230 research outputs found
Development of probes for assessment of ion heat transport and sheath heat flux in the boundary of the Alcator C-Mod Tokamak
No full textThesis: Ph. D., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2013.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis. Pages [228-229] blank.Includes bibliographical references.Progress towards a viable fusion reactor will require comprehensive understanding of boundary plasma physics. Knowledge in this area has been growing, yet there are critical gaps. Measurements of the sheath heat flux transmission coefficient-a fundamental physical quantity whose theoretical value is ~ 7-have varied from 2 to 20. Values below 5 are physically impossible and have challenged the understanding of this very basic theory. In addition, measurements of ion temperature are sparse and ion energy transport is poorly understood. To this end a set of new diagnostics, including a surface thermocouple, ion sensitive probe, and retarding field analyzer, have been developed that can tolerate the extreme heat fluxes in the Alcator C-Mod boundary plasma. These probes are used to asses issues of heat flux and ion energy transport. Systematic studies with these new tools reveal the following: A comparison of surface thermocouples and Langmuir probes confirms standard sheath heat flux theory in a tokamak for the first time. The measurement of unphysically low sheath heat flux transmission coefficients and an anomalous increase in measured divertor pressure by Langmuir probes, which is also unphysical, are found the be linked. Plasma-neutral simulations indicate that these artifacts are due to the Langmuir probe bias modifying the local plasma. Important space charge limits to measurements with ion sensitive probes are found experimentally and explored in depth with a 1D kinetic simulation. These results clarify the plasma conditions under which an ion sensitive probe may be used to measure ion temperature and/or plasma potential. The retarding field analyzer is demonstrated to be a viable ion temperature diagnostic up to the last closed flux surface in C-Mod. A ₁D fluid simulations is built to interpret edge ion heat transport. At high collisionality-where the fluid approximations are valid: the simulation reproduces the measured edge ion-to-electron temperature ratio (~ 2). However, at low collisionality-where fluid approximation is not valid-the simulation is not able to reproduce the experimental temperature ratio (~ 4). The addition of kinetic heat flux limiters can bring the simulated ratio into agreement with measurements. The value of heat flux limiter is found to be consistent with that expected from kinetic theory.by Daniel Frederic Brunner.Ph. D
Assessment of X-point target divertor configuration for power handling and detachment front control
No full textA study of long-legged tokamak divertor configurations is performed with the edge transport code UEDGE (Rognlien et al., J. Nucl. Mater. 196, 347, 1992). The model parameters are based on the ADX tokamak concept design (LaBombard et al., Nucl. Fusion 55, 053020, 2015). Several long-legged divertor configurations are considered, in particular the X-point target configuration proposed for ADX, and compared with a standard divertor. For otherwise identical conditions, a scan of the input power from the core plasma is performed. It is found that as the power is reduced to a threshold value, the plasma in the outer leg transitions to a fully detached state which defines the upper limit on the power for detached divertor operation. Reducing the power further results in the detachment front shifting upstream but remaining stable. At low power the detachment front eventually moves to the primary X-point, which is usually associated with degradation of the core plasma, and this defines the lower limit on the power for the detached divertor operation. For the studied parameters, the operation window for a detached divertor in the standard divertor configuration is very small, or even non-existent; under the same conditions for long-legged divertors the detached operation window is quite large, in particular for the X-point target configuration, allowing a factor of 5–10 variation in the input power. These modeling results point to possibility of stable fully detached divertor operation for a tokamak with extended divertor legs.United States. Department of Energy (Contract DE-AC52-07NA27344
Three-dimensional simulation of H-mode plasmas with localized divertor impurity injection on Alcator C-Mod using the edge transport code EMC3-EIRENE[superscript a)]
