Showing papers by "Patrick Diamond published in 1992"
TL;DR: In this article, a simple model of long-wavelength drift wave turbulence in the presence of a sheared poloidal flow is analyzed in detail, and it is shown that velocity shear induces a strong stabilizing effect by shifting the eigenmode away from the k⋅B=0 resonant surface.
Abstract: A simple, paradigmatic model of long‐wavelength drift wave turbulence in the presence of a sheared poloidal flow is analyzed in detail. Linear theory predicts that velocity shear induces a strong stabilizing effect by shifting the eigenmode away from the k⋅B=0 resonant surface, thereby enhancing ion damping. However, multiple‐helicity numerical calculations indicate that velocity shear has little or no effect on saturated fluctuation levels. Analysis suggests that this result is related to the incidence of a spiky, radially intermittent profile of the turbulent fluctuation levels, induced by low‐q mode rational surfaces, and occurs when the turbulent diffusivity exceeds the product of diamagnetic frequency and gyroradius squared. An analytical theory that explains the observed suppression of velocity and magnetic shear damping is presented.
82 citations
TL;DR: In this article, the authors used numerical flow simulations to study the effect of coherent structures on transport in the context of a two-field, two-dimensional model of dissipative drift-wave turbulence.
Abstract: Numerical flow simulations are used to study the effect of coherent structures on transport in the context of a two‐field, two‐dimensional model of dissipative drift‐wave turbulence. The presence and nature of structures are found to depend on the adiabaticity parameter α=k∥2 VT2/2νeiωs which controls the degree to which the electrons respond to parallel electric fields. Transport estimates based on quasilinear and mixing‐length models are compared with the simulations. In the regime with long‐lived coherent structures, the turbulent particle transport predicted by a standard quasilinear or mean‐field estimate is found to exceed that actually observed in the presence of coherent structures.
53 citations
TL;DR: In this paper, the theory of ionization-driven drift wave turbulence is presented in the context of a quasilocal model and large fluctuation levels, in excess of mixing length expectations, are predicted.
Abstract: The theory of ionization‐driven drift wave turbulence is presented in the context of a quasilocal model. Linear analysis reveals that ionization effects can destabilize collisional drift waves and can possibly induce parallel shear flow instabilities, as well. Nonlinear analysis indicates that energy is transferred from large to small stable scales and converted to ion kinetic energy. Results indicate mode coupling effects are dominant. Large fluctuation levels, in excess of mixing length expectations, are predicted. The ionization source drives a purely inward particle flux, which can explain the anomalously rapid uptake of particles that occurs in response to gas puffing.
52 citations
TL;DR: In this article, the linear theory of the sonic ion-temperature gradient-driven mode in the presence of sheared poloidal rotation is discussed in the context of a hydrodynamic model and the growth rate increases for weak shear but then decreases when the shearing frequency exceeds the mode frequency.
Abstract: The linear theory of the sonic ion‐temperature‐gradient‐driven mode in the presence of sheared poloidal rotation is discussed in the context of a hydrodynamic model. Analytical and numerical calculations show that the growth rate increases for weak shear, but then decreases when the shearing frequency exceeds the mode frequency. This trend is a consequence of the coupling of radial eigenmodes induced by the asymmetric effective potential and the absorption and damping due to resonance between the wave frequency and shearing frequency. The former dominates at weak shear, resulting in destabilization, while the latter dominates for strong shear, resulting in stabilization. Mixing length estimates of the turbulent diffusivity are given, and a novel bifurcation scenario for the L→H transition is discussed.
50 citations
TL;DR: In this paper, a simple model for drift-thermal instability induced turbulence is derived and studied both analytically and numerically both nonlocal, nonlinear analytical calculations and three-dimensional computations are used.
Abstract: A simple model for drift‐thermal instability‐induced turbulence is derived and studied both analytically and numerically. Both nonlocal, nonlinear analytical calculations and three‐dimensional computations are used. Potential and temperature fluctuation levels and radial correlation lengths are calculated and compared to numerical results. The saturation mechanism and the role of a fluctuation‐generated shear flow are elucidated. The numerical calculations are used to obtain spectra and correlation lengths. A detailed comparison of analytical and numerical results is given.
34 citations
TL;DR: In this article, the authors considered the effect of magnetic shearing instability on the strength and organization of the magnetic field in a thin accretion disk and found that the magnetic energy density saturates at about (H/r) exp 4/3 times the local pressure (H is the disk thickness and r is its radius).
Abstract: We consider the consequences of magnetic buoyancy and the magnetic shearing instability (MSI) on the strength and organization of the magnetic field in a thin accretion disk. We discuss a model in which the wave-driven dynamo growth rate is balanced by the dissipative effects of the MSI. As in earlier work, the net helicity is due to small advective motions driven by nonlinear interactions between internal waves. Assuming a simple model of the internal wave spectrum generated from the primary m = 1 internal waves, we find that the magnetic energy density saturates at about (H/r) exp 4/3 times the local pressure (where H is the disk thickness and r is its radius). On very small scales the shearing instability will produce an isotropic fluctuating field. For a stationary disk this is equivalent to a dimensionless 'viscosity' of about (H/r) exp 4/3. The vertical and radial diffusion coefficients will be comparable to each other. Magnetic buoyancy will be largely suppressed by the turbulence due to the MSI. We present a rough estimate of its effects and find that it removes magnetic flux from the disk at a rate comparable to that caused by turbulent diffusion.
30 citations
TL;DR: In this paper, a perturbative analysis of toroidally localized and helically localized eigenfunctions in the ballooning space for nonaxisymmetric toroidal confinement devices was performed.
