Electron Temperature Fluctuations Research Articles

First Results of Correlation Electron Cyclotron Emission on Tore Supra

Measurements of electron temperature fluctuations by means of correlation electron cyclotron emission (ECE) diagnostics aid in understanding the nature of the turbulent transport in fusion plasmas. On Tore Supra tokamak, a 32-channel heterodyne ECE radiometer has been upgraded to include two channels for temperature fluctuation measurements. The central frequency of the yttrium iron garnet filter on each channel is remotely monitored by a driver, allowing one to shift the observation volume in the plasma radially. To resolve the fluctuation amplitude of 0.1%, integration times of ~3 s at 1 MHz sampling rate are required for the statistical error level of 0.05%. Together with the 32-channel profile radiometer, correlation ECE is a powerful and unique tool for simultaneous observation of the magnetohydrodynamic phenomena and studies of the plasma transport properties in various heating regimes during long-shot multimegawatt operation. First measurements of temperature fluctuations, of various origins, on Tore Supra (including the observation of the toroidicity-induced Alfvén eigenmodes), as well as a description of the analysis methods used in the data evaluation, are reported in this paper.

Upgrades to the TEXTOR electron cyclotron emission imaging diagnostic

A 128 channel electron cyclotron emission imaging instrument has been routinely used to study magnetohydrodynamics physics such as m=1 and m=2 modes on the TEXTOR tokamak. As currently configured, each of the 16 mixer array elements measures plasma emission at 8 simultaneous frequencies to form a 16×8 image of electron temperature profiles and fluctuations over an area of 16cm (vertical) by 6cm (horizontal). A redesigned mixer array, coupled with new wideband electronics to be installed later this year, will increase the plasma coverage to 17cm(v)×9cm(h). The new arrangement offers increased temperature resolution together with new gain and video bandwidth controls in a highly modular configuration for ease of maintenance and facilitation of future upgrades both in frequency coverage as well as number of channels.

Electron temperature fluctuations in H II regions

Context. The spatial electron temperature fluctuations in ionized nebulae that were initially proposed to explain the discrepancies among temperatures obtained from different sensors have been pointed to as the cause of huge inconsistencies among abundances of heavy elements calculated from recombination and forbidden emission lines. Recently, there have been some attempts of direct detection and quantification of spatial temperature fluctuations with point-to-point temperature measurements across the nebula.

Comparison Study of 2D Images of Temperature Fluctuations during Sawtooth Oscillation with Theoretical Models

High temporal and spatial resolution two-dimensional (2D) images of electron temperature fluctuations were employed to study the sawtooth oscillation in the Toroidal Experiment for Technically Oriented Research tokamak plasmas. The 2D images are directly compared with the expected 2D patterns of the plasma pressure (or electron temperature) from various theoretical models. The observed experimental 2D images are only partially in agreement with the expected patterns from each model: The image of the initial reconnection process is similar to that of the ballooning mode model. The intermediate and final stages of the reconnection process resemble those of the full reconnection model. The time evolution of the images of the hot spot or island is partially consistent to those from the full reconnection model but is not consistent with those from the quasi-interchange model.

Observation of High-Field-Side Crash and Heat Transfer during Sawtooth Oscillation in Magnetically Confined Plasmas

High resolution (temporal and spatial), two-dimensional images of electron temperature fluctuations during sawtooth oscillations were employed to study the crash process and heat transfer in magnetically confined toroidal plasmas. The combination of kink and local pressure driven instabilities leads to a small poloidally localized puncture in the magnetic surface at both the low and the high field sides of the poloidal plane. This observation closely resembles the "fingering event" of the ballooning mode model with the high- mode only predicted at the low field side.

