Download or read book The L to H mode Transition and Momentum Confinement in Alcator C Mod Plasmas written by John Reel Walk (Jr.) and published by . This book was released on 2010 with total page 64 pages. Available in PDF, EPUB and Kindle. Book excerpt: Using a spatially-resolving x-ray spectrometer system, toroidal impurity rotation in Alcator C-Mod plasmas was measured. The propagation of the rotational velocity from the edge to the core of the plasma column was measured during the L- to H-mode transition. Momentum transport was measured in both Ohmic and ICRF-heated discharges, which produced EDA and ELM-free H-modes. The momentum transport was modeled by a simplified diffusion model, in which momentum diffusivity was substantially higher than neoclassically predicted values.
Download or read book Global Momentum Confinement Times in Alcator C Mod H and I Regime Plasmas written by Michelle M. Victora and published by . This book was released on 2014 with total page 45 pages. Available in PDF, EPUB and Kindle. Book excerpt: Using a spatially-resolving x-ray spectrometer system, the toroidal rotation velocity in Alcator C-Mod plasmas is measured and analyzed. At the L-H and L-I transition, there is a co-current movement in the toroidal rotation velocity.The propagation of this rotational velocity from the edge to the core of the torus was measured following the L- to H- and L- to I-mode transitions. A hyperbolic tangent fit was used to determine a single variable for rise time in rotational velocity. The hyperbolic tangent fit parameter acts as a proxy for the global momentum confinement time, which is then compared to other plasma parameters. Through this compaxison, we found an decrease in rise time in correlation with increasing density(n)/current(I), particularly distinctive in the range of 1-2.5 x 1020 (MA x m3)-1. Due to a lack of overlap in density between I- and H-mode, we find this may be indicative of an overall decrease in rise time between L- to I- and L- to H-mode transitions. In order to explore this possibility, we must achieve I-mode runs with the same current and density as Hmode to determine if there is still a decrease between the two transitions.
Download or read book Driven Rotation Self Generated Flow and Momentum Transport in Tokamak Plasmas written by John Rice and published by Springer Nature. This book was released on 2022-01-13 with total page 158 pages. Available in PDF, EPUB and Kindle. Book excerpt: This book provides a comprehensive look at the state of the art of externally driven and self-generated rotation as well as momentum transport in tokamak plasmas. In addition to recent developments, the book includes a review of rotation measurement techniques, measurements of directly and indirectly driven rotation, momentum sinks, self-generated flow, and momentum transport. These results are presented alongside summaries of prevailing theory and are compared to predictions, bringing together both experimental and theoretical perspectives for a broad look at the field. Both researchers and graduate students in the field of plasma physics will find this book to be a useful reference. Although there is an emphasis on tokamaks, a number of the concepts are also relevant to other configurations.
Download or read book Spontaneous Toroidal Rotation in Alcator C Mod Plasmas with No Momentum Input written by John E. Rice and published by . This book was released on 2005 with total page 102 pages. Available in PDF, EPUB and Kindle. Book excerpt: Spontaneoustoroidal rotation of impurity ions has been observed in the core of Alcator C-Mod plasmas with no external momentum input. The magnitude of the rotation rangesfrom -60 km/s(coun ter-current) in limiter L-mode discharges to +140 km/s (co-current) in ICRF heated H-mode plasmas. The core rotation in L-mode plasmasisgenerally counter-current and isfound to depend strongly on the magnetic topology; in near double null discharges, the core rotation changes by 25 km/swith a variation of a few millimetersin the distance between the primary and secondary separatrices. In H-mode plasmas, the rotation increments in the co-current direction with the toroidal rotation velocity increase proportional to the corresponding stored energy increase, normalized to the plasma current. These discharges exhibit a positive Er in the core. Immediately following the transition from L-mode into enhanced Da (EDA) H-mode, the co-current rotation appears near the plasma edge and propagates to the center on a time scale similar to the energy confinement time, but much less than the neo-classical momentum diffusion time, indicating both the role of the plasma boundary in the dynamics of the H-mode transition and the anomalous nature of momentum transport. Rotation velocity profilesare flat in EDA H-mode plasmasand centrally peaked for ELM-free Hmodes, demonstrating the effects of an inward momentum pinch. In EDA H-mode discharges that develop internal transport barriers (ITBs), the core toroidal rotation inside of the barrier foot is observed to drop on a time scale similar to the core pressure profile peaking (100s of ms), indicating a negative Er well in the core region.
