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The coupling of the short wavelength ion temperature gradient mode (SWITG) and trapped electron mode (SWTEM) in the presence of impurity ions is numerically studied in tokamak plasmas. It is found that the coupling of SWITG and SWTEM modes is divided into hybrid and coexistent cases. The impurity ions always have stabilizing effects on the hybrid mode (namely, SWTE-ITG) with small ηi (ηi < 0.5), which is different from the conventional TE-ITG mode. In the lager ηi (ηi > 0.5) regime, the impurity ions with the density profiles peaked inwardly (outwardly) stabilize (destabilize) the SWTE-ITG mode. Another new finding in this paper is that the impurity ions stabilize the SWTEM mode independent of whether their density profile is peaked inwardly or outwardly. The result for the SWITG mode is similar to the SWTE-ITG mode in the lager ηi regime, except that the excitation of the SWTEM mode requires ηi higher than a certain threshold. In addition, the kθρs spectra, eigenmode structures, and the effects of different impurity ion species on the modes are discussed. [ABSTRACT FROM AUTHOR]

Two m/n = 1/1 kink modes excited by energetic electrons (called double e-fishbone) have been observed near the q = 1 flux surfaces in the HL-2A discharges. The negative magnetic central shear configuration was achieved with localized electron cyclotron resonance heating and electron cyclotron current drive during plasma current ramp-up. The features of the modes have been first shown by advanced 2D electron cyclotron emission imaging (ECEI) system. From ECEI, two m/n = 1/1 modes propagating in the opposite directions poloidally have been clearly observed. These modes can be found only in low density discharge, and their frequencies are close to the precessional frequency of the trapped energetic electrons. More interestingly, the thermal energy transfer between the two modes was revealed by this new diagnostic, which is found to be related to the nonlinear interaction of the two modes and local electron thermal transport. [ABSTRACT FROM AUTHOR]

The effects of impurity ions on the trapped electron mode (TEM) in tokamak plasmas are numerically investigated with the gyrokinetic integral eigenmode equation. It is shown that in the case of large electron temperature gradient (ge), the impurity ions have stabilizing effects on the TEM, regardless of peaking directions of their density profiles for all normalized electron density gradient R=Lne. Here, R is the major radius and Lne is the electron density gradient scale length. In the case of intermediate and/or small ge, the light impurity ions with conventional inwardly (outwardly) peaked density profiles have stabilizing effects on the TEM for large (small) R=Lne, while the light impurity ions with steep inwardly (outwardly) peaked density profiles can destabilize the TEM for small (large) R=Lne. Besides, the TEM driven by density gradient is stabilized (destabilized) by the light carbon or oxygen ions with inwardly (outwardly) peaked density profiles. In particular, for flat and/or moderate R=Lne, two independent unstable modes, corresponding respectively to the TEM and impurity mode, are found to coexist in plasmas with impurity ions of outwardly peaked density profiles. The high Z tungsten impurity ions play a stronger stabilizing role in the TEM than the low Z impurity ions (such as carbon and oxygen) do. In addition, the effects of magnetic shear and collision on the TEM instability are analyzed. It is shown that the collisionality considered in this work weakens the trapped electron response, leading to a more stable TEM instability, and that the stabilizing effects of the negative magnetic shear on the TEM are more significant when the impurity ions with outwardly peaked density profile are taken into account. [ABSTRACT FROM AUTHOR]

High harmonic fast waves (HHFW) are among the candidates for non-inductive current drive (CD), which is essential for long-pulse or steady-state operation of tokamaks. Current driven with HHFW in EAST tokamak plasmas is numerically studied. The HHFW CD efficiency is found to increase non-monotonically with the wave frequency, and this phenomenon is attributed to the multi-pass absorption of HHFW. The sensitivity of CD efficiency to the value of the parallel refraction index of the launched wave is confirmed. The quasilinear effects, assessed as significant in HHFW current drive with the GENRAY/CQL3D package, cause a significant increase in CD efficiency as RF power is increased, which is very different from helicon current drive. Simulations for a range of toroidal dc electric fields, in combination with a range of fast wave powers, are also presented and indicate that the presence of the DC field can also enhance the CD efficiency. [ABSTRACT FROM AUTHOR]

