Abstract: Two novel nonlinear mode coupling processes for reversed shear Alfven eigenmode (RSAE) nonlinear saturation are proposed and investigated. In the first process, RSAE nonlinearly couples to a co-propagating toroidal Alfven eigenmode (TAE) with the same toroidal and poloidal mode numbers, and generates a geodesic acoustic mode (GAM). In the second process, RSAE couples to a counter-propagating TAE and generates an ion acoustic wave quasi-mode (IAW). The condition for the two processes to occur is favored during current ramp. Both processes contribute to effectively saturate the Alfvenic instabilities, as well as nonlinearly transfer of energy from energetic fusion alpha particles to fuel ions in burning plasmas.

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Topics: Ion acoustic wave (52%), Toroidal and poloidal (50%)

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6 results found

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Nonlinear gyrokinetic equations for low-frequency electromagnetic waves in general plasma equilibria

01 Oct 1981-

Abstract: A nonlinear gyrokinetic formalism for low‐frequency (less than the cyclotron frequency) microscopic electromagnetic perturbations in general magnetic field configurations is developed. The nonlinear equations thus derived are valid in the strong‐turbulence regime and contain effects due to finite Larmor radius, plasma inhomogeneities, and magnetic field geometries. The specific case of axisymmetric tokamaks is then considered and a model nonlinear equation is derived for electrostatic drift waves. Also, applying the formalism to the shear Alfven wave heating scheme, it is found that nonlinear ion Landau damping of kinetic shear‐Alfven waves is modified, both qualitatively and quantitatively, by the diamagnetic drift effects. In particular, wave energy is found to cascade in wavenumber instead of frequency.

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Topics: Landau damping (59%), Wavenumber (56%), Nonlinear system (56%) ... show more

587 Citations

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Yan-Cheng Wang^{1}, Meng Cheng^{2}, William Witczak-Krempa^{3}, William Witczak-Krempa^{4} +1 more•Institutions (5)

Abstract: The experimental discovery of the fractional Hall conductivity in two-dimensional electron gases revealed new types of quantum particles, called anyons, which are beyond bosons and fermions as they possess fractionalized exchange statistics. These anyons are usually studied deep inside an insulating topological phase. It is natural to ask whether such fractionalization can be detected more broadly, say near a phase transition from a conventional to a topological phase. To answer this question, we study a strongly correlated quantum phase transition between a topological state, called a $\mathbb{Z}_2$ quantum spin liquid, and a conventional superfluid using large-scale quantum Monte Carlo simulations. Our results show that the universal conductivity at the quantum critical point becomes a simple fraction of its value at the conventional insulator-to-superfluid transition. Moreover, a dynamically self-dual optical conductivity emerges at low temperatures above the transition point, indicating the presence of the elusive vison particles. Our study opens the door for the experimental detection of anyons in a broader regime, and has ramifications in the study of quantum materials, programmable quantum simulators, and ultra-cold atomic gases. In the latter case, we discuss the feasibility of measurements in optical lattices using current techniques.

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Topics: Quantum phase transition (68%), Topological quantum computer (68%), Quantum critical point (64%) ... show more

6 Citations

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Yan-Cheng Wang^{1}, Meng Cheng^{2}, William Witczak-Krempa^{3}, William Witczak-Krempa^{4} +1 more•Institutions (5)

Abstract: The experimental discovery of the fractional Hall conductivity in two-dimensional electron gases revealed new types of quantum particles, called anyons, which are beyond bosons and fermions as they possess fractionalized exchange statistics. These anyons are usually studied deep inside an insulating topological phase. It is natural to ask whether such fractionalization can be detected more broadly, say near a phase transition from a conventional to a topological phase. To answer this question, we study a strongly correlated quantum phase transition between a topological state, called a $${{\mathbb{Z}}}_{2}$$
quantum spin liquid, and a conventional superfluid using large-scale quantum Monte Carlo simulations. Our results show that the universal conductivity at the quantum critical point becomes a simple fraction of its value at the conventional insulator-to-superfluid transition. Moreover, a dynamically self-dual optical conductivity emerges at low temperatures above the transition point, indicating the presence of the elusive vison particles. Our study opens the door for the experimental detection of anyons in a broader regime, and has ramifications in the study of quantum materials, programmable quantum simulators, and ultra-cold atomic gases. In the latter case, we discuss the feasibility of measurements in optical lattices using current techniques. Conventional quantum particles can break up into fractionalized excitations under the right conditions; however, their direct experimental observation is challenging. Here, the authors predict strong optical conductivity signatures of such excitations in the vicinity of a topological phase transition.

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Topics: Quantum phase transition (68%), Topological quantum computer (66%), Topological order (66%) ... show more

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Abstract: General nonlinear equations describing reversed shear Alfven eigenmode (RSAE) self-modulation via zero frequency zonal structure (ZFZS) generation are derived using nonlinear gyrokinetic theory, which are then applied to study the spontaneous ZFZS excitation as well as RSAE nonlinear saturation. It is found that both electrostatic zonal flow (ZF) and electromagnetic zonal current (ZC) can be preferentially excited by finite amplitude RSAE, depending on specific plasma parameters. The modification to local shear Alfven wave continuum is evaluated using the derived saturation level of ZC, which is shown to play a comparable role in saturating RSAE with the ZFZS scattering.

