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Macroscopic Stability

Macroscopic stability research in NSTX aims to understand and verify stabilization physics and develop active control methods applicable to the ST development path and to tokamaks in general, leveraged by a unique low-A, low-l_i, and high-beta plasma regime.

Research Priorities:

• Study ideal stability stability dependence on plasma aspect ratio and boundary shaping
• Understand and improve RWM control at increased aspect ratio and boundary shaping
• Study RWM physics in reduced density and collisionality regime
• Study 3D field physics of locking, neoclassical tearing, and magnetic braking at reduced density and collisionality
• Study mode-induced disruption physics and mitigation

Milestones:

R(11-2): Assess ST stability dependence on plasma aspect ratio and boundary shaping

Responsible TSGs: Macro-stability, Advanced Scenarios and Control

"Next-step ST conceptual designs assume aspect ratio A ³ 1.6 and/or high elongation (k = 3-3.5) to maximize projected fusion performance. These aspect ratio and elongation values are higher than commonly accessed on NSTX (A < 1.5 and k  = 2.4-2.8), and to narrow this gap, NSTX Upgrade is designed to overlap next-step configurations by operating with higher aspect ratio (A=1.6-1.7) and k up to 3. This combination of increased aspect ratio and higher elongation is projected to increase vertical instability growth rates by up to a factor of 3 and degrade kink marginal stability normalized beta by -0.5 to -1 relative to present NSTX performance. In this milestone, the integrated plasma scenarios previously developed in NSTX will be extended to plasma geometries closer to those of the Upgrade and next-step devices and the stability properties systematically explored. The maximum sustainable normalized beta will be determined versus aspect ratio (up to A=1.7) and elongation (up to 3) and compared to ideal stability theory using codes such as DCON and PEST. Both passive and actively-controlled RWM stability will be assessed both experimentally and theoretically using codes such as MISK and VALEN, and the viability of previously developed control techniques will be tested. The vertical stability margin will also be determined, and vertical motion detection and control improvements will be implemented. Boundary shape parameters (including squareness) will be varied to assess the impact of shaping and plasma-wall coupling on global stability. Edge NTV rotation damping is also expected to vary with aspect ratio and will be investigated. Plasma profile modifications and the impact on NSTX integrated performance (confinement, non-inductive fraction, pedestal stability, and recycling and divertor dynamics) will also be documented. Overall, these results will help guide stability control development for both NSTX Upgrade and next-step STs."

IR(12-1): Investigate magnetic braking physics and develop toroidal rotation control at low collisionality (incremental)

Responsible TSGs: Macroscopic Stability, Advanced Scenarios and Control

"Plasma rotation and its shear affect plasma transport, stability and achievable bootstrap current and thereby impact the performance of integrated ST scenarios. In order to explore the role of rotation in transport and stability, the physics governing the plasma rotation profile will be assessed over a range of collisionality and rotation by exploiting the tools of NBI momentum input and resonant and non-resonant braking from externally applied 3D fields. Possible tools for varying the plasma collisionality include using density/fueling variation, pumping by lithium, and electron heating by High Harmonic Fast Waves. Key aspects of this study include the behavior of the Neoclassical Toroidal Viscosity at low collisionality and rotation, and the detailed modeling of the plasma response to applied non-axisymmetric fields, including self-shielding. A prerequisite for accomplishing the rotation control assessment of this milestone is the implementation of real-time rotation measurements in FY2011. The effectiveness of various inputs in achieving controllability of the rotation profile will be assessed in order to develop and implement optimized real-time rotation control algorithms. In support of these goals, the IPEC code will be further developed to examine the impact of 3D fields on the plasma, and the more general theory will be converted to simpler models for the real-time rotation control. MISK code analysis will be used to determine rotation profiles that are optimized for plasma stability, and these profiles will in turn be used as targets for the rotation control system. This research will provide the required understanding of rotation control and plasma stability critical for NSTX-U, ITER, and next-step STs."

ITPA and BPO Participation:

• MDC-1 Disruption mitigation by massive gas jets
• MDC-2 Joint experiments on resistive wall mode physics
• MDC-4 Neoclassical tearing mode physics – aspect ratio comparison
• MDC-12 Non-resonant magnetic braking
• MDC-14 Rotation effects on neoclassical tearing modes
• MDC-15 Disruption database development
• MDC-17 Active disruption avoidance