Parametric turbulence-sustained gas discharges

A brief review is given of papers on the RF production of a plasma whose electrons are heated due to the parametric turbulence driven by an alternating electric pump field and maintain the discharge by ionizing the working gas atoms. Results are summarized from studies of low-frequency parametric turbulence, specifically, ion-acoustic plasma turbulence in a magnetic field, ion-cyclotron turbulence associated with the excitation of ion Bernstein modes, and lower hybrid turbulence in a plasma with ions of one or two species. The turbulence level and the rate of turbulent heating of the electrons and ions are presented, and the results of modeling of these phenomena are described. Attention is focused on experiments in which low-frequency parametric turbulence may be observed.     

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.     

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.     

Decay instability of a lower hybrid wave

A study is made of the decay instability of a lower hybrid wave with a finite wave vector (k ₀≠0) and a large amplitude such that the oscillatory velocity of the electrons with respect to the ions cannot be neglected. It is shown that, depending on the angle between the propagation direction of the lower hybrid wave and the external magnetic field and the angle through which the wave is scattered, the decay instability is primarily governed either by the oscillatory electron motion with respect to the ions or by the nonlinear response of the plasma to the lower hybrid wave propagating in it. The role of the nonlinear frequency shift in the saturation of the lower hybrid decay instability is clarified.     

Threshold Energy Density of Lower Hybrid Waves in the Freja Experiment

Data from the Freja satellite experiment on the lower hybrid turbulence in the Earth’s magnetosphere are analyzed. It is shown that the observed threshold energy density of lower hybrid waves required for the excitation of localized wave packets is in good agreement with theoretical predictions.     

Vortex electron dynamics in a high-current plasma lens

An analytic study is made of the following problems: the instability of a plasma against the excitation of vortex turbulence, the turbulence saturation amplitude, the types and spatial structures of the nascent vortices, and their nonlinear growth rates in an electrostatic plasma lens for focusing high-current ion beams.     

Suppression of the current instability by decay processes

A study is made of the instability of lower hybrid waves in an isothermal magnetized current-carrying plasma. It is assumed that the dynamic suppression of the current instability is associated with the excitation of a quasi-monochromatic wave near the instability threshold and its subsequent decay into two strongly damped lower hybrid waves. It is shown that the decay process results in the onset of either a quasi-periodic or stochastic nonlinear stabilization regime involving a small number of modes. The anomalous resistance is estimated.     

Nonlinear properties of two-dimensional wake waves excited by relativistic electron bunches in plasma

The properties of a nonlinear plasma wake wave excited by an axially symmetric relativistic electron bunch are studied. It is shown that the nonlinear dependence of the wake wavelength on the transverse coordinate leads to distortion of the phase front of the wake wave and to steepening and oscillations of the transverse profile of the wakefield. The magnetic field of the wake wave is nonzero and oscillates at a frequency higher than the plasma electron frequency. Because of nonlinearity, the amplitude of the excited wake wave changes with distance from the bunch. The increase in nonlinearity leads to the development of turbulence and chaotization of the wakefield and results in the switching-on of the thermal effects and plasma heating.     

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.     

Energy transfer to a wire from a surrounding Joule-heated corona

This paper considers the early behavior of the current-carrying coronal plasma formed around the relatively colder liquid-vapor core of a wire. This has applications to both a wire array before global effects dominate and a single wire. An analytic, theoretical model is developed where the Joule heating in the coronal plasma is thermally conducted to the cold core. The balance of both energy and pressure are assumed, and it is further assumed that the Hall parameter is much less than one throughout the domain. This last assumption will be violated near the outside radius of the corona where runaway conditions and lower hybrid turbulence can also occur. The nonlinear second-order differential equation for the normalized temperature variation with a normalized radius has only one free dimensionless parameter, which is the ratio of the applied axial electric field to the mean radial temperature gradient (in electronvolts/m). The inverse of this ratio scales essentially as \({{T^2 } \mathord{\left/ {\vphantom {{T^2 } {\sqrt n }}} \right. \kern-\nulldelimiterspace} {\sqrt n }}\), thus showing that both a low Hall parameter and a low magnetic Reynolds' number can occur when the mean free path is less than the collisionless skin depth, a criterion for the onset of current or heat-flow driven electrothermal instabilities.     

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.     

Excitation of inertial Alfvén waves via modulational interaction with lower hybrid waves

The concept of modulational instability, which results from the coupling of waves modes of very different time and space scales, was introduced to plasma physics through an elegant paper by Vedenov and Rudakov in 1964 [1]. Our paper is devoted to the theory of modulational instability resulting from the interaction of lower hybrid waves and slow density perturbations associated with inertial Alfvén waves. The nonlinear set of equations describing the modulational coupling of these two types of waves is constructed. The lower hybrid wave trajectories are analyzed within predefined density structures and it is shown that these waves can be trapped in the vicinity of the density extremum. The density modulations, originally being associated with inertial Alfvén waves, deepen due to the trapping of lower hybrid waves; this leads to modulational instability. A dispersion relation describing the modulational instability is constructed and analyzed. The threshold intensity of the lower hybrid waves for the onset of instability is obtained and it is shown that instability can serve as an efficient mechanism for the excitation of inertial Alfvén waves in the auroral ionosphere.     

