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1. Electron Temperature Gradient Driven Transport Model for Tokamak Plasmas

Author(s):
Rafiq, Tariq; Wilson, Christopher; Luo, Lixiang; Weiland, Jan; Schuster, Eugenio; Pankin, Alexei; Guttenfelder, Walter; Kaye, Stan
Abstract:
A new model for electron temperature gradient (ETG) modes is developed as a component of the Multi-Mode anomalous transport module [T. Rafiq \textit{et al.,} Phys Plasmas \textbf{20}, 032506 (2013)] to predict a time dependent electron temperature profile in conventional and low aspect ratio tokamaks. This model is based on two-fluid equations that govern the dynamics of low-frequency short- and long-wavelength electromagnetic toroidal ETG driven drift modes. A low collisionality NSTX discharge is used to scan the plasma parameter dependence on the ETG real frequency, growth rate, and electron thermal diffusivity. Electron thermal transport is discovered in the deep core region where modes are more electromagnetic in nature. Several previously reported gyrokinetic trends are reproduced, including the dependencies of density gradients, magnetic shear, $\beta$ and gradient of $\beta$ $(\betap)$, collisionality, safety factor, and toroidicity, where $\beta$ is the ratio of plasma pressure to the magnetic pressure. The electron heat diffusivity associated with the ETG mode is discovered to be on a scale consistent with the experimental diffusivity determined by power balance analysis.
Type:
Dataset
Issue Date:
30 August 2022

2. Nonlinear growth of magnetic islands by passing fast ions in NSTX

Author(s):
Yang, James; Fredrickson, Eric; Podestà, Mario; Poli, Francesca
Abstract:
The growth of magnetic islands in NSTX is modeled successfully, with the consideration of passing fast ions. It is shown that a good quantitative agreement between simulation and experimental measurement can be achieved when the uncompensated cross-field current induced by passing fast ions is included in the island growth model. The fast ion parameters, along with other equilibrium parameters, are obtained self-consistently using the TRANSP code with the assumptions of the ‘kick’ model (Podestà et al 2017 Plasma Phys. Control. Fusion 59 095008). The results show that fast ions can contribute to overcoming the stabilizing effect of polarization current for magnetic island growth.
Type:
Dataset
Issue Date:
2022

3. Dynamics of filaments during the edge-localized mode crash on NSTX

Author(s):
Lampert,Mate; Diallo,Ahmed; Myra,James R.; Zweben, Stewart J.
Abstract:
Edge localized modes (ELMs) are routinely observed in H-mode plasma regimes of the National Spherical Torus Experiment (NSTX). Due to the explosive nature of the instability, only diagnostics with high temporal and spatial resolution could provide a detailed insight into the dynamics associated with the ELMs. Gas-puff imaging (GPI) at NSTX provides 2D measurements of the magnetic field aligned fluctuations (e.g. ELM filaments) in the scrape-off layer and the at the plasma edge with 2.5 us temporal and 10 mm optical resolution.A novel analysis technique was developed to estimate the frame-by-frame velocities and the spatial parameters of the dominant structures associated with the ELMs. The analysis was applied to single ELM events to characterize the ELM crash dynamics, and then extended to a database of 169 ELM events.Statistical analysis was performed in order to find the characterizing dynamics of the ELM crash. The results show that on average an ELM crash consists of a filament with a circular cross-section which is propelled outwards with a characterizing peak radial velocity of ~3.3 km/s. The radial velocity was found to be linearly dependent on the distance of the filament from the separatrix, which has never been seen before. The ELM filament is characterized by propagation in the ion-diamagnetic direction poloidally with a peak velocity of 11.4 km/s. The ELM crash lasts for approximately 100us until the radial propulsion settles back to the pre-ELM level. The experimental findings were compared with analytical theory. Two possible mechanisms were identified for explaining the observations: the curvature interchange model and the current-filament interaction model.
Type:
Dataset
Issue Date:
January 2021

4. Enhanced Pedestal H-mode at low edge ion collisionality on NSTX

Author(s):
Battaglia, D.J.; Guttenfelder, W.; Bell, R.E.; Diallo, A.; Ferraro, N.;, Fredrickson, E.; Gerhardt, S.P.; Kaye, S.M.; Maingi, R.; Smith, D.R.
Abstract:
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.
Type:
Dataset
Issue Date:
June 2020

5. Application of Townsend avalanche theory to tokamak startup by coaxial helicity injection

Author(s):
Hammond, K.C.; Raman, R.; Volpe, F.A.
Abstract:
Townsend avalanche theory is employed to model and interpret plasma initiation in NSTX by Ohmic heating and coaxial helicity injection (CHI). The model is informed by spatially resolved vacuum calculations of electric field and magnetic field line connection length in the poloidal cross-section. The model is shown to explain observations of Ohmic startup including the duration and location of breakdown. Adapting the model to discharges initiated by CHI offers insight into the causes of upper divertor (absorber) arcs in cases where the discharge fails to initiate in the lower divertor gap. Finally, upper and lower limits are established for vessel gas fill based on requirements for breakdown and radiation. It is predicted that CHI experiments on NSTX-U should be able to use as much as four times the amount of prefill gas employed in CHI experiments in NSTX. This should provide greater flexibility for plasma start-up, as the injector flux is projected to be increased in NSTX-U.
Type:
Dataset
Issue Date:
September 2017