Download or read book Observations of Anomalous Transport in Tokamak Plasmas with No Momentum Input written by William Davis Lee and published by . This book was released on 2003 with total page 16 pages. Available in PDF, EPUB and Kindle. Book excerpt: Anomalous momentum transport has been observed in Alcator C-Mod tokamak plasmas. The time evolution of core impurity toroidal rotation velocity profiles has been measured with a crystal x-ray spectrometer array. Following the L-mode to EDA (enhanced D[/sub/ alpha]) H-mode transition in both Ohmic and ICRF heated discharges, the ensuing co-current toroidal rotation velocity, which is generated in the absence of any external momentum source, is observed to propagate in from the edge plasma to the core with a time scale of order of the observed energy confinement time, but much less than the neo-classical momentum confinement time. The steady state toroidal rotation velocity profiles in EDA H-mode plasmas are relatively flat and the momentum transport can be simulated with a simple diffusion model. Assuming the L-H transition produces an instantaneous edge velocity source (which disappears at the H- to L-mode transition), the momentum transport may be characterized by a diffusivity, with values of 0.07 m2/s during EDA H-mode and 0.2 m2/s in L-mode. These values are large compared to the calculated neo-classical momentum diffusivities. Velocity profiles during ELM-free H-modes are centrally peaked, which suggests inward momentum convection; the observed profiles are matched with simulations including an inward convection velocity of 10 m/s. In EDA H-mode discharges which develop internal transport barriers, the velocity profiles become hollow in the center indicating the presence of a negative radial electric field well in the vicinity of the barrier foot.
Download or read book Transport driven Scrape off Layer Flows and the X point Dependence of the L H Power Threshold in Alcator C Mod written by Brian LaBombard and published by . This book was released on 2004 with total page 62 pages. Available in PDF, EPUB and Kindle. Book excerpt: Factor of ~2 higher power thresholds for low- to high-confinement mode transitions (LH) with unfavorable x-point topologies in Alcator C-Mod [Phys. Plasmas 1, 1511 (1994)] are linked to flow boundary conditions imposed by the scrape-off layer (SOL). Ballooning-like transport drives flow along magnetic field lines from low- to high-field regions with toroidal direction dependent on upper/lower x-point balance; the toroidal rotation of the confined plasma responds, exhibiting a strong counter-current rotation when Bx B points away from the x-point. Increased auxiliary heating power (rf, no momentum input) leads to an L-H transition at approximately twice the edge electron pressure gradient when Bx B points away. As gradients rise prior to the transition, toroidal rotation ramps toward the co-current direction; the H-mode is seen when the counter-current rotation imposed by the SOL flow becomes compensated. Remarkably, L-H thresholds in lower-limited discharges are identical to lower x-point discharges; SOL flows are also found similar, suggesting a connection.
Download or read book The Dependence of Core Rotation on Magnetic Configuration and the Relation to the H mode Power Threwhold in Alcator C Mod Plasmas with No Momentum Input written by John E. Rice and published by . This book was released on 2004 with total page 28 pages. Available in PDF, EPUB and Kindle. Book excerpt: The observed toroidal rotation in Alcator C-Mod Ohmic L-mode plasmas has been found to depend strongly on magnetic configuration. For standard discharges with a lower single null and the ion BxNB drift downward, and with BT = 5.4 T, IP = 0.8 MA and ne = 1.4 x 1020/m3, the core toroidal rotation is measured to be in the range of 10-20 km/s (counter-current). Similar plasmas with upper single null have significantly stronger counter-current rotation, in the range of 30-50 km/s. The rotation depends very sensitively on the distance between the primary and secondary separatrices in near double null plasmas, with changes of ~ 25 km/s occurring over a variation of a few millimeters in this distance. Application of ICRF power has been found to increase the rotation in the co-current direction. The transition to H-mode is seen to occur in these standard plasmas when the core rotation reaches a characteristic value, near 0 km/s, hence higher input power is needed to induce the transition in the upper single null configuration.
