Download or read book Simulations of Reversed Shear Alfv n Eigenmodes in Fusion Plasmas written by Wenjun Deng and published by LAP Lambert Academic Publishing. This book was released on 2012-07 with total page 148 pages. Available in PDF, EPUB and Kindle. Book excerpt: In fusion plasmas, generated from heating sources or fusion products, energetic particles can excite Alfven eigenmodes, which undermines energetic particle confinement. To reduce or avoid this problem, behaviors of energetic particles and Alfven eigenmodes need to be studied in detail. In this book, a nonlinear gyrokinetic simulation model, which recovers the ideal magnetohydrodynamic (MHD) theory in the linear long-wavelength regime, is formulated for studying kinetic MHD processes in magnetized plasmas. This comprehensive formulation enables gyrokinetic simulation of both pressure gradient-driven and current-driven instabilities including ideal and kinetic ballooning modes, kink modes, and shear Alfven waves, as well as their nonlinear interactions in multi-scale simulations. Implemented in the gyrokinetic toroidal code (GTC), the new formulation is verified in simulations of reversed shear Alfven eigenmode (RSAE). The verified model is then applied to studying the linear properties of RSAE driven by density gradient of neutral beam injected fast ions in a well-diagnosed DIII-D tokamak experiment.

Download or read book Gyrokinetic Particle Simulations of Reversed Shear Alfv n Eigenmodes in Fusion Plasmas written by Wenjun Deng and published by . This book was released on 2011 with total page 160 pages. Available in PDF, EPUB and Kindle. Book excerpt: A nonlinear gyrokinetic simulation model, which recovers the ideal magnetohydrodynamic (MHD) theory in the linear long-wavelength regime is formulated for studying kinetic MHD processes in magnetized plasmas. This comprehensive formulation enables gyrokinetic simulation of both pressure gradient-driven and current-driven instabilities including ideal and kinetic ballooning modes, kink modes, and shear Alfvén waves, as well as their nonlinear interactions in multi-scale simulations. Implemented in the gyrokinetic toroidal code (GTC), the new formulation is verified in simulations of reversed shear Alfvén eigenmode (RSAE) in fusion plasmas. The antenna excitation of RSAE provides verifications of its mode structure, frequency and damping rate from the initial perturbation simulation with kinetic thermal ions. When excited by fast ions, their non-perturbative contributions modify the mode structure relative to the ideal MHD theory. With inclusion of thermal plasma pressure, the mode frequency increases due to the elevation of the Alfvén continuum by the geodesic compressibility. The GTC simulations have been benchmarked with extended hybrid MHD-gyrokinetic simulations. The verified gyrokinetic simulation model is applied to studying the linear properties of RSAE driven by density gradient of neutral beam injected fast ions in a well-diagnosed DIII-D tokamak experiment (discharge #142111). GTC simulations find that weakly damped RSAE exists due to toroidal coupling and other geometric effects. Various damping and driving mechanisms are identified and measured in the simulations, which shows that accurate damping and growth rate calculation requires true mode structure from non-perturbative, fully self-consistent simulation. The mode structure has no up-down symmetry mainly due to the radial symmetry breaking by the radial variation of fast ion density gradient, as measured in the experiment by electron cyclotron emission imaging. The RSAE frequency up-sweeping and the mode transition from RSAE to toroidal Alfvén eigenmode are in good agreement with the experimental results when scanning the values of the minimum safety factor in simulations. Good agreements in frequencies, growth rates, and mode structures are obtained among simulations of gyrokinetic codes GTC and GYRO, and an MHD-hybrid code TAEFL, which provide further verification and validation of the gyrokinetic model for simulating the kinetic MHD processes. As a prelude to nonlinear simulations of RSAE and associated fast ion transport, properties of microturbulence in reversed shear plasmas are also studied.

Download or read book A Study of Reversed Shear Alfv n Eigenmodes in Alcator C Mod with Phase Contrast Imaging written by Eric Matthias Edlund and published by . This book was released on 2009 with total page 209 pages. Available in PDF, EPUB and Kindle. Book excerpt: Shear Alfvén waves are a fundamental mode of magnetized plasma oscillation and may exist in tokamak plasmas as eigenmodes with a global structure and discrete frequencies. The inhomogeneity of the plasma profiles in conjunction with tokamak geometry tends to focus the Alfvén waves in regions of near uniformity defined by local extrema in the Alfvén continuum, a quantity which describes the local Alfvén resonance frequency for particular mode numbers. Modes excited in the vicinity of these near uniform regions may be weakly damped and excited by energetic ions. The reversed shear Alfvén eigenmode (RSAE), localized deep in the plasma core, is typically associated with a minimum in q in reversed magnetic shear configurations. RSAEs have proven especially useful for MHD spectroscopy, that is the inference of plasma equilibrium properties through their frequency spectra, due to their high sensitivity to the value of qmin. Reversed shear equilibria during the current ramp-up phase and sawtoothing phase are studied through the spatial and temporal characteristics of the RSAEs. Analysis of the spatial structures of RSAEs measured by phase contrast imaging, and interpreted with a synthetic diagnostic using numerical results from the ideal MHD code NOVA, provides constraints on the evolution of qmi,. Additional observations including mode coupling, mode tunneling, and minimum frequency scaling are discussed in light of theoretical and numerical models, with commentary on possible future experiments.