6. Drift kinetic effects on the plasma response in high beta spherical tokamak experiments

Author(s):
Wang, Z.R.; Park, J.-K.; Menard, J.E.; Liu, Y.Q.; Kaye, S.M.; Gerhardt, S.
Abstract:
High $\beta$ plasma response to the rotating n=1 external magnetic perturbations is numerically studied and compared with National Spherical Torus eXperiment (NSTX). The hybrid magnetohydrodynamic(MHD)-kinetic modeling shows the drift kinetic effects are important to resolve the disagreement of plasma response between the ideal MHD prediction and the NSTX experimental observation when plasma pressure reaches and exceeds the no-wall limit [F. Troyon et al., Plasma Phys. Control. Fusion \textbf{26}, 209 (1984)]. Since the external rotating fields and high plasma rotation are presented in NSTX experiments, the importance of resistive wall effect and plasma rotation on determining the plasma response is also identified, where the resistive wall suppresses the plasma response through the wall eddy current. The inertial energy, due to plasma rotation, destabilizes the plasma. The complexity of plasma response, in this study, indicates that MHD modeling, including comprehensive physics e.g. the drift kinetic effects, resistive wall and plasma rotation, is essential to reliably predict the plasma behavior in high beta spherical tokamak device.
Type:
Dataset
Issue Date:
September 2017

7. Sensitivity of WallDYN material migration modeling to uncertainties in mixed-material surface binding energies

Author(s):
Nichols, J.H.; Jaworski, M.A.; Schmid, K.
Abstract:
The WallDYN package has recently been applied to a number of tokamaks to self-consistently model the evolution of mixed-material plasma facing surfaces. A key component of the WallDYN model is the concentration-dependent surface sputtering rate, calculated using SDTRIM.SP. This modeled sputtering rate is strongly influenced by the surface binding energies (SBEs) of the constituent materials, which are well known for pure elements but often are poorly constrained for mixed-materials. This work examines the sensitivity of WallDYN surface evolution calculations to different models for mixed-material SBEs, focusing on the carbon/lithium/oxygen/deuterium system present in NSTX. A realistic plasma background is reconstructed from a high density, H-mode NSTX discharge, featuring an attached outer strike point with local density and temperature of 4e20 m^-3 and 4 eV, respectively. It is found that various mixed-material SBE models lead to significant qualitative and quantitative changes in the surface evolution profile at the outer divertor, with the highest leverage parameter being the C-Li binding model. Uncertainties of order 50%, appearing on time scales relevant to tokamak experiments, highlight the importance of choosing an appropriate mixed-material sputtering representation when modeling the surface evolution of plasma facing components. These results are generalized to other fusion-relevant materials with different ranges of SBEs.
Type:
Dataset
Issue Date:
2017

8. Stabilizing effect of resistivity towards ELM-free H-mode discharge in lithium-conditioned NSTX

Author(s):
Banerjee, D.; Zhu, P.; Maingi, R.
Abstract:
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.
Type:
Dataset
Issue Date:
July 2017

9. Two-dimensional turbulence cross-correlation functions in the edge of NSTX

Author(s):
Zweben, S.J.; Stotler, D.P.; Scotti, F.; Myra, J.R.
Abstract:
The 2-D radial vs. poloidal cross-correlation functions of edge plasma turbulence were measured near the outer midplane using the gas puff imaging (GPI) diagnostic on NSTX. These correlation functions were evaluated at radii r= 0 cm, ±3 cm, and ±6 cm from the separatrix and poloidal locations p=0 cm and ±7.5 cm from the GPI poloidal center line for 20 different shots. The ellipticity ε and tilt angle φ of the positive cross- correlation regions, and the minimum negative cross-correlation “cmin” and total negative over positive values “neg/pos” were evaluated for each of these cases. The average results over this data set were ε=2.2±0.9, φ=87±34o (i.e. poloidally oriented), cmin= -0.30±0.15, and neg/pos=0.25±0.24. Thus there was significant variation in these correlation results within this database, with dependences on the location within the image, the magnetic geometry, and the plasma parameters. Possible causes for this variation are discussed, including the misalignment of the GPI view with the local B field line, the magnetic shear of field lines in the edge, the poloidal flow shear of the turbulence, blob-hole correlations, and the neutral density ‘shadowing’ effect in GPI.
Type:
Dataset
Issue Date:
September 2017

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