Guttenfelder W.; S.M. Kaye; Y. Ren; W. Solomon; R.E. Bell; J. Candy; S.P. Gerhardt; B.P. LeBlanc; H. Yuh
This paper presents quasi-linear gyrokinetic predictions of the Coriolis momentum pinch for low aspect-ratio NSTX H-modes where previous experimental measurements were focused. Local, linear calculations predict that in the region of interest (just outside the mid-radius) of these relatively high-beta plasmas, profiles are most unstable to microtearing modes that are only effective in transporting electron energy. However, sub-dominant electromagnetic and electrostatic ballooning modes are also unstable, which are effective at transporting energy, particles and momentum. The quasi-linear prediction of transport from these weaker ballooning modes, assuming they contribute transport in addition to that from microtearing modes in a nonlinear turbulent state, leads to a very small or outward convection of momentum, inconsistent with the experimentally measured inward pinch, and opposite to predictions in conventional aspect ratio tokamaks. Additional predictions of a low beta L-mode plasma, unstable to more traditional electrostatic ion temperature gradient-trapped electron mode instability, show that the Coriolis pinch is inward but remains relatively weak and insensitive to many parameter variations. The weak or outward pinch predicted in NSTX plasmas appears to be at least partially correlated to changes in the parallel mode structure that occur at finite beta and low aspect ratio, as discussed in previous theories. The only conditions identified where a stronger inward pinch is predicted occur either in the purely electrostatic limit or if the aspect ratio is increased. As the Coriolis pinch cannot explain the measured momentum pinch, additional theoretical momentum transport mechanisms are discussed that may be potentially important.
An important goal of stellarator optimization is to achieve good confinement of
energetic particles such as, in the case of a reactor, alphas created by Deuterium-Tritium
(D-T) fusion. In this work, a fixed-boundary stellarator equilibrium was re-optimized for
energetic particle confinement via a two-step process: first, by minimizing deviations from quasi-axisymmetry (QA) on a single flux surface near the mid-radius, and secondly by maintaining
this improved quasi-axisymmetry while minimizing the analytical quantity ΓC , which represents
the angle between magnetic flux surfaces and contours of J||, the second adiabatic invariant.
This was performed multiple times, resulting in a group of equilibria with significantly reduced
energetic particle losses, as evaluated by Monte Carlo simulations of alpha particles in scaled-up
versions of the equilibria. This is the first time that energetic particle losses in a QA stellarator
have successfully been reduced by optimizing ΓC . The relationship between energetic particle
losses and metrics such as QA error (Eqa) and ΓC in this set of equilibria were examined via
statistical methods and a nearly linear relationship between volume-averaged ΓC and prompt
particle losses was found.
Choi, W.; Poli, F. M.; Li, M. H.; Baek, S. G.; Gorenlenkova, M.; Ding, B. J.; Gong, X. Z.; Chan, A.; Duan, Y. M.; Hu, J. H.; Lian, H.; Lin, S. Y.; Liu, H. Q.; Qian, J. P.; Wallace, G.; Wang, Y. M.; Zang, Q.; Zhao, H. L.
Synergistic effects between two frequencies of lower hybrid (LH) waves—operating at 2.45 and 4.6 GHz—were observed in experiment on EAST for the first time. At low density (n_e,lin ≈ 2.0 × 10^19m^−3), simultaneous injection of a 65/35 mix of 2.45 GHz/4.6 GHz power achieved an LHCD efficiency that was 25% higher than what should be expected from the linear combination of the two sources. The experiment was interpreted with time-dependent simulations, using the equilibrium and transport solver TRANSP, coupled with the ray-tracing code GENRAY and the Fokker-Planck solver CQL3D. For each discharge, profiles of current and hard x-ray from simulation and measurement agree within uncertainties. An examination of the electron distribution function indicates that the LH synergy is supported by the increased width of the LH resonance plateau in the simultaneous injection case compared to independent injection.
