Sheared Poloidal Flow Research Articles

The excitation of geodesic acoustic mode flows by a resonant magnetic field and by resonant heating

Toroidal magnetohydrodynamic (MHD) simulations demonstrate that sheared poloidal flows in tokamaks can be generated by the resonant excitation of the geodesic acoustic mode (GAM). Poloidal flows are generated by two resonant excitation methods: oscillating currents in an external coil and an oscillating heat source. The coil current and the heat source oscillate in time at the local GAM frequency. The sheared poloidal flow generated by the excitation of the GAM may be useful for the suppression of plasma instabilities.

Impact of a shear flow on double tearing nonlinear dynamics

The dynamics of global reconnection in the presence of a poloidal shear flow located in between magnetic islands is investigated. Different linear and nonlinear regimes are identified depending on the resistivity, the equilibrium velocity amplitude, and the distance between the low-order resonant surfaces. It is found that nonlinearly, the shear flow can significantly delay DTM generation and global reconnection. It is shown that this delay is linked to a symmetry breaking imposed by the shear flow and the generation of mean poloidal flows in the resistive layers. It is also found that turbulence can be generated by Kelvin-Helmholtz instability in between the resonance layers and enhance magnetic reconnection processes.

Semi-analytic approach to diverted tokamak equilibria with incompressible toroidal and poloidal flows

Generalized Grad–Shafranov equation for tokamak equilibrium with incompressible toroidal and poloidal flows is solved to obtain a double-null diverted configuration based on an approach presented before (Shi 2008 Plasma Phys. Control. Fusion 50 085006). This solution consists of only two terms of base functions obtained from the variable-separating method and suitable for describing both the internal region within the separatrix and a scrape-off layer region. Dependences of the main equilibrium properties, such as the magnetic field, plasma pressure and the equilibrium radial electric field and the plasma current on flows are revealed. In particular, we find that the presence of poloidal flow causes a deviation of the current surface from the magnetic surface and the sheared poloidal flow produces a non-zero toroidal current component that possibly affects the peeling–ballooning stability of the pedestal.

Dependence of the L- to H-mode power threshold on toroidal rotation and the link to edge turbulence dynamics

The injected power required to induce a transition from L-mode to H-mode plasmas is found to depend strongly on the injected neutral beam torque and consequent plasma toroidal rotation. Edge turbulence and flows, measured near the outboard midplane of the plasma (0.85 < r/a < 1.0) on DIII-D with the high-sensitivity 2D beam emission spectroscopy (BES) system, likewise vary with rotation and suggest a causative connection. The L–H power threshold in plasmas with the ion ∇B drift directed away from the X-point decreases from 4–6 MW with co-current beam injection, to 2–3 MW near zero net injected torque and to <2 MW with counter-injection in the discharges examined. Plasmas with the ion ∇B drift directed towards the X-point exhibit a qualitatively similar though less pronounced power threshold dependence on rotation. 2D edge turbulence measurements with BES show an increasing poloidal flow shear as the L–H transition is approached in all conditions. As toroidal rotation is varied from co-current to balanced in L-mode plasmas, the edge turbulence changes from a uni-modal character to a bi-modal structure, with the appearance of a low-frequency (f = 10–50 kHz) mode propagating in the electron diamagnetic direction, similar to what is observed as the ion ∇B drift is directed towards the X-point in co-rotating plasmas. At low rotation, the poloidal turbulence flow near the edge reverses prior to the L–H transition, generating a significant poloidal flow shear that exceeds the measured turbulence decorrelation rate. This increased poloidal turbulence velocity shear appears to facilitate the L–H transition. No such reversal is observed in high rotation plasmas. The high-frequency poloidal turbulence velocity spectrum exhibits a transition from a geodesic acoustic mode zonal flow to a higher-power, lower frequency zero-mean-frequency zonal flow as rotation varies from co-current to balanced during a torque scan at constant injected neutral beam power, perhaps also facilitating the L–H transition. This reduced power threshold at lower toroidal rotation may benefit inherently low-rotation plasmas such as ITER.

Dependence of the low to high confinement mode transition power threshold and turbulence flow shear on injected torque

The power required to induce a bifurcation from a low-confinement mode to a high-confinement mode in DIII-D tokamak [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] plasmas is found to depend sensitively on the injected neutral beam torque and consequent toroidal rotation. Plasmas exhibit a factor of 2–4 reduction in this power threshold, dependent on ion ∇B drift direction. Correlated with this change, turbulence velocity measurements near 0.9&amp;lt;r/a&amp;lt;1.0 for balanced injection demonstrate significantly larger poloidal flow shear at a given injection power, relative to cocurrent injection, facilitating the confinement transition.

