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FY2019 NSTX-U Research Milestones

JRT-2019:  Conduct research to understand the role of neutral fueling and transport in determining the pedestal structure.

The edge pedestal is a key component in achieving overall high confinement in a magnetic fusion device. Therefore, obtaining a physics understanding and predictive capability for the pedestal height and structure is a major goal of fusion research and requires advances in the understanding of the separate structure of density and temperature profiles in the pedestal region. A key challenge is to understand the importance of particle sources in determining the density pedestal and project to burning plasma scenarios. Experiments on DIII-D and archived data from C-Mod, DIII-D, and NSTX will be used to test how fueling, reduced recycling, and transport affect the density pedestal structure. The role of divertor geometry and its effect upon the pedestal structure will also be investigated. U.S. researchers involved in collaborative activities on other relevant experiments may also contribute to this effort.

R(19-1):  Validate transport models for high-beta ST H-mode plasmas and assessing the importance of multi-scale effects in NSTX/NSTX-U turbulent transport

Description:  This Milestone consists of two components: (1) Modeling electron thermal transport in STs is a critical step in developing predictive capability. Following progress in R18-3, this milestone activity will continue to develop and validate transport models specifically capable of addressing high-beta, low-aspect-ratio H-modes. The models must be capable of treating microtearing modes (MTM), as well as kinetic ballooning modes (KBM) and energetic particle modes (EPM), which presently include MMM and TGLF. After validating the models using available ST experimental data, they will be used to make projections for NSTX-U plasmas, as well as evaluate the physics assumptions used in the design of next-step ST-based devices. (2) Electron scale (kθρi >> 1) ETG turbulence has been predicted to be important in various NSTX and NSTX-U L-mode and H-mode plasmas. In some of these cases, gyrokinetic simulations indicate that ion scale (kθρi < 1) turbulence should be completely suppressed by E×B shear such that only electron scale turbulence should remain. However, in many cases the ion scale turbulence is predicted to not be completely suppressed, or the predicted transport from ETG is insufficient to account for experimental electron heat flux levels. Gyrokinetic simulations and recently updated reduced models will be used to clarify when and where ETG can be an important contributor in ST transport, and to begin addressing whether ETG can be considered in isolation from ion scale turbulence. Systematic parameter scans in driving gradients and E×B shearing rates will be performed in electron-scale linear gyrokinetic simulations to determine the ETG instability threshold as a function of these parameters. These results will be used to better quantify when ion scale transport dominates electron-scale transport, or vice versa. The region between these limits of single-scale transport should help demarcate where multiscale effects are expected to be important.

R(19-2): Develop optimized ramp-up scenarios in spherical tokamaks

Description:  This milestone leverages the simulation capabilities developed as part of the R(18-2) milestone to realize optimized ramp-up scenarios on NSTX-U and MAST-U. The reduced model framework (TOKSYS) will be extended to include the evolution of the q-profile, temperature, density and fast-ion pressure.  The reduced models for the kinetic profiles will be derived from transport models, such as TRANSP, and experimental results. These advances in the model will be leveraged to examine the resiliency of the ramp-up scenarios to expected experimental variations, such as the timing of the L-H transition and the temporary loss or delayed turn-on of a neutral beam.  This framework will allow the development of real-time algorithms that can improve the resiliency of the scenarios.  For example, experiments on NSTX-U demonstrated that a new control scheme that alters the outer boundary of the plasma shape to achieve a target inner gap distance improved the resiliency of the time of diverting to variations in the neutral beam heating. The transport modeling framework (TRANSP) will be extended to remove assumptions from the initial studies pursued in FY18. For example, the evolution of Te and Ti would be based on a flux-driven transport model that was benchmarked on a database of ST ramp-up results. This capability would be leveraged to examine the plasma resistivity and current profile relaxation, which are significant drivers of the internal inductance, ohmic flux consumption and MHD stability. Another goal would be to implement a neutral fueling model that would alter the edge fueling to match a target L-mode density, similar to what can be achieved in the experiment.  If possible, targeted experiments on MAST-U would be conducted to test the simulation results and provide continued refinement to the assumptions of the models.    

R(19-3)  Validate tearing mode physics for tearing avoidance in high-performance scenarios

Description:  Tearing modes (TMs) and neoclassical tearing modes (NTMs) can significantly limit the access to high-performance regimes in spherical tokamaks (STs) and standard-aspect-ratio advanced tokamaks (ATs). Particularly in high βP and/or low-torque ITER baseline scenarios it is critical to predict the path to mitigate and avoid TM/NTMs throughout the entire discharge evolution including the early ramp-up phase. The goal of this milestone is to first validate TM/NTM stability physics models and simulations in ST and AT regimes, and to accelerate the development of reliable mitigation and avoidance strategies of TM/NTMs. Existing experimental data from NSTX, NSTX-U, MAST, DIII-D and KSTAR will be investigated for validation, and predictive simulations will be performed for DIII-D and KSTAR advanced scenarios. Each of the key physics components influencing the onset of tearing instabilities will be systematically examined and compared with applicable codes. The effects of shaping, aspect ratio, and current density profile will be analyzed with resistive MHD codes such as resistive DCON and PEST-III on TM databases, and the effects of kinetic profiles including rotation or viscosity will be studied with extended MHD codes such as M3D-C1 or MARS on selected TM/NTM datasets. The toroidally-generalized TM stability index in the simulations will be compared with the onset conditions observed in experiments, and will be used in conjunction with integrated modeling based on TRANSP to test predictive TM avoidance. TM/NTM evolution with islands in experiments will also be analyzed, and compared with extended MHD codes to develop various stabilizing techniques including NTM entrainment. Any reduced TM/NTM physics models developed will be utilized for TM/NTM mitigation and avoidance in various future device operations to allow access to high-performance operating scenarios in NSTX-U.

