On the dynamics of large-scale structures in electron temperature gradient turbulence

The electron temperature gradient mode has been proposed to be a source of experimentally relevant electron thermal transport, via a variety of non-linear phenomena such as the generation of streamers. The question of streamer stability and saturation is revisited, with the effects of geometry and perturbation stability highlighted. It is shown that the streamer saturation level is not determined by the balance of Kelvin–Helmholtz rate vs. linear growth rate, but by balancing the non-linear Kelvin–Helmholtz drive against damping mechanisms of the Kelvin–Helmholtz perturbation, suggesting a significantly lower streamer saturation level. In addition, random shear suppression of ETG turbulence by drift-ion temperature gradient (DITG) modes is studied, and it is found that streamers will be sensitive to shearing by short-wavelength DITG modes. An additional interaction mechanism, modulations of the electron temperature gradient induced by the DITG turbulence, is considered and shown to be quite significant. These considerations are used to motivate a discussion of the requirements for a credible theory of streamer transport.     

Spontaneous generation of self-organized solitary wave structures at earth's magnetopause

Spontaneous formation of solitary wave structures has been observed in Earth's magnetopause, and is shown to be caused by the breakup of a zonal flow by the action of drift wave turbulence. Here we show matched observations and modeling of coherent, large-scale solitary electrostatic structures, generated during the interaction of short-scale drift wave turbulence and zonal flows at the Earth's magnetopause. The observations were made by the Cluster spacecraft and the numerical modeling was performed using the wave-kinetic approach to drift wave-zonal flow interactions. Good agreement between observations and simulations has been found, thus explaining the emergence of the observed solitary structures as well as confirming earlier theoretical predictions of their existence.     

Change of zonal flow spectra in the JIPP T-IIU tokamak plasmas

When Ohmically heated low-density plasmas are additionally heated by higher-harmonics ion-cyclotron-range-of frequency heating, heated by neutral beam injection, or strongly gas puffed, the intensity of zonal flows in the geodesic acoustic mode frequency range in the tokamak core plasma decreases sharply and that of low-frequency zonal flow grows drastically. This is accompanied by a damping of the drift wave propagating in the electron diamagnetic drift direction, turbulence by trapped electron mode (TEM), and the increase of the mode propagating to ion diamagnetic drift direction (ITG). In the half-radius region, TEM and high-frequency zonal flows remain intense in both OH and heated phases. ITG and low-frequency zonal flows grow in heated plasmas, suggesting a strong coupling between ITG and low-frequency zonal flow.     

Drift wave turbulence in a dense semi-classical magnetoplasma

A semi-classical nonlinear collisional drift wave model for dense magnetized plasmas is developed and solved numerically. The effects of fluid electron density fluctuations associated with quantum statistical pressure and quantum Bohm force are included, and their influences on the collisional drift wave instability and the resulting fully developed nanoscale drift wave turbulence are discussed. It is found that the quantum effects increase the growth rate of the collisional drift wave instability, and introduce a finite de Broglie length screening on the drift wave turbulent density perturbations. The relevance to nanoscale turbulence in nonuniform dense magnetoplasmas is discussed.     

Generation of a sheared plasma rotation by emission, propagation, and absorption of drift wave packets

Collisional electron drift wave turbulence generates drift wave packet structures with density and vorticity fluctuations in the central plasma pressure gradient region of a linear plasma device. Tracking these packets reveals that they follow an outward directed spiral-shaped trajectory in the (r,θ) plane, are azimuthally stretched, and develop anisotropy as they approach an axisymmetric, radially sheared azimuthal flow located at the plasma boundary. Nonlinear energy transfer measurements and time-delay analysis confirm that structure absorption amplifies the sheared flow. Similar mechanisms likely operate at the edge of confined toroidal plasmas and should lead to the amplification of sheared flows at the boundary of these devices as well.     

Dynamics of fully nonlinear drift wave-zonal flow turbulence system in plasmas

We present numerical simulations of fully nonlinear drift wave-zonal flow (DW-ZF) turbulence systems in a nonuniform magnetoplasma. In our model, the drift wave (DW) dynamics is pseudo-three-dimensional (pseudo-3D) and accounts for self-interactions among finite amplitude DWs and their coupling to the two-dimensional (2D) large amplitude zonal flows (ZFs). The dynamics of the 2D ZFs in the presence of the Reynolds stress of the pseudo-3D DWs is governed by the driven Euler equation. Numerical simulations of the fully nonlinear coupled DW-ZF equations reveal that short scale DW turbulence leads to nonlinear saturated dipolar vortices, whereas the ZF sets in spontaneously and is dominated by a monopolar vortex structure. The ZFs are found to suppress the cross-field turbulent particle transport. The present results provide a better model for understanding the coexistence of short and large scale coherent structures, as well as associated subdued cross-field particle transport in magnetically confined fusion plasmas.     