The effects of impurity ions on the short wavelength ion temperature gradient (SWITG) driven instability in elongated tokamak plasmas are numerically investigated with the gyrokinetic integral eigenmode equation. It is found that for a moderate electron density gradient, the SWITG mode is first destabilized and then stabilized with increasing elongation κ, which is different from the conventional long wavelength ITG mode. For a large electron density gradient, the elongation can effectively stabilize the SWITG mode. Moreover, the low Z impurity ions with inwardly (outwardly) peaked density profiles have stabilizing (destabilizing) effects on the SWITG modes in elongated plasmas. Interestingly, the high Z tungsten impurity ions with inwardly peaked density profiles play a stronger stabilizing role in the SWITG modes than the low Z impurity ions (such as carbon and oxygen) do. In particular, the high Z tungsten impurity ions with a weakly outwardly peaked density profile still have a stabilizing effect. Finally, the critical threshold of impurity density gradient scale length for exciting impurity mode is also numerically obtained, indicating that the impurity mode is harder to be excited in elongated plasmas than in circular ones. [ABSTRACT FROM AUTHOR]

Energetic-particle-induced kinetic electromagnetic geodesic acoustic modes (EKEGAMs) are numerically studied in low β (=plasma pressure/magnetic pressure) tokamak plasmas. The parallel component of the perturbed vector potential is considered along with the electrostatic potential perturbation. The effects of finite Larmor radius and finite orbit width of the bulk and energetic ions as well as electron parallel dynamics are all taken into account in the dispersion relation. Systematic harmonic and ordering analysis are performed for frequency and growth rate spectra of the EKEGAMs, assuming (kpi) ~ q-3 ~ β ≪ 1, where q, k, and qi are the safety factor, radial component of the EKEGAMs wave vector, and the Larmor radius of the ions, respectively. It is found that there exist critical βh/βi values, which depend, in particular, on pitch angle of energetic ions and safety factor, for the mode to be driven unstable. The EKEGAMs may also be unstable for pitch angle λ0B < 0.4 in certain parameter regions. Finite β effect of the bulk ions is shown to have damping effect on the EKEGAMs. Modes with higher radial wave vectors have higher growth rates. The damping from electron dynamics is found decreasing with decrease of the temperature ratio Te/Ti. The modes are easily to be driven unstable in low safety factor q region and high temperature ratio Th/Ti region. The harmonic features of the EKEGAMs are discussed as well. [ABSTRACT FROM AUTHOR]

Features of fishbone-like trapped energetic electrons driven modes (TEEMs), which are a particular manifestation of the fishbone gap modes, are investigated taking into account model radial profile and the pitch angle distribution of the energetic electrons (EEs). The TEEMs are found unstable only when the beta values of EEs βh (¼pressure of the energetic electrons/pressure of magnetic field) are higher than certain critical values which are proportional to perturbed energy δWc of background plasma and much lower than that for traditional fishbone modes. In addition, TEEMs become stable again and enter a second stable regime whenβh values are higher than second critical values. Furthermore, the modes can only be excited in plasmas which are stable for MHD instabilities. The real frequency and growth rate of the modes are approximately linear and parabolic functions of βh, respectively. The real frequency is very low but not zero in the vicinity of lower beta region, whereas it is comparable to the toroidal precession frequency of the EEs in higher critical beta region. The numerical results show that the second stable regime is easy to form when q=1 flux surface is close to the magnetic axis. Suitable density gradient of EEs and magnetic shear are other two necessary conditions for excitation of the TEEMs. [ABSTRACT FROM AUTHOR]

The pellet pacing ELM mitigation concept is being tested in some tokamaks such as ASDEX Upgrade, DIII-D and JET. By increasing the ELM frequency, the ELM size can be reduced and eventually suppressed to meet the lifetime requirements on ITER target plates. In the HL-2A tokamak, ELMy H-mode operation is routinely performed and small type-III ELMs with a high repetition rate and some type-I (or possibly large type-III) ELM events are observed. Large ELMs are often preceded by strong coherent magnetic oscillations, and produce obvious perturbations on plasma current Ip, electron density ñedge at the edge, stored energy WE, etc. The coherent magnetic oscillations before an ELM crash or during the ELM are measured by toroidal and poloidal Mirnov coils and analysed by the wavelet technique to study the spectral characteristics of the short time ELM events. Pellet injection experiments are performed in type-III ELMy H-mode plasmas and ELM-free H-mode plasmas to study the physics of pellet triggering ELM. The analyses of pellet-induced ELMs and spontaneous ELMs are presented. Because the pellet size is relatively large, it induces magnetic oscillations lasting longer than that of a natural ELM. [ABSTRACT FROM AUTHOR]

