Bulletin of the American Physical Society
50th Annual Meeting of the Division of Plasma Physics
Volume 53, Number 14
Monday–Friday, November 17–21, 2008; Dallas, Texas
Session NO3: Magnetic Confinement II |
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Chair: Donald Batchelor, Oak Ridge National Laboratory Room: Reunion A |
Wednesday, November 19, 2008 9:45AM - 9:57AM |
NO3.00001: Spheromak Simulation in 3+1 Dimensions Using Multiscale Space/Time Spectral Elements Charles Weatherford The NIMROD code is the standard simulation tool for modeling spheromaks. The current implementation of NIMROD is a two-fluid code, parallelized for high performance computers and uses direct solution methods for the matrix equations. The present work involves an investigation of various possible improvements to the NIMROD code. In particular, an iterative solver will replace the direct solver in present use. The iterative solver will employ Jacobian-free Newton-Krylov Methods. A procedure composed of four pieces is used: (1) An iterative correction is accomplished with each step. Optimal polynomials lead to various preconditioned Krylov methods; (2) Each error component is treated in an optimal subspace; (3) The Jacobian is avoided. In the Jacobian-Free Newton-Krylov method, a Krylov method solves the linear Newton correction equation, which requires Jacobian-vector products. These are approximated by the Frechet derivatives so that the actual Jacobian elements are never explicitly needed; (4) The solver is used as a preconditioner. This defines a weakly converging nonlinear method. Next, a new method for time-advance will be employed which involves the use of spectral-elements in space and time. [Preview Abstract] |
Wednesday, November 19, 2008 9:57AM - 10:09AM |
NO3.00002: Plasma Liner Compression of Compact Toroids to Fusion Conditions John Slough, Chris Pihl, George Votroubek To make fusion practical at small scale, an efficient method for repetitively compressing the FRC to fusion gain conditions is required. A promising approach, and the one is being explored at the MSNW facility employs a plasma shell to radially compress and heat the FRC plasmoid to fusion conditions. The energy that is required for the implosion compression and heating of the FRC plasmoid is derived from both the radial kinetic energy of the plasma liner used to compress it, and the axial kinetic energy of the FRC's motion prior to compression. The timescale for forming and accelerating both the FRC and liner can be much longer than the time it takes for the energy to be thermalized in the implosion. This greatly reduces the demand on the power delivery systems as the energy can be accumulated over a time interval of several microseconds far from the interaction region. The design and testing of the key components for the formation of the plasma liner have been successfully completed, as well as the numerical and analytical work to optimize the liner compression. The results from this work will be presented as well as plans for a proof of concept experiment. [Preview Abstract] |
Wednesday, November 19, 2008 10:09AM - 10:21AM |
NO3.00003: Compressional Heating of Dynamically Formed and Merged FRCs in the IPA-C Experiment George Votroubek, John Slough, Chris Pihl Pulsed, linear, fusion reactors have been investigated due to their appealing geometry, low cost and fairly straight-forward engineering and technical challenges. The FRC's inherent high beta, translatability and simple linear geometry make it an ideal source plasma for such a reactor. The goal of the Inductive Plasma Acceleration and Compression (IPA-C) experiment is to develop the FRC plasma source required. In the IPA-C, a stable, hot, and dense plasmoid is formed through collisional merging and compression of two FRCs dynamically formed $\sim $1.5 m from their final location. This method of remote formation and translation of FRCs into a central section has many advantages when scaled up for fusion burn: large stand-off distances can be maintained between the plasma source and the fusion burn region, the simply connected geometry is ideal for blankets and shielding, and the rapid, repetitive cycling of the fusion burn becomes viable. Recent upgrades of the IPA-C include: increase in formation energy and coil radius for a higher flux FRC; addition of an acceleration section; and doubling of the maximum compression field. Recent results will be discussed. [Preview Abstract] |
Wednesday, November 19, 2008 10:21AM - 10:33AM |