Nonlinear broadband doubling of the extraordinary wave frequency in inhomogeneous magnetoactive plasma

The nonlinear resonance doubling of radio wave frequencies in inhomogeneous plasma is studied as applied to the ionosphere under the conditions of the phase synchronism between an extraordinary pump wave and its second harmonic. The synchronism is not related to plasma resonances, but is determined by the magnetic field and plasma electron density in the transparency region. The generation efficiency of the second harmonic of a transversely propagating wave is calculated for a wide frequency band lying higher than the lower hybrid resonance frequency. It is shown that this effect is physically analogous to the generation of the second harmonic of laser radiation in a nonlinear crystal. The generation efficiency of the second harmonic is determined for inhomogeneous ionospheric plasma in which the synchronism condition is satisfied in a limited frequency range. It is shown that this effect can be used for remote nonlinear diagnostics of the upper ionospheric plasma, in which the characteristic size of the synchronism region can reach several kilometers. It is proposed to use a combination of satellite and ground-based ion probes in experiments on transionospheric probing. Even if the frequency of the wave emitted from the satellite is lower than the critical frequency in the ionosphere, the frequency of its second harmonic can exceed the critical frequency, so that it can be recorded by a ground-based ion probe or a specially designed receiver. The reflected second-harmonic signal can also be detected at the satellite by using a broadband radio-frequency spectrometer.     

Generalized scale invariance and log-Poisson statistics for turbulence in the scrape-off-layer plasma in the T-10 tokamak

Results are presented from experimental observations of the statistical properties of scrape-off-layer plasma turbulence in the T-10 tokamak. The experimentally observed fluctuations in the fluxes and plasma density are intermittent in nature and obey a non-Gaussian statistics. The generalized property of plasma turbulence is its scale invariance. The experimental scalings for the moments of the distribution function of the difference in the amplitudes of fluctuations in the fluxes and plasma density are described by the log-Poisson model of strong turbulence. The self-similarity properties of turbulence that are associated with the topology of dissipative structures are investigated.     

Heat transport from the region of local electron heating in a plasma

The initial stage of the one-dimensional expansion of a hot electron cloud into a “warm” plasma (i.e., into a plasma with a finite electron temperature) is studied with allowance for plasma turbulence. It is shown that, in a nonturbulent plasma or in a plasma with sufficiently weak turbulence, counterstreaming warm plasma flows interpenetrate one another; in this process, the plasma flows are accelerated and form beams escaping from the heating region. A stronger turbulence gives rise to electron heat waves that also propagate away from the heating region.     

Microwave enhanced-scattering correlation diagnostics of a turbulent plasma

A study is made of the influence of large-scale plasma turbulence on the results from a diagnostic method that is based on enhanced scattering of microwaves near the upper hybrid resonance and is highly sensitive to small-scale fluctuations. The resolution in radial wavenumbers that is provided by an enhanced-scattering correlation analysis of small-scale fluctuations with allowance for multiple small-angle scatterings of the probing and scattered waves along their paths is determined. The frequency spectrum of a wave that is backscattered by the small-scale fluctuations involved in large-scale turbulent motion and undergoes multiple smallangle scatterings is analyzed.     

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.     

Resistive drift-alfvén turbulence in a tokamak edge plasma

The behavior of turbulent fluxes in the vicinity of a resonant point m/n = q(x _res) in a plane edge plasma layer in a tokamak is studied by numerically analyzing the nonlinear MHD equations in a five-field electromagnetic model. Simulations show that the heat and electron turbulent fluxes decrease with increasing ion temperature at the plasma edge. It is shown that these fluxes are suppressed due to the stabilization mechanism associated with an increase in the shear of the E × B drift velocity, which in turn increases with increasing ion pressure gradient. The effect of the zonal magnetic field on turbulent transport is also investigated. It is shown that an increase in this field stabilizes edge plasma turbulence.     

Langmuir turbulence in ionospheric plasma

A kinetic theory is developed for strong Langmuir turbulence in the region of the reflection of a high-power ordinary radiowave in ionospheric plasma. The structure and quantity of the cavitons that form in the stage of well-developed turbulence are determined. The acceleration of electrons is investigated, and it is found that the electron distribution function acquires a significant tail with an effective temperature T_eff of 50 to 100 times the plasma temperature. The region occupied by fast electrons is hundreds of times thicker than the layer of Langmuir turbulence. The theoretical results are shown to correlate well with the observational data on the electron acceleration and plasma emission in ionospheric experiments.