Electromagnetic effects of the kinetic geodesic acoustic modes (KGAMs) are numerically studied in low β(= plasma pressure/magnetic pressure) tokamak plasmas. The parallel component of the perturbed vector potential is considered along with the electrostatic potential perturbation. The finite Larmor radius and finite orbit width of the ions as well as electron parallel dynamics are all taken into account. Systematic harmonic and ordering analysis is performed for collisionless damping of the KGAMs, assuming β~(κρi)2, where κand ρiare the radial component of the KGAM wave vector and the Larmor radius of the ions, respectively. It is found that the electron parallel dynamics enhances the damping of the electrostatic KGAM modes when the safety factor q is high. In addition, the electromagnetic (finite β effect is revealed to enhance and weaken the damping of the modes in plasmas of low and high safety factor q~2.0 and 5.5, respectively. The harmonic features of the KGAMs are discussed as well. [ABSTRACT FROM AUTHOR]

The linear behaviors of the double tearing mode (DTM) mediated by parallel electron viscosity in cylindrical plasmas with reversed magnetic shear and thus two resonant rational flux surfaces is numerically investigated. The distance between the two surfaces is found to play an important role for modes with poloidal mode number m>1. Two modes, one of which is centered at the inner rational surface and the other is located between the two surfaces, are simultaneously unstable and the growth rates show the standard single tearing mode (STM) scaling as γ∝R-1/3 when the distance is large (here, the Reynolds number R≡τυ/τh, τυ, and τh are, respectively, the viscosity penetration time of the magnetic field and the Alfvén time for a plasma sheet of width a). The latter is unstable only and the growth rate transits to the standard DTM scaling as γ∝R-1/5 for low-m (e.g., m<4) modes and keeps the STM scaling γ∝R-1/3 for high-m (e.g., m∼10) modes, which are found dominant, when the distance is decreased. In contrast, two unstable modes extending from plasma center to the two rational surfaces, respectively, coexist and the growth rates always show the scaling of γ∝R-1/5, independent of the distance, when the poloidal mode number m=1. The DTMs mediated by electron viscosity are enhanced by plasma resistivity of the range where the growth rate of the mode induced by the latter alone is comparable with that mediated by the former alone and vice versa. Otherwise, the growth rate of the mode is equal to the higher of the modes mediated by resistivity or electron viscosity alone when both of them are taken into account. [ABSTRACT FROM AUTHOR]

Strong correlation between high frequency microturbulence and low frequency geodesic acoustic mode (GAM) has been observed in the edge plasmas of the HL-2A tokamak, suggesting possible GAM generation via three wave coupling with turbulence, which is in turn modulated by the GAM. In this work, we use the gyrokinetic toroidal code to study the linear and nonlinear development of the drift instabilities, as well as the generation of the GAM (and low frequency zonal flows) and its interaction with the turbulence for realistic parameters in the edge plasmas of the HL-2A tokamak for the first time. The simulation results indicate that the unstable drift wave drives strong turbulence in the edge plasma of HL-2A. In addition, the generation of the GAM and its interaction with the turbulence are all observed in the nonlinear simulation. The simulation results are in reasonable agreement with the experimental observations. [ABSTRACT FROM AUTHOR]

The internal kink (fishbone) modes, driven by barely passing energetic ions (EIs), are numerically studied with the spatial distribution of the EIs taking into account. It is found that the modes with frequencies comparable to the toroidal precession frequencies are excited by resonant interaction with the EIs. Positive and negative density gradient dominating cases, corresponding to off- and near-axis depositions of neutral beam injection (NBI), respectively, are analyzed in detail. The most interesting and important feature of the modes is that there exists a second stable regime in higher βh (=pressure of EIs/toroidal magnetic pressure) range, and the modes may only be excited by the barely passing EIs in a region of βth1<βh<βth2 (βth is threshold or critical beta of EIs). Besides, the unstable modes require minimum density gradients and minimum radial positions of NBI deposition. The physics mechanism for the existence of the second stable regime is discussed. The results may provide a means of reducing or even preventing the loss of NBI energetic ions and increasing the heating efficiency by adjusting the pitch angle and driving the system into the second stable regime fast enough. [ABSTRACT FROM AUTHOR]