A novel mechanism for exciting intrinsic toroidal rotation

Beginning from a phase space conserving gyrokinetic formulation, a systematic derivation of parallel momentum conservation uncovers two physically distinct mechanisms by which microturbulence may drive intrinsic rotation. The first mechanism, which emanates from E×B convection of parallel momentum, has already been analyzed [O. D. Gurcan et al., Phys. Plasmas 14, 042306 (2007); R. R. Dominguez and G. M. Staebler, Phys. Fluids B 5, 3876 (1993)] and was shown to follow from radial electric field shear induced symmetry breaking of the spectrally averaged parallel wave number. Thus, this mechanism is most likely active in regions with steep pressure gradients or strong poloidal flow shear. The second mechanism uncovered, which appears in the gyrokinetic formulation through the parallel nonlinearity, emerges due to charge separation induced by the polarization drift. This novel means of driving intrinsic rotation, while nominally higher order in an expansion of the mode frequency divided by the ion cyclotron frequency, does not depend on radial electric field shear. Thus, while the magnitude of the former mechanism is strongly reduced in regions of weak radial electric field shear, this mechanism remains unabated and is thus likely relevant in complementary regimes.

Shear flows induced by nonlinear evolution of double tearing modes

Shear flows induced by nonlinear evolution of double tearing modes are investigated in a resistive magnetohydrodynamic model with slab geometry. It is found that intensive and thin poloidal shear flow layers are generated in the magnetic island region driven by coupled reconnection process at both rational surfaces. The structure of the flow layers keeps evolving after the merging of magnetic separatrices and forms a few narrow vortices along the open field lines in the final stage of magnetic reconnection. The effects of the distance between both rational surfaces and the initial magnetic shear on the nonlinear evolution of the plasma flows are also taken into consideration and the relevant mechanism is discussed.

Core barrier formation near integer q surfaces in DIII-D

Recent DIII-D experiments have significantly improved the understanding of internal transport barriers (ITBs) that are triggered close to the time when an integer value of the minimum in q is crossed. While this phenomenon has been observed on many tokamaks, the extensive transport and fluctuation diagnostics on DIII-D have permitted a detailed study of the generation mechanisms of q-triggered ITBs as pertaining to turbulence suppression dynamics, shear flows, and energetic particle modes. In these discharges, the evolution of the q profile is measured using motional Stark effect polarimetry and the integer qmin crossings are further pinpointed in time by the observation of Alfvén cascades. High time resolution measurements of the ion and electron temperatures and the toroidal rotation show that the start of improved confinement is simultaneous in all three channels, and that this event precedes the traversal of integer qmin by 5–20ms. There is no significant low-frequency magnetohydrodynamic activity prior to or just after the crossing of the integer qmin and hence magnetic reconnection is determined not to be the precipitant of the confinement change. Instead, results from the GYRO code point to the effects of zonal flows near low order rational q values as playing a role in ITB triggering. A reduction in local turbulent fluctuations is observed at the start of the temperature rise and, concurrently, an increase in turbulence poloidal flow velocity and flow shear is measured with the beam emission spectroscopy diagnostic. For the case of a transition to an enduring internal barrier the fluctuation level remains at a reduced amplitude. The timing and nature of the temperature, rotation, and fluctuation changes leading to internal barriers suggests transport improvement due to increased shear flow arising from the zonal flow structures.

Analyses of edge plasma characteristics in HL-2A

The edge plasma characteristics are studied by both a movable array of Mach/Reynolds stress/Langmuir 10-probes in the boundary region and the fixed flush probe arrays on the 4 divertor neutralization plates at the same toroidal cross-section in the HL-2A tokamak. The dependence of the Reynolds stress on poloidal flow in the edge plasma is analysed. The result indicates that the sheared poloidal flow in tokamak plasma can be induced by the radial gradient of Reynolds stress. In the divertor experiments of HL-2A, the profiles of the electron temperature, density and floating potential on divertor plates are measured by the flush probe arrays. The edge electron temperature in divertor configuration is higher than that in limiter configuration. The temperature asymmetry between outer and inner target plates is observed. The result of magnetic surface reconstructed from 18 Mirnov coils signals is presented. Both the particle recycling and the impurity flux in the bulk plasma during divertor discharges are discussed. Neutral gas pressure in divertor chamber, measured by fast ionization gauge during divertor discharge, is given.

Assessment of Flow Drive by Use of Ion Bernstein Wave on Heliotron J and EAST Devices

The possibility of driving poloidal flows by use of ion Bernstein wave is assessed for Heliotron J and EAST devices by means of ray tracing analysis. Sheared poloidal flow is expected to suppress plasma turbulences due to the decorrelation of the waves. In Heliotron J and EAST plasma, the rays of Ion Bernstein Wave travel into the central region with oscillations along the magnetic lines of force and their power is absorbed by ions at the cyclotron resonance layers. The momentum input has been estimated by calculating the momentum change of rays and the poloidal flow has been estimated using neoclassical viscosities. The wave momentum changes its sign as it propagates inward, depositing sheared momentum to the plasma, and therefore causes sheared poloidal flows.

