| 1. |
- Eriksson, Frida, 1986, et al.
(author)
-
Kinetic theory of phase space plateaux in a non-thermal energetic particle distribution
- 2015
-
In: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 22:9, s. art.no. 092126-
-
Journal article (peer-reviewed)abstract
- The transformation of kinetically unstable plasma eigenmodes into hole-clump pairs with temporally evolving carrier frequencies was recently attributed to the emergence of an intermediate stage in the mode evolution cycle, that of an unmodulated plateau in the phase space distribution of fast particles. The role of the plateau as the hole-clump breeding ground is further substantiated in this article via consideration of its linear and nonlinear stability in the presence of fast particle collisions and sources, which are known to affect the production rates and subsequent frequency sweeping of holes and clumps. In particular, collisional relaxation, as mediated by e.g. velocity space diffusion or even simple Krook-type collisions, is found to inhibit hole-clump generation and detachment from the plateau, as it should. On the other hand, slowing down of the fast particles turns out to have an asymmetrically destabilizing/stabilizing effect, which explains the well-known result that collisional drag enhances holes and their sweeping rates but suppresses clumps. It is further demonstrated that relaxation of the plateau edge gradients has only a minor quantitative effect and does not change the plateau stability qualitatively, unless the edge region extends far into the plateau shelf and the corresponding Landau pole needs to be taken into account.
|
|
| 2. |
|
|
| 3. |
|
|
| 4. |
- Lilley, Matthew, 1983, et al.
(author)
-
Collective fast ion effects in tokamaks
- 2010
-
In: Meeting of the Swedish Research Unit, RUSA, November 10-11 2010, Stockholm.
-
Conference paper (other academic/artistic)
|
|
| 5. |
- Lilley, Matthew, 1983, et al.
(author)
-
Drag induced energetic particle modes
- 2011
-
In: International Sherwood Fusion Theory Conference, Austin, Texas, May 2 – 4, 2011.
-
Conference paper (peer-reviewed)
|
|
| 6. |
- Nyqvist, Robert, 1983, et al.
(author)
-
Adiabatic Description of Long Range Frequency Sweeping
- 2012
-
In: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 52:9
-
Journal article (peer-reviewed)abstract
- A theoretical framework is developed to describe long range frequency sweeping events in the 1D electrostatic bump-on-tail model with fast particle sources and collisions. The model includes three collision operators (Krook, drag (dynamical friction) and velocity space diffusion), and allows for a general shape of the fast particle distribution function. The behaviour of phase space holes and clumps is analysed in the absence of diffusion, and the effect of particle trapping due to separatrix expansion is discussed. With a fast particle distribution function whose slope decays above the resonant phase velocity, hooked frequency sweeping is found for holes in the presence of drag collisions alone.
|
|
| 7. |
|
|
| 8. |
- Nyqvist, Robert, 1983, et al.
(author)
-
Modeling of Long-Range Sweeping Phenomena
- 2011
-
In: Joint US-EU TTF Workshop, San Diego, California, April 6 – 9, 2011 (oral presentation).
-
Conference paper (peer-reviewed)
|
|
| 9. |
|
|
| 10. |
- Sharapov, S.E., et al.
(author)
-
Energetic particle instabilities in fusion plasmas
- 2013
-
In: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 53:10, s. 104022-
-
Journal article (peer-reviewed)abstract
- Remarkable progress has been made in diagnosing energetic particle instabilities on present-day machines and in establishing a theoretical framework for describing them. This overview describes the much improved diagnostics of Alfvén instabilities and modelling tools developed world-wide, and discusses progress in interpreting the observed phenomena. A multi-machine comparison is presented giving information on the performance of both diagnostics and modelling tools for different plasma conditions outlining expectations for ITER based on our present knowledge.
|
|
| 11. |
|
|
| 12. |
- Lilley, Matthew, 1983, et al.
(author)
-
Effect of dynamical friction on nonlinear energetic particle modes
- 2010
-
In: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 17:9, s. Art. no. 092305-
-
Journal article (peer-reviewed)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.
|
|
| 13. |
- Lilley, Matthew, 1983, et al.
(author)
-
Nonlinear evolution of beam driven waves on MAST
- 2010
-
In: IAEA Fusion Energy Conference, Daejon, South Korea 11 - 16 October 2010.
-
Conference paper (peer-reviewed)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.
|
|
| 14. |
- Lloyd, B., et al.
(author)
-
Overview of physics results from MAST
- 2011
-
In: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 51:9, s. 094013 (paper no.)-
-
Journal article (peer-reviewed)abstract
- Major developments on the Mega Amp Spherical Tokamak (MAST) have enabled important advances in support of ITER and the physics basis of a spherical tokamak (ST) based component test facility (CTF), as well as providing new insight into underlying tokamak physics. For example, L-H transition studies benefit from high spatial and temporal resolution measurements of pedestal profile evolution (temperature, density and radial electric field) and in support of pedestal stability studies the edge current density profile has been inferred from motional Stark effect measurements. The influence of the q-profile and E x B flow shear on transport has been studied in MAST and equilibrium flow shear has been included in gyro-kinetic codes, improving comparisons with the experimental data. H-modes exhibit a weaker q and stronger collisionality dependence of heat diffusivity than implied by IPB98(gamma, 2) scaling, which may have important implications for the design of an ST-based CTF. ELM mitigation, an important issue for ITER, has been demonstrated by applying resonant magnetic perturbations (RMPs) using both internal and external coils, but full stabilization of type-I ELMs has not been observed. Modelling shows the importance of including the plasma response to the RMP fields. MAST plasmas with q > 1 and weak central magnetic shear regularly exhibit a long-lived saturated ideal internal mode. Measured plasma braking in the presence of this mode compares well with neo-classical toroidal viscosity theory. In support of basic physics understanding, high resolution Thomson scattering measurements are providing new insight into sawtooth crash dynamics and neo-classical tearing mode critical island widths. Retarding field analyser measurements show elevated ion temperatures in the scrape-off layer of L-mode plasmas and, in the presence of type-I ELMs, ions with energy greater than 500 eV are detected 20 cm outside the separatrix. Disruption mitigation by massive gas injection has reduced divertor heat loads by up to 70%.
|
|
| 15. |
|
|