Self-organized Te redistribution during driven reconnection processes in high-temperature plasmas

Two-dimensional (2D) images of electron temperature fluctuations with high temporal and spatial resolution were employed to study the sawtooth oscillation in Toroidal EXperiment for Technology Oriented Research [S. S. Abdallaev et al., Nucl. Fusion 43, 299 (2003)] tokamak plasmas. The new findings are: (1) 2D images revealed that the reconnection is localized and permitted the determination of the physical dimensions of the reconnection zone in the poloidal and toroidal planes. (2) The combination of a pressure bulge due to finite pressure effects or a kink instability accompanied with a sharp pressure point leads to an “X-point” reconnection process. (3) Reconnection can take place anywhere along the q∼1 rational magnetic surface (both high- and low-field sides). (4) Heat flow from the core to the outside of the inversion radius during the reconnection time is through the finite opening on the poloidal and toroidal planes and the flow is highly collective. These new findings are compared with the characteristics of various theoretical models and experimental results for the study of the sawtooth oscillation in tokamak plasmas.

Chemical abundances in seven galactic planetary nebulae

An observational study of chemical abundances in the galactic planetary nebulae NGC 1535, NGC 2438, NGC 2440, NGC 3132, NGC 3242, NGC6302, and NGC 7009 based on long-slit spectra of high signal-to-noise ratio in the 3100 to 6900 A range is presented. We determined the N, O, Ne, S, and Cl abundances from collisionally excited lines and the He and O ++ abundances from recombination lines. The O ++ /H + estimates derived from recombination lines are about a factor of four and two higher than those derived from forbidden lines for NGC 7009 and NGC 3242, respectively. Spatial profiles of O ++ /H + abundance from OII permitted lines and from [OIII] forbidden lines were obtained for the planetary nebula NGC 7009. The differences between O ++ /H + derived from recombination and from forbidden lines present smooth variations along the nebular surface of NGC 7009, with the differences decreasing from the center to the edges of the nebula. If these abundance differences are explained by the presence of electron temperature fluctuations, quantified by the parameter t 2 , a value of about t 2 = 0.09 is required for NGC 3242 and NGC 7009.

A photoionization-modelling study of 30 Doradus: the case for small-scale chemical inhomogeneity

Photoionization models of the giant H II region (GHIIR) 30 Doradus are built and confronted to available ultraviolet (UV), optical, infrared (IR) [Infrared Space Observatory (ISO)] and radio spectra, under blackbody or CoStar spectral energy distributions for the primary source and various density distributions for the nebular gas. Chemically homogeneous models show very small rms electron-temperature fluctuations and fail to reproduce the heavy-element optical recombination line (ORL) spectrum of the nebula. Dual-abundance models incorporating small-scale chemical inhomogeneities in the form of hydrogen-deficient inclusions which are in pressure balance with the normal-composition ambient gas, provide a better fit to the observed heavy-element ORLs and other nebular lines, while most spectral features are satisfactorily accounted for. The inclusions, whose mass is ∼2 per cent of the total gaseous mass, are 2‐ 3 times cooler and denser than the ambient nebula. Their O/H abundance ratio is ∼0.9 dex larger than in the normal-composition gas and have typical mass fractions of X = 0.687, Y = 0.273 and Z = 0.040. Helium is found to be about as deficient as hydrogen in the inclusions, while elements heavier than neon, such as sulphur and argon, are quite possibly enhanced in proportions similar to oxygen, as indicated by the most satisfactory dual-abundance model obtained. This suggests that the posited hydrogen-deficient inclusions may have arisen from partial mixing of matter which was nucleosynthetically processed in a supernova event with gas of normal Large Magellanic Cloud composition. The average gaseous abundances of the chemically inhomogeneous models are ∼0.08 dex higher than those of the homogeneous models, yet they are lower by a similar ∼0.08 dex than those derived from standard empirical methods (fully corrected for inaccuracies in ionization correction factors and differences in atomic data) which postulate temperature fluctuations in a chemically homogeneous medium. Attention is drawn to a bias in the determination of H II region (HIIR) helium abundances in the presence of hydrogen-deficient inclusions. It is argued that these results provide evidence for incomplete small-scale mixing of the interstellar medium (ISM). The case for the existence of abundance inhomogeneities in HIIRs is examined in the light of current theoretical considerations regarding the process of chemical homogenization in the ISM. Ke yw ords: ISM: abundances ‐ ISM: evolution ‐ H II regions ‐ ISM: individual: 30 Doradus ‐ galaxies: individual: LMC.

