Small-scale Plasma Research Articles

Investigation of small-scale tokamak plasma turbulence by correlative UHR backscattering diagnostics

Fine scale turbulence is considered nowadays as a possible candidate for the explanation of anomalous ion and electron energy transport in magnetized fusion plasmas. The unique correlative upper hybrid resonance backscattering (UHR BS) technique is applied at the FT-2 tokamak for investigation of density fluctuations excited in this turbulence. The measurements are carried out in Ohmic discharge at several values of plasma current and density and during current ramp up experiment. The moveable focusing antennas set have been used in experiments allowing probing out of equatorial plane. The radial wave number spectra of the small-scale component of tokamak turbulence are determined from the correlation data with high spatial resolution. Two small-scale modes possessing substantially different phase velocities are observed in plasma under conditions when the threshold for the electron temperature gradient mode excitation is overcome. The possibility of plasma poloidal velocity profile determination using the UHR BS signal is demonstrated.

New mechanism for the formation of discrete stripes in the solar radio spectrum

Dispersion relations are derived for the eigenfrequency spectrum of a spatially periodic nonlinear plasma resonators created in the solar atmosphere due to the development of thermal instability. The eigenfrequency spectra of such resonators are calculated, and it is shown that they are capable of generating tens of discrete stripes (a so-called zebra structure) the number of which is independent of the ratio of the plasma frequency to the gyrofrequency in the source. This may help to overcome all difficulties in explaining the large number of stripes in the zebra structure, as well as the small magnitude of the magnetic field. The spatially periodic plasma resonators under consideration act as a filter with numerous transparency windows separated from one another by opaque regions. The number of stripes and their frequencies in the zebra structure depend on the spatial period of plasma nonuniformity, which is equal to meters or decameters for conditions typical of the solar atmosphere. The high brightness temperature of radio emission in the zebra structure is attributed to coherent emission from a large number of identical small-scale plasma sources. Some regular properties of the observed zebra structure are explained.

A study of the hydrogen production from a small plasma converter

In this paper, the hydrogen production characteristics of a purpose-designed and built small scale plasma converter for small engines was investigated. Hydrogen was produced through the reformation of ionised hydrocarbon fuel and air mixture by means of a spark discharge. The experimental results showed that a suitable size of the reaction chamber can increase the concentration of the produced hydrogen and that under a given methane supply rate, a low O 2/C ratio resulted in high hydrogen production concentration as well as high hydrogen volume flow rate. As the methane supply rate was increased, however, the hydrogen concentration reduced while the hydrogen volume flow rate increased. Hydrogen concentration was also improved when the intake gas mixture resided longer in the reaction channel, although the hydrogen volume flow rate revealed an inversed trend. It was also shown that by combining an intake air swirl with an appropriate O 2/C ratio and methane supply rate, both the methane conversion efficiency and the hydrogen production concentration were improved. In general, under the optimal operating condition, the plasma converter produced a maximum hydrogen concentration of 48% and a hydrogen volume flow rate of 70 mL/min.

