L-H transition in the mega-amp spherical tokamak

Area of Science:

• Plasma physics
• Fusion energy research
• Magnetic confinement fusion

Background:

• High-mode (H-mode) plasma confinement is crucial for achieving sustained fusion reactions.
• Conventional threshold scalings often underestimate the power required for H-mode transitions.
• The Mega Ampere Spherical Tokamak (MAST) is a key facility for studying tokamak physics.

Purpose of the Study:

• To investigate the achievement of H-mode plasmas in the MAST spherical tokamak at high input power.
• To analyze the energy confinement properties and plasma edge dynamics following the L-H transition.
• To compare experimental results with existing international scaling laws.

Main Methods:

• Operation of the MAST spherical tokamak.
• High-power plasma heating and fueling.
• Diagnostic measurements of plasma parameters, including density, temperature, turbulence, and rotation.

Main Results:

• H-mode plasmas were successfully achieved at input power significantly exceeding conventional threshold predictions.
• A substantial improvement in energy confinement was observed post-transition, outperforming international scalings.
• The transition was characterized by increased edge-density gradients, reduced turbulence, efficient wave conversion, and edge poloidal rotation.

Conclusions:

• The MAST spherical tokamak can achieve H-mode plasmas under conditions not predicted by standard scalings.
• The observed energy confinement improvements highlight the potential for enhanced performance in spherical tokamaks.
• The study provides valuable insights into the physics of the L-H transition and edge plasma dynamics.