Electron temperature fluctuations in planetary nebulae

An observational study of the spatial variation of the electron temperature and density in 10 galactic planetary nebulae is presented. The data consist of long-slit spectra of high signal-to-noise ratio in the 3100 to 6900 A range. Electron temperatures were determined from the [Oiii](λ 4959 + λ 5007)/λ 4363 and [Nii](λ 6548 + λ 6583)/λ 5755 ratios and from the Balmer discontinuity. Electron densities were estimated from the [Sii]λ 6716/λ 6731, [Cliii]λ 5517/λ 5537, and [Ariv]λ 4711/λ 4740 ratios. Electron temperature variations of low amplitude were found across the nebular surface in the planetary nebulae studied. The temperature distribution across each nebula presents a variance relative to the mean corresponding to 0.0003 ≤ t 2 (Bal) ≤ 0.0078, 0.0003 ≤ t 2 (Nii) ≤ 0.0097, and 0.0011 ≤ t 2 (Oiii) ≤ 0.0050. A systematic spatial variation of electron density has been detected in most of objects (NGC 1535, NGC 2438, NGC 2440, NGC 3132, NGC 3242, NGC 6302, NGC 6563, and NGC 7009). The remaining objects (NGC 6781 and NGC 6853) have not shown any significant electron density dependence on position. NGC 2438, NGC 6563, NGC 6781, and NGC 6853 are in general the most diffuse and probably evolved objects studied here, with low mean densities in the range Ne(Sii) ≈ 95–158 cm −3 . An anti-correlation between temperature and density was found for NGC 2438 and NGC 3132, with the electron temperature increasing with the decrease of electron density and a correlation between temperature and density was found for NGC 2440, NGC 3242, NGC 6302, and NGC 7009, with the electron temperature increasing with the increase of electron density. These relationships seem to be associated with the structure of the nebula. The nebulae in which the correlation between temperature and density is present are ring shaped. The anti-correlation between temperature and density is found in bipolar planetary nebulae that are denser in the centre of the nebula.

Simultaneous measurements of fluctuations of ion current, electron temperature, and floating potential with a tunnel probe

The tunnel probe is a new kind of Langmuir probe for use in the tokamak scrape-off layer. Two-dimensional kinetic analysis of the ion current distribution on the concave conductors is used to calibrate the probe for making fast dc measurements of ion flux and electron temperature. Qualitative agreement with classical Langmuir-probe measurements is found, but the electron temperature given by the tunnel probe is several times lower. This discrepancy is partly caused by secondary electron emission, and can easily be corrected. Strong eduction of ion-flux and electron-temperature fluctuations is observed during electrode biasing.

Simulation of turbulence in the divertor region of tokamak edge plasma

Results are presented for turbulence simulations with the fluid edge turbulence code BOUT [X.Q. Xu, R.H. Cohen, Contr. Plas. Phys. 36 (1998) 158]. The present study is focussed on turbulence in the divertor leg region and on the role of the X-point in the structure of turbulence. Results of the present calculations indicate that the ballooning effects are important for the divertor fluctuations. The X-point shear leads to weak correlation of turbulence across the X-point regions, in particular for large toroidal wavenumber. For the saturated amplitudes of the divertor region turbulence it is found that amplitudes of density fluctuations are roughly proportional to the local density of the background plasma. The amplitudes of electron temperature and electric potential fluctuations are roughly proportional to the local electron temperature of the background plasma.