Ion temperature gradient driven mode in presence of transverse velocity shear in magnetized plasmas

The effect of sheared poloidal flow on the toroidal branch of the ion temperature gradient driven mode of magnetized nonuniform plasma is studied. A novel “nonmodal” calculation is used to analyze the problem. It is shown that the transverse shear flow considerably reduced the growth of the instability. A small but finite amount of viscosity and/or diffusion enhanced the stabilization process.

Dynamics of poloidal flows in enhanced reverse shear bifurcation

A simple reduced enhanced reverse shear (RERS) model is constructed to study the dynamics of poloidal flows during the ERS transition. This model predicts that a reversal of poloidal flow shear occurs just prior to the transition, as seen in experiment [R. E. Bell et al., Phys. Rev. Lett. 81, 1429 (1998)]. This transition front propagates until the radial location where the safety factor (q) is minimum and becomes locked there due to insufficient input power to overcome the threshold requirement for the bifurcation. This study also reveals that there can be many routes to ERS transition depending upon various tunable parameters.

Effect of ion ∇B drift direction on density fluctuation poloidal flow and flow shear

The divertor magnetic geometry has a significant effect on the poloidal velocity and resulting velocity shear of turbulent density fluctuations in the outer region of L-mode tokamak plasmas, as determined via two-dimensional measurements of density fluctuations with beam emission spectroscopy on DIII-D [J. L. Luxon, Nucl. Fusion 42, 614 (2002)]. Plasmas with similar parameters, except that in one case the ion ∇B drift points towards the divertor X point (lower-single-null, LSN), and in the other case, the ion ∇B drift points away from the divertor X point (upper-single-null, USN), are compared. Inside r∕a=0.9, the turbulence characteristics (density fluctuation amplitude, flow direction, correlation lengths) are similar in both cases, while near r∕a=0.92, a dramatic reversal of the poloidal velocity of turbulent eddies relative to the core flow direction is observed in plasmas with the ion ∇B drift pointing towards the divertor X point. No such velocity reversal is observed in plasmas with the ion ∇B drift pointing away from the divertor X point. This poloidal velocity reversal results in a significantly larger local shear in the poloidal velocity of density fluctuations in plasmas with the ion ∇B drift pointing towards the divertor X point. Additionally, these plasmas locally exhibit significant dispersion with two distinct and counterpropagating turbulence modes. Likewise, the radial correlation length of the density fluctuations is reduced in these plasmas, consistent with biorthogonal decomposition measurements of dominant turbulence structures. The naturally occurring density fluctuation poloidal velocity shear in these LSN plasmas may facilitate the L-H transition that occurs at an input power of roughly one-half to one-third that of corresponding plasmas with the ion ∇B drift pointing away from the X point.

Turbulence and intermittent transport at the boundary of magnetized plasmas

Numerical fluid simulations of interchange turbulence for geometry and parameters relevant to the boundary region of magnetically confined plasmas are shown to result in intermittent transport qualitatively similar to recent experimental measurements. The two-dimensional simulation domain features a forcing region with spatially localized sources of particles and heat outside which losses due to the motion along open magnetic-field lines dominate, corresponding to the edge region and the scrape-off layer, respectively. Turbulent states reveal intermittent eruptions of hot plasma from the edge region, propagating radially far into the scrape-off layer in the form of field-aligned filaments, or blobs. This results in positively skewed and flattened single-point probability distribution functions of particle density and temperature, reflecting the frequent appearance of large fluctuations. The conditional fluctuation wave forms and transport statistics are also in a good agreement with those derived from the experiments. Associated with the turbulence bursts are relaxation oscillations in the particle and heat confinements as well as in the kinetic energy of the sheared poloidal flows. The formation of blob structures is thus related to profile variations, which are here triggered in a quasiperiodic manner by a global dynamical regulation due to the self-sustained sheared flows.

Statistical description of the radial structure of turbulence in the JET plasma boundary region

The statistical properties of the radial structure of fluctuations and E×B turbulent transport have been investigated in the plasma boundary of the JET tokamak. PDFs of fluctuations and turbulent transport have shown evidence of multiple radial scale lengths in the plasma boundary region. Radial effective velocities and turbulence radial coherence are modified in the presence of sheared poloidal flows which remains near marginal stability. These findings show a link between the structure of SOL profiles, sheared flows and turbulence statistical properties.

