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 TP6: Poster Session VII: DIII-D II; KSTAR, FDF, ITER, Diagnostics; FRC and Spheromak; Basic Turbulence and Transport; Gyrokinetics |
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Room: Marsalis A/B, 9:30am - 12:30pm |
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TP6.00001: DIII-D |
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TP6.00002: Advanced Scenario Development Using Off-Axis Neutral Beam Current Drive in DIII-D M. Murakami, J.M. Park, T.C. Luce, C.C. Petty, R. Prater, T.S. Taylor, M.R. Wade A goal of the DIII-D AT program is the development of Advanced Tokamak scenarios in support of ITER and future tokamak reactors. Research on DIII-D has focused on the stationary fully noninductive, high-bootstrap fraction scenario development. One-dimensional self-consistent scenario modeling using both scaled experimental transport and theory-based (GLF23) models shows that the proposed 10-MW off-axis NBCD with high power electron cyclotron and fast wave heating and current drive will allow full noninductive operation at high beta with flat safety factor profile with $q_{min}>2$ for twice the current relaxation time, consistent with $Q=5$ steady-state operation of ITER. The modification of the DIII-D NB system for off-axis NBCD will provide a flexible scientific tool for understanding transport, energetic particles, heating and CD physics, and validating the off-axis NBCD in support of scenarios for ITER and FDF. [Preview Abstract] |
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TP6.00003: Tearing Mode Stability of Steady-State Scenario Plasmas in DIII-D F. Turco, T.C. Luce, J.R. Ferron, C.C. Petty, P.A. Politzer, A.D. Turnbul, D.P. Brennan, T.A. Casper, C.T. Holcomb, M. Murakami Tearing mode stability is crucial for high-performance scenarios intended for steady-state operation. The appearance of tearing modes in DIII-D discharges leads to loss of energy confinement, but more importantly to redistribution of the current profile that is not recoverable with the available non-inductive current drive sources. Tearing modes can appear after 1-2 s at constant pressure (i.e., on the resistive evolution time scale). The stability is strongly affected by the location and distribution of the applied electron cyclotron current drive (ECCD), but not through direct interaction with the mode rational surface. The local evolution of the current density (as measured by motional Stark effect spectroscopy) will be shown for a sequence of similar discharges that vary only in the applied ECCD distribution. Comparison to ideal and resistive MHD linear stability calculations will be carried out. [Preview Abstract] |
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TP6.00004: Internal Mode Structure of Resonant Field Amplification in DIII-D M.J. Lanctot, G. Navratil, H. Reimerdes, I.N. Bogatu, Y. In, M.S. Chu, A.M. Garofalo, G.L. Jackson, R.J. La Haye, E.J. Strait, A.D. Turnbull, Y.Q. Liu, M. Okabayashi, W.M. Solomon The sensitivity of high-$\beta$ plasmas to error fields is caused by a paramagnetic plasma response to error fields with a topology that is resonant with the structure of weakly-damped resistive wall modes (RWM), a phenomenon referred to as resonant field amplification (RFA) [1]. The RFA has been driven in DIII-D H-mode plasmas by applying slowly-rotating, low-n magnetic fields with a set of 12 coils located inside the vacuum vessel. Measurements of the RFA mode structure have been obtained using a pair of soft x-ray photodiode cameras. A virtual diagnostic has been developed to compare the measurements to the eigenfunctions for the free boundary external kink and the RWM, which were calculated using the stability codes GATO and MARS-F. Details of the analysis will be presented.\par \vskip6pt \noindent [1]~A.H.\ Boozer, Phys.\ Rev.\ Lett.\ {\bf 86}, 5059 (2001).\par [Preview Abstract] |
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TP6.00005: Resistive Wall Modes With ECCD-NTM Suppressed High Beta Plasmas in DIII-D M. Okabayashi, M.S. Chance, H. Takahashi, M.J. Lanctot, H. Reimerdes, Y. In, M.S. Chu, A.M. Garofalo, G.L. Jackson, R.J. La Haye, E.J. Strait, A.S. Welander, R.J. Buttery ECCD at $q\sim 2$ was applied to stabilize the 2/1 NTM to clarify the role of the NTM in RWM stability near the no-wall kink beta limit at low rotation. The plasmas under NTM suppression were more stable even with near zero plasma rotation, except for a few cases where global mode with zero mode rotation was excited by an ELM growing with wall time scale. After the ECCD pulse, global MHD modes were excited leading to the plasma termination. The mode time-evolution and structures were similar both for slow or zero mode rotation, indicating that NTMs are dominant and easily excited in the condition broader than RWM. The plasma rotation at the termination was lower than reported in 2007. In the transient period, fishbone instabilities were excited with a strongly-deformed spatial structure due to the coupling to marginal RWM (Fishbone-driven RWM). [Preview Abstract] |
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TP6.00006: Comparison of RWM Stabilization Strategies in DIII-D S. Yang, E. Schuster, D.A. Humphreys, M.L. Walker, Y. In, J.S. Kim One of the major non-axisymmetric instabilities under study in the DIII-D tokamak is the resistive wall mode (RWM), a form of plasma kink instability whose growth rate is moderated by the influence of a resistive wall. The FAR-TECH/General Atomics RWM dynamic model represents the plasma surface as a toroidal current sheet and represents the wall using an eigenmode approach. This dynamic model is used for the design of model-based controllers that have the potential of outperforming present proportional-derivative (PD) controllers. We report on validation of this dynamic model, a required step before implementation of any model-based controller in the DIII-D plasma control system. In addition, simulation results are presented comparing the performance of advanced controllers synthesized using the validated dynamic model and present non-model-based PD controllers. [Preview Abstract] |
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TP6.00007: Transport Changes Near $q$=1 Surfaces in the DIII-D Tokamak M.E. Austin, K.W. Gentle, C.T. Holcomb, G.R. McKee, M.W. Shafer, C.C. Petty, T.L. Rhodes Spontaneous improvement in electron energy transport is routinely seen in the core of DIII-D discharges as the safety factor $q$ approaches $1$. For a range of discharge types with constant heating conditions, core $\chi_e$ is seen to decrease just before the first sawtooth, as evidenced by a sharp rise in central electron temperature. The behavior is similar to barriers observed in reverse shear plasmas near $q_{min}=2,3$; however, the picture is made more complicated by the onset and decay of a variety of MHD modes. Changes in turbulent fluctuation amplitudes are noted as well as the presence of high frequency coherent modes. Further evidence of $q=1$ transport barriers is exhibited in an off-axis EC-heated discharge where $q_{min}$ is driven above 1 and unusual hollow $T_e$ profiles with sharp changes in gradients are observed. We compare the data with models of transport barriers near low-order rational $q$ surfaces. [Preview Abstract] |
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TP6.00008: Long-Wavelength Turbulence Scaling Properties in DIII-D G.R. McKee, D.J. Schlossberg, M.W. Shafer, C.H. Holland, P. Gohil The scaling properties of long-wavelength density fluctuations are investigated in DIII-D L-mode and H-mode plasmas utilizing the expanded high-sensitivity 2D Beam Emission Spectroscopy (BES) system. BES employs a 64-channel system that utilizes a radially-scannable 8x8 array sampling multiple radial and poloidal correlation lengths, allowing for full sampling of the 2D wavenumber spectrum. Measurements of turbulence as a function of several important dimensionless parameters ($\kappa$, $T_e/T_i$, ion mass, $\rho_\star$) are obtained, showing that fluctuation intensity increases strongly with decreasing plasma elongation (at constant $q$), consistent with increased thermal transport and reduced energy confinement. In contrast, increasing $T_e/T_i$ increases momentum and thermal transport with little change in low-$k$ density fluctuations. Measurements obtained during a $\rho_\star$ ($\rho_i/a$) scan in hydrogen will also be presented. Together, these measurements will be crucial for comparing with transport simulations, such as GYRO and TGLF. [Preview Abstract] |
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TP6.00009: Multi-scale, Multi-field Turbulence Response During Electron Cyclotron Heating (ECH) W.A. Peebles, T.L. Rhodes, A.E. White, G. Wang, J.C. Hillesheim, L. Schmitz, L. Zeng, E.J. Doyle, G.R. McKee, M.W. Shafer, J.C. DeBoo, M.A. Van Zeeland ECH at $r/a \sim 0.4$ significantly modifies the electron temperature of LSN Ohmic plasmas with minimal effect on local ion temperature and electron density. A unique array of turbulence diagnostics was used to study the turbulence response across all turbulent scales ($0 < k\rho_s < 10$) and for two distinct turbulent fields. At $r/a \sim 0.6$, low-$k$ electron temperature fluctuations increased significantly ($\sim 3$) with ECH. In contrast, low and intermediate-$k$ density fluctuations remained unchanged or reduced slightly. High-$k$ ($\sim$35 cm$^{-1}$) density fluctuations, associated with the electron temperature gradient driven mode, increased by $>$30\%. Interestingly, low-$k$ density and electron temperature fluctuations were found to be locally correlated across the frequency range $\sim$10-100 kHz. This unique data set can be utilized to rigorously test the turbulence physics inherent in nonlinear gyrokinetic turbulence codes. [Preview Abstract] |
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TP6.00010: Multi-scale Fluctuation Behavior During Balanced NBI H-mode in the DIII-D Tokamak T.L. Rhodes, W.A. Peebles, L. Schmitz, A.E. White, J. Hillesheim, G. Wang, L. Zeng, E.J. Doyle, G.R. McKee, M.W. Shafer Using a unique array of diagnostics, the behavior of fluctuations over a broad range in wavenumber ($0 \leq k\rho_s \leq 10$) and for two different fields (density and electron temperature) during balanced NBI is examined. Simultaneous co- and counter-NBI is utilized to access H-mode with a minimum of total torque. The diagnostic set utilized includes FIR scattering, Doppler backscattering, correlation ECE, BES, reflectometry, and high-$k$ mm-wave backscattering. Near $r/a=0.6$ high-$k$ fluctuations ($k\sim 35$ cm$^{-1}$) increase substantially with NBI (x3) prior to and during the H-mode. Low-$k$ temperature fluctuations also increase with NBI ($r/a$=0.6-0.7) prior to decreasing at the LH transition. In contrast, intermediate $k$ density fluctuations appear relatively unchanged just prior to the LH. Analysis of flow activity ($E_r$, $E_r$ shear, and zonal flow), transport behavior from power balance, and comparison to linear gyrokinetic stability calculations (including sensitivity studies) will be presented. [Preview Abstract] |
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TP6.00011: Sawtooth Crash Modifications to Intermediate-k Turbulence and Flows on DIII-D J.C. Hillesheim, W.A. Peebles, T.L. Rhodes, L. Schmitz, T.A. Carter, P.A. Gourdain, G. Wang Initial data from the new multichannel Doppler backscattering and reflectometry diagnostic system on DIII-D are presented including results showing increased intermediate-$k$ ($k_\perp \rho_s \sim 2$, $k_\perp \sim$ 6-7 cm$^{-1}$) density fluctuations and flow velocity modifications associated with the sawtooth crash. In Ohmic plasmas at radial location $\rho \stackrel{\sim}{=} 0.4$, density fluctuation propagation velocity excursions of up to $\sim$9 km/s from an equilibrium of \hbox{$\sim0 $ km/s} and relaxation back to equilibrium in less than 200 $\mu$s are observed. Density fluctuation levels are observed to increase within the same period and to remain at a higher level for an additional 200-500 $\mu$s. Details and capabilities of the new multichannel (currently 4-channel) system for simultaneous multipoint measurements with frequencies separated by 350 MHz and tunable in the range 53-78 GHz are described. [Preview Abstract] |
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TP6.00012: Phase Contrast Imaging Measurements of Turbulence in ELMing Plasmas J.C. Rost, M. Porkolab, J.R. Dorris, K.H. Burrell The Phase Contrast Imaging (PCI) diagnostic on DIII-D has been upgraded since the last study of ELMs. Signal-to-noise and bandwidth have been increased, and a rotating mask system added to spatially localize measurements of electrostatic modes is used here to determine if modes are aligned with the magnetic field. During Type I ELMs, a burst of turbulence coincides with the magnetic perturbation but lasts longer, several milliseconds compared to a few 100 microseconds. ELM-like modes seen between Type I ELMs (possibly Type II ELMs) are smaller and shorter on the magnetics and D$_\alpha$, but the density perturbations on the PCI are similar in amplitude to that during the Type I ELMs. The turbulence during the ELM-like modes has a phase velocity about one third that of the turbulence during the Type I ELM, indicating a different radial location or different driving instability. Measurements with the mask show that the ELM-related turbulence is aligned with the total background magnetic field, similar to electrostatic modes. [Preview Abstract] |
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TP6.00013: Localized Measurement of Short Wavelength Plasma Fluctuations Using the DIII-D Phase Contrast Imaging Diagnostic J.R. Dorris, J.C. Rost, M. Porkolab, K.H. Burrell The DIII-D Phase Contrast Imaging (PCI) turbulence diagnostic measures density fluctuations in two operational configurations: (1) line-integrated over the entire viewing chord or (2) using a rotating mask system that takes advantage of the vertical variation of radial magnetic field to make localized measurements along the PCI chord. The localized length of chord is inversely proportional to wavenumber, making this technique more favorable for short wavelength modes ($k >$ 8/cm). Improvements in PCI S/N have allowed measurements to be obtained showing broadband turbulent fluctuations to 20/cm. Initial analysis of localized data during ECH heated L-mode discharges shows a change in turbulence propagation direction from ion to electron diamagnetic direction with increasing density. Gyrokinetic simulations using the GYRO code are being performed to investigate the mechanism for this propagation direction reversal. Further refinements of the localization analysis will allow a more robust quantitative comparison to simulation. Details of analysis enhancements and comparison to simulation will be presented. [Preview Abstract] |
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TP6.00014: Radial Correlation Length of Turbulent Density Fluctuations in DIII-D Plasmas, G. Wang, W.A. Peebles, T.L. Rhodes, J.C. Hillesheim, E.J. Doyle, L. Schmitz, A.E. White, L. Zeng The radial correlation length (L$_r$) of turbulent density fluctuations is an important quantity for understanding turbulent transport in tokamak plasmas. In DIII-D, a correlation reflectometer and a tunable multi-channel reflectometer system allow L$_r$ measurement with both high time and spatial resolutions. In this presentation, results will be reported from two recent areas of study: (1) Measurements of L$_r$ in Ohmic, ECH, and NBI heated L-mode plasmas, and comparisons to predictions from nonlinear gyrokinetic codes; and (2) Measurements of fast changes in L$_r$ during the L- to H-mode transition. Preliminary results show that: (1) in general, L$_r$ increases from the edge to core and scales as (5-10) $\rho_s$ ($\rho_s$ is the ion gyroradius using $T_e$); (2) L$_r$ decreases with ECH in otherwise Ohmic plasmas; and (3) at the L-H transition, cross-correlations between reflectometer channels close to separatrix increase simultaneously as divertor D$_\alpha$ signal starts to decrease, while in the bulk plasma, they begin decreasing after a propagation delay. [Preview Abstract] |
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TP6.00015: Neoclassical Poloidal and Toroidal Velocities of Impurity Ions S.K. Wong, V.S. Chan, W.M. Solomon The poloidal and toroidal velocities of impurity ions in a two-ion species plasma for large aspect ratio circular flux surfaces are calculated in the banana and Pfirsch-Schulter regimes of neoclassical theory. The toroidal velocity is allowed to be comparable to the thermal speed of the impurity ions. Closed form expressions are obtained for these velocities in terms of the radial electric field as well as density and temperature gradients. The standard kinetic derivation adopted is compared with the moment approach to the same problems in the case of small toroidal velocities. Comparisons of the calculated poloidal velocity with experimental observations in DIII-D [1] show improved agreement due to the allowance of larger toroidal flows. \vskip6pt \noindent [1] W.M. Solomon, Phys. Plasmas 13, 056116 (2006). [Preview Abstract] |
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TP6.00016: Hybrid TGLF-GYRO Steady-State Transport Calculations with TGYRO J. Candy, R.E. Waltz, M. Fahey We report on the status and development of TGYRO, a steady-state transport manager which enables GYRO [1] to be used for steady-state transport calculations. This project is partner to the larger FACETS SciDAC project. Currently, TGYRO has two distinct operational modes: a $\bf{local}$ and a $\bf{global}$ mode. In this presentation we focus on the local mode only, for which a Newton-type iterative scheme is applied to local transport flux computations, including GYRO gyrokinetic simulations. Iteration continues until the turbulent fluxes match target fluxes determined by (a) self-consistent thermonuclear sources, radiation, exchange, etc., or (b) imposed target fluxes derived from experimental power balance. This approach makes significant use of the TGLF [2] transport model. In this presentation we will discuss the success and limitations of the iterative solver as applied to GYRO simulations, and the use of hybrid TGLF-GYRO flux calculations to enhance robustness and speed convergence of the method. \vskip6pt \noindent [1] J. Candy, R.E. Waltz, J. Comput. Phys. 186, 545 (2003).\par \noindent [2] J.E. Kinsey, et al., Phys. Plasmas 15, 055908 (2008). [Preview Abstract] |
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TP6.00017: Rotating Gyrokinetics with Poloidal and Toroidal Rotation G.M. Staebler A canonical transformation was found in Ref. [1] that includes poloidal and toroidal rotation in a Vlasov equilibrium distribution function. The magnetic field is required to have closed nested flux surfaces but does not need to be axisymmetric. This starting point is extended to a gyro-kinetic equation in the transformed Lagrangian phase space. It is found to be much easier to include rotation first into the Vlasov equation and then gyro-average to obtain a reduced gyro-kinetic equation than it is to try and add rotation to the gyrokinetic equation directly. The contribution to the gyrokinetic equation obtained has a simple form in vector notation. There are velocity shear and Coriolis terms for both the toroidal and poloidal rotation components. Higher order ``obit squeezing'' terms are also found from the canonical transformation. Similarities and differences with previous work will be presented. \vskip6pt \noindent [1] G.M. Staebler, Phys. Plasmas 11, 1064 (2004). [Preview Abstract] |
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TP6.00018: Gyrokinetic Simulation Tests of Tracer and Quasilinear Transport R.E. Waltz, A. Casati The GYRO gyrokinetic code is used to test quasilinear theory via a one-step plasma-tracer simulation and additionally via two-step linear and nonlinear plasma simulations convoluting the quasilinear and field spectral weights. The one-step simulations have ion and electron``plasma species'' at full densities and ``tracer species" at trace densities making no contribution to the Poisson field equation. If the tracer and plasma gyrokinetic equations are identical, then so are their respective (energy and particle channel) diffusivities. Comparing tracer and plasma diffusivities when the tracer equation nonlinearity is deleted provides a quantifiable test of the quasilinear transport approximation (QLTA). [Similarly, other approximations like ``passive scalar transport'' can be tested by plasma-tracer simulations.] The two-step test preserves ambipolarity but includes only the leading linear modes at each wave number. Quasilinear transport spectra appear to breakdown first at high wave numbers. Net quasilinear energy diffusivities are typically $1.4-1.8$ larger. Possible nonlinear modifications to improve QLTA are explored. [Preview Abstract] |
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TP6.00019: Development of electron thermal transport model in DIII-D discharges T. Rafiq, A.Y. Pankin, G. Bateman, A.H. Kritz, F.D. Halpern, J.D. Callen The electron thermal transport in tokamak plasmas is investigated with predictive integrated modeling simulations using a choice of different electron thermal transport models. Two models for transport driven by Electron Temperature Gradient (ETG) modes are considered: (1) the ETG part of the GLF23 transport model; and (2) the Horton model for the the electromagnetic part of the ETG anomalous transport [1]. These models are combined with the paleoclassical model [2] for electron thermal transport. ASTRA predictive simulation results obtained using these models are compared with one another and compared with experimental data from DIII-D H-mode discharges in an effort to discriminate among the models. It is found that the electromagnetic limit of the Horton model is important near the magnetic axis where the ETG mode in the GLF23 model is below threshold. The paleoclassical model is found to be needed to produce the observed edge pedestal in the \mbox{DIII-D} simulations. \newline [1] W. Horton, B. G. Hong, and W. M. Tang, \newline Phys. Fluids 31, 2971 (1988). \newline [2] J. D. Callen, Nucl. Fusion 45, 1120 (2005). [Preview Abstract] |
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TP6.00020: Development of an IMFIT Fast-Ion Physics Module and Study of Low-Frequency Sound-Wave Gap Structure W. Guo, M.S. Chu, L.L. Lao, H.E. St. John, G. Abla, M. Choi, A.D. Turnbull, M.A. Van Zeeland, Y.M. Jeon, J.M. Park, G.Q. Li, Q. Ren A predictive understanding of fast-ion transport and stability is essential for burning plasma experiments. Recently, much lower frequency fast-ion instabilities in the sound-wave frequency range were observed in several tokamaks. The CONT code that has been extensively applied for investigation of Alfven frequency gap structure is being enhanced to study these lower frequency acoustic-wave gap structures. An equilibrium database with various plasma geometry, pressure, and current profiles is being compiled for this study. To facilitate the study of fast-ion transport and stability, a fast-ion physics module is being developed for integration into the Integrated Modeling and Fitting (IMFIT) tool. IMFIT provides a convenient platform for interactions among various fast-ion physics codes such as CONT, NUBEAM, ORBIT-RF, and NOVA-K and testing of fast-ion transport and stability theory against experimental observations. Details will be presented. [Preview Abstract] |