The MaCWAVE program to study gravity wave influences on the polar mesosphere

Abstract. MaCWAVE (Mountain and Convective Waves Ascending VErtically) was a highly coordinated rocket, ground-based, and satellite program designed to address gravity wave forcing of the mesosphere and lower thermosphere (MLT). The MaCWAVE program was conducted at the Norwegian Andøya Rocket Range (ARR, 69.3° N) in July 2002, and continued at the Swedish Rocket Range (Esrange, 67.9° N) during January 2003. Correlative instrumentation included the ALOMAR MF and MST radars and RMR and Na lidars, Esrange MST and meteor radars and RMR lidar, radiosondes, and TIMED (Thermosphere Ionosphere Mesosphere Energetics and Dynamics) satellite measurements of thermal structures. The data have been used to define both the mean fields and the wave field structures and turbulence generation leading to forcing of the large-scale flow. In summer, launch sequences coupled with ground-based measurements at ARR addressed the forcing of the summer mesopause environment by anticipated convective and shear generated gravity waves. These motions were measured with two 12-h rocket sequences, each involving one Terrier-Orion payload accompanied by a mix of MET rockets, all at ARR in Norway. The MET rockets were used to define the temperature and wind structure of the stratosphere and mesosphere. The Terrier-Orions were designed to measure small-scale plasma fluctuations and turbulence that might be induced by wave breaking in the mesosphere. For the summer series, three European MIDAS (Middle Atmosphere Dynamics and Structure) rockets were also launched from ARR in coordination with the MaCWAVE payloads. These were designed to measure plasma and neutral turbulence within the MLT. The summer program exhibited a number of indications of significant departures of the mean wind and temperature structures from ``normal" polar summer conditions, including an unusually warm mesopause and a slowing of the formation of polar mesospheric summer echoes (PMSE) and noctilucent clouds (NLC). This was suggested to be due to enhanced planetary wave activity in the Southern Hemisphere and a surprising degree of inter-hemispheric coupling. The winter program was designed to study the upward propagation and penetration of mountain waves from northern Scandinavia into the MLT at a site favored for such penetration. As the major response was expected to be downstream (east) of Norway, these motions were measured with similar rocket sequences to those used in the summer campaign, but this time at Esrange. However, a major polar stratospheric warming just prior to the rocket launch window induced small or reversed stratospheric zonal winds, which prevented mountain wave penetration into the mesosphere. Instead, mountain waves encountered critical levels at lower altitudes and the observed wave structure in the mesosphere originated from other sources. For example, a large-amplitude semidiurnal tide was observed in the mesosphere on 28 and 29 January, and appears to have contributed to significant instability and small-scale structures at higher altitudes. The resulting energy deposition was found to be competitive with summertime values. Hence, our MaCWAVE measurements as a whole are the first to characterize influences in the MLT region of planetary wave activity and related stratospheric warmings during both winter and summer.

On small-scale plasma inhomogeneities of the traveling ionospheric disturbances

We report on the results of the first special experiment on radio sounding of the midlatitude ionosphere by signals from in-orbit satellites at a frequency of 150 MHz under quiet geophysical conditions. Along with the conventional correlation processing, fractal processing of the received signals was also performed. Using the fractal approach, we obtained first data on the sources and generation mechanisms of small-scale plasma inhomogeneities of the traveling ionospheric disturbances (TIDs) in the upper ionosphere. It is noted that the phenomenon of nonlinear “ breaking” of the acoustic-gravity waves entering the ionosphere from the underlying atmosphere plays the crucial role in the formation of plasma inhomogeneities of the TIDs.

Preliminary design of a 150 kJ repetitive plasma focus for the production of 18-F

Experiments in the past five years have demonstrated production of short-lived radioisotopes with a Plasma Focus device, using the so-termed Endogenous Mode. So far radioisotope activities of only a few microcuries have been obtained from single discharges in small scale Plasma Focus machines (capacitor bank energies of approximately 7 kJ). It is expected that higher activities could be obtained with larger bank energies, operating at high pulse repetition rates, e.g. 1 Hz. However, many scientific and technological issues must be addressed for a high-energy Plasma Focus device to run at one pulse per second. Aim of this paper is to present preliminary results pertaining to the plasma, electrical, fluid-dynamical, thermal, material and mechanical design of a 150 kJ Plasma Focus, capable of a repetition rate of 1 Hz, that will be operated at 30 kV with a 350 μF capacitor bank and a maximum total current of 1.5 MA. This device will be used to breed 18-F for the synthesis of drugs used in positron-emission medical examinations, such as FDG for PET.

Radiowave fluctuations in the polar ionosphere on the satellite-to-satellite paths under high solar activity

We analyze ionospheric fluctuations of decimeter radio waves on occultation polar paths between the navigational GPS satellites and the satellite CHAMP. Time dependences of the variance of the signal amplitude and amplitude fluctuation spectra under high solar activity in October–November 2003 are presented. The behavior of the signal amplitude fluctuations during occultation ionospheric sounding in the polar regions in different time of the day and in the equatorial regions in the daytime are considered. Radio-wave fluctuations are related to the solar-activity manifestations. It is shown that during strong solar-flare activity, intense small-scale plasma irregularities are excited in the polar ionosphere.

