Low-frequency Electrostatic Fluctuations Research Articles

Direct measurements of classical and enhanced gradient-aligned cross-field ion flows in a helicon plasma source using laser-induced fluorescence

Direct laser induced fluorescence measurements are shown of cross-field ion flows normal to an absorbing boundary that is aligned parallel to the axial magnetic field in a helicon plasma. We show Langmuir and emissive probe measurements of local density and plasma potential in the same region, as well as floating probe spectra near the boundary. With these measurements, we investigate the influence of ion-neutral collisionality on radial ion transport by varying the ratio of the ion gyro-radius, ρi, to the ion-neutral collision length, λ, over the range 0.34 ≤ ρiλ−1 ≤ 1.60. Classical drift-diffusion transport along density and potential gradients is sufficient to describe flow profiles for most cases. For two parameter regimes (ρiλ−1 = 0.65 and 0.44), low-frequency electrostatic fluctuations (f < 10 kHz) and enhanced cross-field bulk ion flow to the boundary are observed.

Imaging of Turbulent Structures and Tomographic Reconstruction of GyM Plasma Emissivity

In GyM, a linear magnetized plasma device, low-frequency electrostatic fluctuations are measured by means of Langmuir probes. To complement electrostatic probe measurements of plasma turbulence and the study of plasma structures, a nonperturbative direct imaging system has been used on GyM, including a fast-framing Photron APX-RS camera and an image intensifier unit. From the line-integrated camera images, we compute time-resolved emissivity profiles of the plasma by applying a tomographic reconstruction technique using a pixel method and solving an overdetermined set of equations by singular value decomposition. The validity and robustness of the tomographic reconstruction technique are examined with respect to noise and wall reflection effects.The tomographic reconstruction is applied to fast camera movies acquired with a frame rate of 75 kHz and 4 μs of exposure time to obtain the temporal evolution of the emissivity fluctuations. Plasma structures can be detected and tracked in the reconstructed emissivity movies with a finest spatial resolution of 2 cm. Conditional average sampling is used to determine the size and speed of turbulent structures.

Turbulent transport in a toroidal magnetized plasma

Turbulent plasma transport due to low-frequency electrostatic fluctuations in a toroidal plasma is studied experimentally. The data are obtained in a magnetized toroidal plasma with no toroidal transform. The plasma is generated by a discharge from a hot electron emitting filament and diagnosed by conventional Langmuir probes measuring densities by electron or ion saturation currents and floating potentials. We present results for the statistical properties of the fluctuating radial transport caused by low-frequency electrostatic turbulence in the device. The turbulent plasma flux is identified as the product of the fluctuating density and the E × B/B2-velocity. Even though the probability densities of the fluctuating electric fields and plasma densities are close to Gaussians, we find strongly intermittent features in the flux signal obtained as the product of these two fluctuating quantities. A conditional statistical analysis gives insight in detail of the turbulent transport. The intermittency studies are extended by analyzing the excess statistics, i.e. the average duration of time intervals in the flux signal spent above a given reference level. We find that this analysis offers a very effective measure for intermittency effects. In our case, the signal is characterized by an excess of temporally narrow, large amplitude bursts, when compared with an equivalent Gaussian random signal.

Territorial characteristics of low frequency electrostatic fluctuations in a simple magnetized torus

This paper presents an experimental investigation of turbulence in simple toroidal plasma devices without rotational transform. It is argued that Rayleigh–Taylor (flute interchange) mode may be one of the source mechanisms for the observed turbulence but is not sufficient to explain its observed global characteristics. Taking BETA device as an example, we show that pure Rayleigh–Taylor mode cannot explain (i) the observation of mode maximum at the location other than where density scale length is minimum, (ii) the comparable value of amplitude level of fluctuations in good curvature region, and (iii) the decrease in the mode amplitude with increasing magnetic field. Investigations have revealed that there exists not only poloidal plasma flow but also that it is sheared. Including this effect explains the first observation. However, modification brought about by velocity shear in the Rayleigh–Taylor mode still does not explain our second and third observations. We have taken an approach that since Rayleigh–Taylor is not excited in a good curvature region, it cannot be the source of turbulence there. Nor is it defensible to say that turbulence born in a bad curvature region is carried over through E×B rotation to the good curvature region. Consequently, we have invoked cross-field Simon–Hoh instability for this region. Experimental evidence supporting our proposal is presented. This paper concludes that toroidal devices have simultaneous existence of different self-consistent sources of turbulence in different regions of the device.

