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.
The Enhanced Pedestal (EP) H-mode regime is an attractive wide-pedestal ELM-free high-betap scenario for NSTX-U and next-step devices as it achieves enhanced energy confinement (H98y,2 > 1.5), large normalized pressure (betaN > 5) and significant bootstrap fraction (f_BS > 0.6) at I_p/B_T = 2 MA/T. This regime is realized when the edge ion collisionality becomes sufficiently small that a positive feedback interaction occurs between a reduction in the ion neoclassical energy transport and an increase in the particle transport from pressure-driven edge instabilities. EP H-mode was most often observed as a transition following a large ELM in conditions with low edge neutral recycling. It is hypothesized that the onset of pressure-driven instabilities prior to the full recovery of the neutral density leads to a temporary period with elevated ion temperature gradient that triggers the transition to EP H-mode. Linear CGYRO and M3D-C1 calculations are compared to beam emission spectroscopy (BES) and magnetic spectroscopy in order to describe the evolution of the edge particle transport mechanisms during the ELM recovery and the saturated EP H-mode state. The observations are consistent with the hypothesis that the onset of pressure-driven edge instabilities, such as the KBM and kink-peeling, can be responsible for the increased particle transport in EP H-mode.
We implement unsupervised machine learning techniques to identify characteristic evolution patterns and associated parameter regimes in edge localized mode (ELM) events observed on the National Spherical Torus Experiment. Multi-channel, localized measurements spanning the pedestal region capture the complex evolution patterns of ELM events on Alfven timescales. Some ELM events are active for less than 100~microsec, but others persist for up to 1~ms. Also, some ELM events exhibit a single dominant perturbation, but others are oscillatory. Clustering calculations with time-series similarity metrics indicate the ELM database contains at least two and possibly three groups of ELMs with similar evolution patterns. The identified ELM groups trigger similar stored energy loss, but the groups occupy distinct parameter regimes for ELM-relevant quantities like plasma current, triangularity, and pedestal height. Notably, the pedestal electron pressure gradient is not an effective parameter for distinguishing the ELM groups, but the ELM groups segregate in terms of electron density gradient and electron temperature gradient. The ELM evolution patterns and corresponding parameter regimes can shape the formulation or validation of nonlinear ELM models. Finally, the techniques and results demonstrate an application of unsupervised machine learning at a data-rich fusion facility.
The control of divertor heat loads - both steady state and transient - remains a key challenge for the successful operation of ITER and FNSF. Magnetic perturbations provide a promising technique to control ELMs (transients), but understanding their detailed impact is difficult due to their symmetry breaking nature. One approach for reducing steady state heat loads are so called 'advanced divertors' which aim at optimizing the magnetic field configuration: the snowflake and the (super-)X-divertor. It is likely that both concepts - magnetic perturbations and advanced divertors - will have to work together, and we explore their interaction based on the NSTX-U setup. An overview of different divertor configurations under the impact of magnetic perturbations is presented, and the resulting impact on plasma edge transport is investigated with the EMC3-EIRENE code.
Variations in size of the magnetic footprint of the perturbed separatrix are found, which is related to the level of flux expansion on the divertor target. Non-axisymmetric peaking of the heat flux related to the perturbed separatrix is found at the outer strike point, but only in locations where flux expansion is not too large.
The Far-infrared Tangential Interferometer/Polarimeter (FIReTIP) system has been refurbished and
is being reinstalled on the National Spherical Torus Experiment-Upgrade (NSTX-U) to supply
real-time line-integrated core electron density measurements for use in the NSTX-U plasma control
system (PCS) to facilitate real-time density feedback control of the NSTX-U plasma. Inclusion
of a visible light heterodyne interferometer in the FIReTIP system allows for real-time vibration
compensation due to movement of an internally mounted retroreflector and the FIReTIP front-end
optics. Real-time signal correction is achieved through use of a National Instruments CompactRIO
field-programmable gate array.
