4 results on '"Rice, John E."'
Search Results
2. Improved profile fitting and quantification of uncertainty in experimental measurements of impurity transport coefficients using Gaussian process regression
- Author
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Chilenski, Mark Alan, Greenwald, Martin J., Marzouk, Youssef M., Howard, Nathaniel Thomas, White, Anne E., Rice, John E., Walk, John R., Jr., Walk Jr., John R., Massachusetts Institute of Technology. Department of Aeronautics and Astronautics, Massachusetts Institute of Technology. Department of Nuclear Science and Engineering, Massachusetts Institute of Technology. Plasma Science and Fusion Center, Chilenski, Mark Alan, Greenwald, Martin J., Marzouk, Youssef M., Howard, Nathaniel Thomas, White, Anne E., Rice, John E., and Walk Jr., John R.
- Subjects
Nuclear and High Energy Physics ,Propagation of uncertainty ,Quality (physics) ,Rate of convergence ,Kriging ,Computer science ,Ground-penetrating radar ,Context (language use) ,Statistical physics ,Sensitivity (control systems) ,Condensed Matter Physics ,Regression - Abstract
The need to fit smooth temperature and density profiles to discrete observations is ubiquitous in plasma physics, but the prevailing techniques for this have many shortcomings that cast doubt on the statistical validity of the results. This issue is amplified in the context of validation of gyrokinetic transport models (Holland et al 2009 Phys. Plasmas 16 052301), where the strong sensitivity of the code outputs to input gradients means that inadequacies in the profile fitting technique can easily lead to an incorrect assessment of the degree of agreement with experimental measurements. In order to rectify the shortcomings of standard approaches to profile fitting, we have applied Gaussian process regression (GPR), a powerful non-parametric regression technique, to analyse an Alcator C-Mod L-mode discharge used for past gyrokinetic validation work (Howard et al 2012 Nucl. Fusion 52 063002). We show that the GPR techniques can reproduce the previous results while delivering more statistically rigorous fits and uncertainty estimates for both the value and the gradient of plasma profiles with an improved level of automation. We also discuss how the use of GPR can allow for dramatic increases in the rate of convergence of uncertainty propagation for any code that takes experimental profiles as inputs. The new GPR techniques for profile fitting and uncertainty propagation are quite useful and general, and we describe the steps to implementation in detail in this paper. These techniques have the potential to substantially improve the quality of uncertainty estimates on profile fits and the rate of convergence of uncertainty propagation, making them of great interest for wider use in fusion experiments and modelling efforts., United States. Dept. of Energy. Office of Fusion Energy Sciences (Award DE-FC02-99ER54512), United States. Dept. of Energy. Office of Science (Contract DE-AC05-06OR23177), United States. Dept. of Energy. Office of Advanced Scientific Computing Research (Award DE-SC0007099)
- Published
- 2015
3. Non-local heat transport in Alcator C-Mod ohmic L-mode plasmas
- Author
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Gao, Chi, Rice, John E., Sun, H. J., Reinke, Matthew Logan, Howard, Nathaniel Thomas, Mikkelson, D., Hubbard, Amanda E., Chilenski, Mark Alan, Walk Jr, John R., Hughes, Jerry W., Jr., Ennever, Paul Chappell, Porkolab, Miklos, White, Anne E., Sung, Choongki, Delgado-Aparicio, Luis, Baek, Seung Gyou, Rowan, William L., Brookman, M. W., Greenwald, Martin J., Granetz, Robert S., Wolfe, Stephen M., Marmar, Earl S., Alcator C-Mod Team, Massachusetts Institute of Technology. Department of Nuclear Science and Engineering, Massachusetts Institute of Technology. Department of Physics, Massachusetts Institute of Technology. Plasma Science and Fusion Center, Gao, Chi, Rice, John E., Reinke, Matthew Logan, Howard, Nathaniel Thomas, Hubbard, Amanda E., Chilenski, Mark Alan, Walk Jr, John R., Hughes, Jerry W., Jr., Ennever, Paul Chappell, Porkolab, Miklos, White, Anne E., Sung, Choongki, Delgado-Aparicio, Luis, Baek, Seung Gyou, Rowan, William L., Greenwald, Martin J., Granetz, Robert S., Wolfe, Stephen M., and Marmar, Earl S.
