Number of results to display per page
Search Results
192. Laboratory study of low-beta forces in arched, line-tied magnetic flux ropes
- Author(s):
- Myers, Clayton; Yamada, Masaaki; Ji, Hantao; Yoo, Jongsoo; Jara-Almonte, Jonathan; Fox, William
- Abstract:
- The loss-of-equilibrium is a solar eruption mechanism whereby a sudden breakdown of the magnetohydrodynamic force balance in the Sun's corona ejects a massive burst of particles and energy into the heliosphere. Predicting a loss-of-equilibrium, which has more recently been formulated as the torus instability, relies on a detailed understanding of the various forces that hold the pre-eruption magnetic flux rope in equilibrium. Traditionally, idealized analytical force expressions are used to derive simplified eruption criteria that can be compared to solar observations and modeling. What is missing, however, is a validation that these idealized analytical force expressions can be applied to the line-tied, low-aspect-ratio conditions of the corona. In this paper, we address this shortcoming by using a laboratory experiment to study the forces that act on long-lived, arched, line-tied magnetic flux ropes. Three key force terms are evaluated over a wide range of experimental conditions: (1) the upward hoop force; (2) the downward strapping force; and (3) the downward toroidal field tension force. First, the laboratory force measurements show that, on average, the three aforementioned force terms cancel to produce a balanced line-tied equilibrium. This finding validates the laboratory force measurement techniques developed here, which were recently used to identify a dynamic toroidal field tension force that can prevent flux rope eruptions [Myers et al., Nature 528, 526 (2015)]. The verification of magnetic force balance also confirms the low-beta assumption that the plasma thermal pressure is negligible in these experiments. Next, the measured force terms are directly compared to their corresponding analytical expressions. While the measured and analytical forces are found to be well correlated, the low-aspect-ratio, line-tied conditions in the experiment are found to both reduce the measured hoop force and increase the measured tension force with respect to analytical expectations. These two co-directed effects combine to generate laboratory flux rope equilibria at lower altitudes than are predicted analytically. Such considerations are expected to modify the loss-of-equilibrium eruption criteria for analogous flux ropes in the solar corona.
- Type:
- Dataset
- Issue Date:
- November 2016
193. Real-time Radiative Divertor Feedback Control Development for the NSTX-U Tokamak using a Vacuum Ultraviolet Spectrometer
- Author(s):
- Soukhanovskii, V.A.; Kaita, R.; Stratton, B.
- Abstract:
- A radiative divertor technique is planned for the NSTX-U tokamak to prevent excessive erosion and thermal damage of divertor plasma-facing components in H-mode plasma discharges with auxiliary heating up to 12 MW. In the radiative (partially detached) divertor, extrinsically seeded deuterium or impurity gases are used to increase plasma volumetric power and momentum losses. A real-time feedback control of the gas seeding rate is planned for discharges of up to 5 s duration. The outer divertor leg plasma electron temperature Te estimated spectroscopically in real time will be used as a control parameter. A vacuum ultraviolet spectrometer McPherson Model 251 with a fast charged-coupled device detector is developed for temperature monitoring between 5 and 30 eV, based on the delta n=0;1 line intensity ratios of carbon, nitrogen or neon ions lines in the spectral range 300 to 1600 A. A collisional-radiative model-based line intensity ratio will be used for relative calibration. A real-time Te-dependent signal within a characteristic divertor detachment equilibration time of ~ 10-15 ms is expected.
- Type:
- Dataset
- Issue Date:
- November 2016
194. Saturation of Alfven modes in tokamaks
- Author(s):
- White, R; Gorelenkov, N.; Gorelenkova, M.; Podesta, M.; Ethier, S.; Chen, Y.
- Abstract:
- Growth of Alfven modes driven unstable by a distribution of high energy particles up to saturation is investigated with a guiding center code, using numerical eigenfunctions produced by linear theory and a numerical high energy particle distribution, in order to make detailed comparison with experiment and with models for saturation amplitudes and the modification of beam profiles. Two innovations are introduced. First, a very noise free means of obtaining the mode-particle energy and momentum transfer is introduced, and secondly, a spline representation of the actual beam particle distribution is used.
- Type:
- Dataset
- Issue Date:
- November 2016
195. Theory based scaling of edge turbulence and implications for the scrape-off layer width
- Author(s):
- Myra, J.R.; Russell, D.A.; Zweben, S.J.