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TP6.00021: Development of an IMFIT Energy Transport Module and Modeling of DIII-D Energy Transport With and Without MHD Activity Y.M. Jeon, L.L. Lao, H.E. St. John, M.S. Chu, G.M. Staebler, R. Prater, J.M. Park, G.Q. Li, Q. Ren, W. Guo A predictive understanding of energy transport is crucial for tokamak and future burning plasma experiments. MHD activities such as sawteeth oscillations and tearing modes can significantly increase the energy transport. Preliminary results from the analysis of a DIII-D giant-sawtooth discharge show that before the application of FW when the sawteeth are tiny both electron and ion energy transport are reasonably described by the GLF23 and MM95 models. To facilitate the analysis, an energy transport module is being developed for integration into the IMFIT tool. IMFIT provides a convenient platform for testing of theory-based transport models against experimental measurements. Details will be presented including analysis of 3/2 tearing mode effects on the energy flux from the recent DIII-D counter ECCD resistive MHD experiments and testing of the new TGLF transport model that is being implemented into the ONETWO transport code. [Preview Abstract] |
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TP6.00022: Developments of an Integrated Modeling Tool to Support DIII-D and EAST Research L.L. Lao, G. Abla, M.S. Chu, H.E. St. John, R. Prater, V.S. Chan, W. Guo, G. Li, C. Pan, Q. Ren, J. Qian, S.J. Wang, B. Wan, J. Li, Y.M. Jeon, J.M. Park, R. Srinivasan Integrated modeling is an important element of tokamak research essential to the interpretation and planning of experiments, and the design and construction of next step devices. This presentation summarizes the development of a modern efficient integrated modeling and fitting tool IMFIT to support key elements of the DIII-D and EAST experimental programs. The goal is to develop and to validate the capability to understand and predict the behavior of tokamak discharges. The IMFIT framework is based on Python and Common Component Architecture (CCA). IMFIT consists of a Task Manager that interacts with users through GUI and manages various tasks using a number of Component Managers responsible for different physics and service areas such as Equilibrium, Transport, and Stability. The framework makes use of Task List, Task Flow, and Communication files to dynamically generate action sequence to accomplish various tasks. Details will be presented. [Preview Abstract] |
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TP6.00023: Development of an IMFIT Current Profile Module and Analysis of Current Profiles in DIII-D G.Q. Li, L.L. Lao, M.S. Chu, G. Abla, H.E. St. John, R. Prater, M.A. Makowski, J.M. Park, M. Murakami, J.M. Jeon, Q. Ren A predictive understanding of current profile evolution is crucial for tokamak research. In a tokamak, the current density is composed of an inductive component, a bootstrap current contribution, and a non-inductively driven portion. Accurate determination of these current components from experimental measurements is challenging, particularly in the edge region due to the large contribution of radial electric field to the MSE signals. To facilitate the analysis, a current profile physics module is being developed for integration into the IMFIT tool. IMFIT provides a convenient platform for testing of Ohmic, NBI, FWCD, and ECCD current models against results deduced from experimental measurements. Detailed will be presented including the evolution of the ECCD profiles with and without tearing mode activities from the recent DIII-D counter ECCD resistive MHD experiments. [Preview Abstract] |
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TP6.00024: Development of an IMFIT Particle Transport Module and Modeling of Tokamak Particle Transport C. Pan, L.L. Lao, M.S. Chu, H.E. St. John, R. Prater, G.Q. Li, Q. Ren, W. Guo, J.M. Park, W.M. Stacey A predictive understanding of particle transport is essential to present tokamak and future burning plasma experiments. It will shed light on plasma fueling, impurity control, and removal of helium ash. Particle flux can be deduced from experimental measurements using the particle balance equation and the ONETWO transport code. To improve the analysis, the GTNEUT neutral transport code is being coupled to ONETWO. Neutral source from the surrounding wall can contribute significantly to the particle balance, although it is less well-determined. One way to proceed is to vary the neutral particle flux at the boundary to match the measured line-average density. To facilitate the modeling, a particle transport module is being developed for integration into the IMFIT tool. IMFIT provides a convenient platform for interactions among particle transport physics codes and testing of particle transport theory against experimental observations. Details will be presented. [Preview Abstract] |
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TP6.00025: Automated and Between-Shot Analysis of DIII-D Discharges R. Prater, X. Lee, S. Flanagan, R.J. Groebner, L.L. Lao, Q. Peng, D.P. Schissel, H.E. St. John, M.R. Wade Productivity of experiments in DIII-D has been improved by developing more detailed profile analysis between shots. The ONETWO transport code is now run routinely between shots to provide reasonably accurate evaluation of profiles as a function of time of heating power, torques, fast ion density, bootstrap current fraction, noninductive fraction, Mach number, and many other quantities of interest to the experimenter. For example, the alignment of electron cyclotron current drive with a rational $q$-surface and an evaluation of the peak driven current density compared with the local bootstrap current as a function of time as the plasma beta is increased is extremely useful in guiding experiments on control of neoclassical tearing modes. Overnight the ONETWO runs are repeated with improved models that are too lengthy for between-shot use. User runs with different profiles or equilibria may also be stored in MDSplus for general use. These results provide an excellent starting point for off-line analysis, for example using GYRO. [Preview Abstract] |
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TP6.00026: Turbulent Confinement Models and Transport Solvers for PTRANSP H.E. St. John, L.L. Lao, G.M. Staebler, D. McCune The PTRANSP development project is a multi-institutional, multi-year, effort designed to ensure that PTRANSP has the best physics models and computational support possible. Here we describe the first year's contribution that the collaborators at GA have made to this project. The primary result is a standalone module, GCNMP, that is coupled to PTRANSP and makes possible the analysis and simulation of tokamak discharges currently using the most promising confinement models, TGLF and GLF23. The TGLF model is particularly challenging in terms of computational effort required and several MPI based parallel schemes were implemented in order to minimize the time required to obtain solutions to the particle and energy balance equations. Using a client server approach the single cpu environment of PTRANSP remains intact. The reformulation of the transport equations strictly in terms of flows rather than the awkward, unnatural decomposition into diffusivities is presented. Application of TGLF to ITER, DIII-D, and benchmarks against XPTOR are discussed. [Preview Abstract] |
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TP6.00027: Determination of Edge Current in the DIII-D Tokamak B. Hudson, C.C. Petty, K.H. Burrell, L.L. Lao, P.B. Snyder, D.M. Thomas, S.L. Allen, C.T. Holcomb, M.A. Makowski A statistical study of the measurement requirements necessary to infer the current profile in the pedestal of the DIII-D tokamak via motional Stark effect (MSE) and lithium beam diagnostics has been performed. Error in the edge magnetic probes and flux loops, Thomson scattering, and MSE and lithium beam polarimetry was systematically varied to determine the uncertainty in inference of the current profile via magnetic reconstructions using EFIT. It will be shown that edge MSE or lithium beam data accurate to 0.1 deg is sufficient to resolve the peak edge current density to within 20\%. This accuracy in the current profile is needed for validation of the peeling-ballooning stability model. Future work using a Hall probe to directly measure the magnetic field components in the plasma edge and SOL to further constrain edge current reconstruction will be presented. [Preview Abstract] |
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TP6.00028: Measurements of the Internal Magnetic Field on DIII-D Using Intensity and Spacing of the Motional Stark Multiplet N.A. Pablant, K.H. Burrell, M.F. Gu, C.T. Holcomb We describe a version of a motional Stark effect diagnostic based on the relative line intensities and spacing of Stark split D$_{\alpha}$ emission from the neutral beams. Using this technique both the magnitude and direction of the internal magnetic field can be measured, in contrast to motional Stark effect (MSE) polarimetry, which can only measure the direction. This system, named B-Stark, has been recently installed on the DIII-D tokamak. To find the magnetic pitch angle, we use the ratio of the intensities of the $\pi_3$ and $\sigma_1$ lines. These lines originate from the same upper level, and so are not dependent on the population levels. The magnitude of the internal B-field is determined from the wavelength separation of the various Stark components. We fit the spectra using a simple Stark model in which the upper level populations of the D$_{\alpha}$ transition are treated as free variables. The magnitude and direction of the magnetic field obtained using this diagnostic technique compare well with measurements from MSE polarimetry and EFIT. [Preview Abstract] |
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TP6.00029: Upgraded Thomson Scattering Diagnostic on DIII-D D.M. Ponce, B.D. Bray, T.M. Deterly, C.L. Hsieh, C. Liu A new final-design polychromator assembly and data acquisition test system is being deployed during plasma operations at DIII-D for electron temperature and density measurement. The new polychromator features APD detectors that incorporate 500 MHz bandwidth amplifiers in a trans-impedance amplification circuit with low input bias current with an overall amplification of 360 times and an integration and a sample-and-hold circuit to provide analog output into a data acquisition digitizer. The APD detector box has high voltage bias supplies and temperature sensing incorporated in a single box. The SNR will be improved at least by a factor of 2$^{1/2}$. The data acquisition system is a D-TACQ DT100 system with a 96 channel 250 kSPS ACQ196CPCI board. We present gain calibration, spectral calibration, plasma run, and calculated electron temperature and density data with respective comparison of the data from the existing Thomson system. We also present the plan for complete deployment of an upgraded system that will incorporate two new 50 Hz lasers and a new Field Programmable Gate Array (FPGA) control and timing module. [Preview Abstract] |
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TP6.00030: Stability Modeling of the Internal Kink and Application to Tokamak Sawteeth A.D. Turnbull, M. Choi, M.S. Chu, L.L. Lao, N.N. Gorelenkov, G.J. Kramer The tokamak sawtooth is widely understood to be due to the ideal or resistive internal kink with the stability threshold modified by kinetic effects. Kinetic effects are usually modeled using generalizations of the Porcelli theory, or with more sophisticated numerical calculations such as from the NOVA-K code. These models, which contain complementary physics, are compared to each other and against DIII-D experiments. The ideal contribution is found to be sensitive to the equilibrium - especially the radius of $q=1$. Investigation of the effect on the fast particle contribution of varying radial and pitch angle distribution with NOVA-K reveals that the stability is also sensitive to these. Yet using reconstructed equilibria, the Porcelli model, which assumes simplified models for the fast ions, yields results in agreement with experiment. The reasons for this are investigated by studying the dependence of the NOVA-K results on these and the equilibrium parameters. Important physics - especially rotation - is also missing from both models and these effects are also investigated. [Preview Abstract] |
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TP6.00031: Edge Turbulence Analysis of DIII-D Single Null L-Mode Discharges with BOUT, M.A. Makowski, M. Groth, M. Umansky, X. Xu, J.A. Boedo The BOUT code, based on the 2-fluid Braginskii equations, has demonstrated good qualitative agreement with the measured characteristics of edge turbulence [1]. In this study, we extend these results by making detailed comparisons between BOUT turbulence simulations and experimental measurements from two DIII-D single null L-mode discharges. The shots are well diagnosed and include probe measurements of the turbulence and time-averaged multiple charge state density profiles. The UEDGE code is used to obtain the poloidally dependent profiles of $T_e$, $T_i$, $n_i$, $\phi$ the plasma potential, and the ion flow velocity, $V_i$ that are needed as initial conditions for the BOUT code. Synthetic diagnostics are being developed to compare the BOUT predictions with the experimental measurements. Results of the BOUT runs and their comparison with the measurements will be presented and the poloidal dependence of the simulated turbulent fluxes will be compared with the diffusive fluxes predicted by UEDGE. \vskip6pt \noindent [1] M.V. Umansky, presented at 21st US Transport Taskforce Workshop, Boulder, Colorado (2008). [Preview Abstract] |
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TP6.00032: BOUT++: A framework for parallel plasma fluid simulations B.D. Dudson, H.R. Wilson, M.V. Umansky, X.Q. Xu, P.B. Snyder A modular code for the solution of 3D fluid equations in curvilinear coordinates is presented. Aimed at simulating Edge Localised Modes (ELMs) in tokamak x-point geometry, the code is able to simulate a wide range of fluid models (magnetised and unmagnetised) involving an arbitrary number of scalar and vector fields. Time evolution is fully implicit, and 3rd-order WENO schemes are implemented for accurate capturing of shocks. Benchmarks are presented for linear and non-linear problems (the Orszag-Tang vortex) showing good agreement. Performance of the code is tested by scaling with problem size and processor number. Linear and non-linear simulations of ELMs are presented, and compared to the linear ELITE code. These initial results, show that BOUT++ recovers many of the features expected from analytic theory of peeling-ballooning modes. [Preview Abstract] |
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TP6.00033: Relationships Between ELM Period and Scrape-Off-Layer Current (SOLC) Density at Divertor Tiles in DIII-D H. Takahashi, E.D. Fredrickson, M.J. Schaffer, J.G. Watkins The current spike from the electrical break-down of the SOL plasma sheath at the divertor tiles as the ion saturation current ($I_{sat}$) density falls toward the thermo-electrically-driven SOLC has been suggested [1] as a mechanism involved in ELM triggering process. Currents flowing in the SOL plasma are limited to ion-saturation current density at the sheath until the driving potential results in sheath breakdown. Density pump-out, following the ELM-produced SOL density rise, provides a robust mechanism of generating a cyclical phenomenon by reducing the ion saturation current density toward SOLC values. The present work, based mainly on Langmuir probe measurements, reports on investigations of the quantitative relationship between SOLC and $I_{sat}$ densities at the ELM onset time as well as how they influence the length of inter-ELM period. \vskip6pt \noindent [1] H. Takahashi, et al., 32nd EPS Conf. on Control. Fusion and Plasma Phys., Tarragona, Spain (2005). [Preview Abstract] |
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TP6.00034: A SOL and Divertor Plasma Calculation Model W.M. Stacey A computationally tractable calculation of the tokamak SOL and divertor is being developed to provide background edge plasma parameters for neutral atom transport codes, separatrix boundary conditions on the electrostatic potential and plasma flows for pedestal calculations, edge ``boundary conditions'' for core transport codes, etc. The 1D parallel particle, momentum, energy, current and electrostatic potential balance equations are solved in a strip connecting inner and outer divertor target plates, subject to sheath boundary conditions. Perpendicular ExB and gradB/curvature drifts and currents associated with the latter are included. The calculation provides poloidally dependent distributions of the density, temperature, parallel flow and electrostatic potential at the separatrix, from which a number of other quantities may be estimated. [Preview Abstract] |
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TP6.00035: OEDGE Modeling of SOL Flow Experiments on DIII-D J.D. Elder, P.C. Stangeby, S.L. Allen, M.E. Fenstermacher, M. Groth, J.A. Boedo, D.L. Rudakov, B.D. Bray, N.H. Brooks, A.W. Leonard, W.P. West, J.G. Watkins, E.A. Unterberg A series of SOL flow experiments was conducted on DIII-D in an upper single-null configuration. The plasma density, temperature and flow were measured at the outer midplane and the crown of the plasma using fast reciprocating probes. Methane was puffed toroidally symmetrically through the lower pumping plenum at a rate which did not perturb the plasma conditions. We present initial OEDGE modeling results of the empirical plasma reconstructions and carbon emissions. Source terms in the empirical reconstruction were imposed to match both the plasma conditions and the flow measurements. This background plasma solution is used as the basis for the carbon emission modeling. The OEDGE code was enhanced for this study by the addition of classical and neoclassical drifts acting on the impurity ions, as well as simultaneous plasma interaction with the divertor targets and sections of the main wall. [Preview Abstract] |
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TP6.00036: Implementation of GTNEUT for Analysis of the Effect of Neutral Particles on Edge Phenomena in DIII-D Z.W. Friis, W.M. Stacey, R.J. Groebner, T.D. Rognlien In order to establish an accurate and computationally economical code for routine analysis of the effects of neutral atoms on edge phenomena in DIII-D, the Georgia Tech 2D Neutral Transport (GTNEUT) code [1], which can use an arbitrarily complex two-dimensional grid to represent the plasma edge geometry, is being implemented in an integrated system. The grid generation capability built into the UEDGE code [2], which utilizes equilibrium-fitting data taken from experiment, is being incorporated. GTNEUT requires the background plasma temperature and density distributions. Measured background plasma data (Thomson, Langmuir probe, CER, and reflectometer) will be supplemented with calculated parameters from a coupled core plasma and ``2-point'' divertor model. \vskip6pt \noindent [1] J. Mandrekas, J. Computer Phys. Comm. 161, 36 (2004).\par \noindent [2]T.D. Rognlien, et al., User Manual of UEDGE Edge-Plasma Transport Code (2007). [Preview Abstract] |
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TP6.00037: Assessment of Collateral Effects to Tokamak Systems During Planned Air Baking of DIII-D to Simulate ITER Tritium Removal B.W.N. Fitzpatrick, J.W. Davis, A.A. Haasz, P.C. Stangeby, S.L. Allen, R. Ellis, W.P. West Thermo-oxidation is a method for removing carbon-based co-deposits and is unique in its ability to remove deuterium from tokamak co-deposits, including tile gaps and shaded areas. This is a possible technique for tritium removal on ITER. In these experiments, we examine the potential collateral (deleterious) effects of a thermo-oxidation experiment planned for DIII-D. Experiments at Toronto have set the process parameters to be 10 Torr air exposure at 250$^{\circ}$ -350$^{\circ}$C for two hours. Components of interest were placed in a vacuum chamber filled with O$_2$ or air and baked at 250$^{\circ}$ and 350$^{\circ}$C. Components were examined for visual or mechanical changes, and when appropriate, mass change. In special cases, optical or electromagnetic diagnostics were performed. Components tested spanned a wide variety of materials and functions, e.g., cryopump components, structural, mechanical and diagnostic components, and fast wave antennae. To date, nearly all DIII-D systems have passed these tests. Detailed results will be presented. [Preview Abstract] |
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TP6.00038: Operation of a Six Gyrotron System on the DIII-D Tokamak J. Lohr, M. Cengher, I.A. Gorelov, D. Ponce The gyrotron installation on the DIII-D tokamak has been completed. The system comprises six 110 GHz 1 MW gyrotrons limited administratively to generating pulses up to 5 s in length. Measured transmission line efficiencies are about 75\% for 100 m line length, and injected power $\leq$3.5 MW has been achieved. The rf beams can be directed anywhere in the tokamak upper half plane for co- and counter current drive with complete control of the elliptical polarization. The gyrotrons are modulated by tetrodes either with pre-programmed time dependence of output power or under control of the DIII-D Plasma Control System (PCS) responding in real time to changing experimental requirements. System reliability has consistently exceeded 80\%. Upgrade plans include installation of two more waveguide lines, bringing the total to eight, gradual replacement of the 1.0 MW diode gyrotrons by \hbox{1.5 MW} depressed collector tubes, construction of additional high voltage power supplies, and modification of the launchers to accommodate real time steering of the rf beams under PCS control. [Preview Abstract] |
