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FY2014 NSTX Research Milestones

JRT-2014: FES 3 Facility Joint Research Target / Milestone

Conduct experiments and analysis to investigate and quantify the plasma response to non-axisymmetric (3D) magnetic fields in tokamaks. The effects of 3D fields can be both beneficial and detrimental and the research will aim to validate theoretical models in order to predict plasma response to varying levels and types of externally imposed 3D fields. The dependence of the response to multiple plasma parameters will be explored in order to gain confidence in the predictive capability of the models.

R(14-1): Assess access to reduced density and collisionality in high-performance scenarios

The high performance scenarios targeted in NSTX-U and next-step ST devices are based on operating at lower Greenwald density fraction and/or lower collisionality than routinely accessed in NSTX. Collisionality plays a key role in ST energy confinement, non-inductive current drive, pedestal stability, resistive wall mode (RWM) stability, neoclassical toroidal viscosity that affects plasma torque balance, and plasma response and transport with 3D fields. Lower density and/or higher temperature are required to access lower nu*. Potential means identified in NSTX to access lower nu* included high harmonic fast wave heating, reduced fueling and/or Li pumping. However, while D pumping from lithium has been observed, additional gas fueling was typically required to avoid plasma disruption during the current ramp and/or in the high beta phase of the highest performance plasmas of NSTX. The goal of this milestone is to identify the stability boundaries, characterize the underlying instabilities responsible for disruption at reduced density, and develop means to avoid these disruptions in NSTX-U. In support of this goal, tearing mode, RWM, neoclassical toroidal viscosity transport, disruption physics, and scrape-off-layer current (SOLC) in low density and collisionality will be investigated through analysis of NSTX data. This analysis will be used to project to NSTX-U scenarios and will include analysis of the potential impact of proposed/new non-axisymmetric control coils (NCC), and related research will also be carried out in other devices such as DIII-D, KSTAR, and MAST. These physics studies will be utilized to prepare for high-performance scenarios using methods such as current ramp-rate (li and q(r) evolution), H-mode transition timing, shape evolution, heating/beta evolution and control, optimized tearing mode and RWM control, rotation control, error field correction, fueling control (SGI, shoulder injector), and optimized Li pumping. This milestone will also aid development of MISK, VALEN, IPEC, and 3D transport models, as well as TRANSP and TSC integrated predictive models for NSTX-U and next-step STs.

R(14-2): Develop models for *AE mode-induced fast-ion transport

Good confinement of fast ions from neutral beam injection and fusion reactions is essential for the successful operation of ST-CTF, ITER, and future reactors. Significant progress has been made in characterizing the Alfvénic modes (AEs) driven unstable by fast ions and the associated fast ion transport. However, models that can consistently reproduce fast ion transport for actual experiments, or provide predictions for new scenarios and devices, have not yet been validated against a sufficiently broad range of experiments. In order to develop a physics-based parametric fast ion transport model that can be integrated in general simulation codes such as TRANSP, results obtained from NSTX and during collaborations with other facilities (MAST, DIII-D) will be analyzed. Information on the mode properties (amplitude, frequency, radial structure) and on the fast ion response to AEs will be deduced from Beam Emission Spectroscopy, Reflectometers, Fast-Ion D-alpha (FIDA) systems, Neutral Particle Analyzers, Fast Ion Loss Probes and neutron rate measurements. The fast ion transport mechanisms and their parametric dependence on the mode properties will be assessed through comparison of experimental results with theory using both linear (e.g., NOVA-K) and non-linear (e.g., M3D-K, HYM) codes, complemented by gyro-orbit (ORBIT) and full-orbit (SPIRAL) particle-following codes. Based on the general parametric model, the implementation of reduced models in TRANSP will then be assessed. For instance, the existing Anomalous Fast Ion Diffusion (AFID) and radial fast ion convection models in TRANSP could be improved by implementing methods to calculate those transport coefficients consistently with the measured (or simulated) mode properties. Further improvements will also be considered, for instance to include a stochastic transport term or quasi-linear models.

R(14-3): Develop advanced axisymmetric control in sustained high performance plasmas

Next step tokamaks and STs will need high-fidelity axisymmetric control. For instance, magnetic control of the plasma boundary and divertor impact the global stability, power handling, and particle control from poloidally localized pumps. Control of the current and rotation profiles will be critical for avoiding resistive wall modes and tearing modes, thus maximizing the achievable beta. The 2nd neutral beamline for NSTX-U will provide considerable flexibility in the neutral beam driven current profile, while additional divertor coils will allow a wide range of divertor geometries; it is thus an appropriate facility for the development of these critical control techniques. As part of this milestone, realtime control algorithms for the snowflake divertor will be designed; these will likely use methods for rapid tracking of multiple X-points, and additions will be made to the ISOFLUX boundary control algorithm to target specific divertor quantities for control. These divertor control algorithm will be prepared for use in NSTX-U, and may be tested in DIII-D. For profile control, a real-time Motional Stark Effect diagnostic will be developed for NSTX, and the data provided to the NSTX-U implementation of rtEFIT for constrained reconstruction of the current profile; the feasibility of realtime rotation measurements in NSTX-U will be determined and that system implemented as appropriate. Real-time control algorithms will be developed for the current profile using the various neutral beams as actuators; integrated modeling of the current profile evolution with codes such as PTRANSP and TSC will be used for system identification. Similarly, algorithms for control of the rotation profile will be developed, using the neutral beams and magnetic braking as actuators. This profile control development may be based on existing DIII-D control algorithms, but with NSTX-U specific constraints. The ability of the proposed non-axisymmetric control (NCC) coils to provide improved actuator capability for rotation control compared to the existing mid-plane coils will be addressed using NTV calculations. The feasibility of simultaneous rotation, current, and beta control will be assessed. This research will provide a considerable head start developing the required control algorithms for NSTX-U, as well as provide valuable guidance on the axisymmetric control designs for next-step tokamaks and STs, including ITER.