Download or read book Double Gap Alfv n Eigenmodes written by and published by . This book was released on 2005 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: A new type of global shear Alfvén Eigenmode is found in tokamak plasmas where the mode localization is in the region intersecting the Alfvén continuum. The eigenmode is formed by the coupling of two solutions from two adjacent gaps (akin to potential wells) in the shear Alfvén continuum. For tokamak plasmas with reversed magnetic shear it is shown that the toroidiciy-induced solution tunnels through the continuum to match the ellipticity-induced Alfvén eigenmode (TAE and EAE, respectively) so that the resulting solution is continuous at the point of resonance with the continuum. The existence of these Double Gap Alfvén Eigenmodes (DGAEs) allows for potentially new ways of coupling edge fields to the plasma core in conditions where the core region is conventionally considered inaccessible. Implications include new approaches to heating and current drive in fusion plasmas as well as its possible use as core diagnostic in burning plasmas.

Download or read book Combined Ideal and Kinetic Effects on Reversed Shear Alfven Eigenmodes written by and published by . This book was released on 2011 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: A theory of Reversed Shear Alfven Eigenmodes (RSAEs) is developed for reversed magnetic field shear plasmas when the safety factor minimum, qmin, is at or above a rational value. The modes we study are known sometimes as either the bottom of the frequency sweep or the down sweeping RSAEs. We show that the ideal MHD theory is not compatible with the eigenmode solution in the reversed shear plasma with qmin above integer values. Corrected by special analytic FLR condition MHD dispersion of these modes nevertheless can be developed. Large radial scale part of the analytic RSAE solution can be obtained from ideal MHD and expressed in terms of the Legendre functions. The kinetic equation with FLR effects for the eigenmode is solved numerically and agrees with the analytic solutions. Properties of RSAEs and their potential implications for plasma diagnostics are discussed.

Download or read book Controlled Stabilization of Alfv n Eigenmodes in DIII D and Validation of Theory and Simulations written by Shawn Tang and published by . This book was released on 2021 with total page 123 pages. Available in PDF, EPUB and Kindle. Book excerpt: Understanding the interaction between wave excitation and damping is essential in the advancement of research on magnetized plasmas in space, laboratory, and astrophysical settings. Alfv\'en waves can be excited across all of these settings, and in fusion research plasmas such as tokamak plasmas, they are present due to energetic particles from neutral beam injection and fusion reactions. The interplay between wave-particle interaction and damping processes is at the core of understanding wave excitation. \\ In the work presented in this dissertation, an experiment was designed to investigate high frequency Alfv\'en waves excited through Doppler-shifted cyclotron resonance with energetic particles from neutral beam injection in the DIII-D tokamak. These waves, compressional (CAE) and global (GAE) Alfv\'en eigenmodes, typically have frequencies close to the ion cyclotron frequency $f_{ci}$; the frequency and amplitude of these modes was measured with the Ion Cyclotron Emission (ICE) diagnostic. The experiment utilized the unique capability of the DIII-D neutral beams to separately control the energy and injection rate. A parametric scan across many magnetic fields and beam geometries was performed to study the dependencies of these modes on various plasma parameters.\\ An energetic ion density threshold was observed during a discharge in which the voltage of an off-axis co-injecting beam was held constant while the current was ramped down by $40\%$. During this discharge, a spectrum of high frequency AEs at $f=0.58f_{ci}$ was stabilized via a controlled energetic ion density ramp for the first time in a fusion research plasma. This observation demonstrates an important property of resonant AEs: that the growth rate of these waves is set by the balance between fast-ion drive and damping processes. The controlled stabilization of this mode also validates previous simulations done on high frequency AEs in which an instability threshold was observed by varying the beam density without changing the shape of the distribution. \\ The scaling of the amplitude of this wave with the beam injection rate was found to be consistent with predictions for single mode collisional saturation near marginal stability. Analytic theory found that for the observed beam injection rate threshold, the mode was near marginal stability throughout the entire beam ramp. This is notably different from previous simulations of CAEs/GAEs that were in the collisionless regime and often far from marginal stability. \\ Modeling codes such as TRANSP and ORB\_GC were used to model and analyze the fast-ion distribution for this discharge. This analysis found that the modes were likely excited by a high energy subset of the fast-ion population with strong gradients in parallel velocity space. Resonance analysis of this subset of the fast-ion population, in conjunction with considerations from dispersion relations, shows that the mode is likely a shear-polarized GAE. This marks the first identification of a GAE excited through Doppler-shifted cyclotron resonance with sub-Alfv\'enic energetic ions, a first in fusion research plasmas.