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TP6.00039: Extension of Gyrotron Pulse Length on the DIII-D ECH System I.A. Gorelov, J. Lohr, D. Ponce, M. Cengher There are six 110 GHz gyrotrons in operation on the DIII-D ECH system. All of these tubes have been designed for 10 s pulses at 1 MW, 80 kV and 40 A. The gyrotrons are routinely operated on DIII-D for up to 5 s pulse duration, consistent with physics requirements for the typical DIII-D discharge duration of $\sim$6 s. Following collector failures in the first production run of three gyrotrons, it was found that the collectors were developing leaks due to cyclic fatigue. Improved sweeping of the electron beam along the collector and modest restriction on gyrotron operating space have resolved the collector issue, and thermal model calculations have been verified by collector temperature measurements. Future DIII-D program plans call for 10 s discharge durations to validate the physics basis for stationary discharges significantly exceeding the current redistribution time in support of ITER. These physics requirements will require full design pulse lengths from the gyrotrons, therefore we are planning to increase the pulse duration of the gyrotrons on DIII-D to 10 s. Technical considerations and model calculations associated with the longer pulse lengths will be presented. [Preview Abstract] |
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TP6.00040: Progress on Measurements of the ECH Power on DIII-D M. Cengher, J. Lohr, I.A. Gorelov, D. Ponce Power injected by the electron cyclotron heating (ECH) system in the DIII-D tokamak is measured on a shot to shot basis for the six 110 GHz, 1 MW class gyrotrons of the ECH system. The rf power generated by each gyrotron is determined calorimetrically from cooling circuits of cavity, matching optics unit and dummy loads. Injected power is then determined from measured transmission line efficiencies. The rf pulse length and time dependence are measured using a diode at the first miter bend in the transmission line. For the database, its signal is normalized to the injected power measurement. Two new approaches to power measurement are being tested. The first is an inline power monitor that uses the rf leaked through a small gap in the waveguide to measure the power transmitted. Test results show proportionality to the input power. The second method uses an in-vessel bolometric measurement of the radiated power from a low temperature, low density plasma generated by injected rf with toroidal magnetic field, but no Ohmic or other heating sources. Conditions affecting the accuracy of each method will be discussed. [Preview Abstract] |
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TP6.00041: KSTAR, FEF, ITER, DIAGNOSTICS |
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TP6.00042: Pre-ionization and plasma startup by second harmonic electron cyclotron wave for KSTAR first plasma J.H. Jeong, S. Park, W. Namkung, M.H. Cho, Y.S. Bae, S.H. Hahn, J. Kim, M. Joung, H.L. Yang Second harmonic electron cyclotron heating (ECH)-assisted startup has been carried out for the fully superconducting KSTAR first plasma. The 84 GHz electron cyclotron (EC) wave, which is the second harmonic EC resonance frequency at R~1.7 m with the toroidal magnetic field of 1.5-T, is launched in the linearly polarized pure extraordinary (X2) mode from the low field side. An EC power $\sim$350 kW was sufficient to obtain a reliable KSTAR first plasma with a loop voltage of 2 V (0.23V/m) at the innermost vacuum vessel wall and the hydrogen pre-fill gas pressure of 3.5e-5 mbar. In this paper, the experimental results of the second harmonic ECH pre-ionization will be presented. [Preview Abstract] |
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TP6.00043: Equilibrium Reconstruction of KSTAR First Plasma K.-I. You, D.K. Lee, B.H. Park, S.G. Lee, J.G. Bak, S.H. Seo, S.H. Hahn, L.L. Lao To reconstruct the plasma equilibrium of KSTAR (Korea Superconducting Tokamak Advanced Research), we have made some modification to the EFIT code and installed it on our computing system. An MDSplus system is used for the data storage of KSTAR; thus, the EFIT reads experimental data from the MDSplus server and writes the results to it. We have modified some subroutines of the EFIT code for direct link with the MDSplus server. KSTAR PF and TF coil systems use a CICC (Cable-In-Conduit Conductor) type superconductor. The CICC jacket material for most PF and all TF coils is Incoloy 908, which is a magnetic material with relative magnetic permeability greater than 10 in low external field. The Incoloy 908 effects should, therefore, be considered in analyzing the magnetic diagnostics data. In this paper, we present our efforts to reconstruct the plasma equilibrium with EFIT code, including the compensation of Incoloy 908 effects. [Preview Abstract] |
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TP6.00044: Analysis of KSTAR first plasma with using a 0-D plasma evolution code Jayhyun Kim, S.W. Yoon, Y.S. Bae, S.H. Hahn, Y.U. Nam, Jinil Chung, H.K. Na, W.C. Kim, H.L. Yang, K.R. Park, Y.K. Oh, S.H. Jeong, D. Mueller, J.A. Leuer During the commissioning of KSTAR, the time evolution of the plasma current is essential to check the performance of the device. Thus plasma variables, which determine the electromagnetic properties of the plasma column, are calculated by solving the particle and power balance of plasma in a volume- averaged manner. In this work, the plasma and vessel currents, which are obtained from coupled circuit equations, are compared with measurements. A series of comparisons enable us to evaluate the range of operational parameters such as energy confinement time, absorbed heating power, and impurity radiation loss. In addition, simulated evolutions of electron density and temperature are also checked with interferometer measurements (electron density) and electron cyclotron emission measurements (electron temperature) in order to reduce the uncertainty of analysis. [Preview Abstract] |
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TP6.00045: Real-Time Plasma Control During KSTAR First Plasma N.W. Eidietis, S.H. Hahn, Y.K. Oh, D.A. Humphreys, A.W. Hyatt, J.A. Leuer, M.L. Walker Real-time control of Korea Superconducting Tokamak Advanced Research (KSTAR) discharges was successfully demonstrated during the KSTAR coil commissioning and first plasma period of May-June 2008. KSTAR, located at the National Fusion Research Institute (NFRI) in Daejeon, South Korea, is designed to explore steady-state, high-performance tokamak plasmas. The KSTAR plasma control system (PCS) was developed in a collaboration between General Atomics and NFRI, and derives from the PCS originally developed for DIII-D and currently in use at NSTX, MAST, EAST, Pegasus, and MST. A suite of electromagnetic analysis and plasma control design tools, closely integrated with the KSTAR PCS, were used extensively to support the startup campaign. Initial coil commissioning was completed successfully, utilizing the power supply voltage and current feedback algorithms. Plasma current, density, and rudimentary control of major radius was demonstrated during startup discharges. [Preview Abstract] |
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TP6.00046: Unique Scenario Development Issues for KSTAR First Plasma J.A. Leuer, D.A. Humphreys, A.W. Hyatt, M.L. Walker, S.W. Yoon, S.H. Hahn, Y.K. Oh, N.W. Eidietis KSTAR commissioning constraints including site power constraints, low vacuum baking temperature, magnetic diagnostics limitations, and coil magnetic properties combine to produce a very small operational window for plasma breakdown and initial plasma operation. We describe these challenges and unique solutions developed for KSTAR machine commissioning. KSTAR is the first major tokamak to use high performance Nb$_3$Sn superconductor in its magnets. However, this required use of magnetic material (Incoloy 908) in the magnet construction. Analysis methods, including 2-D and 3-D finite element methods, were developed to simulate the influence of this nonlinear magnetic material. Methods were developed to include the nonlinear material influence in standard reconstruction models. Successful first plasma required inclusion of these unique features in the KSTAR scenario development. The analysis methods will be discussed and aspects of the first plasma campaign will be described. [Preview Abstract] |
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TP6.00047: Magnetic diagnostic measurements of the first plasma operation for KSTAR S.G. Lee, J.G. Bak, E.M. Ka Magnetic diagnostics include Rogowski coils, flux loops, and magnetic field probes have been utilized for the first plasma operation on KSTAR. These magnetic diagnostics provide the plasma current, loop voltage and poloidal flux, and local poloidal magnetic field which are essential measurements for the first plasma operation. The expected plasma current of about 100 kA for the first plasma on the KSTAR tokamak was successfully achieved with an ECH pre-ionization. Magnetic diagnostic measurements for the plasma current, loop voltage and flux, and poloidal magnetic field under various plasma discharges will be discussed. [Preview Abstract] |
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TP6.00048: Diamagnetic loop measurement in the KSTAR first plasma J.G. Bak, S.G. Lee, E.M. Ka, S.W. Yoon, D.K. Lee The diamagnetic loop (DL) measurement by using two concentric poloidal loops as a loop pair enclosing a plasma column is done during the first plasma campaign in the Korea Superconducting Tokamak Advanced Research (KSTAR) machine. The compensation of vacuum fluxes in the DL measurement can be done by using a slight difference between the pick-up areas of the two loops. For the evaluation of a diamagnetic flux from the two loop signals, the contributions of toroidal field (TF) and poloidal field (PF) coils in the DL measurement are investigated from vacuum flux measurements using the two loops. In this work, experimental results from the initial DL measurement and the studies on the evaluation of the diamagnetic flux from the two loop signals will be presented. [Preview Abstract] |
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TP6.00049: Fusion Development Facility Mission R.D. Stambaugh, V.S. Chan, A.M. Garofalo, J.P. Smith, C.P.C. Wong A Fusion Development Facility (FDF) is proposed to fill the gaps between ITER and current experiments and a fusion demonstration power plant (DEMO). FDF should carry forward Advanced Tokamak physics and enable development of fusion's energy applications. Near term advanced tokamak physics will be used to achieve steady-state with burn, producing 100-250 MW fusion power with modest energy gain ($Q<5$) in a modest sized device (between DIII-D and JET). FDF will further develop all elements of AT physics for an advanced performance DEMO. With neutron flux at the outboard midplane of 1-2 MW/m$^2$, continuous operation for periods up to two weeks, and a goal of a duty factor of 0.3 on a year, FDF can produce fluences of 3-6 MW-yr/m$^2$ in ten years of operation. The development of blankets suitable for tritium, electricity, and hydrogen production will be done in port modules. The most promising candidates will be deployed as full blankets in FDF. FDF will have a goal of demonstrating closure of the fusion fuel cycle, producing its own tritium. FDF, ITER, IFMIF, and other AT devices will provide the basis for a fusion DEMO power plant of the ARIES-AT type. [Preview Abstract] |
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TP6.00050: Fusion Development Facility Machine Design Aspects J.P. Smith, V.S. Chan, A.M. Garofalo, R.D. Stambaugh, C.P.C. Wong To fill the gap prior to building a fusion demonstration power plant (DEMO), a Fusion Development Facility (FDF) is proposed. As currently configured, FDF is a copper, water-cooled coil machine capable of running continuously for several weeks with the goal to test several blanket configurations in its lifetime. To accommodate multiple changes in blankets, a machine configuration must be chosen that allows for the efficient remote exchange. The TF coil configuration drives the primary maintenance approach decision. A TF coil with joints similar to DIII-D and Alcator C-Mod, allows for one maintenance approach while a continuously wound TF coil drives a different approach. The base machine design parameters are described. The different machine configuration options are presented which consider the design aspects for the machine including alignment of the first wall and divertor, coolant access, and exchange of the blanket. [Preview Abstract] |
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TP6.00051: Fusion Development Facility Divertor Design A.M. Garofalo, T.W. Petrie, J.P. Smith, M.R. Wade, V.S. Chan, R.D. Stambaugh, R. Bulmer, D.N. Hill, C.J. Lasnier, T.D. Rognlien, D. Ryutov, M. Umansky, J. Canik, R. Maingi, M. Kotschenreuther, S.M. Mahajan, P. Valanju The Fusion Development Facility (FDF) is proposed as an aspect ratio 3.5 tokamak with major radius of 2.5 m and total heating power of $\sim$100 MW, that would make possible a fusion demonstration power plant (DEMO) of the ARIES-AT type as the next step after ITER. One of the main issues that needs to be resolved on the path from ITER to DEMO is that of power exhaust. Requirements for the FDF divertor include: heat flux reduction to levels manageable in steady-state; adequate fuel exhaust for density control; adequate helium exhaust to maintain $f_{He}<\,$10\%; compatibility with good confinement and stability in order to achieve high level mission goals. FDF will have to show integrated physics/technical solutions compatible with a 14 MeV neutron environment. A compact, symmetric double null divertor addressing these requirements is being designed. UEDGE and SOLPS modeling of divertor performance will be presented. Other divertor concepts, including Snowflake and SX divertors, are being considered for FDF. [Preview Abstract] |
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TP6.00052: Optimization of Transport for the FDF V.S. Chan, A.M. Garofalo, L.L. Lao, R. Prater, H.E. St. John, R.D. Stambaugh To meet the goal of optimizing fusion reactors and enabling development of fusion's energy applications, the Fusion Development Facility (FDF) [1] has to operate with advanced tokamak physics to achieve high confinement and stability. Theoretical modeling suggests this can be achieved with a high edge pedestal and strong plasma shaping. The fusion gain can be enhanced by a peaked density profile consistent with high-performance discharges with low collisionality in existing tokamaks. One of the challenges for FDF is to sustain the plasma in steady state which requires judicious use of external current and flow drive to supplement a large bootstrap current fraction. The high pedestal pressure and peaked density pose challenges for penetration of neutral beam and rf waves for profile control. In this study, a 1-D transport code is used to evaluate the tradeoff between pedestal pressure, confinement, stability and the ability to control and maintain a steady-state current profile. An optimized parametric space for FDF operation will be presented and experimental data supporting the projected FDF performance will be discussed.\par \vskip6pt \noindent [1] V.S. Chan, et al., ``Physics Validation of a Fusion Development Facility Based on the Tokamak Approach,'' submitted to Fusion Sci.\ Technol. [Preview Abstract] |
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TP6.00053: Fusion Blanket Development in FDF C.P.C. Wong, J.P. Smith, R.D. Stambaugh To satisfy the electricity and tritium self-sufficiency missions of a Fusion Development Facility (FDF), suitable blanket designs will need to be evaluated, selected and developed. To demonstrate closure of the fusion fuel cycle, 2-3 main tritium breeding blankets will be used to cover most of the available chamber surface area in order to reach the project goal of achieving a tritium breeding ratio, TBR~$> 1$. To demonstrate the feasibility of electricity and tritium production for subsequent devices such as the fusion demonstration power reactor (DEMO), several advanced test blankets will need to be selected and tested on the FDF to demonstrate high coolant outlet temperature necessary for efficient electricity production. Since the design goals for the main and test blankets are different, the design criteria of these blankets will also be different. The considerations in performing the evaluation of blanket and structural material options in concert with the maintenance approach for the FDF will be reported in this paper. [Preview Abstract] |
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TP6.00054: How DEMO Should Look if ITER Fails Paul Garabedian Runs of the NSTAB equilibrium and stability code show there are many 3D solutions of the advanced tokamak problem subject to axially symmetric boundary conditions. These numerical simulations based on mathemetical equations in conservation form predict that the ITER project will encounter pervasive disruptions and ELMs crashes. Test particle runs of the TRAN Monte Carlo code show that for quasineutrality to prevail in tokamaks a certain minimum level of 3D asymmetry of the magnetic spectrum is required which is comparable to that found in quasiaxially symmetric (QAS) stellarators. The computational theory suggests that a QAS stellarator with proportions like those of ITER might make a better magnetic fusion reactor. There is an attractive candidate with major radius 8m, plasma radius 3m, and magnetic field 5T. The external field is generated by just twelve moderately twisted coils that seem not hard to construct. Papers about this configuration have been published recently by the author in the Proc. Natl. Acad. Sci. U.S.A. [Preview Abstract] |
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TP6.00055: Plans for low and high power testing of 170 GHz Corrugated waveguide for ITER T.S. Bigelow, J.B. Caughman, D.A. Rasmussen, M.A. Shapiro, J.A Sirigiri, R.J. Temkin A high power waveguide test stand for testing and qualifying corrugated waveguide components is being developed at ORNL. The US is committed to supplying 24 complete waveguide transmission lines to ITER. ITER has requested that the system be capable of operating at 2 MW cw due to the likelihood that a higher power gyrotron be developed. A number of prototype components have been procured commercially and studied at low power at MIT and ORNL. High power tests have been performed at 1 MW cw power levels at JAEA on similar components. A full-length prototype system test is desirable to confirm the overall performance of the system including such issues as losses, waveguide heating, mode purity, interaction between components, polarization control, vacuum pumping, and arcing. Plans are to perform these tests at a power level of 2 MW cw or higher using a resonant ring principle and a lower power gyrotron such as a 140 GHz 300 kw cw tube on hand at ORNL and a 170 GHz 500 kw cw tube that will be procured. Although progress has been delayed by funding for ITER projects, considerable work on completing the test stand, procurement of some prototype components, vacuum testing and some low power testing of components and a shorter resonant-ring have been completed. Work Supported by US DOE under contract DE-AC05-OR22725. [Preview Abstract] |
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TP6.00056: Measurement of 170 GHz Corrugated Waveguide Transmission Line and Components for ITER ECRH D.S. Tax, E.N. Comfoltey, S.T. Han, M.A. Shapiro, J.R. Sirigiri, R.J. Temkin, P.P. Woskov, T.S. Bigelow, D.A. Rasmussen The US will build 24 Transmission Lines (TL) connecting the 170 GHz gyrotrons to the ITER tokamak. A set of 170 GHz corrugated waveguide TL components built by General Atomics is under test at MIT. As critical TL components the miter bends were tested, as they are responsible for the largest mode conversion and loss. A loss of 0.05$\pm $0.02dB per miter bend has been measured using a vector network analyzer. Mode content has been measured in the HE$_{11}$ mode cold-test launcher and after the miter bend. These measurements have motivated revisiting the theoretical mode conversion losses in ITER TLs. It is shown that the mode conversion loss of the power in an HE$_{11}$ mode at a miter bend is greatly altered by the presence of even a small proportion of higher order modes (HOMs) in the TL, and is a strong function of HOM's magnitude and phase relative to the HE$_{11}$ mode. The resulting total loss in the ITER TLs is expected to be very different from the loss previously predicted using single mode theory. [Preview Abstract] |
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TP6.00057: Design of a tangential Phase Contrast Imaging diagnostic for the TCV tokamak Stefano Coda, Alessandro Marinoni, Rene Chavan, Jean Claude Magnin, Guy Pochon A PCI diagnostic has been designed, built and installed in the TCV tokamak, employing a 7-cm wide CO2 laser beam in a near-toroidal launch direction. The system can resolve wavelengths in the range 0.1 to 7 cm, sampling 32 chords at 12.5 Msamples/sec, thus appraising microinstabilities ranging from ion to electron spatial scale lengths. Being an imaging technique it does not face difficulties in investigating highly inhomogeneous regions, such as transport barriers. The tangential configuration, combined with appropriate spatial filtering techniques, provides an excellent spatial resolution, of the order of 1\% of the minor radius. The spatial filtering also allows the selection of different spatial regions (e.g. deep core or edge). Wavelengths and correlation properties can be recovered from the spatial mapping. First data will be presented along with preliminary interpretation and comparisons with linear gyrokinetic simulations. [Preview Abstract] |
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TP6.00058: Large Area Black Body Source for ITER ECE In Situ Calibration P.E. Phillips, H-P. Liu, M.E. Austin, J. Beno, R.F. Ellis, A. Ouroua, W.L. Rowan Here we report on the development of a prototype hot calibration source for the ITER ECE system. The source is intended to meet the requirements described in DDD55 Overview of Diagnostics. The requirement is for two \textit{200 mm} diameter sources, one operating at atmospheric pressure and the other operating in vacuum near the plasma. Both will operate at temperatures up to \textit{800 $^{\circ}$C} and have an emissivity $>0.7$ for frequencies greater than \textit{120 GHz}. A realistic thermal model of a SiC hot calibration source with a surface of pyramids has been constructed. The model results will be compared with data from a prototype SiC source. The microwave emissivity of a full size (\textit{200 mm} OD) source was measured in the range \textit{100-500 GHz} with a calibrated Michelson radiometer. Heating methods under consideration will be discussed.. [Preview Abstract] |
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TP6.00059: The Motional Stark Effect Diagnostic for ITER E.L. Foley, F.M. Levinton, H.Y. Yuh, L.E. Zakharov The United States has been tasked with the development and implementation of a Motional Stark Effect (MSE) system on ITER. In the harsh ITER environment, MSE is particularly susceptible to degradation, as it depends on polarimetry, and the polarization reflection properties of surfaces are highly sensitive to thin film effects due to plasma deposition and erosion of a first mirror. Here we present the results of a comprehensive study considering a new MSE-based approach to internal plasma magnetic field measurements for ITER. The proposed method uses the Line Shifts in the MSE spectrum (MSE-LS) to provide a radial profile of the magnetic field magnitude. To determine the utility of MSE-LS for equilibrium reconstruction, studies were performed using the ESC-ERV code system. A near-term opportunity to test the use of MSE-LS for equilibrium reconstruction is being pursued in the implementation of MSE with Laser-Induced Fluorescence (MSE-LIF) on NSTX. Simulation results for ITER and NSTX will be presented, and the relative merits of the traditional line polarization approach and the new line shift approach will be discussed. [Preview Abstract] |
