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172. Regarding the optimization of O1-mode ECRH and the feasibility of EBW startup on NSTX-U
- Author(s):
- Lopez, N; Poli, F
- Abstract:
- Recently published scenarios for fully non-inductive startup and operation on the National Spherical Torus eXperiment Upgrade (NSTX-U) (Menard et al 2012 Nucl. Fusion 52 083015) show Electron Cyclotron Resonance Heating (ECRH) as an important component in preparing a target plasma for efficient High Harmonic Fast Wave and Neutral Beam heating. The modeling of the propagation and absorption of EC waves in the evolving plasma is required to define the most effective window of operation, and to optimize the launcher geometry for maximal heating and current drive during this window. Here, we extend a previous optimization of O1-mode ECRH on NSTX-U to account for the full time-dependent performance of the ECRH using simulations performed with TRANSP. We find that the evolution of the density profile has a prominent role in the optimization by defining the time window of operation, which in certain cases may be a more important metric to compare launcher performance than the average power absorption. This feature cannot be captured by analysis on static profiles, and should be accounted for when optimizing ECRH on any device that operates near the cutoff density. Additionally, the utility of the electron Bernstein wave (EBW) in driving current and generating closed flux surfaces in the early startup phase has been demonstrated on a number of devices. Using standalone GENRAY simulations, we find that efficient EBW current drive is possible on NSTX-U if the injection angle is shifted below the midplane and aimed towards the top half of the vacuum vessel. However, collisional damping of the EBW is projected to be significant, in some cases accounting for up to 97% of the absorbed EBW power
- Type:
- Dataset
- Issue Date:
- June 2018
173. Detection of an electron beam in a high density plasma via an electrostatic probe
- Author(s):
- Majeski, Stephen; Yoo, Jongsoo; Zweben, Stewart; Yamada, Masaaki
- Abstract:
- An electron beam is detected by a 1D floating potential probe array in a relatively high density (10e12 − 10e13 cm−3) and low temperature (∼ 5 eV) plasma of the Magnetic Reconnection Experiment (MRX). Clear perturbations in the floating potential profile by the electron beam are observed. Based on the floating potential profile and a current balance equation to the probe array tips, the effective width of the electron beam is determined, from which we determine the radial and toroidal beam current density profiles. After the profile of the electron beam is specified from the measured beam current, we demonstrate the consistency of the current balance equation and the location of the perturbation is also in agreement with field line mapping. No significant broadening of the electron beam is observed after the beam propagates for tens of centimeters through the high density plasma. These results prove that the field line mapping is, in principle, possible in high density plasmas.
- Type:
- Dataset
- Issue Date:
- 2018
174. Gas Puff Imaging Diagnostics of Edge Plasma Turbulence in Magnetic Fusion Devices
- Author(s):
- Zweben, S.J.; Terry, J.L.; Stotler, D.P.; Maqueda, R.J.
- Abstract:
- 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.
- Type:
- Dataset
- Issue Date:
- April 2017
175. Toward fusion plasma scenario planning for NSTX-U using machine-learning-accelerated models
- Author(s):
- Mark D. Boyer
- Abstract:
- One of the most promising devices for realizing power production through nuclear fusion is the tokamak. To maximize performance, it is preferable that tokamak reactors achieve advanced operating scenarios characterized by good plasma confinement, improved magnetohydrodynamic (MHD) stability, and a largely non-inductively driven plasma current. Such scenarios could enable steady-state reactor operation with high \emph{fusion gain} --- the ratio of produced fusion power to the external power provided through the plasma boundary. Precise and robust control of the evolution of the plasma boundary shape as well as the spatial distribution of the plasma current, density, temperature, and rotation will be essential to achieving and maintaining such scenarios. The complexity of the evolution of tokamak plasmas, arising due to nonlinearities and coupling between various parameters, motivates the use of model-based control algorithms that can account for the system dynamics. In this work, a learning-based accelerated model trained on data from the National Spherical Torus Experiment Upgrade (NSTX-U) is employed to develop planning and control strategies for regulating the density and temperature profile evolution around desired trajectories. The proposed model combines empirical scaling laws developed across multiple devices with neural networks trained on empirical data from NSTX-U and a database of first-principles-based computationally intensive simulations. The reduced execution time of the accelerated model will enable practical application of optimization algorithms and reinforcement learning approaches for scenario planning and control development. An initial demonstration of applying optimization approaches to the learning-based model is presented, including a strategy for mitigating the effect of leaving the finite validity range of the accelerated model. The approach shows promise for actuator planning between experiments and in real-time.
- Type:
- Dataset
- Issue Date:
- May 2020
176. Compact steady-state tokamak performance dependence on magnet and core physics limits
- Author(s):
- Menard, J.E.
