Effect of the radial electric field on lower hybrid plasma heating in the FT-2 tokamak

Conditions for efficient ion heating in the interaction of lower hybrid waves with plasma are experimentally determined. Experiments show that efficient lower hybrid heating stimulates a transition to the improved confinement mode. The formation of internal and external transport barriers is associated with strong central ion heating, which results in a change of the radial electric field E _r and an increase in the shear of the poloidal plasma velocity. The improved confinement mode in the central region of the discharge is attained under the combined action of lower hybrid heating and an additional rapid increase in the plasma current. A new mechanism for the generation of an additional field E _r is proposed to explain the formation of a transport barrier.     

Parameters of turbulent structures at the periphery of the FT-2 tokamak

Results are presented from probe measurements carried out in the scrape-off layer of the FT-2 tokamak in the course of additional lower hybrid heating, during which an L-H transition was observed. The objective of this study was to obtain information on the parameters of blobs-turbulent structures with enhanced plasma density. The measurements were performed not only on the low-field side of the torus, but also on the high-field side, which is still poorly studied. Coherent structures with radial velocities directed both toward the vessel wall and into the plasma column were revealed at the tokamak periphery. Blobs propagating toward the vessel wall were found to prevail both before and after the L-H transition. The average radial velocity of blobs in the L- and H-modes was determined experimentally. The dependence of the radial blob velocity on the transverse size and density of the structure agrees with the ballooning mode model. It is found that the average value of the poloidal blob velocity is four to five times higher than the average radial velocity. The results of measurements carried out on both sides of the torus indicate the presence of internal poloidal polarization of blobs. The average drift velocity of such polarized structures is directed toward the vessel wall. The L-H transition is accompanied by a reduction in the radial velocity. At the same time, the average plasma density inside the structures observed on the low-field side increases appreciably during the transition. The obtained dependences of the radial blob velocity on the plasma density inside the structure generally agree with predictions of the ballooning mode model.     

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.     

Analysis of the efficiency of lower hybrid current drive in the FT-2 tokamak

Results are presented from experimental studies of the efficiency of lower hybrid current drive (LHCD) in the FT-2 tokamak. The dependence of the LHCD efficiency on the grill phasing Δφ and RF oscillator power was determined experimentally in a wide range of plasma densities. It is shown that, at high plasma currents (i.e., at sufficiently high electron temperatures), current drive is suppressed when the plasma density reaches its resonance value n _LH for the pumping wave frequency, rather than when parametric decay comes into play (as was observed in regimes with lower plasma currents and, accordingly, lower electron temperatures T _e ). In order to analyze the experimentally observed effect of LHCD and its dependence on the value and sign of the antenna phasing, the spectra of the excited LH waves, P(N _z ), were calculated. Simulations using the FRTC code with allowance for the P(N _z ) spectrum and the measured plasma parameters made it possible to calculate the value and direction of the LH-driven current, which are determined by the spectrum of the excited LH waves. It is shown that the synergetic effect caused by the interaction between different spectral components of the excited RF wave plays a decisive role in the bridging of the gap in the wave spectrum.     

Peripheral fluctuations and particle transport during an L-H transition in the FT-2 tokamak

Experimental data on the processes in edge plasma that accompany the transition to an improved confinement regime during lower hybrid heating in the FT-2 tokamak are presented. The poloidal and radial distributions of the plasma parameters and drift particle fluxes were measured with the use of mobile mulitielectrode Langmuir probes and were found to be substantially nonuniform in the poloidal direction. The evolution of the plasma parameters in the course of heating and during an L-H transition is investigated. It is shown that, in FT-2 experiments, the drift of plasma particles in a slowly varying (quasi-steady) electric field and the fluctuation-induced particle fluxes make comparable contributions to the radial particle transport, whereas the contribution of fluctuations to poloidal plasma fluxes is negligibly small. The effective coefficient of radial diffusion is determined. The measurement results show that the L-H transition is accompanied by a substantial decrease in this coefficient.     

