Slow L-H transitions in DIII-D plasmas

Area of Science:

• Plasma Physics
• Fusion Energy Research

Background:

• The L-mode to H-mode (low to high) confinement transition is crucial for achieving sustained fusion reactions.
• Understanding the dynamics of this transition is key to optimizing fusion reactor performance.

Purpose of the Study:

• To investigate the detailed mechanisms governing the L-H transition in plasma confinement.
• To analyze the role of turbulent instabilities and shear flows during the transition process.

Main Methods:

• Utilized an experimental technique enabling arbitrarily slow L-H transitions in the DIII-D tokamak.
• Observed and analyzed periodic turbulent instability bursts near the separatrix.
• Modeled burst damping using self-generated shear flows and a predator-prey relationship.

Main Results:

• Initial transitions were inhibited by periodic turbulent bursts near the separatrix.
• These bursts were effectively damped by self-generated shear flows, exhibiting a predator-prey dynamic.
• Increased neutral beam power led to a shift from initial bursts to Type III Edge Localized Modes, preceding a stable H-mode.

Conclusions:

• Self-generated shear flows play a critical role in suppressing turbulent bursts and enabling the L-H transition.
• The observed predator-prey relationship provides a valuable model for understanding burst dynamics.
• The findings contribute to a more comprehensive understanding of plasma confinement transitions for future fusion devices.