- Subjects
Nuclear and High Energy Physics ,Materials science ,Alcator C-Mod ,Physics::Plasma Physics ,Pinch ,Electron temperature ,Plasma ,Electron ,Collisionality ,Atomic physics ,Condensed Matter Physics ,Thermal diffusivity ,Ohmic contact - Abstract
Non-local heat transport experiments were performed in Alcator C-Mod ohmic L-mode plasmas by inducing edge cooling with laser blow-off impurity (CaF2) injection. The non-local effect, a cooling of the edge electron temperature with a rapid rise of the central electron temperature, which contradicts the assumption of 'local' transport, was observed in low collisionality linear ohmic confinement (LOC) regime plasmas. Transport analysis shows this phenomenon can be explained either by a fast drop of the core diffusivity, or the sudden appearance of a heat pinch. In high collisionality saturated ohmic confinement (SOC) regime plasmas, the thermal transport becomes 'local': the central electron temperature drops on the energy confinement time scale in response to the edge cooling. Measurements from a high resolution imaging x-ray spectrometer show that the ion temperature has a similar behaviour as the electron temperature in response to edge cooling, and that the transition density of non-locality correlates with the rotation reversal critical density. This connection may indicate the possible connection between thermal and momentum transport, which is also linked to a transition in turbulence dominance between trapped electron modes (TEMs) and ion temperature gradient (ITG) modes. Experiments with repetitive cold pulses in one discharge were also performed to allow Fourier analysis and to provide details of cold front propagation. These modulation experiments showed in LOC plasmas that the electron thermal transport is not purely diffusive, while in SOC the electron thermal transport is more diffusive like. Linear gyrokinetic simulations suggest the turbulence outside r/a = 0.75 changes from TEM dominance in LOC plasmas to ITG mode dominance in SOC plasmas., United States. Dept. of Energy (DoE Contract No DE-FC02-99ER54512), Oak Ridge Institute for Science and Education (DOE Fusion Energy Postdoctoral Research Program)
- Published
- 2014
4. Non-neoclassical up/down asymmetry of impurity emission on Alcator C-Mod
- Author
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Matthew Reinke, Nathaniel Thomas Howard, J.L. Terry, James Irby, John Rice, Anne White, Martin Greenwald, Yuri Podpaly, Jerry Hughes, Ian H. Hutchinson, Massachusetts Institute of Technology. Department of Nuclear Science and Engineering, Massachusetts Institute of Technology. Plasma Science and Fusion Center, Hutchinson, Ian, Reinke, Matthew Logan, Rice, John E., Hutchinson, Ian H., Greenwald, Martin J., Howard, Nathaniel Thomas, Hughes, Jerry W., Irby, James Henderson, Podpaly, Yuri, Terry, James L., and White, Anne E.
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Physics ,Nuclear and High Energy Physics ,Toroid ,Condensed matter physics ,media_common.quotation_subject ,Tore Supra ,Condensed Matter Physics ,Computer Science::Digital Libraries ,Asymmetry ,Alcator C-Mod ,Physics::Plasma Physics ,Impurity ,Atomic physics ,Ohmic contact ,Scaling ,Computer Science::Databases ,Dimensionless quantity ,media_common - Abstract
We demonstrate that existing theories are insufficient to explain up/down asymmetries of argon x-ray emission in Alcator C-Mod ohmic plasmas. Instead of the poloidal variation, ñ[subscript z]/〈n[subscript z]〉, being of order the inverse aspect ratio, ϵ, and scaling linearly with B[subscript t][superscript _ over n][subscript e]/I[2 over p], it is observed over 0.8 < r/a < 1.0 to be of order unity and exhibits a threshold behaviour between 3.5, United States. Dept. of Energy (Contract DE-FC02-99ER54512), United States. Dept. of Energy (Fusion Research Postdoctoral Research Program)
- Published
- 2013
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