Fusion Energy Sciences
Geodesic acoustic modes in plasma turbulence
Researchers in the SciDAC Plasma Microturbulence Project have observed a coherent oscillation in the poloidal flow of density fluctuations that shows remarkable similarity to predicted characteristics of geodesic acoustic modes with beam emission spectroscopy in DIII-D plasmas. Two methods of analyzing the data, one based on wavelets and one based on time-resolved cross-correlation, reveal similar features in the resulting flow field. These experimental observations and the corresponding simulations provide compelling evidence that geodesic acoustic modes are active in the turbulence in the outer regions of tokamak plasmas and play an integral role in affecting and mediating the fully saturated state of turbulence and resulting transport.
G. R. McKee, R. J. Fonck, M. Jakubowski, K. H. Burrell, K. Hallatschek, R. A. Moyer, D. L. Rudakov, W. Nevins, G. D. Porter, P. Schoch, and X. Xu, “Experimental characterization of coherent, radially-sheared zonal flows in the DIII-D tokamak,” Phys. Plasmas 10, 1712 (2003). FES, SciDAC
Computational atomic physics for fusion energy
Colgan and Pindzola have applied the time-dependent close-coupling theory to the study of the electron-impact ionization of helium from the excited (1s2s) configuration. They made the calculations in an effort to resolve the discrepancy between theoretical calculations and existing experimental measurements for electron scattering from the metastable states of helium. The relative difference between the non-perturbative close-coupling and the perturbative distorted-wave results grew larger as the principal quantum number was increased. This difference has important implications in the collisional-radiative modeling of many astrophysical and laboratory plasmas that use perturbation theory.
J. P. Colgan and M. S. Pindzola, “Time-dependent close-coupling studies of the electron-impact ionization of excited-state helium,” Phys. Rev. A 66, 062707 (2002). FES, SciDAC
Ballooning instability in the Earth’s magnetotail
The Earth’s magnetotail, the main source of the polar aurora, is the part of the magnetosphere that is pushed away from the sun by the solar wind. The ballooning instability of the magnetotail interests researchers because of its possible relevance to the dynamics of magnetic substorms. Zhu et al. studied this ballooning instability within the framework of Hall magnetohydrodynamics, which enter the stability analysis through changes introduced in the plasma compressibility.
P. Zhu, A. Bhattacharjee, and Z. W. Ma, “Hall magnetohydrodynamic ballooning instability in the magnetotail,” Phys. Plasmas 10, 249 (2003). FES, SciDAC, NSF
Size scaling of turbulent transport
Lin et al. have developed a nonlinear model for turbulence radial spreading based on the modified porous-medium equation. The model offers a phenomenological understanding of the transition from Bohm to gyro-Bohm scaling when the parameter (minor radius/gyroradius) is larger than 500. They also estimated the role of the trapped electron nonlinearity in zonal flow generation in trapped-electron-mode turbulence in the context of parametric instability theory.
Z. Lin, T. S. Hahm, S. Ethier, W. W. Lee, J. Lewandowski, G. Rewoldt, W. M. Tang, W. X. Wang, L. Chen, and P. H. Diamond, “Size scaling of turbulent transport in tokamak plasmas," Proceedings of 19th IAEA Fusion Energy Conference, Lyon, France (2002). FES, SciDAC
Electron-impact ionization of oxygen ions
Loch et al. made experimental measurements of the ionization cross section for Oq+, where q = 1–4, performed with a crossed-beams apparatus, and compared them with theoretical calculations. For O+, the experimental measurements are in very good agreement with configuration-average time-dependent close-coupling calculations. For the remaining oxygen ions, the experimental measurements are in good agreement with time-independent distorted-wave calculations. As expected, the accuracy of the perturbative distorted-wave calculations improves with increasing ion charge.
S. D. Loch, J. P. Colgan, M. S. Pindzola, W. Westermann, F. Scheuermann, K. Aichele, D. Hathiramani, and E. Salzborn, “Electron-impact ionization of Oq+ ions for q = 1–4,” Phys. Rev. A 67, 042714 (2003). FES, SciDAC, DFG
Simulation of intense beams for heavy-ion fusion
A multibeamlet approach to a high-current-ion injector, whereby a large number of beamlets are accelerated and then merged to form a single beam, offers a number of potential advantages over a monolithic single-beam injector. These advantages include a smaller transverse footprint, more control over the shaping and aiming of the beam, and more flexibility in the choice of ion sources. A potential drawback, however, is a larger emittance. Grote et al. are studying the merging of beamlets (Figure 6) and how this merging determines emittance. Their results suggest that a multibeamlet injector can be built with a normalized emittance less than 1 pmm mrad.