Baldwin, Jane W; Dessy, Jay Benjamin; Vecchi, Gabriel A; Oppenheimer, Michael; Jia, Liwei; Gudgel, Richard G; Paffendorf, Karen
This data is compiled to support a publication in the journal Earth's Future: Baldwin et al 2019 "Temporally Compound Heat Waves and Global Warming: An Emerging Hazard".
The GCM GFDL CM2.5-FLOR was used to produce the raw climate model data. The model code for FLOR is freely available and can be downloaded at https://www.gfdl.noaa.gov/cm2-5-and-flor/. Code used to calculate the derived heat wave statistics data and produce figures in the paper is available at https://github.com/janewbaldwin/Compound-Heat-Waves
The heat wave statistics derived output for only one definition is provided (daily minimum temperature, 90th percentile threshold, temporal structure 3114) which is the definition used the most in the paper figures. Statistics for the other definitions can be created by running the HWSTATS code provided in the corresponding github folder, which includes python scripts which do the analysis and PBS job scheduling and submission scripts which show how to run the python scripts. For more information on this, please see the github readme.
Linear stability analysis of the national spherical torus experiment (NSTX) Li-conditioned
ELM-free H-mode equilibria is carried out in the context of the extended
magneto-hydrodynamic (MHD) model in NIMROD. The purpose is to investigate the physical
cause behind edge localized mode (ELM) suppression in experiment after the Li-coating of
the divertor and the first wall of the NSTX tokamak. Besides ideal MHD modeling, including
finite-Larmor radius effect and two-fluid Hall and electron diamagnetic drift contributions,
a non-ideal resistivity model is employed, taking into account the increase of Z eff after
Li-conditioning in ELM-free H-mode. Unlike an earlier conclusion from an eigenvalue code
analysis of these equilibria, NIMROD results find that after reduced recycling from divertor
plates, profile modification is necessary but insufficient to explain the mechanism behind
complete ELMs suppression in ideal two-fluid MHD. After considering the higher plasma
resistivity due to higher Z eff , the complete stabilization could be explained. A thorough
analysis of both pre-lithium ELMy and with-lithium ELM-free cases using ideal and
non-ideal MHD models is presented, after accurately including a vacuum-like cold halo
region in NIMROD to investigate ELMs.
Diallo, A.; Banerjee, S.; Zweben, S.; Stoltzfus-Dueck, T.
We studied the energy exchange dynamics across the low-to-high-confinement (L-H) in NSTX discharges using the gas-puff imaging (GPI) diagnotic. The investigation focused on the energy exchange between flows and turbulence, to help clarify the mechanism of the L-H transition. We apply this study to three type of heating schemes, including a total of 17 shots from the NSTX 2010 campaign run. Results show that the edge fluctuation characteristics (fluctuation levels, radial and poloidal correlation lengths) measured using GPI do not vary just prior to the H-mode transition, but change after the transition. Using a velocimetry approach (orthogonal-programming decomposition), velocity fields of a 24 $\times$ 30 cm GPI view during the L-H transition were obtained with good spatial ($\sim$1 cm) and temporal ($\sim$2.5 $\mu$s) resolutions. Analysis using these velocity fields shows that the production term is systematically negative just prior to the L-H transition indicating transfer from mean flows to turbulence, which is inconsistent with the predator-prey paradigm. Moreover, using the inferred absolute value of the production term, an estimate of the L-H transition duration is found to be 25 ms, which is much larger than the measured duration. These discrepancies are further reinforced by consideration of the ratio between the kinetic energy in the mean flow to the thermal free energy, which is estimated to be much less than 1, suggesting again that turbulence depletion mechanism may not be playing an important role in the transition to the H-mode. Although the Reynolds work is too small to directly deplete the turbulent free energy reservoir, order-of-magnitude analysis shows that the Reynolds stress may still make a non-negligible contribution to the observed poloidal flows.
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.