NO3.00004: Diamagnetic measurements in the STOR-M tokamak Dallas Trembach Diamagnetic measurements of poloidal beta have been successfully performed on the Saskatchewan Torus-Modified (STOR-M) using a compensated coil system mounted exterior to the vacuum chamber wall. A significant challenge in performing these measurements on STOR-M is the presence of a decaying toroidal magnetic field over the duration of the discharge. A simple method for compensating these measurements based on independently measuring the vacuum field signal and correcting during post-processing is presented. Measurements of poloidal beta using the diamagnetic coil arrangement are compared to calculations of poloidal beta based on the Spitzer conductivity corrected for trapped electrons. [Preview Abstract] |
Wednesday, November 19, 2008 10:33AM - 10:45AM |
NO3.00005: Real time feedback control of the sawtooth period using ECRH launchers James Paley, Federico Felici, Timothy Goodman, Francesco Piras, Stefano Coda The sawtooth instability is both necessary to remove helium ash from the plasma core in a fusion device as well as a trigger for unwanted, confinement limiting neoclassical tearing modes. This has motivated the development of sawtooth control techniques. One of the more promising control methods is to target localised ECRH/ECCD in the vicinity of the q=1 surface. We have demonstrated on TCV the ability to control the sawtooth period in real time by actuating the ECRH launcher mirror angle. Sawtooth control is complicated by the non-linear, multi-parameter nature of the instability and effect of the ECRH beam. For example, moving the ECRH beam not only modifies the local current profile, but also influences the plasma temperature and position of the q=1 surface. An effective control system should be able to track such changes. The control objective was simple -- maintain the sawtooth period at a reference value. The controller was able to locate and maintain the sawtooth period at the reference, despite period modifying disturbances such as plasma density variations. [Preview Abstract] |
Wednesday, November 19, 2008 10:45AM - 10:57AM |
NO3.00006: Coupled Plasma-Wall Modeling with WALL-PSI/EDGE1D A.Yu. Pigarov, S. Krasheninnikov We developed the Wall and Plasma-Surface Interactions (WALL- PSI) code which is the 1-D code for particle and heat transport inside plasma-facing components. WALL-PSI incorporated new approaches in continuum, diffusion-advection modeling of hydrogen species in wall materials. Some results on WALL-PSI benchmarking against experimental data on hydrogen retention, permeation, and erosion rates for major fusion related materials will be highlighted. To study basic physics processes, WALL-PSI was coupled to our EDGE1D code that mimics cross-field transport of plasma and neutrals in tokamaks with SOL. We report on the results of self-consistent plasma- neutrals-wall simulation showing: (1) examples of strong plasma- wall coupling, (2) nonlinear variation of hydrogen inventory and recycling coefficient vs. incident plasma flux, and (3) featured instabilities. Saturation of wall in long duration discharge without external gas pumping feedback resulted in MARFE in agreement with experiments [T.Fujita etal, Nucl. Fusion 46(2006)S3]. The modeled transitional effects include sawteeth-like oscillations in edge plasma due to switching from wall pumping to gas release. Here 20$\%$ variation in recycling coefficient corresponds to transitions between cold deeply- detached and hot sheath-limited edge plasmas due to thermal plasma instability. Work supported by DoE Grants DE-FC0207- ER54908, DE-FG0204-ER54739. [Preview Abstract] |
Wednesday, November 19, 2008 10:57AM - 11:09AM |
NO3.00007: Dust radiation and imaging in tokamak plasmas R.D. Smirnov, J.H. Yu, S.I. Krasheninnikov, A.Yu. Pigarov, M. Rosenberg, D.A. Mendis Dust in fusion devices presents safety and operational concerns for future reactors such as ITER. One of the techniques used for surveying and studying dust in tokamak plasmas is imaging of the dust particles by fast cameras. The imaging is possible because heating and ablation of the dust by impinging plasma particles creates two sources of radiation: thermal radiation from the heated dust particle and radiation from the ablation cloud interacting with the surrounding plasma. The spatial and spectral characteristics of the radiation can provide information as on dust (size, temperature, composition), as well as on dust-plasma interaction processes (ablation dynamics, local plasma parameters). In this work we calculate the spectrum and intensity of both the thermal radiation from a carbon dust particle and the radiation from the ablation cloud surrounding it for various plasma parameters. The possibility to detect each component of the radiation by modern fast framing cameras for different dust sizes is evaluated. This allows to estimate minimal size of the visible dust and the ablation cloud characteristics, which are important for evaluation of dust contribution in the plasma contamination. [Preview Abstract] |
Wednesday, November 19, 2008 11:09AM - 11:21AM |