D. P. Grote, E. Henestroza, and J. W. Kwan, “Design and simulation of a multibeamlet injector for a high-current accelerator,'' Phys. Rev. ST AB 6, 014202 (2003). FES
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Figure 6 |
Simulating the thermal structure of solar storms
Solar storms can potentially disrupt the operation of orbiting satellites, endanger astronauts, and cause failure of long-distance power grids on land. Understanding the physical mechanisms that originate these storms is of fundamental importance if they are to be forecasted. Mok et al. simulated the thermal structure of the solar atmosphere in the neighborhood of a sunspot group in 3D for the first time. They found that the strong, structured magnetic field from the sunspots guides the heat flow and plasma flow, resulting in fine structures in temperature and density because of the complicated field-line topology.
Y. Mok, R. Lionello, Z. Mikic, and J. Linker, “Calculating the thermal structure of solar active regions in three dimensions,” Astrophys. J. (submitted, 2003). FES, NSF, NASA
Modeling the plasma-density limit
Understanding the plasma-density limit is crucial for projecting the performance of future fusion reactors. Xu et al. have developed a model for the density limit in a tokamak plasma based on the edge simulation results from BOUT and UEDGE codes. Simulations of turbulence in tokamak boundary plasmas show that turbulent fluctuation levels and transport increase with collisionality. The simulations also show that it is easier to reach the density limit as the density increases while holding pressure constant than holding temperature constant.
X. Q. Xu, W. M. Nevins, T. D. Rognlien, R. H. Bulmer, M. Greenwald, A. Mahdavi, L. D. Pearlstein, and P. Snyder, “Transitions of turbulence in plasma density limits,” Phys. Plasmas 10, 1773 (2003). FES, MIT, GA
Microstructure evolution in irradiated materials
The promise of fusion as a viable energy source depends on the development of structural materials capable of sustaining operation in harsh radiation conditions. The structure and mobility of self-interstitial atom (SIA) clusters has profound significance on the microstructural evolution of irradiated materials. Marian et al. have studied the effect of substitutional copper solute atoms on the mobility of self-interstitial clusters in Fe-Cu alloys. The effect of this oversized substitutional solute is to enhance the three-dimensional character of small-cluster diffusion and to affect general cluster diffusion properties.
J. Marian, B. D. Wirth, A. Caro, B. Sadigh, G. R. Odette, J. M. Perlado, and T. Diaz de la Rubia, “Dynamics of self-interstitial cluster migration in pure alpha-Fe and Fe-Cu alloys," Phys. Rev. B 65, 144102 (2002). FES
Designing the National Compact Stellerator Experiment
The U.S. fusion community is constructing a compact, moderate-cost experiment, the National Compact Stellerator Experiment (NCSX), to investigate 3D confinement and stability. The configurations being studied are compact, are passively stable to known instabilities, and are designed to have a “hidden” symmetry to give good transport properties. In addition, because of their enhanced stability, they should be free of disruptions. These configurations are designed to combine the best features of the tokamak and the stellerator. The NCSX has achieved a successful conceptual design review.
G. H. Neilson and the NCSX Team, “Physics considerations in the design of NCSX,” Proceedings of 19th IAEA Fusion Energy Conference, Lyon, France (2002). FES
Stabilizing microturbulence in plasmas
Bourdelle et al. have shown that microturbulence can be stabilized in the
presence of steep temperature and density profiles. High values of |
´|
have a stabilizing influence on drift modes. This may form the basis for a
positive feedback loop in which high core b values lead to improved confinement,
and to further increase in . In high- spherical tokamak plasmas,
high |´| rather than low aspect ratio is a source of stabilization.
Therefore, the effect of high |´| should be stabilizing in the
plasmas of the National Spherical Torus Experiment.
C. Bourdelle, W. Dorland, X. Garbet, G. W. Hammett, M. Kotschenreuther,
G. Rewoldt, and E. J. Synakowski, "Stabilizing impact of high gradient
of on microturbulence," Phys. Plasmas 10, 2881
(2003). FES
Trapped electron mode turbulence
Gyrokinetic simulations of turbulent particle transport in the Alcator C-Mod by Ernst et al. have led to an understanding of the observed spontaneous particle transport barrier formation, and its control using radio-frequency heating in the core. As the density gradient steepens, nonresonant, collisionless, trapped electron modes (TEMs) are driven unstable. Gyrokinetic stability analysis shows that the modes are driven by the density gradient, have wavelengths around twice the ion gyroradius, and are partially damped by collisions. After the TEM onset, the turbulent outflow strongly increases with the density gradient. This outflow ultimately balances the inward Ware pinch, leading to a steady state.
