either bear the same or different signs. It is found that a stable EMEC wave can be destabilized due to its nonlinear interactions with an unstable one, while a pair of unstable EMEC waves yields an increased instability growth rate. Individually stable waves remain stable while interacting with one another. Stationary nonlinear solutions of the coupled equations are presented. The relevance of our investigation to nonlinear phenomena in space plasmas is discussed.

Transit-time instability in Hall thrusters

Longitudinal waves characterized by a phase velocity of the order of the velocity of ions have been recurrently observed in Hall thruster experiments and simulations. The origin of this so-called ion transit-time instability is investigated with a simple one-dimensional fluid model of a Hall thruster discharge in which cold ions are accelerated between two electrodes within a quasineutral plasma. A short-wave asymptotics applied to linearized equations shows that plasma perturbations in such a device consist of quasineutral ion acoustic waves superimposed on a background standing wave generated by discharge current oscillations. Under adequate circumstances and, in particular, at high ionization levels, acoustic waves are amplified as they propagate, inducing strong perturbation of the ion density and velocity. Responding to the subsequent perturbation of the column resistivity, the discharge current generates a standing wave, the reflection of which sustains the generation of acoustic waves at the inlet boundary. A calculation of the frequency and growth rate of this resonance mechanism for a supersonic ion flow is proposed, which illustrates the influence of the ionization degree on their onset and the approximate scaling of the frequency with the ion transit time. Consistent with experimental reports, the traveling wave can be observed on plasma density and velocity perturbations, while the plasma potential ostensibly oscillates in phase along the discharge.

Monitoring Alfvén Cascades with Interferometry on the JET Tokamak

A microwave interferometry technique is applied for the first time for detecting a discrete spectrum of Alfvén cascade (AC) eigenmodes excited with fast ions in reversed magnetic shear plasmas of the Joint European Torus. The interferometry measurements of plasma density perturbations associated with ACs show an unprecedented frequency and time resolution superior to that obtained with external magnetic coils. The measurements of ACs are used for monitoring the evolution of the safety factor and density of rational magnetic surfaces in the region of maximum plasma current.

Geomagnetic conjugate observations of medium‐scale traveling ionospheric disturbances at midlatitude using all‐sky airglow imagers

We report for the first time simultaneous observations of medium‐scale traveling ionospheric disturbances (MSTIDs) at geomagnetic conjugate points in both hemispheres, using two all‐sky airglow imagers at midlatitudes. A 630‐nm all‐sky CCD imager at Sata, Japan, detected MSTIDs with a wavefront elongated from NW to SE on the night of August 9, 2002. During this event, MSTIDs with a wavefront elongated from SW to NE were observed at the geomagnetic conjugate point, Darwin, Australia. To investigate geomagnetic conjugacy of the MSTID structures, the Darwin images were mapped The MSTID structures mapped from Darwin to its magnetic conjugate points along the geomagnetic field lines (B) coincide closely with those in the Sata images. This result suggests that polarization electric field (Ep) plays an important role in the generation of MSTIDs. Ep maps along B and moves the F region plasma upward or downward by E × B drifts, causing plasma density perturbations with structures mirrored in the northern and southern hemispheres.

Nonlinear model for magnetosonic shocklets in plasmas

Exact nonlinear equations for magnetosonic shocklets in a uniform hot magnetoplasma are derived by using the nonlinear magnetohydrodynamic equations. Analytic, as well as numerical, solutions of the nonlinear equations are presented. Shocklike structures of the ion fluid velocity and magnetic field (or the plasma density) perturbations are obtained. The results may have relevance to the understanding of fast magnetosonic shocklets that have been recently observed by onboard instruments of the Cluster spacecraft at the Earth’s bow shock.

