Plasma Flows In The Alcator C Mod Scrape Off Layer
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| Author | : Noah M. Smick |
| Publisher | : |
| Total Pages | : 28 |
| Release | : 2004 |
| Genre | : |
| ISBN | : |
A novel, magnetically-driven swing probe was recently installed near the midplane on the high-field side SOL in Alcator C-Mod. The probe collects plasma from co- and counter-current directions during its respective 0-90 and 90-180 degrees of motion, thus providing profiles of density, electron temperature and plasma flow parallel to magnetic field lines (Mach number ... ) up to the separatrix. Results are reported from discharges with different magnetic topologies: lower single-null, upper single-null, and double-null. In single-null, a strong parallel flow ( ... M ...~ 1) is detected, which is always directed from the low- to high-field SOL. In double-null discharges, e-folding lengths in the high-field SOL are a factor of ~4 shorter than the low-field SOL. Thus, plasma appears to 'fill-in' the high-field SOL in single-null plasmas, not by cross-field transport but by parallel flow from the low-field SOL -- a picture consistent with a very strong ballooning-like component to the cross-field transport.
| Author | : James L. Terry |
| Publisher | : |
| Total Pages | : 68 |
| Release | : 2005 |
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(cont.) Evidence that this turbulent transport may play an important role in the core-plasma density limit is presented. Much lower levels of turbulence and no blobs are observed in the high-field-side scrape-off-layer. For single-null magnetic configurations, plasma in the inboard scrape-off-layer appears to be almost entirely a result of plasma flow along field lines from the low-field side. Strong parallel flows with sensitivity to magnetic topology are found, along with strong evidence for momentum coupling between these scrape-off-layer flows and core toroidal rotation
| Author | : Noah M. Smick |
| Publisher | : |
| Total Pages | : 234 |
| Release | : 2009 |
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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 | : Sanjay Gangadhara |
| Publisher | : |
| Total Pages | : 20 |
| Release | : 2002 |
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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.
| Author | : Brian LaBombard |
| Publisher | : |
| Total Pages | : 20 |
| Release | : 2002 |
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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 | : Sanjay Gangadhara |
| Publisher | : |
| Total Pages | : 86 |
| Release | : 2003 |
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| ISBN | : |
The physics of impurity transport in response to a local gas injection in the scrape-off-layer (SOL) of Alcator C-Mod is investigated. Carbon "plumes'' are formed at variable locations in the SOL -- up to the separatrix -- by puffing deuterated ethylene gas (C2D4) through the end of a reciprocating fast-scanning probe. CCD cameras are used to simultaneously record C+1 and C+2 emission patterns from two near-perpendicular views. The plume dispersal patterns are found to yield direct qualitative information about plasma flow, including the direction of VExB near the separatrix. Impurity transport and plasma-surface interaction physics implicit in the 3-D plume structure is explored in detail using a Monte Carlo impurity transport code, with the aim of extracting background plasma-flow quantities. A number of important local effects involving plasma-probe interaction have been identified: a vertical ExB drift near the probe surface, a parallel electric field above the probe tip arising from plasma recycling off the probe surface, and sputtering of a carbon layer that dynamically forms on the probe surface. The emission patterns are also found to yield important information on flows in the SOL: radial electric field (Er) in the near SOL and volume-averaged values of the parallel Mach number in the far SOL. Er values obtained from plume data compare favorably with estimates of Er based on the poloidal propagation velocity of edge plasma fluctuations measured by the scanning probe. Comparisons between parallel Mach numbers obtained from the plume data and probe measurements indicate that the probe over-estimates the parallel flow towards the divertor in the far SOL. This result supports the picture of particle balance in the SOL of Alcator C-Mod being dominated by main-chamber recycling, with weak plasma flow into the divertor.
| Author | : |
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| Total Pages | : |
| Release | : 2008 |
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Recent interest in the experimental study of tokamak plasma flow for different magnetic field geometries calls for theoretical understanding of the effects of tokamak magnetic topology changes on the flow. The consequences of total magnetic field reversal and/or X-point reversal on divergence-free plasma flow within magnetic flux surfaces are considered and the results are applied to interpret recent Alcator C-Mod scrape-off layer flow measurements. In his comment to that work, Aydemir asserted that poloidal plasma flow reversal is not a valid response to toroidal magnetic field reversal in an up-down symmetric tokamak, and that the toroidal plasma flow must reverse instead. We show that this assertion is wrong due to his misunderstanding of the corresponding symmetry transformation.
| Author | : Brian LaBombard |
| Publisher | : |
| Total Pages | : 62 |
| Release | : 2004 |
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| ISBN | : |
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.
| Author | : Roman Igorevitch Ochoukov |
| Publisher | : |
| Total Pages | : 187 |
| Release | : 2013 |
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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 | : Andreas Dinklage |
| Publisher | : Springer Science & Business Media |
| Total Pages | : 524 |
| Release | : 2005-06-09 |
| Genre | : Science |
| ISBN | : 9783540252740 |
Plasma Physics: Confinement, Transport and Collective Effects provides an overview of modern plasma research with special focus on confinement and related issues. Beginning with a broad introduction, the book leads graduate students and researchers – also those from related fields - to an understanding of the state-of-the-art in modern plasma physics. Furthermore, it presents a methodological cross section ranging from plasma applications and plasma diagnostics to numerical simulations, the latter providing an increasingly important link between theory and experiment. Effective references guide the reader from introductory texts through to contemporary research. Some related exercises in computational plasma physics are supplied on a special web site
