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2011
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2010
Instabilities of Alfvén Eigenmodes (AEs) are often excited by a super-Alfvénic population of fast ions accelerated with ion cyclotron resonance heating (ICRH) or produced by neutral beam injection (NBI) in present-day tokamaks. In view of the next-step burning plasma experiments with super-Alfvénic alpha-particles, the experimental data on AE in…
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2010
Recently, Bayesian probability theory has been used at a number of experiments to fold uncertainties and interdependencies in the diagnostic data and forward models, together with prior knowledge of the state of the plasma, to increase accuracy of inferred physics variables. A new probabilistic framework, MINERVA, based on Bayesian graphical models…
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2010
Plasma rotation in tokamaks is of special interest for its potential stabilising effect on micro- and macro instabilities, leading to increased confinement. In MAST, the torque from Neutral Beam Injection (NBI) can spin the plasma to a core velocity ~ 300km.s -1 (Alfven Mach number ~ 0.3). Low density plasmas often exhibit a weakly non-monotonic sa…
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2010
A fully nonlinear model is developed for the bump-on-tail instability including the effects of dynamical friction drag and velocity space diffusion on the energetic particles driving the wave. The results show that drag provides a destabilizing effect on the nonlinear evolution of waves. Specifically, in the early nonlinear phase of the instability…
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2010
MAST plasmas with a safety factor above unity and a profile with either weakly reversed shear, or broad low shear regions, regularly exhibit long-lived saturated ideal magnetohydrodynamic (MHD) instabilities. The toroidal rotation is flattened in the presence of such perturbations and the fast ion losses are enhanced. These ideal modes, distinguish…
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2009
Over the past two decades, the increase in neutral-beam heating and particle production in magnetically confined fusion plasmas has led to an increase in energetic particle driven mode activity, much of which has an electromagnetic signature which can be detected by the use of external Mirnov coils. Typically, the frequency and spatial wave number …
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2009
In respect of their symmetry properties, toroidal magnetically confined plasmas have much in common with the Taylor-Couette flow. A symmetry-based analysis (equivalent bifurction theory) has proved very powerful in the analysis of the latter problem. This Letter discusses the applicability of the method to nuclear fusion experiments such as tokamak…
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2009
The nonlinear evolution of waves excited by the resonant interaction with energetic particles, just above the instability threshold, is shown to depend on the type of relaxation process that restores the unstable distribution function. When dynamical friction dominates over diffusion in the phase space region surrounding the wave-particle resonance…
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2009
Recent observations of magnetic fluctuation activity in the Mega Ampere Spherical Tokamak (MAST) reveal the presence of plasmas with bands of both low and high frequency magnetic fluctuations. Such plasmas exhibit a spectrum of low frequency modes with adjacent toroidal mode numbers, for which the measured frequency is near the Doppler shifted rota…
Showing 91 - 100 of 147 UKAEA Paper Results