Quasiparticle approach to the modulational instability of drift waves coupling to zonal flows

The interaction between broadband drift mode turbulence and zonal flows has been studied through the wave-kinetic approach. Simulations have been conducted in which a particle-in-cell representation is used for the quasiparticles, while a fluid model is employed for the plasma. The interactions have been studied in a plasma edge configuration which has applications in both tokamak physics and magnetopause boundary layer studies. Simulation results show the development of a zonal flow through the modulational instability of the drift wave distribution, as well as the existence of solitary zonal flow structures about an ion gyroradius wide, drifting towards steeper relative density gradients.     

有旋湍流场中湍流模型应用的研究

本文通过对三种双时间尺度湍流模型的理论分析，选择出最合理的模型，并在其基础上引入非线性Reynolds应力─应变关系式及梯度Richardson数修正，对有旋湍流场进行计算，其结果表明，双时间尺度模型能较好地预测出复杂湍流场中平均物理量的分布，尤其对回流区大小和强度的预测较常规k-ε模型结果有很大改进。因此，本文将双时间尺度模型应用于旋流燃烧器中气体颗粒两相流动的计算。     

Response of plasma turbulence against externally-controlled perturbations

A theory of the dynamic response of the turbulent plasma against the externally-controlled perturbations is reported. Based on Mori’s method [Prog. Theor. Phys. 33 423 (1965)], the nonlinear force is assumed to be separated into the memory function and the nonlinear fluctuating force. The former corresponds to the damping term, and the latter is categorized into the noise term. The response of the turbulent plasma against the externally-controlled source is formulated. The response kernel, which connects the externally-controlled source and the response of the turbulent field, is shown to have both the nonlocal property (in space) and the non-Markovian response (in time). A discussion is made on the nonlocal and non-Markovian response, including the case of disparate-scale interactions. A new method is proposed to observe experimentally the nonlocal interaction in the drift wave turbulence via the zonal flows.     

Recent work at Hiroshima on drift wave turbulence and associated transport

Recent developments in the theory of drift wave turbulence and associated transport phenomena are reviewed within the framework of the weak turbulence theory. Basic physical effects leading to anomalous cross-field transport of particles and energies are discussed in detail for the case of collisionless electrostatic drift waves in a uniform magnetic field. Effects of electro-magnetic perturbations (finite-β effect), magnetic shear and toroidal geometry are also discussed.     

Nonlinear interactions between drift waves and zonal flows

Phase coherent interactions between drift waves and zonal flows are considered. For this purpose, mode coupling equations are derived by using a two-fluid model and the guiding center drifts. The equations are then Fourier analyzed to deduce the nonlinear dispersion relations. The latter depict the excitation of zonal flows due to the ponderomotive forces of drift waves. The flute-like zonal flows with insignificant density fluctuations have faster growth rates than those which have a finite wavelength along the magnetic field direction. The relevance of our investigation to drift wave driven zonal flows in computer simulations and laboratory plasmas is discussed. Received 5 April 2002 Published online 28 June 2002     

Nonlinear triad interactions and the mechanism of spreading in drift-wave turbulence

We present the results of a derivation of the fluctuation energy transport matrix for the two-field Hasegawa-Wakatani model of drift wave turbulence. The energy transport matrix is derived from a two-scale direct interaction approximation assuming weak turbulence. We examine different classes of triad interactions and show that radially extended eddies, as occurs in penetrative convection, are the most effective in turbulence spreading. We show that in the near-adiabatic limit internal energy spreads faster than the kinetic energy. Previous theories of spreading results are discussed in the context of weak turbulence theory.     

Taming Drift Wave Turbulence

Drift waves in magnetized plasmas often occur in a turbulent form and are often considered as being responsible for anomalous cross‐field particle transport. It is thus very appealing to achieve active control of the drift wave dynamics. A control scheme acting both in space and in time is developed to synchronize drift wave turbulence. It consists of an arrangement of eight electrodes (octupole exciter) in flush‐mounted geometry in the edge region of the magnetized plasma column. The electrodes of the octupole exciter are driven by sinusoidal signals. Between each two neighbouring electrodes, the phase shift of the exciter signal is kept fixed. It is demonstrated experimentally that the exciter signals have strong influence on the different drift wave states, i.e. the turbulent states can be synchronized to a single preselected drift wave mode. The efficiency of the spatiotemporal synchronization is sensitively dependent on both driver frequency and phase shift.     

