| 21. |
- 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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| 22. |
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| 23. |
- Lloyd, B., et al.
(författare)
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Overview of physics results from MAST
- 2011
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 51:9, s. 094013 (paper no.)-
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Tidskriftsartikel (refereegranskat)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%.
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| 24. |
- Mader, P., et al.
(författare)
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Experimental validation of fringing field effects for the multipactor phenomenon
- 2012
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Ingår i: 15th International Symposium on Antenna Technology and Applied Electromagnetics, ANTEM 2012, Toulouse, 25-28 June 2012. - 9781467302913
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Konferensbidrag (refereegranskat)abstract
- The multipactor effect is a constraining phenomenon in the design of RF components. Due to an increasing number of channels and power levels for multicarrier signals, specific requirements of design must be taken into account in order to prevent the multipactor effect. The current methods of analysis are based on the Hatch and Williams diagrams, which provide the multipaction threshold voltage for a parallel plate geometry as a function of the frequency gap product. However, whereas experimental results have shown good correlation with the theoretical charts for the parallel plate geometry, discrepancies have been found for complex geometries such as irises.
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| 25. |
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| 26. |
- Nyqvist, Robert, 1983, et al.
(författare)
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Alfvén Waves in Tokamaks
- 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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| 27. |
- 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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| 28. |
- Nyqvist, Robert, 1983, et al.
(författare)
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On wave-particle interaction in axisymmetric toroidal systems
- 2011
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Ingår i: Physica Scripta. - : IOP Publishing. - 1402-4896 .- 0031-8949. ; 84:1, s. 015503-
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Tidskriftsartikel (refereegranskat)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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| 29. |
- Nyqvist, Robert, 1983, et al.
(författare)
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Radiative Damping of Low Shear Toroidal Alfvén Eigenmodes
- 2010
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Ingår i: 37th EPS Conference on Plasma Physics, June 21 – 25 2010, Dublin, Ireland.
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Konferensbidrag (refereegranskat)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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| 30. |
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