Toroidal Rotation as an Experimentation for Plasma Flow Observations in the Alcator C-Mod Scrape-off Layer PDF Download

Author: Brian LaBombard
Publisher:
ISBN:
Category :
Languages : en
Pages : 20

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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.

Author: Noah M. Smick
Publisher:
ISBN:
Category :
Languages : en
Pages : 234

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(Cont.) Toroidal rotation, Pfirsch-Schlilter and transport-driven contributions are unambiguously identified. Parallel flows are found to dominate the high-field particle fluxes; the total poloidally-directed flow carries one half of the particle flux arriving on the inner divertor. As a result, convection is also found to be an important player in high-field side heat transport. In contrast, E_r x B plus parallel flows yield a mostly-toroidal flow component in the low-field SOL. The magnitude of the transport-driven flow component is found to be quantitatively consistent with radial fluctuation-induced particle fluxes measured on the low-field side, identifying this as the primary driver. In contrast, fluctuation-induced flux measurements on the high-field side midplane are found to be essentially zero, thereby excluding an 'inward pinch' effect as the mechanism that closes the mass-flow loop in this region.

Author:
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Category : Aeronautics
Languages : en
Pages : 704

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Author: John E. Rice
Publisher:
ISBN:
Category :
Languages : en
Pages : 102

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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.

Author: MITSURU KIKUCHI
Publisher: International Atomic Energy
ISBN:
Category : Antiques & Collectibles
Languages : en
Pages : 1158

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Humans do not live by bread alone. Physically we are puny creatures with limited prowess, but with unlimited dreams. We see a mountain and want to move it to carve out a path for ourselves. We see a river and want to tame it so that it irrigates our fields. We see a star and want to fly to its planets to secure a future for our progeny. For all this, we need a genie who will do our bidding at a flip of our fingers. Energy is such a genie. Modern humans need energy and lots of it to live a life of comfort. In fact, the quality of life in different regions of the world can be directly correlated with the per capita use of energy [1.1–1.5]. In this regard, the human development index (HDI) of various countries based on various reports by the United Nations Development Programme (UNDP) [1.6] (Fig. 1.1), which is a parameter measuring the quality of life in a given part of the world, is directly determined by the amount of per capita electricity consumption. Most of the developing world (~5 billion people) is crawling up the UN curve of HDI versus per capita electricity consumption, from abysmally low values of today towards the average of the whole world and eventually towards the average of the developed world. This translates into a massive energy hunger for the globe as a whole. It has been estimated that by the year 2050, the global electricity demand will go up by a factor of up to 3 in a high growth scenario [1.7–1.9]. The requirements beyond 2050 go up even higher.

Author: John E. Rice
Publisher:
ISBN:
Category :
Languages : en
Pages : 30

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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.

Author:
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ISBN:
Category :
Languages : en
Pages :

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A theoretical model is developed to explain the plasma rotations induced by lower hybrid waves in Alcator C-Mod. In this model, torodial rotations are driven by the Lorentz force on the bulk electron flow across flux surfaces, which is a response of the plasma to the resonant-electron flow across flux surfaces induced by the lower hybrid waves. The flow across flux surfaces of the resonant electrons and the bulk electrons are coupled through the radial electric fi eld initiated by the resonant electrons, and the friction between ions and electrons transfers the toroidal momentum to ions from electrons. An improved quasilinear theory with gyrophase dependent distribution function is developed to calculate the perpendicular resonant-electron flow. Toroidal rotations are determined using a set of fluid equations for bulk electrons and ions, which are solved numerically by a fi nite- difference method. Numerical results agree well with the experimental observations in terms of flow pro file and amplitude. The model explains the strong correlation between torodial flow and internal inductance observed experimentally, and predicts both counter-current and co-current flows, depending on the perpendicular wave vectors of the lower hybrid waves. __________________________________________________

Author: Daniel Joowon Kwak
Publisher:
ISBN:
Category :
Languages : en
Pages : 193

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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.

Author: Roman Igorevitch Ochoukov
Publisher:
ISBN:
Category :
Languages : en
Pages : 187

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ICRF-heated discharges on Alcator C-Mod are associated with enhanced sputtering of molybdenum plasma facing surfaces and increased levels of core impurity contents, which subsequently degrade the core plasma performance. RF sheath rectification on open magnetic field lines that intercept material surfaces is currently suspected of causing an enhancement of molybdenum impurity sources by increasing the energy with which incident plasma ions strike material surfaces. While it has previously been observed that plasma potentials on open magnetic field lines are enhanced in ICRF-heated discharges on Alcator C-Mod, a direct link between local RF wave fields and plasma potentials has yet to be established. Experimental measurements reveal that regions that directly magnetically map and do not map to the active antennas experience plasma potential enhancement. The "mapped" results are consistent with the slow wave rectification mechanism where the plasma potential enhancement is a result of rectification of the slow ICRF wave electric field launched directly by the antenna. This rectification mechanism is localized to regions directly magnetically mapped to the active antennas and occurs over a narrow plasma density range where the slow waves can propagate. The potential enhancement in the "unmapped" regions (inaccessible to directly launched slow waves) correlates well with the local fast wave fields and has multiple features that are consistent with the theory that involves fast waves coupling to a slow wave at a conducting surface, which then leads to rectification of the plasma potential. Cross field profile measurements reveal that the plasma density profile is also affected by ICRF power and it is suspected that the gradients in the plasma potential profile are responsible for the density profile changes through E x B plasma flows along equipotential surfaces. The implications are that the absolute plasma potentials and the plasma potential gradients are capable of affecting molybdenum sputtering and sources by modifying the sputtering yield and the incident ion flux, respectively.

Author: Sanjay Gangadhara
Publisher:
ISBN:
Category :
Languages : en
Pages : 20

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Accurate measurements of plasma flows in the scrape-off layer (SOL) are a necessary requirement for understanding the physics of tokamak edge plasmas. A system is being developed on Alcator C-Mod for inferring flows parallel (v) and perpendicular (vE x B) to local magnetic field lines from impurity emission patterns ("plumes") generated by local gas injection. Carbon plumes are generated at variable location in the SOL by puffing deuterated ethylene gas (C2D4) through the end of a reciprocating fast-scanning probe. Two intensified CCD cameras are used to record C+1 and C+2 emission patterns simultaneously from near-perpendicular views. Plumes are modeled using a Monte Carlo impurity transport code, from which values for the background flows may be extracted. The sensitivity of the plume structure is investigated for a number of code inputs, including radial electric field (Er) and the neutral launch dynamics. Initial modeling results indicate discrepancies between values of v and Er extracted from the plumes and measurements obtained from probe data. Key words: Alcator C-Mod; Scrape-Off Layer Flows; Plumes.