Publisher’s Note: “Effect of electron temperature fluctuations on slowly swept Langmuir probe measurements” [Rev. Sci. Instrum. 75, 4334 (2004)

Publisher’s Note: “Effect of electron temperature fluctuations on slowly swept Langmuir probe measurements” [Rev. Sci. Instrum. 75, 4334 (2004)

Two-dimensional electron cyclotron emission imaging diagnostic for TEXTOR

A two-dimensional electron cyclotron emission imaging (ECEI) diagnostic has been developed in which broad bandwidth radiation is collected by a vertically aligned mixer array and separated by frequency band. Using a 16 element array, and an 8 band receiver attached to each array element, time-resolved 16×8 images of electron temperature profiles and fluctuations of the TEXTOR plasma are acquired with vertical and horizontal channel spacings of 11 and 8 mm, and spot sizes of 13 and 9 mm, respectively. The system is wideband tunable from 95 to 130 GHz, and the focus location may be shifted horizontally via translation of one of the optical imaging elements. System design and laboratory testing details of the ECE Imaging optics and multichannel wideband electronics are presented, together with TEXTOR plasma data.

Effect of electron temperature fluctuations on slowly swept Langmuir probe measurements

Swept Langmuir probes are widely used to measure electron temperature (Te) in laboratory plasmas by performing an exponential fit to the measured volt–ampere (I–V) characteristic. Often the probe voltage sweep frequency is much lower than the characteristic frequencies of the plasma fluctuations and a time-averaged I–V characteristic is used for the fit. We show by numerical modeling that in the presence of Te fluctuations with frequencies well above the voltage sweep frequency this standard technique applied to a swept single probe tends to read higher than the actual time-averaged Te provided no correlated plasma potential (Vp) fluctuations are present. In the presence of coupled Te and Vp fluctuations a slowly swept single probe may read either higher or lower than the average Te, depending on the relative amplitude and phase of the temperature and potential fluctuations. In contrast, swept double probe measurements of Te are virtually unaffected by either Te or Vp fluctuations.

Fluctuation measurements using a five-pin triple probe in the Joint European Torus boundary plasma

A multi-probe system has been developed to investigate the importance of electron temperature fluctuations turbulent transport in the Joint European Torus boundary plasma. The compact five-pin triple probe has been designed to reduce the phase delay of fluctuations due to the finite pin separation in the standard triple probe technique, while still avoiding the effects of probe shadowing. This probe has been found to have advantages over the standard triple probe in the region close to the separatrix (r−rsep<0.01 m), where the fluctuation wavelength is large. Importantly, results obtained with the probe support the standard estimation of cross-field particle flux, which ignores temperature fluctuations. They cannot be ignored, however, in the estimation of energy losses since measurements show that the conducted energy may be as large as the convected component.

Comparison of different methods of electron cyclotron emission-correlation radiometry for the measurement of temperature fluctuations in the plasma core

ECE correlation radiometry is a technique for measuring high frequency electron temperature fluctuations in the plasma core of fusion relevant devices. Two correlation methods, based on different physical principles, have been developed at W7-AS and TEXT-U. Depending on the required application, the two techniques can offer complementary advantages. At W7-AS a comparison of both techniques has been carried out. In all measurements both techniques yield identical results to within measurement error. The advantages of each method are discussed.

Chemical abundances of planetary nebulae from optical recombination lines - II. Abundances derived from collisionally excited lines and optical recombination lines