- Abstract:
- Compact tokamak fusion reactors utilizing advanced high-temperature superconducting magnets for the toroidal field coils have received considerable recent attention due to the promise of more compact devices and more economical fusion energy development. Facilities with combined Fusion Nuclear Science (FNS) and Pilot Plant missions to provide both the nuclear environment needed to develop fusion materials and components while also potentially achieving sufficient fusion performance to generate modest net electrical power are considered. The performance of the tokamak fusion system is assessed using a range of core physics and toroidal field magnet performance constraints to better understand which parameters most strongly influence the achievable fusion performance.
- Type:
- Dataset
- Issue Date:
- December 2018
177. Understanding the dynamics and energetics of magnetic reconnection in a laboratory plasma: Review of recent progress on selected fronts
- Author(s):
- Yamada, M.; Yoo, J.; Myers, C. E.
- Abstract:
- Magnetic reconnection is a fundamental process at work in laboratory, space and astrophysical plasmas, in which magnetic field lines change their topology and convert magnetic energy to plasma particles by acceleration and heating. One of the most important problems in reconnection research has been to understand why reconnection occurs so much faster than predicted by MHD theory. Following the recent pedagogical review of this subject [M. Yamada, R. Kulsrud, and H. Ji, Rev. Mod. Phys. {\bf 82}, 603 (2010)], this paper presents a review of more recent discoveries and findings in the research of fast magnetic reconnection in laboratory, space, and astrophysical plasmas. In spite of the huge difference in physical scales, we find remarkable commonality between the characteristics of the magnetic reconnection in laboratory and space plasmas. In this paper, we will focus especially on the energy flow, a key feature of the reconnection process. In particular the experimental results on the energy conversion and partitioning in a laboratory reconnection layer [M. Yamada {\it et al.}, Nat. Commu. {\bf 5}, 4474 (2014)] are discussed and compared with quantitative estimates based on two-fluid analysis. In the Magnetic Reconnection Experiment (MRX), we find that energy deposition to electrons is localized near the X-point and is mostly from the electric field component perpendicular to the magnetic field. The mechanisms of ion acceleration and heating are also identified and a systematic and quantitative study on the inventory of converted energy within a reconnection layer with a well-defined but variable boundary. The measured energy partition in a reconnection region of similar effective size ($L \approx$ 3 ion skin depths) of the Earth's magneto-tail [J. Eastwood {\it et al.}, Phys. Rev. Lett. {\bf 110}, 225001 (2013)] is notably consistent with our laboratory results. Finally, to study the global aspects of magnetic reconnection, we have carried out a laboratory experiment on the stability criteria for solar flare eruptions, including {\textquotedblleft}storage and release{\textquotedblright} mechanisms of magnetic energy. We show that toroidal magnetic flux generated by magnetic relaxation (reconnection) processes generates a new stabilizing force which prevents plasma eruption. This result has lead us to discovery of a new stabilizing force for solar flares [C. E. Myers {\it et al.}, Nature {\bf 528}, 526 (2015)]
- Type:
- Dataset
- Issue Date:
- May 2016
178. Blob-hole correlation model for edge turbulence and comparisons with NSTX GPI data
- Author(s):
- Myra, J.R.; Zweben, S.J.; Russell, D.A.
- Abstract:
- Gas puff imaging (GPI) observations made in NSTX [Zweben S J, et al., 2017 Phys. Plasmas 24 102509] have revealed two-point spatial correlations of edge and scrape-off layer turbulence in the plane perpendicular to the magnetic field. A common feature is the occurrence of dipole-like patterns with significant regions of negative correlation. In this paper, we explore the possibility that these dipole patterns may be due to blob-hole pairs. Statistical methods are applied to determine the two-point spatial correlation that results from a model of blob-hole pair formation. It is shown that the model produces dipole correlation patterns that are qualitatively similar to the GPI data in several respects. Effects of the reference location (confined surfaces or scrape-off layer), a superimposed random background, hole velocity and lifetime, and background sheared flows are explored and discussed with respect to experimental observations. Additional analysis of the experimental GPI dataset is performed to further test this blob-hole correlation model. A time delay two-point spatial correlation study did not reveal inward propagation of the negative correlation structures that were postulated to correspond to holes in the data nor did it suggest that the negative correlation structures are due to neutral shadowing. However, tracing of the highest and lowest values (extrema) of the normalized GPI fluctuations shows strong evidence for mean inward propagation of minima and outward propagation of maxima, in qualitative agreement with theoretical expectations. Other properties of the experimentally observed extrema are discussed.
- Type:
- Dataset
- Issue Date:
- July 2018
179. 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
180. 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