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Topics: Zonal flow (plasma) (57%)

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Abstract: The neutral particle analyzer (NPA) is one of the crucial diagnostic devices in a Tokamak facility. The stripping unit is one of the main parts of the NPA. A windowless gas-stripping room with two differential pipes has been constructed in a parallel electric and magnetic fields (E//B) NPA. The pressure distributions in the stripping chamber are simulated by ANSYS Fluent together with MolFlow+. Based on the pressure distributions obtained from the simulation, the stripping efficiency of the E//B NPA is studied using GEANT4. Hadron reaction physics is modified to track the charge state of each particle in a cross-section-based method in GEANT4. The transmission rates (R) and stripping efficiencies $$f_{+1}$$
are examined for particle energies ranging from 20 to 200 keV with the input pressure (
$$P_0$$
), ranging from 20 to 400 Pa. According to the combined global efficiency, $$R \times f_{+1}$$
, $$P_0$$
= 240 Pa is obtained as the optimum pressure for the maximum global efficiency in the incident energy range investigated.

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Topics: Neutral particle (56%), Stripping (chemistry) (54%)

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42 results found

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Nonlinear gyrokinetic equations for low-frequency electromagnetic waves in general plasma equilibria

Abstract: A nonlinear gyrokinetic formalism for low‐frequency (less than the cyclotron frequency) microscopic electromagnetic perturbations in general magnetic field configurations is developed The nonlinear equations thus derived are valid in the strong‐turbulence regime and contain effects due to finite Larmor radius, plasma inhomogeneities, and magnetic field geometries The specific case of axisymmetric tokamaks is then considered and a model nonlinear equation is derived for electrostatic drift waves Also, applying the formalism to the shear Alfven wave heating scheme, it is found that nonlinear ion Landau damping of kinetic shear‐Alfven waves is modified, both qualitatively and quantitatively, by the diamagnetic drift effects In particular, wave energy is found to cascade in wavenumber instead of frequency

... read more

Topics: Landau damping (59%), Wavenumber (56%), Nonlinear system (56%) ... show more

703 Citations

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Abstract: A procedure which reconciles long parallel wavelength, characteristic of plasma instabilities, with periodicity in a sheared toroidal magnetic field is described. Applied to the problem of high-$n$ ballooning modes in tokamaks it makes possible a full minimization of the potential energy functional $\ensuremath{\delta}W$ and shows that previous calculations overestimated stability.

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Topics: Tokamak (50%)

656 Citations

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Abstract: In toroidal systems with geodesic curvature an electrostatic acoustic mode occurs with plasma motion in the magnetic surfaces, perpendicular to the field. In typical stellarators this mode should dominate ordinary sound waves associated with motion along the field.

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Topics: Ion acoustic wave (63%), Acoustic wave (61%), Physical acoustics (56%) ... show more

578 Citations

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Ambrogio Fasoli^{1}, C. Gormenzano, Herbert L Berk^{2}, Boris Breizman^{2} +15 more•Institutions (9)

Abstract: This chapter reviews the progress accomplished since the redaction of the first ITER Physics Basis (1999 Nucl Fusion 39 2137-664) in the field of energetic ion physics and its possible impact on burning plasma regimes New schemes to create energetic ions simulating the fusion-produced alphas are introduced, accessing experimental conditions of direct relevance for burning plasmas, in terms of the Alfvenic Mach number and of the normalised pressure gradient of the energetic ions, though orbit characteristics and size cannot always match those of ITER Based on the experimental and theoretical knowledge of the effects of the toroidal magnetic field ripple on direct fast ion losses, ferritic inserts in ITER are expected to provide a significant reduction of ripple alpha losses in reversed shear configurations The nonlinear fast ion interaction with kink and tearing modes is qualitatively understood, but quantitative predictions are missing, particularly for the stabilisation of sawteeth by fast particles that can trigger neoclassical tearing modes A large database on the linear stability properties of the modes interacting with energetic ions, such as the Alfven eigenmode has been constructed Comparisons between theoretical predictions and experimental measurements of mode structures and drive/damping rates approach a satisfactory degree of consistency, though systematic measurements and theory comparisons of damping and drive of intermediate and high mode numbers, the most relevant for ITER, still need to be performed The nonlinear behaviour of Alfven eigenmodes close to marginal stability is well characterized theoretically and experimentally, which gives the opportunity to extract some information on the particle phase space distribution from the measured instability spectral features Much less data exists for strongly unstable scenarios, characterised by nonlinear dynamical processes leading to energetic ion redistribution and losses, and identified in nonlinear numerical simulations of Alfven eigenmodes and energetic particle modes Comparisons with theoretical and numerical analyses are needed to assess the potential implications of these regimes on burning plasma scenarios, including in the presence of a large number of modes simultaneously driven unstable by the fast ions

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Topics: Marginal stability (52%), Magnetic confinement fusion (52%), Normal mode (50%)

456 Citations

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Abstract: Ideal and resistive MHD equations for the shear Alfven waves are studied in a low-β toroidal model by employing the high-n ballooning formalism. The ion sound effects are neglected. For an infinite shear slab, the ideal MHD model gives rise to acontinuous spectrum of real frequencies and discrete eigenmodes (Alfven-Landau modes) with complex frequencies. With toroidal coupling effects due to nonuniform toroidal magnetic field, the continuum is broken up into small continuum bands and new discrete toroidal eigenmodes can exist inside the continuum gaps. Unstable ballooning eigenmodes are also introduced by the bad curvature when β > βc. The resistivity (η) can be considered perturbatively for the ideal modes. In addition, four branches of resistive modes are induced by the resistivity: (1) resistive entropy modes which are stable with frequencies going to zero with resistivity as η 1 3 ; (2) tearing modes which are stable (Δ′ η 3 5 ; (3) resistive periodic shear Alfven waves which approach the finite frequency end points of the continuum bands as η 1 2 ; and (4) resistive ballooning modes which are purely growing with growth rate proportional to η 1 3 β 2 3 as η → 0 and β → 0.

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Topics: Resistive touchscreen (50%)

415 Citations