In Paper I, we presented spectrophotometric measurements of emission lines from the ultraviolet (UV) to the far-infrared for 12 Galactic planetary nebulae (PNe) and derived nebular thermal and density structures using a variety of plasma diagnostics. The measurements and plasma diagnostic results are used in the current paper to determine elemental abundances in these nebulae. Abundance analyses are carried out using both strong collisionally excited lines (CELs) and weak optical recombination lines (ORLs) from heavy element ions. Assuming electron temperatures and densities derived from H i recombination spectra (line and continuum), we are able to determine the ORL C abundance relative to hydrogen for all the PNe in our sample, N and O abundances for 11 of them and Ne abundances for nine of them. In all cases, ORL abundances are found to be systematically higher than the corresponding values deduced from CELs. In NGC 40, the discrepancy between the abundances derived from the two types of emission line reaches a factor of 17 for oxygen. For the other 10 PNe, the discrepancies for oxygen vary from 1.6 to 3.1. In general, collisionally excited infrared fine-structure lines, which have excitation energies less than 103 K and consequently emissivities that are insensitive to electron temperature and temperature fluctuations, yield ionic abundances comparable to those derived from optical/UV CELs. For a given nebula, the discrepancies between the ORL and CEL abundances are of similar magnitude for different elements. In other words, relative abundance ratios such as C/O, N/O and Ne/O deduced from the traditional method based on strong CELs are comparable to those yielded by ORLs, for a wide range of ORL to CEL oxygen abundance ratios, varying from near unity to over a factor of 20. We have also determined ORL abundances relative to hydrogen for the third-row element magnesium for 11 nebulae in our sample. In strong contrast to the cases for second-row elements, Mg abundances derived from the Mg ii 3d–4f λ4481 ORL are nearly constant for all the PNe analysed so far and agree within the uncertainties with the solar photospheric value. In accordance with results from previous studies, the ORL to CEL abundance ratio is correlated with the difference between the electron temperatures derived from the [O iii] forbidden-line ratio, on the one hand, and from the hydrogen recombination Balmer discontinuity, on the other. We find that the discrepancy between the ORL and CEL abundances is correlated with nebular absolute diameter, surface brightness, the electron density derived from [S ii] CELs, and excitation class. The results confirm that the dichotomy of temperatures and heavy elemental abundances determined from the two types of emission line, which has been widely observed in PNe, is a strong function of nebular evolution, as first pointed out by Garnett and Dinerstein. Our analyses show that temperature fluctuations and/or density inhomogeneities are incapable of explaining the large discrepancies between the heavy elemental abundances and electron temperatures determined from the two types of emission line. Our analyses support the bi-abundance model of Liu et al., who have proposed that PNe contain another previously unseen component of ionized gas which, highly enriched in heavy elements, has an electron temperature of ≲103 K and emits strongly in recombination lines but not in CELs. Our determinations of low average emission temperatures from the observed line intensity ratios of He i and O ii ORLs lend further support to this scenario.

Upper limit on turbulent electron temperature fluctuations in the core of Alcator C-Mod

Electron cyclotron emission correlation radiometry is used to measure the local turbulent electron temperature fluctuations in the plasma core of the Alcator C-Mod tokamak. Using standard analysis techniques, we see no evidence of the broadband fluctuations seen in other devices. From an analysis of the diagnostic resolution, coupled with these data, we can place an upper limit on the measurable fluctuation amplitude and set bounds for the wave number spectrum required for the electrostatic turbulence model of anomalous heat flux to be valid.

Effect of poloidal velocity shear on the edge fluctuations in the SINP tokamak

The fluctuations of potential, density and electron temperature have been measured in the edge of the SINP tokamak using Langmuir probes. A naturally occurring extended E × B velocity shear region has been found to exist near the limiter. The poloidal and radial correlation lengths decrease in this region, indicating a spatial decorrelation induced by the velocity shear. The phase angles of the fluctuating signals are also different in the shear region. The turbulent transport drops, causing a steepening of the density and temperature profiles, resembling the characteristics of the high confinement modes in tokamaks.

Stimulated scattering of intense radio waves in partially ionized space dusty plasmas

The nonlinear interaction between intense ordinary mode (O‐mode) radio waves and modified magnetoacoustic waves in partially ionized space dusty plasmas is considered, including the combined action of the radio wave pressure and the electron Joule heating caused by the O‐mode electric field. A set of equations containing the wave equation for the radio waves and the electron density and temperature fluctuations associated with the modified magnetoacoustic waves is derived. The set is then used to investigate decay and modulational instabilities of a constant amplitude O‐mode radio wave in the Earth's ionosphere and mesosphere.