D. R. Ernst et al., "Role of trapped electron mode turbulence in density profile control with ICRH in Alcator C-Mod," International Sherwood Fusion Theory Conference, Corpus Christi, Texas, April 28–30, 2003. FES
Designing compact tokamak-stellarator hybrids
Ware et al. described for the first time the complete physics properties
of compact, drift-optimized tokamak-stellarator hybrids with a high-shear
tokamak-like rotational transform profile and |B| that is quasipoloidally
symmetric (Figure 7). The rotational transform in these hybrid configurations
is produced primarily by a self-consistent bootstrap current. The nonaxisymmetric
components of |B| yield a bootstrap current lower than that in an axisymmetric
device, which leads to good magnetohydrodynamic stability properties without
the need for a conducting wall. The neoclassical confinement improves with
increasing , leading to a new form of configurational invariance in
these stellarators.
A. S. Ware, S. P. Hirshman, D. A. Spong, L. A. Berry, A.
J. Deisher, G. Y. Fu, J. F. Lyon, and R. Sanchez, “High- equilibria
of drift-optimized compact stellarators,” Phys. Rev. Lett. 89, 125003
(2002). FES
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Figure 7 |
Understanding neoclassical tearing modes
Tearing modes are magnetic islands formed by the topological rearrangement of magnetic field lines through reconnection. The prevention of neoclassical tearing modes (NTMs) in tokamak plasmas is a major challenge because they can degrade plasma confinement. Ideal modes can seed NTMs through forced reconnection, yet in sawtoothing discharges it is not well understood why a particular sawtooth crash seeds an NTM after several preceding sawteeth did not. Also, tearing modes sometimes appear and grow without an obvious ideal mode, causing a seed island. Based on theoretical and experimental results, Brennan et al. have proposed and tested a new mechanism for tearing-mode onset that explains these puzzling observations.
D. P. Brennan, R. J. La Haye, A. D. Turnbull, M. S. Chu, T. H. Jensen, L. L. Lao, T. C. Luce, P. A. Politzer, E. J. Strait, S. E. Kruger, and D. D. Schnack, “A mechanism for tearing onset near ideal stability boundaries,” Phys. Plasmas 10, 1643 (2003). FES
Simulating pseudochaotic dynamics
Nonlinear dynamics and chaos simulations are important for understanding the microscale of plasma turbulence, as well as for understanding general nonlinear dynamical effects. A family of random systems with zero Lyapunov exponent is called pseudochaos. Zaslavsky and Edelman have shown how the fractional kinetic equation can be introduced for pseudochaos and how the main critical exponents of fractional kinetics can be evaluated from the dynamics. Pseudochaotic dynamics and fractional kinetics can be applied to the behavior of streamlines or magnetic field lines.
G. M. Zaslavsky and M. Edelman, “Pseudochaos,'' in Perspectives and Problems in Nonlinear Science, edited by E. Kaplan, J. E. Marsden, and K. R. Sreenivasan (Springer-Verlag, New York, 2003), pp. 421–443. FES, ONR, NSF
Turbulent transport with kinetic electrons
Chen et al. have developed a new electromagnetic kinetic electron-simulation
model in 3D toroidal flux-tube geometry that uses a generalized split-weight
scheme, where the adiabatic part is adjustable. This model includes electron–ion
collisional effects. For H-mode parameters, the nonadiabatic effects of kinetic
electrons increase linear growth rates of the ion-temperature-gradient-driven
(ITG) modes, mainly due to trapped-electron drive. The ion heat transport
is also increased from that obtained with adiabatic electrons. The linear
behavior of the zonal flow is not significantly affected by kinetic electrons.
The ion heat transport decreases to below the adiabatic electron level when
finite plasma is included due to finite- stabilization of the ITG modes.
Y. Chen, S. E. Parker, B. I. Cohen, A. M. Dimits, W. M. Nevins, D. Shumaker, V. K. Decyk, and J. N. Leboeuf, "Simulations of turbulent transport with kinetic electrons and electromagnetic effects," Nucl. Fusion 43, 1121 (2003). FES, NASA