Correlation properties of black-body radiation in the context of the electron cyclotron emission of a magnetized plasma

Summary The quest for the cause of the anomalously large heat transport in magnetically confined thermonuclear plasmas has motivated detailed studies of fluctuations of various plasma parameters and, among others, of the electron temperature fluctuations. For an optically thick plasma the fluctuations of the electron cyclotron emission, ECE, are proportional to the electron temperature fluctuations up to a critical frequency where this ceases to be true. For the ultimate purpose of recovering, by correlation analysis, the actual plasma temperature fluctuations from the fluctuations of the measured radiation at frequencies above the critical, the mutual correlation properties of two ECE beams are analyzed and a criterion is obtained to decide when the thermal radiation fluctuations are uncorrelated and hence ignorable. The author of this paper has agreed to not receive the proofs for correction.     

Observation of the coherence between the plasma density fluctuations in the core and at the edge of the plasma column in the L-2M stellarator

Results are presented from the measurements of the radial coherence between the plasma density fluctuations in the core and at the edge of the plasma column in the L-2M stellarator. It is found that the radial coherence between these fluctuations is fairly high: the coherence coefficient attains a value of higher than 0.5 for the spectral wavelet components with frequencies lying in the range from several kilohertz to 30 kHz. The statistical characteristics of density fluctuations are also studied. It is shown that the power distribution function of the plasma density fluctuations in the plasma core differs from Gaussian.     

Adiabatic electron thermal pressure fluctuations in tokamak plasmas

Electron thermal pressure fluctuations measured in the edge plasma of the Texas Experimental Tokamak Upgrade are a fundamental component of plasma turbulence on both sides of the velocity shear layer. The ratio of specific heats, estimated from fluctuations in electron temperature and electron number density measured simultaneously at the same electrode, indicates that observed fluctuations are adiabatic. The observations are made by means of a novel Langmuir probe technique, the time domain triple-probe method, which concurrently measures multiple plasma properties at each of two electrodes with the temporal and the spatial resolution required to estimate thermodynamic properties in a turbulent plasma.     

Measurement of density fluctuations on the JIPP T-IIU tokamak via an HCN laser scattering

Observation of density fluctuations in tokamak plasmas is important to study the plasma confinement and to perform high power heating of the plasma. We observed the density fluctuations by means of an HCN laser scattering method during rf heating in the ion-cyclotron range of frequency on a tokamak plasma.The density fluctuations at the drift wave frequency are not enhanced so much by the heating, but the frequency spectrum is shifted to higher frequency. The increase of the density fluctuation level during the heating has been observed only in low-frequency region owing to MHD activity.     

Procedure for phase discrimination applicable to homodyne reflectometers for plasma density measurements

A phase discrimination method for homodyne reflectometers applied to plasma density fluctuations is proposed and tested in the laboratory simulating the plasma fluctuations by a vibrating metallic mirror.Applicability conditions for plasma measurements are established. The method can be used in broadband operation. Application to density profile measurements is also possible.     

Arc root dynamics in high power plasma torches — Evidence of chaotic behavior

Although plasma torches have been commercially available for about 50 years, areas such as plasma gun design, process efficiency, reproducibility, plasma stability, torch lives etc. have remained mostly unattended. Recent torch developments have been focusing on the basic understanding of the plasma column and its dynamics inside the plasma torch, the interaction of plasma jet and the powders, the interaction of the plasma jet with surroundings and the impingement of the jet on the substrate. Two of the major causes of erratic and poor performance of a variety of thermal plasma processes are currently identified as the fluctuations arising out of the arc root movement on the electrodes inside the plasma torch and the fluid dynamic instabilities arising out of entrainment of the air into the plasma jet. This paper reviews the current state of understanding of these fluctuations as well as the dynamics of arc root movement in plasma torches. The work done at the author’s laboratory on studying the fluctuations in arc voltage, arc current, acoustic emissions and optical emissions are also presented. These fluctuations are observed to be chaotic and interrelated. Real time monitoring and controlling the arc instabilities through chaos characterization parameters can greatly contribute to the understanding of electrode erosion as well as improvement of plasma torch lifetime.     

Velocity Fluctuations and Transport in the JET Boundary Region

A new approach for turbulent fluxes and _E×B measurements in the bulk plasma is proposed. It is based in the measurement of fluctuations in the phase velocity of fluctuations. The structure of turbulence has been investigated in the JET plasma boundary region with a fast reciprocating Langmuir probe system. Fluctuations in the radial and poloidal phase velocity have been computed from floating potential and ion saturation current measurements. The correlation between density fluctuations and fluctuations in the radial velocity of fluctuations signals show a good agreement with the turbulent transport computed from the correlation between density and poloidal electric field fluctuations. These results suggest that turbulent transport might be computed in the plasma core from measurement of density fluctuations. E×B sheared flows, both constant and varying in time, are close to the critical value to trigger the transition to improve confinement regimes below the power threshold to trigger the formation of transport barriers.     

