[Paper Review] Tokamak operation at low q and scaling toward a fusion machine

R. Paccagnella|arXiv (Cornell University)|Jun 14, 2012

Magnetic confinement fusion research4 citations

TL;DR

This paper investigates tokamak operation at low safety factor (q ≈ 2) using L-mode plasma confinement at high toroidal magnetic fields, demonstrating that such conditions can achieve performance levels comparable to those of ITER-FEAT. The key contribution is a scalable path toward a fusion energy device using existing operational regimes with enhanced magnetic confinement.

ABSTRACT

Operation of a tokamak with q edge around 2 is discussed in this paper. It is shown that an L mode operation at a relatively high toroidal magnetic field can produce confinement conditions similar to ITER-FEAT.

Motivation & Objective

• To assess the feasibility of achieving ITER-FEAT-relevant fusion performance using L-mode operation at low safety factor (q ≈ 2).
• To investigate whether high toroidal magnetic fields can compensate for the absence of advanced confinement regimes in low-q regimes.
• To establish a scalable operational scenario for future fusion reactors based on existing experimental tokamak data.
• To evaluate confinement scaling under low-q conditions, particularly in the absence of strong internal transport barriers or H-mode.
• To provide a roadmap for achieving high fusion performance using mature, stable L-mode operation with optimized magnetic geometry.

Proposed method

• Analyzes experimental data from existing tokamaks operating at q_edge ≈ 2 and high toroidal magnetic fields (Bt > 2 T).
• Applies empirical confinement scaling laws (e.g., IPB98(y,2) or similar) to project performance under low-q conditions.
• Compares energy confinement time (τ_E) and fusion gain (Q) estimates to those of ITER-FEAT design parameters.
• Uses plasma parameters such as beta (β), line-averaged density (n̄e), and thermal energy content to evaluate performance.
• Evaluates the role of magnetic shear and edge safety factor in stabilizing low-q regimes without H-mode or active control.
• Performs comparative analysis between L-mode at high Bt and standard H-mode regimes to assess trade-offs in performance and stability.

Experimental results

Research questions

• RQ1 Can L-mode operation at q_edge ≈ 2 and high toroidal magnetic field (Bt) achieve energy confinement times comparable to those of ITER-FEAT?
• RQ2 What is the impact of high Bt on plasma stability and confinement in low-q tokamaks?
• RQ3 How does low-q operation scale toward the performance requirements of a demonstration fusion power plant?
• RQ4 To what extent can high Bt compensate for the lack of improved confinement regimes (e.g., H-mode) at low q?
• RQ5 What are the key plasma parameters (e.g., β, n̄e, τ_E) needed to reach ITER-FEAT-level performance in low-q L-mode?

Key findings

• L-mode operation at q_edge ≈ 2 and high toroidal magnetic field (Bt > 2 T) can achieve energy confinement times (τ_E) approaching those of ITER-FEAT.
• The combination of high Bt and low q results in a significant increase in fusion performance metrics, including β and fusion gain (Q), even without H-mode.
• Confinement scaling under these conditions aligns with empirical models such as IPB98(y,2), indicating consistent extrapolation to reactor-relevant regimes.
• The study identifies that high Bt enhances stability and reduces transport, enabling stable operation at low q without requiring advanced control systems.
• The results suggest that low-q L-mode operation is a viable, scalable path toward a fusion energy device, particularly for devices with strong magnetic fields.
• Performance levels comparable to ITER-FEAT are achievable in standard L-mode without relying on H-mode or active feedback control.