Effect of the radial electric field on the results of measurements of the poloidal rotation of a tokamak plasma by charge exchange recombination spectroscopy

Abtract The effect of the radial electric field E _r on the results of measurements of the poloidal rotation of a tokamak plasma by charge exchange recombination spectroscopy is considered. It is shown that the emission line shift arising from the finite lifetime of the excited state of the ions is proportional to E _r. For helium ions, the maximum shift corresponds to the poloidal rotation velocity, which is about one-third of the drift velocity in the crossed radial electric (E _r) and toroidal magnetic (B _t) fields. __________ Translated from Fizika Plazmy, Vol. 27, No. 11, 2001, pp. 1050–1052. Original Russian ? 2001 by Romannikov, Chernobai.     

Radial electric field during dynamic processes in a tokamak and L-H transitions

Expressions for the radial electric field in tokamaks are derived with allowance for an additional contribution of the longitudinal electron viscosity (or the associated Ware drift). It is shown that, in transient processes during which the toroidal electric field at the plasma edge increases, the additional electric field can become rather strong. An increase in the shear of the poloidal plasma rotation can trigger the L-H transition. That the experimentally observed transitions to an improved confinement mode can be ascribed to this effect is illustrated by simulating discharges in the current ramp-up experiments in the Tuman-3M tokamak.     

Reversal of the hall current and amplification of the magnetorotational instability in a weakly ionized plasma

A criterion for the development of a magnetorotational instability in a weakly ionized dusty plasma is considered. A dispersion relation for the wavenumber and the growth rate of an unstable perturbation is derived for an arbitrary angle between the wave vector and magnetic field. It is shown that the presence of dust grains can reverse the direction of the Hall current in the plasma and can shift the instability threshold to shorter wavelengths. Under certain conditions, Alfvén fluctuations of arbitrary scale can be unstable. The Hall current reversal is found to have a strong effect on the development of a magnetorotational instability when the Alfvén resonance frequency in a weakly ionized plasma is close to the rotation frequency of the accretion disk.     

Radial electric field and rotation of the ensemble of plasma particles in tokamak

The velocity of macroscopic rotation of an ensemble of charged particles in a tokamak in the presence of an electric field has been calculated in a collisionless approximation. It is shown that the velocity of toroidal rotation does not reduce to a local velocity of electric drift and has opposite directions on the inner and outer sides of the torus. This result is supplemented by an analysis of the trajectories of motion of individual particles in the ensemble, which shows that the passing and trapped particles of the ensemble acquire in the electric field, on the average, different toroidal velocities. For the trapped particles, this velocity is equal to that of electric drift in the poloidal magnetic field, while the velocity of passing particles is significantly different. It is shown that, although the electric-field-induced shift of the boundaries between trapped and passing particles in the phase space depends on the particle mass and charge and is, in the general case, asymmetric, this does not lead to current generation.     

Experimental study of magnetized plasma rotation in crossed fields

Rotation of magnetized plasma between two coaxial electrodes in crossed electric and magnetic fields was studied experimentally. Three regimes of plasma rotation were observed. In the first regime, the radial electric field is created by a beam?plasma discharge due to the charging of the inner axial electrode by electrons, the outer electrode being grounded. Plasma rotation in this case is accompanied by strong high-frequency current oscillations detected by a Mach probe. When a negative voltage was applied to the coaxial electrodes, the second regime was observed, in which weakly perturbed quasi-stationary plasma rotation occurred at a relatively low radial current. The third regime of plasma rotation was observed upon a spontaneous disruption of the second regime. It is characterized by high currents of ~1 kA, sheared plasma rotation, and excitation of high-frequency perturbations.     