A study of low-frequency microturbulence by CO2-laser collective scattering in the FT-2 tokamak

Results are presented from studies of small-scale plasma density fluctuations in the FT-2 tokamak by the method of far-forward CO₂-laser collective scattering. The frequency and wavenumber spectra of fluctuations are measured using parallel k analysis at various positions of the scattering volume in the plane of the minor cross section of the torus. The data obtained are interpreted using numerical simulations. In phenomenological models, plasma fluctuations are substituted by a superposition of two-dimensional noninteracting cells with Gaussian profiles. A comparison of the calculated and experimental spectra shows that plasma fluctuations should be described based on the concept of strong microturbulence. The poloidal rotation velocity and the characteristic scale length of the scattering fluctuations, as well as the radial position of the region where they are located, are determined. The diffusion coefficient of the cells introduced in the model turns out to be close to the thermal diffusivity determined from the electron energy balance in the ohmic phase of the discharge.     

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.     

Investigation of statistical properties of peripheral fluctuations during an L-H transition in the FT-2 tokamak

The statistical properties of fluctuations of the plasma density and radial drift particle flux in the peripheral region of the FT-2 tokamak are analyzed using data from probe measurements. It is found that the probability distribution functions of the quantities under study vary over the radius and poloidal angle and change significantly after a transition to an improved confinement mode during auxiliary lower hybrid heating. Using experimental data and existing theoretical models, an analytic expression for the probability distribution function of the plasma density fluctuations is derived in a strongly nonlinear approximation. The expression is shown to agree well with experimental observations.     

Use of additional helium puffing for the diagnostics of plasma parameters at the FT-2 tokamak

The experiments carried out at the FT-2 tokamak in which additional pulsed puffing of helium into the hydrogen plasma was used for diagnostic purposes are considered. To estimate the necessary content of helium ions in the experiments on studying short-scale plasma oscillations, the ionization-recombination balance was simulated numerically under the assumption of a toroidally homogeneous influx of the working gas onto the boundary of the plasma column. In these simulations, the effective density of the neutral gas incident on the plasma boundary was determined by the iteration method, which made it possible to provide agreement between the obtained solution and the experimental discharge conditions. In particular, the correspondence of the determined admixture content to both the plasma quasineutrality condition and the value of the effective charge Z _eff, as well as agreement between the calculated and measured plasma density profiles, was ensured. The simulations were performed under the assumption of anomalous diffusion coefficients for all plasma components. The temporal variations of the ionization-recombination balance were checked by comparing them with the measured spectra of radiation in the HeI, HeII, and H_?? lines. In the current drive experiments, variations in n _e (r) at the discharge periphery were examined by the method based on the proportionality of the intensity ratio of the helium spectral lines, HeI(668 nm)/HeI(728 nm), to the plasma density. In these calculations, the factors relating the intensity ratio of these lines to the plasma density were taken from the literature on spectral diagnostics.     

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.     

Transition into the improved core confinement mode as a possible mechanism for additional electron heating observed in the lower hybrid current drive experiments at the FT-2 tokamak

In experiments on lower hybrid current drive (LHCD) carried out at the FT-2 tokamak, a substantial increase in the central electron temperature T _e (r = 0 cm) from 550 to 700 eV was observed. A complex simulation procedure is used to explain a fairly high LHCD efficiency and the observed additional heating, which can be attributed to a transition into the improved core confinement (ICC) mode. For numerical simulations, data obtained in experiments with deuterium plasma at 〈n _e 〉 = 1.6 × 10¹⁹ m^–3 were used. Simulations by the GRILL3D, FRTC, and ASTRA codes have shown that the increase in the density and central temperature is apparently caused by a significant suppression of heat transport in the electron component. The mechanism for transition into the improved confinement mode at r < 3 cm can be associated with the broadening of the plasma current channel due to the lower hybrid drive of the current carried by superthermal and runaway electrons. In this case, the magnetic shear s = (r/q)(dq/dr) in the axial region of the plasma column almost vanishes during the RF pulse. In this study, the effect of lower hybrid waves on the plasma parameters, resulting in a transition into the ICC mode, is considered. New experimental and calculated data are presented that evidence in favor of such a transition. Special attention is paid to the existence of a threshold for the transition into the ICC mode in deuterium plasma.     