The first cluster and double star symposium

Cluster, a joint project between the European Space Agency (ESA) and NASA, is the first mission that is composed of four spacecraft flying in formation, and thus the first able to separate spatial and temporal features inside smallscale plasma structures within, and the boundaries between, many regions of the Earth's upper atmosphere (from 19,000–119,000 kilometers altitude).During the past two years, the Cluster mission has been enhanced by the Double Star (DSP) mission, a magnetospheric mission composed of two satellites that is a joint project between the China National Space Administration (CNSA) and ESA.The orbits have been chosen so that interesting conjunctions occur frequently between Cluster and the two DSP satellites. Conducting coordinated studies using simultaneous Cluster and Double Star observations increases the overall scientific return from both missions.

Materials processing at atmospheric pressure: Nonequilibrium effects on nanotechnology and mega-industries

Abstract Applications of atmospheric pressure nonequilibrium plasmas, because of their special advantages of forming reactive plasmas in a simple reactor, are spreading into various engineering fields, not only of materials processing, but also into energy and environment areas. Our group has explored new applications of both filamentary and diffuse barrier discharges, including the establishment of appropriate modeling, which enables better optimization of given plasma processes. More recently, microplasmas produced in submillimeter to micrometer reactors are also highlighted in association with atmospheric pressure nonequilibrium plasma because such small-scale plasmas frequently require high-density media to produce. This paper overviews our recent projects: (1) steam reforming of methane using filamentary barrier discharge; (2) deposition of carbon nanotubes in atmospheric pressure radio frequency discharge (APRFD); and (3) synthesis of silicon nanoparticles using microplasma.

Spatial Clustering of Photospheric Structures

The study of large-scale structures existing in solar photospheric flows provides an essential tool for constraining the models of solar and stellar convection and for understanding physical processes that are at the basis of solar variability. Recent convection models and N-body dynamic simulations predict the evolution of small-scale features (i.e., granules) into large-scale ones. In this work we address the question of the emergence of large spatial scale patterns as a direct consequence of the organization of small-scale plasma flows on time periods longer than the mean granular lifetime. Our analysis reveals that the photospheric dynamics plays a key role in structuring stable intensity features and suggests that surface flows organize small-scale plasma structures, sweeping them up to form clusters of recurrent and stable granular features. Using a quite novel statistical method, the hexagonal normalized information entropy, we establish that sites where recurrent and stable granular features are observed exhibit a clustering spatial scale of about 8 Mm and timescale around 10 minutes.

Optimum detrending of raw GPS data for scintillation measurements at auroral latitudes

Small-scale plasma density irregularities may produce fluctuations in both amplitude and phase of radio waves propagating through the ionosphere. This phenomenon is observed by means of radio signals broadcast by orbiting satellites. A powerful experimental tool is provided by the observation of GPS signals, which allows scintillation measurements from different azimuthal sectors, due to their spatial diversity. Ionospheric scintillation is also responsible for transionospheric signal degradation which can affect the performance of satellite navigation systems. GPS scintillation monitors are widely used for measuring ionospheric scintillation indices by means of GPS satellites signals. An independent experiment for measuring ionospheric scintillation effects on GPS signals has been set up at auroral latitudes in the European sector, in order to investigate the occurrence of phase without amplitude scintillation events, as measured by standard GPS scintillation monitors at high latitudes. The results obtained by such an experiment seem to confirm that erroneous data detrending can be responsible for high-phase scintillation with low-intensity scintillation events, which take place when the fixed filtering window, used to detrend raw GPS signal components, is not appropriate to actual plasma dynamics at small scales.

In situ observations of small scale neutral and plasma dynamics in the mesosphere/lower thermosphere at 79°N

From June 29 to July 6 2003, the German-Norwegian ROMA-SvalRak campaign (ROMA = Rocket borne Observations in the Middle Atmosphere) took place at the SvalRak rocket range, Ny-Ålesund (78.9°N, 11.9°E; Spitzbergen). The main scientific aim of this campaign was to study polar mesosphere summer echoes (PMSE) and mesospheric turbulence. During this campaign a total of three instrumented sounding rockets was launched. In addition, ground based observations with a VHF radar and a potassium lidar at Longyearbyen complemented the in situ measurements. All three sounding rockets were equipped with the CONE (COmbined sensor for Neutrals and Electrons) instrument to measure small scale structure of neutral air and electron density, and neutral temperature. The PIP (positive ion probe) instrument was used to measure small scale structure of the positive ion density. Furthermore, two cold plasma probes were flown to measure electron temperature, and a particle detector was employed to detect signatures of charged aerosols. During the first launch, an electric field experiment was also incorporated, while during the other two launches, Faraday rotation experiments yielded absolute electron number densities. During all three rocket flights a PMSE was observed by the VHF radar, whereas the potassium lidar detected a noctilucent cloud (NLC) only during the second launch. Signatures of charged particles forming the PMSE and NLC layers were recorded by the onboard particle detectors.

Investigations of short-scale fluctuations in a helicon plasma by cross-correlation enhanced scattering

Correlation enhanced scattering (CES) near the upper hybrid resonance has been applied for studying small-scale plasma density fluctuations excited by the rf fields in a helicon discharge. The turbulent fluctuations are diagnosed for conditions where the electron plasma frequency exceeds the electron cyclotron frequency considerably. The frequency and wave number spectra of the fluctuations are measured both in the plasma core as well as in outer region of the helicon discharge. The spectral measurements evidence the short-scale fluctuations to originate from a parametric decay instability. The low-frequency fluctuations obey the ion-sound dispersion relation while the lower sideband of the helicon wave frequency satisfies the Trivelpiece–Gould wave dispersion relation. In order to gain more insight in the experimental results and, in particular, to estimate the fluctuation level the backscattering process is analyzed both numerically and analytically for high-density plasma conditions. A fully electromagnetic model was developed that takes into account the radial density distribution of the plasma column as well as the antenna diagram of the rectangular emitting/receiving horn. Using this model the relative amplitude of ion-sound density fluctuations in the core of the helicon discharge is estimated as 11%. The role of nonlinear effects on formation of the scattering spectra due to the high fluctuation level in the plasma center is discussed. The findings also demonstrate that the CES diagnostic can be applied to diagnose fluctuations in spherical tokamak plasmas where the probing conditions resemble those of high-density helicon discharges.

Polar mesosphere summer echoes (PMSE): Review of observations and current understanding

Abstract. Polar mesosphere summer echoes (PMSE) are very strong radar echoes primarily studied in the VHF wavelength range from altitudes close to the polar summer mesopause. Radar waves are scattered at irregularities in the radar refractive index which at mesopause altitudes is effectively determined by the electron number density. For efficient scatter, the electron number density must reveal structures at the radar half wavelength (Bragg condition for monostatic radars; ~3 m for typical VHF radars). The question how such small scale electron number density structures are created in the mesopause region has been a longstanding open scientific question for almost 30 years. This paper reviews experimental and theoretical milestones on the way to an advanced understanding of PMSE. Based on new experimental results from in situ observations with sounding rockets, ground based observations with radars and lidars, numerical simulations with microphysical models of the life cycle of mesospheric aerosol particles, and theoretical considerations regarding the diffusivity of electrons in the ice loaded complex plasma of the mesopause region, a consistent explanation for the generation of these radar echoes has been developed. The main idea is that mesospheric neutral air turbulence in combination with a significantly reduced electron diffusivity due to the presence of heavy charged ice aerosol particles (radii ~5–50 nm) leads to the creation of structures at spatial scales significantly smaller than the inner scale of the neutral gas turbulent velocity field itself. Importantly, owing to their very low diffusivity, the plasma structures acquire a very long lifetime, i.e., 10 min to hours in the presence of particles with radii between 10 and 50 nm. This leads to a temporal decoupling of active neutral air turbulence and the existence of small scale plasma structures and PMSE and thus readily explains observations proving the absence of neutral air turbulence at PMSE altitudes. With this explanation at hand, it becomes clear that PMSE are a suitable tool to permanently monitor the thermal and dynamical structure of the mesopause region allowing insights into important atmospheric key parameters like neutral temperatures, winds, gravity wave parameters, turbulence, solar cycle effects, and long term changes.