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TP6.00060: ITER MSE Calibration System Investigation Alissa Mezzacappa, Fred Levinton, Elizabeth Foley, Howard Yuh, Brian Daly The ITER MSE calibration system will stream information about primary mirror polarization properties which continuously evolve. Details for the system are investigated by reflecting linearly polarized light of known polarization on an aluminum or gold mirror at varying angles of incidence and tilt angle creating a three dimensional matrix. Reflected light then passes though a polarimeter. Mueller matrix algebra is used to reconstruct polarization previous to reflection. Mueller matrix algebra provides a one-to-one correlation between experimental and input quantities for measurements with no tilt. For measurements taken with a tilt angle our Mueller matrix algebra does not yet yield proper results. A linear relationship between tilt and calculated experimental values of polarization for the gold mirror specimen is found. Further work is required to understand the relationship of tilt to calculated polarization. Requirements for the calibration system may need to include a calibration beam capable of multiple solid angles to reconstruct the mirror properties at the solid angle of the heating beam. [Preview Abstract] |
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TP6.00061: Local 3-D Toroidal Plasma Tomography Using the Phillips-Tikhonov Regularization Method Seung Hun Lee, Junghee Kim, Wonho Choe Tomography is one of a powerful diagnostic method for obtaining the local information from the line-integrated plasma emission in fusion devices. The 3-D tomography is a complicated task compared to the 2-D tomography. Because of the limitation of the spatial distribution of the array detectors around a torus, the regularization algorithm such as the Phillips-Tikhonov method is advantageous to achieve more reliable reconstruction. In this work, we performed a feasibility study of 3-D tomography for toroidal plasmas. Four tangentially-viewing array detectors of each array consisting of 16x16 detector elements were assumed to be implemented. The reconstruction area is configured as 70 cm x 50 cm of poloidal cross-section and 40 toroidal layers, which has spatial resolution of 5 cm. We chose the phantoms which are KSTAR plasma-like profiles combined with the equilibrium flux surfaces with n = 0, 1, 2, 3 modes. The change of the emission peak in each layer in the reconstruction result agrees reasonably well with that of the phantom, with relative error of 5 - 10 {\%}. [Preview Abstract] |
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TP6.00062: High Resolution Nd:YAG Thomson Scattering Diagnostic system using ICCD Jianshan Mao, Xiaoqi Xi, Junyu Zhao, Tao Zhang Nd:YAG Thomson Scattering Diagnostic system are installed on EAST Tokamak. The signal intensity, collection ability and the accuracy are expected to be improved using the new designed polychromators and collection lens system. Also, the range of the measurable Te and Ne are extended. A high resolution Thomson scattering (simply called TVTS) system has been designed on EAST tokamak. New system will employ the Littrow type grating spectrometer and ICCD cameras equipped with intensifier III, which can make simultaneous measurements of the electron temperature and density in plasma at many positions along the laser chord. The spatial and temporal resolution can reach 10mm and several ms, respectively. [Preview Abstract] |
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TP6.00063: Diagnostics for Heavy Ion Beam Driven Warm Dense Matter Experiments Pavel Ni, Francis Bieniosek, Matthaeus Leitner, William Waldron A set of diagnostic has been developed for the WDM experiments at the NDCX linear accelerator which is used as a driver for heating metallic targets. The diagnostics are aimed at the in-situ measurement of temperature, expansion velocity and pressure of a WDM sample. A specially developed three-channel pyrometer probes color temperatures at 750 nm,1000 nm and 1500 nm, with 75 ps temporal resolution. The system has a broad dynamic range (black body), with a lower limit $\sim $2000 K and upper limit $\sim $100000 K. Continuous target emission from 450 nm to 850 nm is recorder by a custom spectrometer, consisting of a high dynamic range Hamamatsu streak camera and a holographic grating. The system is calibrated absolutely with a tungsten ribbon lamp (NIST traceable). Hydrodynamic expansion velocity of a target's free surface is measured by a commercially available all- fiber Doppler shift laser interferometer (VISAR). [Preview Abstract] |
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TP6.00064: Digital Holography for Fast High-Spatial-Resolution Density Measurements C.E. (Tommy) Thomas Jr., L.R. Baylor, S.K. Combs, S.J. Meitner, D.A. Rasmussen A digital holography demonstration system has been used on the ORNL Disruption Mitigation Test Stand to make extremely high spatial resolution (less than one mm) 2-D density measurements. Although holographic interferometry has long been used as a density diagnostic, this is the first application of digital holography (no plates, no darkroom, no chemicals, no waiting) to high-resolution spatial measurements of density. In the present experiments a CO2 laser and 320 x 256 pixel ($\sim $82,000 pixels) infrared-camera based digital holography system has measured the expansion plume from a fast-valve high-pressure gas injection system similar to the disruption mitigation system presently being tested on DIII-D. The present system is capable of exposures as short as 20 microseconds at 30 frames-per-second (FPS). The experimental design and data from the ORNL Disruption Mitigation Test Stand experiments will be presented, along with discussion of the design and specifications for a second generation system with pulse exposures as short as one microsecond and frame rates as high as 10,000 FPS, proposed for use as a density and fluctuation diagnostic on the LTX experiment at PPPL. [Preview Abstract] |
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TP6.00065: Fitting of spectrometer parameters for precise wavelength calibration F. Scotti, R.E. Bell A prototype high-throughput lens-based visible spectrometer is being developed as a general-purpose tool to study plasma discharges in visible wavelengths. The stigmatic imaging with the f/1.8 200 mm lenses will allow many spectra to be stacked vertically. The curvature of the emission lines in the focal plane increases with slit height and changes with wavelength. A precise wavelength calibration is desired to accurately determine wavelengths of all spectra on a 2D detector using only the grating angle as input. A stepping-motor controlled sine-drive is used to adjust grating position. Techniques are being developed to aid in the alignment of each spectrometer component: lenses, grating, sine drive, slit, and detector. All the parameters in the grating equation as well as slit orientation and focal plane tilt are determined by using multiple calibration spectra and regression methods. Ne, Ar, and Hg pen lamps are used to produce calibration spectra. The reproducibility of the calibration and effects of temperature on the wavelength accuracy will be tested. Supported by U.S. DOE Contract DE-AC02-76CH03073. [Preview Abstract] |
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TP6.00066: Experimental and Theoretical Studies of the Soft X-ray Emission of Tungsten from Tokamak Plasmas Joel Clementson, Peter Beiersdorfer, Gregory Brown, Ming Gu, Albert Wynn, Charles Weatherford Tungsten is presently making a comeback as a plasma facing material in magnetic fusion devices. The need for atomic data is paramount in order to diagnose W seeded plasmas. We present a measurement of nickel-like tungsten using crystal spectroscopy, where we have resolved the strong spectral feature at 7.93 {\AA}, previously observed at the ASDEX Upgrade tokamak. We show the feature to be made up of an electric quadrupole and a magnetic octupole component. The measurements were done using the SuperEBIT electron beam ion trap in Livermore and include line positions, intensities, and polarization. We compare our measurements with calculations carried out using the FAC modeling code and show that the two lines are sensitive to electron density. This work was performed under the auspices of the United States Department of Energy by Lawrence Livermore National Laboratory under contracts W-7405-ENG-48 and DE-AC52-07NA-27344. [Preview Abstract] |
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TP6.00067: Expanded Capability of the Edge CXRS System on JET T.M. Biewer, D.L. Hillis, Y. Andrew, N.C. Hawkes, K.-D. Zastrow, K. Crombe, C. Strege A new instrument has been added to the Joint European Torus (JET) edge charge-exchange recombination spectroscopy (CXRS) suite of diagnostics. The new instrument consists of a short focal length spectrometer coupled to a fast framing CCD camera. With the addition of this instrument, the number of sightlines is increased by 20 to a total of 58 views. The radial range of the edge CXRS system extends from $r/a \quad \sim $ 0.5 to $\sim $ 1.0. The time resolution of this instrument is improved to 10 ms. This diagnostic observes simultaneously the neutral-beam induced charge-exchange emission of C VI at 529.1 nm and of Ne X at 524.8 nm, complementing the existing edge CXRS instruments, which can be tuned to observe any visible wavelength of interest. An overview of the CXRS diagnostic system on JET will be presented. Preliminary data will be shown from the current JET campaign. [Preview Abstract] |
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TP6.00068: Edge current density measurement using Li-beam on JT-60U Kensaku Kamiya, Takaaki Fujita, Atsushi Kojima, Hirotaka Kubo A 10keV Li-beam probing system has been developed on JT-60U for the edge current density measurement using Zeeman polarimetry (ZP), which has 20 channels having spatial resolution of up to 1cm at r/a$\sim $0.8-1. The diagnostic makes it possible to measure the electron density and its fluctuation by BES, ion temperature and rotation velocity by CXRS, simultaneously. Recent experiment on JT-60U, we have achieved that the Li-beam injection to the NBI heated plasma with beam current of up to 5mA. One of advantages on JT-60U is high Bt operation up to 4T, which enables us to measure the edge current density with high precision using ZP. By using a high throughput spectrometer, we confirmed that each circular polarized components are fully separated by +/-0.1 nm from linear polarized one at Bt=4T. Doppler broadening is found to be small as 0.04 nm. In order to select the sigma component of the Zeeman triplet of the Doppler shifted Li emission, the etalon filter having FWHM of $\sim $0.1 nm is utilized. A new tuning method of central wavelength has been demonstrated, scanning the beam acceleration voltage during a single shot using both temperature and rotation control. Details of the design and performance are presented. [Preview Abstract] |
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TP6.00069: A Low Cost Photo-Electric Detector for an Arched Flux Tube Experiment Rory Perkins, Paul Bellan A low cost EUV detector is being developed for use in a laboratory experiment where two plasma-filled flux tubes merge in either a co-helicity or counter-helicity arrangement (J.F. Hansen, S.K.P. Tripathi, and P.M. Bellan, Phys. Plasma 2, 3177(2004)). The detector utilizes the photo-electric effect to measure EUV radiation from 10 to 120 nm (S.J. Zweben, R.J. Taylor, Plasma Physics, Vol. 23, No. 4(1981)). The detector geometry is coaxial. A cylindrical collimator capped in wire mesh was placed around the magnesium disk to collimate the field of view and reduce capacitive pick-up. Magnets placed outside the collimator deflect incoming charged particles. The detector was tested in a vacuum chamber with a flash lamp located 50 cm from the detector. A current-to-voltage amplifier with a 1 microsecond rise-time read the detector's output on the test chamber. The detector output on the main experimental chamber was sent directly into 50 ohms with no amplification and produced signals above 200 mV, well above the observed noise. The rise-time for this configuration is well below 1 microsecond. An array of such detectors is currently being designed to image the flux tubes in this EUV range. [Preview Abstract] |
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TP6.00070: Innovative Plasma Imaging Array Concept Benjamin Tobias, Calvin Domier, Xiangyu Kong, Tianran Liang, Neville Luhmann, Jr., M.J. van de Pol, I.G.J. Classen, J. Boom, R. Jaspers, A.J.H. Donne, Hyeon Park A new lens/antenna array concept has been developed for millimeter-wave plasma imaging applications with dramatic increases in RF bandwidth and sensitivity. In this arrangement, an array of tightly coupled miniatured substrate lenses is fabricated such that each antenna has a dedicated substrate lens. The new arrangement exhibits low sidelobe levels over a bandwidth spanning 90 to 140 GHz for use in electron cyclotron emission imaging and microwave imaging reflectometry. An innovative ``vertical zoom'' control is also supported, which the vertical extent of the imaged plasma can be varied from 20 to 30 cm. The first plasma implementation of the new concept will take place on the TEXTOR tokamak in Fall 2008, with systems for DIII-D and ASDEX to follow in 2009. Experimental details regarding the imaging arrays and the new TEXTOR optical design will be presented. [Preview Abstract] |
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TP6.00071: FRC AND SPHEROMAK |
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TP6.00072: Field Reversed Configuration (FRC) Formation Studies with Variable Magnetic Geometry G.A. Wurden, T.P. Intrator, P.E. Sieck, R.J. Cortez, W.J. Waganaar FRX-L has been recently rebuilt with conical theta pinch coils, fast cusp coils, and a translation/capture coils. We report on studies of optimizing the formation of FRC's in the 4 degree conical theta pinch region, while doing parameter scans of the adjacent fast cusp coil B fields, and downstream translation coils. The visible interferometer is configured for 7 chords, and is located at the exit of the theta coil region, for quantitative density information. Filtered visible light fibers provide positional information, while sets of B-dot and flux loop probes yield excluded flux. This work is supported by the Office of Fusion Energy Sciences, and DOE/LANL contract DE- AC52-06NA25396. [Preview Abstract] |
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TP6.00073: Capture of a high density field reversed configuration in a flux conserver P.E. Sieck, T.P. Intrator, G.A. Wurden, X. Sun, R.J. Cortez, W.J. Waganaar A physics demonstration of Magnetized Target Fusion (MTF) is being pursued by a collaborative team from Los Alamos National Laboratory and Air Force Research Laboratory. The approach is to form a high density Field Reversed Configuration (FRC), translate it into a liner, and adiabatically compress the FRC by imploding the liner. Capture of the FRC in the liner is critical to the success of the experiment. Several interesting phenomena are possible. The FRC can undergo heating when it bounces off the end mirror. Also, partial capture of the FRC can occur such that a smaller FRC remains captured while some plasma and magnetic energy ejects from the liner. Magnetic mirrors at both the upstream and downstream ends of the capture section help to determine the prominence of these behaviors. The translation and capture regions of the experiment are instrumented with flux loops and surface magnetic field diagnostics. Plans will be presented for internal magnetic field probes in these regions. Implications for optimum capture geometry for compression experiments will be discussed. This work is supported by the Office of Fusion Energy Sciences, and DOE/LANL contract DE-AC52-06NA25396. [Preview Abstract] |
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TP6.00074: Formation of FRCs on the Pulsed High Density Experiment Samuel Andreason, John Slough The Pulsed High Density (PHD) experiment has been reassembled at a new facility with sufficient space to continue through the full acceleration and compression stages to reach breakeven. The intention here is to produce a large FRC, but remain in the kinetic regime where the FRC is stable and the transport sufficiently low that a Q $>$ 1 plasma can be attained at moderate densities $\sim $ 10$^{23}$ m$^{-3}$. During reassembly a more complete analysis of previous experimental results has been made. One of the issues in the early phase of the experiment was inefficient flux trapping during field reversal due to the large scale of the FRC source (0.4 m radius). The on-axis seed plasma was unable to diffuse out to the walls on a timescale commensurate with the introduction of bias fields. This resulted in more than half of the initial bias flux lost before sheath formation halted flux loss. An annular array of plasma sources has been constructed that solves this problem and greatly enhances the flux retention. Dynamic formation has been employed on PHD and analysis tools capable of interpreting the magnetic loop diagnostic array have been developed. Results with comparison to numerical models will be presented. [Preview Abstract] |
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TP6.00075: New perspectives on FRC confinement Loren Steinhauer Four factors relevant to FRC's motivate a rethinking the nature of confinement in FRCs. (1) Particle loss at the separatrix is regulated by drift turbulence with an oblique wave vector and perpendicular component comparable to the inverse of the ion gyroradius. The resulting transport scaling is better than gyroBohm, This mechanism gives a reasonable prediction of the particle confinement observed in FRC experiments. (2) The scrape-off layer can be modeled as a quasi-steady balance between radial particle diffusion and streaming endloss to the divertor region. (3) ``Tearing relaxation,'' i.e. tearing that maintains the internal profile at the marginal stability condition appears to explain the anomalous flux loss rate in FRCs. Application of the so-called nearby-equilibrium analysis led to this implication. If so, then the ``apparent'' resistivity at the O-point is only the manifestation of a completely different flux annihilation mechanism, namely tearing. (4) The transport of energy is convective, i.e. proportional to the particle diffusivity. Past extrapolations of FRC confinement were based on empirical scalings; these new advances allow, for the first time, a physics-based model of transport. This should foster more convincing extrapolations to next-generation experiments. [Preview Abstract] |
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TP6.00076: Design aid diagram based on dimensionless variables of reactor-like FRCs S.A. Cohen A variety of FRC reactor designs are being discussed within the US MFE community. These differ in choices of fuel, heating method, and operational mode, resulting in widely varying device sizes (r) and magnetic fields (B) and different demands on stability and energy confinement. Based on considerations of dimensionless parameters descriptive of stability and energy confinement, we place present and proposed FRCs in the r-B parameter space and show the boundaries that indicate achievement of various reactor-like parameters. In FRC devices operating above log r(cm) + 0.8 log B(G) = 4.6, reactor-like stability and energy-confinement issues can be simultaneously addressed. Dimensional parameters and criteria for pulse length requirements are also described for reactor-like FRCs. [Preview Abstract] |
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TP6.00077: Observation and Analysis of Fluctuations in an RMF Driven FRC Katy Ghantous, Sam Cohen, Sasha Landsman Using electrostatic and capacitive probes, we have observed fluctuations in the divertor and main-plasma scrapeoff-layer regions of the PFRC, an RMF-driven high-beta device. The fluctuations, in the range 10 kHz to 400 MHz, show sharp peaks at the RMF fundamental frequency, 14 MHz, and its harmonics, up to the limit of the detection system. Lower frequency peaks occur near 50 and 175 kHz. Sidebands often occur above and below the RMF harmonics, separated from it by frequencies near the low frequency fluctuations. The low frequency fluctuations and the associated higher frequency sidebands are seen to depend on RMF coupling and penetration. We will present analysis and interpretation of these fluctuations using correlation, FFT and symbolic dynamics techniques. [Preview Abstract] |
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TP6.00078: Development of High-Temperature Superconducting Flux Conservers for the Princeton FRC Experiment Clayton E. Myers, B. Berlinger, S.A. Cohen The Princeton Field-Reversed Configuration (PFRC) experiment employs an odd-parity rotating-magnetic-field (RMF) current-drive and plasma-heating system to initiate and sustain high-beta plasmas. Passive, discretized flux-conserver (FC) rings are used to apply magnetic pressure to the FRC while still allowing the RMF to penetrate to the plasma. The duration of confined high-beta plasma pulses is limited by the skin time of the FCs. Presently, the PFRC is equipped with solid copper FCs that have individual skin times of 3 ms. An upgraded PFRC facility will produce plasma pulses in excess of 10 ms, which requires FCs with skin times exceeding 100 ms. In this paper we shall review the evolution of PFRC FC arrays, culminating in new experimental studies of high-temperature (HiT) superconductor (SC) FCs. SC-FCs have been produced by embedding Hi-T SC tapes in OFHC copper FC rings. Several SC tape configurations have been studied experimentally and a wide range of extended SC-FC skin times have been produced (400 ms-12 s). Due to the high FC current required to balance the plasma pressure, critical current saturation of the SC tapes has also been closely studied. [Preview Abstract] |
