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Multiple space and time scales arise in plasma turbulence in magnetic confinement fusion devices because of the smallness of the square root of the electron-to-ion mass ratio and the consequent disparity of the ion and electron thermal gyroradii and thermal speeds. Direct simulations of this turbulence that include both ion and electron space–time scales indicate that there can be significant...
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It is shown that in low-beta, weakly collisional plasmas, such as the solar corona, some instances of the solar wind, the aurora, inner regions of accretion discs, their coronae and some laboratory plasmas, Alfvénic fluctuations produce no ion heating within the gyrokinetic approximation, i.e. as long as their amplitudes (at the Larmor scale) are small and their frequencies stay below the...
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The strong, sharp flow structures that are seen frequently in tokamak cores, and large amplitude spontaneous global toroidal rotation are both surprising in light of current theories where toroidal flow evolution is dominantly diffusive. Mechanisms for spontaneously generating strong poloidal shear flows have been extensively investigated, but these processes were thought not to apply to...
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Planar periodic surface lattice (PSL) structures based on thin, subwavelength substrates have been studied experimentally and numerically. Coupled eigenmode resonances composed of partial volume and surface modes are observed for PSLs with lattice periodicities of 1.50 mm and 1.62 mm etched onto thin copper-backed, substrates. We show that the copper backing is essential for mode-selection in...
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The cold ion limit of the local gyrokinetic model is rigorously taken to produce a nonlinear system of fluid equations that includes background flow shear. No fluid closure is required. By considering a simple slab geometry with magnetic drifts, but no magnetic shear, these fluid equations reduce to the Charney–Hasegawa–Mima model in the presence of flow shear. Analytic solutions to this model...
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This paper describes a model of electron energization and cyclotron-maser emission applicable to astrophysical magnetized collisionless shocks. It is motivated by the work of Begelman, Ergun and Rees [Astrophys. J. 625, 51 (2005)] who argued that the cyclotron-maser instability occurs in localized magnetized collisionless shocks such as those expected in blazar jets. We report on recent...
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Spatially non-local aspects of turbulent transport in tokamak plasmas are examined with global gyrokinetic simulations using the ORB5 code. Inspired by very accurate measurements in the TCV tokamak in L-mode, we initialise plasma profiles with constant logarithmic gradients in the core and constant linear gradients in the ‘pedestal’ (). The main finding is that transport in the core is...
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A numerical simulation is presented concerning an L/O mode electromagnetic wave propagating normally into an overdense magnetised plasma with a smooth density gradient leading to excitation of Langmuir turbulence in the vicinity of the reflection point. The simulation parameters are chosen to represent an ionospheric radio frequency heating experiment but may have relevance to other...
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We propose that pressure anisotropy causes weakly collisional turbulent plasmas to self-organize so as to resist changes in magnetic-field strength. We term this effect ‘magneto-immutability’ by analogy with incompressibility (resistance to changes in pressure). The effect is important when the pressure anisotropy becomes comparable to the magnetic pressure, suggesting that in collisionless,...
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The tokamak is the most advanced approach to fusion and is approaching operation under power-plant conditions, promising sustainable, low-emission, baseload power to the grid. As the heating power of a tokamak is increased above a threshold, the plasma suddenly bifurcates to a state of high confinement, creating a region of plasma with a large pressure gradient at its edge. This bifurcation...
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Microwave undulators have great potential to be used in short-wavelength free-electron lasers. In this paper, the properties of a corrugated waveguide and its performance as an undulator cavity for a UK X-ray free-electron laser were systematically studied. The equations presented in this paper allow a fast estimation of the dimensions of the corrugated waveguide. An undulator cavity operating...
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Differential rotation is induced in tokamak plasmas when an underlying symmetry of the governing gyrokinetic-Maxwell system of equations is broken. One such symmetry-breaking mechanism is considered here: the turbulent acceleration of particles along the mean magnetic field. This effect, often referred to as the ‘parallel nonlinearity’, has been implemented in the δf gyrokinetic code stella...
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Linear accelerators operating at millimeter or sub-terahertz frequencies and short pulse duration have the advantages of lower power consumption and high repetition rate. In this paper planar metallic accelerating structures with different modes operating at 210 GHz were designed. A tolerance study was also carried out to determine the sensitivities of the geometric parameters to the wakefield...
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A study of turbulent impurity transport by means of quasilinear and nonlinear gyrokinetic simulations is presented for Wendelstein 7-X (W7-X). The calculations have been carried out with the recently developed gyrokinetic code stella. Different impurity species are considered in the presence of various types of background instabilities: ion temperature gradient (ITG), trapped electron mode...
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Exhaust power components due to ELMs, radiation and heat transport across the edge transport barrier (ETB) between ELMs are quantifed for H-mode plasmas in JET-C and JET-ILW for comparison with simulations of pedestal heat transport. In low-current, JET-ILW pulses with a low rate of gas fuelling, the pedestal heat transport is found not to be stiff, i.e. the effective, mean heat diffusivity...
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