Electrostatic instability in magnetically confined inhomogeneous plasma driven by nonlinear force

A theoretical investigation on amplification of electrostatic ion acoustic wave in magnetically confined plasma has been presented in this paper. This investigation considers nonlinear wave–particle interaction process, called plasma maser effect, in presence of drift wave turbulence supported by magnetically confined inhomogeneous plasma. The role of associated nonlinear dissipative force in this effect in a confined plasma has been analyzed. The nonlinear force, which arises as a result of resonant interaction between electrons and modulated fields, is shown to drive the instability. Using the ion fluid equation and the ion equation of continuity, the nonlinear dispersion relation of a test ion acoustic wave has been derived, and the growth rate of ion acoustic wave in presence of low frequency drift wave turbulence has been estimated using Helimak data.     

Step propagation of a streamer in an electronegative gas

It is shown on the basis of numerical simulation and an experimental investigation that a streamer can propagate in a step manner in an electronegative gas. The experiments and most calculations were performed for air under close to normal conditions. The step motion is associated with the appearance of a secondary ionization wave near the electrode and propagation of this wave along the channel of the streamer; this wave maintains the channel in a conducting state and allows for the propagation of the streamer in a nonuniform external field over distances which are inaccessible under ordinary conditions of a streamer discharge. Simulation in heated air, oxygen, and SF₆ demonstrated that the phenomenon studied is common for various gases and that the special features of its manifestation remain in a wide range of decay rates of the streamer channel.     

On Modulational Interaction of the Lower-Hybrid Drift Oscillations

The general nonlinear equation of the third order in field strength for the lower-hybrid drift waves in inhomogeneous plasma is obtained on the basis of kinetic theory. This equation enables us to describe strong turbulence effects (modulational instability, soliton-like solutions, etc.) as well as weak turbulence effects (decays, scattering). The investigation of the modulational instability of the lower-hybrid drift waves is carried out. It is demonstrated that the development of the lower-hybrid drift wave modulational instability is possible only when the wavevector of the modulational perturbations is less or of the order of the wavevector of the pump wave. The condition on the wave vectors, when the nonlinear response defining the character of the modulational instability is determined by the inhomogeneity effects, is obtained.     

Modelling of the Effect of the Sheared Poloidal Flow on the Electrostatic Turbulence on the CASTOR Tokamak

Electrostatic drift turbulence of the edge plasma in the CASTOR tokamak is studied numerically by using the Hasegawa-Wakatani equations. The fluctuations of plasma density and potential as well as the corresponding fluctuation-induced particle flux are calculated for regimes with various plasma poloidal flows. Results of numerical simulations are in a reasonable agreement with experimental results.     

Inverse energy transfer by near-resonant interactions with a damped-wave spectrum

The interaction of long-wavelength anisotropic drift waves with the plasma turbulence of electron density advection is shown to produce the inverse energy transfer that condenses onto zonal modes, despite the expectation of forward transfer on the basis of nonconservation of enstrophy. Wave triads with an unstable wave and two waves of a separate, damped spectrum carry the transfer, provided they satisfy a near-resonance condition dependent on turbulence level and wave number.     

On applying the extended intrinsic mean spin tensor to modelling the turbulence in non-inertial frames of reference

Modelling the turbulent flows in non-inertial frames of reference has long been a challenging task. Recently we introduced the notion of the “extended intrinsic mean spin tensor” for turbulence modelling and pointed out that, when applying the Reynolds stress models developed in the inertial frame of reference to modelling the turbulence in a non-inertial frame of reference, the mean spin tensor should be replaced by the extended intrinsic mean spin tensor to correctly account for the rotation effects induced by the non-inertial frame of reference, to conform in physics with the Reynolds stress transport equation. To exemplify the approach, we conducted numerical simulations of the fully developed turbulent channel flow in a rotating frame of reference by employing four non-linear K-ε models. Our numerical results based on this approach at a wide range of Reynolds and Rossby numbers evince that, among the models tested, the non-linear K-ε model of Huang and Ma and the non-linear K-ε model of Craft, Launder and Suga can better capture the rotation effects and the resulting influence on the structures of turbulence, and therefore are satisfactorily applied to dealing with the turbulent flows of practical interest in engineering. The general approach worked out in this paper is also applied to the second-moment closure and the large-eddy simulation of turbulence.     

Thermistor controlled gunn oscillator at Ka-band

Frequency drift of Gunn oscillators is a major cause of concern in most of the Millimeter wave communication systems. This paper describes a simple and cost effective technique to arrest the frequency drift of a Ka band Gunn oscillator within 15 MHz for the operating temperature range of 0°C to 60°C as against a typical drift of about 50 to 100 MHz for free running Gunn oscillator for the same temperature range. At the ambient, the oscillator remains within ±1 MHz from switching on to stabilization. The temperature variation is sensed with a small thermistor bead placed close to the diode and a correction voltage is applied to the bias to compensate for the frequency drift. This compensation circuit also takes into account the non-linear behaviour of the thermistor and the Gunn oscillator with the temperature.