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Micro-instabilities, turbulence, and zonal flows are central to achievable tokamak performance. Knowledge of their poloidal structure would improve theoretical models and experimental interpretation. The poloidal structure of micro-instabilities may be altered by global effects. Global simulations can be computationally expensive, but global behaviour can be recovered from an array of local...
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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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An investigation on the onset of low n kinetic ballooning mode (KBM) in a plasma environment relevant to the spherical reactors is carried out. The low n modes can be quite global in nature and can set an upper limit on the achievable β, a critical parameter for better performance and energy production in spherical reactors. The gyrokinetic simulations for a set of numerically generated...
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We present new results of multi-scale simulations of Langmuir turbulence during heating experiments where a powerful electromagnetic wave is injected into the overhead ionosphere where it excites parametric instabilities leading to turbulence on multiple spatial and temporal scales. The simulations are carried out using a generalized Zakharov model [1, 2] in which the complex phase and...
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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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An electromagnetic wave employed to introduce energy into plasma non-linearly exchanges energy via complex interactions with particles and plasma wave modes. Examples include Raman scattering where two electromagnetic waves are coupled via a Langmuir wave and Brillouin scattering where the coupling is via an ion-acoustic wave. One electromagnetic wave may be externally driven with the...
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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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