Download or read book Observations of Toroidal Rotation Suppression with ITB Formation in ICRF and Ohmic H mode Alcator C Mod Plasmas written by John E. Rice and published by . This book was released on 2000 with total page 23 pages. Available in PDF, EPUB and Kindle. Book excerpt: Co-current central toroidal rotation has been observed in Alcator C-Mod plasmas with on-axis ICRF heating. The rotation velocity increases with plasma stored energy and decreases with plasma current. Very similar behavior has been seen during Ohmic H-modes, which suggests that the rotation, generated in the absence of an external momentum source, is not principally an ICRF effect. A scan of the ICRF resonance location across the plasma has been performed in order to investigate possible influences on the toroidal rotation. With a slight reduction of the toroidal magnetic field from 4.7 to 4.5 T and a corresponding shift of the ICRF resonance from r/a = -0.36 to -0.48, the central toroidal rotation significantly decreased in concert with the formation of an internal transport barrier (ITB). During the ITB period, the electrons and impurities peaked continuously for . Comparison of the observed rotation and neo-classical predictions indicates that the core radial electric field changes from positive to negative during the ITB phase. Similar rotation suppression and ITB formation have been observed during some Ohmic H-mode discharges.

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 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 Radio Frequency Power in Plasmas written by Tak K. Mau and published by American Inst. of Physics. This book was released on 2001-11-09 with total page 570 pages. Available in PDF, EPUB and Kindle. Book excerpt: This proceedings volume includes the most recent research advances in the application of RF power in plasmas, mainly in fusion science. Most papers deal with plasma heating and current drive, profile control, and stabilization using RF techniques, with some on RF sources and launchers. New and exciting results from all major tokamak experiments, and from theory and modeling are included. There is also a paper on RFplasma application in space propulsion by an active NASA astronaut. This book is a valuable reference for the RF researchers in fusion and other disciplines.

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 Nuclear Fusion written by and published by . This book was released on 2006 with total page 1102 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Observations of Central Toroidal Rotation in ICRF Heated Alcator C Mod Plasmas written by John E. Rice and published by . This book was released on 1997 with total page 30 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book X ray Observations of Central Toroidal Rotation in Ohmic Alcator C Mod Plasmas written by John E. Rice and published by . This book was released on 1996 with total page 19 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Toroidal Rotation and 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 30 pages. Available in PDF, EPUB and Kindle. Book excerpt: The time evolution of toroidal rotation velocity profiles has been measured in Alcator C-Mod [I.H. Hutchinson et al., Phys. Plasmas (1), 1511 (1994)] plasmas using a tangentially viewing x-ray spectrometer array. The strong co-current toroidal rotation in enhanced D[alpha] (EDA) high confinement mode (H-mode) plasmas is observed to propagate in from the edge on a time scale similar to the energy confinement time. The ensuing steady state rotation velocity profiles in both Ohmic and ion cyclotron range of frequencies (ICRF) heated EDA H-modes, which are generated in the absence of any external momentum input, are found to be relatively flat. These profiles may be simulated by a simple diffusion model with the boundary condition of an edge rotation, which appears during the H-mode period. The observed profiles are well matched by the simulations using a momentum diffusivity of [approx.]0.1 m2/s, which is much larger than the calculated neo-classical value, and the momentum transport may be regarded as anomalous. The Alcator C-Mod rotation observations have been compared in detail with the calculations of neo-classical and sub-neo-classical theory, to the predictions from modeling of ICRF wave induced energetic ion orbit shifts, and to estimates from turbulence driven mechanisms. The magnitude and scalings of the observed rotation results are in accord with neo-classical and sub-neo-classical calculations, but the measured momentum diffusivity is higher than the predictions by a large fraction. The prediction of rotation reversal with a high magnetic field side resonance location for ICRF wave induced ion orbit shifts has not been observed in the experiments. While the turbulence driven rotation calculations are mostly qualitative, they represent some of the observed features.

Download or read book Impurity Toroidal Rotation and Transport in Alcator C Mod Ohmic H Mode Plasmas written by John E. Rice and published by . This book was released on 1999 with total page 25 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 Investigation of Intrinsic Rotation Dependencies in Alcator C Mod Using a New Data Analysis Workflow written by Daniel Joowon Kwak and published by . This book was released on 2015 with total page 193 pages. Available in PDF, EPUB and Kindle. Book excerpt: Toroidal rotation, important for suppressing various turbulent modes, mitigating MHD instabilities, and preventing locked modes that cause disruptions, may not be sufficiently generated by external devices in larger devices i.e. ITER. One possible solution is intrinsic rotation, self-generated flow without external momentum input, which has been observed in multiple tokamaks. More specifically, rotation reversals, a sudden change in direction of intrinsic rotation without significant change in global plasma parameters, have also been observed and are not yet fully understood. Studying this phenomenon in ohmic L-mode plasmas presents a rich opportunity to gain better understanding of intrinsic rotation and of momentum transport as a whole. The literature presents many different hypotheses, and this thesis explores three in particular. The first two hypotheses each posits a unique parameter as the primary dependency of reversals - the dominant turbulent mode, or the fastest growing turbulent mode(TEM/ITG), and the local density and temperature profile gradients, especially the electron density gradient, respectively. Other studies state that neoclassical effects cause the reversals and one study in particular presents a 1-D analytical model. Utilizing a new data analysis workflow built around GYRO, a gyrokinetic-Maxwell solver, hundreds of intrinsic rotation shots at Alcator C-Mod can be processed and analyzed without constant user management, which is used to test the three hypotheses. By comparing the rotation gradient u', a proxy variable indicative of the core toroidal intrinsic rotation velocity, to the parameters identified by the hypotheses, little correlation has been found between u' and the dominant turbulence regime and the ion temperature, electron temperature, and electron density profile gradients. The plasma remains ITG-dominated based on linear stability analysis regardless of rotation direction and the local profile gradients are not statistically significant in predicting the u'. Additionally, the experimental results in C-Mod and ASDEX Upgrade have shown strong disagreement with the 1 -D neoclassical model. Strong correlation has been found between u' and the effective collisionality Veff. These findings are inconsistent with previous experimental studies and suggest that further work is required to identify other key dependencies and/or uncover the complex physics and mechanisms at play.