Download or read book Interpretation of the Finite Pressure Gradient Effects in the Reversed Shear Alfv n Eigenmode Theory written by and published by . This book was released on 2008 with total page 451 pages. Available in PDF, EPUB and Kindle. Book excerpt: Ideal MHD equations employed in the NOVA code are analyzed analytically and numerically in order to investigate the role of the pressure gradient on global reversed shear Alfvén eigenmodes (RSAEs) or Alfvén cascades. We confirm both numerically and analytically conclusions obtained earlier using the ideal MHD code NOVA and analytically that the plasma pressure gradient plays a key role in the existence condition and in the dispersion relation for the mode. The effect of the plasma pressure gradient is to shift the mode frequency up at the low part of the RSAE frequency chirp and downshift the mode frequency when the frequency approaches the TAE gap This finding is opposite to predictions in a recent publication, where the pressure gradient is found to be always stabilizing by means of downshifting the RSAE frequency and enhancing its in- teraction with the continuum. We resolve this discrepancy by showing that neglecting the pressure gradient effect on the plasma equilibrium (modification of the Shafranov shift and the averaged curvature) leads to conclusions at variance to the numerical and analytical results presented here. A new variational approximation of the RSAE is introduced which compares remarkably well with NOVA solutions. With this new approximation we clearly demonstrate the diagnostic potential and limitations of the RSAE frequency measurement for MHD spectroscopy.

Download or read book The Magentic sic Fusion Theory Effort written by United States. Department of Energy. Office of Fusion Energy and published by . This book was released on 1979 with total page 108 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Resonant Toroidal Alfven Eigenmodes RTAEs in Neutral Beam Heated Reverse Magnetic Shear Plasmas on TFTR written by and published by . This book was released on 1999 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Resonant Toroidal Alfven Eigenmodes (RTAEs) [1, 2] excited by neutral beam ions are observed in the region of the internal transport barrier in enhanced reverse shear (ERS) plasmas on TFTR. These modes occur in multiples of the same toroidal mode number in the range n=2-4 and appear as highly localized structures near the minimum in the q-profile with frequency near to that expected for TAEs. Unlike regular TAEs, these modes are observed in plasmas where the birth velocity of beam ions is well below the fundamental or sideband resonance condition. Theoretical analysis indicates that the Toroidicity induced Alfven Eigenmode (TAE) does not exist in these discharges due to strong pressure gradients (of the thermal and fast ions) which moves the mode frequency down into the lower Alfven continuum. However a new non-perturbative analysis (where the energetic particles are allowed to modify the mode frequency and mode structure) indicates that RTAEs can be driven by neutral beam ions in the weak magnetic shear region of ERS plasma, consistent with observations on TFTR. The importance of such modes is that they may affect the alpha particle heating profile or enhance the loss of energetic alpha particles in an advanced tokamak reactor where large internal pressure gradients and reverse magnetic shear operation are required to sustain large bootstrap current.

Download or read book High n Helicity induced Shear Alfv n Eigenmodes written by N. Nakajima and published by . This book was released on 1992 with total page 17 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Low n Shear Alfven Spectra in Axisymmetric Toroidal Plasmas written by and published by . This book was released on 1985 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: In toroidal plasmas, the toroidal magnetic field is nonuniform over a magnetic surface and causes coupling of different poloidal harmonics. It is shown both analytically and numerically that the toroidicity not only breaks up the shear Alfven continuous spectrum, but also creates new, discrete, toroidicity-induced shear Alfven eigenmodes with frequencies inside the continuum gaps. Potential applications of the low-n toroidicity-induced shear Alfven eigenmodes on plasma heating and instabilities are addressed. 17 refs., 4 figs.