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TP6.00079: 0.5-5 keV X-ray Spectroscopy as a T$_{e}$ Diagnostic of PFRC Plasma A. Stepanov, S.A. Cohen A commercial Si-PIN X-ray detector has been adapted for the non-invasive measurement of the RMF-heated PFRC's electron temperature. The energy scale of the detector was calibrated using X-ray fluorescence, and detector efficiency is being measured using the Bremsstrahlung spectrum of a thick carbon target. The expected plasma emission spectrum was obtained by numerically integrating the Elwert approximation to the exact QM Gaunt factor. The measured X-ray pulse-height distribution, with the detector viewing along a diameter 8 cm from the midplane, often shows an exponential behavior, consistent with a single temperature. Comparing the calculated to the measured spectra yielded electron temperatures between 100 and 300eV. It was observed that a -100 kHz frequency shift (FS) of the RMF from its initial 14 MHz value during a pulse results in a 20{\%} increase in T$_{e}$ and 4-fold increase in X-ray count rate, despite decreases in plasma density and power coupled to the plasma. Time-of-arrival histograms of the X-ray pulses were obtained confirming the existence of a ``dead zone'' before the application of the FS and the subsequent increase in emission after application of the FS. [Preview Abstract] |
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TP6.00080: Ion Energy Distribution Functions in the Irvine FRC W.S. Harris, E.P. Garate, W.W. Heidbrink, R. McWilliams, T. Roche, E. Trask Charge-exchange neutral particle energy distribution function measurements have been made over the 20-500 eV energy range in the Irvine Field Reversed Configuration (IFRC) using a time-of-flight charge exchange analyzer. Neutral flux measurements imply that ion acceleration has been observed as a consequence of the applied toroidal electric field, giving average energies around 65 eV. Radially scanning through different chords results in an increased neutral flux as the chord's minimum radius approaches the null circle. The initial magnetic field has been varied in order to compare ion acceleration in the following cases. In one case, the ion gyroradius is 20 cm, approximately equal to the null circle radius, 25 cm, and the resulting reversed field has a magnitude of 100 Gauss and a lifetime of 20 $\mu$s. In the other case, the ion gyroradius during FRC formation is 4.5 cm and the resulting reversed field has a magnitude of 200 Gauss and lifetime of 80 $\mu$s. Visible emission spectroscopy has also been performed on the H$_{\alpha}$ line resulting in Doppler broadening in the range of 5-10eV. [Preview Abstract] |
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TP6.00081: Dispersion Measurements On the Irvine FRC in the Lower Hybrid Frequency Range E. Trask, E.P. Garate, W.S. Harris, W.W. Heidbrink, R. McWilliams, T. Roche Measurements of the group velocity have been made on the Irvine Field Reversed Configuration (IFRC) by first exciting electromagnetic fields in the plasma with a folded-dipole antenna and then detecting the electric and magnetic fields with probes in the plasma. Key parameters for the plasma are $<$n$> \quad \sim $ 2 10$^{14}$ cm$^{-3}$, B$_{max}\sim $ 250 Gauss, lower hybrid frequency (slow wave) $\sim $ 10 MHz, launch frequency $\sim $ 30 MHz. Radial group velocities with values ranging from 5E8 cm/s to greater than 3E9 cm/s have been inferred from measurement of delay times between shots. This implies a radial group velocity index of refraction of up to 60. This agrees somewhat with the cold plasma dispersion relation, which predicts indices of refraction ranging from 30-300. Next steps include measurement of the group and phase velocities in both axial and radial dimensions. [Preview Abstract] |
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TP6.00082: Density Profiles in the Irvine Field Reversed Configuration T. Roche, F. Brandi, E.P. Garate, F. Giammanco, W.W. Heidbrink, W.S. Harris, R. McWilliams, E. Paganini, E. Trask A mach probe has been used to measure the time-evolved, radial ion density profile and ion flow velocity in the Irvine Field Reversed Configuration (IFRC). The probe consisted of four tungsten tips 0.1mm in diameter and about 1.7mm long. An alumina barrier was placed between 2 of the tips to block ions impinging from opposite directions. The blocked tips were biased 30V negative with respect to the plasma floating potential to draw ion saturation current. The temperature of the ions was measured to be $\sim $10 eV using doppler broadening spectroscopy. Peak densities were measured to be $\sim $5 x 10$^{14}$ cm$^{-3}$. Flow velocity was measured for the plasma source at 5 x 10$^{6}$ cm/s without the presence of magnetic fields. Data gathered during reversal were too noisy to measure the flow velocity of the FRC. These data were compared with two other methods for calculating the density. A CO2 laser interferometer measured a line integrated density of 5 x 10$^{15}$ cm$^{-3}$ over an approximately 10 cm chord length. Previously gathered magnetic field data provided a radial density profile under the assumption of pressure balance. The combination of these two methods verifies both the shape and magnitude of the measured signals. An energy analyzer is being designed to measure the ion velocity distribution function in the IFRC. [Preview Abstract] |
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TP6.00083: Diagnostics for the Colorado FRC A.D. Light, C.L. Ellison, S.J. Fahrenholtz, T. Munsat, J.M. Nuger A collection of fast diagnostics is under development for studies on the Colorado FRC. Current and planned instruments emphasize high spatial and time resolution for detailed measurements of fluctuations and bulk flows. All systems are frequency-limited only by the data acquisition rate ($\ge$ 10 MHz). Diagnostics under development include a seven-channel CO2 quadrature interferometer, a compact, 16-position, three-axis magnetic probe, a localized ion-Doppler spectroscopy instrument, a fast ion gauge for measuring transient gas pressure, and a multi-channel Mach probe array. Details of the instruments and preliminary measurements are presented. [Preview Abstract] |
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TP6.00084: Merging and Flow Experiments on the Colorado FRC T. Munsat, C.L. Ellison, A.D. Light Experiments have begun on the Colorado FRC Experiment, a new machine for the study of turbulence, flow, stability, and cross-field transport in a prolate field-reversed configuration. The experiment is a merged-spheromak device driven by magnetized coaxial plasma guns. We have designed and constructed a two-point biasing probe for driving E x B flows at M~1. Magnetized gun operation is now standard and merging/flow experiments have begun. We present early results from the first merging experiments and attempts to impose and measure bulk rotation. [Preview Abstract] |
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TP6.00085: Oblate Field Reversed Configurations (FRCs) in SSX T. Gray, M.R. Brown, B.S. Gerber-Siff, K.R. Labe, E.H. Dewey, L.D. Bookman, M.J. Shaffer, C.D. Cothran FRCs offer closed magnetic field line confinement and plasma exhaust along open lines; and, as with other true compact tori (CTs), the plasma is not linked by magnet windings. Oblate FRCs would make more compact fusion reactors than prolate ones if plasma diffusivities are equal. Preliminary results in a new oblate flux conserver ($R=0.25~m, L=0.4~m$) designed to stabilize long-wave m=1 kink instabilities in SSX demonstrated magnetically quiet FRCs. However, the excitation of unstable MHD modes occurs during most discharges. The SSX FRCs are formed by merging two spheromaks of opposite magnetic helicities, enabling the creation of FRCs with high internal poloidal flux. The FRC equilibria are diagnosed by inserted magnetic probes. Ion Doppler spectroscopy shows T$_i$ $\approx$ 80 eV shortly after reconnection and T$_e$ $\approx$ 20 eV, but temperature profiles have not yet been resolved. Ion gyro orbits are much smaller than the plasma radius, so these plasmas are fluid, not kinetically, dominated. Work on stabilizing the FRCs will be discussed. [Preview Abstract] |
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TP6.00086: Field Reversed Configuration (FRC) formation and compression using liners J.H. Degnan, P. Adamson, M. Domonkos, E.L. Ruden, C. Grabowski, D. Brown, D. Gale, M. Kostora, D. Ralph, W. Sommars, M.H. Frese, S.D. Frese, D.J. Amdahl, J.F. Camacho, S.K. Coffey, T.P. Intrator, G.A. Wurden, S.C. Hsu, P. Sieck, P.J. Turchi, W.J. Waganaar, R.E. Siemon, T.J. Awe, A.G. Lynn, N.F. Roderick AFRL and LANL are developing Magnetized Target Fusion (MTF) using the Shiva Star capacitor bank at AFRL to implode an Al liner containing an FRC to raise density and temperature. Experiments at LANL and AFRL explore FRC formation and translation. 2D-MHD calculations with MACH2 look at translation, capturing and compressing the FRC. Extended MHD examines FRC rotation. These guide the design of the compression experiments at AFRL.~Field exclusion, interferometer, radiographic, radiation data, and 2D-MHD simulations will be presented. Supported by DOE-OFES. [Preview Abstract] |
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TP6.00087: Final Results from the SSPX Spheromak Program H.S. McLean, B.I. Cohen, D.N. Hill, E.B. Hooper, B. Hudson, R.J. Jayakumar, L.L. Lodestro, J.D. King, J.M. Moller, C.A. Romero-Talamas, T.L. Stewart, R.D. Wood, E.C. Morse, J.A. Johnson III, E.D. Mezonlin, J. Titus, C.R. Sovinec In its last year of operation, SSPX achieved significant performance including peak electron temperature T$_{e}\sim $500eV, toroidal magnetic field on axis B$>$1T, plasma current I$_{p}\sim $1MA, and core electron thermal diffusivity $\chi _{e}<$ 10 m$^{2}$/sec. We investigated helicity injection with longer pulse lengths ($>$10 msec), and with multiple current pulses. The latter demonstrated quasi-steady-state operation with intervals of improving confinement between field-building pulses. We also explored the relationship of field buildup to kinking of the central plasma column by extending the length of the flux conserver and observed improved field generation. In ongoing numerical work, new visualization tools aid interpretation of NIMROD 3D resistive MHD simulations and comparisons to SSPX data continue to help validate predictive capability. Goals identified for next-generation spheromaks include even longer pulses, auxiliary heating with NBI, higher flux amplification through variable bias flux operation, and thinner walls with active feedback control of external tilt/shift modes. This work performed under the auspices of the U.S. DoE by LLNL under Contract DE-AC52-07NA27344. [Preview Abstract] |
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TP6.00088: Estimation of plasma flow and toroidal rotation on SSPX using a passive ion Doppler spectrometer J.D. King, H.S. McLean, M.J. May, E.C. Morse We present results of ion Doppler spectrometer (IDS) measurements of helium ion velocity coupled with the MIST 1D impurity transport code calculations to estimate plasma flow and toroidal rotation in the SSPX spheromak. Helium discharges ensured sufficient light. Collection optics view a narrow, near-cylindrical volume with radius $\sim $ 1cm, and the intensity of collected light decreases as the inverse square of the distance from the plasma to the optic, or solid angle. Light was collected along a chord near the edge (red-shift) as well as a direct radial (null-shift) view through the center. By considering the helium charge state distribution calculated by MIST, and the solid angle of the collection optic to the plasma location, an estimate of the spatial resolution of the IDS was obtained. It was found that 70{\%} of He-II 468.57 nm light collected by the IDS, was localized in a 6 cm radial region. The analysis of several shots indicate that plasma near the edge of the spheromak flows with a toroidal velocity of 5 to 45 km/s during spheromak formation. These flows correspond to a toroidal rotation frequency of 1.7 to 15.6 kHz. This work performed under the auspices of the U.S. DoE by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
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TP6.00089: Virtual diagnostics in NIMROD simulations for direct comparison to SSPX measurements T.L. Stewart, C.A. Romero-Talamas, H.S. McLean, D.L. Correll, L.L. LoDestro, B.J. Whitlock, B.A. Nelson The visualization code VisIt is being used to analyze numerical simulations of the Sustained Spheromak Physics Experiment [E. B. Hooper, L. D. Pearlstein, and R. H. Bulmer, Nucl. Fusion \textbf{39}, 863 (1999)], obtained using the three-dimensional, resistive magnetohydrodynamic code NIMROD [C. R. Sovinec, A. H. Glasser, T. A. Gianakon, et al., J. Comput. Phys. \textbf{195}, 355 (2004)]. Virtual diagnostics, such as insertable and edge magnetic probes, and Thomson scattering, are being installed in the simulation domain at locations corresponding to the experimental diagnostics, in order to directly compare simulated and real measurements. Initial results of these comparisons will be presented. Work performed by Lawrence Livermore National Laboratory under the auspices of the U.S. Department of Energy, Contract DE-AC52-07NA27344. [Preview Abstract] |
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TP6.00090: Numerical and experimental investigations of helicity sustainment in spheromaks C.A. Romero-Talamas, E.B. Hooper, H.S. McLean, T.L. Stewart, B. Hudson, L.L. LoDestro, R.D. Wood, J.M. Moller, R.W. Geer, R.O. Kemptner Numerical simulations of SSPX plasmas [E.B. Hooper, et al., Nucl. Fusion 39, 863 (1999)] using the NIMROD code [C.R. Sovinec, et al., J. Comput. Phys. 195, 355 (2004)] are being analyzed with the 3D visualization code VisIt. Simulations show that constant T surfaces approximately coincide with magnetic flux surfaces, except in regions where magnetic reconnection occurs, thus providing an insight of helicity and topology evolution before and after reconnection. Stellarator-like T surfaces might be formed before these reconnection events and lead to hollow profiles (measured along radial lines) similar to profiles measured at SSPX. Simulations also show how helicity increases in the open flux before reconnection, and where reconnection occurs; virtual measurements of these events are being compared to experimental ones. The authors are grateful to B.A. Nelson from the PSI center at the University of Washington, and to B.J. Whitlock from the VisIt team for their software support. Work performed under the auspices of the U.S. Department of Energy under contract DE-AC52-07NA27344 at LLNL. [Preview Abstract] |
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TP6.00091: Spheromak aspect-ratio effects on poloidal flux amplification E.B. Hooper, H.S. McLean, C.A. Romero-Talamas, R.D. Wood A short experimental run at the end of SSPX operation examined the effect of increasing the flux conserver length-to-aspect ration, L/R, from 1 to 1.2, thereby reducing the formation threshold for $\lambda _{gun} =\mu _0 I_{gun} /\Psi _{gun} $ from 10 m$^{-1}$ to $\approx $ 7.5 m$^{-1}$ with a corresponding increase in power efficiency [1]. Resistive MHD (NIMROD) simulations of flux amplification which agreed well with experiment at L/R=1 [2] agree fairly well with L/R=1.2 and have been extended to L/R=1.6, just under the tilt-mode stability limit (1.67) for an isolated spheromak. At the longest length, helicity injection changes from a chaotic relaxation process to a steady, high amplitude n=1 mode which opens the field lines throughout most of the flux conserver. Calculations are presented to elucidate the characteristics of the chaos for the standard flux-conserver dimensions. Comparisons are made among the simulations to determine the ``optimum'' L/R based on a trade-off between spheromak buildup efficiency and low mode activity. [1] R. D. Wood, et al., submitted to Phys. Rev. Letters. [2] E. B. Hooper, et al., Nucl. Fusion \textbf{47}, 1064 (2007). [Preview Abstract] |
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TP6.00092: The Relationship of the n=1 Column Mode to Spheromak Formation B.I. Cohen, E.B. Hooper, L.L. LoDestro, D.D. Ryutov, H.S. McLean, C.A. Romero-Talamas, R.D. Wood Observations from spheromaks and simulations [1] using the time-dependent, nonlinear, resistive magnetohydrodynamic code NIMROD indicate that in the formation phase an n=1 instability (toroidal Fourier mode) is excited by the current driven down the plasma column by electrostatic current injection into the spheromak. Toroidal harmonics are generated, partly by nonlinear effects; and the growth of the column mode terminates in a violent reconnection event that forms the spheromak: toroidal flux reconnects and poloidal flux emerges. By means of NIMROD simulations, detailed diagnostics, visualizations, and analytical calculations, we investigate the relationship between the n=1 column mode and the reconnection event that forms the spheromak. [1] C. R. Sovinec, B. I. Cohen, et al., Phys. Rev. Lett. 94, 035003 (2005); B.I. Cohen, E.B. Hooper, et al., Phys. Plasmas 12, 056106 (2005); E. B. Hooper, B. I. Cohen, et al., J. Fusion Energy 26, 71 (2007); E. B. Hooper, B. I. Cohen, et al., Phys. Plasmas 15, 032502 (2008). [Preview Abstract] |
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TP6.00093: Multipulsed edge-current drive in a spheromak L.L. LoDestro, B.I. Cohen, E.B. Hooper, H.S. Mclean, T.L. Stewart, R.D. Wood Flux amplification (A)--the ratio of poloidal magnetic flux enclosed by a spheromak's toroidal core-plasma to an applied edge flux--is a critical parameter for an economic spheromak-based fusion reactor. In [1], experimental measurements of A in SSPX found good agreement with numerical studies [2] carried out with the NIMROD code over a range of ``extended formation" discharge parameters. However, SPPX appeared to be limited to A $\sim$ 3 while the simulated A continued to rise with increasing gun-current (\def\ig{I_{\rm gun}}$\ig$). Early experiments with the SSPX modular capacitor bank, on the other hand, gave some indication that $d\ig/dt$ also played a role and that, perhaps, multiple pulses with faster current swings in both directions could build magnetic field more efficiently. Experiments to explore this were, however, limited by gun discharge circuit inductance. In this paper we investigate the effect numerically using multipulse scenarios in the SSPX geometry with NIMROD. \hfill\break [1] B.\ Hudson et al., Phys.\ Plasmas {\bf 15}, 056112 (2008). \hfill\break [2] E.B.\ Hooper et al., Nucl.\ Fusion {\bf 47}, 1064 (2007). \hfill\break Work performed by LLNL under the auspices of the U.S. DoE, Contract DE-AC52-07NA27344. [Preview Abstract] |
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TP6.00094: Analysis of EUV Oxygen Spectra from LLNL SSPX and UNR Laser Plasma Source of ``Sparky'' P.G. Wilcox, A.S. Safronova, V.L. Kantsyrev, U.I. Safronova, K.M. Williamson, M.E. Weller, J. Clementson, P. Beiersdorfer, K.W. Struve In our recent work [Wilcox et al, RSI (2008, in press)] we applied a non- LTE kinetic model of oxygen to analyze an oxygen spectrum between 140 and 240 {\AA}, produced on LLNL SSPX. Here we study the whole collection of recent experimental SSPX EUV oxygen spectra produced under different plasma conditions, specifically in the broad range of temperatures from as low as 15 eV up to 280 eV, and at an electron density of around 10 $^{14 }$cm$^{-3}$. In addition, we analyze new experimental data from EUV oxygen and carbon spectra, recorded at much higher density at the compact laser plasma source of ``Sparky'' at UNR . The comprehensive comparison of EUV oxygen spectra from both experiments with theoretical calculations was accomplished, and temperature and density sensitive lines were identified.This work is relevant to diagnostics of plasma with low -- Z ions and Tokamak plasma in particular. Work is supported by DOE under grant DE-FG02-08ER54951 and in part under NNSA Coop. Agr. DE-FC52-06NA27588 and DE-FC52-06NA27586. Work at LLNL was performed under auspices of the DOE under contract DE-AC52-07NA2344. [Preview Abstract] |
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TP6.00095: Magnetic field compression of an accelerated compact toroid in a conical drift section R.D. Horton, D.Q. Hwang, R.W. Evans, F. Liu, R. Klauser, Glenn Umont There are numerous applications for spheromak-like compact toroids (SCTs)with high plasma density and internal magnetic field. Previous experiments have demonstrated density and field compression of SCTs using coaxial conical electrodes [1,2]. For some applications, however, use of a central electrode may not be practical, and compression must be performed by tapering the outer electrode alone. A tapered conical electrode has been added to the CTIX device to measure magnetic field compression in this geometry. The absence of a center electrode allows magnetic field to be measured via magnetic probes at an adjustable range of axial positions, or by conventional recessed probes on the outer electrode at fixed positions. The field data serves as a benchmark for a smoothed-particle hydrodynamics (SPH) code currently under development. Results will be used to optimize compression cone geometry for the best conversion of SCT kinetic energy into thermal and magnetic energy. [1] J. H. Hammer, et al., PRL 61, 2843 (1988) [2] A.W. Molvik et al., PRL 66, 165 (1991) [Preview Abstract] |
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TP6.00096: Efficiency of flux build-up with high repetitive Spheromak injection Hirotomo Itagaki, Kaori Kishi, Hiroto Numazawa, Yuki Terashima, Tomohiko Asai, Tsutomu Takahashi, Haruhisa Koguchi A spheromak plasma generated by magnetic coaxial plasma gun is able to be accelerated easily by Lorentz force due to self magnetic field and discharge current. Then the plasmoid is expected as a magnetic helicity and particle injector for nuclear fusion core. To supply significant amount of helicity and particles with minimum disturbance on a target plasma, a plasmoid has to be generated with higher repetitive ratio of 50 kHz. Currently we are focusing on to estimate the efficiency of helicity injection in a high repetitive operatio case. In a preliminary experiments with pyrex discharge tube with guide magnetic field, increased decay time of poloidal magnetic flux and emission of line spectrum of He?U have been observed. To investigate generation efficiency of magnetic flux in more detail, metal flux conserver was installed and detailed internal probe measurements have been conducted. [Preview Abstract] |
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TP6.00097: Alfv\'enic plasma flow during spheromak preformation stage Deepak Kumar, Paul Bellan Spheromak formation consists of a series of dynamic steps whereby highly localized plasma near the electrodes evolves towards a Taylor state equilibrium. The dynamical evolution stage is often modeled as a series of equilibrium states. However, the experiments at the Caltech Spheromak facility have revealed that non-equilibrium Alfv\'enic flows are driven during these preliminary stages by unbalanced $\vec{J}\times\vec{B}$ forces. The flow velocity was measured using time of flight measurements using a novel He-Ne density interferometer with low phase ambiguity $\sim 1^\circ$(D. Kumar and P. M. Bellan, Rev. Sci. Instrum. {\bf 77}, 083503 (2006)). The flow velocities depend on the gas species inertia and lead to a collimated plasma jet with $\beta \sim 1$(P. M. Bellan, Phys. Plasmas {\bf 10} Pt2, 1999 (2003)). Experiments are underway to characterize how the flow velocity depends on the initial neutral gas density profile. Under some conditions, a layer of neutral gas with density $\sim 10^{23}/\textrm{m}^3$ and thickness $\sim 1$ cm is observed to move in front of the plasma jet. The neutral gas density in the layer was estimated using the Gladstone-Dale relation(F. J. Weinberg, Optics of flames (1963)). [Preview Abstract] |
