D Dickinson C M Roach J M Skipp H R Wilson
Solutions to a model global 2D eigenmode equation describing micro-instabilities in tokamak plasmas are presented that demonstrate a sensitivity of the mode structure and stability to plasma profiles. In narrow regions of parameter space, with special plasma profiles, a maximally unstable mode is found that balloons on the outboard side of the toka…
PublishedE. G. Highcock A. A. Schekochihin S. C. Cowley M. Barnes F. I. Parra C. M. Roach W. Dorland
Sheared toroidal flows can cause bifurcations to zero-turbulent-transport states in tokamak plasmas. The maximum temperature gradients that can be reached are limited by subcritical turbulence driven by the parallel velocity gradient. Here it is shown that magnetic field pitch/inverse aspect ratio is a critical control parameter for sheared tokamak…
PublishedS. Saarelma P. Hill A. Bottino G. Colyer A.R. Field B. Mcmillan A. Peeters C.M. Roach Mast Team
Electrostatic gyrokinetic analyses are presented for an L-mode discharge with an internal transport barrier, from the spherical tokamak, MAST. Local and global microstability analysis finds similar linear growth rates for ion temperature gradient (ITG) driven modes. When the electron response is assumed to be adiabatic, growth rates are found to be…
PublishedD. Dickinson C. M. Roach S. Saarelma R. Scannell A. Kirk H. R. Wilson
Plasma equilibria reconstructed from the Mega-Amp Spherical Tokamak have sufficient resolution to capture plasma evolution during the short period between edge-localized modes (ELMs). Immediately after the ELM, steep gradients in pressure, P , and density, n e , form pedestals close to the separatrix, and they then expand into the core. Local gyrok…
PublishedM. Barnes F.I. Parra E.G. Highcock A.A. Schekochihin S.C. Cowley C.M. Roach
Nonlinear gyrokinetic simulations are conducted to investigate turbulent transport in tokamak plasmas with rotational shear. At sufficiently large flow shears, linear instabilities are suppressed, but transiently growing modes drive subcritical turbulence whose amplitude increases with flow shear. This leads to a local minimum in the heat flux, ind…
PublishedD. Dickinson S. Saarelma R. Scannell A.Kirk C.M.Roach H.R.Wilson
Pedestal profiles that span the ELM cycle have been obtained and used to test the idea that the pedestal pressure gradient in MAST is limited by the onset of Kinetic Ballooning Modes (KBMs). During the inter-ELM period of a regularly type I ELM-ing discharge on MAST, the pressure pedestal height and width increase together while the pressure gradie…
PublishedE. G. Highcock M. Barnes F. I. Parra A. A. Schekochihin C. M. Roach et al.
First-principles numerical simulations are used to describe a transport bifurcation in a differentially rotating tokamak plasma. Such a bifurcation is more probable in a region of zero magnetic shear than one of finite magnetic shear, because in the former case the component of the sheared toroidal flow that is perpendicular to the magnetic field h…
PublishedE. G. Highcock M. Barnes A. A. Schekochihin F. I. Parra C. M. Roach S.C. Cowley
The effect of flow shear on turbulent transport in tokamaks is studied numerically in the experimentally relevant limit of zero magnetic shear. It is found that the plasma is linearly stable for all nonzero flow shear values, but that subcritical turbulence can be sustained nonlinearly at a wide range of temperature gradients. Flow shear increases …
PublishedS. A. Arshad J. G. Cordey D. C. Mcdonald J. Farthing E. Joffrin M. Von Hellermann C. M. Roach J. Svensson
Principal techniques and trends in the validation and analysis of data in magnetic fusion research are described and examples of applications are given. Wellestablished methods to obtain key physical quantities are outlined, as well as newer techniques employing integrated analysis of multiple diagnostics to improve quality and extract additional i…
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