Finite ion Larmour radius effects in the problem of zonal flow generation by kinetic drift-Alfvén turbulence

A theory of spontaneous generation of zonal flows by kinetic drift-Alfvén turbulence in the finite-pressure plasma (β > me/mi) is generalized to include the ion diamagnetic effects and the finite ion Larmour radius effects. In the framework of the corresponding set of generalized two-fluid magnetohydrodynamic equations and on the assumption of a distinct time- and space-scale separation between the turbulent oscillations and the zonal flow, a set of coupled equations is derived to describe the interaction between the turbulence and the flow, consisting of the evolution equation for the spectral function of turbulence and the mean-field equations for zonal flow. The possibility of spontaneous zonal flow generation by the kinetic drift-Alfvén turbulence is investigated in details in several cases. In the case of kinetic drift-Alfvén turbulence with the space scale of the order of the ion Larmour radius or below, the instability caused by the resonant interaction of the wave packet with the slow modulations of zonal flow has been analysed, and the criterion for the onset of the zonal flow instability has been derived. In the case of short-wavelength turbulence, two regimes are considered. It is shown, that, when the frequency of short-wavelength oscillations is close to the electron-drift frequency and the zonal perturbation of plasma density can be described by the Boltzmann Law, the instability criterion is a generalization of the previously obtained result to the case of non-equal temperatures of the ions and electrons. The new regime is found, in which the zonal perturbation of plasma density is negligible. The condition for the onset of resonant instability is obtained.

Stability of surface waves at magnetized plasma – metal interface

AbstractThe stability of high‐frequency potential surface waves at a dense magnetized plasma – metal interface with respect to a low‐frequency plasma density modulation is studied in the point of view of the surface waves control. The discussion is addressed to the situation, when an external steady magnetic field is directed perpendicularly to the interface. The nonlinear interaction process of the high‐frequency surface wave, its satellites and the low‐frequency plasma density perturbation is investigated. It is shown that the low‐frequency plasma density perturbation can be represented as a superposition of forced waves of surface and volume types and can lead to an additional attenuation of the surface waves. This attenuation arises when the surface wave amplitude exceeds the threshold value. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetic field perturbations correlated with large amplitude lower-hybrid waves in a high-voltage linear plasma discharge

The results of theoretical and experimental studies on excitation of magnetic field perturbations correlated with large amplitude lower-hybrid (LH) wave bursts in a high-voltage linear plasma discharge are presented. It is shown that the magnetic field perturbations, which are excited in the experiments mostly in the paramagnetic sense, are associated with the development of the magneto-modulational processes. The equations describing the relationship between the quasistationary magnetic field perturbations, plasma density perturbations, and the fields of LH waves are derived using the general nonlinear formalism developed for description of the modulational effects in arbitrary media. The results obtained on the basis of these equations are compared with the experimental data using the following parameters: the values of the magnitude of the magnetic field perturbations; the direction of the vector of the magnetic field perturbation; the correlation between the magnetic field perturbations and the electron density perturbations. It is shown that for LH waves propagating in one plane theoretical predictions are in a good agreement with the experimental results.

Global view of the nighttime low-latitude ionosphere by the IMAGE/FUV 135.6 nm observations

The FUV Spectrographic Imager (FUV/SI) onboard the IMAGE satellite takes images at two wavelengths (121.8 nm: SI‐12 and 135.6 nm: SI‐13). The SI‐13 images of the low latitude region at night clearly show the intertropical FUV arcs corresponding to the equatorial anomaly (EA) in the ionosphere. The two‐minute cadence of the instrument provides a unique data set of the nighttime low‐latitude ionosphere. We compare the IMAGE observations of the OI 135.6 nm nightglow with the model emission intensities calculated by using the SAMI2 ionosphere model. The comparison shows that although both the FUV observation and the model give comparable emission intensities near the EA peak, the two differ in terms of their latitudinal and local time dependences. Large‐scale (∼1000 km) longitudinal wavy structures are often seen in OI 135.6 nm images. These structures drift eastward with a phase velocity of ∼100 m/s. The structure may be due to the plasma density perturbations generated by acoustic gravity waves through amplification by the spatial resonance and the Rayleigh‐Taylor (R‐T) instability. This is a common structure seen in the pre‐midnight sector of FUV/SI images regardless of longitude.