NO3.00008: Fullwave Simulation of Doppler Reflectometry in Turbulent Plasmas Carsten Lechte, Ulrich Stroth, Garrard Conway Doppler reflectometry is a microwave diagnostic for plasma density fluctuations and flow velocities. In a fusion plasma, the radial electric field can then be estimated from the poloidal plasma velocity. In the presence of strong density fluctuations, the response from the plasma is dominated by dispersion and multiple scattering, and hence becomes non-linear. Quantitative investigations are then only possible with 2D or 3D simulations of the wave propagation using the Maxwell equations and the electron equations of motion. IPF-FD3D is the finite difference time domain code used to investigate the dependence of the scattering efficiency from the various plasma conditions. First results in slab geometry indicate a strong dependence on the density gradient, the turbulent fluctuation strength, and the wave polarisation. In addition, the actual plasma conditions in ASDEX-Upgrade are recreated in the simulation in order to interpret experimental measurements. These investigations will help achieve the main physics goal of determining the absolute density fluctuation wavenumber spectrum of the plasma. [Preview Abstract] |
Wednesday, November 19, 2008 11:21AM - 11:33AM |
NO3.00009: Large-orbit $\delta f$ simulation of non-local neoclassical effects in tokamaks R.A. Kolesnikov, W.X. Wang, F.L. Hinton, W.M. Tang, W.W. Lee In collisionless limit, the neoclassical equilibrium distribution is a function of three Constants of Motion (CoM). In standard $\delta f$ simulation, a local Maxwellian is used as a background distribution. The difference between CoM and local Maxwellian is due to particle orbit width, which can become very large for high temperature ions and steep pressure gradients. Analogous to the conventional local Maxwellian, the new equilibrium makes no contribution to the radial particle, energy and toroidal angular momentum fluxes [1], thus the radial transport is obtained from moments of $\delta f$. Taking the CoM function as equilibrium in PIC simulations eliminates the large orbit contribution to the rapid growth of the particle weights. We illustrate this by simulating the radial transport using GTC-NEO code [2] which has been modified to support the new equilibrium. In presence of density, temperature and toroidal flow gradients the new simulation shows much smaller particle weights compared to the original Maxwellian based results. The new algorithm may be especially useful for simulation of systems with sharp gradients as well as in presence of impurities, and energetic particles. [1] F. L. Hinton, TTF, San Diego, CA~(2008). [2] W. X. Wang et. al., Physics of Plasmas 13, 082501 (2006). [Preview Abstract] |
Wednesday, November 19, 2008 11:33AM - 11:45AM |
NO3.00010: Transport of Energetic Particles by Microturbulence in Fusion Plasmas Wenlu Zhang, Zhihong Lin, Liu Chen The diffusion of the the energetic particles by the microscopic ion temperature gradient (ITG) turbulence is investigated in large scale first-principle simulations of fusion plasmas using the global gyrokinetic toroidal code (GTC) [Lin, Science 1998]. The ion radial spread as a function of energy and pitch angle is measured in the steady state ITG turbulence. The probability density function of the radial excursion is found to be very close to a Gaussian, indicating a diffusive transport from a random walk process. The radial diffusivity as a function of the energy and pitch angle can thus be calculated using the random walk model. We find that the diffusivity decreases drastically for high energy particles due to the averaging effects of the large gyroradius and banana width, and the fast decorrelation of the energetic particles with the ITG oscillations. By performing the integration in phase space, we can calculate the diffusivity for any distribution function. The diffusivity driven by the ITG turbulence for the NBI ions with a slowing-down distribution function is found to decrease rapidly for the born energy up to an order of magnitude of the electron temperature and more gradually to a very low level for higher born energy. [Preview Abstract] |
Wednesday, November 19, 2008 11:45AM - 11:57AM |
