Download or read book Advances in Measurement and Modeling of the High confinement mode Pedestal on the Alcator C Mod Tokamak written by Jerry Wayne Hughes and published by . This book was released on 2005 with total page 52 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.
Download or read book Fusion Science and Technology written by and published by . This book was released on 2007 with total page 890 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Evolution of Pedestal Profiles Through the L H and H L Transitions in Alcator C Mod written by Amanda E. Hubbard and published by . This book was released on 2001 with total page 14 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.
Download or read book Study of High Performance Mode Access Conditions on the Alcator C Mod Tokamak written by Yunxing Ma and published by . This book was released on 2013 with total page 210 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.
Download or read book Enhanced D Alpha H mode Studies in the Alcator C Mod Tokamak written by Earl S. Marmar and published by . This book was released on 2000 with total page 6 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.
Download or read book Edge Transport Barrier Studies on the Alcator C Mod Tokamak written by Jerry Wayne Hughes and published by . This book was released on 2005 with total page 452 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.
Download or read book H mode Pedestal and L H Transition Studies on Alcator C Mod written by Jerry Wayne Hughes and published by . This book was released on 2005 with total page 104 pages. Available in PDF, EPUB and Kindle. Book excerpt: H-mode research on Alcator C-Mod is described, with a focus on the edge transport barrier (ETB). ETB pedestals are characterized using several diagnostics, leading to a thorough description of profile structure in H-mode. L-H transition criteria are discussed, along with the fast evolution of the pedestal following the L-H transition. H-mode regimes are described in terms of their edge transport characteristics and the local edge parameters favoring each. Empirical scalings of the pedestal with operational parameters are found, helping to illuminate physics governing the pedestal structure, and the relationship between edge transport and global confinement is discussed. Dimensionless comparisons between discharges on different tokamaks are discussed. Finally, ongoing work and directions for the future are described.
Download or read book ICRF heated Enhanced Confinement Modes in the Alcator C mod Tokamak written by Yuichi Takase and published by . This book was released on 1996 with total page 24 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book High Resolution Bolometry on the Alcator C Mod Tokamak written by Réjean Louis Boivin and published by . This book was released on 1998 with total page 18 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Impurity Asymmetries in the Pedestal Region of the Alcator C Mod Tokamak written by Randy Michael Churchill and published by . This book was released on 2014 with total page 216 pages. Available in PDF, EPUB and Kindle. Book excerpt: In an effort to illuminate the effects of the strong plasma gradients in the pedestal region on impurity transport, research was conducted to measure complete sets of impurity density, poloidal and parallel velocity, and temperature at two separate poloidal locations in the pedestal region of the Alcator C-Mod tokamak. To this end, the diagnostic technique gas puff-CXRS was refined and expanded on, allowing for the first time in a tokamak complete measurements of impurities at the high-field side (HFS). Large in-out B5+ impurity density asymmetries were measured in H-mode plasmas with strong boundary electron density gradients, with a build-up of impurity density at the HFS. Impurity temperatures were also found to be asymmetric in the pedestal region, with larger temperatures at the low-field side (LFS). Such temperature asymmetries suggest a significant asymmetry in electron density near the separatrix. In contrast to these H-mode results, plasmas with low boundary electron density gradients, such as L-mode and I-mode, exhibit constant impurity density on a flux surface, even if strong electron temperature gradients are present. Mechanisms which could drive such poloidal asymmetries are explored. Experiments provide evidence against localized impurity sources and fluctuation-induced transport as primary causes. Particle transport timescales are compared, showing that the radial transport becomes comparable to or faster than the parallel transport in the pedestal region. Additionally, modelling of impurity transport using conventional, one-dimensional neoclassical physics fails to correctly reproduce the measured flux-surface averaged impurity density, suggesting along with the timescale estimates that a more complete two-dimensional treatment of impurity particle transport is required. The measured impurity velocities at the LFS and HFS are compared to the canonical form for particle flow velocity within the flux surface of a tokamak. Within the error bars of the measurement, agreement is found with the canonical form. The implications of exact matches to the canonical form are low radial transport, and the E x B drift dominating the perpendicular impurity flow. Further work is motivated into more precise velocity measurements to determine if the velocities exactly match this canonical form.
Download or read book The High Resolution Video Capture System on the Alcator C Mod Tokamak written by Aaron J. Allen and published by . This book was released on 1996 with total page 14 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book High Resolution Visible Continuum Imaging Diagnostic on the Alcator C Mod Tokamak written by Earl S. Marmar and published by . This book was released on 2000 with total page 13 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Pedestal Profiles and Measurements in C Mod Enhanced D alpha H modes written by Amanda E. Hubbard and published by . This book was released on 2000 with total page 18 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.
Download or read book The Alcator C Mod Program written by Earl S. Marmar and published by . This book was released on 2005 with total page 24 pages. Available in PDF, EPUB and Kindle. Book excerpt: Alcator C-Mod is a high magnetic field tokamak with strong shaping capabilities. While compact in physical dimensions, C-Mod produces plasmas which overlap, in dimensionless parameters and absolute performance, with those produced in much larger devices. Auxiliary heating and current drive systems for C-Mod exclusively employ radio-frequency tools (Ion-Cyclotron and Lower Hybrid), naturally decoupling the heating, fueling and momentum sources. Routine operation at high absolute density, enabled by large B/R, allows C-Mod to explore regimes with fully equilibrated electrons and ions. Compactness also yields very high power densities and particle fluxes, expanding the available parameter space for scrape-off-layer and divertor physics and technology studies. C-Mod has always and exclusively used high Z metallic plasma facing components for all high heat flux regions. C-Mod produces the highest absolute pressure plasmas (at the ITER magnetic field and [beta]), with the current tokamak record for volume average plasma pressure (1.8x10^5 Pa).
Download or read book Core Internal Transport Barriers on Alcator C Mod written by Catherine L. Fiore and published by . This book was released on 2000 with total page 26 pages. Available in PDF, EPUB and Kindle. Book excerpt: The formation of internal transport barriers (ITB) has been observed in the core region of Alcator C-Mod under a variety of conditions. The improvement in core confinement following pellet injection (pellet enhanced performance or PEP mode) has been well documented on Alcator C-Mod in the past. Recently three new ITB phenomena have been observed which require no externally applied particle or momentum input. Short lived ITBs form spontaneously following the high confinement (H) to low confinement (L) mode transition and are characterized by a large increase in the global neutron production (enhanced neutron or EN modes.) Experiments with ICRF (ion cyclotron range of frequencies) power injection to the plasma off-axis on the high field side results in the central density rising abruptly and becoming peaked. The ITB formed at this time lasts for 10 energy confinement times. The central toroidal rotation velocity decreases and changes sign as the density rises. Similar spontaneous ITBs have been observed in ohmically heated H-mode plasmas. All of these ITB events have strongly peaked density profiles with a minimum in the density scale length occurring near r/a = 0.5 and have improved confinement parameters in the core region of the plasma. Keywords: Alcator C-Mod; confinement; tokamaks; transport phenomena; neutrons.
Download or read book Doppler Measurements in the Edge of the Alcator C Mod Tokamak Using a High resolution Visible Spectrometer written by Alexander Turner Graf and published by . This book was released on 2008 with total page 390 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book High Resolution Edge Thomson Scattering Measurements of the Alcator C Mod Tokamak written by Jerry Wayne Hughes and published by . This book was released on 2000 with total page 11 pages. Available in PDF, EPUB and Kindle. Book excerpt:
