Accessing the density-free regime with ECRH-assisted ohmic start-up on EAST

Area of Science:

• Fusion energy research
• Plasma physics
• Magnetic confinement fusion

Background:

• High plasma density is essential for achieving energy breakeven and burning plasma in tokamaks.
• The Greenwald density limit is an empirical upper bound on electron density, beyond which tokamaks typically disrupt.
• Exceeding this density limit is a significant challenge in magnetic confinement fusion.

Purpose of the Study:

• To investigate methods for achieving high plasma density operation above the Greenwald limit in tokamaks.
• To explore the feasibility of operating in a predicted density-free regime.

Main Methods:

• Experiments were conducted on the Experimental Advanced Superconducting Tokamak (EAST).
• Electron cyclotron resonance heating (ECRH)-assisted ohmic start-up was employed.
• A sufficiently high initial neutral density was maintained.

Main Results:

• Achieved line-averaged electron density in the range of (1.3 to 1.65) x 10^20 m^-3, significantly exceeding EAST's typical operational range (0.8 to 1.0) x 10^20 m^-3.
• Experiments operated in the density-free regime, consistent with recent plasma-wall self-organization theory.
• Demonstrated a viable method for substantially increasing tokamak density limits.

Conclusions:

• The findings suggest a promising approach for increasing tokamak density limits.
• This advancement is critical for progressing toward achieving a burning plasma and fusion energy.
• The successful operation in the density-free regime validates theoretical predictions and opens new avenues for fusion research.