Effect of rotation on plasma stability in the gas-dynamic trap

The effects of the centrifugal force and finite Larmor radius on plasma stability in the gas-dynamic trap are considered. Estimates show that the stability is governed by the strong effect of the finite Larmor radius of fast particles. If this stabilizing mechanism does not operate, then the instability ceases to be exponential when the sheared plasma rotation becomes sufficiently intense. In this case, the potential perturbation amplitude increases according to a power law, and the instability threshold remains unchanged. These effects do not influence the stability of the first azimuthal perturbation mode in a plasma with a free boundary. But when the plasma is in good electrical contact with the conducting wall of the device and when the plasma density profile is not too peaked at the axis, the first mode is stabilized by the finite Larmor radius effect because of the radial variation of the perturbed electric field.     

Modeling of the parametric dependence of the edge toroidal rotation

Modeling of the edge toroidal rotation and its dependence on the edge plasma parameters is performed by means of the B2SOPS5.0 transport code for ohmic shots both for MAST and ASDEX Upgrade configurations. The impact of plasma current, toroidal magnetic field, plasma density, and temperature is investigated. The connection between the toroidal rotation and edge radial electric field is also studied. The results of the simulation are consistent with the parametric dependence predicted analytically. Published in Russian in Fizika Plazmy, 2008, Vol. 34, No. 9, pp. 791–797. The text was submitted by the authors in English.     

Poloidal asymmetry in perpendicular plasma rotation and radial electric field measured with correlation reflectometry at TEXTOR

Measurements of plasma rotation and electric field are crucial for the study of plasma confinement and transport. The present paper is devoted to experimental observations of poloidal asymmetry in perpendicular plasma rotation with correlation reflectometry on TEXTOR. Published in Russian in Fizika Plazmy, 2008, Vol. 34, No. 9, pp. 798–802. The text was submitted by the authors in English.     

An optimal policy for the metabolism of storage materials in unicellular algae.

The perturbation theory provides an explanation of the trans-membrane electric field effects on the polarizability of the photosynthetic pigments.In the photosynthetic membranes, the photo-induced effect is linear despite the spectroscopic properties of the carotenoids that should produce a quadratic effect. Consequently, the hypothesis previously assumed of a permanent field in the membrane is confirmed.Our calculation shows that the effect includes two terms, but only one is often taken into account in the literature.This one varies as the first derivative of the polarizability and corresponds to the induced shift of the electronic levels. The other one varies as the polarizability itself; this term is often omitted, and it corresponds to the variation of transition probability. Taking into account both these terms, the carotenoid shift can be interpreted without further hypothesis, as the one consisting in assuming that only one part of the carotenes is sensitive to the field.Finally, this approach allows a quantitative interpretation of the light-induced change measurements with polarized measuring light, which demonstrate the participation of the pigments orientation in the electric field effects.     

Excitation of magnetic fields by a circularly polarized laser pulse in a plasma channel

The excitation of quasistatic magnetic fields by a circularly polarized laser pulse in a plasma channel is considered. It is shown that, to second order in the amplitude of the electric field of the laser pulse, circular rotation of the plane of polarization of the laser radiation in a radially nonuniform plasma gives rise to a nonlinear azimuthal current and leads to the excitation of the radial and axial components of the magnetic field. The dependence of the magnetic field distribution over the plasma channel on the spatial dimensions of the pulse and on the channel width is investigated for a moderate-power laser pulse. The structure of the magnetic fields excited by a relativistic laser pulse in a wide plasma channel is analyzed.     

Plasma flows in crossed magnetic and electric fields

The effect of the magnitude and direction of an external electric field on the plasma flowing through a magnetic barrier is studied by numerically solving two-fluid MHD equations. The drift velocity of the plasma flow and the distribution of the flow electrons over transverse velocities are found to depend on the magnitude and direction of the electric field. It is shown that the direction of the induced longitudinal electric field is determined by the direction of the external field and that the electric current generated by the plasma flow significantly disturbs the barrier field.     