Forecasting the occurrence of ionospheric scintillation activity in the equatorial ionosphere on a day-to-day basis

In the low-latitude ionospheric F region, ambient conditions after sunset are often conducive to the generation of small-scale plasma density structures by the Rayleigh–Taylor instability mechanism. Radio wave signals passing through these electron density irregularities experience amplitude fluctuations that lead to radio wave “scintillation”. The scintillation index, S4, is a measure of the severity of these amplitude fluctuations. It is well known that the occurrence frequency of scintillation activity depends on solar cycle, season, longitude and local time, with a large day-to-day variability. In this paper, we present the results of a continuing study that utilizes a ground-based digital sounder at the magnetic equator to provide a “forecast” of the subsequent occurrence of scintillation activity and a network of UHF and L-band receivers that measure the S4 index, on a night-to-night basis. To establish the “forecasting” relationship, observations were carried out in 1998 and 1999 in the Peruvian/Chilean longitude sector. The relationship between the magnitude of the vertical E × B drift velocity enhancement just after sunset as determined by the Jicamarca Digisonde and the magnitude of the S4 index subsequently observed an hour or two later by the scintillation receivers at Ancon, Peru and Antofagasta, Chile was established. In most cases at both UHF and L-band frequencies, this relationship is not linear, but there appears to be a “threshold” in E × B drift of ∼ 20 m / s . On nights when E × B drift was less than 20 m/s, the UHF S4 index generally was less than 0.5, while on nights when E × B drift exceeded 20 m/s, the S4 index was greater than 0.5. The day-to-day variability of this relationship will be presented and discussed. Since October 1, 2001, the Jicamarca Digisonde has been used to “forecast” the occurrence of UHF S 4 > 0.5 or S 4 < 0.5 on a night-to-night basis. The validity of the “forecast” is subsequently confirmed by UHF S4 observations at Ancon, Peru and Antofagasta, Chile. The resulting statistics for these nightly S4 forecasts from November 1, 2001 to April 30, 2002 will be presented and discussed.

Analysis of the small-scale plasma perturbations during density limit disruptions in the T-10 tokamak

Small-scale quasi-coherent oscillations of the x-ray emissivity and poloidal magnetic field perturbations are identified in high density plasmas in the T-10 tokamak with the use of a toroidal view x-ray imaging system, in-vessel hard x-ray detector and fast magnetic probes. Small-scale x-ray oscillations are associated with forward bremsstrahlung emission of the suprathermal electrons (Eγ ∼ 25–100 keV) from the plasma region where the m = 2, n = 1 magnetohydrodynamic (MHD) mode is localized. The oscillations are typically represented by a solitary high frequency (15–85 kHz) harmonic in the spectrum of perturbations accompanying MHD modes during density limit disruptions. During a disruption energy quench, the small-scale oscillations are often transformed to repetitive bursts of the nonthermal x-ray radiation localized in the outer part of the plasma. Analysis indicates a possible connection of the oscillations with nonthermal electron beams induced during magnetic reconnection around X-points of the m = 2, n = 1 magnetic islands. It is argued that the small-scale oscillations can be generated due to the modulation of the electron beams moving through an equilibrium magnetic field with ripples during rotation of the m = 2, n = 1 mode.

MHD plasma stability in alternative toroidal systems and the local dispersion relation

The local dispersion relation obtained for an inhomogeneous anisotropic high-pressure plasma in the Chew-Goldberger-Low approximation is used to qualitatively study small-scale MHD plasma instabilities in alternative magnetic configurations in which the plasma compressibility plays a significant stabilizing role. It is established that it is important to satisfy the Bernstein-Kadomtsev condition in order to reduce the growth rate of the quasi-flute oscillations. Moderate plasma anisotropy is shown not to have a substantial destabilizing effect on the MHD plasma stability under the Bernstein-Kadomtsev condition in alternative systems. The situation in which the electron compressibility vanishes while the ion compressibility is nonzero is discussed; it is shown that, in this situation, the Bernstein-Kadomtsev condition becomes more stringent as the longitudinal wavenumber increases.