Imaging of turbulent structures and tomographic reconstruction of TORPEX plasma emissivity

In the TORPEX [A. Fasoli et al., Phys. Plasmas 13, 055902 (2006)], a simple magnetized plasma device, low frequency electrostatic fluctuations associated with interchange waves, are routinely measured by means of extensive sets of Langmuir probes. To complement the electrostatic probe measurements of plasma turbulence and study of plasma structures smaller than the spatial resolution of probes array, a nonperturbative direct imaging system has been developed on TORPEX, including a fast framing Photron-APX-RS camera and an image intensifier unit. From the line-integrated camera images, we compute the poloidal emissivity profile of the plasma by applying a tomographic reconstruction technique using a pixel method and solving an overdetermined set of equations by singular value decomposition. This allows comparing statistical, spectral, and spatial properties of visible light radiation with electrostatic fluctuations. The shape and position of the time-averaged reconstructed plasma emissivity are observed to be similar to those of the ion saturation current profile. In the core plasma, excluding the electron cyclotron and upper hybrid resonant layers, the mean value of the plasma emissivity is observed to vary with (Te)α(ne)β, in which α=0.25–0.7 and β=0.8–1.4, in agreement with collisional radiative model. The tomographic reconstruction is applied to the fast camera movie acquired with 50 kframes/s rate and 2 μs of exposure time to obtain the temporal evolutions of the emissivity fluctuations. Conditional average sampling is also applied to visualize and measure sizes of structures associated with the interchange mode. The ω-time and the two-dimensional k-space Fourier analysis of the reconstructed emissivity fluctuations show the same interchange mode that is detected in the ω and k spectra of the ion saturation current fluctuations measured by probes. Small scale turbulent plasma structures can be detected and tracked in the reconstructed emissivity movies with the spatial resolution down to 2 cm, well beyond the spatial resolution of the probe array.

Life cycle of density structures in a simple magnetized torus

This paper presents an experimental investigation of spatio-temporal structures embedded in the low-frequency electrostatic fluctuations in a toroidal plasma device without rotational transform. The structures are extracted using the conditional averaging technique. The fundamental features and life cycle of the structures from their evolution to demise have been studied. The dipole structures formed rotate in poloidal cross-section with a propagation velocity of ∼9×104 cm s−1. The direction of propagation is the same as the electron diamagnetic drift direction and the E×B rotation direction. We show that these structures evolve in two phases. In the first phase, the spatial extent and the measure of intensity of the structures increase considerably, indicating that ingestion of plasma is more than the loss to the limiter due to diffusion. In the second phase, decay of the structures begins. There are two time scales; the longer one precedes the shorter one. The decay is exponential, indicating that it is solely due to diffusion of plasma to the limiter. This indicates that what appears to be a perpetual periodicity-dominated time series actually consists of a series of large spatio-temporal structures having a definite life cycle of secular growth and decay phases. We interpret this result to mean that BETA plasma possesses a dynamic equilibrium in which the plasma oscillates between two non-equilibrium states.

Fast visible imaging of turbulent plasma in TORPEX

Fast framing cameras constitute an important recent diagnostic development aimed at monitoring light emission from magnetically confined plasmas, and are now commonly used to study turbulence in plasmas. In the TORPEX toroidal device [A. Fasoli et al., Phys. Plasmas 13, 055902 (2006)], low frequency electrostatic fluctuations associated with drift-interchange waves are routinely measured by means of extensive sets of Langmuir probes. A Photron Ultima APX-RS fast framing camera has recently been acquired to complement Langmuir probe measurements, which allows comparing statistical and spectral properties of visible light and electrostatic fluctuations. A direct imaging system has been developed, which allows viewing the light, emitted from microwave-produced plasmas tangentially and perpendicularly to the toroidal direction. The comparison of the probability density function, power spectral density, and autoconditional average of the camera data to those obtained using a multiple head electrostatic probe covering the plasma cross section shows reasonable agreement in the case of perpendicular view and in the plasma region where interchange modes dominate.

