Evidence for Electromagnetic Fluid Drift Turbulence Controlling the Edge Plasma State in the Alcator C-Mod Tokamak PDF Download
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Author: Publisher: ISBN: Category : Languages : en Pages : 120
Book Description
Plasma profiles across the separatrix and scrape-off layer (SOL) in Alcator C-Mod are examined for a range of plasma densities, currents and magnetic fields in ohmic L-mode discharges and for a subset of conditions in ohmic H-mode discharges. In all plasmas, electron pressure gradient scale lengths (Lp) exhibit a minimum value just outside the separatrix (i.e., in the near SOL), forming the base of a weak (strong) pedestal in L-mode (H-mode) plasmas. Over a wide range of conditions in ohmic L-mode discharges, Lp at this location are found to track with a monotonic function of electron collision frequency, when this quantity is normalized according to the framework of electromagnetic fluid drift turbulence theory. Moreover, at fixed values of normalized collisionality (characterized as the 'diamagnetic parameter', alpha_d), electron pressure gradients in the near SOL increase with plasma current squared, holding the MHD ballooning parameter, alpha_MHD, unchanged. Thus, the state of the near SOL is restricted to a narrow region within this two-parameter phase-space. An implication is that cross-field heat and particle transport are strong functions of these parameters. Indeed, as alpha_d is decreased below ~0.3, cross-field heat convection increases sharply and competes with parallel heat conduction along open field lines, making high plasma density regions of alpha_MHD-alpha_d space energetically inaccessible. These observations are consistent with the idea that the operational space of the edge plasma, including boundaries associated with the tokamak density limit, are controlled by electromagnetic fluid drift turbulence.
Author: Publisher: ISBN: Category : Languages : en Pages : 120
Book Description
Plasma profiles across the separatrix and scrape-off layer (SOL) in Alcator C-Mod are examined for a range of plasma densities, currents and magnetic fields in ohmic L-mode discharges and for a subset of conditions in ohmic H-mode discharges. In all plasmas, electron pressure gradient scale lengths (Lp) exhibit a minimum value just outside the separatrix (i.e., in the near SOL), forming the base of a weak (strong) pedestal in L-mode (H-mode) plasmas. Over a wide range of conditions in ohmic L-mode discharges, Lp at this location are found to track with a monotonic function of electron collision frequency, when this quantity is normalized according to the framework of electromagnetic fluid drift turbulence theory. Moreover, at fixed values of normalized collisionality (characterized as the 'diamagnetic parameter', alpha_d), electron pressure gradients in the near SOL increase with plasma current squared, holding the MHD ballooning parameter, alpha_MHD, unchanged. Thus, the state of the near SOL is restricted to a narrow region within this two-parameter phase-space. An implication is that cross-field heat and particle transport are strong functions of these parameters. Indeed, as alpha_d is decreased below ~0.3, cross-field heat convection increases sharply and competes with parallel heat conduction along open field lines, making high plasma density regions of alpha_MHD-alpha_d space energetically inaccessible. These observations are consistent with the idea that the operational space of the edge plasma, including boundaries associated with the tokamak density limit, are controlled by electromagnetic fluid drift turbulence.
Author: Ben Zhu Publisher: ISBN: Category : Languages : en Pages : 206
Book Description
A new global 3D two-fluid code, GDB, based on the drift-reduced Braginskii model has been developed and tested to study the turbulent transport across the entire tokamak edge region: from plasma sources in the inner core to plasma sinks in the outer-most scrape-off layer (SOL). In this code, profiles of plasma density, electron and ion temperature, electric potential, magnetic flux and parallel flow are evolved self-consistently. Milliseconds-long simulations are carried out in a shifted-circle magnetic configuration with realistic Alcator C-Mod tokamak inner wall limited (IWL) discharge parameters. The resistive ballooning instability is identified as the predominant driver of edge turbulence in the L-mode regime. Simulations show, in agreement with experimental observations, as the simulation moves towards density limit regime by increasing density, the turbulent transport is drastically enhanced and the plasma profiles are relaxed; on the other hand, as the simulation approaches to the H-mode regime by increasing temperature, the turbulent transport is suppressed and plasma profiles are steepened with a pedestal-like structure forming just inside of the separatrix. Radial transport level and turbulence statistics of these simulations also qualitatively match the experimental measurements. Spontaneous E x B rotation in the electron diamagnetic drift direction in the closed flux region are observed in all cases. It can be explained based on the steady state ion continuity relation [mathematical equation]. E x B rotation in the closed flux region is found mostly cancels the ion diamagnetic drift as H-mode-like regimes are approached, and exceeds it by a factor of two or more at lower temperatures due to parallel ion flows.
Author: Victoria R. Winters Publisher: ISBN: Category : Languages : en Pages : 36
Book Description
Understanding the underlying dynamics of turbulence in magnetic confinement fusion experiments is extremely important. Turbulence greatly reduces the confinement time of these devices and therefore greater knowledge of turbulent dynamics can help with its mitigation. Experiments from the Alcator C-Mod tokamak [18] provide support for a theory that edge turbulence in tokamak fusion plasmas is the result of deterministic chaos, rather than stochastic processes [15]. Using readily available reflectometer data from Alcator C-Mod (C-Mod), analysis of C-Mod edge turbulence in Ohmic plasmas and Ion Cyclotron Range of Frequencies (ICRF) heated L-Mode plasmas shows that density fluctuations just inside or at the Last Closed Flux Surface (LCFS) exhibit exponential power spectra. Theoretically, the characteristic slope of the data on a semi-log plot gives the full width of the underlying Lorentzian pulses, which give rise to the exponential power spectra due to the dynamics of deterministic chaos. Using a separate fitting routine, individual Lorentzian pulses in the reflectometer time series data are identified, and the widths of the Lorentzian pulses match the inverse characteristic frequency of the exponential power spectra. Analysis of the waiting times between pulses and the pulse amplitudes indicate these are randomly distributed yet the pulse widths have a narrow distribution. These characteristics are consistent with a chaotic process. There is also a preliminary comparison of GPI data and a discussion of limitations of the analysis presented here and plans for future work. Overall, the experimental results in this study are consistent with edge turbulence that is at least partially generated by chaotic dynamics.
Author: Jerry Wayne Hughes Publisher: ISBN: Category : Languages : en Pages : 52
Book Description
Edge transport barrier (ETB) studies on the Alcator C-Mod tokamak [Phys. Plasmas 1, 1511, (1994)] investigate pedestal scalings and radial transport of plasma and neutrals. Pedestal profiles show trends with plasma operational parameters such as total current IP . A ballooning-like I2P dependence is seen in the pressure gradient, despite calculated stability to ideal ballooning modes. A similar scaling is seen in the near scrape-off-layer for both low-confinement (L-mode) and H-mode discharges, possibly due to electromagnetic fluid drift turbulence setting transport near the separatrix. Neutral density diagnosis allows examination of D0 fueling in H-modes, yielding profiles of effective particle diffusivity in the ETB, which vary as IP is changed. Edge neutral transport is studied using a 1D kinetic treatment. In both experiment and modeling, the C-Mod density pedestal exhibits a weakly increasing pedestal density and a nearly invariant density pedestal width as the D0 source rate increases. Identical modeling performed on pedestal profiles typical of DIII-D [Nucl. Fusion 42, 614, (2002)] reveal differences in pedestal scalings qualitatively similar to experimental results.
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Author: S. J. Zweben
Author: S. J. Zweben
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Author: Ben Zhu
Author: Maxim V. Umansky
Author: Victoria R. Winters
Author: Jerry Wayne Hughes
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