Advances in Measurement and Modeling of the High-confinement-mode Pedestal on the Alcator C-Mod Tokamak PDF Download
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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.
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.
Author: Amanda E. Hubbard Publisher: ISBN: Category : Languages : en Pages : 14
Book Description
Local edge electron parameters are measured in the Alcator C-Mod tokamak, during discharges in which input power is continuously ramped up and down, leading to transitions from the low-confinement (L) to high confinement (H) mode and back to L-mode. This allows measurement of the accessible portions of the non-monotonic flux-gradient relationship proposed by models of the H-mode as a critical transition. Results are consistent with a dependence of conductivity x on temperature gradient, having a very sharp decrease above a critical value. Other possible flux-gradient relations are also examined, and the data are compared with theoretical formulations. The initial transient of pedestal energy in the first few ms after the L-H transition is also analyzed and found to be consistent with a sudden decrease in x across the pedestal region.
Author: Yunxing Ma Publisher: ISBN: Category : Languages : en Pages : 210
Book Description
Usually when sufficient heating power is injected, tokamak plasma will make an abrupt transition into a state with improved confinement, known as the high-confinement mode, or H-mode. Given the greatly enhanced fusion yield, H-mode is foreseen as the baseline scenario for the future plasma operation of the International Thermonuclear Experimental Reactor (ITER). Many research efforts have been given to understand the criteria for H-mode access. To further contribute to this research, a primary focus of this thesis is characterizing the H-mode access conditions in the Alcator C-Mod tokamak, across a broad range of plasma density, magnetic field, and plasma current. In addition, dedicated experiments were designed and executed on C-Mod, to explore the effects of divertor geometry, ICRF resonance location, and main ion species on H-mode access conditions. Results from these experiments will be included in this thesis. The underlying physics of H-mode access is very complex, and the critical mechanisms remain largely unresolved. To promote our understanding, some models proposed for the H-mode transition are tested, using well documented local plasma conditions, obtained in C-Mod experiments. In particular, this thesis pioneers the test of a recently developed model for H-mode threshold power predictions.
Author: Earl S. Marmar Publisher: ISBN: Category : Languages : en Pages : 6
Book Description
A favorable regime of H-mode confinement, seen on the Alcator C-Mod tokamak is described. Following a brief period of ELM-free H-mode, the plasma evolves into the Enhanced D-Alpha (EDA) H-mode which is characterized by very good energy confinement, the complete absence of large, intermittent type I ELMs, finite impurity and majority species confinement, and low radiated power fraction. Accompanying the EDA H-mode, a quasi-coherent (QC) edge mode is observed, and found to be responsible for particle transport through the edge confinement barrier. The QC-mode is localized within the strong density gradient region, and has poloidal wavenumber and lab-frame frequency of 100 kHz. Parametric studies show that the conditions which promote EDA include moderate safety factor, high triangularity (d>0.35) and high target density (ne>1.2x20 m-3). EDA H-mode is readily obtained in purely ohmic and well as in ICRF auxiliary-heated discharges.
Author: United States. Congress. House. Committee on Appropriations. Subcommittee on Energy and Water Development Publisher: ISBN: Category : Energy development Languages : en Pages : 1440
Author: Amanda E. Hubbard Publisher: ISBN: Category : Languages : en Pages : 18
Book Description
High resolution measurements on the Alcator C-Mod tokamak [I.H. Hutchinson et al, Phys. Plasmas 1, 1551 (1994)] of the transport barrier in the "Enhanced Da" (EDA) regime, which has increased particle transport without large edge localized modes, show steep density and temperature gradients over a region of 2-5 mm, with peak pressure gradients up to 12 MPa/m. Evolution of the pedestal at the LH transition is consistent with a large, rapid drop in thermal conductivity across the barrier. A quasi-coherent fluctuation in density, potential and Bpol, with fo%7E50-150 kHz and kq%7E 4 cm-1, always appears in the barrier during EDA, and drives a large particle flux. Conditions to access the steady-state EDA regime in deuterium include d> 0.35, q95> 3.5 and L-mode target line average density> 1.2 x 1020 m-3. A reduced q95 limit is found for hydrogen discharges.
Author: Jerry Wayne Hughes Publisher: ISBN: Category : Languages : en Pages : 452
Book Description
Edge transport barriers (ETBs) in tokamak plasmas accompany transitions from low confinement (L-mode) to high confinement (H-mode) and exhibit large density and temperature gradients in a narrow pedestal region near the last closed flux surface (LCFS). Because tokamak energy confinement depends strongly on the boundary condition imposed by the edge plasma pressure, one desires a predictive capability for the pedestal on a future tokamak. On Alcator C-Mod, significant contributions to ETB studies were made possible with edge Thomson scattering (ETS), which measures profiles of electron temperature (20 [leq] Te[eV] [leq] 800) and density (0.3 [leq] ne[10^20m^-3] [leq] 5) with 1.3-mm spatial resolution near the LCFS. Profiles of Te, ne, and pe = neTe are fitted with a parameterized function, revealing typical pedestal widths [delta] of 2-6mm, with [delta]Te [geq] [delta]ne , on average. Pedestals are examined to determine existence criteria for the enhanced D[alpha] (EDA) H-mode. A feature that distinguishes this regime is a quasi-coherent mode (QCM) near the LCFS. The presence or absence of the QCM is related to edge conditions, in particular density, temperature and safety factor q. Results are consistent with higher values of both q and collisionality [nu]* giving the EDA regime. Further evidence suggests that increased abs([nabla]pe) may favor the QCM; thus EDA may have relevance to low-[nu]* reactor regimes, should sufficient edge pressure gradient exist.
Author: Jerry Wayne Hughes
Author: Jerry Wayne Hughes
Author: 
Author: Amanda E. Hubbard
Author: Yunxing Ma
Author: 
Author: Earl S. Marmar
Author: Yuichi Takase
Author: United States. Congress. House. Committee on Appropriations. Subcommittee on Energy and Water Development
Author: Amanda E. Hubbard
Author: Jerry Wayne Hughes