Observation of long-distance radial correlation in toroidal plasma turbulence

This Letter presents the discovery of macroscale electron temperature fluctuations with a long radial correlation length comparable to the plasma minor radius in a toroidal plasma. Their spatiotemporal structure is characterized by a low frequency of ～1-3 kHz, ballistic radial propagation, a poloidal or toroidal mode number of m/n=1/1 (or 2/1), and an amplitude of ～2% at maximum. Nonlinear coupling between the long-range fluctuations and the microscopic fluctuations is identified. A change of the amplitude of the long-range fluctuation is transmitted across the plasma radius at the velocity which is of the order of the drift velocity.     

Electrostatic fluctuations in nonequilibrium plasmas with particle collisions

We present a method for calculation of plasma fluctuations in a nonequilibrium plasma with stable particle distribution functions. The method takes into account particle collisions and collective particle interactions which affect the collision process. In contrast to the known approaches, our method makes no reference to a specific form of the collision integrals and distribution functions. Using the developed method, we calculate the high-frequency spectrum of the electric field fluctuations in a collisional plasma with arbitrary particle distribution functions.     

Observation of the effects of dust particles on plasma fluctuation spectra

Charged dust particles are theoretically expected to modify the amplitude and spectrum of plasma fluctuations, and this can eventually provide novel diagnostic tools. Direct experimental evidence of the effects of dust particles on the fluctuations of a low collisionality plasma is reported, in agreement with the expectations of kinetic theory.     

Investigation on the Hard X-ray Radiations of the IR-T1 Tokamak Plasma: Electric and Magnetic Perspectives

In this experiment, the effect of magnetohydrodynamic (MHD) fluctuations in the hard X-ray radiation from the IR-T1 tokamak plasma is investigated. To reach this goal, the main parameters of plasma such as plasma current and loop voltage are measured. Also, the rake and poloidal Langmuir probes are used to calculate the radial and poloidal electric fields. To detect the hard X-ray radiation, a NaI-scintillator detector is used. To study on the MHD fluctuations, an array of 12 Mirnov coils is used. The obtained data are analyzed by using the singular value decomposition (SVD) algorithm. The wavelet spectrum of the dominant principal components of Mirnov coils is drawn. The results of wavelet and SVD analysis show that the hard X-ray radiation is increased with increasing the fluctuations of the dominant principal components (at the same time). It is also shown that the rate of hard X-ray radiation emitted from the tokamak plasma increased with increasing the MHD fluctuations. The energy of the system is wasted and reduced by these radiations. This an increase in the particle pressure of the plasma.     

Collective Scattering of Electromagnetic Waves from a Relativistic Magnetized Plasma

Recently, laser and microwave scatterings have become one of the important diagnostic means for plasma. Laser and microwave correlative scattering spectrum is determined by particle-density fluctuations in a weak turbulent plasma. In a relativistic plasma, on the basis of complete electromagnetic interaction between particles, a general expression for particle density fluctuations and spectra of laser and microwave scattering from a magnetized plasma are derived. The laser and microwave scattering spectra provide information on electron density and temperature, ion temperature, resonance and nonresonance effects.     

Magnetic properties of a nearly classical one-component plasma in three or two dimensions: II. Strong field

The equilibrium statistical mechanics of a nearly classical one-component plasma, submitted to a strong magnetic field, is studied, in three or two dimensions, by a suitable expansion of the Wigner distribution function. A strong magnetic field quenches the quantum fluctuations transverse to the field. The situation is especially simple for a two-dimensional plasma, which has a classical behaviour in the strong-field limit; as a consequence, a classical Wigner crystallization can be induced by the magnetic field.     

Recurrence quantification analysis of electrostatic fluctuations in fusion plasmas

We have investigated plasma turbulence at the edge of a tokamak plasma using data from electrostatic potential fluctuations measured in the Brazilian tokamak TCABR. Recurrence quantification analysis has been used to provide diagnostics of the deterministic content of the series. We have focused our analysis on the radial dependence of potential fluctuations and their characterization by recurrence-based diagnostics. Our main result is that the deterministic content of the experimental signals is most pronounced at the external part of the plasma column just before the plasma radius. Since the chaoticity of the signals follows the same trend, we have concluded that the electrostatic plasma turbulence at the tokamak plasma edge can be partially explained by means of a deterministic nonlinear system.     