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TP6.00098: Overview of the Steady-Inductive Helicity Injected Torus D.A. Ennis, C. Akcay, W.T. Hamp, T.R. Jarboe, G.J. Marklin, B.A. Nelson, A.J. Redd, R.J. Smith, B.T. Stewart, B.S. Victor, J.S. Wrobel The Helicity Injected Torus-Steady Inductive (HIT-SI) investigates steady inductive helicity injection in a high-beta spheromak geometry. Semi-toroidal injectors external to the ``bow-tie'' shaped axisymmetric confinement region form and sustain spheromaks with over 30 kA of toroidal current using $\sim$ 8 MW of input power. Recent upgrades of the plasma facing surfaces have increased the spheromak lifetime up to 10 ms and application of a background vertical field reduces current flipping. Further experiments demonstrate the scaling of toroidal current with injector voltage and flux amplitude, optimum phasing of the injector voltage and flux waveforms between 15$^{\circ}$ and 35$^{\circ}$ and reduced plasma wall interaction with constant gas flow. The HIT-SI diagnostic suite includes: multi-point Thomson scattering, FIR interferometry, Doppler, VUV and SPRED spectroscopy, bolometry, internal and surface magnetic probes. The magnetic field spatial structure and temporal evolution measured by internal and surface probes are well described by a fully-relaxed Taylor-state equilibrium model. Additional results of parameter scaling and spectroscopic measurements during steady state current drive will be presented. Work supported by USDoE. [Preview Abstract] |
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TP6.00099: Surface Magnetics on the HIT-SI Experiment J.S. Wrobel, T.R. Jarboe, B.A. Nelson, R.J. Smith, B.T. Stewart An array of 96 surface magnetic probes sensitive to the poloidal and toroidal B field are embedded in the HIT-SI spheromak equilibrium flux conserver, a 12.7mm thick chromium copper alloy shell with an L/R time of 100ms. An extensive calibration campaign has been completed to correct for the frequency dependent attenuation of the magnetic field by the shell and provide plasma edge field measurements over a 10Hz-200kHz bandwidth. The system is expected to provide several important results: 1) A measurement of the non-Taylor part of the equilibrium which may reveal details of the small scale, high frequency magnetic relaxation process. 2) A measurement of the MHD mode amplitudes and evolution in the equilibrium region. 3) Provide insight into injector effects which are important for future injector designs. Comparison of experimental vector field results to computational simulations will explore the dominant physics involved in steady inductive helicity injection current drive. Analysis, progress and methods will be presented. [Preview Abstract] |
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TP6.00100: Density Analysis of the HIT-SI Experiment Brian Victor, Roger Smith, Cihan Akcay, George Andexler, David Ennis, Tom Jarboe, Brian Nelson A far-infrared (FIR) interferometer is used to measure the line-integrated density of the Helicity Injected Torus with Steady Inductive current drive (HIT-SI) plasmas. An overview of the FIR system is discussed and recent upgrades to the system are presented. The system uses dual optically pumped difluoromethane gas lasers, with a wavelength of 184.3 $\mu $m, to produce a heterodyne signal with a beat of approximately 2 MHz. This 2 MHz beat allows for the resolution of high-frequency fluctuations of the HIT-SI plasma density. Plasma density is compared to the initial background fill gas pressure and to the ``puff'' gas pressure to obtain particle confinement information. Based upon the high initial density due to the background fill gas and the subsequent decrease in density, the particle confinement time is estimated. The steady-state density compared to the flow rate from the ``puff'' gas valves provides a second means of estimating the particle confinement time. Comparison of the FIR density data to the Langmuir probe density data is also presented. [Preview Abstract] |
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TP6.00101: NIMROD Simulations of Decaying and Driven Hit-SI Plasmas Cihan Akcay, Charlson Kim, Thomas Jarboe, Brian Nelson, Valerie Izzo The Steady Inductive Helicity Injected Torus (HIT-SI) is a spheromak that uses two semi-toroidal injectors to provide steady inductive helicity injection (SIHI), which produces and sustains a spheromak with significant toroidal current by generating poloidal flux using relaxation current drive. NIMROD's resistive MHD model is employed to study the relaxation behavior of decaying and SIHI-driven spheromaks. For decaying spheromaks, our results indicate an increase in poloidal flux amplification with increasing Lundquist number (S). However, when a highly resistive edge layer is imposed, it dominates the overall resistivity and reduces the flux amplification at a given S. But, we observe more energy going into the non-axisymmetric modes in this case. For driven spheromaks we model the injectors as RFPs injecting flux and current at the appropriate locations on the annular regions of the machine. The first series of simulations will chart the spheromak sustainment and relaxation behavior as a function of S using the resistive MHD model. [Preview Abstract] |
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TP6.00102: MHD simulation of relaxation transition to a flipped relaxed state in spherical torus Takashi Kanki, Masayoshi Nagata, Yasuhiro Kagei Recently, it has been demonstrated in the HIST device that in spite of the violation of the Kruskal-Shafranov stability condition, a normal spherical torus (ST) plasma has relaxed to a flipped ST state through a transient reversed-field pinch-like state when the vacuum toroidal field is decreased and its direction is reversed [1]. It has been also observed during this relaxation transition process that not only the toroidal field but also the poloidal field reverses polarity spontaneously and that the ion flow velocity is strongly fluctuated and abruptly increased up to $>$ 50 km/s. The purpose of the present study is to investigate the plasma flows and the relevant MHD relaxation phenomena to elucidate this transition mechanism by using three-dimensional MHD simulations [2]. It is found from the numerical results that the magnetic reconnection between the open and closed field lines occurs due to the non-linear growth of the $n$=1 kink instability of the central open flux, generating the toroidal flow $\sim $ 60 km/s in the direction of the toroidal current. The $n$=1 kink instability and the plasma flows driven by the magnetic reconnection are consider to be responsible for the self-reversal of the magnetic fields. [1] M. Nagata \textit{el al}., Phys. Rev. Lett. \textbf{90}, 225001 (2003). [2] Y. Kagei \textit{el al}., Plasma. Phys. Control. Fusion \textbf{45}, L17 (2003). [Preview Abstract] |
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TP6.00103: Adiabatic Compression of Compact Tori for Current Drive and Heating Simon Woodruff, Angus Mcnab, Kenneth Miller, Tim Ziemba Several critical issues stand in the development path for compact tori. An important one is the production of strong magnetic fields, (or large flux amplifications) by use of a low current source. The Pulsed Build-up Experiment is a Phase II SBIR project in which we aim to show a new means for generating strong magnetic fields from a low current source, namely, the repetitive injection of helicity-bearing plasma that also undergoes an acceleration and compression. In the Phase I SBIR, advanced computations were benchmarked against analytic theory and run to determine the best means for the acceleration and compression of a compact torus plasma. The study included detailed simulations of magnetic reconnection. In Phase II, an experiment has been designed and is being built to produce strong magnetic fields in a spheromak by the repetitive injection of magnetic helicity from a low current coaxial plasma source. The plasma will be accelerated and compressed in a similar manner to a traveling wave adiabatic compression scheme that was previously applied to a mirror plasma [1]. [1] P. M. Bellan Scalings for a Traveling Mirror Adiabatic Magnetic Compressor Rev. Sci. Instrum. 53(8) 1214 (1982) Work supported by DOE Grant No. DE-FG02-06ER84449. [Preview Abstract] |
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TP6.00104: Experimental investigation of high-speed gas flow generation by a compact toroid injection into a gas neutralizer R. Nakanishi, D. Liu, T. Nakatsuka, N. Fukumoto, Y. Kikuchi, M. Nagata, T. Takahashi We have investigated production of high-speed gas flow by using a compact toroid (CT) injector as a new technique for fuelling future reactors [1]. An accelerated CT plasmoid penetrates into a drift tube as a neutralizer cell, then high-speed neutral particle flow is generated through charge-exchange between CT plasma and neutral gas. In the preliminary experiment, a single-stage CT injector produced CT plasmas with the density of 1$\sim $4 x10$^{21}$ m$^{-3}$ and the speed of 30$\sim $70 km/s. The plasmas penetrated into the neutralizer cell filled with hydrogen gas up to about 10$^{-3}$ Torr by using a piezoelectric valve. However remarkable results were not obtained. We have also calculated neutralization efficiencies of CT plasma using the model of the axial NBI into a FRC plasma, and which is equivalent to that of CT injection into neutral gas. The result indicated that neutralization efficiency could be enhanced to be 40 {\%} by increasing the gas pressure up to 0.1 Torr. Thus, in order to obtain the higher pressure, the piezoelectric valve was replaced with a fast solenoid. As a diagnostic tool of neutral gas flow, a pressure sensor with high time resolution was newly equipped. We will present the experimental demonstration on the new setup. [1] Y. Ito, et al., Bull. Am. Phys. Soc., \textbf{52}, No. 11, CP8-62 (2007). [Preview Abstract] |
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TP6.00105: Extended MHD Simulations of the Formation, Merging, and Heating of Compact Tori Angus Macnab, Simon Woodruff We examine the formation, compression, merging, and stability of compact tori (CT) for magnetic field generation and heating by use of the 3D extended MHD code, NIMROD [C.R. Sovinec et al. J. Comp. Phys. 355, 195, (2004)]. Recent advances in the NIMROD code allow us to study plasmas, including the effects of Hall physics and highly anisotropic and field dependent transport. The physics of CT formation and acceleration requires numerical models that can effectively treat plasma flows in systems that are often far from equilibrium. The formation of plasmas with strong magnetic fields by use of a low power source still remains a critical issue. Recently, a novel means for generating strong B from a low current source was developed, and relies on the repetitive injection of plasma from a coaxial gun, leading to the step-wise increase in both total circulating current and core plasma temperature. A natural limit is encountered much as in CT injection for fueling into tokamaks, namely the injected plasma must penetrate the target plasma. To reach high fields, this then will require compression before injection. Stability of the configuration to fluid (e.g. Rayleigh-Taylor) and ideal modes (e.g. tilt/shift) are examined. [Preview Abstract] |
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TP6.00106: Flow profile measurement with multi-Mach probes on the HIST spherical torus device S. Hashimoto, T. Nishioka, K. Ando, Y. Kikuchi, N. Fukumoto, M. Nagata Role of plasma flow during MHD relaxation and magnetic reconnection processes is still underlying physics. The HIST spherical torus can generate various spherical torus (ST) configurations by changing the external toroidal magnetic field. Especially, the flipped ST (F-ST) configuration has been for the first time found in the HIST device [1]. In the present study, plasma flow measurements were performed by multi-Mach probes in the ST and the F-ST configurations. In addition, the measured plasma flow was compared with that evaluated by an ion Doppler spectrometer (IDS) system and plasma images measured by a high-speed camera. As the result, it was shown that the toroidal plasma flow ($\sim $ 20 km/s) at the location far from the plasma gun was clearly reversed after the transition from the ST to the F-ST. However, the direction of the toroidal flow was not changed near the plasma gun. Therefore, it can be considered that there are flipped and non-reversal regions in the plasma. The result agrees well with a magnetic configuration predicted by magnetic field measurements. The plasma images measured by the high-speed camera also indicated that a helically twisted structure appeared from the gun region, and it localized at the edge region. [1] M. Nagata et al., Phys. Rev. Lett. \textbf{90}, pp. 225001-225004 (2003). [Preview Abstract] |
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TP6.00107: Ion flow measurements during the MHD relaxation processes in the HIST spherical torus device T. Nishioka, S. Hashimoto, K. Ando, Y. Kikuchi, N. Fukumoto, M. Nagata Plasma flow is one of the key roles in self-organization and magnetic reconnection processes of helicity-driven spherical torus (ST) and spheromak. The HIST spherical torus can form the standard ST and the flipped ST plasmas by utilizing the variation of the external toroidal field coil current. The flipped ST plasma can be generated by changing the polarity of the toroidal magnetic field during the standard ST discharge [1]. We have developed an ion Doppler spectrometer (IDS) system using a compact 16 channel photomultiplier tube (PMT) in order to measure the spatial profile of ion temperature and rotation velocity in the HIST device. The IDS system consists of a light collection system including optical fibers, 1 m-spectrometer and the PMT detector. As the results, it was observed that ion velocity was about 10 km/s in the same direction as the toroidal current and ExB direction in the standard ST discharge. The observed ion velocity agrees with Mach probe measurements. During the transition from the standard ST to the flipped ST state, the ion temperature was fluctuated and increased. The result implies an ion heating during magnetic reconnections. In addition, the toroidal direction of the ion flow was reversed. The detail physics of the observed phenomenon will be shown. [1] M. Nagata et al., Phys Rev. Lett. \textbf{90}, pp. 225001-225004 (2003). [Preview Abstract] |
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TP6.00108: Simulation Studies of Field-Reversed Configurations with Rotating Magnetic Field Current Drive E.V. Belova, R.C. Davidson Results of 3D kinetic simulations of rotating magnetic field (RMF) current drive in field-reversed configuration (FRC) are presented. Self-consistent hybrid simulations have been performed using the HYM code for even- and odd-parity RMF and different FRC parameters and RMF frequencies. Simulations show that the RMF pushes the plasma radially inward, resulting in a reduced plasma density outside separatrix. Lower plasma density and larger RMF amplitudes result in faster RMF field penetration, in agreement with previous two-fluid studies. Generation of axisymmetric toroidal magnetic field during the RMF current drive has been observed. Numerical study of the effects of the applied RMF field on particle confinement shows that the rate of particle losses increases for RMF frequency close to the ion cyclotron frequency. It is also shown that high-frequency even-parity RMF reduces ion losses when it is fully penetrated. It is also found that fully-penetrated, odd-parity RMF forces particles away from the midplane toward the FRC ends. The observed changes in particle confinement are related to ponderomotive forces. Partially penetrated RMF results in mostly radial ponderomotive forces which improve particle confinement in both cases. [Preview Abstract] |
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TP6.00109: Effect of Plasma Shaping Magnetic Field Coil on RMF Plasmas Xiaokang Yang, Yuri Petrov, Tian-Sen Huang Experiments focusing on the response of plasma parameters to the change of external magnetic field have been conducted in 40 ms RMF plasma discharges, by using a magnetic coil in the mid-plane of the cylindrical chamber rotamak. The feeding current in the middle coil can be programmed to have different pulse duration, amplitude and triggering time. When the current in the middle coil is below a threshold level of 600A, plasma current responds with a growth to the increase of coil current. Depending on the initial conditions, plasma current can be enhanced by a maximum 3.1 kA; the peak plasma current is about 250{\%} of the original plasma current. When middle coil is energized with current below disruptive level, in most area of plasma column n$_{e}$ increases significantly with I$_{p}$, but there is only small change in T$_{e}$. In FRC regime, MHD instability can be completely suppressed by feeding middle coil with suitable current and triggering time. [Preview Abstract] |
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TP6.00110: Experimental study of driven magnetic relaxation in a laboratory plasma S.C. Hsu, X.Z. Tang The Driven Relaxation Experiment (DRX) has been built at LANL to investigate the possibility of exploiting resonances in the nonlinear force-free equation [1] to optimize magnetic flux amplification and current multiplication for driven-relaxed spheromak-like plasmas, and to explore the application of these ideas to plasma astrophysics problems [2]. It is also our goal to see whether relaxed states with $\lambda > \lambda_1$ can be formed and sustained. The experiment uses a planar magnetized coaxial gun (100--180~kA, 1--7~mWb) to generate driven- relaxed plasmas within a cylindrical flux-conserving boundary (0.9~m diameter). Unique features of DRX include high $\lambda_{\rm gun}$ up to 3$\lambda_1$, and a continuously adjustable boundary elongation. The gun is powered by a 3- stage capacitor bank to form (10~kV, 500~$\mu$F) and sustain (5~kV, 8~mF) the plasma for up to 500~$\mu$s, corresponding to $>10$ Sweet-Parker times which allows the plasma to reach a quasi-steady-state. The primary diagnostic is a 48- channel 2D magnetic probe array that will map out a poloidal cross-section of the magnetic field configuration at one toroidal position. The full equilibrium magnetic field will be constructed using a combination of the experimental data and a nonlinear force-free equilibrium solver. We will present details of the experimental setup and the first experimental data. Supported by LANL LDRD. [1] Tang \& Boozer, PRL~{\bf 94}, 225004 (2005); PRL~{\bf 98}, 175001 (2007) [2] Tang, ApJ~{\bf 679}, 1000 (2008). [Preview Abstract] |
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TP6.00111: Poloidal magnetic flux control experiments in Rotamak Yuri Petrov, Xiaokang Yang, Tian-Sen Huang A new set of equilibrium coils has been installed in cylindrical chamber rotamak to allow for an active control of poloidal magnetic flux in 40-ms plasma discharges. The coils are powered by programmable current source with 1-3 ms response time. The coils allow controlling both the shape of the plasma and the boundary flux magnitude. Without the current in the coils, the boundary flux drops from vacuum value of 0.30 mWb to 0.05-0.10 mWb after the plasma current is generated. If the current is applied to the coils, the boundary flux can be maintained within 0.22-0.26 mWb range, thus keeping the separatrix away from chamber walls during plasma shot. The new system also includes passive flux conserving rings that help to eliminate fast variations of the boundary flux. [Preview Abstract] |
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TP6.00112: Recent results in the TCS-Upgrade device J.A. Grossnickle, A.L. Hoffman, P.A. Melnik, K.E. Miller, R.D. Milroy, G.C. Vlases The original TCS experiment has demonstrated the robust ability to form and sustain FRCs in steady-state using Rotating Magnetic Fields (RMF). However, temperatures were limited by impurity radiation to 10s of eV. A new device, TCSU, was built with a bakable ultra-high vacuum chamber in order to reduce impurities and overall recycling. Within the first few weeks of operation TCSU achieved sustained temperatures much higher than those in TCS. Impurity seeding experiments implied very low levels of oxygen, carbon, and silicon, using only glow discharge wall conditioning. This suggests very low radiated power, in agreement with bolometric measurements. To further lower impurity content, siliconization and Ti-gettering have also been used for wall conditioning. A 2-D internal magnetic probe has been installed that gives radial profiles of both toroidal and poloidal fields. Results from the wall conditioning and internal probe studies will be reported. [Preview Abstract] |
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TP6.00113: Ion Temperature Measurements and Preliminary Results from Ion Flow Measurements in TCSU Chris Deards Using a Princeton Instruments ICCD spectroscopic system the ion temperature of the TCSU plasma is determined within the instrument temperature limit. Since the ICCD system is not time resolved the temperature will be determined by examining the Si III, C III, and O III impurity lines at various times and comparing the results to the time resolved total temperature, which is inferred from magnetic field and density measurements. In addition, devices designed to measure the toroidal and poloidal ion flow when attached to the ICCD will be presented. The overall system's intended uses are to study the forces acting on the ions (ion spin-up) and ion flow with regard to velocity shear stabilization. Preliminary results will be discussed. [Preview Abstract] |
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TP6.00114: BASIC TURBULENCE AND TRANSPORT |