No full textExperiments in Alcator C-Mod to assess the level of toroidal asymmetry in divertor conditions resulting from poloidally and toroidally localized extrinsic impurity gas seeding show a weak toroidal peaking (∼1.1) in divertor electron temperatures for high-power enhanced D-alpha H-mode plasmas. This is in contrast to similar experiments in Ohmically heated L-mode plasmas, which showed a clear toroidal modulation in the divertor electron temperature. Modeling of these experiments using the 3D edge transport code EMC3-EIRENE [Y. Feng et al., J. Nucl. Mater. 241, 930 (1997)] qualitatively reproduces these trends, and indicates that the different response in the simulations is due to the ionization location of the injected nitrogen. Low electron temperatures in the private flux region (PFR) in L-mode result in a PFR plasma that is nearly transparent to neutral nitrogen, while in H-mode the impurities are ionized in close proximity to the injection location, with this latter case yielding a largely axisymmetric radiation pattern in the scrape-off-layer. The consequences for the ITER gas injection system are discussed. Quantitative agreement with the experiment is lacking in some areas, suggesting potential areas for improving the physics model in EMC3-EIRENE.United States. Department of Energy (Contract DE-AC05-00OR22725)United States. Department of Energy (Contract DE-FC02-99ER54512
Fast imaging of filaments in the X-point region of Alcator C-Mod
No full textA rich variety of field-aligned fluctuations has been revealed using fast imaging of Dαemission from Alcator C-Mod's lower X-point region. Field-aligned filamentary fluctuations are observed along the inner divertor leg, within the Private-Flux-Zone (PFZ), in the Scrape-Off Layer (SOL) outside the outer divertor leg, and, under some conditions, at or above the X-point. The locations and dynamics of the filaments in these regions are strikingly complex in C-Mod. Changes in the filaments’ generation appear to be ordered by plasma density and magnetic configuration. Filaments are not observed for plasmas with n/nGreenwald≲ 0.12 nor are they observed in Upper Single Null configurations. In a Lower Single Null with 0.12 ≲ n/nGreenwald ≲ 0.45 and Bx∇B directed down, filaments typically move up the inner divertor leg toward the X-point. Reversing the field direction results in the appearance of filaments outside of the outer divertor leg. With the divertor targets “detached”, filaments inside the LCFS are seen. These studies were motivated by observations of filaments in the X-point and PFZ regions in MAST, and comparisons with those observations are made. Keywords: Alcator C-Mod; Turbulence; Divertor; X-point; Filament
Constraining the divertor heat width in ITER
No full textA model is developed which constrains heat width, λr based on global power balance, momentum conservation, pedestal stability and sheath heat transmission. The model relies on measurements of the ratio of separatrix to pedestal pressure; a ratio ∼5% is found to be expected for ITER. Applying this model indicates a constraint that the allowed λr ∼ 10–30 mm for ITER if the divertor is in the high-recycling regime as expected (T 100 eV, a condition which would have very negative consequences for the divertor, but has never been observed experimentally.United States. Department of Defense (US DOE award DE-SC00-02060)United States. Department of Defense (contract DE-FC02-99ER54512)United States. Department of Defense (cooperative agreement DE-FC02- 04ER54698
Low-power techniques for video decoding
No full textThesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2009.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student submitted PDF version of thesis.Includes bibliographical references (p. 149-156).The H.264 video coding standard can deliver high compression efficiency at a cost of large complexity and power. The increasing popularity of video capture and playback on portable devices requires that the energy of the video processing be kept to a minimum. This work implements several architecture optimizations that reduce the system power of a high-definition video decoder. In order to decode high resolutions at low voltages and low frequencies, we employ techniques such as pipelining, unit parallelism, multiple cores, and multiple voltage/frequency domains. For example, a 3-core decoder can reduce the required clock frequency by 2.91 x, which enables a power reduction of 61% relative to a full-voltage single-core decoder. To reduce the total memory system power, several caching techniques are demonstrated that can dramatically reduce the off-chip memory bandwidth and power at the cost of increased chip area. A 123 kB data-forwarding cache can reduce the read bandwidth from external memory by 53%, which leads to 44% power savings in the memory reads. To demonstrate these low-power ideas, a H.264/AVC Baseline Level 3.2 decoder ASIC was fabricated in 65 nm CMOS and verified. It operates down to 0.7 V and has a measured power down to 1.8 mW when decoding a high definition 720p video at 30 frames per second, which is over an order of magnitude lower than previously published results.by Daniel Frederic Finchelstein.Ph.D