- Abstract:
- Turbulence and plasma parameter data from the National Spherical Torus Experiment NSTX [M. Ono, S.M. Kaye, Y.-K.M. Peng, G. Barnes et al., Nucl. Fusion 40, 557 (2000)] is examined and interpreted based on various theoretical estimates. In particular, quantities of interest for assessing the role of turbulent transport on the midplane scrape-off layer heat flux width are assessed. Because most turbulence quantities exhibit large scatter and little scaling within a given operation mode, this paper focuses on length and time scales and dimensionless parameters between operational modes including Ohmic, low (L), and high (H) modes using a large NSTX edge turbulence database [S.J. Zweben, W.M. Davis, S.M. Kaye, J.R. Myra et al., Nucl. Fusion 55, 093035 (2015)]. These are compared with theoretical estimates for drift and interchange rates, profile modification saturation levels, a resistive ballooning condition, and dimensionless parameters characterizing L and high H mode conditions. It is argued that the underlying instability physics governing edge turbulence in different operational modes is in fact similar, and is consistent with curvature-driven drift ballooning. Saturation physics, however, is dependent on the operational mode. Five dimensionless parameters for drift-interchange turbulence are obtained and employed to assess the important of turbulence in setting the scrape-off layer heat flux width lambda_q and its scaling. An explicit proportionality of the width lambda_q to safety factor and major radius (qR) is obtained under these conditions. Quantitative estimates and reduced model numerical simulations suggest that the turbulence mechanism is not negligible in determining lambda_q in NSTX, at least for high plasma current discharges.
- Type:
- Dataset
- Issue Date:
- November 2016
196. Three New Extreme Ultraviolet Spectrometers on NSTX-U for Impurity Monitoring
- Author(s):
- Weller, M.E.; Beiersdorfer, P.; Soukhanovskii, V.; Magee, E.W.; Scotti, F.
- Abstract:
- Three extreme ultraviolet (EUV) spectrometers have been mounted on the National Spherical Torus Experiment-Upgrade (NSTX-U). All three are flat-field grazing-incidence spectrometers and are dubbed X-ray and Extreme Ultraviolet Spectrometer (8 ñ 70 ≈), Long-Wavelength Extreme Ultraviolet Spectrometer (190 ñ 440 ≈), and Metal Monitor and Lithium Spectrometer Assembly (MonaLisa, 50 ñ 220 ≈). XEUS and LoWEUS were previously implemented on NSTX to monitor impurities from low- to high-Z sources and to study impurity transport while MonaLisa is new and provides the system increased spectral coverage. The spectrometers will also be a critical diagnostic on the planned laser blow-off (LBO) system for NSTX-U, which will be used for impurity edge and core ion transport studies, edge-transport code development, and benchmarking atomic physics codes.
- Type:
- Dataset
- Issue Date:
- November 2016
197. Time dependent analysis of visible helium line-ratios for electron temperature and density diagnostic using synthetic simulations on NSTX-U
- Author(s):
- Munoz Burgos, J.M.; Barbui, T.; Schmitz, O.; Stutman, D.; Tritz, K.
- Abstract:
- Helium line-ratios for electron temperature (Te) and density (ne) plasma diagnostic in the Scrape-Off-Layer (SOL) and Edge regions of tokamaks are widely used. Due to their intensities and proximity of wavelengths, the singlet 667.8 and 728.1 nm, and triplet 706.5 nm visible lines have been typically preferred. Time- dependency of the triplet line (706.5 nm) has been previously analyzed in detail by including transient effects on line-ratios during gas-puff diagnostic applications. In this work, several line-ratio combinations within each of the two spin systems are analyzed with the purpose of eliminating transient effects to extend the application of this powerful diagnostic to high temporal resolution characterization of plasmas. The analysis is done using synthetic emission modeling and diagnostic for low electron density NSTX SOL plasma conditions for several visible lines. This analysis employs both quasi-static equilibrium and time-dependent models in order to evaluate transient effects of the atomic population levels that may affect the derived electron temperatures and densities as a helium gas-puff penetrates the plasma. Ratios between the most intense lines are usually preferred due to their higher signal to noise ratio. The analysis of a wider range of spectral lines will help to extend this powerful diagnostic to experiments where the wavelength range of the measured spectra may be constrained either by limitations of the spectrometer, or by other conflicting lines from different ions.
- Type:
- Dataset
- Issue Date:
- November 2016
198. Fusion Nuclear Science Facilities and Pilot Plants Based on the Spherical Tokamak
- Author(s):
- Menard, J.E.; Brown, T.; El-Guebaly, L.; Boyer, M.; Canik, J.; Colling, B.; Raman, R.; Wang, Z.; Zhai, Y.; Buxton, P.; Covele, B.; D'Angelo, C.; Davis, A.; Gerhardt, S.; Gryaznevich, M.; Harb, M.; Hender, T.C.; Kaye, S.; Kingham, D.; Kotschenreuther, M.; Mahajan, S.; Maingi, R.; Marriott, E.; Meier, E.T.; Mynsberge, L.; Neumeyer, C.; Ono, M.; Park, J.-K.; Sabbagh, S.A.; Soukhanovskii, V.; Valanju, P.; Woolley, R.
