Coherent Drift Wave Research Articles

High sensitive vectorial $\dot{B}$Ḃ-probe for low frequency plasma waves

A miniaturized multidimensional magnetic probe is developed for application in a low-temperature plasma environment. A very high sensitivity for low-frequency magnetic field fluctuations with constant phase run, a very good signal-to-noise ratio combined with an efficient electrostatic pickup rejection, renders the probe superior compared with any commercial solution. A two-step calibration allows for absolute measurement of amplitude and direction of magnetic field fluctuations. The excellent probe performance is demonstrated by measurements of the parallel current pattern of coherent electrostatic drift wave modes in the VINETA (versatile instrument for studies on nonlinearity, electromagnetism, turbulence, and applications) experiment.

Effect of parallel currents on drift-interchange turbulence: Comparison of simulation and experiment

Two-dimensional (2D) turbulence simulations are reported in which the balancing of the parallel and perpendicular currents is modified by changing the axial boundary condition (BC) to vary the sheath conductivity. The simulations are carried out using the 2D scrape-off-layer turbulence (SOLT) code. The results are compared with recent experiments on the controlled shear de-correlation experiment (CSDX) in which the axial BC was modified by changing the composition of the end plate. Reasonable qualitative agreement is found between the simulations and the experiment. When an insulating axial BC is used, broadband turbulence is obtained and an inverse cascade occurs down to low frequencies and long spatial scales. Robust sheared flows are obtained. By contrast, employing a conducting BC at the plate resulted in coherent (drift wave) modes rather than broadband turbulence, with weaker inverse cascade, and smaller zonal flows. The dependence of the two instability mechanisms (rotationally driven interchange mode and drift waves) on the axial BC is also discussed.

Laser induced fluorescence measurements of ion velocity and temperature of drift turbulence driven sheared plasma flow in a linear helicon plasma device

Using laser induced fluorescence (LIF), radial profiles of azimuthal ion fluid velocity and ion temperature are measured in the controlled shear de-correlation experiment (CSDX) linear helicon plasma device. Ion velocities and temperatures are derived from the measured Doppler broadened velocity distribution functions of argon ions. The LIF system employs a portable, high power (>300 mW), narrowband (∼1 MHz) tunable diode laser-based system operating at 668.614 nm. Previous studies in CSDX have shown the existence of a radially sheared azimuthal flow as measured with time delay estimation methods and Mach probes. Here, we report the first LIF measurements of sheared plasma fluid flow in CSDX. Above a critical magnetic field, the ion fluid flow profile evolves from radially uniform to peaked on axis with a distinct reversed flow region at the boundary, indicating the development of a sheared azimuthal flow. Simultaneously, the ion temperature also evolves from a radially uniform profile to a profile with a gradient. Measurements in turbulent and coherent drift wave mode dominated plasmas are compared.

Comparison of electrostatic and electromagnetic synchronization of drift waves and suppression of drift wave turbulence in a linear device

Experiments in a cylindrical magnetized plasma on the control of drift waves by means of two different spatiotemporal open-loop control systems—an electrostatic and an electromagnetic exciter—are reported. The drift wave dynamics is controlled by a mode-selective signal created with azimuthal arrangements of eight electrodes and eight saddle coils, respectively. Nonlinear interaction between the control signals and drift waves is observed, leading to synchronization of coherent drift waves and suppression of broadband drift wave turbulence. The cross-phase between density and potential fluctuations reduces from ≈π/2 in turbulence to ≈0 in controlled turbulence. Hence, the cross-field transport is reduced to the level of coherent drift waves. For both control systems the coupling to the drift wave can be ascribed to the drive of parallel currents, on the one hand via direct electric contact and, on the other hand, via electromagnetic induction.

Nonlinear interaction of drift waves with driven plasma currents

In a cylindrical magnetized plasma, coherent drift wave modes are synchronized by a mode selective drive of plasma currents. Nonlinear effects of the synchronization are investigated in detail. Frequency pulling is observed over a certain frequency range. The dependence of the width of this synchronization range on the amplitude of the driven plasma currents forms Arnold tongues. The transition between complete and incomplete synchronization is indicated by the onset of periodic pulling and phase slippage. Synchronization is observed for driven current amplitudes, which are some percent of the typical value of parallel currents generated by drift waves.

Formation, Spatiotemporal Dynamics, and Control of Structures in Weakly Developed Drift Wave Turbulence

Experimental investigations of the evolution of the dynamics of drift wave turbulence in a linear high-density helicon plasma device are presented. The turbulent density fluctuations in the plasma edge are characterized by large intermittent events caused by radially propagating turbulent structures, which are formed in the radial plasma density region due to increased cross-field transport by a quasi-coherent drift wave mode. Similar to coherent drift wave modes the turbulent structures are correlated with fluctuating parallel currents. The role of fluctuating currents parallel to the ambient magnetic field in the evolution of coherent drift wave modes and drift turbulence is highlighted by investigations of the interaction of drift wave fluctuations with externally driven currents. Frequency pulling of coherent drift wave modes over a frequency range of up to 30% of the natural drift wave frequency is demonstrated. Furthermore, the drive of mode-selective current patterns allow for complete synchronization of drift wave turbulence and consequently leads to a strong reduction of the associated fluctuation-induced transport. c

Laboratory studies of drift waves: nonlinear mode interaction and structure formation in turbulence

Coherent drift modes and drift wave turbulence are studied in a collisionality dominated high-density helicon plasma. The drift wave instability is identified by comparison of the drift mode density fluctuation characteristics with a linear dispersion relation, which reproduces the experimentally observed bent eigenmode structures due to the radial gradients of the collisionality profile. Attention is especially paid to the parallel currents associated with the drift mode density fluctuations, which are in agreement with the basic drift wave instability mechanism. A coupling of the drift mode to the externally excited Alfvén waves is found, which results in the frequency changes of the drift mode. The parallel currents associated with radially propagating density fluctuation structures in drift wave turbulence are similar to the drift mode case, which strongly suggests that polarization of the structures is a result of the parallel electron dynamics.

Recent Progress in the GAMMA 10 Tandem Mirror

Following the 2002 Conference on Open Magnetic Systems for Plasma Confinement, a three times progress in the formation of ion-confining potential height is achieved in the hot-ion mode. The advance in the potential formation leads to a finding of remarkable effects of radially produced shear of electric fields dEr/dr on the suppression of not only coherent drift waves but turbulence-like fluctuations for the first time in GAMMA 10. Also, the progress in the potential formation is made in line with the extension of our proposed physics scaling of potential formation covering over representative tandem-mirror operational modes, characterized in terms of (a) a high-potential mode having kV-order plasmaconfining potentials and (b) a hot-ion mode yielding fusion neutrons with 10-20 keV bulk-ion temperatures (Ti).

X-ray tomography systems for observations of the effects of radially sheared electric fields on fluctuations in plasmas

Experimental verification of the effects of radially sheared electric-field (or potential) formation in plasmas is one of the most critical issues to understand the physics basis for plasma confinement improvements. In the GAMMA 10 tandem mirror, recent experimental results show shear formation effects on the suppression of not only coherent drift waves but turbulence-like fluctuations without any coherent phasing relation during the ion-confining potential formation period. Contours of the central-cell soft x-ray brightness show spatially and temporally fluctuated structures during a weak sheared period by the use of the 50 channel microchannel plate system. A new x-ray tomography system is developed for analyzing temporally and spatially resolved plasma behavior in the presence or absence of these shear formation effects in GAMMA 10. The system consists of two 48-channel silicon semiconductor detector arrays with different viewing angles. X-ray energy responses of the new detector arrays along with response uniformity of detector channels have been characterized using synchrotron radiation at the Photon Factory.

Spatial mode structures of electrostatic drift waves in a collisional cylindrical helicon plasma

In a cylindrical helicon plasma, mode structures of coherent drift waves are studied in the poloidal plane, the plane perpendicular to the ambient magnetic field. The mode structures rotate with a constant angular velocity in the direction of the electron diamagnetic drift and show significant radial bending. The experimental observations are compared with numerical solutions of a linear nonlocal cylindrical model for drift waves [Ellis et al., Plasma Phys. 22, 113 (1980)]. In the numerical model, a transition to bended mode structures is found if the plasma collisionality is increased. This finding proves that the experimentally observed bended mode structures are the result of high electron collisionality.