Laser Propagation and Compton Scattering in Parabolic Plasma Channel

A Gaussian laser beam propagating in a parabolic plasma channel is discussed in this paper. For a weak laser, plasma density perturbation induced by interaction between the laser field and plasma is very small, the refractive index can be assumed to be constant with respect to time variable. For a parabolic plasma channel, through the static propagation equation, we obtain an analytical solution of the profile function of the Gaussian laser beam for an unmatched case and give the general condition for the matched case. As the laser intensity increases, an effect due to strong laser fields is included. We discuss how to design and select the distribution of plasma density for a certain experiment in which a plasma channel is utilized to guide a laser beam. The number of scattered photons (X-rays) generated through Compton backscattering in a plasma channel is discussed.

Current and Future Status of Pulse Elecromagnetic Wave Generation Using Plasma

Novel frequency upshift phenomena, a flash ionization and a photon acceleration process, are reviewed. The flash ionization is the frequency upshift that the quick creation of the plasma around a electromagnetic wave, on time scales on the order of a cycle of the wave. The photon acceleration is that the frequency of the electromagnetic wave propagating with the plasma density perturbation is upshifted because of the difference of the phase velocity. Furthermore, the generation of the short pulse electromagnetic wave in the THz region is also introduced.

Large scale structuring of plasma density perturbations in ionospheric modifications

The theory of nonlinear large-scale structuring in ionospheric modifications is developed. The results of an experiment in the northern ionosphere are presented. The ionospheric modification was produced by HAARP facility (Alaska), and the plasma disturbances were measured using the polar-orbiting satellite Oscar 27. The comparison shows a reasonable agreement between observational data and the theory. For the first time the existence of artificially produced plasma depletion structure elongated along magnetic field lines at hundreds kilometers is established.

Alfvén wave tomography for cold magnetohydrodynamic plasmas

Alfvén wave propagation in slightly nonuniform cold plasmas is studied by means of ideal magnetohydrodynamics (MHD) nonlinear equations. The evolution of the MHD spectrum is shown to be governed by a matrix linear differential equation with constant coefficients determined by the spectrum of quasistatic plasma density perturbations. The Alfvén waves are shown not to affect the plasma density inhomogeneities, as they scatter off of them. The application of the MHD spectrum evolution equation to the inverse scattering problem allows tomographic measurements of the plasma density profile by scanning the plasma volume with Alfvén radiation.

Gravitational-to-electromagnetic wave conversion and gamma-ray bursts calorimetry: The GRB980425/SN 1998bw∼1049ergradio emission