Research of edge plasma structure on the HL-1M tokamak

In this paper, experimental results of plasma characteristics in several discharges have been measured in the plasma boundary region of the HL-1M tokamak using a multiarray of Mach/Langmuir 6 probes, electrostatic 4 probes and 20 group of 3 probes. In the experiments of Ohmic discharge, lower hybrid current drive, supersonic molecular beam injection, multi-shot pellet injection and neutral beam injection, etc., the correlation between the Reynolds stress and poloidal flow in the edge plasma is obtaimed. The experimental results suggest that the confinement improvement is related to the increasing rotation velocity and to the decreasing edge fluctuation. The results indicate that the sheared poloidal flow can be generated in tokamak plasma due to the radially varying Reynolds stress. The particle confinement with low-density discharge can increase 1—2 times in LHCD. The particle confinement time and poloidal rotation velocity can at least be doubled after MPI, and that the particle confinement time can increase one order of magnitude and can obtain high performance plasma during the experiment of SMBI.

Effects of an Anomalous Resistivity on the Power Deposition by Alfvén Waves in Pre-Heated Spherical Tokamaks

As it is the case with tokamaks in general, and moreover, due to their specific geometry (limited space for inboard solenoid magnets), low aspect ratio (spherical) tokamaks (STs) require additional auxiliary non-ohmic current startup and maintenance, generation of internal transport barriers (associated with underlying sheared poloidal flows and quasi-stationary radial electric fields), plasma heating, etc. One of the options to generate these necessary effects in STs is by the aid of rf waves launched from a suitable external antenna; in this option the effects just mentioned are a consequence of ponderomotive forces resulting from the interaction of the rf waves with the plasma.Since experimental data on STs (viz., the START-device) reveal the presence of an anomalous plasma resistivity (about four times Spitzer's one), we carried out a systematic parametric investigation of the effects of an increased plasma resistivity on the magnitude and spatial localization of the resulting power deposition.

Edge plasma electrostatic fluctuation and anomalous transport characteristics in the Sino-united spherical tokamak

In this paper, the edge plasma parameters, including the electron temperature (Te), electron density (ne), plasma potential (ϕp), radial electric field (Er) and the corresponding fluctuations in the Sino-united spherical tokamak (SUNIST) have been systematically measured using Langmuir probe arrays. The characteristics of the edge electrostatic fluctuations and the turbulence-induced transport are experimentally investigated. The results show that a naturally formed Er × Bϕ poloidal velocity shear layer (VSL) exists near the radial location of the limiter. Wavenumber spectrum analyses show that edge fluctuations have a radial propagation character of the drift wave turbulence, with a characteristic radial phase velocity vphr ∼ 0.7 km s−1 in the scrape-off-layer and vphr ∼ 0.9–1.4 km s−1 at plasma edge, suggesting that the edge turbulence may originate from the core and propagate to the edge in an intermittent pattern. A radial profile of the toroidal flow velocity Mach number has been observed whose maximum is just located at the VSL position, which may imply a possible relation between the poloidal shear flow and the toroidal plasma flow in SUNIST.

Study of internal transport barrier triggering mechanism in tokamak plasmas

Sheared flow layers driven by magnetic energy, released in tearing-reconnection processes inherent in dissipative magnetohydrodynamics, are proposed as a triggering mechanism for the creation of the internal transport barrier (ITB) in tokamak plasmas. The double tearing mode, mediated by anomalous electron viscosity in configurations with a nonmonotonic safety factor, is investigated as an example. Particular emphasis is placed on the formation of sheared poloidal flow layers in the vicinity of the magnetic islands. A quasilinear simulation demonstrates that the sheared flows induced by the mode have desirable characteristics (lying just outside the magnetic islands), and sufficient levels required for ITB formation. A possible explanation is also proffered for the experimental observation that the transport barriers are preferentially formed in the proximity of low-order rational surfaces.

Effect of poloidal sheared flow on the long-range correlation characters of edge plasma turbulent transport

The long-range correlation characteristics of the turbulence-induced particle transport fluxes measured in the plasma edge region of the Chinese Tokamak-6B (CT-6B) [X. Yang et al., Nucl. Fusion 36, 1669 (1996)] have been investigated. The Hurst exponent calculations and the power spectra exhibit the self-similarity and long-range correlation character of the particle transport fluxes. The rescaled probability distribution functions (PDF’s) of the local fluxes are found to be self-similar, with the self-similarity parameter well above 0.5 in the range of time scales between the turbulence decorrelation and confinement time. The decorrelation effect of the E×B flow shear on the long-range correlation character of the flux has been identified by the changes in the radial dependences of the Hurst parameter, spectral decay index, and the outward flux tail of the PDF near the flow shear layer. This suggests that the mechanism for these changes is the simple decorrelation of the large-scale transport events by the shear flow.