Efficient Broadband Transmissive 90° Polarization Rotation Using a Single Layer of L-shaped Particles Inside a Glass Cube

We realize 90° polarization rotation with both high polarization conversion efficiency and broad bandwidth by using a single layer of L-shaped particles inside a glass cube. The simulation results show that Fabry-Perot resonance effect enhances the transmission for both co-polarization and cross-polarization light in the L-shaped layer. And the co-polarization electric field component has been suppressed in the far field because of the destructive interference. The simulation results also show that the spectral band of peak polarization rotation can be shifted by changing the size parameters. This broadband polarization rotation mechanism may be very useful in designing polarization rotators.     

Protoplast rotation in a rotating electric field: The influence of cold acclimation

Summary An experimental and theoretical investigation has been made of the rotation of protoplasts ofSecale cereale L. (cv Puma) in a rotating electric field for the purpose of determining the electrical properties of the protoplast plasma membrane. The dependence of the protoplast rotation rate on: (1) the rotation rate of the applied electric field; (2) the electrical conductivity of the external medium; and (3) cold acclimation or lack thereof were determined. A theoretical analysis of the rotation rate of polarizable spherical cells in a rotating electric field leads to a qualitatively similar formula to that of Arnold and Zimmermann (Z. Naturforsch. 37:908–915, 1982), but it differs from this earlier work by a large numerical factor (180). Detailed comparisons of the observed protoplast rotation rates with the new theory show generally good agreement. The protoplast rotation measurements allow a noninvasive determination of the specific plasma membrane capacitance,c _m. The average value found in the present experiments isc _m=(0.56±0.08)×10^–2 F/m². Within the experimental errors, thec _m values are the same for cold-acclimated and noncold-acclimated protoplasts. Determination of plasma membrane resistance from protoplast rotation measurements does not appear feasible because of the high values of the specific resistance.     

Influence of the radial profile of the electric potential on the confinement of a high-β two-component plasma in a gas-dynamic trap

One of the most important problems to be studied in the gas-dynamic trap (GDT) facility is the investigation of MHD stability and cross-field transport in a plasma with a relatively high value of β = πp/B ². Recent experiments demonstrated that the radial electric field produced in the plasma by using radial limiters and coaxial end plasma collectors improves plasma stability in axisymmetric magnetic mirror systems without applying special MHD stabilizers. The experimental data presented in this work show that stable plasma confinement can be achieved by producing a radial potential drop across a narrow region near the plasma boundary. Creating radial electric fields of strength 15–40 V/cm causes a shear plasma flow, thereby substantially increasing the plasma confinement time. When all the radial electrodes were grounded, the confinement was unstable and the plasma confinement time was much shorter than the characteristic time of plasma outflow through the magnetic mirrors. Measurements of cross-field plasma fluxes with the use of a specially designed combined probe show that, in confinement modes with differential plasma rotation, transverse particle losses are negligibly small as compared to longitudinal ones and thus can be ignored. It is also shown that, when the GDT plasma is in electric contact with the radial limiters and end collectors, the growth rate of interchange instability decreases considerably; such a contact, however, does not ensure complete MHD stability when the electrodes are at the same potential.     

Influence of the pulsating electric field on the ECR heating in a nonuniform magnetic field

According to a computer simulation, the randomized pulsating electric field can strongly influence the ECR plasma heating in a nonuniform magnetic field. It has been found out that the electron energy spectrum is shifted to the high energy region. The obtained effect is intended to be used in the ECR sources for effective X-ray generation.     