This paper examines a method for real-time control of non-inductively sustained scenarios in NSTX-U by using TRANSP,
a time-dependent integrated modeling code for prediction and interpretive analysis of tokamak experimental data, as a
simulator. The actuators considered for control in this work are the six neutral beam sources and the plasma boundary
shape. To understand the response of the plasma current, stored energy, and central safety factor to these actuators
and to enable systematic design of control algorithms, simulations were run in which the actuators were modulated and
a linearized dynamic response model was generated. A multi-variable model-based control scheme that accounts for the
coupling and slow dynamics of the system while mitigating the effect of actuator limitations was designed and
simulated. Simulations show that modest changes in the outer gap and heating power can improve the response time of
the system, reject perturbations, and track target values of the controlled values.
Bertelli, N; Valeo, E.J.; Green, D.L.; Gorelenkova, M.; Phillips, C.K.; Podesta, M.; Lee, J.P.; Wright, J.C.; Jaeger, E.
At the power levels required for significant heating and current drive
in magnetically-confined toroidal plasma, modification of the particle distribution
function from a Maxwellian shape is likely [T. H. Stix, Nucl. Fusion, 15 737
(1975)], with consequent changes in wave propagation and in the location and
amount of absorption. In order to study these effects computationally, both the
finite-Larmor-radius and the high-harmonic fast wave (HHFW), versions of the
full-wave, hot-plasma toroidal simulation code TORIC [M. Brambilla, Plasma Phys.
Control. Fusion 41, 1 (1999) and M. Brambilla, Plasma Phys. Control. Fusion
44, 2423 (2002)], have been extended to allow the prescription of arbitrary velocity
distributions of the form f(v||, v_perp, psi , theta). For hydrogen (H) minority heating of a
deuterium (D) plasma with anisotropic Maxwellian H distributions, the fractional
H absorption varies significantly with changes in parallel temperature but is
essentially independent of perpendicular temperature. On the other hand, for
HHFW regime with anisotropic Maxwellian fast ion distribution, the fractional
beam ion absorption varies mainly with changes in the perpendicular temperature.
The evaluation of the wave-field and power absorption, through the full wave
solver, with the ion distribution function provided by either aMonte-Carlo particle
and Fokker-Planck codes is also examined for Alcator C-Mod and NSTX plasmas.
Non-Maxwellian effects generally tends to increase the absorption with respect to
the equivalent Maxwellian distribution.
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.
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.
Gas puff imaging (GPI) is a diagnostic of plasma turbulence which uses
a puff of neutral gas at the plasma edge to increase the local visible
light emission for improved space-time resolution of plasma
fluctuations. This paper reviews gas puff imaging diagnostics of edge
plasma turbulence in magnetic fusion research, with a focus on the
instrumentation, diagnostic cross-checks, and interpretation
issues. The gas puff imaging hardware, optics, and detectors are
described for about 10 GPI systems implemented over the past ~15
years. Comparison of GPI results with other edge turbulence diagnostic
results are described and many common features are observed. Several
issues in the interpretation of GPI measurements are discussed, and
potential improvements in hardware and modeling are suggested.
Sharma, A. Y.; Cole, M. D. J.; Görler, T.; Chen, Y.; Hatch, D. R.; Guttenfelder, W.; Hager, R.; Sturdevant, B. J.; Ku, S.; Chang, C. S.
Plasma shaping may have a stronger effect on global turbulence in tight-aspect-ratio tokamaks than in conventional-aspect-ratio tokamaks due to the higher toroidicity and more acute poloidal asymmetry in the magnetic field. In addition, previous local gyrokinetic studies have shown that it is necessary to include parallel magnetic field perturbations in order to accurately compute growth rates of electromagnetic modes in tight-aspect-ratio tokamaks. In this work, the effects of elongation and triangularity on global, ion-scale, linear electromagnetic modes are studied at NSTX aspect ratio and high plasma beta using the global gyrokinetic particle-in-cell code XGC. The effects of compressional magnetic perturbations are approximated via a well-known modification to the particle drifts that was developed for flux-tube simulations [N. Joiner et al., Phys. Plasmas 17, 072104 (2010)], without proof of its validity in a global simulation. Magnetic equilibria are re-constructed for each distinct plasma profile that is used. Coulomb collision effects are not considered. Within the limitations imposed by the present study, it is found that linear growth rates of electromagnetic modes (collisionless microtearing modes and kinetic ballooning modes) are significantly reduced by NSTX-like shaping. For example, growth rates of kinetic ballooning modes at high beta are reduced to the level of that of collisionless trapped electron modes.