Characteristics of ion temperature gradient (ITG) instability in the presence of a magnetic island are investigated numerically using a gyrofluid model. It is shown that when the magnetic island is wide enough to produce a broad distribution of rational surfaces near the O-point region, the ITG perturbations at these rational surfaces form a radially global-type eigenmode with a fast growth rate, which is referred to as the magnetic-island-induced ITG mode. Moreover, the magnetic island also causes both radial and poloidal mode couplings, which play a stabilizing role. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The three-dimensional characteristics of the geodesic acoustic mode zonal flows (GAMZFs) and the ambient turbulence (AT) at the edge of the HuanLiuqi-2A tokamak [Y. Liu et al., Nucl. Fusion 45, S203 (2005)] are investigated with Langmuir probe arrays and the results are presented in detail. The toroidal and poloidal symmetries, and the radial scale of the GAMZFs are simultaneously identified. The envelopes of the high frequency components of the AT in the presence of the GAMZFs are analyzed. The GAM frequency components (GAMFCs) of the coherent envelopes are also shown to have poloidal and toroidal symmetries, and similar radial scales as the GAMZF does. The correlation between the GAMFCs of the envelopes and the GAMs is high, with phase shifts between π/2 to π, indicating that the GAMZFs may regulate the AT and the regulation is embodied in the envelopes. Three-wave coupling between GAM and AT is found to be a plausible formation mechanism for the former, which acts on the whole spectra of the latter within its scale length. The temporal evolutions of the total fluctuation power, the GAM and the AT powers show that the AT power decreases when GAM power increases and vice versa, indicating possible regulating effects of the latter on the former. [ABSTRACT FROM AUTHOR]

Series of ion temperature gradient (ITG or ηi) driven modes in the short wavelength region, |k⊥ρi|>1, are investigated with a gyrokinetic integral equation code in toroidal plasmas. These instabilities exist even if electrons are assumed adiabatic. However, nonadiabatic electron response can influence these short wavelength ITG (SWITG) modes, especially the fundamental l=0 mode. At typical parameters, excitation of the l=0 mode requires that both ηi and ηe exceed thresholds, while the l=1 and l=2 modes with higher harmonic eigenfunctions persist unstable even at ηe=0. Dependence of the SWITG modes on other parameters is also investigated. The l=1 mode with an odd potential eigenfunction grows faster than the l=0 mode and may be dominant in low ηe, high β, weak positive magnetic shear, and/or weak toroidicity regions. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The effects of flow shear on the temperature gradient driven short wavelength ion (SWITG) modes and electron temperature gradient (ETG) modes are investigated in a sheared slab. The SWITG mode can be stabilized at arbitrary β when the E×B velocity shear, VE′, reaches above a critical value. Since the SWITG mode has a lower frequency, a lower VE′ is needed to stabilize the SWITG mode than to stabilize the conventional ITG mode. However, the critical values of VE′ for stabilization of both SWITG and conventional ITG modes are much less than vti/Ln, where vti and Ln are ion thermal speed and the scale length of density gradient. Contrastively, the ETG mode cannot be stabilized until the VE′ is larger than vti/Ln. Similarly, a parallel velocity shear with order vti/Ln has significant effects on the SWITG mode but is too small to influence the ETG mode. The different behaviors of flow shear effects on the SWITG and ETG modes may indicate that the ETG mode is more reasonable than the SWITG as the candidate responsible for anomalous electron thermal transport. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The linear behavior of the double tearing mode in plasmas with a phenomenological anomalous electron viscosity is investigated within the framework of magnetohydrodynamic theory. In the large Reynolds number R = τ[sub v]/τ[sub h] (τ[sub v] and τ[sub h] are, respectively, the viscosity penetration time of the magnetic field and the Alfvén time for a plasma sheet of width a) limit, the growth rate is found to scale as R[sup -1/5] if the two resonant surfaces, at x = ±x[sub s], are close enough to satisfy x[sub s]/a ... (k[sub y]a)[sup -ll/15]R[sup -1/15]. For larger separation between the resonant surfaces, the growth rate transits to a R[sup -1/3] scaling. The transition occurs at x[sub s]/a∼(k[sub y]a)[sup -l l/15]R[sup -1/15]. The R[sup -1/5] is shown to be closely correlated with the violation of the constant-ψ approximation. The nonlinear velocity perturbations associated with the unstable double tearing mode are estimated to saturate at a level high enough to serve as a trigger for the formation of transport barriers observed in advanced tokamaks. [ABSTRACT FROM AUTHOR]