Dynamics of the electron thermal diffusivity at improved energy confinement during lower hybrid plasma heating in the FT-2 tokamak

The dynamics of electron heat transport at improved energy confinement during lower hybrid plasma heating in the FT-2 tokamak was studied experimentally. Evolution of the profiles of the electron temperature and density was thoroughly investigated under conditions of fast variation in the plasma parameters. The energy balance in the electron channel is calculated with the help of the ASTRA code by using the measured plasma parameters. Correlation is revealed between the dynamics of electron heat transport and the behavior of small-scale drift turbulence measured using the enhanced scattering correlation diagnostics. The suppression of heat transfer and turbulence agrees well with the increase in the shear of poloidal plasma rotation calculated from experimental data in the neoclassical approximation.     

Effect of viscosity on the plasma decay in a magnetic field

The effect of viscosity on the evolution of an axisymmetric plasma column in a longitudinal magnetic field is considered. It is found that, under the action of viscosity, the plasma density profile tends to become Gaussian.     

Strong Isotope Effect in Energy Confinement at the FT-2 Tokamak

Plasma Physics Reports - The effect of the isotope content of plasma (hydrogen/deuterium) on energy confinement time was studied at the FT-2 tokamak. A strong isotope effect is discovered, which is...     

Time evolution of the exponential wavenumber spectra of turbulence upon helium injection into a hydrogen discharge at the FT-2 tokamak

The effect of variations in the key parameter of short-wavelength turbulence—the ion-acoustic Larmor radius ρ _s , which determines the position of the maximum of the drift instability growth rate over poloidal wavenumbers—was studied experimentally at the FT-2 tokamak. For this purpose, helium was injected to hydrogen plasma, which resulted in a change in the electron temperature at the plasma edge. The universality of the exponential shape of the turbulence spectra over radial wavenumbers q and a substantial excess of the characteristic turbulence scale L over the ion-acoustic Larmor radius was confirmed with the help of correlative diagnostics of enhanced scattering. This excess at the discharge periphery reaches a value of 3–5 at a low electron temperature, apparently, due to an increase in the dissipation of drift waves upon their cascade transfer toward short scale-lengths.     

Drift and geodesic effects on the ion sound eigenmode in tokamak plasmas

A kinetic treatment of geodesic acoustic modes (GAMs), taking into account ion parallel dynamics, drift and the second poloidal harmonic effects is presented. It is shown that first and second harmonics of the ion sound modes, which have respectively positive and negative radial dispersion, can be coupled due to the geodesic and drift effects. This coupling results in the drift geodesic ion sound eigenmode with a frequency below the standard GAM continuum frequency. Such eigenmode may be able to explain the split modes observed in some experiments.     

Study of the linear conversion of lower hybrid waves in the FT-1 tokamak by the enhanced time-of-flight scattering technique

The propagation of lower hybrid (LH) waves in a tokamak plasma in the presence of an LH resonance surface is studied experimentally with the use of a specially elaborated technique based on the backscattering of the probing microwave radiation in the upper hybrid resonance region. The technique provides resolution in the wave vectors of the scattering density fluctuations. The conditions are determined under which the LH wave propagates in accordance with the predictions of linear theory and is converted into the short-wave-length ion Bernstein mode. The parameter range is found in which the predictions of linear theory fail to hold and the nonlinear effects come into play during LH wave conversion. The radial wavelengths of the LH and ion Bernstein waves are determined.     

Electrostatic modeling of ion pores. Energy barriers and electric field profiles

This paper presents calculations of the image potential for an ion in an aqueous pore through lipid membrane and the electric field produced in such a pore when a transmembrane potential is applied. The method used is one introduced by Levitt (1978, Biophys. J. 22:209), who solved an equivalent problem, in which a surface charge density is placed at the dielectric boundary. It is shown that there are singularities in this surface charge density if the model system has sharp corners. Numerically accurate calculations require exact treatment of these singularities. The major result of this paper is the development of a projection method that explicitly accounts for this behavior. It is shown how this technique can be used to compute, both reliably and efficiently, the electrical potential within a model pore in response to any electrical source. As the length of a channel with fixed radius is increased, the peak in the image potential approaches that of an infinitely long channel more rapidly than previously believed. When a transmembrane potential is applied the electric field within a pore is constant over most of its length. Unless the channel is much longer than its radius, the field extends well into the aqueous domain. For sufficiently dissimilar dielectrics the calculated values for the peak in the image potential and for the field well within the pore can be summarized by simple empirical expressions that are accurate to within 5%.