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Improved models for neoclassical tearing modes and anomalous transport are developed and validated within integrated modeling codes to predict toroidal rotation, temperature and current density profiles in tokamak plasmas. Neoclassical tearing modes produce helical filaments of plasma, called magnetic islands, which have the effect of degrading tokamak plasma confinement or terminating the discharge. An improved code is developed in order to compute the widths of multiple simultaneous magnetic islands whose shapes are distorted by the radial variation in the magnetic perturbation [F. D. Halpern, et al., J. Plasma Physics 72 (2006) 1153]. It is found in simulations of DIII-D and JET tokamak discharges that multiple simultaneous magnetic islands produce a 10% to 20% reduction in plasma thermal confinement. If magnetic islands are allowed to grow to their full width in ITER fusion reactor simulations, fusion power production is reduced by a factor of four [F. D. Halpern, et al., Phys. Plasmas 13 (2006) 062510]. In addition to improving the prediction of neoclassical tearing modes, a new Multi-Mode transport model, MMM08, was developed to predict temperature and toroidal angular frequency profiles in simulations of tokamak discharges. The capability for predicting toroidal rotation is motivated by ITER simulation results that indicate that the effects of toroidal rotation can increase ITER fusion power production [F. D. Halpern et al., Phys. Plasmas 15 (2008), 062505]. The MMM08 model consists of an improved model for transport driven by ion drift modes [F. D. Halpern et al., Phys. Plasmas 15 (2008) 012304] together with a model for transport driven by short wavelength electron drift modes combined with models for transport driven by classical processes. The new MMM08 transport model was validated by comparing predictive simulation results with experimental data for 32 discharges in the DIII-D and JET tokamaks. It was found that the prediction of intrinsic plasma rotation is consistent with experimental measurements in discharges with zero net torque. A scaling relation was developed for the toroidal momentum confinement time (angular momentum divided by net torque) as a function of plasma current and torque per ion.
Models for large scale instabilities in tokamaks are developed, and they are tested together with models for transport and other physical processes by comparing the results of BALDUR integrated predictive modeling simulations with experimental data from tokamaks. Simulations are carried out for the low aspect ratio tokamaks, Mega Ampere Spherical Tokamak and the National Spherical Torus Experiment, to test the applicability of models that were developed for conventional tokamaks. The results indicate that neoclassical transport dominates over anomalous transport in the inner third of the plasma. Sawtooth oscillations, which are the result of an instability that periodically redistributes the central part of the plasma profiles, play a significant role by radially spreading the neutral beam injection heating profiles across the broad sawtooth mixing region. Two models for sawtooth oscillations, the Porcelli and the Kadomtsev models, are combined and tested against experimental data from the Joint European Torus and the Tokamak Fusion Test Reactor. Most of the sawtooth crashes are triggered by the m = 1 resistive internal kink mode in the semi-collisional regime. The median sawtooth period increases with increasing magnetic reconnection fraction. The best overall agreement with experimental data is obtained with a magnetic reconnection fraction of approximately 55%. Finally, plasma pressure effects are included in a quasi-linear model for saturated tearing modes, which are instabilities that produce magnetic islands. The model is used in a stand-alone code, and in the BALDUR code, to calculate the widths of saturated magnetic islands in tokamak plasmas with arbitrary cross-section and plasma pressure. The widths of tearing mode islands increase with decreasing aspect ratio and with increasing elongation. Also, the island widths increase when the gradient of the current density increases at the edge of the islands or when the current density inside the islands is suppressed. The widths oscillate in time in response to periodic sawtooth crashes. Local enhancements in the transport produced by magnetic islands have a noticeable effect on global plasma confinement in simulations of low aspect ratio, high beta tokamaks, where saturated tearing mode islands can occur with widths greater than 15% of the plasma minor radius.
This is a graduate textbook on tokamak physics, designed to provide a basic introduction to plasma equilibrium, particle orbits, transport, and those ideal and resistive magnetohydrodynamic instabilities which dominate the behavior of a tokamak discharge, and to develop the mathematical methods necessary for their theoretical analysis.
