H-mode dithering phase studies on ST40.

Area of Science:

• Nuclear Fusion Plasma Physics
• Tokamak Plasma Confinement
• Magnetohydrodynamics

Background:

• The dithering H-mode phase in tokamaks is characterized by oscillations near the L-H transition power threshold and low plasma collisionality.
• Understanding this phase is critical for achieving sustained high-performance plasma regimes in fusion devices.

Purpose of the Study:

• To characterize the dithering H-mode phase in the ST40 tokamak, focusing on oscillation frequencies, density dependencies, and power thresholds.
• To investigate the role of ion thermal transport and magnetohydrodynamic (MHD) activity in the dithering H-mode transition.

Main Methods:

• Experimental measurements on the ST40 low aspect ratio, high magnetic field tokamak.
• Analysis of oscillation frequencies and their relationship with core plasma density.
• Use of the ASTRA code to calculate power coupled to ion species.
• Testing a simple model for self-regulation mechanisms.

Main Results:

• Dithering phase oscillation frequencies ranged from 400 to 800 Hz, inversely related to core plasma density.
• H-modes were observed across a nonlinear, low-density power threshold operational space with distinct low- and high-density branches.
• The minimum power threshold for dithering H-mode access was measured at 4.7(±0.5) × 10¹⁹ m^-3, consistent with multi-machine predictions.
• ASTRA calculations showed nonlinear density dependence of power coupled to ions, highlighting the ion thermal channel's contribution.
• Dithers were accompanied by bursts of MHD activity, suggesting a self-regulation scenario.

Conclusions:

• The ion thermal channel plays a significant role in the L-H phase transition, particularly in the dithering regime.
• A self-regulation mechanism involving turbulence, zonal flows, pressure gradients, and MHD activity may govern the dithering phase.
• Findings contribute to understanding H-mode transitions and pedestal physics in fusion plasmas.