Streaming instabilities in multicomponent interstellar clouds

Streaming instabilities in interstellar clouds have been investigated. It is pointed out that the motion of a lighter dust species relative to a plasma containing a heavier dust species can give rise to low frequency electrostatic fluctuations. The dust ion acoustic and dust acoustic waves can become unstable in such plasmas. The instability regions are influenced by the presence of background dust. Since interstellar clouds have relative motions in many cases and contain dust species, therefore the present results indicate the presence of an electrostatic dust ion acoustic wave and dust acoustic wave in such systems. The results have been applied to interstellar medium using particular data and comparisons have been made with previous works.

Ion streaming instability in a quantum dusty magnetoplasma

It is shown that a relative drift between the ions and the charged dust particles in a magnetized quantum dusty plasma can produce an oscillatory instability in a quantum dust acousticlike wave. The threshold and growth rate of the instability are presented. The result may explain the origin of low-frequency electrostatic fluctuations in semiconductors quantum wells.

Probe measurements of low-frequency plasma potential and electric field fluctuations in a magnetized plasma

A system of two cylindrical probes aligned along the magnetic field, and equipped with insulating end plugs, is proposed for measurement of low-frequency fluctuations of the electrostatic field in a magnetized plasma. It is demonstrated by modeling and experiments that the plug probe floats close to the plasma potential. The electric field component in a given direction is obtained by subtracting the plasma potentials obtained on two spatially separated plug probes. The probe system is applied to low-frequency electrostatic fluctuations in a simple magnetized torus, and reveals the presence of global oscillations, large scale propagating structures (m=1 modes), and developed turbulence with power-law spectra. Two different mode branches for the fluctuations are identified by comparing results from plug probes with results from conventional probes. Sources of errors arising from applying floating potential of conventional probes for electric field measurements are pointed out and discussed.

Experimental investigation of particle heating in a strongly coupled dusty plasma

Highly charged dust particles in a plasma can be strongly coupled when their kinetic temperature is low. This temperature is determined by a balance of heating and gas cooling. The heating is believed to be electrostatic, although its exact nature is still under investigation. Experiments in a multiple-layer plasma crystal were conducted to test proposed heating mechanisms. A method for measuring small-amplitude, low-frequency fluctuations in ion density was developed and, using this, very low-frequency electrostatic fluctuations were found upstream of the particles. These fluctuations should propagate with the ions towards the particles and heat them. However, the fluctuations were uncorrelated with, and too weak to account for, the observed particle temperatures. In the experiment, the temperature increased and then decreased with gas pressure; this result is only partly consistent with an ion wake heating mechanism. These negative findings help narrow the range of possible explanations for the observed temperatures.

Experimental study of low-frequency electrostatic fluctuations in a magnetized toroidal plasma

An experimental study of low-frequency electrostatic fluctuations is presented for a plasma produced by a steady-state discharge in a magnetized toroidal plasma device without a rotational transform. A significant intermittency of the fluctuations is observed. Thus the evolution and propagation of large coherent vortical structures is demonstrated by a conditional sampling technique. The flutelike nature of the structures is explicitly demonstrated. The analysis includes measurements of fluctuations in plasma density, electric potential, and electron temperature. The relevance of the observations to anomalous transport in the device is pointed out. The performance of the conditional sampling technique is compared to a simple correlation analysis by a Monte Carlo simulation.

Role of finite parallel wave number in low frequency fluctuations in toroidal current less plasma

In this paper we present results related to studies of low frequency electrostatic fluctuations in a toroidal plasma of varying length and without a toroidal current. The toroidal length of plasma is varied from fully toroidal (major radius R=45 cm to 278, 212 and 142 cm. The characteristics of fluctuations changed, showing the effect of finite system length on the nature of the fluctuations. The major results are 1 a drastic reduction in the amplitude of fluctuations, 2 this reduction is mainly due to the disappearance of coherent peaks at 3 kHz and its harmonics observed in unbounded system, 3 the presence of a broad coherent feature around 10 kHz, only in bad curvature region, at low magnetic field (200 G) and 4 the appearance of a large number of coherent peaks, in one particular system length, at a higher magnetic field (400 G). These results indicate that the Rayleigh–Taylor instability with finite k‖ may be excitable in the device.

