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FY2020 Research Milestones

JRT20:    Validation of neoclassical and turbulent impurity transport models in reactor relevant plasma conditions


Accumulation of impurities, ranging from light ions (helium ash) to high-Z (such as tungsten) can adversely impact the reactivity of the fusion core through fuel dilution and excessive radiation.  To inform operation of ITER and beyond, transport of impurities from the divertor to the core will be studied, particularly as parameters that are expected to impact the relative balance of turbulent versus neoclassical impurity transport are varied. Experiments will introduce a wide range of low to high Z impurities, while turbulence and transport properties are documented. Integrated modeling tools will be used to validate theoretical models and interpret the physical mechanisms of transport in the core, divertor, and scrape-off layer.


R(20-1): Initial ST pedestal transport simulation and modeling

Description: Models developed to predict pedestal pressure height have been successfully validated on many conventional aspect ratio tokamak discharges. One state-of-the-art pedestal structure model (EPED) for ELMy H-modes uses two constraints: (1) an MHD peeling-ballooning instability limit that sets the ultimate achievable pressure, (2) a transport limit that sets the pressure gradient limit (i.e. relationship between pressure height and width) via a KBM-threshold constraint. However, EPED has so far been unsuccessful in predicting pedestal structure in spherical tokamaks like NSTX and MAST. There is increasing experimental and theoretical evidence that other transport mechanisms (besides KBM) may be important in setting the pedestal gradient below the KBM-limit in STs, as well as in some conventional tokamak H-modes. Gyrokinetic turbulence codes (GS2, CGYRO, GENE, XGC-1) are now capable of treating the majority of challenges presented by the H-mode pedestal including strong shaping, large gradients, wide range of collisionality, and strong electromagnetic effects. This milestone will use several of these codes to predict linear thresholds of various pedestal instabilities based on ST experiments as well as their key parametric dependencies. Nonlinear simulations will also be attempted to predict saturated transport characteristics, focusing on narrow and wide pedestal cases. In addition, contributions from neoclassical transport will also be predicted using state-of-the-art codes (NEO, XGC0). Together, these simulation and modeling efforts will be used to advance our understanding of whether KBM or other mechanisms set the transport limit in the ST H-mode pedestal.

R(20-2): Scenario optimization algorithm development based on reduced models

Description: This milestone builds off of the FY19 development of reduced profile evolution models using machine learning approaches applied to TRANSP and experimental data. It also builds from recent work on applying numerical optimization algorithms to TRANSP for ramp-up actuator trajectory planning. By applying numerical optimization algorithms (e.g., sequential quadratic programming, Bayesian optimization, or reinforcement learning) to the fast reduced models, convergence to solutions will be greatly accelerated compared to the TRANSP-based approach. The resulting approximate solutions to the actuator trajectory planning problem may be suitable for application in experiment (e.g., as feedforward trajectories to be adjusted in real-time by feedback control algorithms) or may be used as an initial guess for refinement using the higher-fidelity TRANSP-based optimization approach. With the goal of eventually enabling the reduced models to be used in real-time applications (observers to estimate the plasma state from available diagnostics, real-time forecasting, or real-time trajectory planning), diagnostic requirements will be assessed. Application of the techniques and real-time diagnostic developments on other devices, including MAST-U and KSTAR will also be pursued.

  

R(20-3):  Integrated Disruption Modeling for NSTX-U

Description: Disruptions represent a major challenge for the tokamak path to magnetic fusion energy due to the significant electromechanical stresses, heat loads, and energetic electron beams they can create.  Models for understanding and characterizing disruptions are critical for engendering confidence in the design and operation of reactor-scale tokamaks.  In order to better understand electromechanical stresses, thermal loads during disruptions, nonlinear simulations of both the thermal quench and current quench phases of disruptions and vertical displacement events (VDE) will be carried out for NSTX and NSTX-U discharges. These 3D MHD simulations will be performed using an integrated physics model that includes models for impurity radiation and transport, halo, and eddy currents. Non-axisymmetric instabilities and magnetic stochasticity will also be included. The effects of mitigation using injected impurities will be considered.  Where practical, simulations will be validated by comparing results with experimental measurements of vertical displacement, magnetic probes, shunt tile currents, and the change in stored thermal energy.


