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Electromagnetic (EM) instabilities and turbulence driven by the electron-temperature gradient (ETG) are considered in a local slab model of a tokamak-like plasma. Derived in a low-beta asymptotic limit of gyrokinetics, the model describes perturbations at scales both larger and smaller than the electron inertial length $d_e$ ...
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Tokamak pedestal electron temperature gradient (ETG) turbulence has a complex three-dimensional structure that is highly inhomogeneous in the poloidal direction, particularly at ion scales. Nonlinear multiscale gyrokinetic simulations of Joint European Torus pedestals reveal that ETG pedestal turbulence has maximum fluctuation amplitudes and transport near the top and bottom of a flux surface,...
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We formulate a generalized periodic boundary condition as a limit of the standard twist-and-shift parallel boundary condition that is suitable for simulations of plasmas with low magnetic shear. This is done by applying a phase shift in the binormal direction when crossing the parallel boundary. While this phase shift can be set to zero without loss of generality in the local flux-tube limit...
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Microtearing instability is one of the major sources of turbulent transport in high-β tokamaks. These modes lead to very localized transport at low-order rational magnetic field lines, and we show that flattening of the local electron temperature gradient at these rational surfaces plays an important role in setting the saturated flux level in microtearing turbulence. This process depends...
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In magnetic confinement fusion devices, the ratio of the plasma pressure to the magnetic field energy, beta, can become sufficiently large that electromagnetic microinstabilities become unstable, driving turbulence that distorts or reconnects the equilibrium magnetic field. In this paper, a theory is proposed for electromagnetic, electron-driven linear instabilities that have current layers...
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High-power microwave beams used for heating and current drive in magnetically confined fusion plasmas can be broadened significantly by plasma turbulence, negatively impacting the efficiency of the machine. The dependence of this beam broadening on plasma and beam parameters is not yet fully understood, particularly where the dependence on one parameter is not separable from the dependence on...
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The properties of the edge ion-scale turbulence are studied using the beam emission spectroscopy (BES) diagnostic on MAST. Evidence of the formation of large-scale high-amplitude coherent structures, filamentary density blobs and holes, 2-4 cm inside the plasma separatrix is presented. Measurements of radial velocity and skewness of the density fluctuations indicate that density holes...
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The first pedestal stability and structure analysis on the new MAST Upgrade (MAST-U) spherical tokamak H-mode plasmas is presented. Our results indicate that MAST-U pedestals are close to the low toroidal mode number (n) peeling branch of the peeling-ballooning instability, in contrast with MAST H-mode pedestals which were deeply in the high-n ballooning branch. This offers the possibility of...
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The properties of supersonic, compressible plasma turbulence determine the behavior of many terrestrial and astrophysical systems. In the interstellar medium and molecular clouds, compressible turbulence plays a vital role in star formation and the evolution of our galaxy. Observations of the density and velocity power spectra in the Orion B and Perseus molecular clouds show large deviations...
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Linear perturbation theory is used to model the ideal magnetohydrodynamic stability of tokamak equilibria under the application of external 3D magnetic perturbations (Hegna 2014 Phys. Plasmas 21 072502). We use the ELITE code (Wilson et al 2002 Phys. Plasmas 9 1277) to produce both a linear plasma response, as well as the linear axisymmetric toroidal eigenmodes which are used as basis...
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The high heat fluxes to the divertor during edge localised mode (ELM) instabilities have to be reduced for a sustainable future tokamak reactor. A solution to reduce the heat fluxes could be the Super-X divertor, which will be tested on MAST-U. ELM simulations for MAST-U Super-X tokamak plasmas have been obtained, using JOREK. A factor 10 decrease in the peak heat flux to the outer target and...
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The poloidal harmonics of the toroidal normal modes of an unstable axisymmetric tokamak plasma are employed as basis functions for the minimisation of the 3D energy functional. This approach presents a natural extension of the perturbative method considered in Anastopoulos Tzaniset al(2019Nucl. Fusion59126028). This variational formulation is applied to the stability of tokamak plasmas subject...
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Ion-gyroradius-scale microinstabilities typically have a frequency comparable to the ion transit frequency. Due to the small electron-to-ion mass ratio and the large electron transit frequency, it is conventionally assumed that passing electrons respond adiabatically in ion-gyroradius-scale modes. However, in gyrokinetic simulations of ion-gyroradius-scale modes in axisymmetric toroidal...
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Nonlinear multiscale gyrokinetic simulations of a Joint European Torus edge pedestal are used to show that electron-temperature-gradient (ETG) turbulence has a rich three-dimensional structure, varying strongly according to the local magnetic-field configuration. In the plane normal to the magnetic field, the steep pedestal electron temperature gradient gives rise to anisotropic turbulence...
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