Transport in the plasma edge of a tokamak with high MHD activity

Summary Electrostatic turbulence and various aspects of magnetic fluctuations have been investigated, in the edge region of the TBR-1 tokamak, by using a set of Langmuir and magnetic probes, and a triple probe. Measurements of plasma parameters such as plasma potential, density, temperature and magnetic field were taken in order to elucidate, the effect on transport of the electrostatic and magnetic fluctuations in the edge. The fluctuations levels are found to be higher than in most tokamaks. The particle flux is outward and slightly higher than that calculated from Bohm diffusion, and occurs in the frequency region typical of the macroscopic MHD oscillations. The Mirnov-oscillation frequencies in TBR-1 are higher than those observed in other tokamaks and, consequently, there is an uncommon superposition between the Mirnov and turbulent density fluctuations spectra. This fact and the presence of high MHD activity may contribute to elucidate the possible influence of the magnetic oscillations on the electrostatic transport observed in the plasma edge. Work partially supported by FAPESP and CNPq.     

Observation of Radial Propagation of Electrostatic Fluctuations in Edge Plasma of the Sino United Spherical Tokamak

Radial propagation of electrostatic fluctuations in the edge plasma of Sino-United Spherical Tokamak (SUNIST) has been measured using Langmuir probes. The propagation characteristics of the floating potential fluctuations are analysed by the two-point correlation technique. The results show radially outward propagation of the turbulent fluctuations at all measured radial positions. The power-average wavenumber profile is approximately constant in plasma edge region and suddenly increases to the limiter. These results are in good agreement with the model predictions proposed by Mattor which suggests that the drift wave propagation may be a source of edge turbulence.     

Polarization fluctuations in the radiation from cosmic sources due to scattering in a random inhomogeneous magnetoactive plasma

We use Feynman integrals along the trajectories to obtain expressions for the fourth-order statistical moments of polarized radiation propagating through a random inhomogeneous plasma. We write down the correlation functions and dispersion of the fluctuations in the Stokes parameters for the case of small fluctuations in the wave filed. We analyze the correlation functions of the Stokes parameters as a function of the polarization of the radiation from the source and the characteristics of the inhomogeneous plasma. We show that for each of the normal waves, the amplitudes and the phases experience different amounts of decorrelation in a random inhomogeneous magnetoactive plasma. As a result, fluctuations occur in the circular polarization.Radio Astronomical Institute, Academy of Sciences of the Ukrainian SSR. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 34, No. 7, pp. 738–747, July, 1991     

Origin of laser-output noise of cataphoretic HeSe+ lasers

We investigated noise and fluctuations of the output power of cataphoretic HeSe⁺ lasers in positive column plasma. Direct coupling of laser output power noise and fluctuations of the local population inversion was found. An investigation of the positive column plasma showed moving striations being responsible for the gain fluctuations. Whereas the local plasma properties are dominated by high frequency striations in the 100 kHz range, integrated quantities such as laser gain per pass are most strongly influenced by low-frequency waves with a continuous noise spectrum below 200 kHz. External modulation of the discharge voltage or current at a frequency near thehf striations reduces laser noise and increases laser output power.     

Role of stochastic fluctuations in the charge on macroscopic particles in dusty plasmas

The currents which charge a macroscopic particle placed in a plasma consist of discrete charges; hence, the charge can undergo random fluctuations about its equilibrium value. These random fluctuations can be described by a simple model which, if the mechanisms for charging of macroscopic particles are known, makes it possible to determine the dependence of the temporal and amplitude characteristics of the fluctuations on the plasma parameters. This model can be used to study the effect of charge fluctuations on the dynamics of the macroscopic particles. The case of so-called plasma-dust crystals (i.e., highly ordered structures which develop because of strong interactions among macroscopic particles) in laboratory gaseous discharge plasmas is considered as an example. The molecular dynamics method shows that, under certain conditions, random fluctuations in the charge can effectively heat a system of macroscopic particles, thereby impeding the ordering process. Zh. éksp. Teor. Fiz. 115, 2067–2079 (June 1999)     

HL-2A��Ե�������估�����˵�ʵ���о�

在一次稳定放电过程中利用快速扫描气动探针系统得到电子温度、密度、等离子体电位、极向电场以及它们的涨落、粒子通量、能量通量在径向的分布。研究了HL-2A装置边缘的静电涨落及其输运。实验结果指出,在等离子体边缘涨落量不服从玻耳兹曼关系,存在由于等离子体空间电位的梯度所产生的径向电场剪切层。分析了涨落对边缘约束的影响。