R(20-4): Assess the effects of neutral beam injection parameters on the fast ion distribution function and neutral beam driven current profile

Description:  Accurate knowledge of neutral beam (NB) ion properties is of paramount importance for many areas of tokamak physics. NB ions modify the power balance, provide torque to drive plasma rotation and affect the behavior of MHD instabilities. Moreover, they determine the non-inductive NB driven current, which is crucial for future devices such as ITER, FNSF and STs with small or no central solenoid. With the additional more tangentially-aimed NB sources, NSTX-U is well equipped to characterize a broad parameter space of fast ion distribution (Fnb) and NB-driven current properties, with significant overlap with other STs such as MAST-U and conventional aspect ratio tokamaks such as DIII-D. The two main goals of this milestone are (i) to characterize the NB ion behavior and compare it with classical predictions, and (ii) to document the operating space of NB-driven current profile. If NSTX-U operations resume in FY20, Fnb will be characterized through the upgraded set of NSTX-U fast ion diagnostics (e.g. fast-ion D-alpha: FIDA, solid-state neutral particle analyzer: ssNPA, scintillator-based fast-lost-ion probe: sFLIP, neutron counters, and possibly a Fusion Products diagnostic) as a function of NB injection parameters (tangency radius, beam voltage) and magnetic field. Building on the initial results obtained in the NSTX-U FY-2016 run campaign, well controlled, single-source scenarios at low NB power will be used to compare fast ion behavior with classical models (e.g. the NUBEAM module of TRANSP) in the absence of fast ion driven instabilities. Collaborations with MAST-U and DIII-D are foreseen for joint studies on NB-CD and validation of the modeling tools. Diagnostics data will be interpreted through the “beam blip” analysis technique and other dedicated codes such as FIDASIM. Then, the NB-driven current profile will be documented for the NB parameter space attainable on the three devices, e.g. by comparing NUBEAM/TRANSP predictions to measurements from Motional Stark Effect (if available), complemented by vertical/tangential FIDA systems, ssNPA and neutron/fusion product diagnostics to assess modifications of the classically expected Fnb. Particular emphasis will be placed on documenting driven current profile variations as a function of injecting beam tangency radius. If NSTX-U cannot support sufficient plasma operations during FY2020, additional emphasis will be placed on collaboration on MAST-U and DIII-D to support the experimental research goals of this milestone on characterization of the fast ion distribution from NBI and of the NB-driven current profile.

 

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(20-1):  Application of expanded disruption prediction and avoidance capability for NSTX-U

Description: Disruption event characterization and forecasting (DECAF) capabilities will be expanded under NSTX-U research activity RA(19-1). These new capabilities will be interfaced with the NSTX-U plasma control system (PCS) along with global mode avoidance tools for disruption avoidance. In support of the NSTX-U device restart, disruption forecasting results will be applied to identify how device actuators can be best used to avoid disruptions. Understanding from DECAF analysis will be implemented into the NSTX-U off-normal event shutdown handler to allow controlled plasma shutdown as desired. Proportional gain and model-based, active n = 1 mode control (with synthetic diagnostics) will be prepared and expanded. These active control capabilities for NSTX-U will provide important suppression of the device error field and its amplification, and mode control. To avoid resistive wall mode onset, rotation profile control algorithm implementation will be started to enable disruption avoidance. The RWM state-space controller observer will also be tested as a criterion for the NSTX-U shutdown handler capability. Real-time MHD spectroscopy (successfully applied to detect the plasma stability and multi-mode plasma response in DIII-D and EAST experiments) will be investigated for use in NSTX-U to actively determine stability to MHD modes while the plasma is stable. The method will be tested and optimized for NSTX-U under this research activity further improving its efficacy for real-time use. The real-time output of the system will also be implemented as a component input to the NSTX-U shutdown handler.

RA(20-2): Global electromagnetic simulations in high-beta NSTX(-U) discharges

The global electromagnetic simulation capability of GTS will address the highly challenging electron transport problem in high-beta NSTX-U experiments. Several advanced EM schemes have been implemented and developed, and these schemes are fully kinetic and cover global finite-beta physics over a wide range of modes, from low-n MHD modes (shear Alfven modes, current-driven tearing modes) to electromagnetic drift-wave instabilities (kinetic ballooning and micro-tearing modes).  These algorithms will be benchmarked against several types of NSTX and NSTX-U data, The benchmarks will include performing linear EM simulations for an NSTX-U L-mode discharge that appears to have strong ITG in the core region, performing linear simulations of microtearing modes with well identified discharges, and attempts to test nonlinear EM simulations. The simulations will focus on single-n, with possible extensions to multiple-n.

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