| 1. |
- Lilley, Matthew, 1983, et al.
(author)
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Collective fast ion effects in tokamaks
- 2010
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In: Meeting of the Swedish Research Unit, RUSA, November 10-11 2010, Stockholm.
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Conference paper (other academic/artistic)
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| 2. |
- Nyqvist, Robert, 1983, et al.
(author)
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Alfvén Waves in Tokamaks
- 2010
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In: Meeting of the Swedish Research Unit, RUSA 2010, November 10 – 11 2010, Stockholm, Sweden.
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Conference paper (other academic/artistic)
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| 3. |
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| 4. |
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| 5. |
- Nyqvist, Robert, 1983, et al.
(author)
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Fast Ion Driven Alfvén Eigenmodes within the q=1 Radius
- 2009
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In: 36th European Physical Society Conference on Plasma Physics 2009, EPS 2009; Sofia; Bulgaria; 29 June 2009 through 3 July 2009. - 9781622763368 ; 33 E2, s. 1527-1530
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Conference paper (peer-reviewed)
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| 6. |
- Nyqvist, Robert, 1983, et al.
(author)
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On wave-particle interaction in axisymmetric toroidal systems
- 2011
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In: Physica Scripta. - : IOP Publishing. - 1402-4896 .- 0031-8949. ; 84:1, s. 015503-
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Journal article (peer-reviewed)abstract
- A general formalism is developed to describe the interaction of charged particles with electromagnetic waves in terms of coupled finite difference mapping equations that incorporate tokamak topology. The approach is based on considering non-adiabatic changes in the constants of particle motion and it covers a range of wave-particle resonance frequencies, from the precessional to cyclotron frequencies of both passing and trapped ions. The concept of overlapping resonances is used to estimate the threshold for a single plane wave to cause stochastic particle motion. In the stochastic regime, the process is Markovian, and particle diffusion in three-dimensional phase space takes place. Estimations of diffusion coefficients are carried out in the two cases of waves interacting with passing and trapped ions by means of the cyclotron and bounce resonances, respectively, and previously known results are recovered in the proper limits.
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| 7. |
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| 8. |
- Nyqvist, Robert, 1983, et al.
(author)
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Radiative Damping of Low Shear Toroidal Alfvén Eigenmodes
- 2010
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In: 37th EPS Conference on Plasma Physics, June 21 – 25 2010, Dublin, Ireland.
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Conference paper (peer-reviewed)abstract
- An important issue for alpha-particle transport due to Alfvén instabilities in burningITER plasmas [1] is the number of unstable Alfvén eigenmodes (AEs) and their properties at high plasma pressure. Multiple low shear toroidal Alfv´en eigenmodes (LSTAEs) [2, 3, 4] are of major concern for ITER scenarios, where extended regions of low magnetic shear may exist in e.g. sawtoothing plasmas or hybrid regimes [1, 5]. In contrast to the conventional TAE, which has a single eigenfrequency per TAE gap, the number of LSTAEs per gap is given by l ~= Є/S [4], and can be quite large in a low-shear region.Moreover, LSTAEs are less affected by high plasma pressure than TAEs: in fact, the very discovery of LSTAEs was galvanised by DT experiments on TFTR, where unstable AEs were detected at very high plasma pressure [2]. More recently, LSTAEs were recognized as important features of sawtoothing tokamak plasmas. On JET, core localized ”tornado” modes (LSTAEs inside the q = 1 radius) were found to precede monster sawtooth crashes [6], and on Alcator C-Mod, frequency chirping AEs associated with very low shear were observed during the sawtooth cycle [7].In this contribution, we develop a theory of LSTAEs by incorporating non-ideal effects associated with finite ion Larmor gyroradius into the MHD model [4]. The radiativedamping for LSTAEs at the top and bottom of the TAE gap is calculated in the limit of high mode number and first order gyroradius, using the characteristic LSTAE ordering d/dr > m/r. We also estimate the LSTAE drive due to ICRH-accelerated fast ions [8] and obtain the mode excitation threshold for ICRH-driven TAEs.
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| 9. |
- Nyqvist, Robert, 1983, et al.
(author)
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Spreading of collimated particle beams within a generalized Fokker-Planck diffusion equation
- 2009
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In: Nuclear Science and Engineering. - 0029-5639 .- 1943-748X. ; 163:1, s. 85-90
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Journal article (peer-reviewed)abstract
- Recently, an expansion of the Boltzmann scattering operator describing the angular spreading of particle beams was given that included the effects of large angle scattering processes, thus generalizing the classical Fokker-Planck equation, valid in the limit of small angle scattering. The present work aims at making an analytical comparison between predictions based on the classical Fokker-Planck equation and those based on a generalized one, which includes a first-order correction term in the expansion of the Boltzmann scattering operator. The analysis is carried out for thin slabs where backscattering effects can be neglected and makes use of a moment approach, which leads to an infinite system of recursively coupled ordinary differential equations. The system is truncated in a consistent manner, and the effects of large angle scattering on the evolution of the moments are determined in explicit analytical form. An approximate similarity solution of the generalized Fokker-Planck equation is also found, and the results of both approaches provide a clear picture of the increased diffusive beam spreading due to large angle scattering. A comparison with previously published Monte Carlo simulation results shows good agreement.
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| 10. |
- Sharapov, S.E., et al.
(author)
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Energetic particle instabilities in fusion plasmas
- 2013
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In: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 53:10, s. 104022-
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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.
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