| 1. |
- Lilley, Matthew, 1983, et al.
(författare)
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Collective fast ion effects in tokamaks
- 2010
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Ingår i: Meeting of the Swedish Research Unit, RUSA, November 10-11 2010, Stockholm.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 2. |
- Nyqvist, Robert, 1983, et al.
(författare)
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Collisional Nonlinear Energetic Particle Modes
- 2010
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Ingår i: Meeting of the Swedish Research Unit, RUSA 2010, November 10 – 11 2010, Stockholm, Sweden.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 3. |
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| 4. |
- Lilley, Matthew, 1983, et al.
(författare)
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Effect of dynamical friction on nonlinear energetic particle modes
- 2010
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 17:9, s. Art. no. 092305-
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Tidskriftsartikel (refereegranskat)abstract
- 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, the drag facilitates the explosive scenario of the wave evolution, leading to the creation of phase space holes and clumps that move away from the original eigenfrequency. Later in time, the electric field associated with a hole is found to be enhanced by the drag, whereas for a clump it is reduced. This leads to an asymmetry of the frequency evolution between holes and clumps. The combined effect of drag and diffusion produces a diverse range of nonlinear behaviors including hooked frequency chirping, undulating, and steady state regimes. An analytical model is presented, which explains the aforementioned diversity. A continuous production of hole-clump pairs in the absence of collisions is also observed.
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| 5. |
- Lilley, Matthew, 1983, et al.
(författare)
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Nonlinear evolution of beam driven waves on MAST
- 2010
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Ingår i: IAEA Fusion Energy Conference, Daejon, South Korea 11 - 16 October 2010.
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Konferensbidrag (refereegranskat)abstract
- Experiments on Alfvénic instabilities driven by super-Alfvénic beams in the spherical tokamak MAST have exhibited a variety of modes excited in a broad range of frequencies from Alfvén Cascade eigenmodes, Toroidal Alfvén Eigenmodes, and chirping modes in thefrequency range 50 - 150 kHz, to compressional Alfvén eigenmodes in the frequency range 0.4 - 3.8 MHz, which is approaching the cyclotron frequency of plasma ions, ω ≈(0.1 ÷ 1)ωBi. For energetic ions produced via neutral beam injection, the unstable distribution function is formed by Coulomb collisions, with dynamical friction (drag) and velocity space diffusion dominating in different regions of phase space (separated by the critical velocity Vcrit).The aim of the present work is to demonstrate that the nonlinear evolution of these modes is determined by the type and strength of relaxation processes. In particular, we validate the recent theoretical finding that drag encourages the beam-driven waves near marginalstability to follow an explosive scenario. An effcient numerical tool has now been created for this task, the results of which show similarities with features that are observed in recent and past experiments, indicating the central role that the drag might play in the evolution of beam or alpha particle driven waves in tokamaks. This has galvanised the current ongoing effort to perform quantitative modelling of Alfvénic instabilities in thepresence of drag using the HAGIS code. The universal feature of these strongly nonlinear scenarios that incorporate drag is the asymmetry of the mode evolution with respect to the wave-particle resonance. As a result we observe a transition from a steady statenon-linear wave to one with a hooked frequency chirp.To connect the bump-on-tail model to the experiments we characterise the relative importance of the drag and the velocity space diffusion by considering the wave particleresonance condition, Ω = k|| V||res - ω + pωBb - kVDb = 0, and calculate the width of the resonance due to diffusion ∆Ωdiff and drag ∆Ωdrag from the the Fokker-Planck operatorfor different types of modes. The importance of the parameter V||res/Vcrit is assessed.
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