This book describes the advanced stability theories for magnetically confined fusion plasmas, especially in tokamaks. As the fusion plasma sciences advance, the gap between the textbooks and cutting-edge researches gradually develops. This book fills in
High confinement (H-mode) discharges in tokamak experiments are characterized by a narrow region of steep pressure gradient called the "pedestal" that forms at the edge of the plasma, and often by an instability called an "Edge Localized Mode (ELM)" that periodically removes energy and particles from the edge of the plasma. The parameters at the top of the pedestal and the characteristics of ELMs have a strong influence on the performance of H-mode discharges. In this study, models for the pedestal are developed initially without including the effects of ELMs. The predictions from these pedestal models yield reasonable agreement with experimental data. These pedestal models are then used to provide boundary conditions in an integrated modeling code in order to simulate plasma profiles, such as temperature and density profiles, in existing H-mode experiments. The simulated profiles obtained using predictive boundary conditions and those obtained using experimental boundary conditions have similar agreement with experimental profiles. A more advanced pedestal model with a dynamic model for ELM crashes is developed using a concept of turbulence suppression at the edge of plasma. These pedestal and ELM models are coupled with a core transport model in an integrated modeling code. An advantage of this approach is that it allows for the evolution of plasma pressure and current profiles in the pedestal region, which can lead to an instability that triggers ELM crashes that limit the growth of the pedestal. Pressure-driven ballooning and current-driven peeling instabilities are considered as the possible instabilities that trigger ELM crashes. The combined core and pedestal models, with the effect of ELMs included, are used to study the dependence of triangularity and heating power on the pedestal. An MHD stability analysis is also performed to confirm the validity of these simulations. It is found that plasma edge stability improves as triangularity increases, as a result of access to the second stability region of ballooning modes. Finally, simulations yield a pedestal height with a dependence on heating power similar to that observed experimentally when the ELM crash model is extended to include ELMS triggered by current-driven peeling instabilities.
In addressing the general issue of anomalous energy transport, this paper reports on results of theoretical studies concerning: (1) the characteristics and relative strength of the dominant kinetic instabilities likely to be present under realistic tokamak operating conditions; (2) specific nonlinear processes relevant to the saturation and transport properties of drift-type instabilities; (3) the construction of semiempirical models for electron thermal transport and the scaling trends inferred from them; and (4) the application of specific anomalous transport models to simulate recent large-scale confinement experiments (TFTR and JET) and current drive experiments.
This book bridges the gap between general plasma physics lectures and the real world problems in MHD stability. In order to support the understanding of concepts and their implication, it refers to real world problems such as toroidal mode coupling or nonlinear evolution in a conceptual and phenomenological approach. Detailed mathematical treatment will involve classical linear stability analysis and an outline of more recent concepts such as the ballooning formalism. The book is based on lectures that the author has given to Master and PhD students in Fusion Plasma Physics. Due its strong link to experimental results in MHD instabilities, the book is also of use to senior researchers in the field, i.e. experimental physicists and engineers in fusion reactor science. The volume is organized in three parts. It starts with an introduction to the MHD equations, a section on toroidal equilibrium (tokamak and stellarator), and on linear stability analysis. Starting from there, the ideal MHD stability of the tokamak configuration will be treated in the second part which is subdivided into current driven and pressure driven MHD. This includes many examples with reference to experimental results for important MHD instabilities such as kinks and their transformation to RWMs, infernal modes, peeling modes, ballooning modes and their relation to ELMs. Finally the coverage is completed by a chapter on resistive stability explaining reconnection and island formation. Again, examples from recent tokamak MHD such as sawteeth, CTMs, NTMs and their relation to disruptions are extensively discussed.
A general methodology for describing the dynamics of transport near marginal stability is formulated. Marginal stability is a special case of the more general phenomenon of self-organized criticality. Simple, one field models of the dynamics of tokamak plasma self-organized criticality have been constructed, and include relevant features such as sheared mean flow and transport bifurcations. In such models, slow mode (i.e. large scale, low frequency transport events) correlation times determine the behavior of transport dynamics near marginal stability. To illustrate this, impulse response scaling exponents (z) and turbulent diffusivities (D) have been calculated for the minimal (Burgers) and sheared flow models. For the minimal model, z= 1 (indicating ballastic propagation) and D[approximately](S[sub 0][sup 2])[sup 1/3], where S[sub 0][sup 2] is the noise strength. With an identically structured noise spectrum and flow with shearing rate exceeding the ambient decorrelation rate for the largest scale transport events, diffusion is recovered with z= 2 and D[approximately] (S[sub 0][sup 2])[sup 3/5]. This indicates a qualitative change in the dynamics, as well as a reduction in losses. These results are consistent with recent findings from[rho] scaling scans. Several tokamak transport experiments are suggested.