Plasma burn-mind the gap

Area of Science:

• Nuclear Fusion Engineering
• Plasma Physics
• Advanced Reactor Design

Background:

• The Spherical Tokamak for Energy Production (STEP) program aims to design a fusion reactor for net electricity generation.
• Current spherical tokamaks (STs) have a significant performance gap compared to future burning plasma concepts.
• Conservative assumptions are made about plasma performance to bridge this gap.

Purpose of the Study:

• To outline plasma scenario designs for the STEP fusion energy project.
• To identify key physics and technical challenges for achieving net electricity production.
• To propose strategies for managing plasma exhaust, core transport, and disruptions.

Main Methods:

• Assessment of plasma configurations, including double null (DN) and positive triangularity (PT).
• Evaluation of external heating and current drive (CD) systems, favoring microwave-based CD.
• Analysis of operational requirements like resistive wall mode (RWM) stabilization and high elongation.

Main Results:

• Double null configurations are favored for manageable plasma exhaust.
• Positive triangularity plasmas with elevated central safety factors enhance core performance.
• Microwaves are identified as the most effective external current drive method.
• Active RWM stabilization and high elongation are crucial for compact STEP designs.

Conclusions:

• Significant challenges remain in core transport due to high normalized plasma pressure, requiring dedicated experiments and advanced models.
• Edge localized modes (ELMs) must be controlled or mitigated to ensure material integrity.
• Novel techniques are needed to manage high runaway electron currents during disruptions for power plant viability.