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TP6.00115: Modeling Laboratory Plasmas with Smoothed Particle Hydrodynamics Jeffrey Johnson, Michael Owen Smoothed Particle Hydrodynamics (SPH) [1], a mesh-free Lagrangian method long used to study astrophysical systems, is increasingly used to study more terrestrial applications in engineering. Here we present results of simulations of plasmas created in the laboratory calculated using a variant of Smoothed Particle Magnetohydrodynamics (SPMHD) [2] to treat the equations of resistive magnetohydrodynamics. We discuss the challenges posed by boundary conditions and the treatment of fields in a vacuum; we then describe simple test problems used to demonstrate how the method handles these challenges. Finally, we show the results of a calculation modeling the acceleration of a plasma with electric currents similar to those created in the Compact Toroid Injection Experiment (CTIX) [3]. References: [1] J. J. Monaghan, Ann. Rev. Astron. Astrophys. 30, 543 (1992) [2] D. J. Price, J. J. Monaghan, MNRAS 348, 139 (2004) [3] K. L. Baker et al, Nucl. Fusion, 42, 94 (2002) [Preview Abstract] |
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TP6.00116: Proper orthogonal decomposition and wavelet methods for noise reduction in particle-based transport calculations Romain Nguyen van ye, Diego del-Castillo-Negrete, D. Spong, S. Hirshman, M. Farge A limitation of particle-based transport calculations is the noise due to limited statistical sampling. Thus, a key element for the success of these calculations is the development of efficient denoising methods. Here we discuss denoising techniques based on Proper Orthogonal Decomposition (POD) and Wavelet Decomposition (WD). The goal is the reconstruction of smooth (denoised) particle distribution functions from discrete particle data obtained from Monte Carlo simulations. In 2-D, the POD method is based on low rank truncations of the singular value decomposition of the data. For 3-D we propose the use of a generalized low rank approximation of matrices technique. The WD denoising is based on the thresholding of empirical wavelet coefficients [Donoho et al., 1996]. The methods are illustrated and tested with Monte-Carlo particle simulation data of plasma collisional relaxation including pitch angle and energy scattering. As an application we consider guiding-center transport with collisions in a magnetically confined plasma in toroidal geometry. The proposed noise reduction methods allow to achieve high levels of smoothness in the particle distribution function using significantly less particles in the computations. [Preview Abstract] |
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TP6.00117: Application of fractional diffusion models to perturbative transport experiments in JET Diego del-Castillo-Negrete, Paola Mantica, Volker Naulin, J.J Rasmussen A class of non-local models based on the use of non-Gaussian stochastic processes and fractional diffusion operators, has been proposed to describe non-diffusive transport in magnetically confined plasmas.\footnote{D. del-Castillo-Negrete, et al., Phys. Plasmas \textbf{11}, 3854 (2004); Phys. Rev. Lett. \textbf{94}, 065003 (2005), Phys. Plasmas \textbf{13}, 082308 (2006).} Previous applications of these models include the study of Lagrangian statistics of tracers in plasma turbulence, and the description of nondiffusive transport phenomenology.\footnote{Ibid.} Here we discuss the applications to perturbative experiments in JET involving fast cold pulse propagation and ICRH power modulation.\footnote{P. Mantica, et al., 19th Intern. Conf. on Fusion Energy, Lyon [IAEA, Vienna, 2002].} Local transport models have found problematic to reconcile the fast propagation of the cold pulses with the comparatively slower propagation of the heat waves generated by power modulation. Here we show that the fractional model can successfully describe both types of perturbations.\footnote{D. del-Castillo-Negrete, et al., Nucl. Fusion \textbf{48} 075009 (2008).} [Preview Abstract] |
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TP6.00118: Numerical methods for anisotropic heat transport in high-temperature plasmas Eric Held, Jeong-Young Ji, Carl Sovinec The highly anisotropic nature of heat transport in high-temperature, magnetized plasmas yields stiff systems of algebraic equations upon spatial discretization. High-order, 2-D finite elements have had substantial success resolving extreme anisotropies even without grid and magnetic field alignment. We discuss further improvements in the spatial representation offered by a mixed finite-element method (MFEM) which solves simultaneously for temperature, $T$, and an auxiliary scalar related to the large parallel heat flux, $q_\|$. The successful application of iterative solves for the ill-conditioned, MFEM system has led to the development of a direct, continuum solution to the lowest-order drift kinetic equation. A fully implicit solve for the coefficients of a basis function expansion in the velocity variables couples to the $T$ advance in the same manner as the auxiliary scalar of the MFEM method. The convergence properties of various velocity bases are discussed and the physical predictions of the continuum and diffusive MFEM solutions are compared for a number of test problems. [Preview Abstract] |
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TP6.00119: Application of the para-real algorithm to a simple fluid drift-wave turbulence model for transport timescale studies Debasmita Samaddar, D.E. Newman, R. Sanchez, B.A. Carreras In this poster, we will present the results of applying the para-real algorithm [1] to a fluid turbulence code. This new technique efficiently parallelizes the time domain and has been found to significantly reduce the computational wall time. This will allow study of the turbulent transport dynamics on transport time scales something that has heretofore been very difficult. By applying this algorithm, we will characterize the fractional transport exponents in simulations of drift-wave turbulence [2] in slab geometry as a function of the model and sheared flow. Several situations will be explored, in which the relative dominance of the polarization and ExB nonlinearities will be tuned artificially and the transport exponents will be explored as the sheared flow changes. References: [1] Martin J. Gander et al, Siam J. Sci. Comput. Vol.29, No.2, pp.556-578 [2] D.E. Newman et al, Phys. Fluids B 5, 1140 (1993) [Preview Abstract] |
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TP6.00120: Understanding the impact of sheared flows on transport dynamics D.E. Newman, R. Sanchez, Debasmita Samaddar, B.A. Carreras, Jean-Noel Leboeuf It has been found in recent gyro-kinetic simulations that the underlying dynamics of transport can change in the presence of a sheared flow. In this paper we show that the effect of the presence of the sheared flow can go beyond the reduction of effective transport coefficients by changing the very nature of the radial transport dynamics. We will explore the physical mechanism for this change using a simple fluid drift wave turbulence model [1]. The change in the dynamics will be shown in the gyro-kinetic and fluid models for imposed and self-consistent flows. Using the fluid model, a simple picture will be developed showing different regimes of the dynamics as the flow changes in which flow stability is also considered. The implication of these changes on transport modeling and large scale turbulence simulations will be discussed. References: [1] D.E. Newman et al, Phys. Fluids B 5, 1140 (1993) [Preview Abstract] |
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TP6.00121: Gyrokinetic particle characterization of core turbulence in tokamaks Jean-Noel Leboeuf, Benjamin Carreras, Viktor Decyk, David Newman, Raul Sanchez We are continuing to characterize transport in gyrokinetic calculations of ion channel turbulence in tokamaks with the three-dimensional global toroidal nonlinear parallel particle-in-cell UCAN code. In particular, we are taking full advantage of the extended particle manager in UCLA's own PLIB library of massively parallel particle and field managing MPI routines. It now automatically handles tracking/tracing of the same active simulation particles through space and time and especially multiple processors, including restarts with different numbers of tagged particles. The particle data thus tracked and stored comprise the complete set of positions and velocities for each tracked particle at each chosen instant of time (typically every 100th time step). These particle data have been analyzed with tools previously applied to passive marker particles in fluid turbulence simulations which are specifically aimed at revealing the non-diffusive aspects of particle and heat transport. The UCAN calculations show that the transport signatures are different without and with zonal flows self-consistently generated from the fluctuations allowed to evolve. These differences are explored in this presentation for tokamak discharges with DIII-D parameters and profiles. [Preview Abstract] |
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TP6.00122: On the nature of transport in near-marginal DTEM turbulence: effect of a subdominant diffusive channel Jose A. Mier, Raul Sanchez, Luis Garcia, David Newman, Ben Carreras In this paper we characterize, using a novel Lagrangian method, the change in nature of radial transport in numerical simulations of near-critical dissipative-trapped-electron-mode turbulence, as the relative strength of an additional diffusive transport channel (subdominant to turbulence) is increased from zero. In its absence, radial transport exhibits the lack of spatial and temporal scales characteristic of self-organized-critical systems. This dynamical regime survives up to diffusivity values which, for the system investigated here, greatly exceeds the expected neoclassical value. Our results complete and extend previous works based instead on the use techniques imported from the study of cellular automata [1]. They shed further light on why some features of self-organized criticality seem to be observed in magnetically confined plasmas in spite of the presence of mechanisms which apparently violate the conditions needed for its establishment.\\ \hspace{0.5cm} $[1]$ J.A. Mier et al, Phys. Plasmas \textbf{13}, 102308 (2006) [Preview Abstract] |
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TP6.00123: Unitary Computational Algorithm for a 3D Hamiltonian System: twisting of filamentary vortex solitons demarcated by fast Poincare recurrence time Jeffrey Yepez, George Vahala, Linda Vahala A mesoscopic lattice representation for the 3D Gross-Pitaevskii (GP) equation is considered using two qubits/spatial node. These qubits are locally entangled by unitary collision operators and the post collision probabilities are then streamed to neighboring nodes by unitary operators. Vortex reconnection occurs without viscosity or resistivity, with Kelvin waves emitted and propagated along the vortex tubes. If the internal energy of this Hamiltonian system is sufficiently low, fast Poincare recurrence time is found -- a result very unexpected for a 3D system. Strong vortex entanglement is found at intermediate times -- not unlike the Arnold Cat map. In this parameter regime there is not a turbulent steady state. However, as the internal energy of the system is increased, twisting of the line vortices is found as the system approaches the zero internal energy Poincare recurrent time. For long times turbulence ensues with the Poincare recurrence time now becoming astronomical. [Preview Abstract] |
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TP6.00124: Basis operator bispectral analysis on CSDX data D.A. Baver, P.W. Terry, G.R. Tynan, S.H. Mueller Basis operator bispectral analysis is a novel approach for inferring parameters of a turbulent system from fluctuation measurements. This method differs from previous approaches to bispectral analysis in that it fits parameters to a model equation that is spatial, rather than spectral, in nature. This results in an algorithm that is much more flexible and adaptable. In particular, it is well-adapted to limited data sets and spatial inhomogeneities, such as shear flow, curvature effects, and boundary conditions. We will demonstrate this method by applying it to experimental data from CSDX. This will comprise both two-field analysis of probe data, and one-field analysis of camera data. By comparing these results with each other and with theoretical models, we can assess the accuracy and utility of this method of data analysis. We will also determine if this method can provide greater understanding of system behavior than simulation-to-experiment or theory-to-experiment comparisons alone. Work supported by USDOE. [Preview Abstract] |
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TP6.00125: Pulse Propagation and Fast Transient Transport Phenomena Models with Self Consistent Noise Z. Wang, P.H. Diamond, O.D. Gurcan, X. Garbet A theoretical understanding of cold pulse propagation and fast transient phenomena has long remained elusive. Recent theoretical advances, such as the developing theory of turbulent spreading, have been motivated by the challenge of quantitatively modeling such pulse propagation phenomena. The most recent theoretical results suggest that both: turbulence spreading, the turbulence diffusion of the fluctuation intensity field; and fluctuation growth, with a critical gradient threshold and self-consistent transport evolution; are required to model pulse propagation. The first capture nonlinear intensity spreading, while the second at least partially represents avalanche phenomena. Here we extend the theory to the interesting questions of noise effects on pulse propagation. Experience with self-organized criticality theory suggests that very modest amounts of noise can nucleate avalanches and other extended transport events. Here we explore the self-consistent treatment of nonlinear noise arising from nonlinear coupling and the effect of noise on the spectrum of transport events. [Preview Abstract] |
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TP6.00126: Zonal Flows from Parametric Decays in ITG Turbulence and the Internal Gravity Wave Paradigm Ken Uzawa, Tomohiko Watanabe, Hideo Sugama, Noriyoshi Nakajima, Wendell Horton The problem of ITG turbulence in a torus is closely related in mathematical structure to the problem of nonlinear internal gravity waves (IGW). The torus has an effective gravity g=c$^{2}_{s}$ cos$\theta $/R which points away from the center of the torus with major radius R. The angle $\theta $ is taken as zero on the outside of the torus where the plasma is unstably stratified in mass density $\rho $ with the buoyancy frequency $\omega ^{2}_{g}$= -g dln$\rho $/dr $<$0 or unstable whereas the inside of the torus where cos$\theta $= -1 the stratification is stable with $\omega ^{2}_{g}$= -gdln$\rho $/dr $>$0. Alfven waves couple the two regions producing the well known ideal beta limit or Tryon limit on stable plasma MHD motions. For drift waves the motions are much smaller scale and thus the local plasma remains stable on the inside and unstable on the outside. We have derived a dispersion relation, which is fourth order algebraic equation with respect to the complex frequency of zonal flow, and find that the conditions for the parametric decay of finite amplitude ITG or IGW into zonal flow modes. We use the pseudo-spectral drift wave code to test the conditions and find how much the energy spreads from a narrow unstable wave packet into a broad spectrum of stable and unstable modes. Work partially supported by the U. S. Department of Energy, National Science Foundation, and the Ministry of Education in Japan. [Preview Abstract] |
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TP6.00127: Momentum Theorems for Zonal Flow Evolution in Drift Wave Turbulence Patrick Diamond, O.D. Gurcan, T.S. Hahm, K. Miki We present the derivation of an \textit{exact} momentum conservation theorem which rigorously constrains the evolution of zonal flows in drift wave turbulence. The theorem is based on the interplay between potential enstrophy conservation and zonal momentum balance, facilitated by the Taylor identity, and constitutes a generalization of the Charney-Drazin theorem, well known in GFD. We present applications to Hasegawa-Wakatani, resistive interchange and fluid ITG systems. The significance of fixed driving flux, dynamic alignment and non-unity Prandtl number (i.e. dynamic alignment) are discussed. Implications for transition models are discussed. This material is based upon work supported by the Department of Energy under Award Numbers DE-FG02-04ER54738 and DE-FC02-08ER54959. [Preview Abstract] |
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TP6.00128: Role of Fluctuation Potential Enstrophy Flux and Turbulence Spreading in Zonal Flow Momentum Balance Y. Kosuga, P.H. Diamond, O.D. Gurcan, T.S. Hahm Analyses of generalized Charney-Drazin theorems for drift wave turbulence indicate that the \textit{unique, explicitly nonlinear effect} controlling flow evolution is the radial transport of fluctuation potential enstrophy. This impacts zonal flow evolution via the divergence of the turbulent potential enstrophy flux, so that turbulence spreading is inexorably linked to zonal flow dynamics. Physically, this follows from the fact that fluctuation pseudo-momentum (generalized wave momentum density) is proportional to potential enstrophy density, so inhomogeneity and transport of the latter must necessarily impact the zonal flow via the balance with pseudo-momentum. Here, we explicitly calculate the potential enstrophy flux for collisional drift wave turbulence and use the result to construct coupled envelope equations for the flow and intensity field. This material is based upon work supported by the Department of Energy under Award Numbers DE-FG02-04ER54738 and DE-FC02-08ER54959. [Preview Abstract] |
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TP6.00129: Study of nonlinear interplay between drift wave and zonal flow Zehua Guo, Liu Chen The size scaling of confinement properties in magnetized plasmas is one of the important and challeging problems of fusion research. In the present work, we both numerically and analytically investigate the nonlinear dynamics of drift wave turbulent transport, adopting the coherent drift wave-zonal flow modulation interaction model [Chen et al., Phys. Plasmas 7, 3129 (2000)] and the slab geometry for simplicity [Guzdar et al., Phys. Plasmas 8, 459 (2001)]. Our model allows both temporal and spatial variations of the turbulence radial envelope. The results demonstrate that the linear drive/damping together with the turbulence spreading, due to finite linear group velocity and nonlinear coupling between the drift wave and zonal flow, cause the device-size dependence of the saturated turbulence intensities and transport coefficients. The coherent and chaotic behavior of the dynamical system are also discussed. [Preview Abstract] |
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TP6.00130: Analysis of feedback loop dynamics in turbulence spreading Kazuhiro Miki, Patrick H. Diamond, Zhihong Lin Turbulence spreading, namely, spatial the spillover of excitation into stable region, is an important mesoscale process and so will naturally couple to zonal flow (ZF) and geodesic acoustic mode (GAM) dynamics. In this work, we examine the feedback loops between the evolving turbulence envelope and ZF/GAMs. One possible mechanism for self-consistent feedback is radial GAM propagation and feedback to turbulence via geodesic-acoustic coupling. Another is the effect of a turbulence potential enstrophy flux on zonal momentum, acting in context with the natural tendency of zonal flows to regulate the enstrophy flux by shearing. These mechanisms are tested using the gyrokinetic PIC simulation code GTC. We compare low and high $q$ evolution cases in order to separate the effect of low frequency zonal flows and higher frequency GAMs. Detailed results will be presented. This material is based upon work supported by the Department of Energy under Award Numbers DE-FG02-04ER54738 and DE-FC02-08ER54959. [Preview Abstract] |
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TP6.00131: Further Experimental Identification of ETG Modes Xiao Wei, Vladimir Sokolov, Amiya K. Sen The electron temperature gradient (ETG) mode, which is believed to be one of the strongest candidates for the anomalous electron energy transport [1, 2], is difficult to detect in experiments because of its high frequency (few MHz) and short wave length ($k_\perp\rho_e \sim 1$). Using a DC bias heating scheme of the core plasma, we are able to produce sufficiently strong electron temperature gradient in Columbia Linear Machine (CLM). A high frequency mode at $\sim 2MHz$, with azimuthal wave number $\it m \sim 15 $, has been observed. This frequency is consistent with the result of a kinetic dispersion relation of slab ETG modes with appropriate $\vec{E} \times \vec {B}$ Doppler shift. The scaling of its fluctuation level with the temperature gradient scale length and the radial structure are found to be roughly consistent with theoretical expectations. The parallel wave number of the mode can be varied via changing the position of the endplate by at least $30 \%$ and the scaling of the fluctuation level with $k_\parallel$ will also be reported. \\ $[1]$ W. Dorland \it {et al}., \it {Phys. Rev. Lett}. 85, 5579 (2000). \\ $[2]$ R.E. Waltz, J. Candy and M. Fahey, \it {Phys. Plasmas}, 14, 056116 (2007). [Preview Abstract] |
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TP6.00132: Diagnostics for Radial Electron Thermal Transport Measurements Vladimir Sokolov, Xiao Wei, Amiya K. Sen Production and identification of electron temperature gradient (ETG) mode have been successfully demonstrated in a basic experiment in Columbia Linear Machine CLM [1]. ETG mode was excited by heating electrons of the core plasma. For local measurement of radial electron thermal transport we use miniature langmuir probes with special resolution $ < 1mm$ and frequency response $ > 2MHz$. Triple probes will be used to measure local temperature fluctuations. For significant improvement of the frequency response of the triple probe we used a capacitive probe as a single probe and used capacitive coupling of double probe's positive tip with the measuring circuit. Then cross- correlation of the electron temperature fluctuations and potential fluctuations will yield local thermal flux. A non-local measurement scheme based on a variation of perturbative transport method is attempted. Here we modulate (at $\sim$ 20-50kHz) the accelerating bias voltage which produces the electron heating. The resulting modulated $ \tilde {T_e}$ in the plasma core will be conducted / convected to the plasma edge, where it will be measured by a triple probe. $[1]$ X. Wei, V. Sokolov and A.K. Sen, Bulletin of 49th APS DPP, p.110, 2007. [Preview Abstract] |
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TP6.00133: Role of stable eignmodes in ETG-driven turbulence Juhyung Kim, Paul W. Terry We investigate the role of the stable eigenmodes in the electron-temperature-gradient driven (ETG) turbulence. The low-wavenumber stable eigenmodes are thought to play a role in the dissipation mechanism leading to the saturation of CTEM [1] and ITG [2] turbulence. Based on the ETG slab fluid model [3], the condition and parameter regime for the saturation by the stable mode are analytically explored through statistical closure theory. In the simulation, the evolution of heat flux and turbulent energy are traced with the correlation of the stable and unstable modes. Possible effect of the stable mode saturation on zonal flow and magnetic fluctuation will be discussed. In the future, we will extend the analysis to the nonlocal ETG fluid model and kinetic model. \newline [1] P. W. Terry, D. A. Baver, and S. Gupta, Phys. Plasmas 13, 022307 (2006). \newline [2] R. Gatto, P. W. Terry, and D. A. Baver, Phys. Plasmas 13, 022306 (2006). \newline [3] W. Horton et. al. Nuclear Fusion 45 (2005). [Preview Abstract] |