NO3.00011: Global Gyrokinetic Simulations of Toroidal Momentum Transport in Tokamak Plasmas Ihor Holod Global gyrokinetic simulations of toroidal momentum transport in turbulent plasmas are performed, using the nonlinear gyrokinetic particle-in-cell code GTC. The momentum transport is driven by the ITG turbulence. The equilibrium plasma rotation is described by shifted Maxwellian parallel velocity distribution for ions. Cases with constant (rigid rotation) and radially sheared angular velocity are considered. The temporal evolution together with the radial profiles of flux surface averaged toroidal momentum and momentum flux are obtained from the simulation. We observe a significant redistribution of toroidal momentum during the ITG instability development and after the nonlinear saturation. The general trend of spinning up of plasma towards a center of tokamak is observed for the rigid rotation cases, which is the manifestation of the off-diagonal (pinch-like) inward flux. For the sheared rotation cases, the competition between diffusive and off-diagonal fluxes takes place. In order to determine toroidal momentum diffusivity we have separated diagonal flux by subtracting the off-diagonal contribution obtained from the corresponding parameter scan. The obtained toroidal momentum conductivity is compared with the ion heat conductivity, giving the ratio in the range 0.3-0.7. This ratio is confirmed by quasilinear estimates based on the obtained fluctuation spectra. The work was supported by SciDAC GPS Center. [Preview Abstract] |
Wednesday, November 19, 2008 11:57AM - 12:09PM |
NO3.00012: Turbulence Induced Diffusion in Conjunction with Gyrocenter Shift K.C. Lee A new concept of turbulence transport and diffusion coefficient based on the theory of gyrocenter shift[1-2] are derived from the microscopic $\tilde {E}xB$ (E is electric field and B is magnetic field) drifts at the boundary of fusion device. It is found that the turbulence flow generated by the ion-neutral collisions induces diffusive transport with the saturation condition where the turbulence induced charge dilation compensates the gyrocenter shift current. The calculations with conventional and modified Boltzmann relations are compared with experiments. The effects of turbulence induced diffusion and the Reynolds number of magnetized plasma to the high confinement mode transitions will be discussed. [1] K. C. Lee, Phys. Plasmas, \textbf{13}, 062505 (2006), [2] K. C. Lee, Phys. Rev. Lett. \textbf{99}, 065003 (2007) [Preview Abstract] |
Wednesday, November 19, 2008 12:09PM - 12:21PM |
NO3.00013: Gyrokinetic $\delta\! f$ particle simulation of energetic particles driven modes Jianying Lang, Yang Chen, Scott Parker The global GEM code [1] is reduced from a fully kinetic model with gyrokinetic ions and drift kinetic electrons to a hybrid model with gyrokinetic ions and massless fluid electrons to study the Toroidicity-Induced Alfven Eigenmodes (TAEs) at finite plasma pressure. This hybrid model can be further reduced to a two fluid model. Using a low-n global TAE as a test case [2], we observed the existence of a global discrete eigenmode in the two fluid simulation. The observed mode frequency is within the gap and close to the lower continuum which agrees well with the expected TAE frequency. A benchmark between the simulation and an eigenmode analysis based on the two fluid model is also accomplished. In the presence of the gyrokinetic hot ions, this TAE is driven unstable with the growth rate scaling linearly with the hot ion pressure $\beta_{hot}$. A mode rotation along the poloidal direction is also observed and the rotation direction depends on $\beta_{hot}$. Turning on the gyrokinetic bulk ions, we observed the growth rate of the unstable TAE being reduced, which is expected because of the kinetic damping effect. We are currently exploring the effects of different saturation mechanisms (wave trapping or mode-mode coupling). Results will be presented. [1] Y. Chen and S. Parker, J. of Comp. Phys. 220, 839 (2007). [2] G. Y. Fu and J. W. Van Dam, Phys. Fluids B 1, 1949 (1989). [Preview Abstract] |
Wednesday, November 19, 2008 12:21PM - 12:33PM |
NO3.00014: Thermonuclear Yield in DIII-D Tokamak Plasma due to the High Harmonics Relativistic Electron Bernstein Modes V. Stefan The scaling laws for the thermonuclear yield, ratio of the thermonuclear power to the external power, for the case of excitation of Electron Bernstein Mode (EB) high harmonics, n(EB), n= 3{\ldots} 6 harmonic number, and lower hybrid waves are obtained for the DIII-D Tokamak plasma environment.\footnote{\textbf{R. Prater}, Phys. Fluids \textbf{11}(5), 2349 (2004).} The plasma-ignition criterion is analyzed in terms of the X-Mode power. An efficient control of anomalous absorption in DIII-D Tokamak plasma is proposed. In this novel model, the external electron cyclotron driver, X-mode, excites relativistic EB mode high harmonics.\footnote{\textbf{V. Stefan, }Anomalous Absorption of High-Harmonic Relativistic EB Modes in Spherical Tokamak Plasmas, 2007. American Physical Society\textbf{, }April 14--17, 2007, Jacksonville, Florida; \textbf{V. Stefan, }Thermonuclear Yield Due to the Relativistic EB Modes in Spherical Tokamak Plasmas, Sherwood Conference, March 30-April 2, 2008, Boulder, Colorado.} Nonlinear relativistic EB harmonics, in turn, are effectively absorbed via nonlinear collisional damping in the electron cyclotron high-harmonic resonance regions. [Preview Abstract] |
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