- Abstract:
- A Fusion Nuclear Science Facility (FNSF) could play an important role in the development of fusion energy by providing the nuclear environment needed to develop fusion materials and components. The spherical torus/tokamak (ST) is a leading candidate for an FNSF due to its potentially high neutron wall loading and modular configuration. A key consideration for the choice of FNSF configuration is the range of achievable missions as a function of device size. Possible missions include: providing high neutron wall loading and fluence, demonstrating tritium self-sufficiency, and demonstrating electrical self-sufficiency. All of these missions must also be compatible with a viable divertor, first-wall, and blanket solution. ST-FNSF configurations have been developed simultaneously incorporating for the first time: (1) a blanket system capable of tritium breeding ratio TBR approximately 1, (2) a poloidal field coil set supporting high elongation and triangularity for a range of internal inductance and normalized beta values consistent with NSTX/NSTX-U previous/planned operation, (3) a long-legged divertor analogous to the MAST-U divertor which substantially reduces projected peak divertor heat-flux and has all outboard poloidal field coils outside the vacuum chamber and superconducting to reduce power consumption, and (4) a vertical maintenance scheme in which blanket structures and the centerstack can be removed independently. Progress in these ST-FNSF missions vs. configuration studies including dependence on plasma major radius R0 for a range 1m to 2.2m are described. In particular, it is found the threshold major radius for TBR = 1 is R0 greater than or equal to 1.7m, and a smaller R0=1m ST device has TBR approximately 0.9 which is below unity but substantially reduces T consumption relative to not breeding. Calculations of neutral beam heating and current drive for non-inductive ramp-up and sustainment are described. An A=2, R0=3m device incorporating high-temperature superconductor toroidal field coil magnets capable of high neutron fluence and both tritium and electrical self-sufficiency is also presented following systematic aspect ratio studies.
- Type:
- Dataset
- Issue Date:
- October 2016
199. Effect of progressively increasing lithium conditioning on edge transport and stability in high triangularity NSTX H-modes
- Author(s):
- Maingi, R.; Canik, J.M.; Bell, R.E.; Boyle, D.P.; Diallo, A.; Kaita, R.; Kaye, S.M.; LeBlanc, B.P.; Sabbagh, S.A.; Scotti, F.; Soukhanovskii, V.A.
- Abstract:
- A sequence of H-mode discharges with increasing levels of pre-discharge lithium evaporation (�dose�) was conducted in high triangularity and elongation boundary shape in NSTX. Energy confinement increased, and recycling decreased with increasing lithium dose, similar to a previous lithium dose scan in medium triangularity and elongation plasmas. Data-constrained SOLPS interpretive modeling quantified the edge transport change: the electron particle diffusivity decreased by 10-30x. The electron thermal diffusivity decreased by 4x just inside the top of the pedestal, but increased by up to 5x very near the separatrix. These results provide a baseline expectation for lithium benefits in NSTX-U, which is optimized for a boundary shape similar to the one in this experiment.
- Type:
- Dataset
- Issue Date:
- August 2016
200. Mitigation of Alfven activity by 3D magnetic perturbations on NSTX
- Author(s):
- Kramer, G.J; Bortolon, A.; Ferraro, N.M.; Spong, D.A.; Crocker, N.A.; Darrow, D.S.; Fredrickson, E.D.; Kubota, S.; Park, J.-K.; Podesta, M.; Heidbrink, W.W.
- Abstract:
- Observations on the National Spherical Torus eXperiment (NSTX) indicate that externally applied non-axisymmetric magnetic perturbations (MP) can reduce the amplitude of Toroidal Alfven Eigenmodes (TAE) and Global Alfven Eigenmodes (GAE) in response to pulsed n=3 non-resonant fields. From full-orbit following Monte Carlo simulations with the 1- and 2-fluid resistive MHD plasma response to the magnetic perturbation included, it was found that in response to MP pulses the fast-ion losses increased and the fast-ion drive for the GAEs was reduced. The MP did not affect the fast-ion drive for the TAEs significantly but the Alfven continuum at the plasma edge was found to be altered due to the toroidal symmetry breaking which leads to coupling of different toroidal harmonics. The TAE gap was reduced at the edge creating enhanced continuum damping of the global TAEs, which is consistent with the observations. The results suggest that optimized non-axisymmetric MP might be exploited to control and mitigate Alfven instabilities by tailoring the fast-ion distribution function and/or continuum structure.
- Type:
- Dataset
- Issue Date:
- August 2016