R(19-4) Assess energetic particle transport by sub-TAE instabilities and develop reduced EP transport modeling tools

Description: Alfvénic instabilities in the toroidal Alfvén eigenmode (TAE) range of frequency are commonly believed to be the greatest concern for enhanced energetic particle (EP) transport in burning plasmas. However, other types of instabilities have also been found to degrade EP confinement in present devices and need to be considered for both optimization of scenarios on present devices and for reliable projections to future devices such as ITER and FNSF. For instance, fishbones and kink modes are known to lead to greatly enhanced EP losses. Tearing mode activity has also been related to decreased EP confinement. Finally, AE modes merging with the acoustic branch, such as beta-induced Alfvénic modes (BAEs & BAAEs) are gaining attention because of their potentially large induced EP transport, see related ITPA Joint Activity. Based on experimental data from NSTX/NSTX-U and other collaborating facilities - e.g. MAST/MAST-U, DIII-D and possibly JET - tools to model EP transport by sub-TAE instabilities will be assessed, with the goal of informing on the development of a comprehensive EP transport module for Integrated Modeling codes such as TRANSP (cf. SciDAC-ISEP project covering FY17-FY21). In particular, the effect of each class of instabilities on EP transport in specific regions of phase space will be characterized, along with the impact on quantities such as Neutral Beam current drive and heating efficiency. At present, an existing framework has been already implemented in TRANSP/NUBEAM to represent EP transport in phase space and its interfacing with models such as the Resonance-Broadening Quasilinear (RBQ) code for Alfvénic instabilities is ongoing. Since it is anticipated that the RBQ approach may not be suitable for instabilities such as fishbones and tearing modes leading to non-diffusive EP transport, alternative approaches will also be explored. The latter include numerically efficient methods to reconstruct the “EP transport probabilities” in terms of phase space variables based on existing theories and/or “reduced” numerical approaches. Once validated, the same methods can then be incorporated in TRANSP/NUBEAM towards self-consistent, time dependent simulations of EP transport by sub-TAE instabilities.

Research Activities carried out in parallel with FWP milestones

Goals denoted as Research Activities (RA) are important NSTX-U research activities that are not FWP milestones and may include substantial NSTX-U collaborator contributions.

RA(19-1):  Expand disruption prediction and avoidance capability for tokamaks

Description: Predicting and avoiding damaging plasma disruptions in fusion-producing tokamaks with high reliability is the present “grand challenge” in tokamak stability research. Meeting this significant goal requires data from a variety of tokamaks and the use of various physical models, analyses, and control methods. The present milestone will greatly expand automated disruption event characterization and forecasting (DECAF) that identifies chains of events that lead to disruptions. Once these chains are determined, methods of breaking the chains using all available control actuators can be defined. Key milestone deliverables include the expansion of the DECAF capability to accurately define disruption event chains across multiple devices and quantitatively increasing the reliability in predicting disruptions with low false positive rate. Through arranged collaborations, the analysis will include input from both national and international tokamaks (e.g. data from DIII-D, KSTAR, MAST-U, NSTX/NSTX-U), which is critical to produce reliable DECAF analysis applicable to ITER and future devices. DECAF code analysis has successfully demonstrated predictive models for global MHD mode onset and automatic determination of rotating tearing mode activity, rotation bifurcation events, and mode locking precursors to disruptions. These successes will be significantly expanded through the further development of physics modules and machine learning capabilities that address the dominant causes of disruptions across several experimental tokamak databases as stated above. Such development will produce and leverage new analysis capabilities and reduced models of results computed by stability analysis codes such as resistive and ideal DCON, M3D-C1, kinetic MHD analysis codes (e.g. MISK), and expanded kinetic equilibrium reconstruction capability. For example, validated physics models and analysis techniques determining tearing mode stability and island growth or decay would be coupled to models of torque balance and applicable code analysis will be tested for their predictive capability of rotating MHD mode locking. A range of models and analyses evaluating density limit-induced disruptions (both low and high) will be evaluated. Technical causes of disruptions will also be assessed. In support of the NSTX-U recovery effort and device restart, these results will be applied to identify how device actuators can be best used to avoid disruptions (e.g. restarting proportional gain and model-based, active n = 1 mode control (with synthetic diagnostics) which will provide important suppression of the device error field, its amplification, and mode control; starting rotation profile control for instability avoidance), and to improve the prediction of disruptions in real-time to inform and directly aid significant improvement of the NSTX-U controlled plasma shutdown handler capability.

RA(19-2):  Assess impact and importance of H species in HHFW-heated NSTX-U full-field plasmas

Description: The goal of NSTX-U is to operate at full field (B = 1 T) for 5 seconds. For this magnetic field, the first and second harmonics of hydrogen (H) are located at the high-field side and in the core plasma, respectively. In principle, part of the high-harmonic fast-wave (HHFW) injected power can be absorbed by the H population reducing the electron and/or the fast-ion heating. For this reason, full wave simulations of NSTX-U scenarios with different H concentrations will be performed. Plasma scenarios with and without neutral beam injection (NBI) will be considered. Furthermore, the possible impact of the tail in the H distribution function (at the 2nd ion cyclotron harmonic) to the electron heating will be investigated by using the combination of RF full wave and Fokker-Planck codes. Finally, an investigation of NSTX-U scenarios in which HHFW might modify either the electron or the ion temperature (through H species) will be analyzed, as it is of particular interest also for transport studies.