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.

Download or read book Central Impurity Toroidal Rotation in ICRF Heated Alcator C Mod Plasmas written by John E. Rice and published by . This book was released on 1999 with total page 36 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Observation of Central Toroidal Rotation Induced by ICRF on EAST supported by the National Magnetic Confinement Fusion Science Program of China Nos 2013GB112004 and 2015GB103002 National Natural Science Foundation of China Nos 11175208 11305212 11375235 11405212 and 11261140328 the Innovative Program of Development Foundation of Hefei Center for Physical Science and Technology 2014FXCX003 and Brain Korea 21 Program for Leading Universities Students BK21 PLUS written by and published by . This book was released on 2016 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Abstract: Core plasma rotation of both L-mode and H-mode discharges with ion cyclotron range of frequency (ICRF) minority heating (MH) scheme was measured with a tangential X-ray imaging crystal spectrometer on EAST (Experimental Advanced Superconducting Tokamak). Co-current central impurity toroidal rotation change was observed in ICRF-heated L- and H-mode plasmas. Rotation increment as high as 30 km/s was generated at ∼1.7 MW ICRF power. Scaling results showed similar trend as the Rice scaling but with significant scattering, especially in L-mode plasmas. We varied the plasma current, toroidal field and magnetic configuration individually to study their effect on L-mode plasma rotation, while keeping the other major plasma parameters and heating unchanged during the scanning. It was found that larger plasma current could induce plasma rotation more efficiently. A scan of the toroidal magnetic field indicated that the largest rotation was obtained for on-axis ICRF heating. A comparison between lower-single-null (LSN) and double-null (DN) configurations showed that LSN discharges rendered a larger rotation change for the same power input and plasma parameters.

Download or read book Toroidal Rotation as an Experimentation for Plasma Flow Observations in the Alcator C Mod Scrape off Layer written by Brian LaBombard and published by . This book was released on 2002 with total page 20 pages. Available in PDF, EPUB and Kindle. Book excerpt: Parallel and ExB plasma flows near the separatrix on the outside midplane of Alcator C-Mod are investigated with a scanning probe for a range plasma densities, currents, and magnetic fields. Strong parallel flows (up to Mach 0.6) are found to peak 2 mm into the scrape-off layer (SOL), reverse nearly symmetrically with magnetic field reversal, and decrease in magnitude with increasing line-averaged density normalized to the Greenwald density. ExB flows in the SOL inferred from the poloidal propagation velocity of plasma fluctuations appear to compensate these parallel flows and scale similarly, i.e., the dominant flow pattern is a pure toroidal rotation. ExB flows inferred by probe-sheath potentials are generally smaller, exhibit more scatter, and do not scale the same, perhaps indicating a less reliable measurement. These measurements suggest a residual poloidal flow (along field lines) of 0.2 to 1 times the electron diamagnetic velocity, depending on plasma conditions.

Download or read book Characterization of Core and Edge Turbulence in L and Enhanced D alpha H mode Alcator C Mod Plasmas written by Nils Plesner Basse and published by . This book was released on 2005 with total page 96 pages. Available in PDF, EPUB and Kindle. Book excerpt: The recently upgraded phase-contrast imaging (PCI) diagnostic is used to characterize the transition from the low (L) to the enhanced D[alpha] (EDA) high (H) confinement mode in Alcator C-Mod [I. H. Hutchinson, et al., Phys. Plasmas, 1511 (1994)] plasmas. PCI yields information on line integrated density fluctuations along vertical chords. The number of channels has been increased from 12 to 32, the sampling rate from 1 MHz to 10 MHz. This expansion of diagnostic capabilities is used to study broadband turbulence in L- and EDA H-mode and to analyze the quasi-coherent (QC) mode associated with EDA H-mode. Changes in broadband turbulence at the transition from L- to EDA H-mode can be interpreted as an effect of the Doppler rotation of the bulk plasma. Additional fluctuation measurements of D[alpha]-light and the poloidal magnetic field show features correlated with PCI in two different frequency ranges at the transition. The back transition from EDA H- to L-mode, the so-called enhanced neutron (EN) mode, is investigated by new high frequency (132 and 140 GHz) reflectometer channels operating in the ordinary (O) mode. This additional hardware has been installed in an effort to study localized turbulence associated with internal transport barriers (ITBs). The EN-mode is a suitable candidate for this study, since an ITB exists transiently as the outer density decreases much faster than the core density in this mode. The fact that the density decays from the outside inward allows us to study fluctuations progressing towards the plasma core. Our results mark the first localized observation of the QC-mode at medium density: 2.2 x 1020 m-3 (132 GHz). Correlating the reflectometry measurements with other fluctuating quantities provides some insight regarding the causality of the EN-mode development.