Download or read book Confinement and Transport Research in Alcator C Mod written by Martin Greenwald and published by . This book was released on 2005 with total page 174 pages. Available in PDF, EPUB and Kindle. Book excerpt: (cont.) This link unified L-mode and H-mode and established a strong connection between local and global transport. Further work on the role of critical gradient lengths and marginal stability lent quantitative support to the ITG theories for ion transport and have helped elucidate nonlinear saturation mechanisms for the turbulence. Local transport studies demonstrated connections between transport channels, with energy, particle and momentum transport varying across regimes in similar ways. Experiments carried out in collaboration with the DIII-D, ASDEX-U and JET groups confirmed the dimensionless scaling approach over the widest available range in machine sizes. These studies suggest that plasma physics is the dominant influence on transport in the core and pedestal for standard L- and H-mode discharges. Dimensionless scaling experiments have shown a strong improvement in confinement with the normalized gyro-size (1/p*). Confinement was found to be Bohm-like in L-mode and gyro-Bohm-like in H-mode. These experiments also showed a strong degradation in confinement with collisionality. Other articles in this issue discuss impurity transport, momentum transport, H-mode pedestal and threshold physics and internal transport barrier regimes.
Download or read book Observations of Anomalous Momentum Transport in Alcator C Mod Plasmas with No Momentum Input written by John E. Rice and published by . This book was released on 2003 with total page 34 pages. Available in PDF, EPUB and Kindle. Book excerpt: (Cont.) In EDA H-mode discharges which develop internal transport barriers, the velocity profiles become hollow in the center, indicating the presence of a negative radial electric field well in the vicinity of the barrier foot. Upper single null diverted and inner wall limited L-mode discharges exhibit strong counter-current rotation (with V[phi](0) [approx.] -60 km/s in some cases), which may be related to the observed higher H-mode power threshold in these configurations. For plasmas with locked modes, the toroidal rotation is observed to stop (V[phi]
Download or read book Study of the Effects of Neutrals in Alcator C Mod Plasmas written by and published by . This book was released on 1999 with total page 4 pages. Available in PDF, EPUB and Kindle. Book excerpt: Recently, much effort has been dedicated to understanding the bifurcation involved in the transition from a low to high confinement regime. While several theories have been brought forward, many factors remain to be elucidated, one of which involves the role played by neutral particles in the evolution of a transport barrier near the edge of the plasma. Alcator C-Mod is especially well suited for the study of neutral particle effects, mainly because of its high plasma and neutral densities, and closed divertor geometry. Alcator C-Mod employs ICRF as auxiiiary heating for obtaining a high confinement regime, although ohmic H-modes are routinely obtained as well. The neutrals can enter the edge dynamics through the particle, momentum and energy balance. In the particle balance, the source of neutrals has to be evaluated vis-8-vis the formation of the edge density pedestal. It is widely believed that plasma rotation is an important factor in reducing transport. In this case, neutrals could act as a momentum sink, through the charge-exchange process. That same process can also modify the energy balance of the plasma near the edge by increasing the cross-field heat flux. These effects are quite difficult to measure experimentally, in large part because neutral particle diagnosis is not an easy task, and because of the inherent 3-dimensional aspect of the problem. Consequently, the neutral s spatial and energy distributions are usually not well known. In Alcator C-Mod, we recently implemented a series of diagnostics for the purpose of measuring these distributions. They include measurements of the neutral pressure at many locations around the tokamak, and spatially resolved measurements of Lyman-a and charge-exchange power emission. A high-resolution multichord (20 channels) tangential view of neutral deuterium emission (Lyman-a) has been recently installed near the midplane. The viewing area covers approximately 4 cm across the separatrix, with a nominal 2 mm radial resolution. Standard Abel inversion techniques give the local Lyman-a emission. In conjunction with electron density and temperature, we can infer the local ionization rate and neutral density profiles. Two types of bolometer arrays (16 channels each), one being sensitive and the other insensitive to power carried by neutrals, view the plasma cross-section, tangentially at the midplane of the tokamak. By Abel inverting the difference between the 2 measurements, we infer the local power emissivity carried by neutrals[11].