Nonlinear paradigm for drift wave–zonal flow interplay: Coherence, chaos, and turbulence

Starting from the coherent four-wave drift wave–zonal flow modulation interaction model of Chen, Lin, and White [Phys. Plasmas 7, 3129 (2000)], nonlinear equations for drift wave–zonal flow interplay are systematically derived from first principles for nonuniform toroidal plasmas. The paradigm model, presented here, is based on a hierarchy in nonlinear wave–wave couplings, which assumes that different drift wave interactions on the shortest nonlinear time scale are mediated by zonal flows. The resulting coherent model demonstrates turbulence spreading to be the cause of transport scaling with system size. The nonlinear saturated state can be either coherent, with limit cycles, or chaotic, depending on the proximity to marginal stability.

Excitation of zonal flow by drift waves in toroidal plasmas

An analytical dispersion relation is derived which shows that, in toroidal plasmas, zonal flows can be spontaneously excited via modulations in the radial envelope of a single-n coherent drift wave, with n the toroidal mode number. Predicted instability features are verified by three-dimensional global gyrokinetic simulations of the ion-temperature-gradient mode. Nonlinear equations for mode amplitudes demonstrate saturation of the linearly unstable pump wave and nonlinear oscillations of the drift-wave intensity and zonal flows, with a parameter-dependent period doubling route to chaos.

On the roles of the density and temperature gradients in the theory of nonlinear coherent drift waves

It was recently claimed that in nonlinear coherent drift wave theory the temperature gradient kappa T=d ln T(x)/dx is irrelevant and that, instead of kappa T, the derivative d kappa n/dx of the density gradient kappa n=d ln n(x)/dx determines the nonlinearity. It is shown that this claim is erroneous in general and is caused by neglecting the space dependence of the phase velocity u(x) approximately=- kappa n(x)T(x) of the waves. The result is derived for quasi-one-dimensional soliton-like waves with e Phi /T<<1. Their duration is limited by the inhomogeneity of the refractive index.

Experimental investigation of coherent drift waves in low magnetic fields

Density gradient-driven coherent drift waves are studied in straight-cylindrical geometry at low magnetic fields, where the finite Larmor corrections of the ions gain increasing influence. For the prominent peaks in multimode spectra the dispersion properties and radial distribution of the density fluctuations are found in reasonable agreement with the nonlocal model of Ellis et al. [Phys. Fluids 22, 2137 (1979)], whereas the local slab model turns out to be inadequate. The azimuthal wavenumbers are determined by digital cross-correlation techniques with fixed probe pairs. The phase velocity of minority peaks in the spectra is found to be close to that of the prominent peaks.

Drift waves and magnetic field oscillations in cylindrical plasmas

A general investigation of linear drift-wave phenomena in cylindrically bounded plasmas, immersed in a magnetic field without shear and curvature, is performed within the two-fluid hydrodynamical approximation, taking into account electron-temperature oscillations and inhomogeneous radial distributions of the undisturbed electron density and temperature. For plasmas in which the electron temperature strongly exceeds the ion temperature the problem is reduced to an ordinary complex second-order differential equation describing the radial distribution of the oscillating electric potential. It is shown that the presence of electron-temperature oscillations (which must always exist in order to satisfy electron-energy conservation) and of radial gradients in the undisturbed electron temperature (which must always exist owing to cooling of the plasma at the boundary) leads to an important modification of the theory of drift waves in cylindrical plasmas (with regard to their stability and the radial distribution of the oscillating quantities) compared with previous papers in which these phenomena were disregarded. A numerical program for solving the corresponding complex-eigenvalue problem has been derived that allows a realistic calculation of all the quantities pertaining to drift-wave phenomena. It has been applied, in particular, to the calculation of the radial distribution of the oscillating coherent magnetic fields accompanying the coherent drift waves. The numerical results prove to be in good agreement with experiments performed with a helium plasma.