Abstract: Trapped electron modes could play an important role in enhancing losses in a toroidal confinement device. However, no direct evidence of these instabilities has been found in such devices. The dissipative trapped electron modes [Sov. Phys. Dokl. 14, 470 (1969)] in l=2 torsatrons using the high‐n ballooning formalism and considering the full three‐dimensional geometry are studied. Using a perturbative analysis, it is found that there are toroidally localized as well as helically localized eigenfunctions in the ballooning space for these nonaxisymmetric devices. The helically localized eigenfunctions give the largest growth rates. It is shown that the helical symmetric limit gives a very good description of the helically induced modes for stellarators such as the Advanced Toroidal Facility [Fusion Technol. 10, 179 (1986)]. In this limit, the criteria for the density gradient and the temperature gradient for the existence of the dissipative trapped electron modes in l=2 stellarators are found. Also, it is found that the radial width of the unstable modes can be very broad, and therefore their effects could possibly be detected in future experimental studies with density control.
27 citations
TL;DR: In this article, the properties of the spectral energy transfer for a two-dimensional fluid representation of dissipative trapped ion convective cell turbulence are studied numerically using a spectral method.
Abstract: The properties of the spectral energy transfer for a two‐dimensional fluid representation of dissipative trapped ion convective cell turbulence are studied numerically using a spectral method. It is established that the spectral energy flow is from long to short wavelength, as governed (under the dynamics of the E×B nonlinearity) by a single quadratic invariant, the energy. This flow is correctly predicted by equilibrium statistical mechanics, as is the equilibrium spectrum. Examining the locality of energy flow, strong nonlocal energy transfer is observed, a process that efficiently transfers the energy of a mode across the spectrum in a correlation time. This transfer process deviates dramatically from the canonical self‐similar cascade dynamics of Kolmogorov that typifies the cascade of two‐ and three‐dimensional Navier–Stokes and Hasegawa–Mima drift wave turbulence. Anisotropy of the spectral transfer dynamics is also observed.
23 citations
TL;DR: In this paper, the authors analyzed the performance of the KSAW current drive with helicity injection by tailoring the sign of poloidal and parallel wave numbers, and showed that the efficiency of the injection is significantly smaller than that by viscoresistive shear Alfven waves.
Abstract: Kinetic shear Alfven wave (KSAW) helicity injection motivated by the goal of simultaneous implementation of radio frequency current and flow [Phys. Rev. Lett. 67, 1535 (1991)] drive is analyzed. The quasilinear helicity flux results in a net helicity increase only via transport through the boundaries. This, in turn, requires a compressional component at the plasma edge boundary and electron dissipation at the Alfven resonance, as well as throughout the region, where helicity transport occurs. A comparison is made to direct KSAW current drive, which can add constructively with helicity injection by tailoring of the sign of poloidal and parallel wave numbers. The KSAW helicity flux is related to the α effect due to MHD (magnetohydrodynamic) resistive kink and shear‐Alfven‐driven helicity fluxes. The helicity flux need not have a zero flux surface average. Finally, the efficiency (IpR/Pabs) of helicity injection by the KSAW is found to be significantly smaller than that by viscoresistive shear Alfven waves, ...
7 citations
TL;DR: In this article, the nonlinear dynamics of kinetic Alfven waves, resonantly excited by energetic ions/alpha particles, is investigated. And the most efficient saturation mechanism is found to be self-induced profile modification, while the background magnetohydrodynamic turbulence results in a nonlinear real frequency shift.
Abstract: The nonlinear dynamics of kinetic Alfven waves, resonantly excited by energetic ions/alpha particles, is investigated. It is shown that α particles govern both linear instability and nonlinear saturation dynamics, while the background magnetohydrodynamic turbulence results only in a nonlinear real frequency shift. The most efficient saturation mechanism is found to be self‐induced profile modification. Expressions for the fluctuation amplitudes and the α‐particle radial flux are self‐consistently derived. The work represents the first self‐consistent, analytical, turbulent treatment of collective α‐particle losses by Alfvenic fluctuations.
6 citations
TL;DR: In this article, it was shown that at certain times, the plasma in the accretion column of the neutron star may form a deep enough pool that the top portion becomes unstable to convective motions in spite of the strong magnetic field.
Abstract: To explain the emission of TeV and PeV gamma rays from accreting X-ray binary sources, protons must be accelerated to several times the gamma-ray energy. It is shown here that at certain times, the plasma in the accretion column of the neutron star may form a deep enough pool that the top portion becomes unstable to convective motions in spite of the strong magnetic field. The resulting turbulence produces fluctuations in the strength of the magnetic field that travel up the accretion column, taking energy out to the region of the energetic protons. The protons resonantly absorb this energy and are accelerated to high energies. Including the synchrotron radiation losses of the protons, it is shown that they can be accelerated to energies that are high enough to explain the gamma-ray observations.
01 Jan 1992
TL;DR: In this article, the radial structure of poloidal flows and radial electric fields in the tokamak plasma edge plays an important role in determining global confinement properties, and self-consistent calculations of plasma turbulence in the presence of flows are needed to identify the mechanism of confinement improvement.
Abstract: The radial structure of poloidal flows and radial electric fields in the tokamak plasma edge plays an important role in determining global confinement properties. In a turbulent plasma, poloidal flow can be generated through the Reynolds stress. At the same time, this poloidal flow controls the level of fluctuations. Thererfore, self-consistent calculations of plasma turbulence in the presence of flows are needed to identify the mechanism of confinement improvement. Here, we study the interaction. Linearly, both the shear flow, V[sub [theta]][sup [prime]] [ne] O, and the curvature flow, V[sub [theta]][sup [double prime]] [ne] O, are stabilizing. Nonlinearly, the effect of the shear flow is weak, while the curvature flow is robust and survives in the nonlinear regime.