Heat-flux footprints for I-mode and EDA H-mode plasmas on Alcator C-Mod
No full textIR thermography is used to measure the heat flux footprints on C-Mod’s outer target in I-mode and EDA H-mode plasmas. The footprint profiles are fit to a function with a simple physical interpretation. The fit parameter that is sensitive to the power decay length into the SOL, λ[subscript SOL], is ~1–3× larger in I-modes than in H-modes at similar plasma current, which is the dominant dependence for the H-mode λ[subscript SOL]. In contrast, the fit parameter sensitive to transport into the private-flux-zone along the divertor leg is somewhat smaller in I-mode than in H-mode, but otherwise displays no obvious dependence on I[subscript p], B[subscript t], or stored energy. A third measure of the footprint width, the “integral width”, is not significantly different between H- and I-modes. Also discussed are significant differences in the global power flows of the H-modes with “favorable” ∇B drift direction and those of the I-modes with “unfavorable” ∇B drift direction.United States. Dept. of Energy (Cooperative Agreement DE-FC02-99-ER54512
Tungsten nano-tendril growth in the Alcator C-Mod divertor
No full textGrowth of tungsten nano-tendrils ('fuzz') has been observed for the first time in the divertor region of a high-power density tokamak experiment. After 14 consecutive helium L-mode discharges in Alcator C-Mod, the tip of a tungsten Langmuir probe at the outer strike point was fully covered with a layer of nano-tendrils. The thickness of the individual nano-tendrils (50–100 nm) and the depth of the layer (600 ± 150 nm) are consistent with observations from experiments on linear plasma devices. The observation of tungsten fuzz in a tokamak may have important implications for material erosion, dust formation, divertor lifetime and tokamak operations in next-step devices.National Science Foundation (U.S.) (Award DMR-08-19762)United States. Dept. of Energy (Award DE-SC00-02060)United States. Dept. of Energy (Contract DE-FC02-99ER54512
Investigation of RF-enhanced plasma potentials on Alcator C-Mod
No full textRadio frequency (RF) sheath rectification is a leading mechanism suspected of causing anomalously high erosion of plasma facing materials in RF-heated plasmas on Alcator C-Mod. An extensive experimental survey of the plasma potential (Φ[subscript P]) in RF-heated discharges on C-Mod reveals that significant Φ[subscript P] enhancement (>100 V) is found on outboard limiter surfaces, both mapped and not mapped to active RF antennas. Surfaces that magnetically map to active RF antennas show Φ[subscript P] enhancement that is, in part, consistent with the recently proposed slow wave rectification mechanism. Surfaces that do not map to active RF antennas also experience significant Φ[subscript P] enhancement, which strongly correlates with the local fast wave intensity. In this case, fast wave rectification is a leading candidate mechanism responsible for the observed enhancement.United States. Dept. of Energy (DE-FC02-99ER54512
The Federal Reserve's operating procedure, nonborrowed reserves, borrowed reserves and the liquidity effect
No full textRecently, there has been considerable interest in identifying the exogenous policy actions of the Fed and a number of identification methods have been proposed. This paper deals with one of these, namely, using nonborrowed reserves in a recursive structural vector autoregression(VAR). A number of researchers [Christiano, Eichenbaum and Evans (1994ab, 1996, 1997), Evans and Marshall(1997), Strongin(1995), Pagan and Robertson(1995) and Brunner(1994)] find evidence of a statistically significant liquidity effect using nonborrowed reserves in a VAR. The success in finding the liquidity effect with nonborrowed reserves in the VAR is attributed to innovations to nonborrowed reserves reflecting supply shocks while innovations to total reserves primarily reflect shocks to demand. The purpose of this paper is to demonstrate that the opposite is true. Evidence of the liquidity effect in recursive structural VARs depends critically on the existence of a negative covariance between the federal funds rate and nonborrowed reserves. Under a variety of operating objectives, the Trading Desk of the Federal Reserve Bank of New York has offset changes in bank-initiated discount window borrowing when implementing the Federal Open Market Committee's policy directive. This practice has created a negative contemporaneous covariance between nonborrowed reserves and the funds rate that has been incorrectly attributed to the liquidity effect. Once the Desk's practice is accounted for, there is no evidence of a statistically significant liquidity effect.Bank reserves ; Open market operations ; Liquidity (Economics)
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