The unusual features of supernova (SN) 1998bw and its apparent association with the gamma-ray burst (GRB) event GRB980425 were highlighted by Kulkarni et al. At its peak SN 1998bw was anomalously superluminous in radio wavelengths with an inferred fluence ${E}_{\mathrm{radio}}g~{10}^{49}\mathrm{erg}$ [S. Kulkarni et al., Nature (London) 395, 663 (1998)], while the apparent expansion velocity of its ejecta $(\ensuremath{\sim}{10}^{\ensuremath{-}5}{M}_{\ensuremath{\bigodot}})$ suggests a shock wave moving relativistically ${(V}_{\mathrm{exp}}\ensuremath{\sim}2c).$ The unique properties of SN 1998bw strengthen the case for it being linked with GRB980425. I present a consistent, novel mechanism to explain the peculiar event SN 1998bw and similar phenomena in GRBs: Conversion of powerful, high frequency (\ensuremath{\sim}2 kHz) gravitational waves (GWs) into electromagnetic waves [M. Johnston, R. Ruffini, and F. Zerilli, Phys. Rev. Lett. 31, 1317 (1973)] might have taken place during SN 1998bw. Yet, conversion of GRB photons into GWs, as advanced by Johnston, Ruffini, and Zerilli [Phys. Lett. 49B, 185 (1974)], may also occur. These processes can produce GRBs depleted in \ensuremath{\gamma} rays but enhanced in x rays, for instance, or even more plausibly induce dark GRBs, those with no optical afterglow. The class of GWs needed to drive the calorimetric changes of these gamma-ray bursts may be generated by (a) the nonaxisymmetric dynamics of a torus surrounding the hypernova (or failed supernova) magnetized stellar-mass black hole (BH) remnant, as in van Putten's mechanism for driving long GRBs powered by the BH spin energy [Phys. Rev. Lett. 87, 091101 (2001)], or in the van Putten and Ostriker mechanism to account for the bimodal distribution in duration in GRBs [Astrophys. J. Lett. 552, L32 (2001)], where the torus magnetohydrodynamics may be dominated by either hyperaccretion onto a slowly spinning BH or suspended accretion onto a fast rotating BH, or (b) the just formed black hole with electromagnetic structure as in the GRB central engine mechanism of Ruffini et al. [Astrophys. J. Lett. 555, L107 (2001); 555, L113 (2001)], provided the issue concerning the origin of the black hole charge can be suitably clarified. In both of these mechanisms the total energy radiated as GWs is about $\ensuremath{\Delta}{E}_{\mathrm{GW}}\ensuremath{\sim}{10}^{53}\mathrm{erg}\ifmmode\times\else\texttimes\fi{}{(M/10M}_{\ensuremath{\bigodot}}),$ which for the conversion efficiency estimated here turns out to be enough to explain the superluminous radio wavelength emission from SN 1998bw. Thus, I argue this process could have induced the enhancement in the radio luminosity of SN 1998bw as evidenced in its light curve [Fig. 2, in S. Kulkarni et al., Nature (London) 395, 663 (1998)] and optical light curves of GRB980326 [J. Bloom et al., Nature (London) 401, 453 (1998)] and GRB990712 [G. Bj\ornsson et al., Astrophys. J. Lett. 552, L121 (2001)]. Moreover, GW-driven plasma density perturbations moving at the speed of light may up- (or down-) convert fireball photons, which could cause further substantial modifications of the gamma-ray burst or supernova calorimetry.

Physical picture for the laser hosing instability in a plasma

The hosing of a light beam centroid is caused by upward or downward tilting of the local wave fronts due to the transverse phase velocity difference across the wave fronts. The phase velocity gradient is caused by the plasma density perturbation, which in turn is driven by the ponderomotive force of the hosing light beam. The hosing equations in both long- and short-wavelength regimes can be heuristically derived from this physical picture.

Study of Pellet-Plasma Interaction in the GAMMA 10 Tandem Mirror

In the GAMMA10 tandem mirror, the first pellet injection experiments in open systems have started.1 The tandem mirror machine is suitable for measuring propagation of pellet-fueled particles along the magnetic field lines. We observed a very low frequency oscillation in various diagnostic signals from the plasmas with a pellet injection. The frequency of the oscillation is a few hundred Hz. It is found that the phase of those signals changes to anti-phase at a certain place. The oscillation continued during plug-ECH for the formation of confining potential and its amplitude decays within a several ms after the end of ECH pulse. Measurements at several locations along the field lines indicates that this oscillation is a standing wave along field lines of plasma density perturbation due to the pellet injection. The wavelength of the oscillation is estimated to be about 6m. The oscillation has mode number n = 3.

Interaction of a shear Alfvén wave with a filamentary density perturbation in a low-β plasma

This analytic study investigates the interaction between a large-scale shear Alfvén wave propagating through a low-β plasma and a pre-existing density perturbation of small transverse scale. The interaction forms an in situ antenna that self-consistently generates two field-aligned current channels of opposite polarity. The expansion of the current channels across the confining field is bounded by the cone trajectories of small-scale inertial Alfvén waves. The spatial patterns of the radiated fields are obtained, and the magnitude of the parallel electric field and its effective phase velocity are assessed. An effective cross section that varies with the parallel and transverse scale lengths of the density perturbation summarizes the efficiency of the direct conversion process.