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Boundary Physics

The NSTX boundary physics research goal is to develop a comprehensive understanding and predictive capability for edge turbulence, H-mode pedestal physics, ELMs, divertor heat flux and power handling, and fueling.

Research Priorities:

• Understand and develop a predictive capability for the physics mechanisms responsible for the structure of the H-mode pedestal
• Study SOL/pedestal heat and particle transport and pedestal MHD stability with the snowflake divertor configuration and compare with 2D fluid and ideal MHD stability models
• Contribute to the understanding of H-mode pedestal transport, turbulence, and stability response to 3D fields

Milestones:

FY2011 OFES Joint Theory-Experiment Research Milestone (FY2011 JRT):

Responsible TSGs: Boundary Physics, Transport & Turbulence

Experiment:

"Improve the understanding of the physics mechanisms responsible for the structure of the pedestal and compare with the predictive models described in the companion theory milestone. Perform experiments to test theoretical physics models in the pedestal region on multiple devices over a broad range of plasma parameters (e.g., collisionality, beta, and aspect ratio). Detailed measurements of the height and width of the pedestal will be performed augmented by measurements of the radial electric field. The evolution of these parameters during the discharge will be studied. Initial measurements of the turbulence in the pedestal region will also be performed to improve understanding of the relationship between edge turbulent transport and pedestal structure."

Theory:

"A focused analytic theory and computational effort, including large-scale simulations, will be used to identify and quantify relevant physics mechanisms controlling the structure of the pedestal. The performance of future burning plasmas is strongly correlated with the pressure at the top of the edge transport barrier (or pedestal height). Predicting the pedestal height has proved challenging due to a wide and overlapping range of relevant spatiotemporal scales, geometrical complexity, and a variety of potentially important physics mechanisms. Predictive models will be developed and key features of each model will be tested against observations, to clarify the relative importance of various physics mechanisms, and to make progress in developing a validated physics model for the pedestal height."

R(11-3): Assess very high flux expansion divertor operation

Responsible TSGs: Boundary Physics, Advanced Scenarios and Control

"The exploration of high flux expansion divertors for mitigation of high power exhaust is important for NSTX-Upgrade, proposed ST and AT-based fusion nuclear science facilities and for Demo. In this milestone, high flux expansion divertor concepts, e.g. the “snowflake”, will be assessed. The magnetic control, divertor heat flux handling and power accountability, pumping with lithium coatings, impurity production, and their trends with engineering parameters will be studied in this configuration. Potential benefits of combining high flux expansion divertors with gas-seeded radiative techniques and ion pumping by lithium will be explored. Two dimensional fluid codes, e.g. UEDGE, will be employed to study divertor heat and particle transport and impurity radiation distribution. Further, H-mode pedestal stability, ELM characterization, as well as edge transport will also be studied in the experiment and modeled with pedestal MHD stability codes, e.g., ELITE, and transport codes, e.g. TRANSP and MIST. This research will provide the foundation for assessing the extrapolability of high flux expansion divertors for heat-flux mitigation in next-step devices."

ITPA and BPO Participation:

• PEP-19 Basic mechanisms of edge transport with resonant magnetic perturbations in toroidal plasma confinement devices
• PEP-23 Quantification of the requirements for ELM suppression by magnetic perturbations from off-midplane coils
• PEP-25 Inter-machine comparison of ELM control by magnetic field perturbations from midplane RMP coils
• PEP-26 Critical parameters for achieving L-H transitions
• PEP-27 Pedestal profile evolution following L-H/H-L transition
• PEP-28 Physics of H-mode access with different X-point height
• PEP-29 Vertical jolts/kicks for ELM triggering and control
• PEP-31 Pedestal structure and edge relaxation mechanisms in I-mode
• PEP-33 Effects of current ramps on the L-H transition and on the stability and confinement of H-modes at low power above the threshold
• PEP-34 Non-resonant magnetic field driven QH-mode
• DSOL-21 Introduction of pre-characterized dust for dust transport studies in divertor and SOL