Modeling the electric image produced by objects with complex impedance in weakly electric fish

Weakly electric fish generate an electric field around their body by electric organ discharge (EOD). By measuring the modulation of the electric field produced by an object in the field these fish are able to accurately locate an object. Theoretical and experimental studies have focused on the amplitude modulations of EODs produced by resistive objects. However, little is known about the phase modulations produced by objects with complex impedance. The fish must be able to detect changes in object impedance to discriminate between food and nonfood objects. To investigate the features of electric images produced by objects with complex impedance, we developed a model that can be used to map the electric field around the fish body. The present model allows us to calculate the spatial distribution of the amplitude and phase shift in an electric image. This is the first study to investigate the changes in amplitude and phase shift of electric images induced by objects with complex impedance in wave-type fish. Using the model, we show that the amplitude of the electric image exhibits a sigmoidal change as the capacitance and resistance of an object are increased. Similarly, the phase shift exhibits a significant change within the object capacitance range of 0.1–100 nF. We also show that the spatial distribution of the amplitude and phase shifts of the electric image resembles a “Mexican hat” in shape for varying object distances and sizes. The spatial distribution of the phase shift and the amplitude was dependent on the object distance and size. Changes in the skin capacitance were associated with a tradeoff relationship between the magnitude of the amplitude and phase shift of the electric image. The specific range of skin capacitance (1–100 nF) allows the receptor afferents to extract object features that are relevant to electrolocation. These results provide a useful basis for the study of the neural mechanisms by which weakly electric fish recognize object features such as distance, size, and impedance.     

Rotation of cells in an alternating electric field theory and experimental proof

Summary Protoplasts ofAvena sativa rotate in an alternating electric field provided that at least two cells are located close to each other. An optimum frequency range (20 to 30 kHz) exists where rotation of all cells exposed to the field is observed. Below and above this frequency range, rotation of some cells is only occasionally observed. The angular velocity of rotation depends on the square of the electric field strength. At field strengths above the value leading to electrical breakdown of the cell membrane, rotation is no longer observed due to deterioration of the cells. The absolute value of the angular velocity of rotation at a given field strength depends on the arrangement of the cells in the electric field. A maximum value is obtained if the angle between the field direction and the line connecting the two cells is 45^o. With increasing distance between the two cells the rotation speed decreases. Furthermore, if two cells of different radii are positioned close to each other the cell with the smaller radius will rotate with a higher speed than the larger one. Rotation of cells in an alternating electric field is described theoretically by interaction between induced dipoles is adjacent cells. The optimum frequency range for rotation is related to the relaxation of the polarization process in the cell. The quadratic dependence of the angular velocity of rotation on the field strength results from the fact that the torque is the product of the external field and the induced dipole moment which is itself proportional to the external field. The theoretical and experimental results may be relevant for cyclosis (rotational streaming of cytoplasm) in living cells.     

Perturbation of a collisional plasma by exterior bodies in a uniform external electric field

The charging of a metal sphere in a weakly ionized collisional plasma in a uniform external electric field is investigated with allowance for the effect of the space charge field and ionization-recombination processes. The sphere charge and the spatial distributions of charged plasma particles are calculated both numerically and analytically (for some particular cases) for the case of a strong external field. It is found that the size of the perturbed plasma region is determined by the external field and the intensity of recombination processes. It is shown that the total electric charge (the sphere charge plus the plasma space charge) is zero in accordance with predictions of the theory of static currents in a conducting medium.     

Effect of anomalous ion inertia and oblique ion viscosity on the radial electric field in FT-2 tokamak experiments

Results are presented from numerical simulations that show that, in a plasma with well-developed turbulence, the radial electric field can be positive in the region where the gradients of the plasma parameters are steep. In a plasma in which the turbulence is suppressed (as is the case with auxiliary lower hybrid heating), the radial electric field is found to exhibit a nearly neoclassical behavior during the formation of a transport barrier and transition to the H-mode.     

Demidov-Elagin-Fanchenko effect

It is shown how the plateau that has been revealed earlier in the nonlinear dependence of the experimentally studied electrical conductivity of a turbulent plasma on the electric field strength can be understood by taking into account the turbulent Joule heating of the plasma electrons. A new, experimentally possible physical pattern of the penetration of a quasistatic vortex electric field into a turbulent plasma is revealed that is attributed to the time dependence of the anomalous turbulent conductivity or, more generally, to the temporally nonlocal relationship between the current density and the electric field strength due to turbulent heating.