Electron temperature gradient (ETG) driven instability in toroidal plasmas is studied with gyrokinetic theory. The full electron kinetics including finite Larmor radius effects, toroidal (curvature and magnetic gradient) drift motion ω[sub D], and transit k[sub ||]v[sub ||], is considered. The upgraded numerical scheme for solving the integral eigenvalue equations allows the study of both growing and damping modes, and thus direct calculation of critical gradient. A systematic parameter study is carried out for low β(=plasma pressure/magnetic pressure) circular flux surface equilibria. The basic characteristics of the modes are discussed. The scaling of the critical gradient with respect to toroidicity and to the ratio of electron temperature over ion temperature is obtained. Estimation for the transport induced by the modes is given. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The ion temperature gradient modes of tearing mode parity are investigated for arbitrary β (=plasma pressure/magnetic pressure) plasmas in a sheared slab. Under the low β limit, the results agree with Reynders’ conclusion that the lowest order (l=1) mode of tearing mode parity persists after the fundamental mode (l=0) of drift mode parity is completely stabilized by finite β [J. V. M. Reynders, Phys. Plasmas 1, 1953 (1994)]. However, when the effects of the magnetic gradient drift and the coupling to the compressional Alfve´n waves are included, the l=0 mode is more difficult to stabilize than the l=1 mode. It is also shown that the l=1 mode is much easier to stabilize by the magnetic shear than the l=0 mode. Generally, the l=1 mode grows faster in the low magnetic shear and low β regime while the l=0 mode is the dominant eigenmode of ion temperature gradient instability in high β plasmas. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A new suppression mechanism of turbulent transport, characteristic of the synergism between safety factor and shear flows, is proposed to explain the internal transport barriers (ITBs) observed in neutral-beam-heated tokamak discharges with reversed magnetic shear. It is shown that the evolution of turbulent transport with the strength of the suppression mechanism reproduces the basic features of the formation and development of ITBs observed in experiments. In addition, the present analyses predict the possibility of global ion and electron heat transport barriers. [ABSTRACT FROM AUTHOR]

A set of integral equations is developed to study drift instabilities in any β (plasma pressure/magnetic pressure) plasmas with the sheared slab magnetic configuration model. Both components of the perturbed vector potential, A˜[sub ||] and A˜[sub ⊥], are considered in the equations, as well as the perturbation of the electrostatic potential &fgr;˜. The magnetic gradient drift effects are taken into account. The ion temperature gradient modes are analyzed and found to be unstable in the high β regime. The stability of the high β modes is very sensitive to the mode frequency. The lower frequency modes are more difficult to be stabilized since the β effects cannot effectively change the frequency and then the particle-wave interaction in the lower frequency regime. The magnetic shear is shown to have strong stabilizing effects on the high β modes. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Effects of β (plasma pressure/magnetic pressure) and T[sub e]/T[sub i] (ratio of electron temperature to ion temperature) on the slab ion temperature gradient modes are studied locally for any β value in anisothermal hydrogenic plasmas with no impurities or fast ions. The stabilizing mechanism of finite β on the modes is investigated in detail. It is shown that increasing T[sub e]/T[sub i] destabilizes the modes and that β[sub i] stabilizes the modes mainly by decreasing T[sub e]/T[sub i], while β[sub e] does so directly. β[sub e] is the dominant stabilizing factor for a fixed T[sub e]/T[sub i]. The numerical results also indicate that the critical β(β[sub c]) for mode stabilization decreases as T[sub e]/T[sub i] decreases. Ion heating favors the suppression of the ion temperature gradient modes in anisothermal plasmas for any β value. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The stability of electron temperature gradient driven modes in a collisionless plasma with magnetic shear has been studied. Two coupled integral equations for the electrostatic and vector potentials have been solved numerically. The properties of these modes derived on the basis of numerical investigation have been utilized to estimate the anomalous electron transport induced by them. [ABSTRACT FROM AUTHOR]

Four different thermal conduction models have been used to calculate the critical density limits in Ohmically heated tokamaks using the electron and ion energy balance equations. The results are compared to experimental density limits for a variety of Ohmically heated tokamaks. [ABSTRACT FROM AUTHOR]

Studies the ion temperature gradient driven microionstabilities for tokamak plasmas with very weak magnetic shear (VWS) using kinetic theory. Physical model and eigenmode equation; Instability and transport; Parallel velocity shear effects; Perpendicular velocity shear effects.

The absolute rate of nonlinear energy transfer among broadband turbulence, low-frequency zonal flows (ZFs) and geodesic acoustic modes (GAMs) was measured for the first time in fusion-grade plasmas using two independent methods across a range of heating powers. The results show that turbulent kinetic energy from intermediate frequencies (20-80 kHz) was transferred into ZFs and GAMs, as well as into fluctuations at higher frequencies (> 80 kHz). As the heating power was increased, the energy transfer from turbulence into GAMs and the GAM amplitudes increased, peaked and then decreased, while the energy transfer into the ZFs and the ZFs themselves increased monotonically with heating power. Thus there exists a competition between ZFs and GAMs for the transfer of turbulent energy, and the transfer into ZFs becomes dominant as the heating power is increased. The poloidal-radial Reynolds stress and the mean radial electric field profiles were also measured at different heating powers and found to be consistent with the energy transfer measurement. The results suggest that ZFs play an important role in the low-to-high (JL-H) plasma confinement transition [ABSTRACT FROM AUTHOR]