Download or read book Magnetohydrodynamics of Laboratory and Astrophysical Plasmas written by Hans Goedbloed and published by Cambridge University Press. This book was released on 2019-01-31 with total page 995 pages. Available in PDF, EPUB and Kindle. Book excerpt: With ninety per cent of visible matter in the universe existing in the plasma state, an understanding of magnetohydrodynamics is essential for anyone looking to understand solar and astrophysical processes, from stars to accretion discs and galaxies; as well as laboratory applications focused on harnessing controlled fusion energy. This introduction to magnetohydrodynamics brings together the theory of plasma behavior with advanced topics including the applications of plasma physics to thermonuclear fusion and plasma- astrophysics. Topics covered include streaming and toroidal plasmas, nonlinear dynamics, modern computational techniques, incompressible plasma turbulence and extreme transonic and relativistic plasma flows. The numerical techniques needed to apply magnetohydrodynamics are explained, allowing the reader to move from theory to application and exploit the latest algorithmic advances. Bringing together two previous volumes: Principles of Magnetohydrodynamics and Advanced Magnetohydrodynamics, and completely updated with new examples, insights and applications, this volume constitutes a comprehensive reference for students and researchers interested in plasma physics, astrophysics and thermonuclear fusion.

Download or read book INIS Atomindex written by and published by . This book was released on 1996 with total page 730 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Electromagnetic Waves For Thermonuclear Fusion Research written by Ernesto Mazzucato and published by World Scientific. This book was released on 2014-04-02 with total page 223 pages. Available in PDF, EPUB and Kindle. Book excerpt: The science of magnetically confined plasmas covers the entire spectrum of physics from classical and relativistic electrodynamics to quantum mechanics. During the last sixty years of research, our initial primitive understanding of plasma physics has made impressive progress thanks to a variety of experiments — from tabletop devices with plasma temperatures of a few thousands of degrees and confinement times of less than 100 microseconds, to large tokamaks with plasma temperatures of up to five hundred million degrees and confinement times approaching one second. We discovered that plasma confinement is impaired by a variety of instabilities leading to turbulent processes with scales ranging from the plasma size to a few millimeters. Understanding these phenomena, which have slowed down progress towards a fusion reactor, requires the use of very sophisticated diagnostic tools, many of which employ electromagnetic waves.The primary objective of this book is to discuss the fundamental physics upon which the application of electromagnetic waves to the study of magnetically confined plasmas is based.

Download or read book Spectral and High Order Methods for Partial Differential Equations written by Jan S. Hesthaven and published by Springer Science & Business Media. This book was released on 2010-10-29 with total page 507 pages. Available in PDF, EPUB and Kindle. Book excerpt: The book contains a selection of high quality papers, chosen among the best presentations during the International Conference on Spectral and High-Order Methods (2009), and provides an overview of the depth and breadth of the activities within this important research area. The carefully reviewed selection of the papers will provide the reader with a snapshot of state-of-the-art and help initiate new research directions through the extensive bibliography.

Download or read book Cosmical Electrodynamics written by H. Alfven and published by Рипол Классик. This book was released on 1963 with total page 245 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Jets From Young Stars III written by Silvano Massaglia and published by Springer. This book was released on 2010-11-25 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: The study of the mechanisms that govern origin and propagation of stellar jets involves the treatment of many concurrent physical processes such as gravitation, hydrodynamics and magnetohydrodynamics, atomic physics and radiation. In the past years, an intensive work has been done looking for so- tions of the ideal MHD equations in the steady state limit as well as studying the stability of out?ows in the linear regime. These kind, of approaches have provided a contribution to the understanding of jets that can hardly be ov- estimated. However, the extension of the analyses to the time-dependent and nonlinear regimes could not be avoided, and the MHD numerical simulations were the only mean to achieve this goal. Intherecentyears,considerableprogresseshavebeenmadebythecom- tational?uiddynamiccommunityinthedevelopmentofnumericaltechniques, theso-calledhighresolutionshockcapturingschemes,wellsuitedforthetre- ment of supersonic ?ows with discontinuities. The numerical simulations of astrophysical jets took advantage of these developments; however new physics needed to be incorporated, such as magnetic ?eld e?ects, radiation losses by diluted gases, and proper astrophysics environments. These needs led to the nontrivial extension of the methods devised for the Euler equations of g- dynamics to the magneto-hydrodynamical system. On the other hand, the possibility of carrying out numerical calculations has been greatly facilitated bytheavailability, ononehand,ofpowerfulsupercomputersand,ontheother hand, of fast processors at low cost. Large scale 3D simulations of jets at high resolution are now possible thanks to supercomputers, but also high reso- tion 2D MHD simulations can be performed routinely on desktop computers.