Browsing by Subject "Drift waves"

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Characterization of instability regimes in the Helimak, a simple magnetic torus
(2017-05) Williams, Chad Blaine; Gentle, Kenneth W.; Bengtson, Roger; Berk, Herbert; Breizman, Boris; Hallock, Gary
Plasma instability regimes are investigated in the Texas Helimak, a toroidal magnetic confinement device that acts as a physical approximation of the theoretical sheared cylindrical slab. The transition from a regime dominated by the ideal interchange instability to one dominated by drift waves is investigated through experimental measurements of the parallel wavenumber, k [parallel symbol], and other statistics, such as the measurement of applicable power spectra. It is shown that the ideal interchange instability dominates in Helimak plasma at high pitch while drift waves dominate at low pitch. Investigations relative to the effects of an applied bias voltage, both positive and negative, are also carried out. In these it is shown that the application of bias greatly influences both the nature and the amplitude of the turbulence.
Drift wave stability and transport in tandem mirrors
(2009-05) Pratt, Jane Lillian; Horton, C. W. (Claude Wendell), 1942-
In recent years experimental advances at the GAMMA-10 facility in Tsukuba, Japan have shown that tandem mirrors should remain an important subject for theoretical study. The absence of toroidal curvature and relatively weak internal plasma parallel currents in a tandem mirror gives the mirror system strongly favorable stability and transport properties compared with toroidal systems. GAMMA-10 experiments (T. Cho et al. PRL (97), 2006) demonstrate that sheared plasma rotation suppresses turbulent radial losses by controlling radial potential profiles. Achievements of the GAMMA-10 include 2.5 keV ion confining potentials and electron temperatures approaching 1 keV (T. Cho, Private correspondence, Dec 24th, 2006). Total energy confinement times for the GAMMA-10 experiment are significantly larger than corresponding empirical confinement times in toroidal devices. At the temperatures currently achieved in the GAMMA-10, the end loss rate [mathematical symbols] 100 ms so that radial losses determine the energy confinement time [mathematical symbol], as intended in tandem mirror reactor designs (R. F. Post, T.K. Fowler, et al., Fusion Science and Technology, (47), 2005). The most current measurements of [mathematical symbol] are on the order of 72 ms. Tandem mirrors exhibit a qualitatively different type of drift wave transport than do toroidal devices, as we have shown by developing confinement time scaling predictions (J. Pratt and W. Horton, Phys. Plasmas (13), 2006. W. Horton, J. Pratt, H.L. Berk, M. Hirata. Proceedings of the Open Magnetic Systems For Plasma Confinement Conference. Tsukuba, Japan, July 17-21, 2006). These predictions use a variety of standard transport models, e.g., Bohm, gyro-Bohm, and electron-temperature gradient models. We analyze electrostatic drift wave eigenmodes for the electrostatic potential and the magnetic perturbation in the GAMMA-10. We use teraFLOPS speed, large scale parallel computers to integrate the orbits in models of the drift wave losses in the GAMMA-10. We extrapolate these results to reactor designs for the kinetically stabilized tandem mirror reactor proposed by Post et al., and discuss implications for its stability, transport, and performance.
Theory and simulation of sheared flows and drift waves in the large plasma device and the helimak
(2006) Perez, Jean Carlos; Horton, Wendell
This work develops a comprehensive understanding of the physical properties of drift waves and vortices in the presence of shear flows in strongly magnetized plasma, including comparison to experimental data. Theoretical modeling and simulation of plasma instabilities is carried out for two important university-scale basic plasma experiments, namely, the Helimak device at The University of Texas at Austin and the LArge Plasma Device (LAPD) at the University of California, Los Angeles. Both machines possess simple geometry while retaining key physical properties of major magnetic confinement devices, making theoretical study amenable to analytical and numerical treatment in a more realistic manner. A large number of probes, near steady state operation of the plasma and relative control of key external physical parameters provide unique plasma data for the study of drift-wave and Kelvin-Helmholtz turbulence. In addition to thorough conventional diagnosis, a new and promising diagnostic tool is introduced to directly measure plasma vorticity. This probe, that we call the vorticity probe, was built and used at the LAPD facility to obtain vorticity data in sheared flow experiments. The data is analyzed in great detail in this work and compared to linear and nonlinear theory. The study in this work is made in several stages. The first stage is to analyze probe data using standard data analysis techniques to obtain meaningful results that can be compared to theoretical models. In the second stage we perform two-fluid linear stability analysis to identify the type of fluctuations that dominate the turbulence. In the linear analysis, analytical and numerical tools are used to illustrate the plasma waves and instabilities. For the nonlinear simulations of the instabilities and vortices we develop a comprehensive fully nonlinear pseudo-spectral code that uses a Chebyshev-Fourier decomposition of the solutions, technique that is widely used in the field of computational fluid dynamics. The new modelling and interpretation of structures developed in this work under well controlled laboratory conditions should help researchers working on large fusion devices and space-astrophysical plasmas to gain a better understanding of the problem of plasma turbulent transport.
Turbulent particle and thermal transport in magnetized plasmas
(2013-05) Fu, Xiangrong; Morrison, Philip J.; Horton, C. W. (Claude Wendell), 1942-
The particle and thermal transport by low-frequency drift waves in magnetized plasmas are studied with theories and simulations. Universal in inhomogeneous plasmas, drift waves in Earth's ionosphere, the GAMMA-10 Tandem Mirror machine, the Columbia Linear Machine and C-Mod tokamak are studied in this thesis. The first investigations are E x B particle transport in the given electric and magnetic fields of the GAMMA-10 mirror machine at the University of Tsukuba in Japan. The results show that the formation of E [subscript r]-shear by local heating of electrons can reduced the radial particle loss. The turbulent impurity particle transport driven by various modes in the MIT tokamak Alcator C-Mod is studied by a quasilinear theory and compared to experimental measurement of Boron density profiles. A code is developed for solving eigensystems of drift wave turbulence equations for the multi-component fusion plasmas and calculating quasilinear particle fluxes. The calculations are much faster than nonlinear simulations and may be suitable for real-time analysis and feedback control of tokamak plasmas. The electron temperature gradient (ETG) mode is a candidate mechanism for anomalous electron thermal transport across various magnetic confinement geometries. This mode was produced in the Columbia Linear Machine (CLM) at Columbia University. Large scale simulations of the ETG mode in the CLM by a gyrokinetic code GTC are carried out on supercomputers at TACC and NERSC. The results show good agreement with experiments in the dominant mode number, wave frequencies and the radial structure. Some nonlinear properties are also analyzed using the code.