Propagation and dispersion of electrostatic waves in the ionospheric E region

Abstract. Low-frequency electrostatic fluctuations in the ionospheric E region were detected by instruments on the ROSE rockets. The phase velocity and dispersion of plasma waves in the ionospheric E region are determined by band-pass filtering and cross-correlating data of the electric-field fluctuations detected by the probes on the ROSE F4 rocket. The results were confirmed by a different method of analysis of the same data. The results show that the waves propagate in the Hall-current direction with a velocity somewhat below the ion sound speed obtained for ionospheric conditions during the flight. It is also found that the waves are dispersive, with the longest wavelengths propagating with the lowest velocity.

Turbulent transport in low-β plasmas

Low-frequency electrostatic fluctuations are studied experimentally in a low-β plasma, with particular attention to their importance for the anomalous plasma transport across magnetic field lines. The presence of large coherent structures in a turbulent background at the edge of the plasma column is demonstrated by a statistical analysis. The importance of these structures for the turbulent transport is investigated. The study is extended by a multichannel conditional analysis to illustrate detailed properties and parameter dependences of the turbulent transport.

Toroidal gyrokinetic particle simulations of core fluctuations and transport

The generation of low frequency electrostatic fluctuations and the formation of anomalous particle and heat diffusion in the core region of tokamak plasmas is investigated using gyrokinetic particle-in-cell simulations in toroidal geometry. We consider the case of H-mode tokamak plasmas characterized by nearly flat core density profiles. The principle source of fluctuations include ion temperature gradient (ITG)-driven instabilities and ITG-dissipative trapped electron modes. We compare the evolution of both branches of instability and find that the electron diffusivity can be larger than the ion thermal diffusivity in the latter case. An externally imposed radial electric field, which gives rise to sheared poloidal flows, is demonstrated to reduce the radial correlation length of the turbulent transport and hence reduce the anomalous thermal flux.

Fluctuations in a magnetized toroidal plasma without rotational transform.

An experimental study of low frequency electrostatic fluctuations is presented for a plasma produced by a steady state discharge in a toroidal device. The evolution and propagation of large coherent vortical structures is demonstrated by a conditional sampling technique. The flutelike nature of the structures is explicitly demonstrated.

Electrostatic fluctuations and turbulent plasma transport in low-β plasmas

Low frequency electrostatic fluctuations are studied experimentally in a low-β plasma, with particular attention to their importance for the anomalous plasma transport across magnetic field lines. The presence of large coherent structures in a turbulent background is demonstrated by a conditional analysis. The importance of these structures for the turbulent transport is demonstrated.

Low-Frequency electrostatic fluctuations in magnetized plasma: microscopic theory

The classical approach together with the quasiparticle method are used to elaborate a microscopic theory of low-frequency (with respect to the ion cyclotron frequency) fluctuations in magnetized plasmas. The spectral distributions of electric field and density fluctuations are calculated in the electrostatic approximation, and the conditions for the transition from incoherent (spontaneous) fluctuations to collective fluctuations in the low-frequency oscillating magnetized plasma are studied. The significance of particle polarization drifts and finite-Larmor-radius effects, as well as of ion motion in such transition is explained.

Vortices in non-uniform dusty plasmas

By means of a two fluid model, we derive a set of non-linear mode coupling equations for weakly interacting low-frequency electrostatic fluctuations that are driven by a sheared ion velocity flow in a non-uniform dusty plasma embedded in an external magnetic field. The linear instability is re-examined. Furthermore, dipolar vortices are shown to be possible stationary solutions of the non-linear equations governing mode coupling of the ion velocity gradient fluctuations in dusty plasmas. The existence regions in parameter space for the dipolar vortex formation are presented. The relevance of our investigation to astrophysical and space plasmas is discussed.