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 Control of Internal Transport Barriers on Alcator C Mod written by Catherine L. Fiore and published by . This book was released on 2003 with total page 58 pages. Available in PDF, EPUB and Kindle. Book excerpt: Recent studies of internal transport and double transport barrier regimes in Alcator C-Mod [I.H. Hutchinson, et al., Phys. Plasmas 1, 1511 (1994)] have explored the limits for forming, maintaining, and controlling these plasmas. C-Mod provides a unique platform for studying such discharges: the ions and electrons are tightly coupled by collisions and the plasma has no internal particle or momentum sources. The double-barrier mode comprised of an edge barrier with an internal transport barrier (ITB) can be induced at will using off-axis ion cyclotron range of frequency (ICRF) injection on either the low or high field side of the plasma with either of the available ICRF frequencies (70 or 80 MHz). When enhanced D [alpha] high confinement mode (EDA H-mode) is accessed in Ohmic plasmas, the double barrier ITB forms spontaneously if the Hmode is sustained for [approx.] high confinement times. The ITBs formed in both Ohmic and ICRF heated plasmas are quite similar regardless of the trigger method. They are characterized by strong central peaking of the electron density, and reduction of the core particle and energy ...
Download or read book Plasma Confinement written by R. D. Hazeltine and published by Courier Corporation. This book was released on 2013-02-20 with total page 484 pages. Available in PDF, EPUB and Kindle. Book excerpt: Graduate-level text examines the essential physics underlying international research in magnetic confinement fusion with accounts of fundamental concepts behind methods of confining plasma at or near thermonuclear conditions. 1992 edition.
Download or read book Semiempirical Models of H mode Discharges written by and published by . This book was released on 1985 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The H-mode transition can lead to a rapid increase in tokamak plasma confinement. A semiempirical transport model was derived from global OH and L-mode confinement scalings and then applied to simulation of H-mode discharges. The radial diffusivities in the model also depend on local density and pressure gradients and satisfy an appropriate dimensional constraint. Examples are shown of the application of this and similar models to the detailed simulation of two discharges which exhibit an H-mode transition. The models reproduce essential features of plasma confinement in the ohmic heating, low and high confinement phases of these discharges. In particular, the evolution of plasma energy content through the H-mode transition can be reproduced without any sudden or ad hoc modification of the plasma transport formulation.
Download or read book Local Plasma Parameters and H mode Threshold in Alcator C mod written by Amanda E. Hubbard and published by . This book was released on 1996 with total page 6 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Internal Transport Barriers in Alcator C Mod written by Catherine L. Fiore and published by . This book was released on 2005 with total page 90 pages. Available in PDF, EPUB and Kindle. Book excerpt: Internal transport barriers, marked by steep density and pressure profiles and reduction of core transport are obtained in Alcator C-Mod. Transient single barriers are observed at the back transition from H- to L- mode, and also when pellet injection is accompanied by ICRF power. Double barriers are induced with injection of off-axis ICRF power deposition. These also arise spontaneously in Ohmic H-mode plasmas when the H-mode lasts for several energy confinement times. C-Mod provides a unique platform for studying such discharges: the ions and electrons are tightly coupled by collisions with Ti/Te=1, and the plasma has no internal particle or momentum sources. ITB plasmas with average pressure greater than 1 atmosphere have been obtained. To form an ITB, particle and thermal flux are reduced in the barrier region, allowing the neoclassical pinch to peak the density while maintaining the central temperature. Gyrokinetic simulation suggests that long wavelength drift wave turbulence in the core is marginally stable at the ITB onset, but steepening of the density profile destabilizes trapped electron (TEM) modes inside the barrier. The TEM ultimately drives sufficient outgoing particle flux to balance the inward pinch and halt further density rise, which allows control of particle and impurity peaking.
