Investigation of Plasma Potential Enhancement in the Scrape-off Layer of Ion Cyclotron Range of Frequencies Heated Discharges on Alcator C-Mod PDF Download
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Author: Roman Igorevitch Ochoukov
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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: Roman Igorevitch Ochoukov
Publisher:
ISBN:
Category :
Languages : en
Pages : 187
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Book Description
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:
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ISBN:
Category : Aeronautics
Languages : en
Pages : 704
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Author: D. K. Smith
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Category :
Languages : en
Pages : 20
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Author: Alexandre Parisot
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Category :
Languages : en
Pages : 26
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Rapid changes in the loading resistance of fast wave antennas can limit high power operations of heating systems in the Ion Cyclotron Range of Frequencies (ICRF). Although novel matching techniques are being developed to reduce their effects, understanding the physics involved in these variations is of interest to guide and facilitate the design effort. We have studied the dependence of the loading resistance upon plasma parameters for the three ICRF antennas in the Alcator C-Mod tokamak. In contrast with similar studies in JET and Tore Supra, the evanescent decay term was not found to play an important role. The dominant variations could be related to changes in the shape of the electron density profiles in the propagating region. In H-mode, the loading resistance decreases as the density at the top of the pedestal is increased, and increases for higher Scrape-Off Layer densities. This dependency on global plasma parameters is generally identical for the three antennas, up to a proportionality constant, while local changes in front of an individual antennas could explain the residual discrepancy. To link the observations with theory, the surface impedance at the Faraday shield was calculated by solving the wave equation in a slab geometry using experimental radial density profiles. This approach leads to a good agreement with measurements over a wide range of operating conditions in L-mode, ELM-free and EDA H-mode plasmas, and it can be interpreted qualitatively in terms of impedance transformation in the scrape-off layer region. Implications for more complex modeling approaches are also discussed.
Author:
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ISBN:
Category : Physics
Languages : en
Pages : 968
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Author: Yijun Lin
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ISBN:
Category :
Languages : en
Pages : 52
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Localized direct electron heating by mode-converted ion cyclotron range of frequencies (ICRF) waves in D(H) tokamak plasmas has been clearly observed for the first time in Alcator C-Mod. Both on- and off-axis (high field side) mode conversion electron heating (MCEH) have been observed. The MCEH profile was obtained from a break in slope analysis of electron temperature signals in the presence of rf (radio frequency) shut-off. The temperature was measured by a 32-channel high spatial resolution (7 mm) 2nd harmonic heterodyne electron cyclotron emission (ECE) system. The experimental profiles were compared with the predictions from a toroidal full-wave ICRF code TORIC. Using the hydrogen concentration measured by a high-resolution optical spectrometer, TORIC predictions were shown qualitatively in agreement with the experimental results for both on- and off-axis MC cases. From the simulations, the electron heating from mode converted ion cyclotron wave (ICW) and ion Bernstein wave (IBW) is examined.
Author: Noah M. Smick
Publisher:
ISBN:
Category :
Languages : en
Pages : 28
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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.
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Category :
Languages : en
Pages :
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This paper presents a characterization of the Alcator C Scrape-Off Layer (SOL) plasma during ICRF hydrogen-minority fast wave heating experiments. The SOL plasma parameters were measured using a multifunctional probe, JANUS, which is capable of simultaneously measuring the ion and electron parameters both parallel and antiparallel with respect to the toroidal magnetic field. The probe data indicate, at low value of injected rf power, there is direct edge heating and density increases at radii greater than that of the antenna Faraday shield. Increasing the injected rf power spreads both the temperature and density increases throughout the edge region, flattening the radial profiles. Varying the position of the resonance layer in the main plasma does not significantly change the effect of ICRF on the SOL parameters. Given this single spatial point characterization of the SOL, a crude estimate of power flow into and through the edge plasma indicate that (approximately)20% of the ICRF power launched from the antenna is absorbed /und directly/ in the SOL plasma. Additional observation of the impurity source rates confirms the conclusions of an earlier paper, which attributed increasing central densities of high-Z impurities to the increase in physical sputtering rate at both the ICRF antenna's Faraday shield and the limiter surface.
Author: Stephen J. Wukitch
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ISBN:
Category :
Languages : en
Pages : 42
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Double transport barrier modes (simultaneous core and edge transport barrier) have been observed with off-axis ion cyclotron range of frequencies (ICRF) heating in the Alcator C-Mod tokamak [I.H. Hutchinson et al., Phys. Plasmas 1, 1511(1994)]. An internal transport barrier (ITB) is routinely produced in enhanced D[alpha] H-mode (EDA) discharges where the minority ion cyclotron resonance layer is at r/a (0.5) during the current flat top phase of the discharge. The density profile becomes peaked without the presence of a particle source in the plasma core and continues to peak until the increased core impurity radiation arrests the improved energy confinement, ultimately leading to a barrier collapse. With the addition of moderate (0.6 MW) central ICRF heating, the double barrier mode was maintained for as long as the ICRF power was applied and modeling shows that the internal thermal barrier was maintained throughout the discharge. The presence of sawteeth throughout most of the ITB discharge allows sawtooth induced heat pulse analysis to be performed. This analysis indicates that there is an abrupt radial discontinuity in the heat pulse time to peak profile when an ITB is present. Furthermore, this discontinuity appears to move into the core plasma from the edge region in about 0.2 sec, several confinement times. The deduced thermal diffusivity, Xhp indicates a barrier exists in the electron thermal transport, the barrier is limited to a narrow radial region, and the transport is unaffected outside this narrow radial extent.
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.