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TP6.00134: Experimental observation of the dynamics of shear flow and self-regulating drift-wave turbulence in a cylindrical plasma device Zheng Yan, George Tynan, Christopher Holland, Min Xu, Jonathan Yu, Stefan Muller We present an experimental observation of the drift-wave/shear flow dynamical system in a cylindrical plasma device. The azimuthal velocity at the shear layer from the multi-tip Langmuir probe measurements shows a low frequency evolution. The turbulent potential, divergence of the turbulent Reynolds stress and the turbulent radial particle flux are also modulated at the same frequency. The turbulence energy and the flow energy are conserved. A detailed ion momentum balance analysis demonstrates that such slow evolving shear flow is sustained by the turbulent Reynolds stress against the collisional and viscous damping. A similar low frequency evolving shear flow is also observed from the fast-frame imaging analysis, which also demonstrates the existence of the shear flow decorrelation process. Taken together, an experimental validation of the theoretical picture of the shear flow dynamics is provided in this work. [Preview Abstract] |
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TP6.00135: Experimental Study of Nonlinear Energy Transfer in Frequency Domain Using a Two-field Model MIN XU, George Tynan, Christopher Holland, Zheng Yan, Stefan Mueller, Jonathan Yu A two-field model for studying quadratic nonlinear energy transfer in frequency domain is proposed, and its theoretical standing, application in experiments and experimental results are presented. The magnitude of quadratically coupled energy among different frequencies is directly computed from nonlinear energy transfer terms in the derived energy balance equations. Experimentally, a 9-tip Langmuir probe array is used to measure all the quantities needed for the calculation. The experiments have been carried out on CSDX (Controlled Shear Decorrelation Experiment) plasma device. The cross-bispectral calculations for both the internal and kinetic energy show clearly that energy transfer from drift wave turbulence to the azimuthally rotating mean flow exists in the region where a shear layer is formed. The measured energy transfer terms (both the kinetic and internal) show a net energy transfer to low frequencies (corresponding to larger azimuthal scales) at the maximum shear location, and a net transfer to higher frequencies (corresponding to smaller azimuthal scales) on either side of the shear layer, indicating that the direction of non-linear energy transfer is closely related to the shear in plasma. [Preview Abstract] |
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TP6.00136: Spatio-Temporal Multiscaling Dynamics of Impurity Transport in Turbulent Plasmas Sadri Benkadda, Shimpei Futatani, Diego del-Castillo-Negrete The spatio-temporal dynamics of the turbulent transport of impurities is studied in the Hasegawa-Wakatani model. Two transport scenarios are studied: a freely decaying case, and a forced case in which the impurity is forced by an externally imposed gradient. The results of the numerical simulations are analyzed using Proper Orthogonal Decomposition (POD) techniques. The low rank spatial POD eigenfunctions capture the large scale coherent structures and the high rank eigenfunctions capture the small scale fluctuations. The temporal evolution at each scale is dictated by the corresponding temporal POD eigenfunctions. In the decaying case the POD reveals the presence of ``bursty'' dynamics in which successively small scales are intermittently activated during the mixing process. In the forced simulations, the temporal dynamics exhibits stationary fluctuations. Spatial intermittency or ``patchiness'' in the mixing process characterizes the distribution of the passive tracer in the decaying quasi-hydrodynamic regime. The spatio-temporal POD scales exhibit a diffusive-type scaling in the quasi-adiabatic regime. However, diffusive scaling seems to be absent in the quasi-hydrodynamic regime. [Preview Abstract] |
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TP6.00137: GYROKINETICS |
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TP6.00138: Theory of fine-scale zonal flow generation Lu Wang, T.S. Hahm Most zonal flow generation theory has been built upon drift wave turbulence with a usual assumption of $q_r \rho _{i\theta } <<1$ [Diamond et al., IAEA-CN-69/TH3/1 (1998), Chen et al., Phys. Plasma 7, 3129 (2000)]. However, recent nonlinear GTC simulations of trapped electron mode (TEM) turbulence exhibit a relatively short radial scale of the zonal flows with $q_r \rho_{i\theta} \sim 1$[Z. Lin et al., IAEA-CN/TH/8-4 (2004)]. This work reports an extension of zonal flow growth calculation via the wave kinetics approach to this short wavelength regime. A generalized expression for the neoclassical polarization shielding [Rosenbluth and Hinton, Phys. Rev. Lett. 80, 724 (1998)] comes from the modern bounce-kinetic equation [Fong and Hahm, Phys. Plasmas 6, 188 (1999)]. [Preview Abstract] |
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TP6.00139: Guiding-center Recursive Vlasov method and Lie-transforms in plasma physics A.J. Brizard, A. Mishchenko The gyrocenter phase-space transformation is rediscovered by a recursive solution of the Hamiltonian dynamics associated with the perturbed guiding-center Vlasov operator. The purpose of the present work is to clarify the relation between the derivation of the gyrocenter phase-space coordinates by the guiding-center Recursive Vlasov method and the method of Lie-transform phase-space transformations. A discussion of a recent application [1] of the Recursive Vlasov method is included. [1] F.I. Parra and P.J. Catto, Plasma Phys. Control. Fusion \textbf{50}, 065014 (2008) [Preview Abstract] |
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TP6.00140: A novel mechanism for driving spontaneous rotation P.I. Chichikov, P.S. Neuvazhay-Koryto, C.J. McDevitt, P.H. Diamond Beginning from a phase space conserving gyrokinetic formulation, a systematic derivation of parallel momentum conservation uncovers two physically distinct mechanisms in which microturbulence may drive spontaneous rotation. The first mechanism, which emanates from ExB convection of parallel momentum, has already been analyzed in [1,2], and was shown to be contingent upon radial electric field shear. Thus, this mechanism is most likely to be active in regions with steep pressure gradients. The second mechanism uncovered, which appears in the gyrokinetic formulation through the parallel nonlinearity, emerges due to charge separation induced by the polarization drift. This novel means of driving spontaneous rotation, while higher order in $\omega_k/\omega_{ci}$, is not dependent on radial electric field shear. Thus, while the magnitude of the former mechanism is strongly diminished in regions of weak radial electric field shear, this mechanism, whose sign is typically in opposition to its ExB counterpart, remains unabated and is thus likely relevant in L-mode plasmas. [1] O. D. Gurcan, et al., Phys.Plasmas 14, 042306 (2007) [2] R. R. Dominguez and G. M. Staebler, Phys. Plasmas 5, 3876 (1993). [Preview Abstract] |
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TP6.00141: Gyrokinetic analysis of vortex structures and distribution functions in slab ETG turbulence Motoki Nakata, Tomohiko Watanabe, Hideo Sugama, Wendell Horton The electron temperature gradient driven (ETG) turbulence is considered as one of the possible candidates causing the anomalous electron heat transport in a core region of magnetic fusion plasmas, and has been actively studied in recent years.In the present study, the ETG turbulence with weak collisionality in a slab configuration are investigated by means of a gyrokinetic Vlasov simulation with high phase-space resolution. Significant reduction of the electron heat flux accompanied with a spontaneous transition of vortex structures from turbulent to coherent states with the self-generated zonal flows has been found even for the large electron temperature gradient parameter \textit{$\eta $}$_{e}$. The detailed comparisons between the stable coherent vortex structure and a steady solution of Hasegawa-Mima equation derived from gyrokinetic equation have been carried out. Fine scale fluctuations in the turbulent vortex state have been identified with high resolution spatial visualizations of the fields, and with spectral analysis of the perturbed distribution function. [Preview Abstract] |
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TP6.00142: Positivity-Preserving Algorithms for Continuum Gyrokinetic and Gyrofluid Simulations of Edge Plasma Turbulence G.W. Hammett, J.L. Peterson The steep density and temperature gradients associated with the edge and scrape off layer regions of a fusion plasma complicate the numerical simulation of plasma turbulence. Spectral methods and Arakawa finite differencing have the useful property of exactly preserving certain conservation properties of Hamiltonian systems and work well for simulating small amplitude fluctuations. However, such algorithms can exhibit Gibbs phenomena, small overshoots in the vicinity of large gradients. While these overshoots are unimportant for small amplitude turbulence in the core region of tokamaks, these algorithms can lead to negative density or temperature in the tokamak edge region. Here we consider a 2-D test case and compare several different methods of solving multi-dimensional hyperbolic equations, including modern shock-capturing algorithms such as 3rd order WENO/UNO, discontinuous Galerkin, and a recent extremum-preserving 4th order method\footnote{P. Colella, M. D. Sekora, J. Comp. Phys. 227, 7069 (2008).}, which combines features of the Piecewise-Parabolic Method and Zalesak's version of Flux-Corrected Transport. [Preview Abstract] |
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TP6.00143: $\delta\!f$ Particle-in-Cell simulation of turbulent transport with Vlasov ions and gyrokinetic electrons Yang Chen, Scott E. Parker There are certain limitations in using gyrokinetic ions for turbulence and reconnection simulations. Applications where Vlasov ions might be more appropriate include ETG turbulence, steep edge gradient turbulence and weak guide-field reconnection. In such a situation the ion gyrokinetic model presently used needs to be extended, but a satisfactory extension valid for fully electromagnetic turbulence is not presently available. Even if an accurate model is found, its numerical implementation could be very challenging, based on past experience of developing gyrokinetic algorithms. The Vlasov ion/gyrokinetic electron hybrid model avoids these difficulties. Numerically, the main constraint on the time step in gyrokinetic ion simulations is due to the electron motion along the magnetic field. We found that for small devices such as NSTX a time step of $\Omega_i\triangle t=0.2$ has to be used for stability. With a time step slightly smaller it is possible to follow the ion gyro-motion accurately. The field equations of the hybrid model are the Faraday's equation and the Ampere's equation for $\delta {\bf B}$ and $\delta {\bf E}$. The scalar and vector potential are not used. We have devised an implicit scheme for this model, demonstrated in 3-D slab for the Alfven waves and the drift Alfven instability. Plans of implementation in toroidal geometry will be discussed. [Preview Abstract] |
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TP6.00144: Gyrokinetic Models for Edge Plasmas A.M. Dimits, R.H. Cohen, X.Q. Xu We have developed practical system of electromagnetic gyrokinetic equations for MFE edge simulations. This system 1) allows for large amplitude perturbations, 2) is consistent with energy conservation, and 3) minimizes the number of (difficult to implement) second-order terms needed. Because the relative perturbation amplitudes may be large, the operator in the gyrokinetic Poisson equation evolves with time, and the gyrocenter equations of motion used must retain specific second-order terms in order to maintain energy conservation. Methods for implementing the second-order terms in the equations of motion, and a useful finite-element discretization of the gyrokinetic Poisson equation have been developed. The latter results from a Galerkin approximation to Brizard's action variational principle. Because the magnetic field inhomogeneity scales are much longer than the radial plasma profile scales in the edge region, only the standard leading order terms (parallel streaming and magnetic drifts) need to be kept in the equilibrium portion of the gyrocenter equations of motion. [Preview Abstract] |
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TP6.00145: Global gyrokinetic simulation of ITER plasmas using coupled flux tubes Michael Barnes, William Dorland, Greg Hammett To faithfully simulate ITER and other modern fusion devices, we must resolve electron and ion fluctuation scales in a five-dimensional phase space and time. Simultaneously, we must account for the interaction of this turbulence with the slow evolution of the large-scale plasma profiles. Because of the enormous range of scales involved and the high dimensionality of the problem, resolved first-principles global simulations are very challenging using conventional (brute force) techniques. We have developed a new approach in which turbulence calculations from multiple gyrokinetic flux tube simulations from GS2 are coupled together using transport equations to obtain self-consistent, steady-state background profiles and corresponding turbulent fluxes. We will present results obtained from coupled flux tube simulations of the core of an ITER plasma, including multiple species, electromagnetic effects, and realistic magnetic geometry. [Preview Abstract] |
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TP6.00146: Advances in Gyrokinetic Particle Simulation Algorithms William Dorland, Ingmar Broemstrup Most widely-used delta-f gyrokinetic particle-in-cell simulation algorithms have two important shortcomings compared to Eulerian algorithms: (1) inaccurate treatment of short wavelength perturbations due to low-order ring-averaging techniques, and (2) unphysical collision operators, and/or collision operators which fail to control the algebraic growth of the particle weights in time. These shortcomings make it difficult to carry out credible, long-time PIC simulations of ITG+ETG, or ITG+TEM, or ITG+microtearing modes. We present an efficient GK PIC algorithm which solves both problems. Linear and nonlinear examples are presented, demonstrating the failures of conventional GK PIC schemes and the success of the new algorithm. Oustanding parallelization of the new algorithm is demonstrated, with successful utilization of many thousands of cores simultaneously. Efforts to port this algorithm to many-core processors such as graphics processing units will also be described. [Preview Abstract] |
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TP6.00147: Self-consistent particle transport in a gyrokinetic Z-pinch with pitch-angle scattering Kyle Gustafson, William Dorland, Diego del-Castillo-Negrete, Ingmar Broemstrup, Michael Barnes We have developed a new nonlinear gyrokinetic $\delta f$ PIC code with a proper pitch-angle scattering collision operator. For the present work, this code is useful for self-consistent particle tracking studies aimed at determining whether non-diffusive transport is relevant in gyrokinetic turbulence. A subset for each value of $k_{\perp}\rho_i$ is selected at random from the entire set of particles that determine the fields. While this technique is more involved than simply calculating flux, it is necessary for discovering evidence of non-diffusive transport. Non-diffusive transport is a consequence of non-Gaussian random walks. Observable results include power-law scaling of the variance of particle displacements and non-Gaussian displacement distribution functions. Here, we examine the nature of particle transport in our $\delta f$ PIC code for multiple values of $k_{\perp}\rho_i$, with and without pitch-angle scattering, in an electrostatic Z-pinch geometry with a temperature gradient. This geometry includes tokamak-relevant curvature effects, but allows for less expensive two-dimensional simulations. Results are compared to analytic expectations and other observations of non-diffusive transport in tracer simulations. [Preview Abstract] |
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TP6.00148: Anisotropies in 3D Driven Plasma Turbulence Kate Despain, William Dorland The energy spectrum dependence on wave number in a turbulent plasma with a background magnetic field remains an unresolved issue in our understanding of plasma turbulence. Most theories assume that the turbulence in the direction perpendicular to the background magnetic field can be treated as isotropic. However, Boldyrev [1] presented a theory in which driven turbulence would have an anisotropic structure in the perpendicular direction. Using a three-dimensional pseudo-spectral code, we investigate the energy flux and spectrum of plasma turbulence as described by the isothermal electron fluid model [2]. Special care is taken to analyze these quantities parallel to the velocity and magnetic field perturbations. \\[0pt] [1] S. Boldyrev, \textit{Spectrum of Magnetohydrodynamic Turbulence}. PRL 96, 115002 (2006). \\ {} [2] A. A. Schekochihin, et al., \textit{Astrophysical gyrokinetics: kinetic and fluid turbulent cascades in magnetized weakly collisional plasmas}, submitted ApJ. Suppl. (2007). [Preview Abstract] |
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TP6.00149: Ohm's law in gyrokinetic magnetic reconnection Ryusuke Numata, William Dorland, Tomoya Tatsuno, Barret Rogers We present numerical results of nonlinear simulations of collisionless magnetic reconnection in a strong guide magnetic field limit ($B_{\perp}/B_{g} \sim \rho/L \ll 1$, where $B_{g}$ and $B_{\perp}$ are the guide magnetic field and the magnetic field perpendicular to the guide field, $\rho$ is the Larmor radius, and $L$ is the macroscopic scale length associated with the parallel dynamics to the guide field using the \texttt{AstroGK} astrophysical gyrokinetics code. In the presence of the guide field, microscopic electron dynamics in the dissipation region, which is believed to break frozen-in condition, may change because of magnetization. We examine the generalized Ohm's law in the gyrokinetic reconnection, which describes the electron dynamics. The results will be compared with other simulation results based on PIC or fluid models. Specifically, we discuss the effect of off-diagonal elements of the electron pressure tensor. [Preview Abstract] |
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TP6.00150: Limitations of the Guiding Center Model for 3D Plasmas Linda Sugiyama In 3D magnetic fields, guiding center (GC) approximations for the charged plasma dynamics that use the particle's guiding center to describe the motion, rather than particle position, encounter geometrical difficulties [1]. For small gyroradius $\epsilon=\rho_i/L<1$. the fast gyration around the field lines is described in terms of a mathematical gyroangle, in local coordinates, and leads to a natural description in terms of an effective magnetic vector potential [2]. In a 2D with straight uniform field lines, such expansions in $\epsilon$ are exact to all orders. For general 3D fields, exact expansion to all orders requires good magnetic flux surfaces, zero magnetic torsion $\mathbf{b}\cdot\nabla\times\mathbf{b}=0$, where $\mathbf{b} \equiv \mathbf{B}/B$, and zero geodesic torsion $\tau_G$ on the flux surfaces. Most confined plasmas, however, have finite parallel current and finite torsion. GC expansions exist to first order in $\epsilon$, but are nonuniform in velocity phase space. If the field possesses exact 2D symmetry, such as toroidal axisymmetry, the expansion can exist at higher order. Ordering the time derivative of the magnetic vector potential in Ohm's law to be small, equivalent to dropping the compressional and/or shear Alfv\'en waves, also introduces geometrical approximations. \vspace{-6pt} \begin{flushleft} [1] L.E. Sugiyama, submitted to Physics of Plasmas (2008). \newline [2] R.J. Littlejohn, \emph{Phys. Fluids} \textbf{24} 1730 (1981). \end{flushleft} [Preview Abstract] |
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TP6.00151: Collision effect on ITG turbulent transport in an L-mode pedestal in real geometry tokmak edge Seung-Hoe Ku, C.S. Chang Even though the Ion temperature gradient (ITG) driven turbulence yields a robust plasma transport, it has not been considered to be a plausible candidate for the transport (and L-H transition) mechanism in the L-mode pedestal due to the weak ion temperature gradient ($\eta_i<2$) compared to the density gradient and the strong background ExB shearing rate. Surprisingly, in a real tokamak edge geometry with a magnetic separatrix, the full-f XGC1 gyrokinetic particle-in-cell code finds that there is a strong and fast ITG turbulence spreading from the density pedestal top ($\eta_i >3$) into the pedestal slope ($\eta_i <2$), with the ion heat conductivity in the entire edge pedestal region roughly similar to the experimentally inferred level. XGC1 uses a numerical magnetic and limiter geometry from a g-eqdsk data. Unlike a delta-f kinetic code, the full-f XGC1 code simulates the turbulence and the background plasma dynamics together. Coulomb collision frequency has a strong radial variation across the pedestal. Emphasis of this presentation will be given to the Coulomb collision effects on the edge ITG turbulence and transport. [Preview Abstract] |
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TP6.00152: Momentum Theorems in Gyrokinetic Plasmas T.S. Hahm, P.H. Diamond, Lu Wang, K. Miki Momentum balance relations for Zonal flows in drift wave turbulence in fusion plasmas have recently been derived using the inviscid invariance of potential vorticity, [P. H. Diamond, et. al., submitted to Plasma Phys. Control. Fusion (2008)] extending the work by Charney and Drazin [J. Geophys. Res. 66, 83 (1961)] in geofluid dynamics. This work reports on the gyrokinetic extension of the momentum theorems. We exploit the equivalence of polarization density and vorticity in the long wavelength limit, as well as the Taylor identity for the Reynolds' stress. [Preview Abstract] |
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