Electron Temperature Effects on Small-Scale Plasma Irregularities Associated With Charged Dust in the Earth's Mesosphere

Polar mesospheric summer echoes (PMSEs) are strong radar echoes in the 50-MHz to 1.3-GHz frequency range produced by scattering from electron irregularities in the earth's mesosphere. The electron irregularities believed to produce PMSEs result from electron charging on subvisible dust that exists in the mesosphere. The study of PMSEs is a forefront issue in near earth space science because of the tremendous remote sensing diagnostic possibilities that exist for studying the earth's middle atmosphere. Recently, experimental results have shown that PMSEs may be modulated by radio wave heating the irregularity source region with a ground-based ionospheric heating facility. The strength of the PMSEs is reduced within second time scales upon turning on the radio wave heating, and there is a similar time scale for recovery of the PMSEs upon turning the radio wave heating off. The numerical model is used to investigate the time evolution of the effects of radio wave heating on the irregularities believed to produce PMSEs. The effects of dust charging and chemistry as well as diffusion is included in the model. The results indicate that diffusion of electrons and ions are of primary importance at early times after turn on or turn off of the radio wave heating while dust charging may have important effects at later times. Also, the scale size of the irregularities has important effects on the reduction and recovery behavior of the irregularities.

CLUSTER observations of lower hybrid waves excited at high altitudes by electromagnetic whistler mode signals from the HAARP facility

[1] We report new observations from the CLUSTER spacecraft of strong excitation of lower hybrid (LH) waves by electromagnetic (EM) whistler mode waves at altitudes ≥20,000 km outside the plasmasphere. Previous observations of this phenomenon occurred at altitudes ≤7000 km. The excitation mechanism appears to be linear mode coupling in the presence of small scale plasma density irregularities. These observations provide strong evidence that EM whistler mode waves are continuously transformed into LH waves as the whistler mode waves propagate at high altitudes beyond L ∼ 4. This may explain the lack of lightning generated whistlers observed in this same region of space.

Development of Small-scale Plasma Experiments for an Undergraduate Laboratory

In this paper we describe some simple low cost experiments for developing plasma physics at undergraduate level. A discharge tube was fabricated to (i) study variation of breakdown voltage with distance and pressure, (ii) estimate charge densities and (iii) estimate electron energies in argon and air discharge. The charge densities measured using Johnson- Malter's double probe method for argon and air were 6.0n1014 electrons m-3 and 4.1n1014 electrons m-3 respectively. The corresponding electron energies determined for argon gas discharge and air discharge were 1.0 n 10-18 J and 9.6 n 10-19 J respectively. These quantities were determined at 1 torr pressure. The minimum breakdown voltage for argon was 900 V at 7 torr. This gave a value of the product of pressure and separation of electrodes10 torr-mm, but at constant 2 mm separation of electrodes the value was 7.5 torr-mm at the same breakdown voltage of 900 V. The Paschen's and the Townsend's laws were also verified under present experimental conditions. Keywords: plasma, electron temperature, electron energy, paschen, townsend Résumé Dans cet article nous décrivons des expériences simples et peu chères de laboratoire de physique des plasmas pouvant être menées dans le cadre des travaux pratiques par les étudiants au niveau du baccalauréat. Un tube de décharge a été fabriqué pour étudier (i) la variation de décharge d'une tension selon la distance et la pression, (ii) évaluer les densités de charge et (iii) évaluer les énergies électroniques dans des décharges d'argon et d'air. Les densités de charge mesurées par la méthode d'un couple de sondes Johnson- Malter dans l'argon et l'air, étaient respectivement de 6.0n1014 électrons par m3 et de 4.1n1014 électrons par m3 Les énergies électroniques correspondantes, déterminées par la décharge pour le gaz d'argon et l'air étaient de 1.0 n 10-18 J et de 9.6 n 10-19 J respectivement. Ces quantités ont été déterminées à 1 torr de pression. La tension de décharge minimale pour l'argon était 900 V à 7 torr. Ceci a donné la valeur du produit de la pression par l'écartement des électrodes de 10 torr-mm, mais avec un écartement constant de 2 mm entre les électrodes, la valeur était de 7,5 torr-mm à la même tension de décharge de 900 V. Les lois de Paschen et Townsend ont été également vérifiées dans des conditions de la présente expérience. Mots-clés: plasma, température électronique, énergie électronique, paschen, townsend (Discovery and Innovation: 2002 14 (3-4): 169-178)